activates====10509564_155871_5594==>Similar findings have been achieved in another study , where it was further shown that <PROT1_155871>  Tat </PROT1_155871> activates the <PROT2_5594>  ERK </PROT2_5594> kinases ( TARGET_CITATION ) . ||10509564_155871_5594==>Because the full - length HIV - 1 <PROT1_155871>  Tat </PROT1_155871> protein is a known transactivator of viral gene expression and activates JNK and <PROT2_5594>  ERK </PROT2_5594> MAPK in CD4 T cells ( TARGET_CITATION ) , we included Tat48 & # x2013 ; 57 as a control peptide in all experiments . ||10509564_155871_5599==>Because the full - length HIV - 1 <PROT1_155871>  Tat </PROT1_155871> protein is a known transactivator of viral gene expression and activates <PROT2_5599>  JNK </PROT2_5599> and ERK MAPK in CD4 T cells ( TARGET_CITATION ) , we included Tat48 & # x2013 ; 57 as a control peptide in all experiments . ||10644332_155871_3725==>In this regard , Lim et al. have recently found that <PROT1_155871>  Tat </PROT1_155871> interaction with SP1 at the human monocyte chemoattractant protein 1 ( hMCP - 1 ) gene promoter may serve as a platform to recruit and stabilize the interaction of <PROT2_3725>  AP1 </PROT2_3725> and NF - { kappa } B proteins to this promoter ( TARGET_CITATION ) . ||10644332_155871_6667==>In this regard , Lim et al. have recently found that <PROT1_155871>  Tat </PROT1_155871> interaction with <PROT2_6667>  SP1 </PROT2_6667> at the human monocyte chemoattractant protein 1 ( hMCP - 1 ) gene promoter may serve as a platform to recruit and stabilize the interaction of AP1 and NF - { kappa } B proteins to this promoter ( TARGET_CITATION ) . ||10644332_155871_6667==><PROT1_155871>  Tat </PROT1_155871> enhances <PROT2_6667>  Sp1 </PROT2_6667> DNA binding and augments <PROT2_6667>  Sp1 </PROT2_6667> phosphorylation , which was shown to be associated with enhanced promoter activity ( CITATION , TARGET_CITATION ) . ||10644332_155871_6667==>Lim and Garzino - Demo provided evidence that ectopic expression of <PROT1_155871>  Tat </PROT1_155871> in the human astrocytoma cell line U - 89 triggered MCP - 1 expression and that this phenomenon relied on the synergistic action of <PROT2_6667>  Sp1 </PROT2_6667> , AP - 1 , and NF - kappa B ( TARGET_CITATION ) . ||11044099_155871_5595==>In an earlier study , we showed that , downstream of the PKC , <PROT1_155871>  Tat </PROT1_155871> could induce the phosphorylation of <PROT2_5595>  ERK1 </PROT2_5595> / 2 , an activation involved in <PROT1_155871>  Tat </PROT1_155871> - induced IL - 10 production ( 7 ) TARGET_CITATION . According to the literature PKC - { delta } , or even PKC - & # 223 ; II , is a potential candidate . ||11156964_155871_5294==>HIV - 1 - <PROT1_155871>  Tat </PROT1_155871> protein has been shown to down - regulate cAMP - response element - binding protein transcription factor expression in PC12 neuronal cells through <PROT2_5294>  PI3K </PROT2_5294> / AKT / cyclic nucleoside phosphodiesterase pathway ( TARGET_CITATION ) . ||11409852_155871_5594==>The mechanisms for ERK1 / 2 activation following viral infection may be induced by direct virus - receptor binding , such as for HIV activation of <PROT2_5594>  ERK </PROT2_5594> ( CITATION ) , or by exposure to a viral protein such as the hepatitis C virus core protein ( CITATION ) or HIV <PROT1_155871>  Tat </PROT1_155871> protein ( TARGET_CITATION ) . ||12023963_155871_5599==>SURE Escherichia coli ( Stratagene ) was transformed with pGEX - 2TK , pGEX - <PROT1_155871>  Tat </PROT1_155871> - 86 , pGEX - <PROT1_155871>  Tat </PROT1_155871> C ( 22 , 25 , 27 ) A , or pGEX - <PROT1_155871>  Tat </PROT1_155871> R ( 49 , 52 , 53 , 55 , 56 , 57 ) A and was induced with IPTG ( isopropyl - & # 223 ; - D - thiogalactopyranoside ) for 3 to 4 h at 37 & # 176 ; C , and the fusion proteins were extracted as previously described ( CITATION , TARGET_CITATION ) , keeping all solutions degassed and at 4 & # 176 ; c. The yield , stability , and biological activity of wild - type <PROT1_155871>  Tat </PROT1_155871> ( assessed as activation of endothelial cell c - Jun N - terminal kinase ( <PROT2_5599>  JNK </PROT2_5599> ) ) were highest when flash - frozen as a glutathione S - transferase ( GST ) fusion . ||12167619_155871_5594==>In this context , it is worth noting that HIV - 1 <PROT1_155871>  Tat </PROT1_155871> protein activates the <PROT2_5594>  ERK </PROT2_5594> pathway ( TARGET_CITATION ) . ||12573582_1026_155807==>Preliminary studies in our laboratory indicate that <PROT2_155807>  Vpr </PROT2_155807> may be responsible , at least in part , for the enhanced expression of macrophage <PROT1_1026>  p21 </PROT1_1026> ( n. V & aacute ; zquez et al. , unpublished observations ) , as recently suggested in T lymphoid and myeloid cell lines ( TARGET_CITATION ) . ||9325171_155871_5594==>In this respect , <PROT1_155871>  Tat </PROT1_155871> protein is expressed in cells obtained from the central nervous system of patients affected by encephalitic AIDS ( 14 ) CITATION . It has also been shown that , at concentrations of 0.1 & # 150 ; 10 nM , <PROT1_155871>  Tat </PROT1_155871> protein activates various signal transduction pathways in neurons , including phosphatidylinositol 3 kinase ( PI - 3K ) ( 15 CITATION , 16 ) CITATION , <PROT2_5594>  ERK </PROT2_5594> / MAPK ( 17 ) TARGET_CITATION , and the cyclic adenosine monophosphate ( cAMP ) - dependent protein kinase A ( PKA ) pathway ( 18 ) CITATION . ||9394803_155871_5290==>Moreover , the <PROT2_5290>  PI3K </PROT2_5290> / AKT pathway was found to be activated by <PROT1_155871>  Tat </PROT1_155871> in T - lymphoblastoid Jurkat cells ( TARGET_CITATION ) . ||9394803_155871_5290==>Interestingly , two human immunodeficiency virus type 1 ( HIV - 1 ) proteins , <PROT1_155871>  Tat </PROT1_155871> ( TARGET_CITATION ) and Nef ( CITATION ) , both activate the <PROT2_5290>  PI3K </PROT2_5290> - AKT pathway , thus providing antiapoptotic signals . ||9560267_155807_5970==>Alternatively , as shown by ( TARGET_CITATION ) , <PROT1_155807>  Vpr </PROT1_155807> may modulate the transcriptional coactivator p300 , which promotes cooperative interactions between the <PROT2_5970>  RelA </PROT2_5970> subunit of NF - kB and cyclin B1.Cdc2 . ||9621077_155871_3791==>The evidence that vitronectin facilitates the phosphorylation of <PROT2_3791>  VEGFR </PROT2_3791> - 2 prompted us to investigate whether the activation of endothelial cells by VEGF - A , as well as by HIV - 1 - <PROT1_155871>  Tat </PROT1_155871> , another ligand of <PROT2_3791>  VEGFR </PROT2_3791> - 2 which stimulates migration and proliferation of endothelial and Kaposi 's sarcoma cells ( CITATION ; TARGET_CITATION ) , was enhanced by specific substrates . ||9621077_155871_3791==>The HIV transcription factor , <PROT1_155871>  tat </PROT1_155871> , has also been reported to bind to <PROT2_3791>  KDR </PROT2_3791> and stimulate receptor autophosphorylation ( TARGET_CITATION ) . ||9621077_155871_3791==>Thus , although <PROT1_155871>  Tat </PROT1_155871> binds the VEGF receptor Flk - 1 / <PROT2_3791>  KDR </PROT2_3791> CITATION , TARGET_CITATION , this does not lead to cell growth . ||9621077_155871_3791==>HIV - 1 <PROT1_155871>  TAT </PROT1_155871> , via its basic domain , can also bind to and activate the Flk - 1 / <PROT2_3791>  KDR </PROT2_3791> receptor in KS cells ( CITATION , TARGET_CITATION ) . ||9621077_155871_3791==><PROT1_155871>  Tat </PROT1_155871> has been shown to bind to and activate <PROT2_3791>  VEGFR </PROT2_3791> - 2 CITATION TARGET_CITATION , to stimulate KS spindle cell growth CITATION , and to induce neovascularization in vivo CITATION CITATION and in transgenic mice CITATION . <PROT2_3791>  VEGFR </PROT2_3791> - 3 is also increased in KS CITATION , and its ligand , VEGF - C , stimulates the proliferation of KS cells in vitro CITATION . It thus seems probable that an autocrine VEGF / <PROT2_3791>  VEGFR </PROT2_3791> activation loop plays a part in the development of AIDS - linked KS . ||9621077_155871_3791==>Additionally , <PROT1_155871>  Tat </PROT1_155871> binds to and activates the tyrosine kinase receptors encoded by the <PROT2_3791>  KDR </PROT2_3791> and Flt - 1 genes in EC and Kaposi 's sarcoma cells ( CITATION , TARGET_CITATION ) and monocytes ( CITATION ) , respectively . ||9621077_155871_3791==>It has been reported that soluble <PROT1_155871>  Tat </PROT1_155871> induces the activation of vascular endothelium and of Kaposi 's sarcoma cells through the engagement of <PROT2_3791>  VEGFR </PROT2_3791> - 2 ( CITATION , TARGET_CITATION ) and the integrin system ( CITATION , CITATION , CITATION ) . ||9621077_155871_3791==>Because <PROT1_155871>  Tat </PROT1_155871> signals inside the cells through the tyrosine kinase <PROT2_3791>  VEGFR </PROT2_3791> - 2 ( CITATION , TARGET_CITATION , CITATION ) , we investigated the tyrosine phosphorylation of <PROT2_3791>  VEGFR </PROT2_3791> - 2 . ||9621077_155871_3791==>The dramatic reduction in ( I125 ) albumin leakage elicited by an anti - <PROT2_3791>  VEGFR </PROT2_3791> - 2 Ab in both <PROT1_155871>  Tat </PROT1_155871> - MBP - and VEGF - A - treated skin indicates that both molecules activated in vivo <PROT2_3791>  VEGFR </PROT2_3791> - 2 , as also reported in vitro on EC and Kaposi & # 146 ; s sarcoma cells ( TARGET_CITATION , CITATION ) . ||9621077_155871_3791==>Previous studies have demonstrated that HIV - 1 <PROT1_155871>  Tat </PROT1_155871> can bind to and activate specific surface receptors including the Flk - 1 / <PROT2_3791>  KDR </PROT2_3791> receptor for vascular endothelial growth factor ( VEGF ) , 3 and the { alpha } 5 { beta } 1 , { alpha } V { beta } 3 , and { alpha } V { beta } 5 integrins ( CITATION , CITATION , TARGET_CITATION , CITATION ) . ||9621077_155871_3791==>HIV - 1 <PROT1_155871>  Tat </PROT1_155871> activation of Flk - 1 / <PROT2_3791>  KDR </PROT2_3791> ( CITATION ) , FLT - 1 ( CITATION , TARGET_CITATION ) , and { alpha } 5 { beta } 1 and { alpha } V { beta } 3 integrins ( CITATION , CITATION , CITATION ) may cause cells to be `` confused '' by the multiplicity of pathways simultaneously activated , whereas these phenomena do not normally occur in the presence of physiological ligands . ||9621077_155871_3791==>Additionally , <PROT1_155871>  Tat </PROT1_155871> binds to and activates the tyrosine kinase receptor encoded by <PROT2_3791>  KDR </PROT2_3791> ( vascular endothelial growth factor ( VEGF ) receptor ( R ) 2 ) in EC and Kaposi 's cells ( CITATION , CITATION , TARGET_CITATION , CITATION , CITATION and CITATION ) . ||9621077_155871_3791==>KS cells express both <PROT2_3791>  VEGFR2 </PROT2_3791> ( TARGET_CITATION , CITATION , CITATION and CITATION ) and small alpha , Greek5small beta , Greek1 and small alpha , Greekvsmall beta , Greek3 integrins ( CITATION , CITATION and CITATION ) that are the receptors for <PROT1_155871>  Tat </PROT1_155871> mediating EC activation . ||9621077_155871_3791==><PROT1_155871>  Tat </PROT1_155871> ligation of these specific surface molecules activates several cellular protein kinases TARGET_CITATION , CITATION , including Flk - 1 / <PROT2_3791>  KDR </PROT2_3791> , Flt - 1 , and MAP kinase . ||9621077_155871_3791==>The RGD and basic domains of <PROT1_155871>  Tat </PROT1_155871> stimulate and modulate the VEGF receptor Flk - 1 / <PROT2_3791>  KDR </PROT2_3791> and components of the focal adhesion kinase in Kaposi 's sarcoma cells ( TARGET_CITATION , CITATION ) . ||9621077_155871_3791==>HIV - <PROT1_155871>  Tat </PROT1_155871> can also activate the VEGF receptor Flk - 1 / <PROT2_3791>  KDR </PROT2_3791> , acting as a pro - angiogenic factor CITATION , TARGET_CITATION . ||9621077_155871_3791==>However , because the proangiogenic factor VEGF - A CITATION and presumably the HIV - <PROT1_155871>  Tat </PROT1_155871> protein which activates the VEGF - Areceptor Flk - 1 / <PROT2_3791>  KDR </PROT2_3791> CITATION , TARGET_CITATION can induce SDF - 1 expression on endothelial cells , the capillary vasculature of the lung and the gastrointestinal tract could also become SDF - 1 positive , perhaps under hypoxic conditions . 
binds====10096020_10524_155871==><PROT1_10524>  Tip60 </PROT1_10524> was discovered as an HIV - 1 <PROT2_155871>  Tat </PROT2_155871> interacting protein ( CITATION ) and is capable of acetylating several lysine residues in the amino - terminal tails of core histones H2A , H3 , and H4 ( TARGET_CITATION ) . ||10096576_155030_5478==>Supporting this hypothesis , several studies suggest that affinities of <PROT2_5478>  CypA </PROT2_5478> for <PROT1_155030>  Gag </PROT1_155030> and for CA are strikingly different ( TARGET_CITATION , CITATION , CITATION ) . ||10096576_155030_5478==><PROT2_5478>  CypA </PROT2_5478> binds the <PROT1_155030>  Gag </PROT1_155030> polyprotein more strongly than it binds mature CA ( TARGET_CITATION , CITATION ) . ||10329126_1025_155871==>The quaternary complex between <PROT1_1025>  CDK9 </PROT1_1025> , CycT1 , <PROT2_155871>  Tat </PROT2_155871> , and TAR RNA induces conformational changes in the enzyme that result in the activation of the <PROT1_1025>  CDK9 </PROT1_1025> kinase ( CITATION , CITATION , CITATION , CITATION , CITATION , TARGET_CITATION ) and , subsequently , the release of TAR ( CITATION ) . ||10329126_1025_155871==>Formation of the quaternary complex between <PROT1_1025>  CDK9 </PROT1_1025> , CycT1 , <PROT2_155871>  Tat </PROT2_155871> , and TAR RNA induces conformational changes in the enzyme complex that result in the constitutive activation of the <PROT1_1025>  CDK9 </PROT1_1025> kinase ( CITATION , CITATION , CITATION , CITATION , CITATION , TARGET_CITATION ) . ||10329126_1025_155871==>The formation of a quaternary complex among <PROT1_1025>  CDK9 </PROT1_1025> , cyclin T1 , <PROT2_155871>  Tat </PROT2_155871> , and TAR RNA determines the recruitment of human P - TEFb to the transcription elongation complex and the efficient synthesis of long productive viral transcripts ( CITATION , CITATION , CITATION , TARGET_CITATION , CITATION , CITATION ) . ||10329126_1025_155871==><PROT2_155871>  Tat </PROT2_155871> is brought to the HIV promoter by interacting with the TAR RNA hairpin located at the 5 ' end of the viral mRNA transcripts and forms a ternary complex with cyclin T1 , which recruits the <PROT1_1025>  Cdk9 </PROT1_1025> kinase ( CITATION , CITATION , CITATION , CITATION , CITATION , TARGET_CITATION , CITATION ) . ||10329126_155871_904==>For example , HIV - 1 <PROT1_155871>  Tat </PROT1_155871> is unable to bind cooperatively with murine <PROT2_904>  CycT1 </PROT2_904> to TAR RNA ( CITATION , CITATION , CITATION , CITATION , CITATION , TARGET_CITATION ) , and TAR RNA - binding and <PROT1_155871>  Tat </PROT1_155871> transactivation could be rescued by expression of human <PROT2_904>  CycT1 </PROT2_904> or a murine <PROT2_904>  CycT1 </PROT2_904> protein containing a point mutation ( Y261C ) in the <PROT1_155871>  Tat </PROT1_155871> - TAR recognition motif ( CITATION , CITATION ) . ||10329126_155871_904==>It later became clear that the mechanistic explanation for the defect was the inability of the murine form of the essential <PROT1_155871>  Tat </PROT1_155871> cofactor , <PROT2_904>  CycT1 </PROT2_904> , to support efficient interaction with the TAR element when bound to <PROT1_155871>  Tat </PROT1_155871> ( CITATION , CITATION , CITATION , TARGET_CITATION , CITATION ) . ||10329126_155871_904==>Alternatively , substitution of a single amino acid in the murine <PROT2_904>  CycT1 </PROT2_904> protein , tyrosine 261 , with its human <PROT2_904>  CycT1 </PROT2_904> counterpart , cysteine , results in a <PROT2_904>  CycT1 </PROT2_904> protein that can be efficiently recruited to TAR by <PROT1_155871>  Tat </PROT1_155871> proteins and , hence , support transactivation ( CITATION , CITATION , CITATION , CITATION , TARGET_CITATION ) . ||10329126_155871_904==><PROT1_155871>  Tat </PROT1_155871> transactivation is abolished in mouse cells because HIV - 1 <PROT1_155871>  Tat </PROT1_155871> is unable to bind cooperatively with murine <PROT2_904>  CycT1 </PROT2_904> to TAR RNA structure ( CITATION - TARGET_CITATION ) . ||10329126_155871_904==>The formation of the tripartite complex between <PROT1_155871>  Tat </PROT1_155871> , <PROT2_904>  CycT1 </PROT2_904> , and TAR depends on the 5 ' bulge and central loop in TAR ( CITATION , CITATION , CITATION , CITATION , CITATION - TARGET_CITATION , CITATION ) . ||10329126_155871_904==>Finally , the species - specific restriction of HIV - 1 <PROT1_155871>  Tat </PROT1_155871> transactivation has been closely linked to the <PROT2_904>  CycT1 </PROT2_904> subunit of P - TEFb ( CITATION - TARGET_CITATION ) . ||10329126_155871_904==>Mouse <PROT2_904>  CycT1 </PROT2_904> differs from its human homologue at several amino acids , and the change from tyrosine to cysteine at position 261 prevents it from interacting with <PROT1_155871>  Tat </PROT1_155871> ( CITATION , CITATION , TARGET_CITATION ) . ||10329126_155871_904==>Interestingly , the substitution of a single amino residue ( tyrosine - 261 ) in mouse <PROT2_904>  CycT1 </PROT2_904> compared to its human homologue was previously identified as the major determinant restricting <PROT1_155871>  Tat </PROT1_155871> - mediated HIV - 1 LTR transactivation in NIH 3T3 cells ( CITATION , CITATION , TARGET_CITATION , CITATION ) . ||10329126_155871_904==>The change of amino residue 261 from cysteine to tyrosine in <PROT2_904>  CycT1 </PROT2_904> had previously been shown to be the major determinant restricting <PROT1_155871>  Tat </PROT1_155871> - mediated HIV - 1 LTR transactivation in NIH 3T3 cells ( CITATION , CITATION , TARGET_CITATION , CITATION ) . ||10329126_155871_904==>Human <PROT2_904>  cycT1 </PROT2_904> markedly stimulates <PROT1_155871>  Tat </PROT1_155871> activation , while cycT2a and T2b fail to form a complex with <PROT1_155871>  Tat </PROT1_155871> bound to the transactivation response RNA element ( TAR ) ( TARGET_CITATION ; CITATION ) . ||10329126_155871_904==>Human <PROT2_904>  cycT1 </PROT2_904> stimulates <PROT1_155871>  Tat </PROT1_155871> activation , while cycT2a and T2b fail to form a complex with <PROT1_155871>  Tat </PROT1_155871> bound to the transactivation response RNA element ( TAR ) ( TARGET_CITATION ; CITATION ) . ||10329126_155871_904==>The quaternary complex between CDK9 , <PROT2_904>  CycT1 </PROT2_904> , <PROT1_155871>  Tat </PROT1_155871> , and TAR RNA induces conformational changes in the enzyme that result in the activation of the CDK9 kinase ( CITATION , CITATION , CITATION , CITATION , CITATION , TARGET_CITATION ) and , subsequently , the release of TAR ( CITATION ) . ||10329126_155871_904==>Formation of the quaternary complex between CDK9 , <PROT2_904>  CycT1 </PROT2_904> , <PROT1_155871>  Tat </PROT1_155871> , and TAR RNA induces conformational changes in the enzyme complex that result in the constitutive activation of the CDK9 kinase ( CITATION , CITATION , CITATION , CITATION , CITATION , TARGET_CITATION ) . ||10329126_155871_904==>Although the C - terminal region of <PROT2_904>  CycT1 </PROT2_904> might play a role in TAR binding and binds the C - terminal domain , the N - terminal 272 amino acids of <PROT2_904>  CycT1 </PROT2_904> are sufficient for <PROT1_155871>  Tat </PROT1_155871> transactivation in vitro and in vivo ( CITATION , CITATION , CITATION - CITATION , CITATION , TARGET_CITATION , CITATION ; k. Fujinaga , d. Irwin , m. Geyer , r. Taube , and b. m. Peterlin , submitted for publication ) . ||10329126_155871_904==>Since the cysteine at position 261 is changed to tyrosine in the murine <PROT2_904>  CycT1 </PROT2_904> ( mCycT1 ) , mCycT1 can not support <PROT1_155871>  Tat </PROT1_155871> transactivation ( CITATION , CITATION , CITATION , CITATION , TARGET_CITATION ) . ||10329126_155871_904==>This intracellular restriction could be partially overcome by the introduction of human cyclin T1 ( <PROT2_904>  CycT1 </PROT2_904> ) ( CITATION , CITATION , CITATION , TARGET_CITATION , CITATION ) , indicating that human <PROT2_904>  CycT1 </PROT2_904> is essential for <PROT1_155871>  Tat </PROT1_155871> - mediated transcription . ||10329126_155871_904==>Human and murine forms of <PROT2_904>  CycT1 </PROT2_904> are 90 % identical at the amino acid level ; a single amino acid change from cysteine to tyrosine at position 261 of murine <PROT2_904>  CycT1 </PROT2_904> prevents it from interacting with <PROT1_155871>  Tat </PROT1_155871> ( CITATION , CITATION , TARGET_CITATION ) . ||10329126_155871_904==>A single amino acid change at residue 261 from cysteine to tyrosine in murine <PROT2_904>  CycT1 </PROT2_904> has been shown to be the major determinant in restriction of <PROT1_155871>  Tat </PROT1_155871> - mediated HIV - 1 LTR trans - activation in NIH 3T3 cells ( CITATION , CITATION , CITATION , TARGET_CITATION , CITATION ) . ||10497173_155871_6383==>We have recently observed that <PROT1_155871>  Tat </PROT1_155871> internalization requires binding of the protein to cell surface HSPGs , since the uptake process does not occur in cells selectively impaired in <PROT2_6383>  HSPG </PROT2_6383> biosynthesis and can be abolished by cell treatment with heparinase III or by competition with soluble , extracellular heparin ( TARGET_CITATION , CITATION ) . ||10661406_1025_155871==>The cyclin domain is responsible for binding <PROT1_1025>  CDK9 </PROT1_1025> and various other proteins , including NF - { kappa } B ( CITATION ) CIITA ( TARGET_CITATION ) , and GRIP1 ( CITATION ) ; the TRM allows the cooperative binding of <PROT2_155871>  Tat </PROT2_155871> and TAR to cyclin T1 ( CITATION ) ; the His - rich region binds the CTD of RNA polymerase II ( CITATION ) as well as granulin ( CITATION ) ; and the PEST domain is required for the interaction with SCFSKP2 ( CITATION ) . ||10661406_1025_155871==>Since the isolation of cyclin T1 as a <PROT1_1025>  CDK9 </PROT1_1025> partner and <PROT2_155871>  Tat </PROT2_155871> - interacting protein , three other cellular proteins have been identified that bind directly to the cyclin box ( amino acids 1 to 254 ) of cyclin T1 , the major histocompatibility complex class II transactivator CIITA , the NF - { kappa } B RelA / p65 subunit , and the glucocorticoid receptor interacting polypeptide 1 GRIP - 1 ( CITATION , TARGET_CITATION , CITATION ) . ||10931842_155807_904==>The association of <PROT1_155807>  Vpr </PROT1_155807> with Tat and <PROT2_904>  CycT1 </PROT2_904> also stimulates LTR - driven transcription ( TARGET_CITATION ) . ||11024024_155871_6383==><PROT2_6383>  HSPG </PROT2_6383> are also involved in the uptake and internalization of small basic molecules such as polyamines ( CITATION ) and basic peptides such as fibroblast growth factor ( CITATION ) and human immunodeficiency virus - 1 <PROT1_155871>  Tat </PROT1_155871> ( TARGET_CITATION ) as well as of virus particles ( CITATION , CITATION ) . ||11024024_155871_6383==>More recently a role for cell - surface heparan sulfate proteoglycans ( <PROT2_6383>  HSPG </PROT2_6383> ) in cellular uptake of a recombinant GST - <PROT1_155871>  Tat </PROT1_155871> - GFP fusion protein was shown by the same group ( TARGET_CITATION ) . ||11024024_155871_6383==>Tyagi et al. ( TARGET_CITATION ) suggested a requirement of <PROT2_6383>  HSPG </PROT2_6383> for cellular uptake of a GST - <PROT1_155871>  Tat </PROT1_155871> - GFP fusion protein . ||11024024_155871_6383==>We have demonstrated the ability of a 14 - amino acid peptide derived from HIV - <PROT1_155871>  Tat </PROT1_155871> to efficiently deliver DNA , HS , and DS into mammalian cells via electrostatic binding and that both CS / DS and <PROT2_6383>  HSPG </PROT2_6383> are involved in peptide - polyanion complex uptake , as opposed to uptake of full - length HIV - <PROT1_155871>  Tat </PROT1_155871> that specifically required <PROT2_6383>  HSPG </PROT2_6383> ( TARGET_CITATION ) . ||11024024_155871_6383==>We have recently observed that <PROT1_155871>  Tat </PROT1_155871> internalization requires binding of the protein to cell surface HSPGs , since the uptake process does not occur in cells selectively impaired in <PROT2_6383>  HSPG </PROT2_6383> biosynthesis and can be abolished by cell treatment with heparinase III or by competition with soluble , extracellular heparin ( CITATION , TARGET_CITATION ) . ||11027346_155871_7852==>Such factors may include the relatively low prevalence of X4 strains in infected populations ; the differential distribution of CCR5 + versus <PROT2_7852>  CXCR4 </PROT2_7852> + target cells in mucosal tissue CITATION ; and , apropos of the subject of this review , a second HIV - encoded chemokine mimic , the <PROT1_155871>  Tat </PROT1_155871> protein CITATION , TARGET_CITATION . ||11027346_155871_7852==>It acts as a chemotactic agonist for neutrophils , basophils ( via CCR3 ) , mast cells ( via CCR3 ) and monocytes ( via CCR2 ) but as an antagonist at <PROT2_7852>  CXCR4 </PROT2_7852> ; importantly <PROT1_155871>  Tat </PROT1_155871> has no activity at CCR5 TARGET_CITATION . ||11027346_155871_7852==><PROT1_155871>  Tat </PROT1_155871> binds and activates vascular endothelial growth factor receptors 1 and 2 ( VEGFR - 1 / Flk - 1 and VEGFR - 2 / Flt - 1 ) in endothelial and other cell types ( CITATION , CITATION and CITATION ) ; interacts with small beta , Greek - chemokine receptors CCR2 and CCR3 and acts as a potent chemoattractant for various leukocytes ( CITATION , CITATION , CITATION and CITATION ) ; binds chemokine receptor <PROT2_7852>  CXCR4 </PROT2_7852> and competes with X4 - HIV - 1 virus infection on T - cells ( CITATION and TARGET_CITATION ) ; and induces cell adhesion through interaction of its RGD domain ( located in its second exon ) with integrin receptors small alpha , Greek5small beta , Greek1 and small alpha , Greekvsmall beta , Greek3 ( CITATION , CITATION and CITATION ) . ||11027346_155871_7852==>Finally , we tested whether the observed pathway of extracellular <PROT1_155871>  Tat </PROT1_155871> internalization also holds true for CD4 + T - cells expressing the <PROT2_7852>  CXCR4 </PROT2_7852> chemokine receptor , which besides acting as a co - receptor for HIV - 1 infection , has also been shown to be a biologically relevant receptor for extracellular <PROT1_155871>  Tat </PROT1_155871> ( TARGET_CITATION , CITATION ) . ||11027346_155871_7852==>In this respect , it is of interest to note that the internalization process in CD4 + T - lymphocytes expressing <PROT2_7852>  CXCR4 </PROT2_7852> , a chemokine receptor that specifically binds extracellular <PROT1_155871>  Tat </PROT1_155871> ( TARGET_CITATION , CITATION ) and mainly resides outside of lipid rafts ( CITATION & # 150 ; CITATION ) , is also severely inhibited by cholesterol depletion , indistinguishable from <PROT2_7852>  CXCR4 </PROT2_7852> - negative cells . ||11027346_155871_7852==><PROT1_155871>  Tat </PROT1_155871> is secreted from virus - infected cells and has been shown to interact extracellularly with the chemokine receptor <PROT2_7852>  CXCR4 </PROT2_7852> ( CITATION and TARGET_CITATION ) . ||11027346_155871_7852==>Provocatively , we and others have recently documented that extracellular soluble full - length <PROT1_155871>  Tat </PROT1_155871> protein binds cell surface <PROT2_7852>  CXCR4 </PROT2_7852> and blocks infection by T - tropic - HIV - 1 ( CITATION and TARGET_CITATION ) . ||11027346_155871_7852==>In the course of studying how <PROT1_155871>  Tat </PROT1_155871> interferes with <PROT2_7852>  CXCR4 </PROT2_7852> - using HIV - 1 subtypes and in investigating the pro - apoptotic and anti - angiogenic properties of linear peptides derived from HIV - 1 - <PROT1_155871>  Tat </PROT1_155871> ( CITATION ) , we were intrigued by the observation that mutations in its cysteine - rich domain ( C25A , C27A ) abolished the anti - HIV effect of <PROT1_155871>  Tat </PROT1_155871> ( TARGET_CITATION ) . ||11027346_155871_7852==>These results are in agreement with the finding of ( TARGET_CITATION ) that <PROT1_155871>  Tat </PROT1_155871> protein behaves as specific <PROT2_7852>  CXCR4 </PROT2_7852> antagonist which inhibits the entry and replication of X4 but not R5 viruses in PBMC . ||11027346_155871_7852==>HIV - 1 <PROT1_155871>  Tat </PROT1_155871> has been reported to function as an antagonist for <PROT2_7852>  CXCR4 </PROT2_7852> TARGET_CITATION . ||11027346_155871_7852==><PROT1_155871>  Tat </PROT1_155871> can bind to several receptors at the plasma membrane , such as CD26 ( Gutheil et al. , 1994 CITATION ) , <PROT2_7852>  CXCR4 </PROT2_7852> ( Xiao et al. , 2000 TARGET_CITATION ) , heparan sulfate proteoglycans ( Tyagi et al. , 2001 CITATION ) , and the low - density lipoprotein receptor & # 150 ; related protein ( LRP ) ( Liu et al. , 2000 CITATION ) . ||11027346_155871_7852==>TARGET_CITATION report that the HIV - 1 <PROT1_155871>  Tat </PROT1_155871> protein , which is secreted from virus - infected cells , is a <PROT2_7852>  CXCR4 </PROT2_7852> - specific antagonist that can selectively inhibit the entry and replication of SI , but not NSI , forms in peripheral blood mononuclear cells , thereby restricting the target cell substrate preferentially for the SI form . ||11087358_155807_5886==>For example , the human immunodeficiency virus <PROT1_155807>  vpr </PROT1_155807> protein can bind to the second uba domain of <PROT2_5886>  hHR23A </PROT2_5886> ( Withers - Ward et al. , 1997 CITATION ) , but not to a number of other uba domains ( Withers - Ward et al. , 2000 TARGET_CITATION ) . ||11250896_155030_5478==>Cyclophilin A ( <PROT2_5478>  CyPA </PROT2_5478> ; Cpr1p ) binds to the human immunodeficiency virus type 1 ( HIV - 1 ) <PROT1_155030>  Gag </PROT1_155030> protein ( CITATION ) and is required for wild - type HIV - 1 replication kinetics ( TARGET_CITATION ) . ||11250896_155030_5478==>This mutant <PROT2_5478>  CypA </PROT2_5478> contains a lesion in the catalytic site ( H126A ) that abolishes both isomerase activity and the capacity to bind to the HIV - 1 <PROT1_155030>  Gag </PROT1_155030> ( TARGET_CITATION , CITATION ) or to CsA ( CITATION ) . ||11250896_155030_5478==>Cyclophilin A ( <PROT2_5478>  CyPA </PROT2_5478> ) is incorporated into HIV - 1 virions via interaction with the CA domain of the <PROT1_155030>  Gag </PROT1_155030> polyprotein and is required for wild - type viral replication kinetics ( TARGET_CITATION ) . ||11250896_155030_5478==>An interaction between the HIV - 1 <PROT1_155030>  Gag </PROT1_155030> - encoded <PROT1_155030>  capsid </PROT1_155030> ( CA ) protein and the prolyl isomerase cyclophilin A ( <PROT2_5478>  CypA </PROT2_5478> ) is required for efficient HIV - 1 replication ( TARGET_CITATION ) . ||11250896_155030_5478==>Consistent with previous reports studying the effects of CsA treatment ( CITATION , CITATION ) , <PROT1_155030>  Gag </PROT1_155030> mutation ( CITATION ) , or <PROT2_5478>  CypA </PROT2_5478> gene targeting ( TARGET_CITATION ) on HIV - 1 replication , <PROT2_5478>  CypA </PROT2_5478> activity was not required for efficient expression and processing of HIV - 1 <PROT1_155030>  Gag </PROT1_155030> polyproteins ( fig. 3A ) . ||11250896_155030_5478==>Samples were normalized for CA protein , because it has been shown that inactivation of <PROT2_5478>  CypA </PROT2_5478> does not affect the content and processing of <PROT1_155030>  Gag </PROT1_155030> in HIV - 1 virions ( TARGET_CITATION ) . ||11250896_155030_5478==><PROT2_5478>  CypA </PROT2_5478> binds to the human immunodeficiency virus type 1 <PROT1_155030>  Gag </PROT1_155030> protein and is required for wild - type human immunodeficiency virus type 1 replication kinetics ( TARGET_CITATION ) . ||11250896_155030_5478==>Aside from minimal effects on the kinetics of <PROT1_155030>  gag </PROT1_155030> processing or virion release ( CITATION , CITATION ) , disruption of <PROT2_5478>  CypA </PROT2_5478> incorporation into virions has no effect on biochemical or ultrastructural characteristics of HIV - 1 virions , including endogenous reverse transcription ( CITATION , TARGET_CITATION , CITATION , CITATION , CITATION , CITATION ) . ||11427703_155030_7251==>In a separate study , VerPlank et al. ( TARGET_CITATION ) used a yeast two - hybrid screen of a B cell library with HIV - 1 <PROT1_155030>  Gag </PROT1_155030> as bait and isolated the protein <PROT2_7251>  Tsg101 </PROT2_7251> , which interacts with the PTAPP L domain sequence of HIV - 1 p6 . ||11427703_155030_7251==>In contrast , VerPlank et al. ( TARGET_CITATION ) reported that mutating the Ub - acceptor sites in p6 had no effect on <PROT1_155030>  Gag </PROT1_155030> / <PROT2_7251>  TSG101 </PROT2_7251> binding in yeast two - hybrid assays . ||11427703_155030_7251==>Although the physiological ligand for p6 has not been definitively identified , a variety of observations suggest that retroviral L domains function by interacting with the host ubiquitination machinery ( reviewed in reference CITATION ) : ( i ) The L domain - containing proteins of MuLV ( p12 ) and HIV ( p6 ) are ubiquitinated ( CITATION ) , ( ii ) depletion of free ubiquitin in virus - producing cells with proteasome inhibitors impairs retrovirus budding ( CITATION , CITATION ) , ( iii ) the presence of L domains in HIV - 1 minimal <PROT1_155030>  Gag </PROT1_155030> constructs induces ubiquitination of the minimal <PROT1_155030>  Gag </PROT1_155030> proteins ( CITATION ) , ( iv ) the ubiquitin ligase Nedd4 interacts with the RSV L domain ( CITATION ) , and ( v ) the cellular protein <PROT2_7251>  TSG101 </PROT2_7251> , which contains at its N terminus a ubiquitin - conjugating enzyme - like sequence , interacts with HIV - 1 <PROT1_155030>  Gag </PROT1_155030> in a p6 - dependent fashion ( TARGET_CITATION ) . ||11427703_155030_7251==>Comparison of the cDNA sequences with the GenBank database revealed that one of the clones was human EF1 { alpha } and another was human <PROT2_7251>  Tsg101 </PROT2_7251> , an inactive homologue of ubiquitin ligase E2 , both of which have been reported to interact with HIV - 1 <PROT1_155030>  Gag </PROT1_155030> ( CITATION , CITATION , TARGET_CITATION ) . ||11427703_155030_7251==><PROT2_7251>  Tsg101 </PROT2_7251> interacted specifically with HIV - 2 <PROT1_155030>  Gag </PROT1_155030> and also with HIV - 1 <PROT1_155030>  Gag </PROT1_155030> , as previously reported ( CITATION , TARGET_CITATION ) . ||11427703_155030_7251==>While we were carrying out this work , there have been reports of an interaction between HIV - 1 <PROT1_155030>  Gag </PROT1_155030> and <PROT2_7251>  Tsg101 </PROT2_7251> ( CITATION , CITATION , TARGET_CITATION ) . ||11427703_155030_7251==>In agreement with studies on the interaction of <PROT2_7251>  Tsg101 </PROT2_7251> with HIV - 1 <PROT1_155030>  Gag </PROT1_155030> ( TARGET_CITATION ) , we have demonstrated that the N - terminal Ubc - conjugation homology domain of <PROT2_7251>  Tsg101 </PROT2_7251> is required for interaction with HIV - 2 <PROT1_155030>  Gag </PROT1_155030> and that overexpression of this portion of <PROT2_7251>  Tsg101 </PROT2_7251> ( amino acid residues 1 to 167 ) inhibits the release of HIV - 2 particles . ||11427703_155030_7251==>Via interaction with the PTAP motif in the p6 domain of <PROT1_155030>  Gag </PROT1_155030> , the ubiquitin - conjugating enzyme homologue <PROT2_7251>  Tsg101 </PROT2_7251> is targeted to the site of virion budding , where it is required for fission of the nascent virion membrane from the host cell membrane ( CITATION , CITATION , TARGET_CITATION ) . ||11427703_155030_7251==>Recently , the product of the tumor suppressor gene 101 ( <PROT2_7251>  TSG101 </PROT2_7251> ) was shown to bind the PTAP motif of p6 <PROT1_155030>  Gag </PROT1_155030> and also to recognize ubiquitin through its ubiquitin enzyme 2 ( UEV ) domain CITATION , TARGET_CITATION . ||11427703_155030_7251==>Finally , ( iv ) the cellular protein <PROT2_7251>  TSG101 </PROT2_7251> , which contains at its N terminus a domain related to Ub conjugating ( E2 ) enzymes , binds <PROT1_155030>  Gag </PROT1_155030> in a p6 - dependent manner ( TARGET_CITATION ) . ||11427703_155030_7251==>We observed that both <PROT2_7251>  TSG101 </PROT2_7251> and HRS contain a tetrapeptide motif ( PTAP in TSG ; PSAP in HRS ) that initially was identified in an HIV <PROT1_155030>  GAG </PROT1_155030> protein late domain that interacts with the UEV domain of <PROT2_7251>  TSG101 </PROT2_7251> ( CITATION , TARGET_CITATION ) . ||11427703_155030_7251==><PROT2_7251>  TSG101 </PROT2_7251> interaction with this motif of <PROT1_155030>  GAG </PROT1_155030> ( CITATION , TARGET_CITATION ) and with a similar motif in the <PROT1_155030>  matrix </PROT1_155030> protein of Ebola virus ( CITATION ) is required for normal viral budding to the cell surface , a process that is topologically equivalent to the removal of mitogenic receptors from the cytoplasm ( CITATION ) . ||11427703_155030_7251==>The budding virus particle is ultimately released from the cell surface in a process that is promoted by an interaction between the late domain in the p6 region of <PROT1_155030>  Gag </PROT1_155030> ( CITATION , CITATION ) and host proteins , most notably the endosomal sorting factor <PROT2_7251>  TSG101 </PROT2_7251> ( tumor susceptibility gene 101 ) ( TARGET_CITATION - CITATION ) . ||11427703_155030_7251==>A ubiquitin binding domain ( designated ubiquitin E2 variant sequence ( UEV ) ) at the NH2 terminus of <PROT2_7251>  Tsg101 </PROT2_7251> binds to the PTAP motif , and the binding is enhanced if the <PROT1_155030>  Gag </PROT1_155030> p6 polypeptide is ubiquitinated ( CITATION ; TARGET_CITATION ; CITATION ) . ||11427703_155030_7251==>The HIV - 1 encoded <PROT1_155030>  Gag </PROT1_155030> protein has been shown to interact with the mammalian orthologue of Vps23 ( <PROT2_7251>  Tsg101 </PROT2_7251> ) and , thereby , recruits ESCRT - I to the plasma membrane where it functions in viral budding ( CITATION ; CITATION ; TARGET_CITATION ) . ||11427703_155030_7251==>This region of Vps27 contains two motifs ( PSDP524 & # 150 ; 527 and PTVP581 & # 150 ; 584 ) that are similar to a peptide sequence within the HIV - 1 <PROT1_155030>  Gag </PROT1_155030> protein ( PTAP ) which is capable of interacting with <PROT2_7251>  Tsg101 </PROT2_7251> ( CITATION ; CITATION ; TARGET_CITATION ) . ||11427703_155030_7251==>The carboxy - terminus of <PROT1_155030>  Gag </PROT1_155030> , p6 , is ubiquitylated CITATION , and it recruits components of multivesicular bodies , such as tumour - susceptibility gene 101 ( <PROT2_7251>  TSG101 </PROT2_7251> ) and vacuolar protein sorting 4 ( VPS4 ) CITATION - TARGET_CITATION , which facilitate the release of progeny virions from the cell . ||11427703_155030_7251==>To determine whether the inhibition of virus release imposed by TSG - 5 ' is dependent upon its interaction with <PROT1_155030>  Gag </PROT1_155030> , we mutated a region of <PROT2_7251>  TSG101 </PROT2_7251> previously shown to be important for <PROT1_155030>  Gag </PROT1_155030> / <PROT2_7251>  TSG101 </PROT2_7251> interaction ( CITATION , CITATION , TARGET_CITATION ) . ||11427703_155030_7251==>There is growing evidence that this process involves the ubiquitin E3 ligase Nedd4 ( CITATION , CITATION , CITATION ) and variant E2 - like protein <PROT2_7251>  TSG101 </PROT2_7251> ( CITATION , CITATION , TARGET_CITATION ) acting through interactions ( CITATION ) with late domain motifs ( CITATION ) in viral <PROT1_155030>  Gag </PROT1_155030> proteins to ligate ubiquitin . ||11427703_155030_7251==>The best - characterized late - domain interaction is that of the PTAP motif in the p6 region of HIV - 1 <PROT1_155030>  Gag </PROT1_155030> with cellular <PROT2_7251>  TSG101 </PROT2_7251> ( CITATION , TARGET_CITATION ) . ||11427703_155030_7251==>Recently , the PTAPP motif , or L domain , of human immunodeficiency virus type 1 ( HIV - 1 ) , which is located in the C - terminal p6 region of the <PROT1_155030>  Gag </PROT1_155030> precursor polyprotein , was found to bind to the protein product of the <PROT2_7251>  tsg101 </PROT2_7251> gene ( CITATION , CITATION , TARGET_CITATION ) . ||11427703_155030_7251==>The N - terminal domain of <PROT2_7251>  Tsg101 </PROT2_7251> also is the minimal binding region required for its interaction with HIV - 1 <PROT1_155030>  Gag </PROT1_155030> ( CITATION , CITATION , TARGET_CITATION ) . ||11427703_155030_7251==>The positive controls confirmed the activities of the constructs : the HIV - 1 <PROT1_155030>  Gag </PROT1_155030> bound <PROT2_7251>  Tsg101 </PROT2_7251> as previously described ( CITATION , CITATION , TARGET_CITATION ) but not with Nedd4 , and MLV <PROT1_155030>  Gag </PROT1_155030> interacted with Nedd4 protein but not with <PROT2_7251>  Tsg101 </PROT2_7251> . ||11427703_155030_7251==>Indeed , many of the virus - encoded proteins have been shown to interact with cellular proteins , e.g. , <PROT1_155030>  Gag </PROT1_155030> and <PROT2_7251>  Tsg101 </PROT2_7251> ( CITATION , CITATION , CITATION , TARGET_CITATION ) , <PROT1_155030>  Gag </PROT1_155030> and cyclophilins ( CITATION ) , integrase and Ini1 ( CITATION ) , Vpr and PP2A ( CITATION ) , Vpr and HHR23A ( CITATION ) , Vpr and UNG ( CITATION ) , Vif and antiviral proteins ( CITATION , CITATION ) , Vif and HP68 ( CITATION ) , and Tat and cyclin T1 ( CITATION , CITATION ) . ||11427703_155030_7251==>In this case , <PROT2_7251>  TSG101 </PROT2_7251> binds the L - domain of the retroviral <PROT1_155030>  gag </PROT1_155030> protein and promotes its monoubiquitination ( CITATION , TARGET_CITATION ) , which is a prerequisite for viral release ( CITATION ) . ||11427703_155030_7251==>In addition , a number of host factors appropriated by the virus ( <PROT2_7251>  Tsg101 </PROT2_7251> , ubiquitin , ERK2 , cyclophilin A ( CypA ) , and topoisomerase I ) interact specifically with elements of the HIV - 1 <PROT1_155030>  Gag </PROT1_155030> polyprotein and contribute to assembly , release , and subsequent postentry events in the viral life cycle ( CITATION , CITATION , CITATION , CITATION , CITATION - CITATION , TARGET_CITATION ) . ||11427703_155030_7251==>Most lentiviruses carry a P ( T / S ) AP motif which is located at the end of <PROT1_155030>  Gag </PROT1_155030> and interacts with <PROT2_7251>  TSG101 </PROT2_7251> ( CITATION , CITATION , CITATION , TARGET_CITATION ) . ||11427703_155030_7251==>The L domain of HIV - 1 <PROT1_155030>  Gag </PROT1_155030> binds to <PROT2_7251>  Tsg101 </PROT2_7251> , which is a component of the ESCRT - 1 endosomal protein sorting complex and is involved in targeting monoubiquitinated proteins to the MVB ( CITATION , CITATION , CITATION , CITATION , TARGET_CITATION ) . ||11427703_155030_7251==>( iii ) <PROT2_7251>  TSG101 </PROT2_7251> can bind p6 in the absence of the <PROT1_155030>  Gag </PROT1_155030> - Ub modification , though ubiquitination may enhance binding ( CITATION , CITATION , TARGET_CITATION ) . ||11427703_155030_7251==>The PTAP motif within the L domain of HIV - 1 <PROT1_155030>  Gag </PROT1_155030> has been shown to interact with <PROT2_7251>  Tsg101 </PROT2_7251> , and this interaction is essential for the final stages of particle budding ( CITATION , TARGET_CITATION ) . ||11427703_155030_7251==><PROT2_7251>  Tsg101 </PROT2_7251> has been identified as a host cell factor that binds the PTAP motif within the p6 region of HIV - 1 <PROT1_155030>  Gag </PROT1_155030> ( CITATION , CITATION , TARGET_CITATION ) . ||11427703_155030_7251==>Recent studies indicated that <PROT1_155030>  Gag </PROT1_155030> interacts directly with <PROT2_7251>  Tsg101 </PROT2_7251> , a ubiquitin enzyme 2 variant family protein that is involved in vesicular sorting pathways in the cell , suggesting that <PROT1_155030>  Gag </PROT1_155030> may utilize components of this pathway to reach the plasma membrane ( CITATION , CITATION , TARGET_CITATION ) . ||11427703_155030_7251==>Functional interactions between <PROT2_7251>  TSG101 </PROT2_7251> and the PTAP motifs present in HIV - 1 <PROT1_155030>  Gag </PROT1_155030> and Ebola virus VP40 have now been well documented ( CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , TARGET_CITATION ) . ||11595185_155030_7251==>Recently , TARGET_CITATION reported that <PROT2_7251>  Tsg101 </PROT2_7251> , which functions in vacuolar protein sorting ( Vps ) , binds to the PTAP motif identified as the L domain within HIV <PROT1_155030>  Gag </PROT1_155030> , and is required for HIV - 1 budding . ||11595185_155030_7251==>Depletion of cellular <PROT2_7251>  Tsg101 </PROT2_7251> levels using small interfering RNA causes defects in both the packaging of <PROT1_155030>  Gag </PROT1_155030> and the budding of viral particles TARGET_CITATION . ||11595185_155030_7251==>The affinity of <PROT2_7251>  Tsg101 </PROT2_7251> for the <PROT1_155030>  Gag </PROT1_155030> protein is increased markedly when <PROT1_155030>  Gag </PROT1_155030> is ubiquitylated TARGET_CITATION , and experiments have identified a region , which includes a beta - hairpin in the UBC - like domain of <PROT2_7251>  Tsg101 </PROT2_7251> , that is essential for this interaction CITATION . ||11595185_155030_7251==>Comparison of the cDNA sequences with the GenBank database revealed that one of the clones was human EF1 { alpha } and another was human <PROT2_7251>  Tsg101 </PROT2_7251> , an inactive homologue of ubiquitin ligase E2 , both of which have been reported to interact with HIV - 1 <PROT1_155030>  Gag </PROT1_155030> ( CITATION , TARGET_CITATION , CITATION ) . ||11595185_155030_7251==><PROT2_7251>  Tsg101 </PROT2_7251> interacted specifically with HIV - 2 <PROT1_155030>  Gag </PROT1_155030> and also with HIV - 1 <PROT1_155030>  Gag </PROT1_155030> , as previously reported ( TARGET_CITATION , CITATION ) . ||11595185_155030_7251==>While we were carrying out this work , there have been reports of an interaction between HIV - 1 <PROT1_155030>  Gag </PROT1_155030> and <PROT2_7251>  Tsg101 </PROT2_7251> ( TARGET_CITATION , CITATION , CITATION ) . ||11595185_155030_7251==>Via interaction with the PTAP motif in the p6 domain of <PROT1_155030>  Gag </PROT1_155030> , the ubiquitin - conjugating enzyme homologue <PROT2_7251>  Tsg101 </PROT2_7251> is targeted to the site of virion budding , where it is required for fission of the nascent virion membrane from the host cell membrane ( TARGET_CITATION , CITATION , CITATION ) . ||11595185_155030_7251==>Recently , the product of the tumor suppressor gene 101 ( <PROT2_7251>  TSG101 </PROT2_7251> ) was shown to bind the PTAP motif of p6 <PROT1_155030>  Gag </PROT1_155030> and also to recognize ubiquitin through its ubiquitin enzyme 2 ( UEV ) domain TARGET_CITATION , CITATION . ||11595185_155030_7251==>We observed that both <PROT2_7251>  TSG101 </PROT2_7251> and HRS contain a tetrapeptide motif ( PTAP in TSG ; PSAP in HRS ) that initially was identified in an HIV <PROT1_155030>  GAG </PROT1_155030> protein late domain that interacts with the UEV domain of <PROT2_7251>  TSG101 </PROT2_7251> ( TARGET_CITATION , CITATION ) . ||11595185_155030_7251==><PROT2_7251>  TSG101 </PROT2_7251> interaction with this motif of <PROT1_155030>  GAG </PROT1_155030> ( TARGET_CITATION , CITATION ) and with a similar motif in the <PROT1_155030>  matrix </PROT1_155030> protein of Ebola virus ( CITATION ) is required for normal viral budding to the cell surface , a process that is topologically equivalent to the removal of mitogenic receptors from the cytoplasm ( CITATION ) . ||11595185_155030_7251==>Interactions between the cellular <PROT2_7251>  Tsg101 </PROT2_7251> ( tumor susceptibility gene 101 ) protein and the PTAP motif present in the p6 domain of HIV - 1 <PROT1_155030>  Gag </PROT1_155030> are required for viral budding from the cell membrane ( TARGET_CITATION ) . ||11595185_155030_7251==>Mutating key residues in the p6 late domain ( CITATION - CITATION , CITATION ) or inhibiting the interaction between p6 and <PROT2_7251>  TSG101 </PROT2_7251> ( TARGET_CITATION , CITATION ) also delays <PROT1_155030>  Gag </PROT1_155030> processing and increases levels of the <PROT1_155030>  Gag </PROT1_155030> cleavage intermediates p25 ( CA - SP1 ) and p41 ( MA - CA ) in virions . ||11595185_155030_7251==>As noted in the Introduction , disruption of the interaction between HIV - 1 <PROT1_155030>  Gag </PROT1_155030> and <PROT2_7251>  TSG101 </PROT2_7251> inhibits the conversion of p25 to p24 ( CITATION , CITATION , TARGET_CITATION , CITATION , CITATION ) . ||11595185_155030_7251==>However , clues have come from the recent observation that HIV <PROT1_155030>  Gag </PROT1_155030> interacts with <PROT2_7251>  TSG101 </PROT2_7251> , a member of the ESCRT complex involved in vesicular trafficking within the cell ( TARGET_CITATION ) . ||11595185_155030_7251==>CEM15 / Apobec 3G interferes with the production of infectious virions in the absence of a functional vif allele ( CITATION , CITATION ) ; <PROT2_7251>  TSG101 </PROT2_7251> interacts with the PTAPP motif in <PROT1_155030>  gag </PROT1_155030> p6 , where it facilitates release of virus from cells ( TARGET_CITATION ) . ||11595185_155030_7251==>A ubiquitin binding domain ( designated ubiquitin E2 variant sequence ( UEV ) ) at the NH2 terminus of <PROT2_7251>  Tsg101 </PROT2_7251> binds to the PTAP motif , and the binding is enhanced if the <PROT1_155030>  Gag </PROT1_155030> p6 polypeptide is ubiquitinated ( TARGET_CITATION ; CITATION ; CITATION ) . ||11595185_155030_7251==>The HIV - 1 encoded <PROT1_155030>  Gag </PROT1_155030> protein has been shown to interact with the mammalian orthologue of Vps23 ( <PROT2_7251>  Tsg101 </PROT2_7251> ) and , thereby , recruits ESCRT - I to the plasma membrane where it functions in viral budding ( TARGET_CITATION ; CITATION ; CITATION ) . ||11595185_155030_7251==>This region of Vps27 contains two motifs ( PSDP524 & # 150 ; 527 and PTVP581 & # 150 ; 584 ) that are similar to a peptide sequence within the HIV - 1 <PROT1_155030>  Gag </PROT1_155030> protein ( PTAP ) which is capable of interacting with <PROT2_7251>  Tsg101 </PROT2_7251> ( TARGET_CITATION ; CITATION ; CITATION ) . ||11595185_155030_7251==>Therefore , it is intriguing that sequences involved in Vps27 function and ESCRT - I recruitment are similar to a conserved motif in the HIV - 1 protein <PROT1_155030>  Gag </PROT1_155030> , which has been shown to interact with <PROT2_7251>  Tsg101 </PROT2_7251> ( TARGET_CITATION ) . ||11595185_155030_7251==>Expression of HIV <PROT1_155030>  Gag </PROT1_155030> & # 150 ; GFP fusion proteins in human cell lines recapitulates many aspects of viral assembly and budding ( CITATION ) , including the requirement for <PROT2_7251>  Tsg101 </PROT2_7251> and other components of the MVB pathway ( TARGET_CITATION ) . ||11595185_155030_7251==><PROT1_155030>  Gag </PROT1_155030> carries a PTAP peptide sequence that interacts with the UEV domain of <PROT2_7251>  Tsg101 </PROT2_7251> , and this interaction is enhanced by <PROT1_155030>  Gag </PROT1_155030> monoubiquitination ( TARGET_CITATION ) . ||11595185_155030_7251==>The carboxy - terminus of <PROT1_155030>  Gag </PROT1_155030> , p6 , is ubiquitylated CITATION , and it recruits components of multivesicular bodies , such as tumour - susceptibility gene 101 ( <PROT2_7251>  TSG101 </PROT2_7251> ) and vacuolar protein sorting 4 ( VPS4 ) TARGET_CITATION , which facilitate the release of progeny virions from the cell . ||11595185_155030_7251==>A functional relevance of the <PROT1_155030>  Gag </PROT1_155030> / <PROT2_7251>  TSG101 </PROT2_7251> interaction in virus release is supported by the observation of Garrus et al. ( TARGET_CITATION ) that depletion of <PROT2_7251>  TSG101 </PROT2_7251> using a small interfering RNA approach blocked HIV - 1 budding , and our demonstration that overexpression of the N - terminal , E2 - like domain of <PROT2_7251>  TSG101 </PROT2_7251> ( referred to as TSG - 5 ' ) impaired particle release ( CITATION ) . ||11595185_155030_7251==>There is growing evidence that this process involves the ubiquitin E3 ligase Nedd4 ( CITATION , CITATION , CITATION ) and variant E2 - like protein <PROT2_7251>  TSG101 </PROT2_7251> ( CITATION , TARGET_CITATION , CITATION ) acting through interactions ( CITATION ) with late domain motifs ( CITATION ) in viral <PROT1_155030>  Gag </PROT1_155030> proteins to ligate ubiquitin . ||11595185_155030_7251==>The best - characterized late - domain interaction is that of the PTAP motif in the p6 region of HIV - 1 <PROT1_155030>  Gag </PROT1_155030> with cellular <PROT2_7251>  TSG101 </PROT2_7251> ( TARGET_CITATION , CITATION ) . ||11595185_155030_7251==>HIV - 1 <PROT1_155030>  Gag </PROT1_155030> recruits an ESCRT - 1 complex protein , <PROT2_7251>  TSG101 </PROT2_7251> , via a PTAP motif in the p6 region and possibly through ubiquitin conjugated to <PROT1_155030>  Gag </PROT1_155030> which may interact with a ubiquitin interaction motif in <PROT2_7251>  TSG101 </PROT2_7251> ( TARGET_CITATION ) . ||11595185_155030_7251==>VPS28 normally binds tightly to <PROT2_7251>  TSG101 </PROT2_7251> ( TARGET_CITATION , CITATION ) , suggesting that budded particles arising from expression of the chimeric ld ( - ) <PROT1_155030>  Gag </PROT1_155030> - VPS28 might incorporate <PROT2_7251>  TSG101 </PROT2_7251> . ||11595185_155030_7251==>Recently , the PTAPP motif , or L domain , of human immunodeficiency virus type 1 ( HIV - 1 ) , which is located in the C - terminal p6 region of the <PROT1_155030>  Gag </PROT1_155030> precursor polyprotein , was found to bind to the protein product of the <PROT2_7251>  tsg101 </PROT2_7251> gene ( TARGET_CITATION , CITATION , CITATION ) . ||11595185_155030_7251==>The N - terminal domain of <PROT2_7251>  Tsg101 </PROT2_7251> also is the minimal binding region required for its interaction with HIV - 1 <PROT1_155030>  Gag </PROT1_155030> ( TARGET_CITATION , CITATION , CITATION ) . ||11595185_155030_7251==>Since <PROT2_7251>  TSG101 </PROT2_7251> shows weak binding to ubiquitin alone ( TARGET_CITATION ) , it could be recruited by binding to ubiquitinated <PROT1_155030>  Gag </PROT1_155030> protein . ||11595185_155030_7251==>The EIAV p9 protein harboring the L - domain motif YXXL was shown to bind the AP - 50 subunit of the AP - 2 complex ( CITATION ) .The PTAP motif present in the HIV - 1 and HIV - 2 <PROT1_155030>  Gag </PROT1_155030> polyprotein and VP40 in Ebola virus ( TARGET_CITATION , CITATION , CITATION , CITATION ) binds the E2 ubiquitin enzyme variant ( UEV ) <PROT2_7251>  Tsg101 </PROT2_7251> . ||11595185_155030_7251==>The positive controls confirmed the activities of the constructs : the HIV - 1 <PROT1_155030>  Gag </PROT1_155030> bound <PROT2_7251>  Tsg101 </PROT2_7251> as previously described ( TARGET_CITATION , CITATION , CITATION ) but not with Nedd4 , and MLV <PROT1_155030>  Gag </PROT1_155030> interacted with Nedd4 protein but not with <PROT2_7251>  Tsg101 </PROT2_7251> . ||11595185_155030_7251==>The current model of viral budding involves binding of the 4 - amino - acid <PROT1_155030>  Gag </PROT1_155030> p6 PTAP motif to cellular <PROT2_7251>  Tsg101 </PROT2_7251> protein and subsequent assortment via the cellular vacuolar protein - sorting pathway ( CITATION , TARGET_CITATION , CITATION , CITATION ) . ||11595185_155030_7251==>While it is possible that ubiquitination at alternative HIV - 1 <PROT1_155030>  Gag </PROT1_155030> sites might facilitate <PROT2_7251>  Tsg101 </PROT2_7251> recruitment ( TARGET_CITATION , CITATION ) , the presence of this proven L - domain cofactor at sites of particle assembly appears insufficient to account for the virion morphogenesis activity exhibited by HIV - 1 p6 . ||11595185_155030_7251==>Indeed , many of the virus - encoded proteins have been shown to interact with cellular proteins , e.g. , <PROT1_155030>  Gag </PROT1_155030> and <PROT2_7251>  Tsg101 </PROT2_7251> ( CITATION , TARGET_CITATION , CITATION , CITATION ) , <PROT1_155030>  Gag </PROT1_155030> and cyclophilins ( CITATION ) , integrase and Ini1 ( CITATION ) , Vpr and PP2A ( CITATION ) , Vpr and HHR23A ( CITATION ) , Vpr and UNG ( CITATION ) , Vif and antiviral proteins ( CITATION , CITATION ) , Vif and HP68 ( CITATION ) , and Tat and cyclin T1 ( CITATION , CITATION ) . ||11595185_155030_7251==><PROT2_7251>  tsg101 </PROT2_7251> has been shown to interact with the PTAP L - domain present in HIV - 1 <PROT1_155030>  gag </PROT1_155030> and Ebola virus VP40 by virtue of its N - terminal ubiquitin enzyme 2 variant domain ( TARGET_CITATION , CITATION ) . ||11595185_155030_7251==>An intriguing area for future research is the interaction of <PROT1_155030>  Gag </PROT1_155030> with components of the vacuolar protein sorting pathway , as recently illustrated by the characterization of <PROT2_7251>  TSG101 </PROT2_7251> as a binding partner for the PTAP domain within p6 ( TARGET_CITATION ) . ||11595185_155030_7251==>As a control we used HIV - <PROT1_155030>  Gag </PROT1_155030> , because the single PTAP motif of this viral protein strongly binds to the UEV of <PROT2_7251>  Tsg101 </PROT2_7251> ( Garrus et al. 2001 TARGET_CITATION ; Martin - Serrano et al. 2001 CITATION ; VerPlank et al. 2001 CITATION ; Demirov et al. 2002 CITATION ; Myers and Allen 2002 CITATION ; Pornillos et al. 2002a CITATION ) . ||11595185_155030_7251==>Unlike EGFR , <PROT1_155030>  Gag </PROT1_155030> contains a PTAP motif , which recruits <PROT2_7251>  Tsg101 </PROT2_7251> to virus budding sites ( Garrus et al. 2001 TARGET_CITATION ; Martin - Serrano et al. 2001 CITATION ; VerPlank et al. 2001 CITATION ; Demirov et al. 2002 CITATION ; Myers and Allen 2002 CITATION ; Pornillos et al. 2002a CITATION ) . ||11595185_155030_7251==>In this case , <PROT2_7251>  TSG101 </PROT2_7251> binds the L - domain of the retroviral <PROT1_155030>  gag </PROT1_155030> protein and promotes its monoubiquitination ( CITATION , CITATION ) , which is a prerequisite for viral release ( TARGET_CITATION ) . ||11595185_155030_7251==>The <PROT2_7251>  Tsg101 </PROT2_7251> UEV domain is also able to recognize a PTAP motif from the late domain p6 protein of HIV - 1 <PROT1_155030>  Gag </PROT1_155030> , and this PTAP binding is necessary for efficient viral budding ( TARGET_CITATION , CITATION ) . ||11595185_155030_7251==>In addition , a number of host factors appropriated by the virus ( <PROT2_7251>  Tsg101 </PROT2_7251> , ubiquitin , ERK2 , cyclophilin A ( CypA ) , and topoisomerase I ) interact specifically with elements of the HIV - 1 <PROT1_155030>  Gag </PROT1_155030> polyprotein and contribute to assembly , release , and subsequent postentry events in the viral life cycle ( CITATION , CITATION , TARGET_CITATION , CITATION , CITATION - CITATION , CITATION ) . ||11595185_155030_7251==>Interaction of HIV - 1 <PROT1_155030>  Gag </PROT1_155030> with <PROT2_7251>  Tsg101 </PROT2_7251> and other components of the MVB ESCRT complex is required for efficient particle production ( TARGET_CITATION , CITATION ) . ||11595185_155030_7251==>Most lentiviruses carry a P ( T / S ) AP motif which is located at the end of <PROT1_155030>  Gag </PROT1_155030> and interacts with <PROT2_7251>  TSG101 </PROT2_7251> ( TARGET_CITATION , CITATION , CITATION , CITATION ) . ||11595185_155030_7251==>It is of note that although the PTAP motif within the p6 <PROT1_155030>  Gag </PROT1_155030> has recently been shown to recruit the human protein <PROT2_7251>  Tsg101 </PROT2_7251> to facilitate HIV - 1 budding ( TARGET_CITATION , CITATION ) significant amounts of the mature p6 protein were seen in all of the virions , suggesting that none of the SRPE , APP , or PTAPPA inserts located near the PTAP motif blocked viral budding ( fig. 3 ) . ||11595185_155030_7251==>The L domain of HIV - 1 <PROT1_155030>  Gag </PROT1_155030> binds to <PROT2_7251>  Tsg101 </PROT2_7251> , which is a component of the ESCRT - 1 endosomal protein sorting complex and is involved in targeting monoubiquitinated proteins to the MVB ( CITATION , TARGET_CITATION , CITATION , CITATION , CITATION ) . ||11595185_155030_7251==>( iii ) <PROT2_7251>  TSG101 </PROT2_7251> can bind p6 in the absence of the <PROT1_155030>  Gag </PROT1_155030> - Ub modification , though ubiquitination may enhance binding ( TARGET_CITATION , CITATION , CITATION ) . ||11595185_155030_7251==>The PTAP motif within the L domain of HIV - 1 <PROT1_155030>  Gag </PROT1_155030> has been shown to interact with <PROT2_7251>  Tsg101 </PROT2_7251> , and this interaction is essential for the final stages of particle budding ( TARGET_CITATION , CITATION ) . ||11595185_155030_7251==><PROT2_7251>  Tsg101 </PROT2_7251> has been identified as a host cell factor that binds the PTAP motif within the p6 region of HIV - 1 <PROT1_155030>  Gag </PROT1_155030> ( CITATION , TARGET_CITATION , CITATION ) . ||11595185_155030_7251==>Recent studies indicated that <PROT1_155030>  Gag </PROT1_155030> interacts directly with <PROT2_7251>  Tsg101 </PROT2_7251> , a ubiquitin enzyme 2 variant family protein that is involved in vesicular sorting pathways in the cell , suggesting that <PROT1_155030>  Gag </PROT1_155030> may utilize components of this pathway to reach the plasma membrane ( CITATION , TARGET_CITATION , CITATION ) . ||11595185_155030_7251==><PROT2_7251>  TSG101 </PROT2_7251> exerts its effect on the budding of HIV - 1 by an interaction with the late domain of the viral protein <PROT1_155030>  Gag </PROT1_155030> ( TARGET_CITATION ) . ||11595185_155030_7251==>Functional interactions between <PROT2_7251>  TSG101 </PROT2_7251> and the PTAP motifs present in HIV - 1 <PROT1_155030>  Gag </PROT1_155030> and Ebola virus VP40 have now been well documented ( TARGET_CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , CITATION ) . ||12009870_155030_5478==>In addition , even though the <PROT1_155030>  Gag </PROT1_155030> proteins of all HIV - 1 and SIVcpz strains tested bind to <PROT2_5478>  CypA </PROT2_5478> CITATION , the replication of some HIV - 1 'O ' group strains is unaffected by its removal CITATION TARGET_CITATION . ||12009870_155030_5478==>The <PROT2_5478>  CypA </PROT2_5478> - <PROT1_155030>  Gag </PROT1_155030> interaction can be blocked by an immunosuppressive drug , cyclosporine A ( CsA ) , and its analogues ( CITATION , CITATION , CITATION ) , and this manipulation inhibits the replication of most HIV - 1 strains in most cells in vitro ( CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , TARGET_CITATION ) . ||12032547_155030_5478==>Comparison of the NMR structures of the N - terminal domains of mature CA and the immature form ( covalently linked to <PROT1_155030>  matrix </PROT1_155030> ( MA ) ) ( TARGET_CITATION ) indicates that formation of the & # 223 ; - hairpin at the N terminus of mature CA , which occurs following proteolytic cleavage of <PROT1_155030>  Gag </PROT1_155030> ( CITATION , CITATION ) , induces both an ~2 - & Aring ; displacement of helix VI and displacement of the cyclophilin A ( <PROT2_5478>  CypA </PROT2_5478> ) binding loop ( fig. 1 ) . ||12032547_155030_5478==>X - ray and nuclear magnetic resonance analysis of <PROT2_5478>  CypA </PROT2_5478> in complex with various <PROT1_155030>  Gag </PROT1_155030> fragments confirmed that these residues participate in direct protein - protein interactions ( CITATION , CITATION , TARGET_CITATION , CITATION , CITATION ) . ||12525642_155807_7374==>We demonstrate that the <PROT1_155807>  Vpr </PROT1_155807> - dependent incorporation of <PROT2_7374>  UNG </PROT2_7374> into HIV - 1 particles is directly responsible for the role of <PROT1_155807>  Vpr </PROT1_155807> in the in vivo modulation of the virus mutation rate ( CITATION , TARGET_CITATION ) . ||12598123_155030_7251==><PROT2_7251>  Tsg101 </PROT2_7251> has been implicated in the budding of HIV - 1 and Ebola virus through its interaction with the PTAP motif present in the L domains of <PROT1_155030>  Gag </PROT1_155030> and VP40 , respectively ( TARGET_CITATION , CITATION ) . ||12598123_155030_7251==>The ability of TSG - 5 ' to inhibit virus budding without any apparent adverse effect on cellular sorting machinery demonstrates that antiviral agents can be developed that interfere with virus replication by specifically targeting the <PROT1_155030>  Gag </PROT1_155030> / <PROT2_7251>  TSG101 </PROT2_7251> interaction ( TARGET_CITATION ) . ||12598123_155030_7251==><PROT2_7251>  Tsg101 </PROT2_7251> has been identified as a host cell factor that binds the PTAP motif within the p6 region of HIV - 1 <PROT1_155030>  Gag </PROT1_155030> ( TARGET_CITATION , CITATION , CITATION ) . ||12598123_155030_7251==>Recent studies indicated that <PROT1_155030>  Gag </PROT1_155030> interacts directly with <PROT2_7251>  Tsg101 </PROT2_7251> , a ubiquitin enzyme 2 variant family protein that is involved in vesicular sorting pathways in the cell , suggesting that <PROT1_155030>  Gag </PROT1_155030> may utilize components of this pathway to reach the plasma membrane ( TARGET_CITATION , CITATION , CITATION ) . ||12663786_155030_7251==>Moreover , expression of HIV - 1 <PROT1_155030>  Gag </PROT1_155030> or Ebola virus VP40 results in PTAP - dependent recruitment of <PROT2_7251>  Tsg101 </PROT2_7251> and VPS28 , another ESCRT - I component , to sites of particle assembly at the plasma membrane ( CITATION , TARGET_CITATION ) . ||12663786_155030_7251==>We have previously described constructs and assays that demonstrated that artificial tethering of <PROT2_7251>  Tsg101 </PROT2_7251> to sites of HIV - 1 particle assembly , by fusion to <PROT1_155030>  Gag </PROT1_155030> , reverses the HIV - 1 budding defect that results from mutational inactivation of the PTAP L - domain ( CITATION , TARGET_CITATION ) . ||12663786_155030_7251==>Because HIV - 1 <PROT1_155030>  Gag </PROT1_155030> efficiently multimerizes at the plasma membrane ( CITATION , CITATION ) , expression of a <PROT1_155030>  Gag </PROT1_155030> { delta } p6 protein that is fused to functional L - domain or is directly fused to <PROT2_7251>  Tsg101 </PROT2_7251> can functionally complement a PTAP - defective proviral construct , resulting in infectious virion production ( CITATION , TARGET_CITATION ) . ||12663786_155030_7251==>In contrast , we have previously shown that similar defects induced by point mutations in the PTAP motif could be reversed by <PROT1_155030>  Gag </PROT1_155030> { delta } p6 - <PROT2_7251>  Tsg101 </PROT2_7251> coexpression ( CITATION , TARGET_CITATION ) . ||12663786_155030_7251==>It has thus been proposed that viral <PROT1_155030>  Gag </PROT1_155030> proteins can delocalize <PROT2_7251>  Tsg101 </PROT2_7251> and the cellular MVB budding machinery to the site of virus budding at the plasma membrane ( Martin - Serrano et al. , 2001 CITATION ; Martin - Serrano et al. , 2003 TARGET_CITATION ) . ||12663786_155030_7251==>It is interesting that we observed a marked redistribution of <PROT2_7251>  Tsg101 </PROT2_7251> in the presence of <PROT1_155030>  Gag </PROT1_155030> , a finding that is consistent with recent studies by Martin - Serrano and colleagues ( TARGET_CITATION ) . ||12663786_155030_7251==>Functional interactions between <PROT2_7251>  TSG101 </PROT2_7251> and the PTAP motifs present in HIV - 1 <PROT1_155030>  Gag </PROT1_155030> and Ebola virus VP40 have now been well documented ( CITATION , CITATION , CITATION , CITATION , TARGET_CITATION , CITATION , CITATION , CITATION ) . ||12663786_155030_7251==>Relocalization of <PROT2_7251>  Tsg101 </PROT2_7251> - VPS37C Complexes to the Plasma Membrane by HIV - 1 <PROT1_155030>  Gag </PROT1_155030> & # 151 ; Previously , we have shown that <PROT2_7251>  Tsg101 </PROT2_7251> can be relocalized to sites of viral budding at the plasma membrane by HIV - 1 <PROT1_155030>  Gag </PROT1_155030> or Ebola virus <PROT1_155030>  matrix </PROT1_155030> proteins ( CITATION , TARGET_CITATION ) , both of which encode PTAP - type L domains . ||12743307_155030_7251==>Likewise , the same siRNAs accelerated virus release ( fig. 6E ) , and overexpression of Tal inhibited two previously described exocytosis activities of <PROT2_7251>  Tsg101 </PROT2_7251> ( fig. 6E ; data not shown ) , namely , elevation of <PROT1_155030>  Gag </PROT1_155030> ubiquitylation ( Myers and Allen 2002 CITATION ) and inhibition of VLP release ( Goila - Gaur et al. 2003 TARGET_CITATION ) . ||12743307_155030_7251==>It was subsequently observed ( TARGET_CITATION ) that a mutation in TSG - 5 ' that blocks the interaction between <PROT1_155030>  Gag </PROT1_155030> and <PROT2_7251>  TSG101 </PROT2_7251> ( the TYN - mutation ) disrupts the inhibitory activity of TSG - 5 ' . ||12743307_155030_7251==>Functional interactions between <PROT2_7251>  TSG101 </PROT2_7251> and the PTAP motifs present in HIV - 1 <PROT1_155030>  Gag </PROT1_155030> and Ebola virus VP40 have now been well documented ( CITATION , TARGET_CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , CITATION ) . ||12900394_155030_7251==>Sundquist and colleagues approached the general problem of targeting of cargo to MVBs from the perspective of the <PROT2_7251>  Tsg101 </PROT2_7251> - recruiting sequence in HIV <PROT1_155030>  Gag </PROT1_155030> ( TARGET_CITATION ) . ||12900394_155030_7251==>Indeed , TARGET_CITATION ( in this issue ) show in a companion paper that the PSAP - containing part of Hrs can functionally replace the PTAP - containing part of the HIV <PROT1_155030>  Gag </PROT1_155030> protein in budding of viruslike particles from the plasma membrane , indicating that HIV <PROT1_155030>  Gag </PROT1_155030> mimics the <PROT2_7251>  Tsg101 </PROT2_7251> - recruiting effect of Hrs . ||12900394_155030_7251==>For example , mutant retroviral <PROT1_155030>  Gag </PROT1_155030> proteins that would otherwise be retained can bud from cells when fused directly to various proteins or domains that can recruit ESCRT - I , including ubiquitin ( CITATION ) , <PROT2_7251>  TSG101 </PROT2_7251> ( CITATION ) , VPS28 ( CITATION ) , and HRS ( TARGET_CITATION ) . ||12900394_155030_7251==><PROT1_155030>  Gag </PROT1_155030> has a sevenfold - higher affinity for <PROT2_7251>  TSG101 </PROT2_7251> than Hrs does and can effectively compete with Hrs for <PROT2_7251>  TSG101 </PROT2_7251> binding in vitro ( TARGET_CITATION ) . ||12900394_155030_7251==>Indeed , fusion of fragments of <PROT2_7251>  Tsg101 </PROT2_7251> or Hrs to an HIV - 1 <PROT1_155030>  Gag </PROT1_155030> protein ( TARGET_CITATION , CITATION , CITATION ) , or VPS28 to an HIV - 1 or EIAV <PROT1_155030>  Gag </PROT1_155030> protein ( CITATION ) , 2 can rescue a budding defect induced by mutation of the <PROT1_155030>  Gag </PROT1_155030> - encoded L domain motif . ||14505569_155030_7251==>Recently , the entire cellular protein network that participates in HIV - 1 budding was mapped , with <PROT2_7251>  TSG101 </PROT2_7251> and AIP1 identified as direct interaction partners of the <PROT1_155030>  Gag </PROT1_155030> p6 domain ( CITATION , TARGET_CITATION , CITATION ) . ||14505570_155030_7251==>Recently , the entire cellular protein network that participates in HIV - 1 budding was mapped , with <PROT2_7251>  TSG101 </PROT2_7251> and AIP1 identified as direct interaction partners of the <PROT1_155030>  Gag </PROT1_155030> p6 domain ( CITATION , CITATION , TARGET_CITATION ) . ||14519844_10015_155030==>They also demonstrate that the <PROT2_155030>  GAG </PROT2_155030> protein from membrane - containing viruses , such as HIV , binds to <PROT1_10015>  Alix </PROT1_10015> / <PROT1_10015>  AIP1 </PROT1_10015> , thereby recruiting the ESCRT machinery to allow budding of the virus from the cell surface ( TARGET_CITATION & # 150 ; CITATION ) . ||14519844_10015_155030==>Recently , the entire cellular protein network that participates in HIV - 1 budding was mapped , with TSG101 and <PROT1_10015>  AIP1 </PROT1_10015> identified as direct interaction partners of the <PROT2_155030>  Gag </PROT2_155030> p6 domain ( TARGET_CITATION , CITATION , CITATION ) . ||2017166_155908_4869==><PROT2_4869>  NPM </PROT2_4869> is known to be enriched in proliferating cells and thought to be involved in ribosome assembly and transport due to its localization to the granular region of the nucleolus ( CITATION ) . <PROT2_4869>  NPM </PROT2_4869> has been ascribed a number of diverse properties , including a potential role in proliferation due to its rapid increase in synthesis during mitogenic stimulation ( CITATION ) ; a role as a cytoplasmic / nuclear shuttle protein ( CITATION ) ; has DNA binding activity ( CITATION , CITATION ) ; relieves transcriptional repression by YY1 ( CITATION ) ; binds the HIV Type 1 <PROT1_155908>  Rev </PROT1_155908> protein ( TARGET_CITATION ) , the human T - cell leukemia virus - 1 Rex protein ( CITATION ) , and another cell cycle - regulated nucleolar protein p120 ( CITATION ) ; is a nuclear substrate of protein kinase C ( CITATION ) , mitotic cdc2 kinase ( CITATION ) , and casein kinase II ( CITATION ) ; can be ADP - ribosylated ( CITATION ) ; and the N - terminal region has been shown to be a part of a fusion protein with a tyrosine kinase , a result of a chromosomal rearrangement , in some non - Hodgkin 's lymphomas ( CITATION ) . ||2017166_155908_4869==>The RNA binding domain of <PROT1_155908>  rev </PROT1_155908> can interact either with the RRE or with the nucleolar <PROT2_4869>  B23 </PROT2_4869> protein , but not with both ( TARGET_CITATION ) . ||2017166_155908_4869==>In addition , protein NO38 / <PROT2_4869>  B23 </PROT2_4869> has been reported to possess ribonuclease activity ( CITATION ) and to associate with transcription factor YY1 ( CITATION ) , protein <PROT1_155908>  Rev </PROT1_155908> of HIV - 1 ( TARGET_CITATION ) , and nucleolar protein p120 ( CITATION ) , suggesting that this protein has multiple functions . ||2017166_155908_4869==>In discussing possible binding partners of protein NO29 , it should further be considered that the related protein NO38 / <PROT2_4869>  B23 </PROT2_4869> has been reported to bind to various other nucleolar proteins such as protein p120 ( CITATION ) or to the nonnucleolar transcription factor YY1 ( CITATION ) as well as to certain viral proteins such as <PROT1_155908>  Rev </PROT1_155908> and Tat of HIV - 1 or the human T - lymphocyte virus type I protein Rex ( TARGET_CITATION , CITATION ) . ||2017166_155908_4869==>The NLS of <PROT1_155908>  Rev </PROT1_155908> has been reported to associate with importin beta , the large subunit of the conventional importin alpha / importin beta NLS receptor heterodimer , as well as with <PROT2_4869>  B23 </PROT2_4869> , a mammalian protein involved in the nuclear import of ribosomal proteins ( TARGET_CITATION ) . ||2017166_155908_4869==>The nucleolar localization signal of human T - cell leukemia virus type 1 Rex and human immunodeficiency virus type 1 <PROT1_155908>  Rev </PROT1_155908> binds specifically to the nucleolar shuttle phosphoprotein <PROT2_4869>  B23 </PROT2_4869> ( CITATION , TARGET_CITATION ) . ||2017166_155908_4869==>In the case of HIV - 1 <PROT1_155908>  Rev </PROT1_155908> , Fankhauser et al. ( TARGET_CITATION ) have proposed that the nucleolar protein <PROT2_4869>  B23 </PROT2_4869> binds to the <PROT1_155908>  Rev </PROT1_155908> basic domain and mediates its nuclear import . ||2017166_155908_4869==>In addition , our data confirm the observation by Henderson and Percipalle ( CITATION ) that the <PROT1_155908>  Rev </PROT1_155908> NLS can directly bind Imp beta and now show , for the first time , that <PROT1_155908>  Rev </PROT1_155908> NLS function is indeed independent of Imp alpha . Finally , we have not observed any evidence in support of the hypothesis of Fankhauser et al. ( TARGET_CITATION ) that <PROT1_155908>  Rev </PROT1_155908> NLS function is mediated by the <PROT2_4869>  B23 </PROT2_4869> protein . ||2017166_155908_4869==>Protein <PROT2_4869>  B23 </PROT2_4869> interacts with other nucleolar proteins , including nucleolin ( CITATION ) , protein p120 ( CITATION ) , and the HIV - 1 <PROT1_155908>  Rev </PROT1_155908> protein ( TARGET_CITATION ) . ||2017166_155908_4869==><PROT2_4869>  B23 </PROT2_4869> has also been reported to bind the arginine - rich basic region of the human T - cell leukemia virus protein Rex ( CITATION ) , and the HIV proteins <PROT1_155908>  Rev </PROT1_155908> ( TARGET_CITATION , CITATION ) and Tat ( CITATION ) . ||2017166_155908_4869==>Furthermore , it is shown that <PROT2_4869>  B23 </PROT2_4869> protein binds a wide diversity of proteins , such as nucleolar phosphoproteins , nucleolin and p120 ( Durban et al. , 1995 CITATION ; Li et al. , 1996 CITATION ) , HIV1 - <PROT1_155908>  Rev </PROT1_155908> protein ( Fankhauser et al. , 1991 TARGET_CITATION ) , retinoblastoma protein ( Takemura et al. , 1999 CITATION ) to stimulate the DNA polymerase alpha activity ( Umekawa et al. , 2001 CITATION ) , and hepatitis delta virus delta antigens ( Huang et al. , 2001 CITATION ) . ||2017166_155908_4869==>Protein <PROT2_4869>  B23 </PROT2_4869> binds RNA and DNA ( CITATION ) and interacts with other nucleolar proteins , including nucleolin ( CITATION ) , protein P120 ( CITATION ) , and the HIV - 1 <PROT1_155908>  Rev </PROT1_155908> protein ( TARGET_CITATION , CITATION ) . ||2017166_155908_4869==>Earlier experiments ( TARGET_CITATION , CITATION ) showed that there is a strong affinity between nonphosphorylated protein <PROT2_4869>  B23 </PROT2_4869> and the HIV <PROT1_155908>  Rev </PROT1_155908> protein and that protein <PROT2_4869>  B23 </PROT2_4869> also acts as a molecular chaperone toward <PROT1_155908>  Rev </PROT1_155908> ( CITATION ) . ||2017166_155908_4869==>To examine the speckle pattern of mutant DD in greater detail , we analyzed DD - transfected cells by confocal microscopy after staining them with the anti - Rex - 2 antibody , an antibody against the nuclear envelope component Nup62 ( added to delineate the nucleus ) , and an antibody recognizing <PROT2_4869>  B23 </PROT2_4869> , a multifunctional nucleolar protein that is proposed to act as a chaperone for nuclear import , nucleolar accumulation , and functional activity of <PROT1_155908>  Rev </PROT1_155908> and Rex ( TARGET_CITATION , CITATION , CITATION - CITATION ) . ||2017166_155908_4869==>Nucleophosmin & # 47 ; <PROT2_4869>  B23 </PROT2_4869> , being more abundant in tumour cells than in normal resting cells , has a potential role in increased nucleolar activity that is necessary for cell proliferation ( CITATION ; CITATION ) ; a role as a cytoplasmic & # 47 ; nuclear shuttle protein ( CITATION ) ; relieves transcription repression by YY1 ( CITATION ) ; binds nuclear and nucleolar localisation signals on the HIV Type 1 <PROT1_155908>  Rev </PROT1_155908> protein ( TARGET_CITATION ) and the human T - cell leukaemia virus - 1 - Rex protein ( CITATION ) ; binds cell cycle - regulated nucleolar protein p120 ( CITATION ) ; inhibits DNA - binding and transcriptional activity of interferon regulatory factor - 1 ( IRF - 1 ) , which is a tumour suppressor ( CITATION ; CITATION ) . ||2017166_155908_4869==>The NLS of <PROT1_155908>  Rev </PROT1_155908> has been reported to associate with importin { beta } , as well as with <PROT2_4869>  B23 </PROT2_4869> , a protein involved in the nuclear import of ribosomal proteins ( CITATION , TARGET_CITATION ) . ||7634337_155908_7514==>Indeed , various cellular proteins which appear to be specific binding partners of the <PROT1_155908>  Rev </PROT1_155908> activation domain have been described ; these include nucleoporin - like protein hRIP / Rab ( TARGET_CITATION , CITATION ) , eukaryotic initiation factor 5A ( eIF - 5A ) ( CITATION ) , and nuclear export factor <PROT2_7514>  CRM1 </PROT2_7514> , which appears to be a general nuclear export signal receptor ( CITATION , CITATION , CITATION , CITATION ) . ||7634337_155908_7514==>These cofactors , including nucleoporin - like proteins such as hRIP / Rab ( TARGET_CITATION , CITATION , CITATION ) , <PROT2_7514>  CRM1 </PROT2_7514> ( CITATION , CITATION , CITATION , CITATION ) , and nuclear eIF - 5A ( CITATION , CITATION ) , are thought to mediate the subsequent nuclear export of <PROT1_155908>  Rev </PROT1_155908> ( reviewed in reference CITATION ) . ||7634337_155908_7514==>In fact , various proteins that are able to bind to this region have been described , including eukaryotic initiation factor 5A ( eIF - 5A ) ( CITATION ) , the nucleoporin - like protein human <PROT1_155908>  Rev </PROT1_155908> interacting protein ( hRIP ) / Rab ( CITATION , TARGET_CITATION ) and , more recently , the nuclear pore - associated factor <PROT2_7514>  CRM1 </PROT2_7514> ( CITATION ) that is critically involved in the translocation of NES - containing proteins through the nuclear pore complex ( CITATION ) . ||7634337_155908_7514==>Hrb is thought to be a recruiter of <PROT1_155908>  Rev </PROT1_155908> to the nuclear pore , either directly ( Bogerd et al. 1995 TARGET_CITATION ) or , more likely , through interaction with <PROT2_7514>  Crm1 </PROT2_7514> ( Neville et al. 1997 CITATION ) . ||7634337_155908_7514==>Nup42 was originally identified as a <PROT1_155908>  Rev </PROT1_155908> - interacting protein in a yeast two - hybrid screen or a copper resistance assay ( TARGET_CITATION , CITATION ) , and the interaction was shown to be mediated by <PROT2_7514>  Crm1 </PROT2_7514> ( CITATION ) . ||7634337_155908_7514==>In the Saccharomyces cerevisiae two - hybrid system , it has been shown that FG - containing repeat domains of different nucleoporins interact with the <PROT1_155908>  Rev </PROT1_155908> NES via <PROT2_7514>  CRM1 </PROT2_7514> with various efficiencies , suggesting that nucleoporins play a role in NES - mediated export ( TARGET_CITATION , CITATION , CITATION , CITATION , CITATION ) . ||7634337_155908_7514==>In contrast , however , the nucleoporin - like protein hRIP / Rab ( TARGET_CITATION , CITATION ) was shown to bind indirectly via <PROT2_7514>  CRM1 </PROT2_7514> / exportin 1 to the <PROT1_155908>  Rev </PROT1_155908> NES ( CITATION ) . ||7634337_155908_7514==>The NES mediates association of <PROT1_155908>  Rev </PROT1_155908> with the general nuclear export factor <PROT2_7514>  CRM1 </PROT2_7514> / exportin1 ( CITATION , CITATION , CITATION , CITATION ) , the nucleoporin - like protein Rab1 / hRIP ( TARGET_CITATION , CITATION ) , and the eukaryotic initiation factor 5A ( eIF5A ) ( CITATION ) , and it is responsible for directing <PROT1_155908>  Rev </PROT1_155908> - RNA complexes through an export pathway used by 5S rRNA and U snRNAs ( CITATION ) . ||7634337_155908_7514==>Several cellular factors have been identified to be directly involved in the <PROT1_155908>  Rev </PROT1_155908> - mediated nuclear export pathway , such as <PROT2_7514>  CRM1 </PROT2_7514> ( CITATION , CITATION ) , eukaryotic initiation factor 5A ( eIF - 5A ) ( CITATION ) , and Rip / Rab ( TARGET_CITATION , CITATION ) . ||7634337_155908_7514==>Several cellular factors have been identified as involved in the <PROT1_155908>  Rev </PROT1_155908> - mediated nuclear export pathway ; these include <PROT2_7514>  CRM1 </PROT2_7514> ( CITATION , CITATION ) , eIF5 - A ( CITATION ) , and Rip / Rab ( TARGET_CITATION , CITATION ) . ||7634337_155908_7514==>Several putative cofactors for <PROT1_155908>  Rev </PROT1_155908> NES function have been identified ; particularly well - documented ones include nucleoporins such as Rab / hRIP and Rip1p ( TARGET_CITATION , CITATION , CITATION ) and the nucleocytoplasmic shuttle protein <PROT2_7514>  CRM1 </PROT2_7514> ( CITATION , CITATION , CITATION ) . ||7637788_155908_3267==>Nucleotide sequence of the longest phage inserts of these cDNAs revealed that they represented partial cDNAs of the human homolog of NUMB , a developmentally regulated gene of Drosophila ( Uemura et al. 1989 CITATION ) ; a NUMB - related gene , which we named NUMB - R ; <PROT2_3267>  RAB </PROT2_3267> , coding the cellular cofactor of the HIV <PROT1_155908>  REV </PROT1_155908> protein ( Bogerd et al. 1995 CITATION ; Fritz et al. 1995 TARGET_CITATION ) ; and a <PROT2_3267>  RAB </PROT2_3267> - related gene , which we named <PROT2_3267>  RAB </PROT2_3267> - R ( see next section ) . ||7637788_155908_3267==>Although <PROT1_155908>  Rev </PROT1_155908> interacts efficiently with Rip1p and hRIP / <PROT2_3267>  Rab </PROT2_3267> in two - hybrid assays ( Bogerd et al. 1995 CITATION ; Fritz et al. 1995 TARGET_CITATION ; Stutz et al. 1995 CITATION ) , the ability of mutant <PROT1_155908>  Rev </PROT1_155908> activation domains to function in vivo closely correlates with their ability to interact with hRIP / <PROT2_3267>  Rab </PROT2_3267> / Rip1p in two - hybrid assays ( Bogerd et al. 1995 CITATION ; Stutz et al. 1996 CITATION ) . ||7637788_155908_3267==>The <PROT1_155908>  rev </PROT1_155908> NES has been reported to interact with several different cellular factors , including the protein termed <PROT2_3267>  Rab </PROT2_3267> ( CITATION ) or hRIP ( TARGET_CITATION ) . ||7637788_155908_3267==>Nuclear pore proteins may also act as a receptor for factors involved in export reactions , since a cellular protein ( <PROT2_3267>  Rab </PROT2_3267> / hRip ) , which contains several FG repeat sequences typically found in nucleoporins , binds to a nuclear export sequence ( NES ) of the HIV <PROT1_155908>  Rev </PROT1_155908> protein which , itself , exports unspliced and partially spliced viral mRNA from the nucleus to the cytoplasm ( CITATION ; CITATION ; TARGET_CITATION ) . ||7637788_155908_3267==>The <PROT1_155908>  Rev </PROT1_155908> protein has been proposed to interact with several cellular proteins such as the <PROT1_155908>  Rev </PROT1_155908> - interacting protein ( <PROT2_3267>  Rip </PROT2_3267> or <PROT2_3267>  Rab </PROT2_3267> ) ( CITATION ; TARGET_CITATION ; CITATION ) , eukaryotic initiation factor 5A ( eIF5A ) ( CITATION ; CITATION ) and proteins of the nuclear pore complex ( NPC ) ( CITATION , CITATION ; CITATION ) . ||7637788_155908_3267==>Nucleoporin - like proteins hRip / <PROT2_3267>  Rab </PROT2_3267> have been shown to interact with the NES of <PROT1_155908>  Rev </PROT1_155908> and are believed to be involved in the nuclear export of <PROT1_155908>  Rev </PROT1_155908> ( CITATION , TARGET_CITATION ) . ||7637788_155908_3267==>In yeast two - hybrid experiments , NESs have been shown to interact , perhaps indirectly , with a variety of proteins such as the <PROT1_155908>  Rev </PROT1_155908> - interacting proteins ( hRip / <PROT2_3267>  Rab </PROT2_3267> ) ( CITATION - TARGET_CITATION ) and several yeast and vertebrate nucleoporins ( nuclear pore proteins ; refs. CITATION , CITATION , CITATION ) . ||7637788_155908_3267==>These transport effects could involve the specific interplay of other cellular factors , such as the nucleoporin - like <PROT2_3267>  Rab </PROT2_3267> / <PROT2_3267>  Rip </PROT2_3267> proteins ( CITATION - TARGET_CITATION ) or eIF 5A ( CITATION , CITATION ) , with the leucine - rich activation domain of <PROT1_155908>  Rev </PROT1_155908> ( CITATION ) , which functions as a nuclear export signal ( CITATION , CITATION ) . ||7637788_155908_3267==>Using the yeast two - hybrid system , a human protein called <PROT2_3267>  Rip </PROT2_3267> ( Fritz et al. 1995 TARGET_CITATION ) or <PROT2_3267>  Rab </PROT2_3267> ( Bogerd et al. 1995 CITATION ) that interacted specifically with <PROT1_155908>  Rev </PROT1_155908> was identified . ||7637788_155908_3267==>The <PROT1_155908>  Rev </PROT1_155908> effector domain contains a nuclear export signal ( NES ) and interacts with the nucleoporin <PROT2_3267>  Rab </PROT2_3267> / <PROT2_3267>  Rip </PROT2_3267> , a protein that mediate nucleocytoplasmic transport ( TARGET_CITATION ) . ||7637788_155908_3267==>They comprised three different , incomplete sequences : two were novel , and the other was identified as the murine homolog of the <PROT1_155908>  Rev </PROT1_155908> - associated binding / <PROT1_155908>  Rev </PROT1_155908> - interacting protein ( <PROT2_3267>  RAB </PROT2_3267> / <PROT2_3267>  Rip </PROT2_3267> ) ( CITATION , TARGET_CITATION ) . ||7637788_155908_3267==>These proteins include the human homologue of the Drosophila NUMB protein ( h - NUMB , refs. CITATION ) , a numb related protein ( h - NUMB - R , ref. CITATION ) , the cellular co - factor of the HIV <PROT1_155908>  REV </PROT1_155908> protein , <PROT2_3267>  RAB </PROT2_3267> ( h - <PROT2_3267>  RAB </PROT2_3267> , refs. CITATION and TARGET_CITATION ) , and a <PROT2_3267>  RAB </PROT2_3267> - related gene ( h - <PROT2_3267>  RAB </PROT2_3267> - R , ref. CITATION ) . ||7637788_155908_3267==>In particular , we note that <PROT2_3267>  RAB </PROT2_3267> , the cellular co - factor of the HIV <PROT1_155908>  REV </PROT1_155908> protein , is an EH interactor that binds to eps15R and exhibits structural and topographical features compatible with those of a nucleoporin ( CITATION , TARGET_CITATION ) . ||7637788_155908_3267==>Initial efforts to define the nuclear export pathway utilized by HIV - 1 <PROT1_155908>  Rev </PROT1_155908> led to the demonstration that the <PROT1_155908>  Rev </PROT1_155908> NES could interact specifically with a cellular nucleoporin - like protein termed hRIP / <PROT2_3267>  Rab </PROT2_3267> in both yeast and mammalian two - hybrid assays ( CITATION , TARGET_CITATION , CITATION ) . ||7637788_155908_3267==>The hRIP / <PROT2_3267>  Rab </PROT2_3267> protein proved able to interact with <PROT1_155908>  Rev </PROT1_155908> when <PROT1_155908>  Rev </PROT1_155908> was bound to the RRE and also specifically bound NESs found in HTLV - 1 Rex and in other , distinct <PROT1_155908>  Rev </PROT1_155908> proteins , such as visna maedi virus ( VMV ) <PROT1_155908>  Rev </PROT1_155908> . Significantly , overexpression of hRIP / <PROT2_3267>  Rab </PROT2_3267> was observed to modestly enhance <PROT1_155908>  Rev </PROT1_155908> function when <PROT1_155908>  Rev </PROT1_155908> expression was limiting or when a <PROT1_155908>  Rev </PROT1_155908> protein bearing an attenuated NES was used ( CITATION , TARGET_CITATION ) . ||7637788_155908_3267==>To further confirm that hCrm1 binding indeed fully correlates with NES function , as also previously demonstrated for hRIP / <PROT2_3267>  Rab </PROT2_3267> binding ( CITATION , CITATION , CITATION , CITATION , TARGET_CITATION , CITATION ) , we examined the ability of hCrm1 to bind to wild - type and NES mutant forms of HIV - 1 <PROT1_155908>  Rev </PROT1_155908> , HTLV - I Rex , and VMV <PROT1_155908>  Rev </PROT1_155908> in a mammalian two - hybrid assay . ||7637788_155908_3267==>This laboratory has previously described ( CITATION ) an extensive set of point missense mutations within the <PROT1_155908>  Rev </PROT1_155908> NES , and it has been demonstrated , by both ourselves and others , that the activity of these NES mutants completely correlates with their ability to bind hRIP / <PROT2_3267>  Rab </PROT2_3267> , as well as certain other nucleoporins such as Nup214 / CAN , in the yeast two - hybrid assay ( CITATION , CITATION , TARGET_CITATION , CITATION , CITATION ) . ||7637788_155908_3267==>The demonstration of a specific interaction between the <PROT1_155908>  Rev </PROT1_155908> NES and hRIP / <PROT2_3267>  Rab </PROT2_3267> , as well as several cellular nucleoporins , in both the yeast and mammalian two - hybrid assays ( CITATION , TARGET_CITATION , CITATION ) appeared to represent an important step forward in understanding how leucine - rich NESs function , given the known importance of nucleoporins in the regulation of nucleocytoplasmic transport ( CITATION , CITATION ) . ||7637788_155908_3267==>Indeed , various cellular proteins which appear to be specific binding partners of the <PROT1_155908>  Rev </PROT1_155908> activation domain have been described ; these include nucleoporin - like protein hRIP / <PROT2_3267>  Rab </PROT2_3267> ( CITATION , TARGET_CITATION ) , eukaryotic initiation factor 5A ( eIF - 5A ) ( CITATION ) , and nuclear export factor CRM1 , which appears to be a general nuclear export signal receptor ( CITATION , CITATION , CITATION , CITATION ) . ||7637788_155908_3267==>These cofactors , including nucleoporin - like proteins such as hRIP / <PROT2_3267>  Rab </PROT2_3267> ( CITATION , TARGET_CITATION , CITATION ) , CRM1 ( CITATION , CITATION , CITATION , CITATION ) , and nuclear eIF - 5A ( CITATION , CITATION ) , are thought to mediate the subsequent nuclear export of <PROT1_155908>  Rev </PROT1_155908> ( reviewed in reference CITATION ) . ||7637788_155908_3267==>In fact , various proteins that are able to bind to this region have been described , including eukaryotic initiation factor 5A ( eIF - 5A ) ( CITATION ) , the nucleoporin - like protein human <PROT1_155908>  Rev </PROT1_155908> interacting protein ( hRIP ) / <PROT2_3267>  Rab </PROT2_3267> ( TARGET_CITATION , CITATION ) and , more recently , the nuclear pore - associated factor CRM1 ( CITATION ) that is critically involved in the translocation of NES - containing proteins through the nuclear pore complex ( CITATION ) . ||7637788_155908_3267==>In particular , the EH network might be involved in the control of nucleocytoplasmic export , as suggested by the finding that three EH - containing proteins , Eps15 , Eps15R , and intersectin , interact with <PROT2_3267>  Hrb </PROT2_3267> ( also called hRip or <PROT2_3267>  RAB </PROT2_3267> , Human Gene Nomenclature Committee approved symbols are used in this paper ) , a cellular cofactor for the HIV - 1 <PROT1_155908>  Rev </PROT1_155908> protein ( Bogerd et al. 1995 CITATION ; Fritz et al. 1995 TARGET_CITATION ) , and with the related protein Hrbl ( Salcini et al. 1997 CITATION ; Yamabhai et al. 1998 CITATION ) . ||7637788_155908_3267==>Efforts to identify cellular cofactors mediating <PROT1_155908>  Rev </PROT1_155908> export led to the isolation of the distantly related human <PROT2_3267>  Hrb </PROT2_3267> ( Bogerd et al. 1995 CITATION ; Fritz et al. 1995 TARGET_CITATION ) and yeast Rip1p ( Stutz et al. 1995 CITATION ) proteins , which were shown to enhance <PROT1_155908>  Rev </PROT1_155908> function in cells . ||7637788_155908_3267==>A yeast two - hybrid screen initially identified the yeast FG - nucleoporin Rip1p and the non - homologous mammalian protein hRip / <PROT2_3267>  RAB </PROT2_3267> as potential targets for <PROT1_155908>  Rev </PROT1_155908> - NES at the nuclear pore ( CITATION ; TARGET_CITATION ; CITATION ) . ||7637788_155908_3267==>A human nucleoporin - like protein called hRIP / <PROT2_3267>  Rab </PROT2_3267> in human and a new yeast nuclear pore - associated protein , <PROT2_3267>  Rip </PROT2_3267> - 1p , were shown to interact specifically with functional NES , which was consistent with a role of <PROT1_155908>  Rev </PROT1_155908> in nuclear export of RNA ( TARGET_CITATION ) . ||7637788_155908_3267==>It has been reported recently that both HIV - 1 <PROT1_155908>  Rev </PROT1_155908> and HTLV - 1 Rex interact in the yeast two - hybrid system with a nucleoporin - like human protein called hRIP / <PROT2_3267>  Rab </PROT2_3267> and several yeast and mammalian FG - rich nucleoporins including yRip1p , hNup214 , and hNup153 ( TARGET_CITATION ) . ||7637788_155908_3267==>Thus , NLP - 1 closely resembles the previously identified <PROT1_155908>  Rev </PROT1_155908> - interacting protein hRIP / <PROT2_3267>  Rab </PROT2_3267> , which also harbors many FG repeats , a high serine content , and a zinc finger of the CCCC type ( TARGET_CITATION , CITATION ) . ||7637788_155908_3267==>CRM - 1 probably bridges the indirect interaction of <PROT1_155908>  Rev </PROT1_155908> with members of the nucleoporin family CITATION , including <PROT2_3267>  Rab </PROT2_3267> / hRIP , reported to be <PROT1_155908>  Rev </PROT1_155908> - binding proteins CITATION , TARGET_CITATION . ||7637788_155908_3267==>In contrast , however , the nucleoporin - like protein hRIP / <PROT2_3267>  Rab </PROT2_3267> ( CITATION , TARGET_CITATION ) was shown to bind indirectly via CRM1 / exportin 1 to the <PROT1_155908>  Rev </PROT1_155908> NES ( CITATION ) . ||7637788_155908_3267==><PROT2_3267>  Rip </PROT2_3267> / <PROT2_3267>  Rab </PROT2_3267> is an HIV <PROT1_155908>  Rev </PROT1_155908> - binding protein with FG repeat motifs ( Bogerd et al. , 1995 CITATION ; Fritz et al. , 1995 TARGET_CITATION ; Stutz et al. , 1995 CITATION ) . ||7637788_155908_3267==>The N - terminal ( DI ) domain contains three copies of the evolutionary conserved EH domain ( CITATION ) , a protein - protein binding module that recognizes NPF ( asparagine - proline - phenylalanine ) motifs ( CITATION ) present in several proteins including the epsins ( CITATION , CITATION ) and <PROT2_3267>  Hrb </PROT2_3267> / hRIP / <PROT2_3267>  RAB </PROT2_3267> ( CITATION ) , a protein involved in the nuclear export of the HIV <PROT1_155908>  Rev </PROT1_155908> protein ( CITATION - TARGET_CITATION ) . ||7637788_155908_3267==>Several cellular factors have been identified to be directly involved in the <PROT1_155908>  Rev </PROT1_155908> - mediated nuclear export pathway , such as CRM1 ( CITATION , CITATION ) , eukaryotic initiation factor 5A ( eIF - 5A ) ( CITATION ) , and <PROT2_3267>  Rip </PROT2_3267> / <PROT2_3267>  Rab </PROT2_3267> ( CITATION , TARGET_CITATION ) . ||7637788_155908_3267==>Several cellular factors have been identified as involved in the <PROT1_155908>  Rev </PROT1_155908> - mediated nuclear export pathway ; these include CRM1 ( CITATION , CITATION ) , eIF5 - A ( CITATION ) , and <PROT2_3267>  Rip </PROT2_3267> / <PROT2_3267>  Rab </PROT2_3267> ( CITATION , TARGET_CITATION ) . ||7637788_155908_3267==>Several putative cofactors for <PROT1_155908>  Rev </PROT1_155908> NES function have been identified ; particularly well - documented ones include nucleoporins such as <PROT2_3267>  Rab </PROT2_3267> / hRIP and Rip1p ( CITATION , TARGET_CITATION , CITATION ) and the nucleocytoplasmic shuttle protein CRM1 ( CITATION , CITATION , CITATION ) . ||7637788_155908_3267==>The human <PROT1_155908>  Rev </PROT1_155908> - interacting protein ( hRIP ) , also known as hRab or <PROT2_3267>  Hrb </PROT2_3267> , was identified using a yeast two - hybrid screen with <PROT1_155908>  Rev </PROT1_155908> as the bait ( Bogerd et al. 1995 CITATION ; Fritz et al. 1995 TARGET_CITATION ) . ||7637788_155908_3267==>Most of these ligands are already known , but a number of new ligands including the HIV - 1 <PROT1_155908>  Rev </PROT1_155908> interacting protein ( <PROT2_3267>  RIP </PROT2_3267> ) , important for nuclear export of viral RNA and infectivity ( TARGET_CITATION ) , are discussed in Supplementary Table 1 . ||7637788_155908_7514==>Indeed , various cellular proteins which appear to be specific binding partners of the <PROT1_155908>  Rev </PROT1_155908> activation domain have been described ; these include nucleoporin - like protein hRIP / Rab ( CITATION , TARGET_CITATION ) , eukaryotic initiation factor 5A ( eIF - 5A ) ( CITATION ) , and nuclear export factor <PROT2_7514>  CRM1 </PROT2_7514> , which appears to be a general nuclear export signal receptor ( CITATION , CITATION , CITATION , CITATION ) . ||7637788_155908_7514==>These cofactors , including nucleoporin - like proteins such as hRIP / Rab ( CITATION , TARGET_CITATION , CITATION ) , <PROT2_7514>  CRM1 </PROT2_7514> ( CITATION , CITATION , CITATION , CITATION ) , and nuclear eIF - 5A ( CITATION , CITATION ) , are thought to mediate the subsequent nuclear export of <PROT1_155908>  Rev </PROT1_155908> ( reviewed in reference CITATION ) . ||7637788_155908_7514==>In fact , various proteins that are able to bind to this region have been described , including eukaryotic initiation factor 5A ( eIF - 5A ) ( CITATION ) , the nucleoporin - like protein human <PROT1_155908>  Rev </PROT1_155908> interacting protein ( hRIP ) / Rab ( TARGET_CITATION , CITATION ) and , more recently , the nuclear pore - associated factor <PROT2_7514>  CRM1 </PROT2_7514> ( CITATION ) that is critically involved in the translocation of NES - containing proteins through the nuclear pore complex ( CITATION ) . ||7637788_155908_7514==>In the Saccharomyces cerevisiae two - hybrid system , it has been shown that FG - containing repeat domains of different nucleoporins interact with the <PROT1_155908>  Rev </PROT1_155908> NES via <PROT2_7514>  CRM1 </PROT2_7514> with various efficiencies , suggesting that nucleoporins play a role in NES - mediated export ( CITATION , CITATION , TARGET_CITATION , CITATION , CITATION ) . ||7637788_155908_7514==>In contrast , however , the nucleoporin - like protein hRIP / Rab ( CITATION , TARGET_CITATION ) was shown to bind indirectly via <PROT2_7514>  CRM1 </PROT2_7514> / exportin 1 to the <PROT1_155908>  Rev </PROT1_155908> NES ( CITATION ) . ||7637788_155908_7514==>The NES mediates association of <PROT1_155908>  Rev </PROT1_155908> with the general nuclear export factor <PROT2_7514>  CRM1 </PROT2_7514> / exportin1 ( CITATION , CITATION , CITATION , CITATION ) , the nucleoporin - like protein Rab1 / hRIP ( CITATION , TARGET_CITATION ) , and the eukaryotic initiation factor 5A ( eIF5A ) ( CITATION ) , and it is responsible for directing <PROT1_155908>  Rev </PROT1_155908> - RNA complexes through an export pathway used by 5S rRNA and U snRNAs ( CITATION ) . ||7637788_155908_7514==>For both HIV - 1 <PROT1_155908>  Rev </PROT1_155908> and influenza A virus NEP , a positive correlation has previously been demonstrated for the ability to bind particular nucleoporins and <PROT2_7514>  Crm1 </PROT2_7514> in yeast and / or mammalian two - hybrid systems and function as a nuclear export chaperon ( CITATION , CITATION , TARGET_CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , CITATION ) . ||7637788_155908_7514==>Several cellular factors have been identified to be directly involved in the <PROT1_155908>  Rev </PROT1_155908> - mediated nuclear export pathway , such as <PROT2_7514>  CRM1 </PROT2_7514> ( CITATION , CITATION ) , eukaryotic initiation factor 5A ( eIF - 5A ) ( CITATION ) , and Rip / Rab ( CITATION , TARGET_CITATION ) . ||7637788_155908_7514==>Several cellular factors have been identified as involved in the <PROT1_155908>  Rev </PROT1_155908> - mediated nuclear export pathway ; these include <PROT2_7514>  CRM1 </PROT2_7514> ( CITATION , CITATION ) , eIF5 - A ( CITATION ) , and Rip / Rab ( CITATION , TARGET_CITATION ) . ||7637788_155908_7514==>Several putative cofactors for <PROT1_155908>  Rev </PROT1_155908> NES function have been identified ; particularly well - documented ones include nucleoporins such as Rab / hRIP and Rip1p ( CITATION , TARGET_CITATION , CITATION ) and the nucleocytoplasmic shuttle protein <PROT2_7514>  CRM1 </PROT2_7514> ( CITATION , CITATION , CITATION ) . ||7778269_155871_22954==>These proteins include TATA - binding protein ( TBP ) ( CITATION , CITATION ) , TAK ( CITATION , CITATION ) , PKR ( CITATION , CITATION ) , T3R ( CITATION ) , <PROT1_155871>  Tat </PROT1_155871> - binding protein 1 ( CITATION , CITATION ) , TAP ( CITATION , CITATION , CITATION ) , TBP - associated factor TAF55 ( CITATION ) , <PROT2_22954>  HT2A </PROT2_22954> ( TARGET_CITATION ) , Tip60 ( CITATION ) , TFIIH ( CITATION , CITATION ) , RNA polymerase II ( CITATION ) , and Sp1 ( CITATION , CITATION ) . ||7778269_155871_22954==>Earlier - reported candidates for <PROT1_155871>  Tat </PROT1_155871> - interacting cofactors include TBP1 ( CITATION ) , TAP ( CITATION ) , Tip60 ( CITATION ) , and <PROT2_22954>  HT2A </PROT2_22954> ( TARGET_CITATION ) . ||7778269_155871_22954==>It was identified in a yeast two - hybrid screening for proteins that interact with the human immunodeficiency virus protein <PROT1_155871>  Tat </PROT1_155871> , and the last 120 amino acids of <PROT2_22954>  HT2A </PROT2_22954> were sufficient for <PROT1_155871>  Tat </PROT1_155871> binding ( TARGET_CITATION ) . ||7778269_155871_22954==>The C - terminal region of <PROT2_22954>  HT2A </PROT2_22954> was used as a positive control and interacted strongly with <PROT1_155871>  Tat </PROT1_155871> , as reported by Fridell et al. ( TARGET_CITATION ) . ||7778269_155871_22954==>In addition to hCycT1 , a number of other potential <PROT1_155871>  Tat </PROT1_155871> interacting proteins and / or cofactors have been proposed including MSS1 , <PROT2_22954>  HT2A </PROT2_22954> , CA150 , TFIID , <PROT1_155871>  Tat </PROT1_155871> - SF1 , TIP30 , and PolII itself ( CITATION - TARGET_CITATION ) , and other groups have reported interactions between <PROT1_155871>  Tat </PROT1_155871> and the coactivator proteins p300 and CREB - binding protein ( CBP ) ( CITATION ) or between <PROT1_155871>  Tat </PROT1_155871> and the transcription factor TFIIH ( CITATION , CITATION , CITATION , CITATION ) . ||7778269_155871_22954==>Another subgroup includes F54G8.4 , a nematode ORF with unknown function , and the human <PROT2_22954>  HT2A </PROT2_22954> protein , identified through its interaction with the human immunodeficiency virus protein <PROT1_155871>  Tat </PROT1_155871> ( TARGET_CITATION ) . ||7778269_155871_22954==>All three factors have ties to RNA metabolism : the nucleoli in Caenorhabditis elegans ncl - 1 mutants are enlarged ( Frank and Roth 1998 CITATION ) ; <PROT2_22954>  HT2A </PROT2_22954> was identified by virtue of interaction with the RNA - binding protein HIV <PROT1_155871>  Tat </PROT1_155871> ( Fridell et al. 1995 TARGET_CITATION ) ; and posttranscriptional regulation of lin - 29 mRNA is abrogated in lin - 41 mutants ( Slack et al. 2000 CITATION ) . ||7778269_155871_22954==>First , the <PROT2_22954>  HT2A </PROT2_22954> human protein interacts with the site - specific RNA - binding <PROT1_155871>  Tat </PROT1_155871> protein ( Fridell et al. 1995 TARGET_CITATION ) , much as Brat interacts with Nos and Pum . ||7778269_155871_22954==>Besides cyclin T , other <PROT1_155871>  Tat </PROT1_155871> - interacting proteins and / or cofactors include Tip60 , <PROT2_22954>  HT2A </PROT2_22954> , CA150 , TFIID , <PROT1_155871>  Tat </PROT1_155871> - SF1 , Tip30 , p300 , and CBP ( CITATION , CITATION - TARGET_CITATION ) . ||7778269_155871_708==>Originally , <PROT2_708>  p32 </PROT2_708> was characterized as being a component of the ASF / SF2 splicing activity purified from HeLa cells ( CITATION ) . Subsequently , it was shown that <PROT2_708>  p32 </PROT2_708> was dispensable for the general splicing activity , although the possibility can not be ruled out that <PROT2_708>  p32 </PROT2_708> has a more specialized role in splicing ( CITATION ) . Recent evidence suggests that <PROT2_708>  p32 </PROT2_708> also interacts with the HIV - 1 <PROT1_155871>  Tat </PROT1_155871> protein ( CITATION , CITATION , TARGET_CITATION ) . In one report , a <PROT1_155871>  Tat </PROT1_155871> - binding protein ( TAP ) was isolated on the basis of <PROT1_155871>  Tat </PROT1_155871> affinity chromatography , and the sequence turned out to be identical to <PROT2_708>  p32 </PROT2_708> except for a few amino acids in the N - terminal precursor segment ( CITATION ) . The same group also found that TAP ( <PROT2_708>  p32 </PROT2_708> ) interacts with the C terminus of TFIIB , and it was suggested that TAP ( <PROT2_708>  p32 </PROT2_708> ) may function as a cellular co - activator that bridges <PROT1_155871>  Tat </PROT1_155871> to the general transcription machinery ( CITATION ) . The significance of the TAP ( <PROT2_708>  p32 </PROT2_708> ) - <PROT1_155871>  Tat </PROT1_155871> interaction was substantiated by a two - hybrid analysis , in vitro binding studies , and a demonstration implying that TAP ( <PROT2_708>  p32 </PROT2_708> ) was able to cooperate with <PROT1_155871>  Tat </PROT1_155871> to synergistically stimulate transcription ( CITATION , CITATION ) . The region involved in <PROT1_155871>  Tat </PROT1_155871> binding was mapped to amino acids corresponding to 247 - 282 in <PROT2_708>  p32 </PROT2_708> , which is outside the region where we see protection by Rev ( amino acids 196 - 208 ) . ||7778269_155871_708==>The <PROT2_708>  p32 </PROT2_708> protein has been shown to interact with human immunodeficiency virus ( HIV ) 1 <PROT1_155871>  Tat </PROT1_155871> protein ( CITATION - TARGET_CITATION ) and Rev protein ( CITATION , CITATION ) by using in vitro binding studies and two - hybrid analyses in yeast cells . ||8628270_1022_155871==>Many transcription activators , including HIV - 1 <PROT2_155871>  Tat </PROT2_155871> , herpes simplex virus VP16 , and human p53 and E2F1 , can bind TFIIH ( CITATION , TARGET_CITATION , CITATION ) whose <PROT1_1022>  CDK7 </PROT1_1022> subunit is a CTD kinase for RPII ( reviewed in refs. CITATION and CITATION ) . ||8628270_1022_155871==>The multi - subunit general transcription factor TFIIH , which has its own tripartite protein kinase , <PROT1_1022>  Cdk7 </PROT1_1022> & # x2013 ; cyclin H & # x2013 ; MAT1 , binds to the same domain of <PROT2_155871>  Tat </PROT2_155871> ( CITATION , CITATION , CITATION and TARGET_CITATION ) . ||8628270_1022_155871==>Transcription activators may remove hnRNP U and / or provide signals to inactivate hnRNP u. It remains to be determined whether a transcription factor such as <PROT2_155871>  Tat </PROT2_155871> , retinoic acid receptor alpha , p53 , or VP16 that binds TFIIH or <PROT1_1022>  Cdk7 </PROT1_1022> ( TARGET_CITATION , CITATION , CITATION , CITATION , CITATION , CITATION ) would affect the interaction of hnRNP U with TFIIH in vivo . ||8628270_1022_155871==>These proteins include the core RNA polymerase II ( CITATION , CITATION ) , whose C - terminal domain is required for <PROT2_155871>  Tat </PROT2_155871> - mediated transactivation ( CITATION ) , TAFII55 ( CITATION ) , and TFIIH and <PROT1_1022>  CDK7 </PROT1_1022> ( TARGET_CITATION , CITATION ) . ||8628270_1022_155871==><PROT2_155871>  Tat </PROT2_155871> , through its activation domain , interacts with the general transcription factor , TFIIH ( TARGET_CITATION ) , and stimulates the phosphorylation of the C - terminal domain of RNA polymerase II by its kinase component , <PROT1_1022>  cdk7 </PROT1_1022> ( CITATION , CITATION ) . ||8628270_155871_2966==><PROT1_155871>  Tat </PROT1_155871> was reported previously to bind to the p62 subunit of <PROT2_2966>  TFIIH </PROT2_2966> ( Blau et al. 1996 TARGET_CITATION ; Parada and Roeder 1996 CITATION ) . ||8628270_155871_2966==>HIV - 1 <PROT1_155871>  Tat </PROT1_155871> has previously been shown to associate with several components comprising the HIV - 1 preinitiation complex including SP1 ( CITATION ) , TATA - binding protein ( TBP ) and associated factors ( TAFs ) ( CITATION ) , the p62 subunit of <PROT2_2966>  TFIIH </PROT2_2966> ( TARGET_CITATION ) in addition to RNA polymerase II ( CITATION ) . ||8628270_155871_2966==>Many transcription activators , including HIV - 1 <PROT1_155871>  Tat </PROT1_155871> , herpes simplex virus VP16 , and human p53 and E2F1 , can bind <PROT2_2966>  TFIIH </PROT2_2966> ( CITATION , TARGET_CITATION , CITATION ) whose CDK7 subunit is a CTD kinase for RPII ( reviewed in refs. CITATION and CITATION ) . ||8628270_155871_2966==>The HIV - 1 <PROT1_155871>  Tat </PROT1_155871> protein stimulates elongation by RNAP II ( reviewed in ref. CITATION ) and interacts with two CTD kinases , <PROT2_2966>  TFIIH </PROT2_2966> ( TARGET_CITATION , CITATION , CITATION ) and P - TEFb ( CITATION , CITATION ) . ||8628270_155871_2966==>Although both <PROT1_155871>  Tat </PROT1_155871> and VP16 bind <PROT2_2966>  TFIIH </PROT2_2966> components ( TARGET_CITATION , CITATION and CITATION ) , <PROT1_155871>  Tat </PROT1_155871> but not VP16 stimulates the ability of kinases in <PROT2_2966>  TFIIH </PROT2_2966> to phosphorylate the RNA polymerase II CTD ( CITATION and CITATION ) . ||8628270_155871_2966==>The multi - subunit general transcription factor <PROT2_2966>  TFIIH </PROT2_2966> , which has its own tripartite protein kinase , Cdk7 & # x2013 ; cyclin H & # x2013 ; MAT1 , binds to the same domain of <PROT1_155871>  Tat </PROT1_155871> ( CITATION , CITATION , CITATION and TARGET_CITATION ) . ||8628270_155871_2966==>All of these factors , like <PROT1_155871>  Tat </PROT1_155871> , bind to <PROT2_2966>  TFIIH </PROT2_2966> ( TARGET_CITATION ) . ||8628270_155871_2966==>DRB may exert its effect by direct inhibition of a CTD kinase activity , and two DRB - sensitive cellular kinases , the CAK component of <PROT2_2966>  TFIIH </PROT2_2966> ( CITATION , TARGET_CITATION ) and <PROT1_155871>  Tat </PROT1_155871> - associated kinase ( TAK ) ( CITATION ) , have been found to interact with the activation domain of <PROT1_155871>  Tat </PROT1_155871> . ||8628270_155871_2966==>Transcription activators may remove hnRNP U and / or provide signals to inactivate hnRNP u. It remains to be determined whether a transcription factor such as <PROT1_155871>  Tat </PROT1_155871> , retinoic acid receptor alpha , p53 , or VP16 that binds <PROT2_2966>  TFIIH </PROT2_2966> or Cdk7 ( TARGET_CITATION , CITATION , CITATION , CITATION , CITATION , CITATION ) would affect the interaction of hnRNP U with <PROT2_2966>  TFIIH </PROT2_2966> in vivo . ||8628270_155871_2966==>In fact , <PROT1_155871>  Tat </PROT1_155871> has been shown to interact with <PROT2_2966>  TFIIH </PROT2_2966> and to stimulate phosphorylation of pol II CTD by the <PROT2_2966>  TFIIH </PROT2_2966> kinase , although different groups have different opinions on which subunit of <PROT2_2966>  TFIIH </PROT2_2966> mediates <PROT1_155871>  Tat </PROT1_155871> binding ( TARGET_CITATION , CITATION , CITATION , CITATION ) . ||8628270_155871_2966==>HIV - 1 <PROT1_155871>  Tat </PROT1_155871> has been shown to interact with <PROT2_2966>  TFIIH </PROT2_2966> ( TARGET_CITATION , CITATION , CITATION , CITATION ) and P - TEFb ( CITATION , CITATION ) in different reports following different protocols . ||8628270_155871_2966==>These proteins include the core RNA polymerase II ( CITATION , CITATION ) , whose C - terminal domain is required for <PROT1_155871>  Tat </PROT1_155871> - mediated transactivation ( CITATION ) , TAFII55 ( CITATION ) , and <PROT2_2966>  TFIIH </PROT2_2966> and CDK7 ( TARGET_CITATION , CITATION ) . ||8628270_155871_2966==>In addition to hCycT1 , a number of other potential <PROT1_155871>  Tat </PROT1_155871> interacting proteins and / or cofactors have been proposed including MSS1 , HT2A , CA150 , TFIID , <PROT1_155871>  Tat </PROT1_155871> - SF1 , TIP30 , and PolII itself ( CITATION ) , and other groups have reported interactions between <PROT1_155871>  Tat </PROT1_155871> and the coactivator proteins p300 and CREB - binding protein ( CBP ) ( CITATION ) or between <PROT1_155871>  Tat </PROT1_155871> and the transcription factor <PROT2_2966>  TFIIH </PROT2_2966> ( TARGET_CITATION , CITATION , CITATION , CITATION ) . ||8628270_155871_2966==>The reported interaction of <PROT1_155871>  Tat </PROT1_155871> with <PROT2_2966>  TFIIH </PROT2_2966> ( TARGET_CITATION , CITATION , CITATION , CITATION ) , which contains a CTD kinase activity , has led to the proposal that <PROT1_155871>  Tat </PROT1_155871> might activate the HIV - 1 LTR by the sequential TAR - independent recruitment of <PROT2_2966>  TFIIH </PROT2_2966> to LTR - bound PolII molecules , followed by the subsequent TAR - dependent recruitment of hCycT1 / P - TEFb ( CITATION , CITATION ) . ||8628270_155871_2966==><PROT2_2966>  TFIIH </PROT2_2966> has been found to associate with <PROT1_155871>  Tat </PROT1_155871> ( TARGET_CITATION , CITATION , CITATION , CITATION ) , and based on differential sensitivity of <PROT2_2966>  TFIIH </PROT2_2966> and P - TEFb to a pseudosubstrate peptide , it was concluded that in addition to P - TEFb , the kinase activity of <PROT2_2966>  TFIIH </PROT2_2966> was required for <PROT1_155871>  Tat </PROT1_155871> to work ( CITATION ) . ||8628270_155871_2966==><PROT1_155871>  Tat </PROT1_155871> , through its activation domain , interacts with the general transcription factor , <PROT2_2966>  TFIIH </PROT2_2966> ( TARGET_CITATION ) , and stimulates the phosphorylation of the C - terminal domain of RNA polymerase II by its kinase component , cdk7 ( CITATION , CITATION ) . ||8628270_155871_2966==>Many transcription activator proteins , including HSV VP16 and human p53 and Androgen Receptor , as well as HIV <PROT1_155871>  Tat </PROT1_155871> , bind to <PROT2_2966>  TFIIH </PROT2_2966> directly , which might stimulate its CTD kinase activity and facilitate entry into productive transcription elongation ( CITATION , CITATION , TARGET_CITATION and CITATION ) . ||8628270_155871_2966==>Perhaps not surprisingly , therefore , the activation domain of <PROT1_155871>  Tat </PROT1_155871> makes direct contact with the protein kinases <PROT2_2966>  TFIIH </PROT2_2966> , P - TEFb , TAFII250 and TIP30 and the protein phosphatase FCP1 ( CITATION , CITATION , CITATION , CITATION , CITATION and TARGET_CITATION ) . ||8628270_155871_2967==><PROT1_155871>  Tat </PROT1_155871> was reported previously to bind to the p62 subunit of <PROT2_2967>  TFIIH </PROT2_2967> ( Blau et al. 1996 TARGET_CITATION ; Parada and Roeder 1996 CITATION ) . ||8628270_155871_2967==>HIV - 1 <PROT1_155871>  Tat </PROT1_155871> has previously been shown to associate with several components comprising the HIV - 1 preinitiation complex including SP1 ( CITATION ) , TATA - binding protein ( TBP ) and associated factors ( TAFs ) ( CITATION ) , the p62 subunit of <PROT2_2967>  TFIIH </PROT2_2967> ( TARGET_CITATION ) in addition to RNA polymerase II ( CITATION ) . ||8628270_155871_2967==>Many transcription activators , including HIV - 1 <PROT1_155871>  Tat </PROT1_155871> , herpes simplex virus VP16 , and human p53 and E2F1 , can bind <PROT2_2967>  TFIIH </PROT2_2967> ( CITATION , TARGET_CITATION , CITATION ) whose CDK7 subunit is a CTD kinase for RPII ( reviewed in refs. CITATION and CITATION ) . ||8628270_155871_2967==>The HIV - 1 <PROT1_155871>  Tat </PROT1_155871> protein stimulates elongation by RNAP II ( reviewed in ref. CITATION ) and interacts with two CTD kinases , <PROT2_2967>  TFIIH </PROT2_2967> ( TARGET_CITATION , CITATION , CITATION ) and P - TEFb ( CITATION , CITATION ) . ||8628270_155871_2967==>Although both <PROT1_155871>  Tat </PROT1_155871> and VP16 bind <PROT2_2967>  TFIIH </PROT2_2967> components ( TARGET_CITATION , CITATION and CITATION ) , <PROT1_155871>  Tat </PROT1_155871> but not VP16 stimulates the ability of kinases in <PROT2_2967>  TFIIH </PROT2_2967> to phosphorylate the RNA polymerase II CTD ( CITATION and CITATION ) . ||8628270_155871_2967==>The multi - subunit general transcription factor <PROT2_2967>  TFIIH </PROT2_2967> , which has its own tripartite protein kinase , Cdk7 & # x2013 ; cyclin H & # x2013 ; MAT1 , binds to the same domain of <PROT1_155871>  Tat </PROT1_155871> ( CITATION , CITATION , CITATION and TARGET_CITATION ) . ||8628270_155871_2967==>All of these factors , like <PROT1_155871>  Tat </PROT1_155871> , bind to <PROT2_2967>  TFIIH </PROT2_2967> ( TARGET_CITATION ) . ||8628270_155871_2967==>DRB may exert its effect by direct inhibition of a CTD kinase activity , and two DRB - sensitive cellular kinases , the CAK component of <PROT2_2967>  TFIIH </PROT2_2967> ( CITATION , TARGET_CITATION ) and <PROT1_155871>  Tat </PROT1_155871> - associated kinase ( TAK ) ( CITATION ) , have been found to interact with the activation domain of <PROT1_155871>  Tat </PROT1_155871> . ||8628270_155871_2967==>Transcription activators may remove hnRNP U and / or provide signals to inactivate hnRNP u. It remains to be determined whether a transcription factor such as <PROT1_155871>  Tat </PROT1_155871> , retinoic acid receptor alpha , p53 , or VP16 that binds <PROT2_2967>  TFIIH </PROT2_2967> or Cdk7 ( TARGET_CITATION , CITATION , CITATION , CITATION , CITATION , CITATION ) would affect the interaction of hnRNP U with <PROT2_2967>  TFIIH </PROT2_2967> in vivo . ||8628270_155871_2967==>In fact , <PROT1_155871>  Tat </PROT1_155871> has been shown to interact with <PROT2_2967>  TFIIH </PROT2_2967> and to stimulate phosphorylation of pol II CTD by the <PROT2_2967>  TFIIH </PROT2_2967> kinase , although different groups have different opinions on which subunit of <PROT2_2967>  TFIIH </PROT2_2967> mediates <PROT1_155871>  Tat </PROT1_155871> binding ( TARGET_CITATION , CITATION , CITATION , CITATION ) . ||8628270_155871_2967==>HIV - 1 <PROT1_155871>  Tat </PROT1_155871> has been shown to interact with <PROT2_2967>  TFIIH </PROT2_2967> ( TARGET_CITATION , CITATION , CITATION , CITATION ) and P - TEFb ( CITATION , CITATION ) in different reports following different protocols . ||8628270_155871_2967==>These proteins include the core RNA polymerase II ( CITATION , CITATION ) , whose C - terminal domain is required for <PROT1_155871>  Tat </PROT1_155871> - mediated transactivation ( CITATION ) , TAFII55 ( CITATION ) , and <PROT2_2967>  TFIIH </PROT2_2967> and CDK7 ( TARGET_CITATION , CITATION ) . ||8628270_155871_2967==>In addition to hCycT1 , a number of other potential <PROT1_155871>  Tat </PROT1_155871> interacting proteins and / or cofactors have been proposed including MSS1 , HT2A , CA150 , TFIID , <PROT1_155871>  Tat </PROT1_155871> - SF1 , TIP30 , and PolII itself ( CITATION ) , and other groups have reported interactions between <PROT1_155871>  Tat </PROT1_155871> and the coactivator proteins p300 and CREB - binding protein ( CBP ) ( CITATION ) or between <PROT1_155871>  Tat </PROT1_155871> and the transcription factor <PROT2_2967>  TFIIH </PROT2_2967> ( TARGET_CITATION , CITATION , CITATION , CITATION ) . ||8628270_155871_2967==>The reported interaction of <PROT1_155871>  Tat </PROT1_155871> with <PROT2_2967>  TFIIH </PROT2_2967> ( TARGET_CITATION , CITATION , CITATION , CITATION ) , which contains a CTD kinase activity , has led to the proposal that <PROT1_155871>  Tat </PROT1_155871> might activate the HIV - 1 LTR by the sequential TAR - independent recruitment of <PROT2_2967>  TFIIH </PROT2_2967> to LTR - bound PolII molecules , followed by the subsequent TAR - dependent recruitment of hCycT1 / P - TEFb ( CITATION , CITATION ) . ||8628270_155871_2967==><PROT2_2967>  TFIIH </PROT2_2967> has been found to associate with <PROT1_155871>  Tat </PROT1_155871> ( TARGET_CITATION , CITATION , CITATION , CITATION ) , and based on differential sensitivity of <PROT2_2967>  TFIIH </PROT2_2967> and P - TEFb to a pseudosubstrate peptide , it was concluded that in addition to P - TEFb , the kinase activity of <PROT2_2967>  TFIIH </PROT2_2967> was required for <PROT1_155871>  Tat </PROT1_155871> to work ( CITATION ) . ||8628270_155871_2967==><PROT1_155871>  Tat </PROT1_155871> , through its activation domain , interacts with the general transcription factor , <PROT2_2967>  TFIIH </PROT2_2967> ( TARGET_CITATION ) , and stimulates the phosphorylation of the C - terminal domain of RNA polymerase II by its kinase component , cdk7 ( CITATION , CITATION ) . ||8628270_155871_2967==>Many transcription activator proteins , including HSV VP16 and human p53 and Androgen Receptor , as well as HIV <PROT1_155871>  Tat </PROT1_155871> , bind to <PROT2_2967>  TFIIH </PROT2_2967> directly , which might stimulate its CTD kinase activity and facilitate entry into productive transcription elongation ( CITATION , CITATION , TARGET_CITATION and CITATION ) . ||8628270_155871_2967==>Perhaps not surprisingly , therefore , the activation domain of <PROT1_155871>  Tat </PROT1_155871> makes direct contact with the protein kinases <PROT2_2967>  TFIIH </PROT2_2967> , P - TEFb , TAFII250 and TIP30 and the protein phosphatase FCP1 ( CITATION , CITATION , CITATION , CITATION , CITATION and TARGET_CITATION ) . ||8628270_155871_2968==><PROT1_155871>  Tat </PROT1_155871> was reported previously to bind to the p62 subunit of <PROT2_2968>  TFIIH </PROT2_2968> ( Blau et al. 1996 TARGET_CITATION ; Parada and Roeder 1996 CITATION ) . ||8628270_155871_2968==>HIV - 1 <PROT1_155871>  Tat </PROT1_155871> has previously been shown to associate with several components comprising the HIV - 1 preinitiation complex including SP1 ( CITATION ) , TATA - binding protein ( TBP ) and associated factors ( TAFs ) ( CITATION ) , the p62 subunit of <PROT2_2968>  TFIIH </PROT2_2968> ( TARGET_CITATION ) in addition to RNA polymerase II ( CITATION ) . ||8628270_155871_2968==>Many transcription activators , including HIV - 1 <PROT1_155871>  Tat </PROT1_155871> , herpes simplex virus VP16 , and human p53 and E2F1 , can bind <PROT2_2968>  TFIIH </PROT2_2968> ( CITATION , TARGET_CITATION , CITATION ) whose CDK7 subunit is a CTD kinase for RPII ( reviewed in refs. CITATION and CITATION ) . ||8628270_155871_2968==>The HIV - 1 <PROT1_155871>  Tat </PROT1_155871> protein stimulates elongation by RNAP II ( reviewed in ref. CITATION ) and interacts with two CTD kinases , <PROT2_2968>  TFIIH </PROT2_2968> ( TARGET_CITATION , CITATION , CITATION ) and P - TEFb ( CITATION , CITATION ) . ||8628270_155871_2968==>Although both <PROT1_155871>  Tat </PROT1_155871> and VP16 bind <PROT2_2968>  TFIIH </PROT2_2968> components ( TARGET_CITATION , CITATION and CITATION ) , <PROT1_155871>  Tat </PROT1_155871> but not VP16 stimulates the ability of kinases in <PROT2_2968>  TFIIH </PROT2_2968> to phosphorylate the RNA polymerase II CTD ( CITATION and CITATION ) . ||8628270_155871_2968==>The multi - subunit general transcription factor <PROT2_2968>  TFIIH </PROT2_2968> , which has its own tripartite protein kinase , Cdk7 & # x2013 ; cyclin H & # x2013 ; MAT1 , binds to the same domain of <PROT1_155871>  Tat </PROT1_155871> ( CITATION , CITATION , CITATION and TARGET_CITATION ) . ||8628270_155871_2968==>All of these factors , like <PROT1_155871>  Tat </PROT1_155871> , bind to <PROT2_2968>  TFIIH </PROT2_2968> ( TARGET_CITATION ) . ||8628270_155871_2968==>DRB may exert its effect by direct inhibition of a CTD kinase activity , and two DRB - sensitive cellular kinases , the CAK component of <PROT2_2968>  TFIIH </PROT2_2968> ( CITATION , TARGET_CITATION ) and <PROT1_155871>  Tat </PROT1_155871> - associated kinase ( TAK ) ( CITATION ) , have been found to interact with the activation domain of <PROT1_155871>  Tat </PROT1_155871> . ||8628270_155871_2968==>Transcription activators may remove hnRNP U and / or provide signals to inactivate hnRNP u. It remains to be determined whether a transcription factor such as <PROT1_155871>  Tat </PROT1_155871> , retinoic acid receptor alpha , p53 , or VP16 that binds <PROT2_2968>  TFIIH </PROT2_2968> or Cdk7 ( TARGET_CITATION , CITATION , CITATION , CITATION , CITATION , CITATION ) would affect the interaction of hnRNP U with <PROT2_2968>  TFIIH </PROT2_2968> in vivo . ||8628270_155871_2968==>In fact , <PROT1_155871>  Tat </PROT1_155871> has been shown to interact with <PROT2_2968>  TFIIH </PROT2_2968> and to stimulate phosphorylation of pol II CTD by the <PROT2_2968>  TFIIH </PROT2_2968> kinase , although different groups have different opinions on which subunit of <PROT2_2968>  TFIIH </PROT2_2968> mediates <PROT1_155871>  Tat </PROT1_155871> binding ( TARGET_CITATION , CITATION , CITATION , CITATION ) . ||8628270_155871_2968==>HIV - 1 <PROT1_155871>  Tat </PROT1_155871> has been shown to interact with <PROT2_2968>  TFIIH </PROT2_2968> ( TARGET_CITATION , CITATION , CITATION , CITATION ) and P - TEFb ( CITATION , CITATION ) in different reports following different protocols . ||8628270_155871_2968==>These proteins include the core RNA polymerase II ( CITATION , CITATION ) , whose C - terminal domain is required for <PROT1_155871>  Tat </PROT1_155871> - mediated transactivation ( CITATION ) , TAFII55 ( CITATION ) , and <PROT2_2968>  TFIIH </PROT2_2968> and CDK7 ( TARGET_CITATION , CITATION ) . ||8628270_155871_2968==>In addition to hCycT1 , a number of other potential <PROT1_155871>  Tat </PROT1_155871> interacting proteins and / or cofactors have been proposed including MSS1 , HT2A , CA150 , TFIID , <PROT1_155871>  Tat </PROT1_155871> - SF1 , TIP30 , and PolII itself ( CITATION ) , and other groups have reported interactions between <PROT1_155871>  Tat </PROT1_155871> and the coactivator proteins p300 and CREB - binding protein ( CBP ) ( CITATION ) or between <PROT1_155871>  Tat </PROT1_155871> and the transcription factor <PROT2_2968>  TFIIH </PROT2_2968> ( TARGET_CITATION , CITATION , CITATION , CITATION ) . ||8628270_155871_2968==>The reported interaction of <PROT1_155871>  Tat </PROT1_155871> with <PROT2_2968>  TFIIH </PROT2_2968> ( TARGET_CITATION , CITATION , CITATION , CITATION ) , which contains a CTD kinase activity , has led to the proposal that <PROT1_155871>  Tat </PROT1_155871> might activate the HIV - 1 LTR by the sequential TAR - independent recruitment of <PROT2_2968>  TFIIH </PROT2_2968> to LTR - bound PolII molecules , followed by the subsequent TAR - dependent recruitment of hCycT1 / P - TEFb ( CITATION , CITATION ) . ||8628270_155871_2968==><PROT2_2968>  TFIIH </PROT2_2968> has been found to associate with <PROT1_155871>  Tat </PROT1_155871> ( TARGET_CITATION , CITATION , CITATION , CITATION ) , and based on differential sensitivity of <PROT2_2968>  TFIIH </PROT2_2968> and P - TEFb to a pseudosubstrate peptide , it was concluded that in addition to P - TEFb , the kinase activity of <PROT2_2968>  TFIIH </PROT2_2968> was required for <PROT1_155871>  Tat </PROT1_155871> to work ( CITATION ) . ||8628270_155871_2968==><PROT1_155871>  Tat </PROT1_155871> , through its activation domain , interacts with the general transcription factor , <PROT2_2968>  TFIIH </PROT2_2968> ( TARGET_CITATION ) , and stimulates the phosphorylation of the C - terminal domain of RNA polymerase II by its kinase component , cdk7 ( CITATION , CITATION ) . ||8628270_155871_2968==>Many transcription activator proteins , including HSV VP16 and human p53 and Androgen Receptor , as well as HIV <PROT1_155871>  Tat </PROT1_155871> , bind to <PROT2_2968>  TFIIH </PROT2_2968> directly , which might stimulate its CTD kinase activity and facilitate entry into productive transcription elongation ( CITATION , CITATION , TARGET_CITATION and CITATION ) . ||8628270_155871_2968==>Perhaps not surprisingly , therefore , the activation domain of <PROT1_155871>  Tat </PROT1_155871> makes direct contact with the protein kinases <PROT2_2968>  TFIIH </PROT2_2968> , P - TEFb , TAFII250 and TIP30 and the protein phosphatase FCP1 ( CITATION , CITATION , CITATION , CITATION , CITATION and TARGET_CITATION ) . ||8628270_155871_902==>DRB may exert its effect by direct inhibition of a CTD kinase activity , and two DRB - sensitive cellular kinases , the <PROT2_902>  CAK </PROT2_902> component of TFIIH ( CITATION , TARGET_CITATION ) and <PROT1_155871>  Tat </PROT1_155871> - associated kinase ( TAK ) ( CITATION ) , have been found to interact with the activation domain of <PROT1_155871>  Tat </PROT1_155871> . ||8830660_155030_5478==>Previous studies have shown that <PROT2_5478>  CyPA </PROT2_5478> binds to the <PROT1_155030>  capsid </PROT1_155030> domain of the <PROT1_155030>  gag </PROT1_155030> polyprotein ( and subsequently to the <PROT1_155030>  capsid </PROT1_155030> protein ) through a proline - rich region in the <PROT1_155030>  capsid </PROT1_155030> domain , Pro ( Xxx ) 4 Pro222 ( Xxx ) 2 Pro ( Xxx ) 5Pro ( fig. 1B ) ( CITATION , CITATION , CITATION , CITATION , TARGET_CITATION ) . ||8830660_155030_5478==>In this regard , an interaction between HIV - 1 <PROT1_155030>  Gag </PROT1_155030> and cyclophilin has been detected by using several approaches , including a yeast - based , two - hybrid system ( CITATION - TARGET_CITATION ) ; the incorporation of cyclophilin A ( <PROT2_5478>  CyPA </PROT2_5478> ) has also been demonstrated in the virus particles ( CITATION ) . ||8830660_155030_5478==>The <PROT2_5478>  CypA </PROT2_5478> binding site on HIV - 1 <PROT1_155030>  capsid </PROT1_155030> is localized to a proline - rich flexible exposed loop that lies in the amino terminal domain ( CITATION , CITATION , TARGET_CITATION , CITATION and CITATION ) and includes residues 85 & # x2013 ; 93 ( TARGET_CITATION , CITATION and CITATION ) , which encompasses the critical residue Pro90 ( CITATION , TARGET_CITATION , CITATION and CITATION ) . ||8830660_155030_5478==>Since cyclophilin A binds to Pro90 of <PROT1_155030>  capsid </PROT1_155030> ( TARGET_CITATION and CITATION ) in the immature state , it is possible that the cis - trans proline isomerase activity of <PROT2_5478>  CypA </PROT2_5478> is important in promoting maturation of the <PROT1_155030>  capsid </PROT1_155030> domain of <PROT1_155030>  Gag </PROT1_155030> . ||9121429_155871_6667==>The mammalian complexes contain a variety of different proteins in addition to Srb homologs and general transcription factors ( GTFs ) ( CITATION , CITATION , CITATION ) , such as proteins involved in recombination and DNA double - strand break repair ( CITATION ) , transcription - coupled repair ( CITATION ) , and RNA processing ( such as polyadenylation and splicing factors ) ( CITATION ) ; elongation factors ( CITATION ) ; coactivators mediating the response to Gal4 - VP16 ( CITATION , CITATION ) , Gal4 - <PROT2_6667>  SP1 </PROT2_6667> ( CITATION ) , and the human immunodeficiency virus type 1 transactivator <PROT1_155871>  Tat </PROT1_155871> ( TARGET_CITATION , CITATION ) ; coactivators that contain histone acetyltransferase activity such as CBP ( CITATION ) ; the breast cancer tumor suppressor gene product BRCA1 ( CITATION ) ; and RNA helicase A ( RHA ) ( CITATION ) . ||9121429_155871_6667==>These functional requirements correlate with physical interactions of <PROT1_155871>  Tat </PROT1_155871> with <PROT2_6667>  Sp1 </PROT2_6667> , TBP , and RNA polymerase II ( CITATION , TARGET_CITATION ) . ||9121429_155871_6667==>For optimal transactivation of HIV gene expression , <PROT1_155871>  Tat </PROT1_155871> requires specific upstream transcription factors , including <PROT2_6667>  Sp1 </PROT2_6667> ( CITATION ) , TATA binding protein ( CITATION , CITATION ) , <PROT1_155871>  Tat </PROT1_155871> - associated kinase ( TAK ) ( CITATION , CITATION ) , TFIIH ( CITATION , CITATION ) , Tip ( CITATION ) , and RNA polymerase II ( TARGET_CITATION , CITATION , CITATION ) . ||9121429_155871_6667==>These proteins include TATA - binding protein ( TBP ) ( CITATION , CITATION ) , TAK ( CITATION , CITATION ) , PKR ( CITATION , CITATION ) , T3R ( CITATION ) , <PROT1_155871>  Tat </PROT1_155871> - binding protein 1 ( CITATION , CITATION ) , TAP ( CITATION , CITATION , CITATION ) , TBP - associated factor TAF55 ( CITATION ) , HT2A ( CITATION ) , Tip60 ( CITATION ) , TFIIH ( CITATION , CITATION ) , RNA polymerase II ( CITATION ) , and <PROT2_6667>  Sp1 </PROT2_6667> ( TARGET_CITATION , CITATION ) . ||9121429_155871_6667==>We and others have previously reported on a role for <PROT2_6667>  Sp1 </PROT2_6667> in <PROT1_155871>  Tat </PROT1_155871> - transactivated expression of the HIV - 1 promoter ( TARGET_CITATION , CITATION ) . ||9121429_155871_6667==>Others have also shown a <PROT1_155871>  Tat </PROT1_155871> - <PROT2_6667>  Sp1 </PROT2_6667> association in transcription complexes ( TARGET_CITATION ) . ||9121429_155871_6667==>We and others have previously demonstrated that in the HIV - 1 system , <PROT2_6667>  Sp1 </PROT2_6667> and <PROT1_155871>  Tat </PROT1_155871> can form a protein - protein complex ( TARGET_CITATION , CITATION ) . 
co-localizes with====10708443_155030_966==>Blots 3 and 4 show that the lipid raft - associated proteins Fyn and <PROT2_966>  CD59 </PROT2_966> ( CITATION , TARGET_CITATION ) , along with gp160 ( blot 5 ) , remain associated with the <PROT1_155030>  Gag </PROT1_155030> / GagPol complex immunoprecipitated from the P100 fraction with anti - IN , i.e. , they band at the same bouyant densities . ||11172097_155459_7431==>While the nature of the proposed cellular inhibitor remains elusive , a number of cellular <PROT1_155459>  Vif </PROT1_155459> - interacting proteins have been identified to date , including <PROT2_7431>  vimentin </PROT2_7431> or a <PROT2_7431>  vimentin </PROT2_7431> - associated factor ( TARGET_CITATION , CITATION ) , HP68 ( CITATION ) , or Hck ( CITATION ) . ||11172097_155459_7431==>However CITATION were unable to detect co - localisation of <PROT1_155459>  Vif </PROT1_155459> and <PROT2_7431>  vimentin </PROT2_7431> in non - permissive cells and further studies with <PROT1_155459>  Vif </PROT1_155459> expression in Cos - 7 cells shows a dramatic effect on <PROT2_7431>  vimentin </PROT2_7431> localisation although <PROT1_155459>  Vif </PROT1_155459> and <PROT2_7431>  vimentin </PROT2_7431> do not co - localise ( TARGET_CITATION ) . ||11172097_155459_7431==>However , it is possible that after synthesis <PROT1_155459>  Vif </PROT1_155459> gradually associates with cellular factors such as CEM15 ( CITATION ) or <PROT2_7431>  vimentin </PROT2_7431> ( TARGET_CITATION , CITATION ) , thereby preventing packaging of <PROT1_155459>  Vif </PROT1_155459> into virus particles . 
competes with====10964507_155807_3308==>This capacity of the HeLa cytosol to support nuclear import of the <PROT1_155807>  Vpr </PROT1_155807> - deficient HIV - 1 PICs was shown to depend on the presence of heat - shock protein 70 ( <PROT2_3308>  Hsp70 </PROT2_3308> ) TARGET_CITATION , a member of the large family of heat - shock proteins . ||10964507_155807_3308==>Given that <PROT2_3308>  Hsp70 </PROT2_3308> stimulates cellular nuclear import in general CITATION , and nuclear import of the HIV - 1 preintegration complex in vitro TARGET_CITATION , it was reasonable to hypothesize that <PROT2_3308>  Hsp70 </PROT2_3308> induced by HIV - 1 infection may assist <PROT1_155807>  Vpr </PROT1_155807> in facilitating HIV - 1 replication in macrophages . ||10964507_155807_3308==>The anti - HIV activity of <PROT2_3308>  Hsp70 </PROT2_3308> reported here seems at odds with our previous observation that <PROT2_3308>  Hsp70 </PROT2_3308> can replace <PROT1_155807>  Vpr </PROT1_155807> in HIV - 1 nuclear import ( TARGET_CITATION ) and is therefore expected to stimulate HIV - 1 nuclear translocation and replication . ||10964507_155807_3308==>Earlier studies showed that <PROT2_3308>  HSP70 </PROT2_3308> competes with <PROT1_155807>  Vpr </PROT1_155807> for its binding to importin { alpha } ( TARGET_CITATION ) and that overexpression of <PROT2_3308>  HSP70 </PROT2_3308> displaces <PROT1_155807>  Vpr </PROT1_155807> from the nuclear membrane ( unpublished data ) . ||11027346_155871_6387==>Finally , the extracellular release of the HIV protein <PROT1_155871>  Tat </PROT1_155871> can exert <PROT2_6387>  CXCL12 </PROT2_6387> - like inhibitory activity on HIV - 1 infection without affecting or even enhancing the replication of R5 viruses ( CITATION , TARGET_CITATION ) . ||9111043_155908_5902==>As <PROT2_5902>  RanBP1 </PROT2_5902> also contains a functional <PROT1_155908>  Rev </PROT1_155908> - like NES ( CITATION ; TARGET_CITATION ) and is exported when injected into nuclei of Xenopus oocytes ( not shown ) we had to rule out that the inhibition of U snRNA export was simply due to saturation of the NES export pathway . ||9111043_155908_5902==>Additional evidence for independent export pathways used by the CTE and RRE / <PROT1_155908>  Rev </PROT1_155908> systems has been presented by TARGET_CITATION , who observed that overexpression of Ran - binding protein 1 ( <PROT2_5902>  RanBP1 </PROT2_5902> ) , both with and without its NES ( CITATION ) , interfered with <PROT1_155908>  Rev </PROT1_155908> but not CTE function . ||9111043_155908_5902==>To further support the specificity of the effect of Nup98 - NP , we also studied the export of a GFP hybrid containing the NES of <PROT1_155908>  Rev </PROT1_155908> as well as the export of <PROT2_5902>  RanBP1 </PROT2_5902> , a protein we previously showed to contain a <PROT1_155908>  Rev </PROT1_155908> - like NES which determines its cytoplasmic accumulation ( TARGET_CITATION ) . ||9111043_155908_5902==>Zolotukhin and Felber ( TARGET_CITATION ) found that the ectopic expression of the <PROT1_155908>  Rev </PROT1_155908> protein ( which contains an NES ) can lead to the nuclear accumulation of <PROT2_5902>  RanBP1 </PROT2_5902> , presumably by competing for Crm1 . ||9111043_155908_5902==>This mutant <PROT2_5902>  RanBP1 </PROT2_5902> also blocks the export of <PROT1_155908>  Rev </PROT1_155908> ( CITATION , TARGET_CITATION ) . ||9111043_155908_5902==>Because <PROT2_5902>  RanBP1 </PROT2_5902> is an accessory factor for the hydrolysis of RanGTP to RanGDP , this result is evidence that the PTB NES , like the leucine - rich NES of <PROT1_155908>  Rev </PROT1_155908> , depends on high levels of RanGTP in the nucleus for export function and hence probably utilizes a transportin family member as export receptor ( TARGET_CITATION ) . ||9342064_155871_2247==>During acute infection of T cells by HIV - 1 , <PROT1_155871>  Tat </PROT1_155871> is released from the cells in an active form ( CITATION , CITATION , TARGET_CITATION ) and via a leaderless secretory pathway that is specific and resembles that of IL - 1 , <PROT2_2247>  bFGF </PROT2_2247> and aFGF ( TARGET_CITATION ) . ||9342064_155871_2247==><PROT1_155871>  Tat </PROT1_155871> is a strong heparin - binding factor and its basic sequence competes with <PROT2_2247>  bFGF </PROT2_2247> for binding to heparan sulfate proteoglycans of the cell surface and ECM ( TARGET_CITATION ) . ||9342064_155871_2247==>After their release from the cells , both <PROT1_155871>  Tat </PROT1_155871> and <PROT2_2247>  bFGF </PROT2_2247> bind to cell surface - and ECM - associated HSPG through their basic region , which has a strong binding affinity for heparin TARGET_CITATION , CITATION , CITATION . ||9342064_155871_2247==>The finding that the binding of <PROT1_155871>  Tat </PROT1_155871> to heparin is competed out by <PROT2_2247>  bFGF </PROT2_2247> TARGET_CITATION suggested that <PROT1_155871>  Tat </PROT1_155871> and <PROT2_2247>  bFGF </PROT2_2247> could compete for the same heparin - binding sites . ||9342064_155871_2247==>Similarly to <PROT2_2247>  bFGF </PROT2_2247> , <PROT1_155871>  Tat </PROT1_155871> binds heparin through its basic sequence and can compete with <PROT2_2247>  bFGF </PROT2_2247> for binding to heparin TARGET_CITATION . ||9342064_155871_2247==>In turn , the <PROT1_155871>  Tat </PROT1_155871> basic sequence competes for heparin - binding sites with extracellular <PROT2_2247>  bFGF </PROT2_2247> bound to HPSG , retrieving it into a soluble form ( CITATION and TARGET_CITATION ) . ||9342064_155871_2247==>Sequestered , extracellularly bound <PROT2_2247>  bFGF </PROT2_2247> can be retrieved into a soluble form by heparin or <PROT1_155871>  Tat </PROT1_155871> or the <PROT1_155871>  Tat </PROT1_155871> basic peptide ( CITATION , CITATION , TARGET_CITATION , CITATION , CITATION ) . ||9342064_155871_2247==>It is known that up to 30 % of the synthesized <PROT2_2247>  bFGF </PROT2_2247> binds to cell - surface and extracellular matrix - associated HSPGs TARGET_CITATION , CITATION . The basic region of <PROT1_155871>  Tat </PROT1_155871> can displace preformed HSPG - bound <PROT2_2247>  bFGF </PROT2_2247> by competing for heparin - binding sites TARGET_CITATION . Indeed , an augmented concentration of <PROT2_2247>  bFGF </PROT2_2247> in the culture supernatants ( 33 to 39 ng / ml ) was observed when podocytes were treated with 30 & # 181 ; g / ml of heparin , in the absence of an enhanced mRNA transcription monitored by reverse transcriptase - PCR . 
complexes with====10454543_155871_904==>However , <PROT2_904>  CYCT1 </PROT2_904> - C only partially removed SPT5 , an elongation factor reported to bind <PROT1_155871>  TAT </PROT1_155871> - SF1 ( ref. TARGET_CITATION ) , and it removed very little Pol II or the TFIIF subunit RAP30 . ||10921877_155807_5524==>It has been recently suggested that <PROT1_155807>  Vpr </PROT1_155807> interaction with protein phosphatase <PROT2_5524>  PP2A </PROT2_5524> contributes to cdc2 hyperphosphorylation ( TARGET_CITATION ) . ||10921877_155807_5524==>Recently , e. Cohen and co - workers ( TARGET_CITATION ) demonstrated that the human immunodeficiency virus <PROT1_155807>  Vpr </PROT1_155807> protein associates with <PROT2_5524>  PP2A </PROT2_5524> via the B subunit ( TARGET_CITATION ) . ||10921877_155807_5524==>The inactivation of cdc25 was recently revealed to be mediated by inhibition of nuclear import of serine / threonine protein phosphatase 2A ( <PROT2_5524>  PP2A </PROT2_5524> ) , known as an activator for cdc25 , through a direct interaction between <PROT1_155807>  Vpr </PROT1_155807> and <PROT2_5524>  PP2A </PROT2_5524> ( TARGET_CITATION ) . ||10921877_155807_5524==>Recently , Hrimech et al. reported that <PROT1_155807>  Vpr </PROT1_155807> mediates G2 arrest by forming a complex with protein phosphatase 2A ( <PROT2_5524>  PP2A </PROT2_5524> ) , an upstream regulator of cdc25 , and enhances the nuclear import of <PROT2_5524>  PP2A </PROT2_5524> TARGET_CITATION . ||10921877_155807_5524==>HIV <PROT1_155807>  Vpr </PROT1_155807> is reported to inactivate cdc25C by physically targeting <PROT2_5524>  PP2A </PROT2_5524> to the nucleus and enhancing the recruitment and dephosphorylation of cdc25C though association with the <PROT2_5524>  PP2A </PROT2_5524> - B55 regulatory subunit ( TARGET_CITATION ) . ||10921877_155807_5524==>More recent publications suggest that the inhibition of Cdc2 activity by the HIV <PROT1_155807>  Vpr </PROT1_155807> protein is due to its direct physical interaction with <PROT2_5524>  PP2A </PROT2_5524> ( TARGET_CITATION , CITATION ) . ||10921877_155807_5524==>Indeed , many of the virus - encoded proteins have been shown to interact with cellular proteins , e.g. , Gag and Tsg101 ( CITATION , CITATION , CITATION , CITATION ) , Gag and cyclophilins ( CITATION ) , integrase and Ini1 ( CITATION ) , <PROT1_155807>  Vpr </PROT1_155807> and <PROT2_5524>  PP2A </PROT2_5524> ( TARGET_CITATION ) , <PROT1_155807>  Vpr </PROT1_155807> and HHR23A ( CITATION ) , <PROT1_155807>  Vpr </PROT1_155807> and UNG ( CITATION ) , Vif and antiviral proteins ( CITATION , CITATION ) , Vif and HP68 ( CITATION ) , and Tat and cyclin T1 ( CITATION , CITATION ) . ||10921877_155807_5524==><PROT1_155807>  Vpr </PROT1_155807> binds to the B55 - type regulatory subunit of the <PROT2_5524>  PP2A </PROT2_5524> phosphatase , activates <PROT2_5524>  PP2A </PROT2_5524> and directs it toward nuclear cdc25A ( TARGET_CITATION ) . ||10921877_155807_5524==>It has also recently been shown that the G2 cell cycle arrest induced by <PROT1_155807>  Vpr </PROT1_155807> that is encoded by the human immunodeficiency virus type 1 , was due to the interaction of this protein with <PROT2_5524>  PP2A </PROT2_5524> via the subunit B55small alpha , Greek , leading to the inhibition of Cdc25C ( TARGET_CITATION ) . ||11704662_1025_155871==>The cyclin domain is responsible for binding <PROT1_1025>  CDK9 </PROT1_1025> and various other proteins , including NF - { kappa } B ( CITATION ) CIITA ( CITATION ) , and GRIP1 ( TARGET_CITATION ) ; the TRM allows the cooperative binding of <PROT2_155871>  Tat </PROT2_155871> and TAR to cyclin T1 ( CITATION ) ; the His - rich region binds the CTD of RNA polymerase II ( CITATION ) as well as granulin ( CITATION ) ; and the PEST domain is required for the interaction with SCFSKP2 ( CITATION ) . ||11704662_1025_155871==>Since the isolation of cyclin T1 as a <PROT1_1025>  CDK9 </PROT1_1025> partner and <PROT2_155871>  Tat </PROT2_155871> - interacting protein , three other cellular proteins have been identified that bind directly to the cyclin box ( amino acids 1 to 254 ) of cyclin T1 , the major histocompatibility complex class II transactivator CIITA , the NF - { kappa } B RelA / p65 subunit , and the glucocorticoid receptor interacting polypeptide 1 GRIP - 1 ( CITATION , CITATION , TARGET_CITATION ) . ||14564014_155459_8065==>It was recently shown that HIV - 1 <PROT1_155459>  Vif </PROT1_155459> interacts with APOBEC3G and induces rapid degradation of APOBEC3G through the proteasomal pathway by recruiting cellular factors <PROT2_8065>  Cul5 </PROT2_8065> , elongins B and C , and Rbx1 to form an Skp1 - cullin - F - box - like complex ( TARGET_CITATION ) . ||14564014_155459_8065==>Direct evidence for this mechanism of <PROT1_155459>  Vif </PROT1_155459> function was recently described by Yu et al. ( TARGET_CITATION ) , who reported that <PROT1_155459>  Vif </PROT1_155459> associates specifically with Elongins B and C and with <PROT2_8065>  Cul5 </PROT2_8065> and Rbx - 1 , as well as with APOBEC3G ( Figure 2d ) . ||14564014_155459_8065==>Additionally , the E3 ligase components Elongins B and C , <PROT2_8065>  Cul5 </PROT2_8065> and Rbx - 1 , which associate with <PROT1_155459>  Vif </PROT1_155459> and are essential for APOBEC3G degradation ( TARGET_CITATION ) , present multiple points of intervention . ||14564014_155459_8065==>The reduction in cellular expression has been attributed to both inhibition of APOBEC3G translation and its degradation in the cytoplasm by <PROT1_155459>  Vif </PROT1_155459> ( CITATION ) , and recent evidence suggests that <PROT1_155459>  Vif </PROT1_155459> interacts with cytoplasmic APOBEC3G as part of a <PROT1_155459>  Vif </PROT1_155459> - <PROT2_8065>  Cul5 </PROT2_8065> - SCF complex , resulting in the ubiquination of APOBEC3G and its degradation ( TARGET_CITATION ) . ||14564014_155459_8065==>The poly - ubiquitination of APOBEC3G ( CITATION ) requires the interaction of <PROT1_155459>  Vif </PROT1_155459> with the <PROT2_8065>  Cul5 </PROT2_8065> ring finger E3 ligase complex ( TARGET_CITATION ) . ||14564014_155459_8065==>This is accomplished by a UBIQUITIN - LIGASE complex that contains <PROT1_155459>  Vif </PROT1_155459> and several cellular proteins such as elongin B and elongin C , cullin - 5 ( <PROT2_8065>  CUL5 </PROT2_8065> ) and ring - box - 1 ( RBX1 ) TARGET_CITATION ( fig. 3 ) . ||14564014_155459_8065==>The HIV - 1 <PROT1_155459>  Vif </PROT1_155459> protein simultaneously binds to APOBEC3G and to the <PROT2_8065>  Cul5 </PROT2_8065> & # 150 ; elongin B & # 150 ; elongin C & # 150 ; Rbx1 ubiquitin ligase complex and , as a result , APOBEC3G becomes polyubiquitinated and degraded by proteasomes TARGET_CITATION CITATION CITATION CITATION CITATION ( fig. 2 ) . ||14564014_155459_8065==>Additional mechanisms have been proposed for <PROT1_155459>  Vif </PROT1_155459> 's activity CITATION CITATION , but the fact that dominant negative mutants of <PROT2_8065>  Cul5 </PROT2_8065> abolish the effect of <PROT1_155459>  Vif </PROT1_155459> indicates that these contribute in relatively minor ways to <PROT1_155459>  Vif </PROT1_155459> function TARGET_CITATION . ||14564014_155459_8065==>HIV - 1 <PROT1_155459>  Vif </PROT1_155459> interacts with cellular proteins <PROT2_8065>  Cul5 </PROT2_8065> , Elongin B , Elongin C , and Rbx1 to form an E3 ubiquitin ligase complex ( TARGET_CITATION ) , similar to the Elongin C - Cul2 - SOCS box ( ECS ) . ||14564014_155459_8065==>The highly conserved SLQXLA motif in <PROT1_155459>  Vif </PROT1_155459> has some similarities with SOCS box sequences and is required for efficient interaction between HIV - 1 <PROT1_155459>  Vif </PROT1_155459> and the <PROT2_8065>  Cul5 </PROT2_8065> - Elongin B - Elongin C complex ( CITATION , TARGET_CITATION ) . ||14564014_155459_8065==>When <PROT2_8065>  Cul5 </PROT2_8065> complex function was inhibited , HIV - 1 <PROT1_155459>  Vif </PROT1_155459> induced polyubiquitination and degradation of human APOBEC3G was blocked ( TARGET_CITATION ) . ||14564014_155459_8065==>When <PROT1_155459>  Vif </PROT1_155459> function was blocked by the mutant <PROT2_8065>  Cul5 </PROT2_8065> ( pCul5 { Delta } Nedd8 ) as previously described ( TARGET_CITATION ) , a significant level of APOBEC3G was detected in the pNL4 - 3 virions ( fig. 1B , lane 11 ) ; however , a reduced level of APOBEC3G was detected in the pNC2 / 2 mutant virions ( fig. 1B , lane 12 ) compared to the level detected in the pNL4 - 3 virions ( lane 11 ) . ||14564014_155459_8065==>Interference with the function of <PROT2_8065>  Cul5 </PROT2_8065> - containing E3 ubiquitin ligase resulted in the inhibition of <PROT1_155459>  Vif </PROT1_155459> - induced APOBEC3G polyubiquitination and degradation , efficient incorporation of APOBEC3G in the presence of <PROT1_155459>  Vif </PROT1_155459> , and compromised viral infectivity ( TARGET_CITATION ) . ||14564014_155459_8065==>The SLQ mutant <PROT1_155459>  Vif </PROT1_155459> remains competent for interaction with APOBEC3G ( CITATION , TARGET_CITATION ) but has a reduced ability to interact with <PROT2_8065>  Cul5 </PROT2_8065> , Elongin B , and Elongin C ( TARGET_CITATION ) . ||14564014_155459_9978==>It was recently shown that HIV - 1 <PROT1_155459>  Vif </PROT1_155459> interacts with APOBEC3G and induces rapid degradation of APOBEC3G through the proteasomal pathway by recruiting cellular factors Cul5 , elongins B and C , and <PROT2_9978>  Rbx1 </PROT2_9978> to form an Skp1 - cullin - F - box - like complex ( TARGET_CITATION ) . ||14564014_155459_9978==>In addition to the interaction of <PROT1_155459>  Vif </PROT1_155459> with the HECT ubiquitin ligases presently described , <PROT1_155459>  Vif </PROT1_155459> was also reported to interact with a ring finger E3 ubiquitin complex containing Elongin B and C , Cullin 5 , and <PROT2_9978>  Rbx1 </PROT2_9978> ( TARGET_CITATION ) . ||14564014_155459_9978==>Indeed , HIV - 1 encodes for the <PROT1_155459>  Vif </PROT1_155459> protein which recruits a RING - finger E3 ubiquitin complex containing Elongin B and C , Cullin 5 , and <PROT2_9978>  Rbx1 </PROT2_9978> TARGET_CITATION and induces the degradation of APOBEC3G by the ubiquitin proteasome pathway CITATION , CITATION and CITATION . ||14564014_155459_9978==>This is accomplished by a UBIQUITIN - LIGASE complex that contains <PROT1_155459>  Vif </PROT1_155459> and several cellular proteins such as elongin B and elongin C , cullin - 5 ( CUL5 ) and ring - box - 1 ( <PROT2_9978>  RBX1 </PROT2_9978> ) TARGET_CITATION ( fig. 3 ) . ||14564014_155459_9978==>The HIV - 1 <PROT1_155459>  Vif </PROT1_155459> protein simultaneously binds to APOBEC3G and to the Cul5 & # 150 ; elongin B & # 150 ; elongin C & # 150 ; <PROT2_9978>  Rbx1 </PROT2_9978> ubiquitin ligase complex and , as a result , APOBEC3G becomes polyubiquitinated and degraded by proteasomes TARGET_CITATION CITATION CITATION CITATION CITATION ( fig. 2 ) . ||14564014_155459_9978==>HIV - 1 <PROT1_155459>  Vif </PROT1_155459> interacts with cellular proteins Cul5 , Elongin B , Elongin C , and <PROT2_9978>  Rbx1 </PROT2_9978> to form an E3 ubiquitin ligase complex ( TARGET_CITATION ) , similar to the Elongin C - Cul2 - SOCS box ( ECS ) . ||14564014_155459_9978==>Recent observations showed that HIV - 1 <PROT1_155459>  Vif </PROT1_155459> recruits E3 ubiquitin ligase , consisting of Cullin5 , Elongin B , Elongin C , and <PROT2_9978>  Rbx1 </PROT2_9978> ( TARGET_CITATION ) , to induce polyubiquitination of APOBEC3G ( CITATION , CITATION , TARGET_CITATION ) . ||8849451_1025_155871==>A number of cellular factors have been proposed to interact with the <PROT2_155871>  Tat </PROT2_155871> activation domain and mediate transcription function , including TATA box - binding protein ( CITATION ) , <PROT2_155871>  Tat </PROT2_155871> - SF1 ( TARGET_CITATION ) , TFIIH ( CITATION , CITATION ) , and a cellular protein kinase activity termed <PROT1_1025>  TAK </PROT1_1025> ( <PROT2_155871>  Tat </PROT2_155871> - associated kinase ) ( CITATION , CITATION ) . ||8849451_1025_155871==>Thus , although other <PROT2_155871>  Tat </PROT2_155871> and TAR interacting factors are also thought to contribute to HIV - 1 transcription ( TARGET_CITATION ) , the interaction of <PROT2_155871>  Tat </PROT2_155871> and TAR with <PROT1_1025>  Cdk9 </PROT1_1025> - CycT on stalled RNA polymerase II holoenzyme complexes and the ensuing phosphorylation of the CTD ( fig. 1 ) appear to be the critical determinants of HIV - 1 transcriptional processivity . ||8849451_1025_155871==>Based on biochemical observations , a number of cofactors that could constitute the cellular interface to <PROT2_155871>  Tat </PROT2_155871> function have been proposed , including cellular general transcription factor TFIIH ( CITATION , CITATION ) , <PROT2_155871>  Tat </PROT2_155871> - SF1 ( TARGET_CITATION ) , and <PROT2_155871>  Tat </PROT2_155871> - associated kinase ( <PROT1_1025>  TAK </PROT1_1025> ) ( CITATION , CITATION , CITATION ) . ||8849451_1025_155871==>It is also likely that <PROT2_155871>  Tat </PROT2_155871> binds other factors in addition to <PROT1_1025>  CDK9 </PROT1_1025> / cyclin T ( TARGET_CITATION ) . ||8849451_1025_155871==>In carrying out its transcriptional activation , <PROT2_155871>  Tat </PROT2_155871> interacts with a number of cellular transcription factors , including TFIID ( TBP ) ( CITATION , CITATION , CITATION ) , TFIID - associated TAFs ( CITATION ) , TFIIH ( CITATION ) , <PROT2_155871>  Tat </PROT2_155871> - associated protein ( TAP ) ( CITATION ) , <PROT2_155871>  Tat </PROT2_155871> - associated kinase ( <PROT1_1025>  TAK </PROT1_1025> / pTEFb ) ( CITATION , CITATION , CITATION , CITATION , CITATION , CITATION ) , TTK ( CITATION ) , Sp1 ( CITATION ) , and SF - 1 ( TARGET_CITATION ) , as well as TAR RNA ( CITATION ) . ||8849451_1025_155871==>Modification of the transcription complex by <PROT2_155871>  Tat </PROT2_155871> and <PROT1_1025>  TAK </PROT1_1025> to make it more processive also requires specific cofactors that have been identified using partially reconstituted cell - free transcription systems ( CITATION , CITATION , TARGET_CITATION ) . ||8849451_1025_155871==>The <PROT2_155871>  Tat </PROT2_155871> transcriptional elongation cofactor <PROT2_155871>  TAT </PROT2_155871> - SF1 ( TARGET_CITATION and CITATION ) associates both with cyclin T1 ( CITATION and CITATION ) in human positive transcription elongation factor b ( P - TEFb , formerly known as <PROT2_155871>  Tat </PROT2_155871> - associated kinase ( <PROT1_1025>  TAK </PROT1_1025> ) ) ( CITATION , CITATION , CITATION , CITATION , CITATION and CITATION ) , a protein kinase heterodimer of cyclin - dependent kinase 9 ( <PROT1_1025>  CDK9 </PROT1_1025> ) and its cyclin T1 ( CYCT1 ) subunit ( CITATION , CITATION , CITATION and CITATION ) , and the snRNPs ( CITATION ) essential for mono - splicing of the vpu & # x2013 ; env mRNA . ||8849451_155871_904==>The <PROT1_155871>  Tat </PROT1_155871> transcriptional elongation cofactor <PROT1_155871>  TAT </PROT1_155871> - SF1 ( TARGET_CITATION and CITATION ) associates both with cyclin T1 ( CITATION and CITATION ) in human positive transcription elongation factor b ( P - TEFb , formerly known as <PROT1_155871>  Tat </PROT1_155871> - associated kinase ( TAK ) ) ( CITATION , CITATION , CITATION , CITATION , CITATION and CITATION ) , a protein kinase heterodimer of cyclin - dependent kinase 9 ( CDK9 ) and its cyclin T1 ( <PROT2_904>  CYCT1 </PROT2_904> ) subunit ( CITATION , CITATION , CITATION and CITATION ) , and the snRNPs ( CITATION ) essential for mono - splicing of the vpu & # x2013 ; env mRNA . 
degrades====12954211_155945_920==>More recently , we showed that removal of the casein kinase II sites in <PROT1_155945>  Vpu </PROT1_155945> , which previous studies have shown to be essential to the interaction of <PROT1_155945>  Vpu </PROT1_155945> with <PROT2_920>  CD4 </PROT2_920> ( CITATION ) , contributes to the severe <PROT2_920>  CD4 </PROT2_920> + T cell loss observed following inoculation with SHIVKU - 1bMC33 ( TARGET_CITATION ) . ||7778293_155945_920==>Such complex formation appears to signal the selective ER degradation of <PROT2_920>  CD4 </PROT2_920> or proteins having the <PROT1_155945>  Vpu </PROT1_155945> response elements ( CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , TARGET_CITATION ) . ||7778293_155945_920==><PROT2_920>  CD4 </PROT2_920> { Delta } 32 is a <PROT2_920>  CD4 </PROT2_920> cytoplasmic domain deletion mutant that is expressed at high levels on the cell surface and is not sensitive to <PROT1_155945>  Vpu </PROT1_155945> - mediated <PROT2_920>  CD4 </PROT2_920> degradation ( TARGET_CITATION ) . ||7778293_155945_920==>Mutagenesis studies delineated a domain extending from residues 416 to 418 ( EKKT ) in the <PROT2_920>  CD4 </PROT2_920> cytoplasmic domain required for degradation and <PROT1_155945>  Vpu </PROT1_155945> binding ( CITATION , TARGET_CITATION , CITATION and CITATION ) . ||8230446_155945_920==>Available evidence suggests that <PROT2_920>  CD4 </PROT2_920> degradation is a multistep process that is initiated by a physical interaction with <PROT1_155945>  Vpu </PROT1_155945> ( ( TARGET_CITATION , CITATION , CITATION and CITATION ) ) . ||8230446_155945_920==>Other viral proteins , such as envelope glycoprotein gp120 , Nef , and <PROT1_155945>  Vpu </PROT1_155945> , have been shown to down - regulate <PROT2_920>  CD4 </PROT2_920> expression in primary monocytes / macrophages and lymphocytes ( CITATION , TARGET_CITATION , CITATION ) . ||8230446_155945_920==>Such complex formation appears to signal the selective ER degradation of <PROT2_920>  CD4 </PROT2_920> or proteins having the <PROT1_155945>  Vpu </PROT1_155945> response elements ( CITATION , CITATION , CITATION , CITATION , TARGET_CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , CITATION ) . ||8230446_155945_920==>ER retention of <PROT2_920>  CD4 </PROT2_920> , however , is not an absolute requirement for <PROT1_155945>  Vpu </PROT1_155945> - induced degradation , since <PROT2_920>  CD4 </PROT2_920> is degraded in a <PROT1_155945>  Vpu </PROT1_155945> - specific manner in the absence of deliberate ER retention in transfected HeLa cells expressing <PROT2_920>  CD4 </PROT2_920> and <PROT1_155945>  Vpu </PROT1_155945> ( TARGET_CITATION ) . ||8230446_155945_920==>In contrast , the cytoplasmic domain of <PROT1_155945>  Vpu </PROT1_155945> is necessary and sufficient to mediate <PROT2_920>  CD4 </PROT2_920> degradation in the ER ( CITATION , TARGET_CITATION , CITATION ) . ||8230446_155945_920==><PROT1_155945>  Vpu </PROT1_155945> appears to interact with the cytoplasmic tail of <PROT2_920>  CD4 </PROT2_920> , targeting it to a proteosome where <PROT2_920>  CD4 </PROT2_920> is degraded ( CITATION , CITATION , TARGET_CITATION , CITATION , CITATION , CITATION , CITATION ) . ||8230446_155945_920==><PROT2_920>  CD4 </PROT2_920> binds to the gp120 surface moiety of Env through its extracellular region ( CITATION ) , whereas the <PROT2_920>  CD4 </PROT2_920> cytoplasmic tail , which associates with the protein tyrosine kinase p56lck ( Lck ) , is the target of Nef and <PROT1_155945>  Vpu </PROT1_155945> ( CITATION , CITATION , CITATION , CITATION , CITATION , TARGET_CITATION , CITATION and CITATION ) . ||8230446_155945_920==>The Env protein sequesters <PROT2_920>  CD4 </PROT2_920> in the endoplasmic reticulum by an interaction mediated by the extracellular domains of <PROT2_920>  CD4 </PROT2_920> and Env ( CITATION , CITATION ) , whereas <PROT1_155945>  Vpu </PROT1_155945> induces the degradation of the HIV receptor by a mechanism which requires an interaction between the cytoplasmic tails of <PROT2_920>  CD4 </PROT2_920> and <PROT1_155945>  Vpu </PROT1_155945> ( CITATION , CITATION , CITATION , TARGET_CITATION ) . ||8230446_155945_920==>Mutagenesis studies delineated a domain extending from residues 416 to 418 ( EKKT ) in the <PROT2_920>  CD4 </PROT2_920> cytoplasmic domain required for degradation and <PROT1_155945>  Vpu </PROT1_155945> binding ( CITATION , CITATION , CITATION and TARGET_CITATION ) . ||8551619_155945_920==>In the absence of <PROT1_155945>  Vpu </PROT1_155945> , <PROT2_920>  CD4 </PROT2_920> exhibited a t1 / 2 ~4 h which is consistent with previously reported half - lives of <PROT2_920>  CD4 </PROT2_920> in human cell lines ( CITATION , TARGET_CITATION ) . ||8551619_155945_920==>It was recently demonstrated ( TARGET_CITATION ) that the two previously defined biological functions of <PROT1_155945>  Vpu </PROT1_155945> , <PROT2_920>  CD4 </PROT2_920> degradation and regulation of virus release , are controlled by two separable structural and functional domains . ||8551619_155945_920==>This bimodal kinetics of <PROT2_920>  CD4 </PROT2_920> loss ( fig. 1A and B ) is similar to results previously obtained using HeLa cells transfected with HIV - 1 subgenomic expression vectors ( TARGET_CITATION , CITATION , CITATION , CITATION ) or infected with rVV expressing <PROT2_920>  CD4 </PROT2_920> and <PROT1_155945>  Vpu </PROT1_155945> ( CITATION ) . ||8551619_155945_920==>For the pulse - chase experiments shown in fig. 6 , target molecules ( wild - type <PROT2_920>  CD4 </PROT2_920> and the mutant CD4KRcyto ) and the effector <PROT1_155945>  Vpu </PROT1_155945> were expressed from HIV - 1 subgenomic expression vectors under control of the HIV - 1 long terminal repeat promoter ( fig. 6C ) , the system previously used to study <PROT1_155945>  Vpu </PROT1_155945> - induced <PROT2_920>  CD4 </PROT2_920> degradation ( TARGET_CITATION , CITATION , CITATION , CITATION ) . ||8551619_155945_920==>Mutational analysis indicates that the hydrophilic cytoplasmic domain of <PROT1_155945>  Vpu </PROT1_155945> is required for <PROT1_155945>  Vpu </PROT1_155945> - mediated <PROT2_920>  CD4 </PROT2_920> degradation ( TARGET_CITATION ) . ||8551619_155945_920==>Whereas the cytoplasmic domain of <PROT1_155945>  Vpu </PROT1_155945> is important for the degradation of <PROT2_920>  CD4 </PROT2_920> , the transmembrane domain of <PROT1_155945>  Vpu </PROT1_155945> is sufficient for partial enhancement of virus release ( TARGET_CITATION ) . ||8551619_155945_920==>The transmembrane region of <PROT1_155945>  Vpu </PROT1_155945> appears to be responsible for the increase in virus release , while its cytoplasmic tail is needed for <PROT2_920>  CD4 </PROT2_920> degradation ( TARGET_CITATION , CITATION ) . ||8551619_155945_920==><PROT1_155945>  Vpu </PROT1_155945> also enhances the degradation of <PROT2_920>  CD4 </PROT2_920> ( CITATION ) , an activity associated with interactions between the cytoplasmic domains of <PROT1_155945>  Vpu </PROT1_155945> and <PROT2_920>  CD4 </PROT2_920> ( CITATION - TARGET_CITATION ) . ||8551619_155945_920==>The TM segment of <PROT1_155945>  Vpu </PROT1_155945> is responsible for particle release / secretion ( TARGET_CITATION ) whereas the cytoplasmic site is essential for degradation of <PROT2_920>  CD4 </PROT2_920> protein ( CITATION , TARGET_CITATION and CITATION ) . ||8551619_155945_920==>Viral protein U ( <PROT1_155945>  Vpu </PROT1_155945> ) is a unique gene product of human immunodeficiency virus ( HIV ) type 1 ( HIV - 1 ) with two well - described functions : <PROT2_920>  CD4 </PROT2_920> degradation and enhancement of viral particle release ( TARGET_CITATION , CITATION ) . ||8551619_155945_920==><PROT1_155945>  Vpu </PROT1_155945> functions described in the literature include both degradation of newly synthesized <PROT2_920>  CD4 </PROT2_920> and enhancement of the release of viral particles from the surface of infected cells ( TARGET_CITATION ) . ||8551619_155945_920==>Studies of the biological function of <PROT1_155945>  Vpu </PROT1_155945> have demonstrated that it is involved in two different activities , namely the enhancement of the release of virus from the infected cell surface ( Schubert et al. , 1996 TARGET_CITATION ) and the triggering of the degradation of the <PROT2_920>  CD4 </PROT2_920> molecule in the endoplasmic reticulum ( ER ) ( Willey et al. , 1992 CITATION ; Schubert and Strebel , 1994 CITATION ) . ||8551619_155945_920==>The cytoplasmic domain of <PROT1_155945>  Vpu </PROT1_155945> , which has two highly conserved phosphorylation sites ( Ser52 and Ser56 ) , is essential for interactions with <PROT2_920>  CD4 </PROT2_920> and induction of <PROT2_920>  CD4 </PROT2_920> degradation in the ER ( Bour et al. 1995 ; TARGET_CITATION ) . ||8551619_155945_920==>It is known that the cytoplasmic part of <PROT1_155945>  Vpu </PROT1_155945> is responsible for direct interaction with <PROT2_920>  CD4 </PROT2_920> and for triggering <PROT2_920>  CD4 </PROT2_920> degradation ( TARGET_CITATION ) . ||8551619_155945_920==>In addition to <PROT2_920>  CD4 </PROT2_920> down - regulation from the surface , <PROT1_155945>  Vpu </PROT1_155945> has also been shown to facilitate virion release from infected cells and this property of <PROT1_155945>  Vpu </PROT1_155945> has been associated with the transmembrane domain which has been reported to have an ion channel activity ( CITATION , TARGET_CITATION and CITATION ) . ||8551619_155945_920==>There is strong evidence that the two principal biological activities of <PROT1_155945>  Vpu </PROT1_155945> are associated with different portions of the protein molecule. ( CITATION and TARGET_CITATION ) The degradation of the <PROT2_920>  CD4 </PROT2_920> / gp160 complexes appears to be affected by the cytoplasmic domain in the C - terminal half of the protein , which consists of two amphipathic helices and the phosphorylation sites . ||8551619_155945_920==>This function involves two distinct effects : <PROT1_155945>  Vpu </PROT1_155945> initiates the degradation of <PROT2_920>  CD4 </PROT2_920> molecules in the endoplasmic reticulum via its cytoplasmic domain ( CITATION , CITATION and TARGET_CITATION ) . ||8551619_155945_920==>In addition , while the determinants for <PROT2_920>  CD4 </PROT2_920> degradation are all contained in the cytoplasmic domain of <PROT1_155945>  Vpu </PROT1_155945> , the transmembrane domain has been shown to play an essential role for the particle release activity ( TARGET_CITATION and CITATION ) . ||8551619_155945_920==>The degradation of <PROT2_920>  CD4 </PROT2_920> involves the cytoplasmic domain of <PROT1_155945>  Vpu </PROT1_155945> , and this <PROT1_155945>  Vpu </PROT1_155945> function is dependent on the phosphorylation of two conserved serine residues ( CITATION , CITATION , CITATION ) ; in contrast , the involvement of <PROT1_155945>  Vpu </PROT1_155945> in virion release depends on the TM domain ( TARGET_CITATION ) . ||8551619_155945_920==><PROT1_155945>  Vpu </PROT1_155945> has two different activities , namely the enhancement of the release of virus from the infected cell surface ( Schubert et al. , 1996 TARGET_CITATION ) and the degradation of the <PROT2_920>  CD4 </PROT2_920> molecule in the endoplasmic reticulum ( ER ) ( Willey et al. , 1992 CITATION ; Schubert and Strebel , 1994 CITATION ) . ||8659106_155945_920==>These phosphorylations are required for the ability of <PROT1_155945>  Vpu </PROT1_155945> to accelerate the decay of <PROT2_920>  CD4 </PROT2_920> ( CITATION , TARGET_CITATION ) . ||8659106_155945_920==>Such complex formation appears to signal the selective ER degradation of <PROT2_920>  CD4 </PROT2_920> or proteins having the <PROT1_155945>  Vpu </PROT1_155945> response elements ( CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , TARGET_CITATION , CITATION , CITATION , CITATION , CITATION , CITATION ) . ||8659106_155945_920==>We reported that such modifications of HIV - 1 <PROT1_155945>  Vpu </PROT1_155945> did not alter the <PROT1_155945>  Vpu </PROT1_155945> activity that induced the ER degradation of <PROT2_920>  CD4 </PROT2_920> or proteins bearing the <PROT1_155945>  Vpu </PROT1_155945> - responsive element ( TARGET_CITATION ) . ||8659106_155945_920==>Like <PROT1_155945>  Vpu </PROT1_155945> , the <PROT2_920>  CD4 </PROT2_920> / <PROT1_155945>  Vpu </PROT1_155945> hybrid proteins are phosphorylated when expressed in HeLa cells , and this phosphoryl modification has been shown to be essential to activate a pathway that lead to the proteolysis of <PROT2_920>  CD4 </PROT2_920> in the ER ( CITATION , TARGET_CITATION ) . ||8659106_155945_920==>We have shown that the <PROT2_920>  CD4 </PROT2_920> / <PROT1_155945>  Vpu </PROT1_155945> hybrid proteins CDVpuF and CDVpuC are delivered to the plasma membrane with kinetics similar to that of <PROT2_920>  CD4 </PROT2_920> ( TARGET_CITATION ) . ||8659106_155945_920==>In <PROT2_920>  CD4 </PROT2_920> / <PROT1_155945>  Vpu </PROT1_155945> , the normally N - terminal end of <PROT1_155945>  Vpu </PROT1_155945> has been transposed to the C terminus , which serves only the anchor function in the <PROT2_920>  CD4 </PROT2_920> / <PROT1_155945>  Vpu </PROT1_155945> context ( TARGET_CITATION ) . ||8659106_155945_920==>We have demonstrated that the <PROT1_155945>  Vpu </PROT1_155945> protein in <PROT2_920>  CD4 </PROT2_920> / <PROT1_155945>  Vpu </PROT1_155945> hybrids is biologically active in inducing the degradation of <PROT1_155945>  Vpu </PROT1_155945> - sensitive proteins in the ER , and this activity of <PROT1_155945>  Vpu </PROT1_155945> is strictly dependent on phosphorylation of the <PROT1_155945>  Vpu </PROT1_155945> cytoplasmic domain at Ser52 and Ser56 in both <PROT1_155945>  Vpu </PROT1_155945> and <PROT2_920>  CD4 </PROT2_920> / <PROT1_155945>  Vpu </PROT1_155945> ( CITATION , TARGET_CITATION , CITATION ) . ||8659106_155945_920==>Such modifications have not altered the biological activities of the <PROT1_155945>  Vpu </PROT1_155945> protein in both <PROT2_920>  CD4 </PROT2_920> proteolysis and Gag release ( TARGET_CITATION ) ( fig. 2 ) . ||8659106_155945_920==><PROT2_920>  CD4 </PROT2_920> / <PROT1_155945>  Vpu </PROT1_155945> chimeric proteins bearing the <PROT1_155945>  Vpu </PROT1_155945> transmembrane and cytoplasmic domains fused to the <PROT2_920>  CD4 </PROT2_920> ectodomain ( CD4U and CD4U2 / 6 ) were engineered with a structure similar to those previously described ( CITATION , TARGET_CITATION ) . ||8659106_155945_920==>Several important biological activities of HIV - 1 <PROT1_155945>  Vpu </PROT1_155945> have been demonstrated in vitro , including the abilities to facilitate the release of virions from infected cells ( CITATION , CITATION , CITATION , CITATION , CITATION ) , induce <PROT2_920>  CD4 </PROT2_920> degradation in the endoplasmic reticulum ( CITATION , TARGET_CITATION , CITATION , CITATION ) , and regulate transport of the viral Env protein from the endoplasmic reticulum to the Golgi apparatus ( CITATION ) . ||8709227_155945_920==>Because <PROT1_155945>  vpu </PROT1_155945> and nef genes can down - modulate <PROT2_920>  CD4 </PROT2_920> expression ( TARGET_CITATION ) , their absence from the HXBII - derived pHIV - gpt provirus ( CITATION ) was presumably a positive factor in enhancing syncytium formation in step 3. It is noteworthy that many of the syncytia contained giant nuclei or nuclei of widely divergent sizes . ||8709227_155945_920==>However , this possibility seems unlikely in light of the fact that HIV makes a significant effort to down - modulate <PROT2_920>  CD4 </PROT2_920> from the surface of infected cells and involves the activities of no fewer than three HIV - 1 gene products , i.e. , Nef , Env , and <PROT1_155945>  Vpu </PROT1_155945> ( TARGET_CITATION ; for reviews , see references CITATION , CITATION , and CITATION ) . ||8709227_155945_920==>The viral proteins <PROT1_155945>  Vpu </PROT1_155945> ( CITATION , CITATION ) , Env ( CITATION ) , and Nef ( CITATION , CITATION ) have all been shown to be independently capable of downmodulating <PROT2_920>  CD4 </PROT2_920> , with Nef active in the early phase of virus infection and <PROT1_155945>  Vpu </PROT1_155945> ( expressed only in HIV - 1 ) ( CITATION ) and Env acting in the later stages of infection ( TARGET_CITATION ) . ||8709227_155945_920==>Productive infection leads to high cellular levels of viral proteins , and the peptide products of nef , env , and <PROT1_155945>  vpu </PROT1_155945> have a requisite and cooperative effect on <PROT2_920>  CD4 </PROT2_920> down - regulation ( TARGET_CITATION ) . ||8709227_155945_920==>Although mechanistically distinct , the effects of Nef , Env and <PROT1_155945>  Vpu </PROT1_155945> are additive , ensuring the almost complete elimination of <PROT2_920>  CD4 </PROT2_920> from the surface of cells productively infected with HIV - 1 ( CITATION and TARGET_CITATION ) . ||8709227_155945_920==>However , although this effect is achieved by most viruses through simple trapping of Env - receptor complexes in the ER , HIV - 1 engages two additional proteins besides Env in <PROT2_920>  CD4 </PROT2_920> down - modulation : Nef and <PROT1_155945>  Vpu </PROT1_155945> ( Chen et al. 1996 TARGET_CITATION ) . ||8709227_155945_920==>Among the HIV - 1 genes <PROT1_155945>  vpu </PROT1_155945> , env , and nef , that have been implicated in down - regulating the levels of cell surface <PROT2_920>  CD4 </PROT2_920> on infected cells , a stronger dependence on Nef function for the reduction of cell surface <PROT2_920>  CD4 </PROT2_920> on primary T lymphocytes has been previously described ( TARGET_CITATION ) . ||8709227_155945_920==><PROT2_920>  CD4 </PROT2_920> down - regulation in these cells is due to the synthesis of two late HIV genes , <PROT1_155945>  Vpu </PROT1_155945> and Env ( TARGET_CITATION ) . ||8709227_155945_920==>The importance of the Nef - dependent <PROT2_920>  CD4 </PROT2_920> downregulation is further emphasized by the fact that in infected cells the concerted , but mechanistically distinct , action of two other HIV genes , env and <PROT1_155945>  vpu </PROT1_155945> , is required to ensure the complete elimination of <PROT2_920>  CD4 </PROT2_920> from the cell surface ( TARGET_CITATION ) . ||8709227_155945_920==>Removal of <PROT2_920>  CD4 </PROT2_920> from the surface of infected cells is achieved by the Nef , <PROT1_155945>  Vpu </PROT1_155945> , and Env products ( CITATION - TARGET_CITATION ) . ||8709227_155945_920==>For instance , the HIV - 1 Nef , Env , and <PROT1_155945>  Vpu </PROT1_155945> proteins are engaged in down - regulating the expression of the surface <PROT2_920>  CD4 </PROT2_920> molecule ( CITATION , TARGET_CITATION ) . ||8709227_155945_920==>Down - modulation of cell surface <PROT2_920>  CD4 </PROT2_920> following HIV - 1 infection is a consequence of coexpression of <PROT2_920>  CD4 </PROT2_920> and the viral <PROT1_155945>  Vpu </PROT1_155945> , Nef , and gp120 envelope proteins within the infected cells ( TARGET_CITATION ) . ||8709227_155945_920==>In vitro evidence indicates that the HIV gene products nef , <PROT1_155945>  vpu </PROT1_155945> , and env are capable of down - regulating expression of <PROT2_920>  CD4 </PROT2_920> ( CITATION , CITATION , TARGET_CITATION ) and that nef can also induce down - regulation of major histocompatibility complex I ( MHC - I ) ( CITATION , CITATION ) and up - regulation of FasL ( CITATION ) from the cellular surface . ||8709227_155945_920==>Nef - independent <PROT2_920>  CD4 </PROT2_920> downregulation was presumably mediated by Env and <PROT1_155945>  Vpu </PROT1_155945> , which are expressed at later times than Nef following HIV - 1 integration and prevent newly synthesized <PROT2_920>  CD4 </PROT2_920> from reaching the plasma membrane ( CITATION , TARGET_CITATION , CITATION , CITATION , CITATION , CITATION ) . ||8709227_155945_920==><PROT1_155945>  Vpu </PROT1_155945> - and Nef - induced <PROT2_920>  CD4 </PROT2_920> Down - modulation Activities Are Required for Optimal HIV - 1 Infectivity & # 151 ; Both <PROT1_155945>  Vpu </PROT1_155945> and Nef down - modulate surface expression of the <PROT2_920>  CD4 </PROT2_920> receptor during HIV - 1 infection ( TARGET_CITATION ) . ||8709227_155945_920==>In HIV - 1 infections , many reports indicate that the viral regulatory gene products <PROT1_155945>  Vpu </PROT1_155945> and Nef also participate in <PROT2_920>  CD4 </PROT2_920> downregulation , in addition to Env ( TARGET_CITATION , CITATION and CITATION ) . ||8709227_155945_920==>Multiple gene products of HIV - 1 , <PROT1_155945>  Vpu </PROT1_155945> , Nef , and Env , act cooperatively in downregulation of <PROT2_920>  CD4 </PROT2_920> , the major receptor for HIV - 1 infection ( TARGET_CITATION ) . ||8709227_155945_920==>Both <PROT1_155945>  Vpu </PROT1_155945> and Nef are known to function in modulation of <PROT2_920>  CD4 </PROT2_920> expression ( TARGET_CITATION ) , suggesting that the phenotype observed with & # x394 ; <PROT1_155945>  vpu </PROT1_155945> / & # x394 ; nef mutant was a consequence of downregulation of <PROT2_920>  CD4 </PROT2_920> expression . ||8709227_155945_920==>Nef , in contrast , is expressed immediately following infection and stimulates the internalization and degradation of <PROT2_920>  CD4 </PROT2_920> molecules already present on the surface of the infected cell prior to Env and <PROT1_155945>  Vpu </PROT1_155945> expression ( CITATION , TARGET_CITATION , CITATION ) . ||8709227_155945_920==>Previous findings have shown that Env can independently down - modulate full - length <PROT2_920>  CD4 </PROT2_920> in the absence of Nef and <PROT1_155945>  Vpu </PROT1_155945> ( TARGET_CITATION ) . 
downregulates====11024150_155807_920==>Recently , it was reported that expression of HIV - 1 <PROT1_155807>  Vpr </PROT1_155807> in a lymphoma T - cell line induces the downmodulation of the <PROT2_920>  CD4 </PROT2_920> receptor and impairs entry of HIV particles into cells ( TARGET_CITATION ) . ||11024150_155807_920==>Our studies included the NL4.3 HIV - 1 <PROT1_155807>  Vpr </PROT1_155807> allele , previously reported to downmodulate surface <PROT2_920>  CD4 </PROT2_920> ( TARGET_CITATION ) , and also the Jurkat E6 T - cell line , used in previous studies . ||11156964_1385_155871==>Finally , addition of extracellular HIV - 1 <PROT2_155871>  Tat </PROT2_155871> protein to PC12 cells elicits a chronic down - regulation of <PROT1_1385>  CREB </PROT1_1385> content and phosphorylation , and a progressive increase in apoptosis ( TARGET_CITATION ) . ||11254713_155871_5966==>Interestingly , the CPSF3 proximal promoter contains several consensus sites for transcription factors such as CREB and c - <PROT2_5966>  Rel </PROT2_5966> , which have been demonstrated to interact physically or functionally with the HIV <PROT1_155871>  Tat </PROT1_155871> protein ( CITATION , TARGET_CITATION ) . ||12746459_155945_920==>It was shown recently that <PROT1_155945>  Vpu </PROT1_155945> exerts a positive effect on HIV - 1 infectivity by down - modulating <PROT2_920>  CD4 </PROT2_920> receptor molecules at the surface of HIV - 1 - producing cells ( TARGET_CITATION ) . ||12746459_155945_920==><PROT1_155945>  Vpu </PROT1_155945> promotes virion release ( CITATION , CITATION ) by counteracting host restriction factors ( CITATION , CITATION ) , downregulates <PROT2_920>  CD4 </PROT2_920> during the late stages of HIV - 1 infection ( TARGET_CITATION , CITATION ) , and inhibits NF - { kappa } B activation ( CITATION ) . ||12746459_155945_920==><PROT1_155945>  Vpu </PROT1_155945> might enhance viral spread by both the same and different mechanisms because it downmodulates <PROT2_920>  CD4 </PROT2_920> ( TARGET_CITATION , CITATION ) but it also increases the release of virus particles ( CITATION , CITATION ) . ||2180064_155871_5610==>Finally , the stable expression of <PROT1_155871>  Tat </PROT1_155871> in HeLa cells treated with interferon is associated with reduced levels of <PROT2_5610>  PKR </PROT2_5610> ( TARGET_CITATION ) . ||2180064_155871_5610==>Finally , <PROT1_155871>  Tat </PROT1_155871> appears to down - regulate <PROT2_5610>  PKR </PROT2_5610> in cells infected with HIV - 1 or stably expressing <PROT1_155871>  Tat </PROT1_155871> ( TARGET_CITATION ) . ||2180064_155871_5610==>Moreover , the ability of <PROT1_155871>  Tat </PROT1_155871> to form a complex with <PROT2_5610>  PKR </PROT2_5610> provides a possible mechanism for the repression of <PROT2_5610>  PKR </PROT2_5610> levels that have been observed in HeLa cells stably expressing <PROT1_155871>  Tat </PROT1_155871> or in T - cells infected with HIV - 1 ( TARGET_CITATION ) . ||2180064_155871_5610==>Also controversial are the findings that the levels and the activity of <PROT2_5610>  PKR </PROT2_5610> are decreased ( TARGET_CITATION ) , increased ( CITATION ) , or unaffected ( CITATION ) in HIV - 1 - infected and in <PROT1_155871>  Tat </PROT1_155871> - expressing cells . ||2180064_155871_5610==>Similar to scenarios seen with HCV infection , the HIV - 1 <PROT1_155871>  Tat </PROT1_155871> protein has demonstrated anti - IFN activity by inhibiting <PROT2_5610>  PKR </PROT2_5610> activity directly ( TARGET_CITATION , CITATION ) . ||2180064_155871_5610==>HIV - 1 <PROT1_155871>  tat </PROT1_155871> can inhibit <PROT2_5610>  PKR </PROT2_5610> , an antiviral effector molecule that , like Mx , is induced by IFN - { alpha } / & # 223 ; ( TARGET_CITATION ) . 
enhances====10400814_155871_5610==>Both functions are restored by cotransfection of <PROT1_155871>  Tat </PROT1_155871> with the cDNA for <PROT2_5610>  PKR </PROT2_5610> ( TARGET_CITATION ) . ||10400814_155871_5610==>HIV <PROT1_155871>  Tat </PROT1_155871> protein also activates transcription factor NF - kappa B through a process that requires the function of <PROT2_5610>  PKR </PROT2_5610> ( TARGET_CITATION ) . ||10400814_155871_5610==>The relative roles during the HIV infective process of the negative ( CITATION ) and positive ( TARGET_CITATION ) effects of <PROT1_155871>  Tat </PROT1_155871> protein on <PROT2_5610>  PKR </PROT2_5610> activity are not yet fully resolved . ||10400814_155871_5610==>Moreover , the ability of <PROT1_155871>  Tat </PROT1_155871> to activate NF - kappa B also seems to require <PROT2_5610>  PKR </PROT2_5610> ( TARGET_CITATION ) . ||10400814_155871_5610==>This notion is supported by the observation that activation of NF - kappa B by <PROT1_155871>  Tat </PROT1_155871> and TNF - alpha is impaired in <PROT2_5610>  PKR </PROT2_5610> - deficient cells ( TARGET_CITATION , CITATION ) . ||10400814_155871_5610==>However , its activities are not restricted to this pathway , and HIV - 1 - associated or exogenous - free <PROT1_155871>  Tat </PROT1_155871> also interacts with protein kinase R ( <PROT2_5610>  PKR </PROT2_5610> ) and the transcriptional coactivators p300 and cyclic AMP response element - binding protein ( TARGET_CITATION , CITATION ) to modulate cellular functions , including cell - cycle and apoptotic pathways . ||10400814_155871_5610==><PROT1_155871>  Tat </PROT1_155871> activates NF - { kappa } B through degradation of the inhibitor I { kappa } B { alpha } , which requires the function of the cellular interferon ( IFN ) - inducible protein kinase <PROT2_5610>  PKR </PROT2_5610> ( TARGET_CITATION ) . ||10400814_155871_5610==>The change in mobility of <PROT1_155871>  Tat </PROT1_155871> - GFP compared to that of M8 are likely due to protein modifications as <PROT1_155871>  Tat </PROT1_155871> can be both acetylated ( on K28 , K50 , and K51 ) and phosphorylated ( S62 , T64 , and S68 by <PROT2_5610>  PKR </PROT2_5610> and requires an intact <PROT1_155871>  tat </PROT1_155871> basic domain ) ( CITATION , TARGET_CITATION , CITATION - CITATION ) . ||10400814_155871_5610==><PROT1_155871>  Tat </PROT1_155871> also interacts with the protein kinase <PROT2_5610>  PKR </PROT2_5610> ( ref. TARGET_CITATION ) and the transcriptional coactivators p300 and CREB - binding protein ( CBP ) CITATION . ||11956210_155871_2648==>In addition to histones , HIV - 1 <PROT1_155871>  Tat </PROT1_155871> itself is a substrate for acetylation by p300 / CBP and the associated protein P / CAF ( TARGET_CITATION , CITATION , CITATION and CITATION ) , and by <PROT2_2648>  hGCN5 </PROT2_2648> ( CITATION ) . ||7716549_1017_155871==>Apoptosis appears linked in some cases to aberrations in the activity of cdks : in HeLa cells , induction of apoptosis through a variety of agents is associated with the activation of cyclin A & # x2013 ; associated kinases ( ( CITATION ) ) , and dominant - negative mutants of cdc2 , <PROT1_1017>  cdk2 </PROT1_1017> , and cdk3 suppress apoptosis ( ( CITATION ) ) ; apoptosis in leukemic T cell lines correlates with cdk1 and <PROT1_1017>  cdk2 </PROT1_1017> activation ( ( CITATION ) ) , and pharmacologic inhibition of cdks prevents growth factor deprivation & # x2013 ; induced apoptosis in PC12 cells ( ( CITATION ) ) ; granzyme B & # x2013 ; induced apoptosis requires cyclin A & # x2013 ; cdc2 and cyclin A & # x2013 ; <PROT1_1017>  cdk2 </PROT1_1017> activation ( ( CITATION , CITATION and CITATION ) ) , and apoptosis mediated through the HIV - 1 <PROT2_155871>  Tat </PROT2_155871> protein is associated with enhanced activation of cyclin A & # x2013 ; associated <PROT1_1017>  cdk2 </PROT1_1017> and cdc2 ( ( TARGET_CITATION ) ) . ||7716549_1017_155871==>Additional evidence for a role of <PROT2_155871>  Tat </PROT2_155871> in cell cycle regulation comes from the evidence that <PROT2_155871>  Tat </PROT2_155871> modulates cell cycle G1 phase of glial cells ( CITATION ) and induces apoptosis by interfering with a proper cyclin E - <PROT1_1017>  CDK2 </PROT1_1017> complex ( TARGET_CITATION ) . ||7716549_155871_983==>For example , cyclin A is transcribed in response to two inducers of apoptosis , c - myc and E1A ( CITATION , CITATION , CITATION , CITATION ) . Elevated cyclin A expression from an inducible promoter can induce apoptosis in serum - starved fibroblasts ( CITATION ) , and cyclin A - dependent kinases are specifically activated when apoptosis is induced in T cell lines by human immunodeficiency virus <PROT1_155871>  tat </PROT1_155871> ( TARGET_CITATION ) and in HeLa cells by a variety of physiological and pharmacological agents ( CITATION ) . In target cells killed by granzyme B , <PROT2_983>  CDC2 </PROT2_983> activity was increased ( CITATION ) , including activity associated with cyclin A . ||7716549_155871_983==>Subsequently , it has been shown that <PROT2_983>  Cdc2 </PROT2_983> kinase activity was required for human immunodeficiency virus - 1 <PROT1_155871>  Tat </PROT1_155871> protein - induced apoptosis in T - lymphocytes ( TARGET_CITATION ) . ||7716549_155871_983==>Apoptosis appears linked in some cases to aberrations in the activity of cdks : in HeLa cells , induction of apoptosis through a variety of agents is associated with the activation of cyclin A & # x2013 ; associated kinases ( ( CITATION ) ) , and dominant - negative mutants of <PROT2_983>  cdc2 </PROT2_983> , cdk2 , and cdk3 suppress apoptosis ( ( CITATION ) ) ; apoptosis in leukemic T cell lines correlates with <PROT2_983>  cdk1 </PROT2_983> and cdk2 activation ( ( CITATION ) ) , and pharmacologic inhibition of cdks prevents growth factor deprivation & # x2013 ; induced apoptosis in PC12 cells ( ( CITATION ) ) ; granzyme B & # x2013 ; induced apoptosis requires cyclin A & # x2013 ; <PROT2_983>  cdc2 </PROT2_983> and cyclin A & # x2013 ; cdk2 activation ( ( CITATION , CITATION and CITATION ) ) , and apoptosis mediated through the HIV - 1 <PROT1_155871>  Tat </PROT1_155871> protein is associated with enhanced activation of cyclin A & # x2013 ; associated cdk2 and <PROT2_983>  cdc2 </PROT2_983> ( ( TARGET_CITATION ) ) . ||7999066_155871_4773==>As HIV - 1 <PROT1_155871>  Tat </PROT1_155871> had been shown to play a role in the regulation of numerous cytokine genes cooperating with different cellular factors ( CITATION , CITATION , CITATION , TARGET_CITATION ) , we asked whether it could affect transactivation by <PROT2_4773>  NFAT1 </PROT2_4773> , a transcription factor involved in regulating the expression of a large number of cytokine genes ( CITATION ) . 
inactivates====10921877_155807_983==>It has been recently suggested that <PROT1_155807>  Vpr </PROT1_155807> interaction with protein phosphatase PP2A contributes to <PROT2_983>  cdc2 </PROT2_983> hyperphosphorylation ( TARGET_CITATION ) . ||10921877_155807_983==>It is suggested that HIV <PROT1_155807>  Vpr </PROT1_155807> can inhibit <PROT2_983>  cdc2 </PROT2_983> activity ( CITATION , CITATION ) via activation of wee1 and inactivation of cdc25C ( TARGET_CITATION , CITATION ) . ||10921877_155807_983==>The HIV <PROT1_155807>  Vpr </PROT1_155807> protein induces G2 arrest during infection and when expressed in s. pombe ( CITATION ) and acts by increasing the inhibitory phosphorylation on <PROT2_983>  Cdc2 </PROT2_983> Tyr15 via its interaction with protein phosphatase 2A ( TARGET_CITATION , CITATION ) . ||10921877_155807_983==>It is suggested that the HIV <PROT1_155807>  Vpr </PROT1_155807> protein can inhibit <PROT2_983>  cdc2 </PROT2_983> kinase activity ( CITATION , CITATION ) through mechanisms requiring wee1 and involving the inactivation of cdc25C ( TARGET_CITATION , CITATION ) . ||10921877_155807_983==>More recent publications suggest that the inhibition of <PROT2_983>  Cdc2 </PROT2_983> activity by the HIV <PROT1_155807>  Vpr </PROT1_155807> protein is due to its direct physical interaction with PP2A ( TARGET_CITATION , CITATION ) . ||10921877_155807_995==>In this regard , it is of interest that the <PROT1_155807>  Vpr </PROT1_155807> protein of human immunodeficiency virus arrests cells at G2 / M through activation of the protein phosphatase 2A and consequent inactivation of <PROT2_995>  Cdc25 </PROT2_995> and the cyclin - B & # x2013 ; p34cdc2 complex ( CITATION and TARGET_CITATION ) . ||10921877_155807_995==>It is suggested that HIV <PROT1_155807>  Vpr </PROT1_155807> can inhibit cdc2 activity ( CITATION , CITATION ) via activation of wee1 and inactivation of <PROT2_995>  cdc25C </PROT2_995> ( TARGET_CITATION , CITATION ) . ||10921877_155807_995==><PROT1_155807>  Vpr </PROT1_155807> may inactivate <PROT2_995>  cdc25C </PROT2_995> through physical interaction with protein phosphatase 2A ( TARGET_CITATION ) , which inhibits <PROT2_995>  cdc25C </PROT2_995> activity by dephosphorylation ( CITATION , CITATION , CITATION ) . ||10921877_155807_995==>The inactivation of <PROT2_995>  cdc25 </PROT2_995> was recently revealed to be mediated by inhibition of nuclear import of serine / threonine protein phosphatase 2A ( PP2A ) , known as an activator for <PROT2_995>  cdc25 </PROT2_995> , through a direct interaction between <PROT1_155807>  Vpr </PROT1_155807> and PP2A ( TARGET_CITATION ) . ||10921877_155807_995==>Recently , Hrimech et al. reported that <PROT1_155807>  Vpr </PROT1_155807> mediates G2 arrest by forming a complex with protein phosphatase 2A ( PP2A ) , an upstream regulator of <PROT2_995>  cdc25 </PROT2_995> , and enhances the nuclear import of PP2A TARGET_CITATION . ||10921877_155807_995==>It is suggested that the HIV <PROT1_155807>  Vpr </PROT1_155807> protein can inhibit cdc2 kinase activity ( CITATION , CITATION ) through mechanisms requiring wee1 and involving the inactivation of <PROT2_995>  cdc25C </PROT2_995> ( TARGET_CITATION , CITATION ) . ||10921877_155807_995==>HIV <PROT1_155807>  Vpr </PROT1_155807> is reported to inactivate <PROT2_995>  cdc25C </PROT2_995> by physically targeting PP2A to the nucleus and enhancing the recruitment and dephosphorylation of <PROT2_995>  cdc25C </PROT2_995> though association with the PP2A - B55 regulatory subunit ( TARGET_CITATION ) . ||10921877_155807_995==>It has also recently been shown that the G2 cell cycle arrest induced by <PROT1_155807>  Vpr </PROT1_155807> that is encoded by the human immunodeficiency virus type 1 , was due to the interaction of this protein with PP2A via the subunit B55small alpha , Greek , leading to the inhibition of <PROT2_995>  Cdc25C </PROT2_995> ( TARGET_CITATION ) . ||10958988_155807_983==><PROT1_155807>  Vpr </PROT1_155807> - mediated cell cycle G2 arrest can be observe in cells from distantly related eukaryotes including human and fission yeast ( Schizosaccharomyces pombe ) and was shown to occur through inhibitory phosphorylation of <PROT2_983>  Cdc2 </PROT2_983> / <PROT2_983>  Cdk1 </PROT2_983> ( CITATION - TARGET_CITATION ) . ||12110603_155807_983==>It has been recently suggested that <PROT1_155807>  Vpr </PROT1_155807> interaction with protein phosphatase PP2A contributes to <PROT2_983>  cdc2 </PROT2_983> hyperphosphorylation ( TARGET_CITATION ) . ||12110603_155807_983==>It is suggested that HIV <PROT1_155807>  Vpr </PROT1_155807> can inhibit <PROT2_983>  cdc2 </PROT2_983> activity ( CITATION , CITATION ) via activation of wee1 and inactivation of cdc25C ( TARGET_CITATION , CITATION ) . ||12110603_155807_983==>The HIV <PROT1_155807>  Vpr </PROT1_155807> protein induces G2 arrest during infection and when expressed in s. pombe ( CITATION ) and acts by increasing the inhibitory phosphorylation on <PROT2_983>  Cdc2 </PROT2_983> Tyr15 via its interaction with protein phosphatase 2A ( TARGET_CITATION , CITATION ) . ||12110603_155807_983==>It is suggested that the HIV <PROT1_155807>  Vpr </PROT1_155807> protein can inhibit <PROT2_983>  cdc2 </PROT2_983> kinase activity ( CITATION , CITATION ) through mechanisms requiring wee1 and involving the inactivation of cdc25C ( TARGET_CITATION , CITATION ) . ||12110603_155807_983==>More recent publications suggest that the inhibition of <PROT2_983>  Cdc2 </PROT2_983> activity by the HIV <PROT1_155807>  Vpr </PROT1_155807> protein is due to its direct physical interaction with PP2A ( TARGET_CITATION , CITATION ) . ||12110603_155807_995==>In this regard , it is of interest that the <PROT1_155807>  Vpr </PROT1_155807> protein of human immunodeficiency virus arrests cells at G2 / M through activation of the protein phosphatase 2A and consequent inactivation of <PROT2_995>  Cdc25 </PROT2_995> and the cyclin - B & # x2013 ; p34cdc2 complex ( CITATION and TARGET_CITATION ) . ||12110603_155807_995==>It is suggested that HIV <PROT1_155807>  Vpr </PROT1_155807> can inhibit cdc2 activity ( CITATION , CITATION ) via activation of wee1 and inactivation of <PROT2_995>  cdc25C </PROT2_995> ( TARGET_CITATION , CITATION ) . ||12110603_155807_995==><PROT1_155807>  Vpr </PROT1_155807> may inactivate <PROT2_995>  cdc25C </PROT2_995> through physical interaction with protein phosphatase 2A ( TARGET_CITATION ) , which inhibits <PROT2_995>  cdc25C </PROT2_995> activity by dephosphorylation ( CITATION , CITATION , CITATION ) . ||12110603_155807_995==>The inactivation of <PROT2_995>  cdc25 </PROT2_995> was recently revealed to be mediated by inhibition of nuclear import of serine / threonine protein phosphatase 2A ( PP2A ) , known as an activator for <PROT2_995>  cdc25 </PROT2_995> , through a direct interaction between <PROT1_155807>  Vpr </PROT1_155807> and PP2A ( TARGET_CITATION ) . ||12110603_155807_995==>Recently , Hrimech et al. reported that <PROT1_155807>  Vpr </PROT1_155807> mediates G2 arrest by forming a complex with protein phosphatase 2A ( PP2A ) , an upstream regulator of <PROT2_995>  cdc25 </PROT2_995> , and enhances the nuclear import of PP2A TARGET_CITATION . ||12110603_155807_995==>It is suggested that the HIV <PROT1_155807>  Vpr </PROT1_155807> protein can inhibit cdc2 kinase activity ( CITATION , CITATION ) through mechanisms requiring wee1 and involving the inactivation of <PROT2_995>  cdc25C </PROT2_995> ( TARGET_CITATION , CITATION ) . ||12110603_155807_995==>HIV <PROT1_155807>  Vpr </PROT1_155807> is reported to inactivate <PROT2_995>  cdc25C </PROT2_995> by physically targeting PP2A to the nucleus and enhancing the recruitment and dephosphorylation of <PROT2_995>  cdc25C </PROT2_995> though association with the PP2A - B55 regulatory subunit ( TARGET_CITATION ) . ||12110603_155807_995==>It has also recently been shown that the G2 cell cycle arrest induced by <PROT1_155807>  Vpr </PROT1_155807> that is encoded by the human immunodeficiency virus type 1 , was due to the interaction of this protein with PP2A via the subunit B55small alpha , Greek , leading to the inhibition of <PROT2_995>  Cdc25C </PROT2_995> ( TARGET_CITATION ) . ||7474080_155807_983==>In mammalian cells , the tyrosine kinase inhibitor K252a induces endoreduplication ( CITATION ) , and viral proteins like human immunodeficiency virus <PROT1_155807>  Vpr </PROT1_155807> ( CITATION , TARGET_CITATION , CITATION , CITATION ) or simian virus 40 large T ( CITATION ) arrest cells in G2 - M by inhibiting the mitotic kinase <PROT2_983>  cdk1 </PROT2_983> ( TARGET_CITATION , CITATION , CITATION ) , resulting in multiple rounds of replication in the absence of mitosis and concomitant polyploidy ( CITATION ) . ||7474080_155807_983==>In support of this hypothesis , HIV - 1 <PROT1_155807>  Vpr </PROT1_155807> has recently been shown to induce G2 arrest ( CITATION , TARGET_CITATION , CITATION , CITATION , CITATION ) by maintaining the cyclin - dependent kinase <PROT2_983>  Cdc2 </PROT2_983> in an inactive , phosphorylated form ( TARGET_CITATION , CITATION , CITATION ) . ||7474080_155807_983==>The characteristics of the cell - cycle block in Lck - deficient T cells are very similar to those induced by the <PROT1_155807>  vpr </PROT1_155807> gene of HIV - 1 where a G2 block is induced also as a result of deficient cdc25C activation and accumulation of hyperphosphorylated <PROT2_983>  cdc2 </PROT2_983> ( CITATION - TARGET_CITATION ) . ||7474080_155807_983==><PROT1_155807>  Vpr </PROT1_155807> , which inhibits the cyclin B1 & # 183 ; <PROT2_983>  Cdc2 </PROT2_983> activity ( CITATION - TARGET_CITATION , CITATION ) , is not directly associated with p300 but instead regulates cyclin B1 & # 183 ; <PROT2_983>  Cdc2 </PROT2_983> activity , which we show interacts with the COOH terminus of this co - activator . ||7474080_155807_983==>The cytostatic effect of <PROT1_155807>  Vpr </PROT1_155807> was shown to result in a specific block in the G2 phase of the cell cycle , which was correlated with the inactivation of the <PROT2_983>  Cdc2 </PROT2_983> kinase ( CITATION , TARGET_CITATION ) . ||7474080_155807_983==>An illustration of the conservation of the <PROT1_155807>  Vpr </PROT1_155807> - induced G2 arrest is that <PROT1_155807>  Vpr </PROT1_155807> induces G2 arrest in both fission yeast and human cells by inhibitory phosphorylation of <PROT2_983>  Cdc2 </PROT2_983> ( CITATION , TARGET_CITATION , CITATION , CITATION , CITATION , CITATION ) . ||7474080_155807_983==>The activity of the <PROT2_983>  Cdc2 </PROT2_983> kinase decreases when <PROT1_155807>  vpr </PROT1_155807> is expressed in human cells ( TARGET_CITATION and CITATION ) , and immunoblot analysis shows that the phosphorylated form of <PROT2_983>  Cdc2 </PROT2_983> , which migrates slower than the dephosphorylated form on a polyacrylamide gel ( CITATION and CITATION ) increases in human cells when <PROT1_155807>  vpr </PROT1_155807> is expressed ( TARGET_CITATION ; CITATION ; CITATION and CITATION ) . ||7474080_155807_983==>Expression of the nonphosphorylatable A14T F15Y mutation of <PROT2_983>  Cdc2 </PROT2_983> overcomes the G2 arrest indicating that <PROT1_155807>  Vpr </PROT1_155807> induces G2 arrest in human cells by preventing dephosphorylation of Thr14 and Tyr15 on <PROT2_983>  Cdc2 </PROT2_983> ( TARGET_CITATION ) . ||7474080_155807_983==>We and others have reported that <PROT1_155807>  Vpr </PROT1_155807> induces hyperphosphorylation of the cyclin - dependent kinase <PROT2_983>  Cdc2 </PROT2_983> in fission yeast and human cells ( TARGET_CITATION ; CITATION ; CITATION ; CITATION and CITATION ) . ||7474080_155807_983==>Other viruses encode proteins which block cell cycle progression : human cytomegalovirus UL69 protein prevents progression from G1 to S phase ( CITATION , CITATION , CITATION ) , herpes simplex virus blocks G1 to S phase progression by blocking pRB phosphorylation ( CITATION , CITATION ) and the human immunodeficiency virus <PROT1_155807>  Vpr </PROT1_155807> protein causes cells to accumulate in G2 - M phase by preventing cyclin B / <PROT2_983>  cdc2 </PROT2_983> activation ( TARGET_CITATION , CITATION , CITATION ) . ||7474080_155807_983==>Expression of HIV <PROT1_155807>  Vpr </PROT1_155807> ( TARGET_CITATION , CITATION ) or HPV E2 protein ( CITATION ) results in inhibition or delayed activation of <PROT2_983>  cdc2 </PROT2_983> kinase activity resulting in an accumulation of cells in the G2 / M phase of the cell cycle . ||7474080_155807_983==>Conversely , human immunodeficiency virus ( HIV ) <PROT1_155807>  Vpr </PROT1_155807> ( TARGET_CITATION , CITATION ) and human papillomavirus E2 ( CITATION ) inhibit or delay the activation of <PROT2_983>  cdc2 </PROT2_983> . ||7474080_155807_983==>It is suggested that HIV <PROT1_155807>  Vpr </PROT1_155807> can inhibit <PROT2_983>  cdc2 </PROT2_983> activity ( TARGET_CITATION , CITATION ) via activation of wee1 and inactivation of cdc25C ( CITATION , CITATION ) . ||7474080_155807_983==>The HIV - 13 <PROT1_155807>  vpr </PROT1_155807> gene encodes a Mr 14 , 000 nuclear protein , <PROT1_155807>  Vpr </PROT1_155807> , which is expressed within infected cells and is packaged into virions CITATION CITATION . The <PROT1_155807>  Vpr </PROT1_155807> is required for importing the viral protein integration complex into the nucleus of nondividing cells CITATION CITATION and induces cell cycle arrest at the gap - 2 checkpoint in a variety of mammalian cells including human squamous cancer cells CITATION , CITATION , CITATION . The cell cycle arrest is characterized by alterations in the activation and phosphorylation state of cell division cycle 2 ( <PROT2_983>  Cdc2 </PROT2_983> ) kinase CITATION TARGET_CITATION and resembles the gap - 2 checkpoint induced by genotoxic agents CITATION , which results in apoptosis CITATION . ||7474080_155807_983==><PROT1_155807>  Vpr </PROT1_155807> - mediated cell cycle G2 arrest can be observe in cells from distantly related eukaryotes including human and fission yeast ( Schizosaccharomyces pombe ) and was shown to occur through inhibitory phosphorylation of <PROT2_983>  Cdc2 </PROT2_983> / <PROT2_983>  Cdk1 </PROT2_983> ( TARGET_CITATION ) . ||7474080_155807_983==>These data also show that the mechanism of action of 16E1 { wedge } E4 differs from that of HIV <PROT1_155807>  Vpr </PROT1_155807> , since <PROT1_155807>  Vpr </PROT1_155807> - induced cell cycle arrest has been shown to be mediated by hyperphosphorylation of <PROT2_983>  Cdc2 </PROT2_983> Tyr15 ( TARGET_CITATION ) and involves Wee1 , Rad24 , and Cdc25 ( CITATION , CITATION ) . ||7474080_155807_983==>It is suggested that the HIV <PROT1_155807>  Vpr </PROT1_155807> protein can inhibit <PROT2_983>  cdc2 </PROT2_983> kinase activity ( TARGET_CITATION , CITATION ) through mechanisms requiring wee1 and involving the inactivation of cdc25C ( CITATION , CITATION ) . ||7474080_155807_983==>Thus , the transfection - enforced expression of <PROT1_155807>  Vpr </PROT1_155807> reportedly causes <PROT2_983>  Cdk1 </PROT2_983> to remain in the phosphorylated , inactive state TARGET_CITATION , an observation that , however , has not been confirmed by other authors CITATION . ||7474080_155807_983==>Coexpression of a constitutively active mutant <PROT2_983>  Cdk1 </PROT2_983> molecule with <PROT1_155807>  Vpr </PROT1_155807> relieved the G2 arrest TARGET_CITATION , indicating that <PROT1_155807>  Vpr </PROT1_155807> might induce apoptosis via its capacity to inhibit <PROT2_983>  Cdk1 </PROT2_983> and to arrest the cell cycle in the G2 phase . ||7474080_155807_983==>The second biological activity associated with <PROT1_155807>  Vpr </PROT1_155807> is its ability to prevent host cell proliferation by arresting cell division in the G2 phase of the cell cycle , ( CITATION , TARGET_CITATION , CITATION , CITATION and CITATION ) correlated with hyperphosphorylation of the cell cycle regulated protein kinase <PROT2_983>  cdc2 </PROT2_983> ( TARGET_CITATION and CITATION ) facilitated by the interaction of <PROT1_155807>  Vpr </PROT1_155807> with protein phosphatase 2A ( PP2A ) . ||7474080_155807_983==>As for RNA viruses , in a human immunodeficiency virus ( HIV ) model system , first investigations indicated that the viral <PROT1_155807>  Vpr </PROT1_155807> protein inhibits <PROT2_983>  Cdc2 </PROT2_983> activity and consequently leads to a G2 - phase cell cycle arrest ( TARGET_CITATION ) . ||7474080_155807_983==>We and others have shown that in various human cell lines <PROT1_155807>  Vpr </PROT1_155807> causes G2 arrest which is associated with <PROT2_983>  Cdc2 </PROT2_983> inactivation and resembles the G2 checkpoint induced by DNA damage ( TARGET_CITATION , CITATION ) . ||7474080_155807_983==>These data support previous work demonstrating the inhibition of cyclin B1 - p34cdc2 complexes by <PROT1_155807>  Vpr </PROT1_155807> ( TARGET_CITATION ) and establish the identity of the upstream regulators of <PROT2_983>  Cdc2 </PROT2_983> . ||7474080_155807_983==>Moreover , <PROT1_155807>  Vpr </PROT1_155807> interrupts the cell cycle at G2 by inhibiting the activation of <PROT2_983>  Cdc2 </PROT2_983> kinase , which is required for entry into the M phase of the cell cycle ( TARGET_CITATION , CITATION and CITATION ) . ||7474080_155807_983==>Although the detailed mechanism of <PROT1_155807>  Vpr </PROT1_155807> - induced cell cycle arrest is as yet unknown , <PROT1_155807>  Vpr </PROT1_155807> - induced G2 arrest is associated with inactivation of <PROT2_983>  Cdc2 </PROT2_983> , the key regulatory kinase element at the G2 / M checkpoint ( TARGET_CITATION , CITATION , CITATION ) . ||7474080_155807_983==>This cell cycle arrest is potentially mediated by the cascade initiated by the interaction of <PROT1_155807>  Vpr </PROT1_155807> with the protein hVIP ( CITATION ) and characterized by accumulation of phosphorylated - inactive <PROT2_983>  Cdc2 </PROT2_983> ( TARGET_CITATION , CITATION , CITATION ) . ||7474100_155807_995==>In cells that express <PROT1_155807>  Vpr </PROT1_155807> , <PROT2_995>  CDC25C </PROT2_995> is in its inactive , hypophosphorylated state ( TARGET_CITATION ) , and wee1 is in its active hypophosphorylated state ( w. Greene , personal communication ) . ||7474100_155807_995==>Indeed , <PROT1_155807>  Vpr </PROT1_155807> expression results in the inactivation of the cyclin - dependent kinase p34cdc2 and of its upstream regulator <PROT2_995>  cdc25 </PROT2_995> ( CITATION and TARGET_CITATION ) . ||7474100_155807_995==>The characteristics of the cell - cycle block in Lck - deficient T cells are very similar to those induced by the <PROT1_155807>  vpr </PROT1_155807> gene of HIV - 1 where a G2 block is induced also as a result of deficient <PROT2_995>  cdc25C </PROT2_995> activation and accumulation of hyperphosphorylated cdc2 ( CITATION - TARGET_CITATION ) . ||7474100_155807_995==>The mechanism by which <PROT1_155807>  Vpr </PROT1_155807> inhibits Cdc2 activity is not completely understood ; however , the <PROT2_995>  Cdc25 </PROT2_995> protein that activates Cdc2 is inactive in <PROT1_155807>  Vpr </PROT1_155807> - expressing cells ( TARGET_CITATION ) , suggesting that proteins which normally control Cdc2 are modulated by <PROT1_155807>  Vpr </PROT1_155807> , which in turn affects HIV transcription . ||7474100_155807_995==>Such a biological function of <PROT1_155807>  Vpr </PROT1_155807> is conserved in primate lentiviruses CITATION and in a variety of cells including mammalian cells CITATION - CITATION ) , yeast ( CITATION , CITATION ) , and bacteria CITATION . In <PROT1_155807>  Vpr </PROT1_155807> - expressing cells , cyclin B - dependent p34cdc2 activity is down - regulated with a nonphosphorylated inactive form of <PROT2_995>  CDC25C </PROT2_995> TARGET_CITATION . It was recently reported that the viral replication increased more than twofold in the cells arrested by <PROT1_155807>  Vpr </PROT1_155807> CITATION , which could explain why the virus with wild - type ( WT ) <PROT1_155807>  Vpr </PROT1_155807> would have a replication advantage in vivo ( CITATION , CITATION ) . ||7474100_155807_995==>In <PROT1_155807>  Vpr </PROT1_155807> - expressing cells , cyclin B - dependent p34cdc2 activity is down - regulated with an inactive form of <PROT2_995>  CDC25C </PROT2_995> TARGET_CITATION . Recently , we have established a stable cell line ( MIT - 23 ) in which <PROT1_155807>  Vpr </PROT1_155807> expression could be tightly regulated by DOX3 , an analogue of tetracycline CITATION CITATION . When <PROT1_155807>  Vpr </PROT1_155807> is expressed in MIT - 23 cells , suppressed p34cdc2 activity with a concomitant multinuclear cell formation is observed . ||7474100_155807_995==>It is suggested that HIV <PROT1_155807>  Vpr </PROT1_155807> can inhibit cdc2 activity ( CITATION , TARGET_CITATION ) via activation of wee1 and inactivation of <PROT2_995>  cdc25C </PROT2_995> ( CITATION , CITATION ) . ||7474100_155807_995==>It is suggested that the HIV <PROT1_155807>  Vpr </PROT1_155807> protein can inhibit cdc2 kinase activity ( CITATION , TARGET_CITATION ) through mechanisms requiring wee1 and involving the inactivation of <PROT2_995>  cdc25C </PROT2_995> ( CITATION , CITATION ) . ||7474100_155807_995==>In response to <PROT1_155807>  Vpr </PROT1_155807> , the Cdc2 - specific phosphatase , <PROT2_995>  Cdc25C </PROT2_995> , is hyperphosphorylated in a pattern consistent with inactivation ( TARGET_CITATION ) . ||7474100_155807_995==>The <PROT1_155807>  Vpr </PROT1_155807> - induced G2 arrest is characterized by inactivation of the Cdc2 kinase and requires the activity of protein phosphatases PP2A and <PROT2_995>  Cdc25 </PROT2_995> ( CITATION , CITATION , CITATION , TARGET_CITATION ) . ||7474100_155807_995==><PROT1_155807>  Vpr </PROT1_155807> does not seem to be a stoichiometric inhibitor of the cdc2p - cyclin B complex , but rather , may exert effects on regulators of cdc2p , such as <PROT2_995>  cdc25C </PROT2_995> , Wee1 , or PP2A ( He et al. , 1995 CITATION ; Re et al. , 1995 TARGET_CITATION ; Zhao et al. , 1996 CITATION ; Elder et al. , 2000 CITATION , 2001 CITATION ; Masuda et al. , 2000 CITATION ) . ||7474100_155807_995==>It has been previously reported that <PROT2_995>  Cdc25C </PROT2_995> is inactive in cells that express <PROT1_155807>  Vpr </PROT1_155807> ( TARGET_CITATION ) . ||7666531_155807_983==>In mammalian cells , the tyrosine kinase inhibitor K252a induces endoreduplication ( CITATION ) , and viral proteins like human immunodeficiency virus <PROT1_155807>  Vpr </PROT1_155807> ( CITATION , CITATION , TARGET_CITATION , CITATION ) or simian virus 40 large T ( CITATION ) arrest cells in G2 - M by inhibiting the mitotic kinase <PROT2_983>  cdk1 </PROT2_983> ( CITATION , TARGET_CITATION , CITATION ) , resulting in multiple rounds of replication in the absence of mitosis and concomitant polyploidy ( CITATION ) . ||7666531_155807_983==>In the case of the human immunodeficiency virus <PROT1_155807>  Vpr </PROT1_155807> protein , the inhibition of <PROT2_983>  cdk1 </PROT2_983> activity is associated with hyperphosphorylation of its catalytic subunit , p34cdc2 ( TARGET_CITATION ) , possibly an indirect effect of <PROT1_155807>  Vpr </PROT1_155807> on regulatory kinases or phosphatases ( CITATION , CITATION , CITATION ) . ||7666531_155807_983==>In support of this hypothesis , HIV - 1 <PROT1_155807>  Vpr </PROT1_155807> has recently been shown to induce G2 arrest ( CITATION , CITATION , TARGET_CITATION , CITATION , CITATION ) by maintaining the cyclin - dependent kinase <PROT2_983>  Cdc2 </PROT2_983> in an inactive , phosphorylated form ( CITATION , CITATION , CITATION ) . ||7666531_155807_983==>The characteristics of the cell - cycle block in Lck - deficient T cells are very similar to those induced by the <PROT1_155807>  vpr </PROT1_155807> gene of HIV - 1 where a G2 block is induced also as a result of deficient cdc25C activation and accumulation of hyperphosphorylated <PROT2_983>  cdc2 </PROT2_983> ( TARGET_CITATION ) . ||7666531_155807_983==><PROT1_155807>  Vpr </PROT1_155807> , which inhibits the cyclin B1 & # 183 ; <PROT2_983>  Cdc2 </PROT2_983> activity ( TARGET_CITATION , CITATION ) , is not directly associated with p300 but instead regulates cyclin B1 & # 183 ; <PROT2_983>  Cdc2 </PROT2_983> activity , which we show interacts with the COOH terminus of this co - activator . ||7666531_155807_983==>An illustration of the conservation of the <PROT1_155807>  Vpr </PROT1_155807> - induced G2 arrest is that <PROT1_155807>  Vpr </PROT1_155807> induces G2 arrest in both fission yeast and human cells by inhibitory phosphorylation of <PROT2_983>  Cdc2 </PROT2_983> ( CITATION , CITATION , TARGET_CITATION , CITATION , CITATION , CITATION ) . ||7666531_155807_983==>The activity of the <PROT2_983>  Cdc2 </PROT2_983> kinase decreases when <PROT1_155807>  vpr </PROT1_155807> is expressed in human cells ( CITATION and CITATION ) , and immunoblot analysis shows that the phosphorylated form of <PROT2_983>  Cdc2 </PROT2_983> , which migrates slower than the dephosphorylated form on a polyacrylamide gel ( CITATION and CITATION ) increases in human cells when <PROT1_155807>  vpr </PROT1_155807> is expressed ( CITATION ; TARGET_CITATION ; CITATION and CITATION ) . ||7666531_155807_983==>We and others have reported that <PROT1_155807>  Vpr </PROT1_155807> induces hyperphosphorylation of the cyclin - dependent kinase <PROT2_983>  Cdc2 </PROT2_983> in fission yeast and human cells ( CITATION ; TARGET_CITATION ; CITATION ; CITATION and CITATION ) . ||7666531_155807_983==>The HIV - 1 <PROT1_155807>  vpr </PROT1_155807> protein prevents <PROT2_983>  Cdc2 </PROT2_983> activation , thereby delaying or preventing replication of infected cells at the G2 / M boundary ( CITATION , TARGET_CITATION , CITATION ) . ||7666531_155807_983==>The HIV - 13 <PROT1_155807>  vpr </PROT1_155807> gene encodes a Mr 14 , 000 nuclear protein , <PROT1_155807>  Vpr </PROT1_155807> , which is expressed within infected cells and is packaged into virions CITATION CITATION . The <PROT1_155807>  Vpr </PROT1_155807> is required for importing the viral protein integration complex into the nucleus of nondividing cells CITATION CITATION and induces cell cycle arrest at the gap - 2 checkpoint in a variety of mammalian cells including human squamous cancer cells TARGET_CITATION , CITATION , CITATION . The cell cycle arrest is characterized by alterations in the activation and phosphorylation state of cell division cycle 2 ( <PROT2_983>  Cdc2 </PROT2_983> ) kinase TARGET_CITATION CITATION and resembles the gap - 2 checkpoint induced by genotoxic agents CITATION , which results in apoptosis CITATION . ||7666531_155807_983==>Early studies demonstrated that <PROT1_155807>  Vpr </PROT1_155807> - induced G2 arrest is associated with inactivation of the cyclin - dependent kinase , <PROT2_983>  Cdc2 </PROT2_983> , by hyperphosphorylation and concomitant suppression of <PROT2_983>  Cdc2 </PROT2_983> - cyclin B kinase activity that is necessary for the G2 to M transition ( CITATION & # 150 ; TARGET_CITATION ) . ||7666531_155807_983==>The second biological activity associated with <PROT1_155807>  Vpr </PROT1_155807> is its ability to prevent host cell proliferation by arresting cell division in the G2 phase of the cell cycle , ( CITATION , CITATION , TARGET_CITATION , CITATION and CITATION ) correlated with hyperphosphorylation of the cell cycle regulated protein kinase <PROT2_983>  cdc2 </PROT2_983> ( CITATION and CITATION ) facilitated by the interaction of <PROT1_155807>  Vpr </PROT1_155807> with protein phosphatase 2A ( PP2A ) . ||7666531_155807_983==>We and others have shown that in various human cell lines <PROT1_155807>  Vpr </PROT1_155807> causes G2 arrest which is associated with <PROT2_983>  Cdc2 </PROT2_983> inactivation and resembles the G2 checkpoint induced by DNA damage ( CITATION , TARGET_CITATION ) . ||7666531_155807_983==>Furthermore , <PROT1_155807>  Vpr </PROT1_155807> can inhibit cell cycle progression at the G2 phase , at least in part via inactivation of the cyclin - dependent kinase <PROT2_983>  cdc2 </PROT2_983> and hyperphosphorylation and the concomitant suppression of <PROT2_983>  cdc2 </PROT2_983> - cyclin B kinase activity ( TARGET_CITATION ) . ||7666531_155807_983==>Moreover , <PROT1_155807>  Vpr </PROT1_155807> interrupts the cell cycle at G2 by inhibiting the activation of <PROT2_983>  Cdc2 </PROT2_983> kinase , which is required for entry into the M phase of the cell cycle ( CITATION , TARGET_CITATION and CITATION ) . ||7666531_155807_983==>Although the detailed mechanism of <PROT1_155807>  Vpr </PROT1_155807> - induced cell cycle arrest is as yet unknown , <PROT1_155807>  Vpr </PROT1_155807> - induced G2 arrest is associated with inactivation of <PROT2_983>  Cdc2 </PROT2_983> , the key regulatory kinase element at the G2 / M checkpoint ( CITATION , TARGET_CITATION , CITATION ) . ||7666531_155807_983==>This cell cycle arrest is potentially mediated by the cascade initiated by the interaction of <PROT1_155807>  Vpr </PROT1_155807> with the protein hVIP ( CITATION ) and characterized by accumulation of phosphorylated - inactive <PROT2_983>  Cdc2 </PROT2_983> ( CITATION , TARGET_CITATION , CITATION ) . ||7666531_155807_983==>It was subsequently shown that it is both HIV - and SIV - derived <PROT1_155807>  Vpr </PROT1_155807> , possibly acting through its interaction with the protein hVIP , that causes this arrest , which is characterized by the accumulation of hyperphosphorylated - inactive <PROT2_983>  Cdc2 </PROT2_983> ( CITATION , TARGET_CITATION , CITATION ) . ||7666531_155807_983==>This in turn may lead to the inactivation of Cdc25C , its inability to activate <PROT2_983>  Cdc2 </PROT2_983> , and cell cycle arrest as observed with <PROT1_155807>  Vpr </PROT1_155807> treatment in vitro ( TARGET_CITATION , CITATION , CITATION ) . ||9094673_155807_983==>In support of this hypothesis , HIV - 1 <PROT1_155807>  Vpr </PROT1_155807> has recently been shown to induce G2 arrest ( CITATION , CITATION , CITATION , CITATION , CITATION ) by maintaining the cyclin - dependent kinase <PROT2_983>  Cdc2 </PROT2_983> in an inactive , phosphorylated form ( CITATION , TARGET_CITATION , CITATION ) . ||9094673_155807_983==>Likewise , the HIV - 1 <PROT1_155807>  Vpr </PROT1_155807> protein , which also induces a G2 arrest in lymphocytes , blocks <PROT2_983>  Cdk1 </PROT2_983> activation by preventing its dephosphorylation ( TARGET_CITATION ) . ||9094673_155807_983==>In this regard , there might be some analogy between the mode of action of CDT and that of a viral protein , the <PROT1_155807>  Vpr </PROT1_155807> of human immunodeficiency virus ( HIV - 1 ) , which induces a G2 block in HeLa cells through inactivation of <PROT2_983>  CDK1 </PROT2_983> by hyperphosphorylation ( TARGET_CITATION ) . ||9094673_155807_983==>The HIV - 13 <PROT1_155807>  vpr </PROT1_155807> gene encodes a Mr 14 , 000 nuclear protein , <PROT1_155807>  Vpr </PROT1_155807> , which is expressed within infected cells and is packaged into virions CITATION CITATION . The <PROT1_155807>  Vpr </PROT1_155807> is required for importing the viral protein integration complex into the nucleus of nondividing cells CITATION CITATION and induces cell cycle arrest at the gap - 2 checkpoint in a variety of mammalian cells including human squamous cancer cells CITATION , CITATION , CITATION . The cell cycle arrest is characterized by alterations in the activation and phosphorylation state of cell division cycle 2 ( <PROT2_983>  Cdc2 </PROT2_983> ) kinase CITATION CITATION and resembles the gap - 2 checkpoint induced by genotoxic agents TARGET_CITATION , which results in apoptosis CITATION . ||9094673_155807_983==>In some situations , e.g. , in response to <PROT1_155807>  Vpr </PROT1_155807> expression , abrogation of G2 arrest by pentoxifylline is associated with decreasing the levels of hyperphosphorylated <PROT2_983>  Cdc2 </PROT2_983> Tyr15 ( TARGET_CITATION ) . ||9094673_155807_983==>The <PROT1_155807>  Vpr </PROT1_155807> - induced G2 arrest is characterized by inactivation of the <PROT2_983>  Cdc2 </PROT2_983> kinase and requires the activity of protein phosphatases PP2A and Cdc25 ( CITATION , CITATION , TARGET_CITATION , CITATION ) . ||9520381_155807_983==>This cell cycle arrest is potentially mediated by the cascade initiated by the interaction of <PROT1_155807>  Vpr </PROT1_155807> with the protein hVIP ( TARGET_CITATION ) and characterized by accumulation of phosphorylated - inactive <PROT2_983>  Cdc2 </PROT2_983> ( CITATION , CITATION , CITATION ) . ||9520381_155807_983==>It was subsequently shown that it is both HIV - and SIV - derived <PROT1_155807>  Vpr </PROT1_155807> , possibly acting through its interaction with the protein hVIP , that causes this arrest , which is characterized by the accumulation of hyperphosphorylated - inactive <PROT2_983>  Cdc2 </PROT2_983> ( CITATION , CITATION , TARGET_CITATION ) . 
incorporates====11932435_155030_3308==>Chaperonins and chaperonin - associated proteins , TriC , <PROT2_3308>  Hsp70 </PROT2_3308> , and Ubp , have been found to interact with MPMV and HIV - 1 <PROT1_155030>  Gag </PROT1_155030> proteins and it has been suggested that these cellular proteins assist in folding <PROT1_155030>  Gag </PROT1_155030> proteins into a conformation ( s ) required for <PROT1_155030>  capsid </PROT1_155030> assembly ( CITATION , TARGET_CITATION , CITATION and CITATION ) . ||11932435_155030_3308==>Some potential consequences for nascent <PROT1_155030>  Gag </PROT1_155030> molecules being exposed to appropriate spatially restricted cofactors include interactions with factors involved in trafficking to the plasma membrane or with chaperones such as <PROT2_3308>  Hsp70 </PROT2_3308> , TRiC and HP68 ( CITATION ; TARGET_CITATION ; CITATION ) . ||11932435_155030_3308==>Previous study ( TARGET_CITATION ) demonstrated that <PROT1_155030>  Gag </PROT1_155030> is sufficient for <PROT2_3308>  Hsp70 </PROT2_3308> incorporation , which , together with an established role of Vpr in the early steps of infection , argues that Vpr does not get in contact with <PROT2_3308>  Hsp70 </PROT2_3308> within the virion . ||11932435_155030_3329==>Chaperonins and <PROT2_3329>  chaperonin </PROT2_3329> - associated proteins , TriC , Hsp70 , and Ubp , have been found to interact with MPMV and HIV - 1 <PROT1_155030>  Gag </PROT1_155030> proteins and it has been suggested that these cellular proteins assist in folding <PROT1_155030>  Gag </PROT1_155030> proteins into a conformation ( s ) required for <PROT1_155030>  capsid </PROT1_155030> assembly ( CITATION , TARGET_CITATION , CITATION and CITATION ) . ||8892894_155030_7430==>Additionally , the cytoskeletal proteins actin , <PROT2_7430>  ezrin </PROT2_7430> , moesin , and cofilin have been localized to the interior of HIV - 1 virions ( TARGET_CITATION ) , and actin has been found to bind <PROT1_155030>  Gag </PROT1_155030> in vitro ( CITATION ) . ||8892894_155030_7430==>These include H03 , a putative histidyl - tRNA synthetase that binds to MA of HIV - 1 ( CITATION ) ; KIF4 , a microtubule - associated motor protein in the kinesin superfamily that interacts with different retroviral <PROT1_155030>  Gag </PROT1_155030> proteins , including that of Mo - MuLV and HIV - 1 ( CITATION , CITATION ) ; actin and other cytoskeletal proteins ( <PROT2_7430>  ezrin </PROT2_7430> , moesin , and cofilin ) found in HIV - 1 virions ( TARGET_CITATION ) ; and translation elongation factor 1alpha , bound to MA and NC domains of HIV - 1 <PROT1_155030>  Gag </PROT1_155030> ( CITATION ) . 
induces====10393859_155871_4843==>Recently , HIV - 1 <PROT1_155871>  Tat </PROT1_155871> has also been shown to induce <PROT2_4843>  iNOS </PROT2_4843> in macrophages ( TARGET_CITATION ) . ||10446807_155871_4843==>Furthermore , NF - small kappa , GreekB has been found to mediate <PROT2_4843>  NOS </PROT2_4843> - II induction by other inducers , including interferon - small gamma , Greek ( IFNsmall gamma , Greek ) ( CITATION and CITATION ) , interleukin - 1small beta , Greek ( IL - 1small beta , Greek ) ( CITATION ) , tumor necrosis factor - small alpha , Greek ( TNFsmall alpha , Greek ) ( CITATION ) , double - stranded RNA ( CITATION ) , HIV - 1 <PROT1_155871>  tat </PROT1_155871> protein ( TARGET_CITATION ) , small beta , Greek - amyloid ( CITATION and CITATION ) , and mixtures containing LPS plus IFNsmall gamma , Greek ( CITATION ) , LPS plus ceramide ( CITATION ) , or LPS plus three cytokines ( i.e. , IL - 1small beta , Greek , TNFsmall alpha , Greek , and IFNsmall gamma , Greek ) ( CITATION ) . ||10446807_155871_4843==>The HIV protein <PROT1_155871>  Tat </PROT1_155871> up - regulates nuclear factor ( NF ) - { kappa } B expression , a transactivator of the <PROT2_4843>  iNOS </PROT2_4843> gene , and exogenous <PROT1_155871>  Tat </PROT1_155871> has been shown to up - regulate NO production in microglial cells ( TARGET_CITATION ) . ||10446807_155871_4843==>The capacity of HIV proteins Env ( gp120 ) and <PROT1_155871>  Tat </PROT1_155871> to induce <PROT2_4843>  iNOS </PROT2_4843> expression in human cell - culture systems ( CITATION , TARGET_CITATION ) had no apparent effect on the murine system , perhaps because of inefficient viral protein expression , inappropriate host cofactors , or its counteraction by yet - undefined mechanisms . ||12167619_155871_4843==>Recently , it was shown that transiently transfected <PROT1_155871>  Tat </PROT1_155871> plasmids induced <PROT2_4843>  iNOS </PROT2_4843> and produced NO in an astrocytoma cell line U373MG ( TARGET_CITATION ) . ||12167619_155871_4843==>Recently we have found that & # x394 ; C / EBPsmall beta , Greek inhibits cytokine - or HIV - 1 <PROT1_155871>  Tat </PROT1_155871> - induced activation of human <PROT2_4843>  iNOS </PROT2_4843> promoter in human astroglial cells ( TARGET_CITATION and CITATION ) suggesting the involvement of C / EBPsmall beta , Greek in cytokine - or <PROT1_155871>  Tat </PROT1_155871> - induced expression of <PROT2_4843>  iNOS </PROT2_4843> . 
inhibits====10064603_1025_155871==>The expression of D167N in trans inhibits strongly <PROT1_1025>  CDK9 </PROT1_1025> - dependent <PROT2_155871>  Tat </PROT2_155871> - activated transcription in a wide variety of cell types ( CITATION , CITATION ) , but it has only minimal effects on <PROT1_1025>  CDK9 </PROT1_1025> - independent transcriptional elongation stimulated by a cellular enhancer ( TARGET_CITATION ) . ||10069809_155908_9972==>In this context it is also important to note that a series of recent studies indeed demonstrated the direct participation of CAN / nup214 , <PROT2_9972>  nup153 </PROT2_9972> , and nup98 in the nuclear export of <PROT1_155908>  Rev </PROT1_155908> and <PROT1_155908>  Rev </PROT1_155908> - mediated viral mRNA export ( Bogerd et al. 1998 CITATION ; Ullman et al. 1999 TARGET_CITATION ; Zolotukhin and Felber 1999 CITATION ) . ||10069809_155908_9972==>Moreover , antibodies to <PROT2_9972>  Nup153 </PROT2_9972> block mRNA , snRNA , and 5S RNA export , as well as NES protein export and <PROT1_155908>  Rev </PROT1_155908> - dependent HIV RNA export ( TARGET_CITATION ) . ||10371501_10657_155908==>More recently , <PROT1_10657>  SAM68 </PROT1_10657> has been identified as a functional homologue of the HIV - 1 <PROT2_155908>  Rev </PROT2_155908> protein ( TARGET_CITATION , CITATION ) , thereby implicating it in the posttranscriptional regulation of complex retroviruses ( CITATION ) . ||10371501_10657_155908==>Along the same lines , <PROT1_10657>  Sam68 </PROT1_10657> , a cellular homologue of <PROT2_155908>  Rev </PROT2_155908> , has been mutated with the aim of forming a non - functional complex with <PROT2_155908>  Rev </PROT2_155908> TARGET_CITATION . ||10393559_10524_155871==>Likewise , the HIV1 transactivator protein , <PROT2_155871>  TAT </PROT2_155871> , inhibits the acetylase activities of two nuclear HATs , <PROT1_10524>  Tip60 </PROT1_10524> , and TAFII250 ( ( CITATION and TARGET_CITATION ) ) . ||10393559_10524_155871==>The HIV transactivator <PROT2_155871>  Tat </PROT2_155871> interacts with several histone acetyltransferases , such as <PROT1_10524>  Tip60 </PROT1_10524> ( TARGET_CITATION , CITATION ) , hTAFII250 ( CITATION ) , p300 , CBP , and P / CAF ( CITATION ) . ||10393559_10524_155871==>For instance , the basic domain of <PROT2_155871>  Tat </PROT2_155871> has been shown to be required for <PROT1_10524>  Tip60 </PROT1_10524> and p300 interaction , whereas the <PROT2_155871>  Tat </PROT2_155871> C - terminal domain is necessary for hTAFII250 interaction ( CITATION , CITATION - TARGET_CITATION ) . ||10393559_10524_155871==><PROT2_155871>  Tat </PROT2_155871> - <PROT1_10524>  Tip60 </PROT1_10524> and <PROT2_155871>  Tat </PROT2_155871> - TAFII250 interactions do not , however , affect transcription from the LTR but repress transcription of cellular genes such as the manganese superoxide dismutase gene ( TARGET_CITATION ) and the major histocompatibility class I genes ( CITATION ) . ||10393559_10524_155871==>In the case of <PROT1_10524>  Tip60 </PROT1_10524> and TAFII250 , <PROT2_155871>  Tat </PROT2_155871> seems to inhibit their HAT activity and consequently modulate the expression of cellular genes ( CITATION , TARGET_CITATION ) . ||10393559_10524_155871==><PROT1_10524>  Tip60 </PROT1_10524> , whose HAT activity was previously shown to be inhibited by <PROT2_155871>  Tat </PROT2_155871> ( TARGET_CITATION ) , was used as a control . ||10393559_10524_155871==><PROT2_155871>  Tat </PROT2_155871> & # x2013 ; <PROT1_10524>  Tip60 </PROT1_10524> and <PROT2_155871>  Tat </PROT2_155871> & # x2013 ; TAFII250 interactions do not affect transcription from the HIV - 1 LTR but repress transcription of cellular genes such as the manganese - dependent superoxide dismutase gene ( TARGET_CITATION ) and the major histocompatibility class I genes ( CITATION ) . ||10393559_10524_155871==>Recently , others have demonstrated that <PROT2_155871>  Tat </PROT2_155871> inhibits acetyltransferase activities of the co - activators <PROT1_10524>  Tip60 </PROT1_10524> and TAFII250 , thereby causing repression of cellular transcription ( TARGET_CITATION , CITATION ) . ||10393559_10524_155871==>Although the targeting of <PROT1_10524>  Tip60 </PROT1_10524> by <PROT2_155871>  Tat </PROT2_155871> was found to interfere with its HAT activity and to disturb the expression of at least one cellular gene ( TARGET_CITATION ) , the function of <PROT1_10524>  Tip60 </PROT1_10524> remained elusive until recently . ||10661406_155871_4261==>Competition between <PROT1_155871>  Tat </PROT1_155871> and <PROT2_4261>  CIITA </PROT2_4261> for binding to the P - TEBFb transcription factor has also been reported in HIV - 1 - infected THP - 1 cells that lost class II expression ( TARGET_CITATION ) . ||10661406_155871_4261==>Kanazawa et al. ( TARGET_CITATION ) have demonstrated that the <PROT1_155871>  Tat </PROT1_155871> protein competed with <PROT2_4261>  CIITA </PROT2_4261> for binding to P - TEFb , an activation factor that is required for class II transcription , and blocked the expression of class II genes in THP - 1 cells . ||10661406_155871_4261==>Thus , <PROT1_155871>  Tat </PROT1_155871> , <PROT2_4261>  CIITA </PROT2_4261> , NF - kappa B , androgen receptor , and c - Myc all recruit P - TEFb to the transcription complex ( CITATION , TARGET_CITATION ) . ||10661406_155871_4261==>Although the viral transactivator <PROT1_155871>  Tat </PROT1_155871> was the first transcription factor shown to require P - TEFb for activity , P - TEFb has recently been shown to bind to a number of cellular transcription factors , including <PROT2_4261>  CIITA </PROT2_4261> , NF - kappa B , androgen receptor , and MyoD ( TARGET_CITATION ) . ||10661406_155871_4261==>Kanazawa et al. ( TARGET_CITATION ) have demonstrated that the <PROT1_155871>  Tat </PROT1_155871> protein competed with <PROT2_4261>  CIITA </PROT2_4261> for binding to P - TEFb , an activation factor that is required for class II transcription , and blocked the expression of class II genes in THP - 1 cells ( fig. 2 ) . ||10661406_155871_4261==>Alternatively , HIV - 1 may prevent the association of the elongation factor P - TEFb with <PROT2_4261>  CIITA </PROT2_4261> through its <PROT1_155871>  Tat </PROT1_155871> protein ( TARGET_CITATION ) . ||10661406_155871_4261==>Examples include the major histocompatibility complex class II transactivator <PROT2_4261>  CIITA </PROT2_4261> ( TARGET_CITATION ) , NF - { kappa } B ( CITATION ) , the p160 coactivator GRIP1 ( CITATION ) , c - Myc ( CITATION , CITATION ) , FBI - 1 ( CITATION ) , Pur { alpha } ( CITATION ) , <PROT1_155871>  Tat </PROT1_155871> - SF1 ( CITATION ) , the androgen receptor ( CITATION ) , granulin ( CITATION ) , and the CTD itself ( CITATION , CITATION ) . ||10661406_155871_4261==>The cyclin domain is responsible for binding CDK9 and various other proteins , including NF - { kappa } B ( CITATION ) <PROT2_4261>  CIITA </PROT2_4261> ( TARGET_CITATION ) , and GRIP1 ( CITATION ) ; the TRM allows the cooperative binding of <PROT1_155871>  Tat </PROT1_155871> and TAR to cyclin T1 ( CITATION ) ; the His - rich region binds the CTD of RNA polymerase II ( CITATION ) as well as granulin ( CITATION ) ; and the PEST domain is required for the interaction with SCFSKP2 ( CITATION ) . ||10661406_155871_4261==>Since the isolation of cyclin T1 as a CDK9 partner and <PROT1_155871>  Tat </PROT1_155871> - interacting protein , three other cellular proteins have been identified that bind directly to the cyclin box ( amino acids 1 to 254 ) of cyclin T1 , the major histocompatibility complex class II transactivator <PROT2_4261>  CIITA </PROT2_4261> , the NF - { kappa } B RelA / p65 subunit , and the glucocorticoid receptor interacting polypeptide 1 GRIP - 1 ( CITATION , TARGET_CITATION , CITATION ) . ||10661406_155871_4261==><PROT1_155871>  Tat </PROT1_155871> competes with the MHC class II master transcriptional regulator <PROT2_4261>  CIITA </PROT2_4261> for binding of CYCT1 ( ref. TARGET_CITATION ) , and levels of another MHC class II regulator , NF - YA , are decreased in HIV - infected cells CITATION . ||10661406_155871_4261==>As Tax - 1 displays many functional similarities with HIV - 1 <PROT1_155871>  Tat </PROT1_155871> , and because it has been recently observed that <PROT1_155871>  Tat </PROT1_155871> and <PROT2_4261>  CIITA </PROT2_4261> can compete each other CITATION , TARGET_CITATION , it was important to investigate whether <PROT2_4261>  CIITA </PROT2_4261> can affect HTLV - 2 productive infection by competing with Tax - 2 , the Tax - 1 homolog of the HTLV - 2 retrovirus . ||10661406_155871_4261==>In this scenario , N - TEF pauses RNAPII at an early step of transcriptional elongation , whereupon P - TEFb can be recruited by many different activators , e.g. , the androgen receptor , <PROT2_4261>  CIITA </PROT2_4261> , c - Myc , NF - { kappa } B , MyoD , and <PROT1_155871>  Tat </PROT1_155871> ( CITATION , CITATION , TARGET_CITATION , CITATION , CITATION ) ( fig. 6 ) . ||10888618_155871_7528==>The overexpression of dnLSF has been shown to relieve <PROT2_7528>  YY1 </PROT2_7528> - mediated repression of <PROT1_155871>  Tat </PROT1_155871> - activated LTR expression ( TARGET_CITATION , CITATION ) . ||10938286_155871_9150==><PROT1_155871>  Tat </PROT1_155871> inhibits <PROT2_9150>  FCP1 </PROT2_9150> , and this inhibition may alleviate <PROT2_9150>  FCP1 </PROT2_9150> - mediated pausing of transcription ( TARGET_CITATION , CITATION ) . ||11243698_155871_8743==>Others have shown that Jurkat cells stably transfected with HIV - <PROT1_155871>  tat </PROT1_155871> are less susceptible to <PROT2_8743>  TRAIL </PROT2_8743> - mediated cell death than are untransfected cells ( TARGET_CITATION ) . ||11273209_155871_9150==>Third , inhibition of <PROT2_9150>  FCP1 </PROT2_9150> phosphatase activity by the human immunodeficiency viral protein <PROT1_155871>  Tat </PROT1_155871> may contribute to stimulate transcriptional elongation from the human immunodeficiency virus promoter ( CITATION , TARGET_CITATION ) . ||11273209_155871_9150==>Similar to the inhibition by <PROT2_9150>  FCP1 </PROT2_9150> on RNPII - mediated <PROT1_155871>  Tat </PROT1_155871> transactivation ( TARGET_CITATION ) , TRBP also may affect gene transcription . ||11483766_10657_155908==>Moreover , a recent study has shown that <PROT1_10657>  Sam68 </PROT1_10657> { Delta } C - mediated inhibition of <PROT2_155908>  Rev </PROT2_155908> activity is due to perinuclear sequestering of unspliced HIV - 1 mRNAs ( TARGET_CITATION ) . ||11483766_10657_155908==>Since our previous studies ( CITATION ) and studies by others ( CITATION , TARGET_CITATION ) have shown that <PROT1_10657>  Sam68 </PROT1_10657> interaction with <PROT2_155908>  Rev </PROT2_155908> may modulate <PROT2_155908>  Rev </PROT2_155908> activity , we then determined whether inhibition of HIV - 1 gene expression and viral replication by down - modulation of constitutive <PROT1_10657>  Sam68 </PROT1_10657> expression is due to a decrease in <PROT2_155908>  Rev </PROT2_155908> activity . ||11483766_10657_155908==>Moreover , our results showed that only marginally synergistic effects between <PROT1_10657>  Sam68 </PROT1_10657> and <PROT2_155908>  Rev </PROT2_155908> were obtained in 293T cells ( fig. 3a ) , which was also confirmed in another recent study ( TARGET_CITATION ) . ||11483766_10657_155908==>In addition , a very recent study has demonstrated that the dominant - negative <PROT1_10657>  Sam68 </PROT1_10657> { Delta } C mutant , in which the C - terminal nuclear localization signal has been deleted , inhibits <PROT2_155908>  Rev </PROT2_155908> activity by causing perinuclear accumulation of unspliced RRE - containing RNAs ( TARGET_CITATION ) , further suggesting that the effects of <PROT1_10657>  Sam68 </PROT1_10657> and the dominant - negative <PROT1_10657>  Sam68 </PROT1_10657> mutant on <PROT2_155908>  Rev </PROT2_155908> function may be mediated by different mechanisms . ||11483766_10657_155908==><PROT1_10657>  Sam68 </PROT1_10657> was found to act as a functional homolog of the human immunodeficiency virus type 1 ( HIV1 ) <PROT2_155908>  Rev </PROT2_155908> protein , which transports RNA from the nucleus to the cytoplasm ( CITATION ; TARGET_CITATION ) . ||11483766_10657_155908==><PROT1_10657>  Sam68 </PROT1_10657> has previously been reported to substitute for , and synergize with , the effects of the HIV <PROT2_155908>  Rev </PROT2_155908> protein ( CITATION , TARGET_CITATION ) . ||11483766_10657_155908==><PROT1_10657>  Sam68 </PROT1_10657> ( Src associated during mitosis of 68 kDa ) , a member of the STAR ( signal transduction and activation of RNA ) family of proteins ( Fumagalli et al. 1994 ; Taylor and Shalloway 1994 ) is able to increase <PROT2_155908>  Rev </PROT2_155908> function in a number of cell lines ( Reddy et al. 1999 , 2000 ; Soros et al. 2001 TARGET_CITATION ; Li et al. 2002a , b ) . ||11483766_10657_155908==>A truncation mutant of <PROT1_10657>  Sam68 </PROT1_10657> ( lacking the C - terminal 100 amino acids , designated <PROT1_10657>  Sam68 </PROT1_10657> { Delta } C ) is a potent inhibitor of <PROT2_155908>  Rev </PROT2_155908> activity ( Reddy et al. 1999 ; Soros et al. 2001 TARGET_CITATION ) . ||11483766_10657_155908==>Consistent with previous work ( Soros et al. 2001 TARGET_CITATION ) , <PROT1_10657>  Sam68 </PROT1_10657> expression resulted in increased production of gp120 from pgTat and pgTat { Delta } E { Delta } S SL1 relative to that seen with <PROT2_155908>  Rev </PROT2_155908> alone ( fig. 3A ) . ||11483766_10657_155908==>One factor that augments <PROT2_155908>  Rev </PROT2_155908> activity is <PROT1_10657>  Sam68 </PROT1_10657> ( Reddy et al. 1999 , 2000 ; Soros et al. 2001 TARGET_CITATION ; Li et al. 2002a ) . ||11483766_10657_155908==>Second , the cellular protein <PROT1_10657>  SAM68 </PROT1_10657> ( src - associated in mitosis ) can partially substitute for <PROT2_155908>  Rev </PROT2_155908> in transient transfection assays ( Reddy et al. 1999 CITATION ; Soros et al. 2001 TARGET_CITATION ) . ||11483766_10657_155908==>Interestingly , a dominant - negative <PROT1_10657>  SAM68 </PROT1_10657> mutant also mislocalizes <PROT2_155908>  Rev </PROT2_155908> - directed RNAs to the nuclear periphery ( Soros et al. 2001 TARGET_CITATION ) . ||11483766_10657_155908==>Although the biological role ( s ) and physiological RNA targets for <PROT1_10657>  Sam68 </PROT1_10657> are unknown , it has been shown to substitute functionally for , and synergize with , the HIV - 1 <PROT2_155908>  Rev </PROT2_155908> protein ( CITATION , CITATION , CITATION ; TARGET_CITATION ) . ||11940654_155871_3065==>In line with this interpretation , He and Margolis showed that activation of LTR expression by <PROT1_155871>  Tat </PROT1_155871> resulted in the displacement of <PROT2_3065>  HDAC1 </PROT2_3065> and in the increased acetylation of H4 ( TARGET_CITATION ) . ||11940654_155871_3065==>YY1 , on the other hand , was found to bind and repress the HIV promoter and <PROT1_155871>  Tat </PROT1_155871> via recruitment of <PROT2_3065>  HDAC1 </PROT2_3065> ( TARGET_CITATION ) . ||11940654_155871_7024==>Recent studies have shown that <PROT1_155871>  Tat </PROT1_155871> activation can be inhibited by the overexpression of the host factors YY1 and <PROT2_7024>  LSF </PROT2_7024> , which recruit histone deacetylase 1 to the LTR ( TARGET_CITATION ) . ||11940654_155871_7024==>A recent report described a novel mechanism of <PROT1_155871>  Tat </PROT1_155871> inactivation by overexpression of the host proteins YY1 and <PROT2_7024>  LSF </PROT2_7024> , which recruit histone deacetylase 1 to the LTR and counteract the positive effect of histone acetyltransferases ( TARGET_CITATION ) . ||11940654_155871_7528==>Recent studies have shown that <PROT1_155871>  Tat </PROT1_155871> activation can be inhibited by the overexpression of the host factors <PROT2_7528>  YY1 </PROT2_7528> and LSF , which recruit histone deacetylase 1 to the LTR ( TARGET_CITATION ) . ||11940654_155871_7528==>A recent report described a novel mechanism of <PROT1_155871>  Tat </PROT1_155871> inactivation by overexpression of the host proteins <PROT2_7528>  YY1 </PROT2_7528> and LSF , which recruit histone deacetylase 1 to the LTR and counteract the positive effect of histone acetyltransferases ( TARGET_CITATION ) . ||11940654_155871_7528==>Counter regulation of chromatin deacetylation and histone deacetylase occupancy at the integrated promoter of human immunodeficiency virus type 1 ( HIV - 1 ) is affected by the HIV - 1 repressor <PROT2_7528>  YY1 </PROT2_7528> and HIV - 1 activator <PROT1_155871>  Tat </PROT1_155871> ( He and Margolis , 2002 TARGET_CITATION ) . ||11940654_155871_7528==><PROT2_7528>  YY1 </PROT2_7528> , on the other hand , was found to bind and repress the HIV promoter and <PROT1_155871>  Tat </PROT1_155871> via recruitment of HDAC1 ( TARGET_CITATION ) . ||12154097_155871_8850==>The arginine - rich motif of <PROT1_155871>  Tat </PROT1_155871> interacts with the catalytic histone acetyltransferase ( HAT ) domains of transcriptional co - activators , p300 / CREB - binding protein ( CBP ) and p300 / CBP - associated factor ( P / <PROT2_8850>  CAF </PROT2_8850> ) : hGCN5 ( CITATION - TARGET_CITATION ) , and <PROT1_155871>  Tat </PROT1_155871> is acetylated by p300 on lysine residues Lys50 / Lys51 ( Lys51 is only weakly acetylated ) and by P / <PROT2_8850>  CAF </PROT2_8850> on Lys28 ( CITATION , CITATION ) . ||12355430_155871_4261==>As Tax - 1 displays many functional similarities with HIV - 1 <PROT1_155871>  Tat </PROT1_155871> , and because it has been recently observed that <PROT1_155871>  Tat </PROT1_155871> and <PROT2_4261>  CIITA </PROT2_4261> can compete each other TARGET_CITATION , CITATION , it was important to investigate whether <PROT2_4261>  CIITA </PROT2_4261> can affect HTLV - 2 productive infection by competing with Tax - 2 , the Tax - 1 homolog of the HTLV - 2 retrovirus . ||12355430_155871_4261==>CBP / p300 and the cyclin T1 of the P - TEFb complex interact also with the HIV - 1 transcriptional activator <PROT1_155871>  Tat </PROT1_155871> CITATION , CITATION , and it has been shown that the inhibition of HIV - 1 replication in <PROT2_4261>  CIITA </PROT2_4261> - positive cells is mediated by competition of <PROT2_4261>  CIITA </PROT2_4261> with <PROT1_155871>  Tat </PROT1_155871> for the binding to cyclin T1 TARGET_CITATION . ||12501250_155871_7157==>Moreover , <PROT1_155871>  Tat </PROT1_155871> impairs the <PROT1_155871>  Tat </PROT1_155871> suppressor functions of <PROT2_7157>  p53 </PROT2_7157> by a dual action : It inhibits the transcription of the <PROT2_7157>  p53 </PROT2_7157> gene ( CITATION ) and the p300 - mediated acetylation of the <PROT2_7157>  p53 </PROT2_7157> protein ( TARGET_CITATION ) . ||12501250_155871_7157==>Indeed , it was recently shown that <PROT2_7157>  p53 </PROT2_7157> was hypo - acetylated at residues K320 in the presence of the HIV - 1 <PROT1_155871>  Tat </PROT1_155871> protein , which represses its transcriptional activity ( TARGET_CITATION ) . ||12588988_155871_2896==>Examples include the major histocompatibility complex class II transactivator CIITA ( CITATION ) , NF - { kappa } B ( CITATION ) , the p160 coactivator GRIP1 ( CITATION ) , c - Myc ( CITATION , CITATION ) , FBI - 1 ( CITATION ) , Pur { alpha } ( CITATION ) , <PROT1_155871>  Tat </PROT1_155871> - SF1 ( CITATION ) , the androgen receptor ( CITATION ) , <PROT2_2896>  granulin </PROT2_2896> ( TARGET_CITATION ) , and the CTD itself ( CITATION , CITATION ) . ||12588988_155871_2896==>The cyclin domain is responsible for binding CDK9 and various other proteins , including NF - { kappa } B ( CITATION ) CIITA ( CITATION ) , and GRIP1 ( CITATION ) ; the TRM allows the cooperative binding of <PROT1_155871>  Tat </PROT1_155871> and TAR to cyclin T1 ( CITATION ) ; the His - rich region binds the CTD of RNA polymerase II ( CITATION ) as well as <PROT2_2896>  granulin </PROT2_2896> ( TARGET_CITATION ) ; and the PEST domain is required for the interaction with SCFSKP2 ( CITATION ) . ||12588988_155871_2896==>Other proteins that have recently been reported to interact with the C terminus of cyclin T1 include <PROT1_155871>  Tat </PROT1_155871> - SF1 ( CITATION ) , the CTD ( CITATION ) , <PROT2_2896>  granulin </PROT2_2896> ( TARGET_CITATION ) , and PIE - 1 ( CITATION ) . ||12588988_155871_2896==>P - TEFb phosphorylates the CTD , thereby enabling processive transcription ( CITATION ) ; <PROT1_155871>  Tat </PROT1_155871> - SF1 is a <PROT1_155871>  Tat </PROT1_155871> - specific cofactor ( CITATION , CITATION ) ; <PROT2_2896>  granulin </PROT2_2896> inhibits <PROT1_155871>  Tat </PROT1_155871> - mediated transactivation ( TARGET_CITATION ) ; and PIE - 1 inhibits transcriptional elongation by P - TEFb ( CITATION ) . ||12588988_155871_2896==>At the molecular level , <PROT2_2896>  PCDGF </PROT2_2896> directly activates mitogen - activated protein kinase , phosphatidylinositol 3' - kinase , and focal adhesion kinase signaling pathways CITATION . In 3T3 mouse embryo fibroblasts deficient in insulin - like growth factor I receptor , <PROT2_2896>  PCDGF </PROT2_2896> overcomes this deficiency and stimulates cell proliferation via the activation of the mitogen - activated protein and phosphatidylinositol 3' - kinase pathways CITATION . It also increases cyclin D1 expression accompanied by increased pRB phosphorylation in breast cancer cells CITATION . <PROT2_2896>  Granulin </PROT2_2896> interacts with HIV <PROT1_155871>  Tat </PROT1_155871> and cyclin T1 to regulate gene transcription TARGET_CITATION . ||2194290_155871_5702==>The discovery of the human immunodeficiency virus <PROT1_155871>  Tat </PROT1_155871> - binding protein <PROT2_5702>  TBP1 </PROT2_5702> ( TARGET_CITATION ) unveiled a new subfamily of putative ATPases of approximately 45 - 50 kDa containing a single ATP binding site called ``P - loop'' domain ( CITATION ) . This ``TBP1 - like'' subfamily initially included the transcription activator <PROT1_155871>  Tat </PROT1_155871> - binding protein TBP7 ( CITATION ) , a modulator of human immunodeficiency <PROT1_155871>  Tat </PROT1_155871> - mediated transactivation MSS1 ( CITATION ) and the yeast protein SUG1 ( CITATION ) . More recently , this subfamily has been expanded to include components of the regulatory subunits of the 26 S protease . ||2194290_155871_5702==>Comparison of these sequences with those in current data bases showed that they exactly matched the sequence of a previously described human protein , <PROT2_5702>  TBP1 </PROT2_5702> ( human immunodeficiency virus <PROT1_155871>  Tat </PROT1_155871> - binding protein ) ( TARGET_CITATION ) ( fig. 6 ) . ||2194290_155871_5702==>These proteins include S4 ( CITATION ) , MSS1 ( CITATION , CITATION ) , TBP7 ( CITATION , CITATION ) , and p45 ( CITATION , CITATION ) . Although <PROT2_5702>  TBP1 </PROT2_5702> , a member of this family originally identified as a human immunodeficiency virus <PROT1_155871>  Tat </PROT1_155871> - binding protein ( TARGET_CITATION ) , has not been identified as a component of PA700 by direct sequencing , an antibody prepared against human <PROT2_5702>  TBP1 </PROT2_5702> was shown to cross - react with a 50 , 000 - Da subunit of PA700 from rat liver ( CITATION ) . We used this antibody to examine the bovine modulator and PA700 proteins . ||2194290_155871_5702==>Two modulator subunits , p50 and p42 , are homologous to one another and are members of a large protein family that contains a consensus sequence for ATP binding ( CITATION ) . The p50 subunit is identical to <PROT2_5702>  TBP1 </PROT2_5702> , previously identified as a human immunodeficiency virus <PROT1_155871>  Tat </PROT1_155871> - binding protein ( TARGET_CITATION ) , while p42 seems to be a new family member . ||2194290_155871_5702==>MSS1 ( mammalian suppressor of sgv1 ) is a modulator of <PROT1_155871>  Tat </PROT1_155871> - mediated transactivation ( CITATION ) , and <PROT2_5702>  TBP1 </PROT2_5702> ( <PROT1_155871>  Tat </PROT1_155871> - binding protein 1 ) was identified by direct protein - protein interaction with <PROT1_155871>  Tat </PROT1_155871> ( TARGET_CITATION ) and hence could be a potential <PROT1_155871>  Tat </PROT1_155871> - binding subunit of the 26 S proteasome . ||2194290_155871_5702==>pGEM - <PROT2_5702>  TBP1 </PROT2_5702> was constructed by inserting the <PROT1_155871>  TAT </PROT1_155871> - binding protein 1 cDNA ( TARGET_CITATION ) into the pGEM7Zf vector ( Promega ) . ||2194290_155871_5702==>Earlier - reported candidates for <PROT1_155871>  Tat </PROT1_155871> - interacting cofactors include <PROT2_5702>  TBP1 </PROT2_5702> ( TARGET_CITATION ) , TAP ( CITATION ) , Tip60 ( CITATION ) , and HT2A ( CITATION ) . ||2194290_155871_5702==><PROT2_5702>  TBP1 </PROT2_5702> has been reported to suppress <PROT1_155871>  tat </PROT1_155871> - mediated transactivation of HIV replication ( TARGET_CITATION ) . ||2194290_155871_5702==><PROT2_5702>  TBP1 </PROT2_5702> was originally described as a transcriptional factor of the HIV 1 by interaction with the <PROT1_155871>  tat </PROT1_155871> protein ( TARGET_CITATION , CITATION ) . ||2194290_155871_5702==><PROT2_5702>  TBP1 </PROT2_5702> binds the HIV <PROT1_155871>  tat </PROT1_155871> transactivator , suppressing its activity in cotransfection experiments ( TARGET_CITATION ) . ||7608968_155871_2959==><PROT1_155871>  Tat </PROT1_155871> has been reported to bind to numerous proteins associated with transcription initiation , including TFIID ( CITATION ; TARGET_CITATION ) , Sp1 ( CITATION ) , TFIIH ( CITATION ) , TAFII - 55 ( CITATION ) and <PROT2_2959>  TFIIB </PROT2_2959> ( CITATION ; TARGET_CITATION ) . ||7608968_155871_2959==><PROT1_155871>  Tat </PROT1_155871> - mediated regulation of cellular gene expression is strongly related to its physical and functional interaction with proteins directly involved in the basal transcriptional process including TFIID , <PROT2_2959>  TFIIB </PROT2_2959> ( TARGET_CITATION ) , and eukaryotic transcription factors such as Sp1 ( CITATION ) , and CAAT enhancer - binding protein ( ( 29 ) . ||7608968_155871_2959==>In addition , <PROT1_155871>  Tat </PROT1_155871> binds directly to several eukaryotic transcription factors including TFIID ( CITATION ) , <PROT2_2959>  TFIIB </PROT2_2959> ( TARGET_CITATION ) , TFIIH ( CITATION ) , Sp1 ( CITATION ) , NF - interleukin - 6 - CAAT enhancer - binding protein ( CITATION ) , and RNA polymerase II ( CITATION ) . ||7621073_155871_6667==>This possibility is supported by the reports showing that <PROT1_155871>  Tat </PROT1_155871> may associate with <PROT2_6667>  Sp1 </PROT2_6667> , TFIID factors , RNA polymerase II , and RNA polymerase II - associated factors ( CITATION , TARGET_CITATION ) . ||7621073_155871_6667==>However , <PROT1_155871>  Tat </PROT1_155871> inhibits the transcription of several <PROT2_6667>  Sp1 </PROT2_6667> - activated cellular promoters by acting directly or indirectly on <PROT2_6667>  Sp1 </PROT2_6667> or <PROT2_6667>  Sp1 </PROT2_6667> - like proteins bound to their specific binding sites in those promoters ( TARGET_CITATION ) . ||7621073_155871_6667==>Although the exact mechanism involved in <PROT1_155871>  Tat </PROT1_155871> - mediated repression of <PROT2_6667>  Sp1 </PROT2_6667> - containing promoters is not known , it has been suggested that <PROT1_155871>  Tat </PROT1_155871> binds to an <PROT2_6667>  Sp1 </PROT2_6667> - related factor , resulting in interference with the assembly of the transcription complexes ( TARGET_CITATION ) . ||8289393_155871_7528==>Initial experiments were performed as previously ( TARGET_CITATION , CITATION ) , demonstrating that cotransfection of <PROT2_7528>  YY1 </PROT2_7528> inhibited <PROT1_155871>  Tat </PROT1_155871> - activated , LTR - driven CAT expression . ||8419915_155871_5702==><PROT2_5702>  TBP1 </PROT2_5702> was originally described as a transcriptional factor of the HIV 1 by interaction with the <PROT1_155871>  tat </PROT1_155871> protein ( CITATION , TARGET_CITATION ) . ||8419915_155871_5702==>It has been reported that besides interacting with three ATPases , MSS1 , <PROT2_5702>  TBP1 </PROT2_5702> and TBP7 , which are highly homologous to the ATPases present in the 26S proteasome , <PROT1_155871>  Tat </PROT1_155871> is able to bind proteasomes and competes with PA28 for binding to proteasomes ( TARGET_CITATION ) . ||9560267_155807_983==>Remarkably , expression of a transdominant form of <PROT2_983>  Cdc2 </PROT2_983> , which also delays or arrests cell in G2 , increased expression of the HIV - 1 LTR to levels similar to those found after transactivation by <PROT1_155807>  Vpr </PROT1_155807> ( CITATION and TARGET_CITATION ) . ||9560267_155807_983==>Up - regulation of viral gene expression may involve binding of <PROT1_155807>  Vpr </PROT1_155807> to the cellular transcription factors SP1 and TFIIB ( CITATION , CITATION ) , as well as modulation of the transcriptional coactivator p300 through regulation of cyclin B1 / <PROT2_983>  cdc2 </PROT2_983> activity ( TARGET_CITATION ) . ||9560267_155807_983==><PROT1_155807>  Vpr </PROT1_155807> - induced G2 arrest , which results in dephosphorylation and inactivation of the <PROT2_983>  Cdc2 </PROT2_983> kinase , may be modulating the transcriptional coactivator p300 , which is involved in NF - { kappa } B and cyclin / <PROT2_983>  Cdc2 </PROT2_983> interaction ( TARGET_CITATION ) . 
interacts with====10082552_155871_2967==>One showed that <PROT1_155871>  Tat </PROT1_155871> transactivation was unaffected after immunodepletion of CAK under conditions that do not deplete <PROT2_2967>  TFIIH </PROT2_2967> ( TARGET_CITATION ) , and the other showed that <PROT2_2967>  TFIIH </PROT2_2967> was lost from the elongation complex , leaving only P - TEFb ( CITATION ) . ||10082552_155871_2967==>Although it has been reported that <PROT1_155871>  Tat </PROT1_155871> enhances CDK7 kinase activity ( CITATION , CITATION , CITATION ) , the role of <PROT2_2967>  TFIIH </PROT2_2967> in <PROT1_155871>  Tat </PROT1_155871> transactivation is controversial ( TARGET_CITATION ) . ||10082552_155871_2967==>Similarly , it has recently been reported that the CAK component of <PROT2_2967>  TFIIH </PROT2_2967> is not required for <PROT1_155871>  Tat </PROT1_155871> - activated transcription ( TARGET_CITATION ) . ||10082552_155871_2967==>Although <PROT2_2967>  TFIIH </PROT2_2967> is dispensable for <PROT1_155871>  Tat </PROT1_155871> function ( TARGET_CITATION ) , our previous results demonstrated that <PROT2_2967>  TFIIH </PROT2_2967> regulates <PROT1_155871>  Tat </PROT1_155871> - dependent CDK9 activation during HIV - 1 transcription ( CITATION ) . ||10082552_155871_2967==>Although the role of <PROT2_2967>  TFIIH </PROT2_2967> in <PROT1_155871>  Tat </PROT1_155871> transactivation is controversial ( TARGET_CITATION , CITATION , CITATION , CITATION ) , <PROT2_2967>  TFIIH </PROT2_2967> has been shown to regulate <PROT1_155871>  Tat </PROT1_155871> - dependent CDK9 activation during HIV - 1 transcription ( CITATION ) . ||10082552_155871_2968==>One showed that <PROT1_155871>  Tat </PROT1_155871> transactivation was unaffected after immunodepletion of CAK under conditions that do not deplete <PROT2_2968>  TFIIH </PROT2_2968> ( TARGET_CITATION ) , and the other showed that <PROT2_2968>  TFIIH </PROT2_2968> was lost from the elongation complex , leaving only P - TEFb ( CITATION ) . ||10082552_155871_2968==>Although it has been reported that <PROT1_155871>  Tat </PROT1_155871> enhances CDK7 kinase activity ( CITATION , CITATION , CITATION ) , the role of <PROT2_2968>  TFIIH </PROT2_2968> in <PROT1_155871>  Tat </PROT1_155871> transactivation is controversial ( TARGET_CITATION ) . ||10082552_155871_2968==>Similarly , it has recently been reported that the CAK component of <PROT2_2968>  TFIIH </PROT2_2968> is not required for <PROT1_155871>  Tat </PROT1_155871> - activated transcription ( TARGET_CITATION ) . ||10082552_155871_2968==>Although <PROT2_2968>  TFIIH </PROT2_2968> is dispensable for <PROT1_155871>  Tat </PROT1_155871> function ( TARGET_CITATION ) , our previous results demonstrated that <PROT2_2968>  TFIIH </PROT2_2968> regulates <PROT1_155871>  Tat </PROT1_155871> - dependent CDK9 activation during HIV - 1 transcription ( CITATION ) . ||10082552_155871_2968==>Although the role of <PROT2_2968>  TFIIH </PROT2_2968> in <PROT1_155871>  Tat </PROT1_155871> transactivation is controversial ( TARGET_CITATION , CITATION , CITATION , CITATION ) , <PROT2_2968>  TFIIH </PROT2_2968> has been shown to regulate <PROT1_155871>  Tat </PROT1_155871> - dependent CDK9 activation during HIV - 1 transcription ( CITATION ) . ||10213687_155348_7520==>Another indication that the retroviral <PROT1_155348>  integrase </PROT1_155348> might be toxic for the host cell comes from a report in which cells deficient for the DNA repair enzyme , DNA - dependent protein kinase , or components of the DNA - dependent protein kinase pathway , <PROT2_7520>  Ku86 </PROT2_7520> or XRCC4 , die through apoptosis upon infection with retroviral vectors ( TARGET_CITATION ) . ||10347184_155908_7514==>As the initially described <PROT1_155908>  Rev </PROT1_155908> & # 150 ; Rip1p two - hybrid interaction was compromised in a <PROT2_7514>  CRM1 </PROT2_7514> mutant background , it was proposed that Crm1p bridges this association ; Crm1p interacts with Rip1p in vitro ( TARGET_CITATION ) but also with many other FG - repeat domains in the two - hybrid assay ( CITATION ) . ||10347184_155908_7514==>Biochemical interaction studies suggest that RanGTP facilitates the specific interaction of the <PROT1_155908>  Rev </PROT1_155908> NES with <PROT2_7514>  CRM1 </PROT2_7514> ( CITATION , TARGET_CITATION , CITATION , CITATION ) , whereas GTP hydrolysis is apparently not required for the formation of the trimeric <PROT1_155908>  Rev </PROT1_155908> - RanGTP - <PROT2_7514>  CRM1 </PROT2_7514> complex ( CITATION ) or for NES - mediated nuclear protein export ( CITATION ) , its role in mediating RRE RNA export is less clear ( CITATION ) . ||10376933_155871_7124==>Expression of <PROT1_155871>  Tat </PROT1_155871> in cells by transient or stable transfection , as well as stimulation of cells with extracellular <PROT1_155871>  Tat </PROT1_155871> , has been demonstrated to have different effects , including inhibition of antigen - induced T - cell responsiveness ( CITATION ) , apoptosis ( CITATION ) , and induction of several cytokines or chemokines , such as tumor necrosis factor alpha ( <PROT2_7124>  TNF </PROT2_7124> - alpha ) , interleukin - 6 ( IL - 6 ) , IL - 8 , and monocyte chemoattractant protein 1 ( MCP - 1 ) ( CITATION , TARGET_CITATION , CITATION , CITATION , CITATION , CITATION , CITATION ) . ||10376933_155871_7124==>The cytokines and chemokines induced by <PROT1_155871>  Tat </PROT1_155871> include IL - 1beta , IL - 2 IL - 6 , IL - 8 , TGF - beta 1 , <PROT2_7124>  TNF </PROT2_7124> - alpha , and MCP - 1 ( CITATION , TARGET_CITATION , CITATION , CITATION , CITATION , CITATION ) . ||10376933_155871_7124==>For example , <PROT1_155871>  Tat </PROT1_155871> activates transforming growth factor - beta expression in human chondrocytes ( CITATION ) , <PROT2_7124>  TNF </PROT2_7124> expression in mononuclear cells ( CITATION ) , IL - 8 secretion in endothelial cells ( TARGET_CITATION ) , and T cell lines following CD3 - and CD28 - mediated co - stimulation ( CITATION ) . ||10773452_155348_6598==>Human <PROT2_6598>  Snf5 </PROT2_6598> was thus given the name <PROT1_155348>  integrase </PROT1_155348> interactor 1. Deletion of <PROT2_6598>  Snf5 </PROT2_6598> in yeast abolishes the lethal phenotype induced by expression of HIV - 1 <PROT1_155348>  integrase </PROT1_155348> ( TARGET_CITATION ) . ||12808466_155459_5694==>( TARGET_CITATION ) reported that point mutants such as E67Q , C100S , E259Q , and C291S showed the similarly impaired antiviral activity on { <PROT2_5694>  Delta </PROT2_5694> } <PROT1_155459>  vif </PROT1_155459> virion . ||12808466_155459_5694==>HuAPOBEC3G introduced more G - to - A hypermutation in the viral DNA of { <PROT2_5694>  Delta </PROT2_5694> } <PROT1_155459>  vif </PROT1_155459> virus than that of wild - type virus , as reported previously ( TARGET_CITATION , CITATION , CITATION ) ( fig. 1B , a and data not shown ) . ||12808466_155459_5694==>This process results in the hypermutation of viral sequences as well as in the premature degradation of viral cDNA ( TARGET_CITATION , CITATION ) , effects which together account for the profound loss of infectivity seen for { <PROT2_5694>  Delta </PROT2_5694> } <PROT1_155459>  vif </PROT1_155459> viruses . ||12808466_155459_5694==>Consistent with earlier findings from the use of similar experimental set - ups ( CITATION , TARGET_CITATION , CITATION , CITATION ) , hA3G was a potent inhibitor of all three { <PROT2_5694>  Delta </PROT2_5694> } <PROT1_155459>  vif </PROT1_155459> viruses ( fig. 2A ) . ||7592727_155807_6667==>Other recent studies indicate that the <PROT1_155807>  Vpr </PROT1_155807> protein can also associate with cellular proteins , such as glucocorticoid receptors ( CITATION ) , the transcription factors <PROT2_6667>  Sp1 </PROT2_6667> and TFIIB ( CITATION , TARGET_CITATION ) , or the uracil DNA glycosylase ( UDG ) enzyme involved in cellular DNA repair ( CITATION ) . ||7592727_155807_6667==>In previous studies , results from in vitro transcription assay suggested that <PROT1_155807>  Vpr </PROT1_155807> may induce transcription of the HIV - 1 promoter through the GC - rich motif of the LTR ( TARGET_CITATION ) , which serves as the binding site for the ubiquitous cellular transcription factor , <PROT2_6667>  Sp1 </PROT2_6667> ( CITATION ) , and that interaction of <PROT1_155807>  Vpr </PROT1_155807> and <PROT2_6667>  Sp1 </PROT2_6667> may be important for the observed activity ( TARGET_CITATION ) . ||7592727_155807_6667==>Several studies have shown that <PROT1_155807>  Vpr </PROT1_155807> can modestly transactivate the HIV long terminal repeat ( LTR ) and heterologous viral promoters in dividing cells ( CITATION , TARGET_CITATION ) , and this may be achieved by direct binding to transcription factors TFIIB and <PROT2_6667>  Sp1 </PROT2_6667> ( CITATION , TARGET_CITATION ) . ||7592727_155807_6667==>Indeed , <PROT1_155807>  Vpr </PROT1_155807> was shown to directly bind to the general transcription factors TF - IIB and <PROT2_6667>  Sp1 </PROT2_6667> in in vitro assays ( CITATION , TARGET_CITATION ) . ||7592727_155807_6667==>Earlier reports also demonstrated that <PROT1_155807>  Vpr </PROT1_155807> can interact with the cellular transcription factors <PROT2_6667>  Sp1 </PROT2_6667> ( TARGET_CITATION ) and TFIIB ( CITATION ) . ||7592727_155807_6667==>Up - regulation of viral gene expression may involve binding of <PROT1_155807>  Vpr </PROT1_155807> to the cellular transcription factors <PROT2_6667>  SP1 </PROT2_6667> and TFIIB ( CITATION , TARGET_CITATION ) , as well as modulation of the transcriptional coactivator p300 through regulation of cyclin B1 / cdc2 activity ( CITATION ) . ||7592727_155807_6667==>Previous reports have indicated that an interaction between <PROT1_155807>  Vpr </PROT1_155807> and the <PROT2_6667>  Sp1 </PROT2_6667> transcription factor is required for <PROT1_155807>  Vpr </PROT1_155807> - mediated transcriptional enhancement of the HIV - 1 LTR ( CITATION , CITATION , TARGET_CITATION ) . ||7592727_155807_6667==>The activation of the HIV - 1 LTR by <PROT1_155807>  Vpr </PROT1_155807> was shown to be mediated , at least in part , through its direct interaction with the cellular transcription factor <PROT2_6667>  Sp1 </PROT2_6667> , which binds to the GC - rich DNA sequence of LTR ( CITATION , TARGET_CITATION ) . ||7592727_155807_6667==>However , it has been suggested that <PROT1_155807>  Vpr </PROT1_155807> may increase transcription by physically interacting with various host factors , such as <PROT2_6667>  Sp1 </PROT2_6667> ( TARGET_CITATION ) , TFIIB ( CITATION , CITATION , CITATION ) , or an activated glucocorticoid receptor ( GR ) ( CITATION , CITATION ) . ||7592727_155807_6667==>It was first shown that the ability of <PROT1_155807>  Vpr </PROT1_155807> to stimulate HIV LTR transcription was mediated mainly through physical interaction between <PROT1_155807>  Vpr </PROT1_155807> and <PROT2_6667>  Sp1 </PROT2_6667> bound to the HIV LTR ( TARGET_CITATION ) . ||7592727_155807_6667==><PROT1_155807>  Vpr </PROT1_155807> transactivation of HIV - 1 is mediated through cis - acting elements within the viral LTR , including NF - { kappa } B , <PROT2_6667>  Sp1 </PROT2_6667> , and GRE enhancer sequences , and <PROT1_155807>  Vpr </PROT1_155807> binds to <PROT2_6667>  Sp1 </PROT2_6667> , TF - IIB , and cyclin T1 in vitro ( CITATION , CITATION , CITATION , TARGET_CITATION ) . ||7592727_155807_6667==>Studies have reported an in vitro association between <PROT1_155807>  Vpr </PROT1_155807> and transcription factors including <PROT2_6667>  Sp1 </PROT2_6667> , TFIIB , and cyclin T1 ( CITATION , CITATION , TARGET_CITATION ) . ||7592727_155807_6667==>In addition to cellular factors , the partnership of <PROT2_6667>  Sp1 </PROT2_6667> with several viral regulatory proteins ( CITATION ) including the HIV - 11 accessory protein , <PROT1_155807>  Vpr </PROT1_155807> , may have a functional consequence on viral and cellular gene expression ( CITATION , TARGET_CITATION ) . ||8419915_155871_5704==>It has been reported that besides interacting with three ATPases , MSS1 , TBP1 and <PROT2_5704>  TBP7 </PROT2_5704> , which are highly homologous to the ATPases present in the 26S proteasome , <PROT1_155871>  Tat </PROT1_155871> is able to bind proteasomes and competes with PA28 for binding to proteasomes ( TARGET_CITATION ) . ||8849451_155871_2962==>HeLa total cell extracts ( 250 & # 181 ; g ) were used for immunoprecipitation with antibodies against preimmune ( PI , lane 2 ) , <PROT1_155871>  Tat </PROT1_155871> - SF1 ( lane 3 ) ( TARGET_CITATION ) , and RAP30 of <PROT2_2962>  TFIIF </PROT2_2962> ( lane 4 ) ( Santa Cruz Biotechnology ) . ||8849451_155871_2962==>Among these are <PROT1_155871>  Tat </PROT1_155871> - SF1 ( CITATION , TARGET_CITATION ) , the human homologue of the Saccharomyces cerevisiae transcription factor SPT5 ( CITATION ) , TFIIH ( CITATION , CITATION , CITATION ) , <PROT2_2962>  TFIIF </PROT2_2962> ( CITATION ) , and a <PROT1_155871>  Tat </PROT1_155871> - associated histone acetyltransferase ( reference CITATION and references therein ) . ||8849451_155871_2962==><PROT1_155871>  Tat </PROT1_155871> - SF1 interacts with <PROT1_155871>  Tat </PROT1_155871> ( TARGET_CITATION ) and P - TEFb ( CITATION ) and was recently found to interact with human SPT5 , Pol II , and the RAP30 subunit of <PROT2_2962>  TFIIF </PROT2_2962> as well ( CITATION ) . ||8849451_155871_2963==>HeLa total cell extracts ( 250 & # 181 ; g ) were used for immunoprecipitation with antibodies against preimmune ( PI , lane 2 ) , <PROT1_155871>  Tat </PROT1_155871> - SF1 ( lane 3 ) ( TARGET_CITATION ) , and <PROT2_2963>  RAP30 </PROT2_2963> of <PROT2_2963>  TFIIF </PROT2_2963> ( lane 4 ) ( Santa Cruz Biotechnology ) . ||8849451_155871_2963==>Among these are <PROT1_155871>  Tat </PROT1_155871> - SF1 ( CITATION , TARGET_CITATION ) , the human homologue of the Saccharomyces cerevisiae transcription factor SPT5 ( CITATION ) , TFIIH ( CITATION , CITATION , CITATION ) , <PROT2_2963>  TFIIF </PROT2_2963> ( CITATION ) , and a <PROT1_155871>  Tat </PROT1_155871> - associated histone acetyltransferase ( reference CITATION and references therein ) . ||8849451_155871_2963==><PROT1_155871>  Tat </PROT1_155871> - SF1 interacts with <PROT1_155871>  Tat </PROT1_155871> ( TARGET_CITATION ) and P - TEFb ( CITATION ) and was recently found to interact with human SPT5 , Pol II , and the <PROT2_2963>  RAP30 </PROT2_2963> subunit of <PROT2_2963>  TFIIF </PROT2_2963> as well ( CITATION ) . ||8849451_155871_2966==>A number of cellular factors have been proposed to interact with the <PROT1_155871>  Tat </PROT1_155871> activation domain and mediate transcription function , including TATA box - binding protein ( CITATION ) , <PROT1_155871>  Tat </PROT1_155871> - SF1 ( TARGET_CITATION ) , <PROT2_2966>  TFIIH </PROT2_2966> ( CITATION , CITATION ) , and a cellular protein kinase activity termed TAK ( <PROT1_155871>  Tat </PROT1_155871> - associated kinase ) ( CITATION , CITATION ) . ||8849451_155871_2966==>In the case of <PROT1_155871>  Tat </PROT1_155871> , it is likely that effects on promoter clearance by modulating the activity of <PROT2_2966>  TFIIH </PROT2_2966> ( CITATION , CITATION , CITATION , CITATION and CITATION ) and potential modulation of the activity of <PROT1_155871>  Tat </PROT1_155871> stimulatory factors ( CITATION and TARGET_CITATION ) are both important for <PROT1_155871>  Tat </PROT1_155871> activation . ||8849451_155871_2966==>Based on biochemical observations , a number of cofactors that could constitute the cellular interface to <PROT1_155871>  Tat </PROT1_155871> function have been proposed , including cellular general transcription factor <PROT2_2966>  TFIIH </PROT2_2966> ( CITATION , CITATION ) , <PROT1_155871>  Tat </PROT1_155871> - SF1 ( TARGET_CITATION ) , and <PROT1_155871>  Tat </PROT1_155871> - associated kinase ( TAK ) ( CITATION , CITATION , CITATION ) . ||8849451_155871_2966==>Among these are the <PROT1_155871>  Tat </PROT1_155871> specific factor <PROT1_155871>  Tat </PROT1_155871> - SF1 ( TARGET_CITATION ) , the positive transcription elongation factor P - TEFb ( CITATION , CITATION , CITATION , CITATION ) , and <PROT2_2966>  TFIIH </PROT2_2966> ( reference CITATION and references therein ; see also references CITATION , CITATION , and CITATION ) . ||8849451_155871_2966==>In addition to hCycT1 , a number of other potential <PROT1_155871>  Tat </PROT1_155871> interacting proteins and / or cofactors have been proposed including MSS1 , HT2A , CA150 , TFIID , <PROT1_155871>  Tat </PROT1_155871> - SF1 , TIP30 , and PolII itself ( CITATION - TARGET_CITATION ) , and other groups have reported interactions between <PROT1_155871>  Tat </PROT1_155871> and the coactivator proteins p300 and CREB - binding protein ( CBP ) ( CITATION ) or between <PROT1_155871>  Tat </PROT1_155871> and the transcription factor <PROT2_2966>  TFIIH </PROT2_2966> ( CITATION , CITATION , CITATION , CITATION ) . ||8849451_155871_2966==>Among these are <PROT1_155871>  Tat </PROT1_155871> - SF1 ( CITATION , TARGET_CITATION ) , the human homologue of the Saccharomyces cerevisiae transcription factor SPT5 ( CITATION ) , <PROT2_2966>  TFIIH </PROT2_2966> ( CITATION , CITATION , CITATION ) , TFIIF ( CITATION ) , and a <PROT1_155871>  Tat </PROT1_155871> - associated histone acetyltransferase ( reference CITATION and references therein ) . ||8849451_155871_2966==>In carrying out its transcriptional activation , <PROT1_155871>  Tat </PROT1_155871> interacts with a number of cellular transcription factors , including TFIID ( TBP ) ( CITATION , CITATION , CITATION ) , TFIID - associated TAFs ( CITATION ) , <PROT2_2966>  TFIIH </PROT2_2966> ( CITATION ) , <PROT1_155871>  Tat </PROT1_155871> - associated protein ( TAP ) ( CITATION ) , <PROT1_155871>  Tat </PROT1_155871> - associated kinase ( TAK / pTEFb ) ( CITATION , CITATION , CITATION , CITATION , CITATION , CITATION ) , TTK ( CITATION ) , Sp1 ( CITATION ) , and SF - 1 ( TARGET_CITATION ) , as well as TAR RNA ( CITATION ) . ||8849451_155871_2966==>In addition to NF - { kappa } B a variety of other factors bind to the HIV - 1 LTR , including SP1 ( CITATION , CITATION , CITATION ) , TBP ( CITATION , CITATION ) , LEF ( CITATION ) , and LBP ( CITATION , CITATION ) , and other factors that have been identified to interact with these proteins , including CBP / p300 ( CITATION , CITATION ) , <PROT1_155871>  Tat </PROT1_155871> - SF1 ( CITATION , CITATION , CITATION , TARGET_CITATION ) , and <PROT2_2966>  TFIIH </PROT2_2966> ( CITATION , CITATION ) . ||9111043_155908_7514==><PROT2_7514>  CRM1 </PROT2_7514> shows homology to importin small beta , Greek - like transport factors and has recently been reported to bind the <PROT1_155908>  Rev </PROT1_155908> NES motif together with Ran GTPase ( CITATION , CITATION , CITATION , CITATION and TARGET_CITATION ) . ||9111043_155908_7514==>Zolotukhin and Felber ( TARGET_CITATION ) found that the ectopic expression of the <PROT1_155908>  Rev </PROT1_155908> protein ( which contains an NES ) can lead to the nuclear accumulation of RanBP1 , presumably by competing for <PROT2_7514>  Crm1 </PROT2_7514> . ||9121429_1022_155871==>( this issue ) TARGET_CITATION find that <PROT2_155871>  Tat </PROT2_155871> binds directly and specifically to recombinant <PROT1_1022>  CDK7 </PROT1_1022> through its transactivation domain . ||9121429_1022_155871==>In the holoenzyme complex , <PROT2_155871>  Tat </PROT2_155871> interacts with TFIIH and activates <PROT1_1022>  CDK7 </PROT1_1022> , a cyclin H - dependent serine / threonine kinase that is known to phosphorylate the C - terminal domain ( CTD ) of RNAPII ( TARGET_CITATION , CITATION , CITATION , CITATION ) . ||9121429_1022_155871==>Transcription activators may remove hnRNP U and / or provide signals to inactivate hnRNP u. It remains to be determined whether a transcription factor such as <PROT2_155871>  Tat </PROT2_155871> , retinoic acid receptor alpha , p53 , or VP16 that binds TFIIH or <PROT1_1022>  Cdk7 </PROT1_1022> ( CITATION , TARGET_CITATION , CITATION , CITATION , CITATION , CITATION ) would affect the interaction of hnRNP U with TFIIH in vivo . ||9121429_1022_155871==>These proteins include the core RNA polymerase II ( TARGET_CITATION , CITATION ) , whose C - terminal domain is required for <PROT2_155871>  Tat </PROT2_155871> - mediated transactivation ( CITATION ) , TAFII55 ( CITATION ) , and TFIIH and <PROT1_1022>  CDK7 </PROT1_1022> ( CITATION , CITATION ) . ||9121429_1022_155871==>More significant to our purpose , <PROT2_155871>  Tat </PROT2_155871> mediates the recruitment of <PROT1_1022>  CDK7 </PROT1_1022> ( TARGET_CITATION ) and CDK9 / PITALRE ( CITATION , CITATION , CITATION ) which phosphorylate the CTD and promote an efficient transcription elongation throughout the entire viral genome . ||9121429_1022_155871==><PROT2_155871>  Tat </PROT2_155871> directly interacts with at least two cellular kinases , <PROT1_1022>  CDK7 </PROT1_1022> ( TARGET_CITATION , CITATION ) and CDK9 ( CITATION , CITATION ) , to stimulate hyperphosphorylation of the C - terminal domain of RNA polymerase II and increase the processivity of the elongating transcription complexes . ||9121429_1022_155871==>The fact that overexpression of the cyclin - dependent kinases CDK9 and <PROT1_1022>  CDK7 </PROT1_1022> , which are believed to be essential for <PROT2_155871>  Tat </PROT2_155871> - mediated transcriptional activation ( TARGET_CITATION , CITATION , CITATION , CITATION , CITATION ) , had no effect on the process of reverse transcription further serves to distinguish the role of <PROT2_155871>  Tat </PROT2_155871> in reverse transcription from its role in transcription . ||9121429_155871_2966==>Recent studies have implicated CAK / <PROT2_2966>  TFIIH </PROT2_2966> as a coactivator for <PROT1_155871>  Tat </PROT1_155871> ( Parada and Roeder 1996 CITATION ; Cujec et al. , this issue TARGET_CITATION ; Garcia - Martinez et al. 1997b CITATION ) . ||9121429_155871_2966==>First , affinity - purified fractions of <PROT2_2966>  TFIIH </PROT2_2966> ( or recombinant CAK ) predominantly induce a low extent of CTD phosphorylation rather than hyperphosphorylation of the CTD , even in the presence of <PROT1_155871>  Tat </PROT1_155871> ( Parada and Roeder 1996 CITATION ; Cujec et al. , this issue TARGET_CITATION ) . ||9121429_155871_2966==>The association of <PROT1_155871>  Tat </PROT1_155871> with <PROT2_2966>  TFIIH </PROT2_2966> could explain why <PROT1_155871>  Tat </PROT1_155871> is associated with the RNAPII holoenzyme and preinitiation complexes prior to transcription ( Cujec et al. 1997a TARGET_CITATION ; Garcia - Martinez et al. 1997a CITATION ) , and could underlie the modest effects of <PROT1_155871>  Tat </PROT1_155871> on transcription initiation that have been observed in several systems . ||9121429_155871_2966==>Furthermore , our finding that <PROT1_155871>  Tat </PROT1_155871> can bind to <PROT2_2966>  TFIIH </PROT2_2966> in vivo is consistent with our previous results demonstrating an interaction between <PROT1_155871>  Tat </PROT1_155871> and the Pol II holoenzyme ( Cujec et al. 1997 TARGET_CITATION ) . ||9121429_155871_2966==>For optimal transactivation of HIV gene expression , <PROT1_155871>  Tat </PROT1_155871> requires specific upstream transcription factors , including Sp1 ( CITATION ) , TATA binding protein ( CITATION , CITATION ) , <PROT1_155871>  Tat </PROT1_155871> - associated kinase ( TAK ) ( CITATION , CITATION ) , <PROT2_2966>  TFIIH </PROT2_2966> ( CITATION , CITATION ) , Tip ( CITATION ) , and RNA polymerase II ( TARGET_CITATION , CITATION , CITATION ) . ||9121429_155871_2966==>These proteins include TATA - binding protein ( TBP ) ( CITATION , CITATION ) , TAK ( CITATION , CITATION ) , PKR ( CITATION , CITATION ) , T3R ( CITATION ) , <PROT1_155871>  Tat </PROT1_155871> - binding protein 1 ( CITATION , CITATION ) , TAP ( CITATION , CITATION , CITATION ) , TBP - associated factor TAF55 ( CITATION ) , HT2A ( CITATION ) , Tip60 ( CITATION ) , <PROT2_2966>  TFIIH </PROT2_2966> ( CITATION , CITATION ) , RNA polymerase II ( CITATION ) , and Sp1 ( TARGET_CITATION , CITATION ) . ||9121429_155871_2966==>In the holoenzyme complex , <PROT1_155871>  Tat </PROT1_155871> interacts with <PROT2_2966>  TFIIH </PROT2_2966> and activates CDK7 , a cyclin H - dependent serine / threonine kinase that is known to phosphorylate the C - terminal domain ( CTD ) of RNAPII ( TARGET_CITATION , CITATION , CITATION , CITATION ) . ||9121429_155871_2966==>The finding that RNA Pol II holoenzyme - containing fractions can not complement the assay system for <PROT1_155871>  Tat </PROT1_155871> function ( Figures 1C and 8B ; data not shown ) is further substantiated by the observation that although a highly purified RNA Pol II holoenzyme interacts with <PROT1_155871>  Tat </PROT1_155871> , in this case probably via interactions with <PROT2_2966>  TFIIH </PROT2_2966> ( TARGET_CITATION ) , it fails to support <PROT1_155871>  Tat </PROT1_155871> function because of an apparent lack of <PROT1_155871>  Tat </PROT1_155871> cofactors . ||9121429_155871_2966==>Transcription activators may remove hnRNP U and / or provide signals to inactivate hnRNP u. It remains to be determined whether a transcription factor such as <PROT1_155871>  Tat </PROT1_155871> , retinoic acid receptor alpha , p53 , or VP16 that binds <PROT2_2966>  TFIIH </PROT2_2966> or Cdk7 ( CITATION , TARGET_CITATION , CITATION , CITATION , CITATION , CITATION ) would affect the interaction of hnRNP U with <PROT2_2966>  TFIIH </PROT2_2966> in vivo . ||9121429_155871_2966==>These proteins include the core RNA polymerase II ( TARGET_CITATION , CITATION ) , whose C - terminal domain is required for <PROT1_155871>  Tat </PROT1_155871> - mediated transactivation ( CITATION ) , TAFII55 ( CITATION ) , and <PROT2_2966>  TFIIH </PROT2_2966> and CDK7 ( CITATION , CITATION ) . ||9121429_155871_2966==><PROT1_155871>  Tat </PROT1_155871> stimulation of <PROT2_2966>  TFIIH </PROT2_2966> phosphorylation of the RNA polymerase II CTD may be responsible for the modest <PROT1_155871>  tat </PROT1_155871> stimulation seen in rodent cells ( CITATION , TARGET_CITATION , CITATION and CITATION ) . ||9121429_155871_2966==>In addition to hCycT1 , a number of other potential <PROT1_155871>  Tat </PROT1_155871> interacting proteins and / or cofactors have been proposed including MSS1 , HT2A , CA150 , TFIID , <PROT1_155871>  Tat </PROT1_155871> - SF1 , TIP30 , and PolII itself ( CITATION - TARGET_CITATION ) , and other groups have reported interactions between <PROT1_155871>  Tat </PROT1_155871> and the coactivator proteins p300 and CREB - binding protein ( CBP ) ( CITATION ) or between <PROT1_155871>  Tat </PROT1_155871> and the transcription factor <PROT2_2966>  TFIIH </PROT2_2966> ( CITATION , CITATION , CITATION , CITATION ) . ||9121429_155871_2968==>Recent studies have implicated CAK / <PROT2_2968>  TFIIH </PROT2_2968> as a coactivator for <PROT1_155871>  Tat </PROT1_155871> ( Parada and Roeder 1996 CITATION ; Cujec et al. , this issue TARGET_CITATION ; Garcia - Martinez et al. 1997b CITATION ) . ||9121429_155871_2968==>First , affinity - purified fractions of <PROT2_2968>  TFIIH </PROT2_2968> ( or recombinant CAK ) predominantly induce a low extent of CTD phosphorylation rather than hyperphosphorylation of the CTD , even in the presence of <PROT1_155871>  Tat </PROT1_155871> ( Parada and Roeder 1996 CITATION ; Cujec et al. , this issue TARGET_CITATION ) . ||9121429_155871_2968==>The association of <PROT1_155871>  Tat </PROT1_155871> with <PROT2_2968>  TFIIH </PROT2_2968> could explain why <PROT1_155871>  Tat </PROT1_155871> is associated with the RNAPII holoenzyme and preinitiation complexes prior to transcription ( Cujec et al. 1997a TARGET_CITATION ; Garcia - Martinez et al. 1997a CITATION ) , and could underlie the modest effects of <PROT1_155871>  Tat </PROT1_155871> on transcription initiation that have been observed in several systems . ||9121429_155871_2968==>Furthermore , our finding that <PROT1_155871>  Tat </PROT1_155871> can bind to <PROT2_2968>  TFIIH </PROT2_2968> in vivo is consistent with our previous results demonstrating an interaction between <PROT1_155871>  Tat </PROT1_155871> and the Pol II holoenzyme ( Cujec et al. 1997 TARGET_CITATION ) . ||9121429_155871_2968==>For optimal transactivation of HIV gene expression , <PROT1_155871>  Tat </PROT1_155871> requires specific upstream transcription factors , including Sp1 ( CITATION ) , TATA binding protein ( CITATION , CITATION ) , <PROT1_155871>  Tat </PROT1_155871> - associated kinase ( TAK ) ( CITATION , CITATION ) , <PROT2_2968>  TFIIH </PROT2_2968> ( CITATION , CITATION ) , Tip ( CITATION ) , and RNA polymerase II ( TARGET_CITATION , CITATION , CITATION ) . ||9121429_155871_2968==>These proteins include TATA - binding protein ( TBP ) ( CITATION , CITATION ) , TAK ( CITATION , CITATION ) , PKR ( CITATION , CITATION ) , T3R ( CITATION ) , <PROT1_155871>  Tat </PROT1_155871> - binding protein 1 ( CITATION , CITATION ) , TAP ( CITATION , CITATION , CITATION ) , TBP - associated factor TAF55 ( CITATION ) , HT2A ( CITATION ) , Tip60 ( CITATION ) , <PROT2_2968>  TFIIH </PROT2_2968> ( CITATION , CITATION ) , RNA polymerase II ( CITATION ) , and Sp1 ( TARGET_CITATION , CITATION ) . ||9121429_155871_2968==>In the holoenzyme complex , <PROT1_155871>  Tat </PROT1_155871> interacts with <PROT2_2968>  TFIIH </PROT2_2968> and activates CDK7 , a cyclin H - dependent serine / threonine kinase that is known to phosphorylate the C - terminal domain ( CTD ) of RNAPII ( TARGET_CITATION , CITATION , CITATION , CITATION ) . ||9121429_155871_2968==>The finding that RNA Pol II holoenzyme - containing fractions can not complement the assay system for <PROT1_155871>  Tat </PROT1_155871> function ( Figures 1C and 8B ; data not shown ) is further substantiated by the observation that although a highly purified RNA Pol II holoenzyme interacts with <PROT1_155871>  Tat </PROT1_155871> , in this case probably via interactions with <PROT2_2968>  TFIIH </PROT2_2968> ( TARGET_CITATION ) , it fails to support <PROT1_155871>  Tat </PROT1_155871> function because of an apparent lack of <PROT1_155871>  Tat </PROT1_155871> cofactors . ||9121429_155871_2968==>Transcription activators may remove hnRNP U and / or provide signals to inactivate hnRNP u. It remains to be determined whether a transcription factor such as <PROT1_155871>  Tat </PROT1_155871> , retinoic acid receptor alpha , p53 , or VP16 that binds <PROT2_2968>  TFIIH </PROT2_2968> or Cdk7 ( CITATION , TARGET_CITATION , CITATION , CITATION , CITATION , CITATION ) would affect the interaction of hnRNP U with <PROT2_2968>  TFIIH </PROT2_2968> in vivo . ||9121429_155871_2968==>These proteins include the core RNA polymerase II ( TARGET_CITATION , CITATION ) , whose C - terminal domain is required for <PROT1_155871>  Tat </PROT1_155871> - mediated transactivation ( CITATION ) , TAFII55 ( CITATION ) , and <PROT2_2968>  TFIIH </PROT2_2968> and CDK7 ( CITATION , CITATION ) . ||9121429_155871_2968==><PROT1_155871>  Tat </PROT1_155871> stimulation of <PROT2_2968>  TFIIH </PROT2_2968> phosphorylation of the RNA polymerase II CTD may be responsible for the modest <PROT1_155871>  tat </PROT1_155871> stimulation seen in rodent cells ( CITATION , TARGET_CITATION , CITATION and CITATION ) . ||9121429_155871_2968==>In addition to hCycT1 , a number of other potential <PROT1_155871>  Tat </PROT1_155871> interacting proteins and / or cofactors have been proposed including MSS1 , HT2A , CA150 , TFIID , <PROT1_155871>  Tat </PROT1_155871> - SF1 , TIP30 , and PolII itself ( CITATION - TARGET_CITATION ) , and other groups have reported interactions between <PROT1_155871>  Tat </PROT1_155871> and the coactivator proteins p300 and CREB - binding protein ( CBP ) ( CITATION ) or between <PROT1_155871>  Tat </PROT1_155871> and the transcription factor <PROT2_2968>  TFIIH </PROT2_2968> ( CITATION , CITATION , CITATION , CITATION ) . ||9121429_155871_6908==>These functional requirements correlate with physical interactions of <PROT1_155871>  Tat </PROT1_155871> with Sp1 , <PROT2_6908>  TBP </PROT2_6908> , and RNA polymerase II ( CITATION , TARGET_CITATION ) . ||9121429_155871_6908==>This is consistent with the mechanism by which <PROT1_155871>  Tat </PROT1_155871> represses MHC class I transcription and with its known ability to bind to components of the transcription machinery , including TAFII250 , TAFII55 , <PROT2_6908>  TBP </PROT2_6908> and RNA polymerase II ( TARGET_CITATION ; CITATION ; CITATION ; CITATION ; CITATION ) . ||9121429_155871_6908==>These proteins include TATA - binding protein ( <PROT2_6908>  TBP </PROT2_6908> ) ( CITATION , CITATION ) , TAK ( CITATION , CITATION ) , PKR ( CITATION , CITATION ) , T3R ( CITATION ) , <PROT1_155871>  Tat </PROT1_155871> - binding protein 1 ( CITATION , CITATION ) , TAP ( CITATION , CITATION , CITATION ) , <PROT2_6908>  TBP </PROT2_6908> - associated factor TAF55 ( CITATION ) , HT2A ( CITATION ) , Tip60 ( CITATION ) , TFIIH ( CITATION , CITATION ) , RNA polymerase II ( CITATION ) , and Sp1 ( TARGET_CITATION , CITATION ) . ||9121429_155871_6908==>The C - terminal domain of <PROT1_155871>  Tat </PROT1_155871> was used as bait to avoid isolating any of the cellular factors ( including <PROT2_6908>  TBP </PROT2_6908> , RNA polymerase II , and TAFII55 ) known to bind the N - terminal transactivation domain of <PROT1_155871>  Tat </PROT1_155871> ( CITATION - TARGET_CITATION ) . ||9121429_155871_6908==><PROT1_155871>  Tat </PROT1_155871> has been shown to interact with a variety of other components of the preinitiation complex ( including <PROT2_6908>  TBP </PROT2_6908> , TAFII55 , RNA polymerase II ) as well as multiple other cellular factors ( CITATION - TARGET_CITATION , CITATION ) . ||9121429_155871_6908==>In addition to hCycT1 , a number of other potential <PROT1_155871>  Tat </PROT1_155871> interacting proteins and / or cofactors have been proposed including MSS1 , HT2A , CA150 , <PROT2_6908>  TFIID </PROT2_6908> , <PROT1_155871>  Tat </PROT1_155871> - SF1 , TIP30 , and PolII itself ( CITATION - TARGET_CITATION ) , and other groups have reported interactions between <PROT1_155871>  Tat </PROT1_155871> and the coactivator proteins p300 and CREB - binding protein ( CBP ) ( CITATION ) or between <PROT1_155871>  Tat </PROT1_155871> and the transcription factor TFIIH ( CITATION , CITATION , CITATION , CITATION ) . ||9121429_155871_6908==>For example , <PROT1_155871>  Tat </PROT1_155871> is found complexed to components of the basal machinery such as the core RNA polymerase II itself ( CITATION , TARGET_CITATION ) , the TATA - binding protein ( <PROT2_6908>  TBP </PROT2_6908> ) subunit of <PROT2_6908>  TFIID </PROT2_6908> ( CITATION , CITATION ) , and the <PROT2_6908>  TFIID </PROT2_6908> associated factor , TAFII55 ( CITATION ) . ||9121429_155871_6908==>Other components of the RNAPII transcription machinery that interact with <PROT1_155871>  Tat </PROT1_155871> include <PROT2_6908>  TBP </PROT2_6908> , RNAPII , <PROT1_155871>  Tat </PROT1_155871> - SF1 and PC4 ( CITATION , CITATION , CITATION , CITATION and TARGET_CITATION ) . ||9121429_155871_9412==>Consistent with an earlier report ( TARGET_CITATION ) that <PROT1_155871>  Tat </PROT1_155871> can associate with an RNA polymerase II holoenzyme complex ( CITATION , CITATION ) , RNA polymerase II ( RPB1 subunit ) and <PROT2_9412>  SRB7 </PROT2_9412> ( a component diagnostic of yeast and mammalian holoenzyme ; ref. CITATION ) , but not RNA polymerase III ( RPC53 subunit ) , were also coimmunoprecipitated with <PROT1_155871>  Tat </PROT1_155871> ( fig. 2e ) . ||9129655_155030_5478==>In this regard , an interaction between HIV - 1 <PROT1_155030>  Gag </PROT1_155030> and cyclophilin has been detected by using several approaches , including a yeast - based , two - hybrid system ( CITATION ) ; the incorporation of cyclophilin A ( <PROT2_5478>  CyPA </PROT2_5478> ) has also been demonstrated in the virus particles ( CITATION - TARGET_CITATION ) . ||9129655_155030_5478==>The immunosuppressive drug cyclosporin ( CsA ) also binds to the hydrophobic pocket of <PROT2_5478>  CypA </PROT2_5478> and competitively inhibits <PROT2_5478>  CypA </PROT2_5478> - CA / <PROT1_155030>  Gag </PROT1_155030> interactions ( CITATION , CITATION , CITATION , CITATION , CITATION , TARGET_CITATION ) . ||9129655_155030_5478==>Disruption of the cyclophilin A / <PROT1_155030>  capsid </PROT1_155030> interaction by either cyclosporin ( CITATION , CITATION and CITATION ) , its analogs ( TARGET_CITATION and CITATION ) , mutagenesis of residues in the binding site ( CITATION , CITATION , CITATION and CITATION ) or deletion of the <PROT2_5478>  CypA </PROT2_5478> gene ( CITATION ) blocks <PROT2_5478>  CypA </PROT2_5478> incorporation into virions and greatly reduces HIV - 1 viral replication and therefore infectivity . ||9292011_155871_6908==>Although <PROT1_155871>  Tat </PROT1_155871> is not a typical activator protein , it does possess the capacity to bind directly several general transcription factors including <PROT2_6908>  TFIID </PROT2_6908> ( CITATION ) , TFIIB ( TARGET_CITATION ) , TFIIH ( CITATION ) , and RNAP II ( CITATION ) . ||9292011_155871_6908==>Both specific and basal cellular transcription factors are key in <PROT1_155871>  Tat </PROT1_155871> - mediated transactivation of virus gene expression , including Sp1 ( CITATION ) , <PROT2_6908>  TBP </PROT2_6908> ( CITATION ) and TAFII 55 ( CITATION ) , TAP , ( CITATION , TARGET_CITATION ) , the kinases TAKs ( CITATION ) , and NF - { kappa } B ( CITATION ) . ||9388268_155871_6667==>Deletion or mutation of the <PROT2_6667>  Sp1 </PROT2_6667> sites in the truncated - 40 / + 80 LTR , in - 283 / + 80 LTRSp1mut , or in - 283 / + 80Delta kappa B / <PROT2_6667>  Sp1 </PROT2_6667> led to the abolition of COUP - TF - induced activation ( fig. 5 , lanes 10 , 14 , and 18 ) because <PROT2_6667>  Sp1 </PROT2_6667> is required for COUP - TF transactivation in microglial cells ( TARGET_CITATION ) ; similarly , <PROT1_155871>  Tat </PROT1_155871> - induced activation was abolished ( lanes 11 , 15 , and 19 ) , because <PROT2_6667>  Sp1 </PROT2_6667> is required for efficient <PROT1_155871>  Tat </PROT1_155871> transactivation ( CITATION , CITATION ) . ||9388268_155871_6667==>However , we show here that a synergistic effect between COUP - TF and <PROT1_155871>  Tat </PROT1_155871> is observed only in the absence of <PROT2_6667>  Sp1 </PROT2_6667> sites , whereas the <PROT2_6667>  Sp1 </PROT2_6667> and COUP - TF proteins lead to a synergistic transcriptional increase in the presence of <PROT2_6667>  Sp1 </PROT2_6667> sites ( TARGET_CITATION ) . ||9388268_155871_6667==>We have previously reported that <PROT1_155871>  Tat </PROT1_155871> also interacts and cooperates with an orphan member of the nuclear receptor superfamily , the chicken ovalbumin upstream promoter transcription factor ( COUP - TF ) that together with <PROT2_6667>  Sp1 </PROT2_6667> activates HIV - 1 LTR - driven transcription ( TARGET_CITATION , CITATION ) . ||9388268_155871_6667==>This was shown to function as an HIV - 1 transcriptional activator by directly interacting with the viral LTR ( CITATION ) , and is facilitated by direct interaction with either <PROT2_6667>  Sp1 </PROT2_6667> ( TARGET_CITATION ) or <PROT1_155871>  Tat </PROT1_155871> ( CITATION ) . 
modulates====11689053_155871_2033==>CBP / <PROT2_2033>  p300 </PROT2_2033> was also recently reported to interact with the HIV - 1 <PROT1_155871>  Tat </PROT1_155871> protein and serve as a coactivator of <PROT1_155871>  Tat </PROT1_155871> - dependent HIV - 1 gene expression ( CITATION , CITATION , CITATION - TARGET_CITATION ) . ||11689053_155871_2033==>It is known that CBP / <PROT2_2033>  p300 </PROT2_2033> acetylates <PROT1_155871>  Tat </PROT1_155871> at a double lysine motif in a highly conserved region ( CITATION , CITATION , TARGET_CITATION ) . ||11689053_155871_2033==>We and others have shown previously that CBP / <PROT2_2033>  p300 </PROT2_2033> acetylates <PROT1_155871>  Tat </PROT1_155871> at a double lysine motif ( amino acids 50 and 51 ) in a highly conserved region , and moreover the minimal HAT domain of CBP / <PROT2_2033>  p300 </PROT2_2033> was capable of performing this acetylation ( CITATION , CITATION , TARGET_CITATION ) . ||11689053_155871_2033==><PROT1_155871>  Tat </PROT1_155871> recruits <PROT2_2033>  p300 </PROT2_2033> & # x2013 ; CBP ( cAMP response element ( CREB ) binding protein ) to the HIV - 1 promoter , where it has been shown to enhance the HAT activity of <PROT2_2033>  p300 </PROT2_2033> and increase transactivation from the HIV - 1 promoter CITATION and TARGET_CITATION . ||12134041_155908_7514==>On the other hand , Li et al. ( TARGET_CITATION and CITATION ) demonstrated that leptomycin B treatment disrupted the <PROT2_7514>  CRM1 </PROT2_7514> - <PROT1_155908>  Rev </PROT1_155908> complex in <PROT1_155908>  Rev </PROT1_155908> / Sam68 overexpressing cells , suggesting that Sam68 - induced <PROT1_155908>  Rev </PROT1_155908> nuclear export was <PROT2_7514>  CRM1 </PROT2_7514> - dependent . ||9334723_155807_7124==>In parallel with recent in vitro studies ( TARGET_CITATION ) that treatment of peripheral blood mononuclear cells with <PROT1_155807>  Vpr </PROT1_155807> suppressed production of certain cytokines ( IL - 2 , IL - 12 and <PROT2_7124>  TNF </PROT2_7124> - { alpha } ) , this study provides in vivo evidence that the <PROT1_155807>  Vpr </PROT1_155807> - mediated immunosuppressive effect is targeted in particular at Th1 - mediated cellular immunity . ||9400615_155871_5524==>Recently , changes in the ratio of <PROT2_5524>  PP2A </PROT2_5524> core enzyme to holoenzyme were found to affect <PROT1_155871>  Tat </PROT1_155871> - dependent HIV - 1 transcription ( TARGET_CITATION ) . ||9400615_155871_5524==>( TARGET_CITATION ) demonstrated that increasing the ratio of <PROT2_5524>  PP2A </PROT2_5524> core enzyme to holoenzyme by expression of a mutant A subunit that binds the C but not the B subunit results in a decrease in <PROT1_155871>  Tat </PROT1_155871> - mediated HIV - 1 transcription and virus production . ||9400615_155871_5524==>To more directly assess the role of <PROT2_5524>  PP2A </PROT2_5524> in <PROT1_155871>  Tat </PROT1_155871> - mediated HIV - 1 transactivation and replication , Ruediger et al. performed experiments using COS , HeLa , and Jurkat T cells and an N - terminal deletion mutant of the A subunit , which lacked the ability to bind the B subunit ( TARGET_CITATION ) . ||9636359_155030_5478==>For example , <PROT2_5478>  CyPA </PROT2_5478> , which is the most abundant cytosolic PPIase , modulates human HIV - 1 infectivity through controlling <PROT1_155030>  Gag </PROT1_155030> processing , in which <PROT2_5478>  CyPA </PROT2_5478> regulates cleavage of <PROT1_155030>  Gag </PROT1_155030> polypeptide by HIV - 1 protease through its ability to catalyze cis - trans interconversion of the proline - peptide bond around the cleavage site of the <PROT1_155030>  Gag </PROT1_155030> polypeptide ( TARGET_CITATION , CITATION , CITATION ) . ||9636359_155030_5478==>Aside from minimal effects on the kinetics of <PROT1_155030>  gag </PROT1_155030> processing or virion release ( TARGET_CITATION , CITATION ) , disruption of <PROT2_5478>  CypA </PROT2_5478> incorporation into virions has no effect on biochemical or ultrastructural characteristics of HIV - 1 virions , including endogenous reverse transcription ( CITATION , CITATION , CITATION , CITATION , CITATION , CITATION ) . 
myristoylated by====2405382_155030_4836==>For example , <PROT2_4836>  NMT </PROT2_4836> inhibitors have been shown to prevent both membrane binding of <PROT1_155030>  Gag </PROT1_155030> as well as virus assembly ( TARGET_CITATION ) . ||2405382_155030_4836==><PROT2_4836>  NMT </PROT2_4836> inhibitors were found to prevent the binding of <PROT1_155030>  Gag </PROT1_155030> to membrane and virus assembly TARGET_CITATION . 
phosphorylates====10074203_155030_5594==>In a second study , they found that the HIV - 1 proteins , Rev , Tat , p17 ( <PROT1_155030>  Gag </PROT1_155030> ) , and Nef , could all be directly phosphorylated in vitro by activated <PROT2_5594>  ERK </PROT2_5594> ( TARGET_CITATION ) . ||10074203_155030_5594==><PROT2_5594>  ERK </PROT2_5594> phosphorylates several HIV - 1 proteins important for viral replication , such as Vif , Rev , Tat , p17 ( <PROT1_155030>  Gag </PROT1_155030> ) , and Nef ( TARGET_CITATION ) . ||10074203_155459_5595==>For example , activation of <PROT2_5595>  ERK1 </PROT2_5595> / ERK2 MAPK in producer cells has been shown to enhance the infectivity of HIV - 1 virions by phosphorylating <PROT1_155459>  Vif </PROT1_155459> ( CITATION ) as well as other mechanisms ( CITATION , TARGET_CITATION ) . ||10074203_155871_5594==>In a second study , they found that the HIV - 1 proteins , Rev , <PROT1_155871>  Tat </PROT1_155871> , p17 ( Gag ) , and Nef , could all be directly phosphorylated in vitro by activated <PROT2_5594>  ERK </PROT2_5594> ( TARGET_CITATION ) . ||10074203_155871_5594==>There is evidence that MAPK / <PROT2_5594>  ERK </PROT2_5594> is able to phosphorylate HIV - 1 proteins in vitro ( i.e. , Vif , Rev , <PROT1_155871>  Tat </PROT1_155871> , p17Gag , and Nef ) ( CITATION , TARGET_CITATION ) . ||10074203_155871_5594==><PROT2_5594>  ERK </PROT2_5594> phosphorylates several HIV - 1 proteins important for viral replication , such as Vif , Rev , <PROT1_155871>  Tat </PROT1_155871> , p17 ( Gag ) , and Nef ( TARGET_CITATION ) . ||10074203_155908_5594==>In a second study , they found that the HIV - 1 proteins , <PROT1_155908>  Rev </PROT1_155908> , Tat , p17 ( Gag ) , and Nef , could all be directly phosphorylated in vitro by activated <PROT2_5594>  ERK </PROT2_5594> ( TARGET_CITATION ) . ||10074203_155908_5594==>There is evidence that MAPK / <PROT2_5594>  ERK </PROT2_5594> is able to phosphorylate HIV - 1 proteins in vitro ( i.e. , Vif , <PROT1_155908>  Rev </PROT1_155908> , Tat , p17Gag , and Nef ) ( CITATION , TARGET_CITATION ) . ||10074203_155908_5594==><PROT2_5594>  ERK </PROT2_5594> phosphorylates several HIV - 1 proteins important for viral replication , such as Vif , <PROT1_155908>  Rev </PROT1_155908> , Tat , p17 ( Gag ) , and Nef ( TARGET_CITATION ) . ||10984616_1457_155908==>Despite the many reports demonstrating that HIV - 1 <PROT2_155908>  Rev </PROT2_155908> can be phosphorylated by <PROT1_1457>  CKII </PROT1_1457> and MAPK in vitro ( TARGET_CITATION , CITATION , CITATION ) , only a single report implicating the relevance of phosphorylation to <PROT2_155908>  Rev </PROT2_155908> function has emerged and suggests that HIV - 1 <PROT2_155908>  Rev </PROT2_155908> phosphorylation accelerates formation of an efficient RNA - binding conformation ( CITATION ) . ||1541298_1457_155945==>A second interesting , but unlikely , candidate is the <PROT2_155945>  Vpu </PROT2_155945> protein of HIV - 1 , which has been shown to be phosphorylated on serine residues 52 and 56 by <PROT1_1457>  CKII </PROT1_1457> ( TARGET_CITATION ) . ||8107101_1457_155945==>A second interesting , but unlikely , candidate is the <PROT2_155945>  Vpu </PROT2_155945> protein of HIV - 1 , which has been shown to be phosphorylated on serine residues 52 and 56 by <PROT1_1457>  CKII </PROT1_1457> ( CITATION - TARGET_CITATION ) . ||8626571_155459_5594==>The lysates were freeze - thawed once and then centrifuged for 10 min at 4 & # 176 ; C at 13 , 000 & # 215 ; g. Samples with equivalent amounts of protein ( 5 - 10 & # 181 ; g ) were resolved by 15 % SDS - PAGE and transferred to PVDF membrane for immunoblotting with rabbit anti - ERK1 or anti - ERK1 and anti - <PROT2_5594>  ERK2 </PROT2_5594> ( 1 : 1500 dilution of each ) , rabbit anti - phosphorylated MAPK ( 1 : 1000 dilution ) , or rabbit anti - <PROT1_155459>  Vif </PROT1_155459> ( 1 : 2000 dilution ) ( TARGET_CITATION ) using the ECL system ( Amersham Pharmacia Biotech ) as described ( TARGET_CITATION ) . ||8626571_155459_5595==>The lysates were freeze - thawed once and then centrifuged for 10 min at 4 & # 176 ; C at 13 , 000 & # 215 ; g. Samples with equivalent amounts of protein ( 5 - 10 & # 181 ; g ) were resolved by 15 % SDS - PAGE and transferred to PVDF membrane for immunoblotting with rabbit anti - <PROT2_5595>  ERK1 </PROT2_5595> or anti - <PROT2_5595>  ERK1 </PROT2_5595> and anti - ERK2 ( 1 : 1500 dilution of each ) , rabbit anti - phosphorylated MAPK ( 1 : 1000 dilution ) , or rabbit anti - <PROT1_155459>  Vif </PROT1_155459> ( 1 : 2000 dilution ) ( TARGET_CITATION ) using the ECL system ( Amersham Pharmacia Biotech ) as described ( TARGET_CITATION ) . ||8626571_155459_5595==>Several protein kinases , including <PROT2_5595>  ERK1 </PROT2_5595> / 2 mitogen - activated protein kinase and protein kinase C , have been shown to enhance human immunodeficiency virus type 1 infectivity by direct phosphorylation of viral proteins <PROT1_155459>  Vif </PROT1_155459> , Tat , Gag , and Nef ( CITATION , CITATION , CITATION , CITATION - TARGET_CITATION ) . ||8806671_1457_155908==>Despite the many reports demonstrating that HIV - 1 <PROT2_155908>  Rev </PROT2_155908> can be phosphorylated by <PROT1_1457>  CKII </PROT1_1457> and MAPK in vitro ( CITATION , TARGET_CITATION , CITATION ) , only a single report implicating the relevance of phosphorylation to <PROT2_155908>  Rev </PROT2_155908> function has emerged and suggests that HIV - 1 <PROT2_155908>  Rev </PROT2_155908> phosphorylation accelerates formation of an efficient RNA - binding conformation ( CITATION ) . ||9621077_155871_2185==>These dramatic effects on actin dynamics are consistent with physiologic and biochemical studies demonstrating an increase in paracellular albumin permeability ( CITATION ) and in phosphorylation of focal adhesion proteins such as FAK , <PROT2_2185>  Pyk2 </PROT2_2185> , paxillin , and p130Cas ( TARGET_CITATION ) after <PROT1_155871>  Tat </PROT1_155871> treatment . ||9621077_155871_5829==>These dramatic effects on actin dynamics are consistent with physiologic and biochemical studies demonstrating an increase in paracellular albumin permeability ( CITATION ) and in phosphorylation of focal adhesion proteins such as FAK , Pyk2 , <PROT2_5829>  paxillin </PROT2_5829> , and p130Cas ( TARGET_CITATION ) after <PROT1_155871>  Tat </PROT1_155871> treatment . ||9621077_155871_9564==>These dramatic effects on actin dynamics are consistent with physiologic and biochemical studies demonstrating an increase in paracellular albumin permeability ( CITATION ) and in phosphorylation of focal adhesion proteins such as FAK , Pyk2 , paxillin , and <PROT2_9564>  p130Cas </PROT2_9564> ( TARGET_CITATION ) after <PROT1_155871>  Tat </PROT1_155871> treatment . 
recruits====10467404_155871_904==>The G5 - 83HIV - Luc reporter containing five GAL4 binding sites at position & # x2212 ; 83 of the HIV - 1 long terminal repeat ( LTR ) , GAL4 & # x2013 ; CDK9 , GAL4 & # x2013 ; CDK9dn , GAL4 & # x2013 ; <PROT2_904>  CycT1 </PROT2_904> , GAL4 & # x2013 ; <PROT2_904>  CycT1 </PROT2_904> ( 1 & # x2013 ; 290 ) , GAL4 & # x2013 ; Sp1 , pSV & # x2013 ; <PROT1_155871>  Tat </PROT1_155871> , GST & # x2013 ; CDK9 , has been previously described ( CITATION ; TARGET_CITATION and CITATION ) . ||10467404_155871_904==>The observation that <PROT1_155871>  Tat </PROT1_155871> - mediated transcriptional activation can be recapitulated simply by tethering <PROT2_904>  CycT1 </PROT2_904> or CDK9 to a promoter - proximal RNA or DNA target argues that P - TEFb recruitment is the sole mechanism by which <PROT1_155871>  Tat </PROT1_155871> proteins activate lentiviral gene expression ( CITATION , CITATION , TARGET_CITATION ) . ||11080476_155871_2959==>A modification of the conformation of CBP / p300 by the <PROT1_155871>  Tat </PROT1_155871> had also been suggested , since the HAT had a better interaction with the basal transcription factors TBP and <PROT2_2959>  TFIIB </PROT2_2959> in the presence of <PROT1_155871>  Tat </PROT1_155871> peptide 41 - 45 ( TARGET_CITATION ) . ||11080476_155871_2959==>In the absence of CBP / p300 and <PROT1_155871>  Tat </PROT1_155871> crystal structures , it was shown previously that <PROT1_155871>  Tat </PROT1_155871> can change the conformation of the CBP / p300 in vitro such that basal transcription factors such as TATA - binding protein ( TBP ) and <PROT2_2959>  TFIIB </PROT2_2959> could bind with higher affinity to CBP / p300 and influence the activation process by aiding CBP / p300 to recruit new partners into the transcription machinery ( TARGET_CITATION ) . ||11282025_1025_155871==>Consistent with this idea , <PROT1_1025>  CDK9 </PROT1_1025> is present together with cyclin T1 and <PROT2_155871>  Tat </PROT2_155871> in nuclear speckles ( TARGET_CITATION ) . ||11430827_155348_6598==>Furthermore the initial isolation of <PROT2_6598>  SNF5 </PROT2_6598> / <PROT2_6598>  INI1 </PROT2_6598> as a protein interacting with and stimulating the HIV <PROT1_155348>  integrase </PROT1_155348> ( CITATION ) and the recent report on the association of <PROT2_6598>  SNF5 </PROT2_6598> with the incoming viral particle ( TARGET_CITATION ) support a role for at least <PROT2_6598>  SNF5 </PROT2_6598> in recombination . ||11430827_155348_6598==>Within 30 minutes of infection , select host proteins including the <PROT1_155348>  integrase </PROT1_155348> interactor 1 ( <PROT2_6598>  INI1 </PROT2_6598> , also known as <PROT2_6598>  SNF5 </PROT2_6598> or <PROT2_6598>  BAF47 </PROT2_6598> ) , a component of the SWI / SNF complex , and PML , a protein present in promyelocytic oncogenic domains or PODs , translocate from the nucleus into the cytoplasm TARGET_CITATION ( fig. 2 ) . ||12055184_155871_1874==>In particular , <PROT1_155871>  Tat </PROT1_155871> protein of HIV has also recently been shown to physically interact with the RB2 & # 47 ; p130 tumor suppressor gene product ( CITATION ) and <PROT2_1874>  E2F4 </PROT2_1874> ( TARGET_CITATION ) . ||14657027_1387_155871==>( TARGET_CITATION ) reported that the histone acetyltransferases ( HATs ) , <PROT1_1387>  CBP </PROT1_1387> , hGCN5 , and P / CAF are recruited to the LTR following activation by TPA and / or <PROT2_155871>  Tat </PROT2_155871> , and this recruitment is associated with the acetylation of histones H3 and H4 . ||14657027_155871_2648==>( TARGET_CITATION ) reported that the histone acetyltransferases ( HATs ) , CBP , <PROT2_2648>  hGCN5 </PROT2_2648> , and P / CAF are recruited to the LTR following activation by TPA and / or <PROT1_155871>  Tat </PROT1_155871> , and this recruitment is associated with the acetylation of histones H3 and H4 . ||14657027_155871_8850==>( TARGET_CITATION ) reported that the histone acetyltransferases ( HATs ) , CBP , hGCN5 , and P / <PROT2_8850>  CAF </PROT2_8850> are recruited to the LTR following activation by TPA and / or <PROT1_155871>  Tat </PROT1_155871> , and this recruitment is associated with the acetylation of histones H3 and H4 . 
regulates====11959860_155871_9733==>Recently , the <PROT2_9733>  SART3 </PROT2_9733> / p110 HAT domain was shown to interact with the HIV - 1 <PROT1_155871>  Tat </PROT1_155871> protein ( TARGET_CITATION ) . ||11959860_155871_9733==><PROT2_9733>  Tip110 </PROT2_9733> Binding to AR & # 151 ; We have recently identified a new protein <PROT2_9733>  Tip110 </PROT2_9733> as a co - transactivator of human immunodeficiency virus type I ( HIV - 1 ) <PROT1_155871>  Tat </PROT1_155871> protein ( TARGET_CITATION ) . ||11959860_155871_9733==>The complex formed by Hiv - 1 TAR - <PROT1_155871>  Tat </PROT1_155871> has been used as bait by two groups and led to the selection of two different proteins , cyclin T1 ( CITATION ) and <PROT2_9733>  Tip110 </PROT2_9733> ( TARGET_CITATION ) . ||12368300_155871_904==>Moreover , in these cells , the state of differentiation regulates viral expression by affecting the level of <PROT2_904>  CycT1 </PROT2_904> and thus , the transactivating function of <PROT1_155871>  Tat </PROT1_155871> ( TARGET_CITATION ) . ||12573582_155807_7157==><PROT1_155807>  Vpr </PROT1_155807> directly acts on mitochondria , on a molecular complex formed by Bax , and on the adenine nucleotide translocase ( CITATION , CITATION ) , and can also elicit <PROT2_7157>  p53 </PROT2_7157> - dependent transcriptional effects ( TARGET_CITATION ) . ||12573582_155807_7157==>The cyclin - dependent kinase inhibitor p21Waf1 was previously shown to be transcriptionally upregulated in a <PROT2_7157>  p53 </PROT2_7157> - dependent fashion in the context of <PROT1_155807>  Vpr </PROT1_155807> expression ( TARGET_CITATION ) . ||1727476_155871_6580==>Overexpression of TBP ( CITATION ) and transcription factors such as AP1 , CTF , USF , and ATF can replace Sp1 in <PROT1_155871>  Tat </PROT1_155871> responsiveness whereas VP16 , E1a , <PROT2_6580>  Oct1 </PROT2_6580> , or NF - kappa B confer only a weak response ( CITATION , TARGET_CITATION , CITATION , CITATION ) . 
requires====7666561_155871_6667==>It has been well established that the <PROT2_6667>  Sp1 </PROT2_6667> transcription factor ( CITATION ) plays an essential role in the regulation of basal transcription as well as in <PROT1_155871>  Tat </PROT1_155871> - mediated transactivation of the HIV - 1 LTR ( CITATION - TARGET_CITATION ) . ||7666561_155871_6667==>Indeed , various studies have shown that <PROT2_6667>  Sp1 </PROT2_6667> contributes to basal ( CITATION , CITATION , CITATION , TARGET_CITATION , CITATION ) and <PROT1_155871>  Tat </PROT1_155871> - activated ( CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , TARGET_CITATION ) expression of the viral LTR . ||7666561_155871_6667==>A cooperative interaction between NF - kappa B bound to the kappa B sites and <PROT2_6667>  Sp1 </PROT2_6667> bound to the adjacent <PROT2_6667>  Sp1 </PROT2_6667> sites is required for HIV - 1 gene expression ( CITATION ) , and <PROT2_6667>  Sp1 </PROT2_6667> is also essential for <PROT1_155871>  Tat </PROT1_155871> - mediated activation of the HIV - 1 LTR ( CITATION , TARGET_CITATION ) . ||7666561_155871_6667==>Our finding that the six <PROT2_6667>  Sp1 </PROT2_6667> sites are beneficial only in the presence of <PROT1_155871>  Tat </PROT1_155871> is consistent with the proposed functional interaction between the <PROT1_155871>  Tat </PROT1_155871> and <PROT2_6667>  Sp1 </PROT2_6667> proteins during LTR - mediated transcription ( CITATION , CITATION , TARGET_CITATION ) . ||7666561_155871_6667==>It is also of particular interest that efficient viral transcription and replication of HIV - 1 requires both <PROT2_6667>  Sp1 </PROT2_6667> sites and the interaction of <PROT2_6667>  Sp1 </PROT2_6667> proteins with the viral transcription factor HIV - 1 <PROT1_155871>  Tat </PROT1_155871> ( TARGET_CITATION , CITATION ) . ||7666561_155871_6667==>The importance of the <PROT2_6667>  Sp1 </PROT2_6667> factor for <PROT1_155871>  Tat </PROT1_155871> - mediated transactivation is well established ( TARGET_CITATION , CITATION ) ; a direct interaction occurs between <PROT2_6667>  Sp1 </PROT2_6667> and amino acids 30 - 72 of <PROT1_155871>  Tat </PROT1_155871> during transactivation ( TARGET_CITATION , CITATION , CITATION ) . ||7666561_155871_6667==>Deletion or mutation of the <PROT2_6667>  Sp1 </PROT2_6667> sites in the truncated - 40 / + 80 LTR , in - 283 / + 80 LTRSp1mut , or in - 283 / + 80Delta kappa B / <PROT2_6667>  Sp1 </PROT2_6667> led to the abolition of COUP - TF - induced activation ( fig. 5 , lanes 10 , 14 , and 18 ) because <PROT2_6667>  Sp1 </PROT2_6667> is required for COUP - TF transactivation in microglial cells ( CITATION ) ; similarly , <PROT1_155871>  Tat </PROT1_155871> - induced activation was abolished ( lanes 11 , 15 , and 19 ) , because <PROT2_6667>  Sp1 </PROT2_6667> is required for efficient <PROT1_155871>  Tat </PROT1_155871> transactivation ( TARGET_CITATION , CITATION ) . ||7666561_155871_6667==>A number of reports have demonstrated that the <PROT2_6667>  Sp1 </PROT2_6667> and kappa B sequences are required for TAR - dependent transactivation by <PROT1_155871>  Tat </PROT1_155871> ( TARGET_CITATION , CITATION , CITATION ) . ||7666561_155871_6667==>The essential role of the <PROT2_6667>  Sp1 </PROT2_6667> transcription factor ( CITATION ) in the regulation of basal transcription and in <PROT1_155871>  Tat </PROT1_155871> - mediated transactivation of the HIV - 1 LTR has been well established ( CITATION , CITATION , TARGET_CITATION ) . ||7666561_155871_6667==>Moreover , <PROT1_155871>  Tat </PROT1_155871> has also been shown to interact directly with <PROT2_6667>  SP1 </PROT2_6667> ( CITATION ) , and both <PROT2_6667>  SP1 </PROT2_6667> and NF - kappa B are required for <PROT1_155871>  Tat </PROT1_155871> - mediated transactivation of the HIV - 1 promoter ( CITATION , TARGET_CITATION ) . ||7666561_155871_6667==>Viral trans - activating protein <PROT1_155871>  Tat </PROT1_155871> is a major positive regulator of this promoter ( CITATION , CITATION and CITATION ) , cooperating with several host transcription factors , such as NF - small kappa , GreekB ( CITATION ) , AP1 ( CITATION ) , <PROT2_6667>  Sp1 </PROT2_6667> and others ( TARGET_CITATION , CITATION and CITATION ) , some of which may share part of the same transcription machinery with the glucocorticoid receptor ( CITATION ) . ||7666561_155871_6667==>The ubiquitous transcription factor <PROT2_6667>  Sp1 </PROT2_6667> is critical for both basal and <PROT1_155871>  Tat </PROT1_155871> - induced transcription of the HIV - 1 LTR ( CITATION and TARGET_CITATION ) . ||7666561_155871_6667==>The transcription factor <PROT2_6667>  Sp1 </PROT2_6667> plays an essential role in the regulation of basal transcription as well as in <PROT1_155871>  Tat </PROT1_155871> - mediated transactivation through HIV - 1 LTR ( CITATION and TARGET_CITATION ) . ||7666561_155871_6667==>Several reports suggest that interaction between <PROT2_6667>  Sp1 </PROT2_6667> and <PROT1_155871>  Tat </PROT1_155871> is required for <PROT1_155871>  Tat </PROT1_155871> - mediated HIV - 1 LTR trans - activation ( CITATION , CITATION , CITATION and TARGET_CITATION ) ; however , the mechanistic puzzle of how <PROT1_155871>  Tat </PROT1_155871> & # x2013 ; <PROT2_6667>  Sp1 </PROT2_6667> functional interaction enhances HIV - 1 transcription has not yet been explained . ||7666561_155871_6667==>The transactivating function of the viral <PROT1_155871>  Tat </PROT1_155871> protein requires the presence of the <PROT2_6667>  Sp1 </PROT2_6667> and NF - { kappa } B sites ( TARGET_CITATION ) and a direct interaction with the <PROT2_6667>  Sp1 </PROT2_6667> protein ( CITATION , CITATION ) ( for review , see refs. ( CITATION , CITATION ) ) . ||7666561_155871_6667==>Although the { kappa } B enhancer has been considered the main inducible cis - acting element ( CITATION ) , several reports suggest that the interaction between <PROT2_6667>  Sp1 </PROT2_6667> and <PROT1_155871>  Tat </PROT1_155871> is required for <PROT1_155871>  Tat </PROT1_155871> - mediated HIV - 1 - LTR transactivation ( TARGET_CITATION , CITATION ) . ||8230415_155871_6667==>A variety of studies indicate that both the sequence and spacing of the NF - kappa B , <PROT2_6667>  SP1 </PROT2_6667> , and TATA elements play an important role in regulating the levels of basal and <PROT1_155871>  Tat </PROT1_155871> - induced transcription ( CITATION - TARGET_CITATION ) . ||8230415_155871_6667==>Findings derived from subgenomic transient transfection studies have shown that promoter upstream binding factors such as <PROT2_6667>  Sp1 </PROT2_6667> are important for <PROT1_155871>  Tat </PROT1_155871> transactivation ( CITATION - TARGET_CITATION ) , and that optimal <PROT1_155871>  Tat </PROT1_155871> transactivation of the LTR requires human cell cofactors ( refs. CITATION and CITATION and reviewed in refs. CITATION ) . ||8230415_155871_6667==><PROT2_6667>  Sp1 </PROT2_6667> activates transcription by cooperative interaction either with itself ( CITATION ) or with other transcriptional factors , such as the papillomavirus E2 protein ( CITATION ) , <PROT1_155871>  Tat </PROT1_155871> protein of the human immunodeficiency virus type 1 ( HIV - 1 ) ( TARGET_CITATION and CITATION ) , NF - kB p65 ( CITATION and CITATION ) , Rb protein ( CITATION ) , GATA - 1 ( CITATION ) , Egr - 1 ( CITATION ) , Ap1 ( CITATION and CITATION ) , cEBPb ( CITATION ) , E2F ( CITATION ) , p53 ( CITATION ) , etc. These observations suggest that modulation of <PROT2_6667>  Sp1 </PROT2_6667> activity may play a critical role in the regulation of cell proliferation and differentiation . ||8230415_155871_6667==>Because the spacing between the 3 & # x2032 ; <PROT2_6667>  Sp1 </PROT2_6667> motif and TATA box of HIV is important for <PROT1_155871>  Tat </PROT1_155871> function and viral replication ( TARGET_CITATION ) , the <PROT2_6667>  Sp1 </PROT2_6667> motif was inserted into the chimeric LTRs with the same spacing as seen in the WT HIV LTR . ||8230415_155871_6667==>While enhanced LTR activity with the addition of the <PROT2_6667>  Sp1 </PROT2_6667> site is consistent with earlier work , suggesting that the presence of a <PROT2_6667>  Sp1 </PROT2_6667> site within the HIV enhancer increases <PROT1_155871>  Tat </PROT1_155871> transactivation ( TARGET_CITATION , CITATION , CITATION and CITATION ) , the <PROT2_6667>  Sp1 </PROT2_6667> site was not necessary for viral replication since MT - & # x394 ; 15 was also infectious . ||8230415_155871_6667==>Supporting this hypothesis , it has recently been shown that the correct spatial arrangement among TAR , <PROT2_6667>  Sp1 </PROT2_6667> - binding sites and TATA motif crucially influences HIV - 1 expression , suggesting that <PROT1_155871>  Tat </PROT1_155871> , <PROT2_6667>  Sp1 </PROT2_6667> and TBP must contact each other for optimal expression and replication of HIV - 1 ( TARGET_CITATION ) . ||8230415_155871_6667==>The <PROT2_6667>  Sp1 </PROT2_6667> elements also seem to play a role in the <PROT1_155871>  Tat </PROT1_155871> inducibility of expression ( TARGET_CITATION ) . ||8230415_155871_6667==>However , <PROT2_6667>  Sp1 </PROT2_6667> binding to the promoter - proximal <PROT2_6667>  Sp1 </PROT2_6667> motifs also promotes <PROT1_155871>  Tat </PROT1_155871> - dependent HIV transcription ( CITATION , CITATION , CITATION , TARGET_CITATION ) . ||8849451_155871_6667==>This possibility is supported by the reports showing that <PROT1_155871>  Tat </PROT1_155871> may associate with <PROT2_6667>  Sp1 </PROT2_6667> , TFIID factors , RNA polymerase II , and RNA polymerase II - associated factors ( CITATION , CITATION - TARGET_CITATION ) . ||8849451_155871_6667==>Findings derived from subgenomic transient transfection studies have shown that promoter upstream binding factors such as <PROT2_6667>  Sp1 </PROT2_6667> are important for <PROT1_155871>  Tat </PROT1_155871> transactivation ( CITATION ) , and that optimal <PROT1_155871>  Tat </PROT1_155871> transactivation of the LTR requires human cell cofactors ( refs. CITATION and TARGET_CITATION and reviewed in refs. CITATION ) . ||8849451_155871_6667==>When this fraction was supplemented with TBP , <PROT2_6667>  SP1 </PROT2_6667> , TFIIA , in vitro transcription analysis indicated that it was competent for basal but not <PROT1_155871>  Tat </PROT1_155871> - induced stimulation of the HIV - 1 LTR ( CITATION and TARGET_CITATION ) . ||8849451_155871_6667==>The complex mediates both <PROT2_6667>  Sp1 </PROT2_6667> and <PROT1_155871>  Tat </PROT1_155871> - activated HIV - 1 transcription in a reconstituted system ; and whereas TARGET_CITATION showed that ectopic <PROT1_155871>  Tat </PROT1_155871> - SF1 represses HIV - 1 transcription in a <PROT1_155871>  Tat </PROT1_155871> - reversible manner , <PROT1_155871>  Tat </PROT1_155871> - SF1 is shown to act positively in mediating efficient <PROT1_155871>  Tat </PROT1_155871> - stimulated transcription in the reconstituted system and , in agreement with a recent report ( CITATION ) , to act at the level of elongation . ||8849451_155871_6667==>In carrying out its transcriptional activation , <PROT1_155871>  Tat </PROT1_155871> interacts with a number of cellular transcription factors , including TFIID ( TBP ) ( CITATION , CITATION , CITATION ) , TFIID - associated TAFs ( CITATION ) , TFIIH ( CITATION ) , <PROT1_155871>  Tat </PROT1_155871> - associated protein ( TAP ) ( CITATION ) , <PROT1_155871>  Tat </PROT1_155871> - associated kinase ( TAK / pTEFb ) ( CITATION , CITATION , CITATION , CITATION , CITATION , CITATION ) , TTK ( CITATION ) , <PROT2_6667>  Sp1 </PROT2_6667> ( CITATION ) , and SF - 1 ( TARGET_CITATION ) , as well as TAR RNA ( CITATION ) . ||8849451_155871_6667==>In addition to NF - { kappa } B a variety of other factors bind to the HIV - 1 LTR , including <PROT2_6667>  SP1 </PROT2_6667> ( CITATION , CITATION , CITATION ) , TBP ( CITATION , CITATION ) , LEF ( CITATION ) , and LBP ( CITATION , CITATION ) , and other factors that have been identified to interact with these proteins , including CBP / p300 ( CITATION , CITATION ) , <PROT1_155871>  Tat </PROT1_155871> - SF1 ( CITATION , CITATION , CITATION , TARGET_CITATION ) , and TFIIH ( CITATION , CITATION ) . ||9334325_1025_155871==>To determine whether the activity bound to the <PROT2_155871>  Tat </PROT2_155871> - affinity column corresponded to a known transcription component , we carried out immunoblot analysis using antibodies directed against various transcription - initiation factors ( TFIIB , TFIID ( TBP subunit ) , TFIIF ( RAP30 subunit ) , TFIIH ( ERCC3 subunit ) ( Drapkin et al. 1994 CITATION ) ) , elongation factors ( SII , Elongin ( SIII ) ( Elongin B subunit ) ( Aso et al. 1995 CITATION ; Garrett et al. 1995 CITATION ) , ELL ( Shilatifard et al. 1996 CITATION ) , P - TEFb ( <PROT1_1025>  CDK9 </PROT1_1025> subunit ) ( Zhu et al. 1997 TARGET_CITATION ) ) , RNA polymerase II , and <PROT2_155871>  Tat </PROT2_155871> - SF1 ( Zhou and Sharp 1996 CITATION ) , a putative <PROT2_155871>  Tat </PROT2_155871> - specific cofactor . ||9334325_1025_155871==>The <PROT2_155871>  Tat </PROT2_155871> - affinity column also bound several general transcription factors to a lesser extent , in particular , TFIIF and the cyclin - dependent kinase / P - TEFb subunit , <PROT1_1025>  CDK9 </PROT1_1025> , which have both been implicated in <PROT2_155871>  Tat </PROT2_155871> function previously ( Kato et al. 1992 CITATION ; Moncebo et al. 1997 CITATION ; Zhu et al. 1997 TARGET_CITATION ; Gold et al. 1998 CITATION ; Wei et al. 1998 CITATION ) . ||9334325_1025_155871==>The <PROT2_155871>  Tat </PROT2_155871> - associated kinase ( <PROT1_1025>  TAK </PROT1_1025> ) complex in nuclear extracts ( Herrmann and Rice 1993 CITATION , 1995 CITATION ; Yang et al. 1996 CITATION ) has been shown to contain cyclin T1 , <PROT1_1025>  CDK9 </PROT1_1025> , and other as yet unidentified subunits of P - TEFb ( Mancebo et al. 1997 CITATION ; Yang et al. 1997 CITATION ; Zhu et al. 1997 TARGET_CITATION ; Wei et al. 1998 CITATION ) . ||9334325_1025_155871==>Studies using dominant - negative proteins and specific kinase inhibitors have shown that <PROT1_1025>  CDK9 </PROT1_1025> kinase activity is critical for <PROT2_155871>  Tat </PROT2_155871> transactivation ( Mancebo et al. 1997 CITATION ; Zhu et al. 1997 TARGET_CITATION ; Gold et al. 1998 CITATION ) , and the multisubunit P - TEFb complex , but not hCycT1 and <PROT1_1025>  CDK9 </PROT1_1025> alone , can restore <PROT2_155871>  Tat </PROT2_155871> trans - activation to <PROT1_1025>  CDK9 </PROT1_1025> - depleted extracts in vitro ( Mancebo et al. 1997 CITATION ; Zhu et al. 1997 TARGET_CITATION ; Zhou et al. 1998 CITATION ) . ||9334325_1025_155871==>Consistent with a role for <PROT1_1025>  CDK9 </PROT1_1025> as a kinase that phosphorylates the carboxy - terminal domain ( CTD ) of RNAPII ( Zhu et al. 1997 TARGET_CITATION ; Peng et al. 1998 CITATION ) , the CTD has also been shown to be required for <PROT2_155871>  Tat </PROT2_155871> activity ( for review , see Jones 1997 CITATION ; Cullen 1998 CITATION ; Emerman and Malim 1998 CITATION ) . ||9334325_1025_155871==>Immunodepletion of <PROT1_1025>  PITALRE </PROT1_1025> ( <PROT1_1025>  CDK9 </PROT1_1025> ) from HeLa nuclear extract eliminated basal transcription elongation and <PROT2_155871>  Tat </PROT2_155871> transactivation in vitro , and the addition of purified Drosophila P - TEFb reversed the block to basal elongation but not <PROT2_155871>  Tat </PROT2_155871> activation ( TARGET_CITATION ) . ||9334325_1025_155871==>When highly purified Drosophila P - TEFb consisting of <PROT1_1025>  CDK9 </PROT1_1025> and cyclin T was added to <PROT1_1025>  CDK9 </PROT1_1025> - depleted HeLa nuclear extract , only basal transcription elongation but not <PROT2_155871>  Tat </PROT2_155871> activation was restored ( TARGET_CITATION ) . ||9334325_1025_155871==>The difference between human and Drosophila P - TEFb in their ability to support <PROT2_155871>  Tat </PROT2_155871> activation could be the result of <PROT1_1025>  CDK9 </PROT1_1025> or cyclin T sequence divergence existing between the two species ( 72 % identity between human and Drosophila <PROT1_1025>  CDK9 </PROT1_1025> , & lt ; 40 % identity between human and Drosophila cyclin T ; TARGET_CITATION ; CITATION ) , which may result in differences in the substrate specificity of the <PROT1_1025>  CDK9 </PROT1_1025> kinase or <PROT2_155871>  Tat </PROT2_155871> & # 150 ; P - TEFb interaction . ||9334325_1025_155871==>DRB has been shown to inhibit various cellular protein kinases including the TFIIH - associated cyclin - dependent kinase ( CITATION ) , the Drosophila - positive transcription elongation factor p - TEFb ( CITATION ) , and a <PROT2_155871>  Tat </PROT2_155871> - associated kinase referred to as <PROT1_1025>  TAK </PROT1_1025> ( CITATION , CITATION ) , which most likely corresponds to the human homolog of p - TEFb ( TARGET_CITATION , CITATION ) . ||9334325_1025_155871==>The TFIIH - associated cyclin dependent kinase , p - TEFb , and <PROT1_1025>  TAK </PROT1_1025> can all associate with <PROT2_155871>  Tat </PROT2_155871> ( TARGET_CITATION ) and phosphorylate the carboxyl - terminal domain of RNA polymerase II ( CITATION , CITATION ) , an event that is thought to be required for the transition into the elongation mode of RNA polymerase II . ||9334325_1025_155871==>Another <PROT2_155871>  Tat </PROT2_155871> associated kinase , <PROT1_1025>  PITALRE </PROT1_1025> , is a component of the transcriptional elongation factor P - TEFb ( CITATION , CITATION , CITATION and TARGET_CITATION ) . ||9334325_1025_155871==>Moreover , like TFIIH ( CITATION , CITATION , CITATION and CITATION ) , <PROT1_1025>  PITALRE </PROT1_1025> is also required for <PROT2_155871>  Tat </PROT2_155871> - mediated effects on HIV - 1 trancriptional elongation ( CITATION and TARGET_CITATION ) . ||9334325_1025_155871==>Thus a current model to explain the ability of <PROT2_155871>  Tat </PROT2_155871> to increase HIV - 1 transcriptional elongation is that <PROT2_155871>  Tat </PROT2_155871> association with RNA polymerase II may alter the ability of cellular kinases such as CDK7 or <PROT1_1025>  PITALRE </PROT1_1025> to phosphorylate specific amino residues in the CTD ( CITATION , CITATION , CITATION , CITATION and TARGET_CITATION ) and this may alter the association of transcription factors such as <PROT2_155871>  Tat </PROT2_155871> - CT1 and <PROT2_155871>  Tat </PROT2_155871> - CT2 , with the HIV - 1 transcription complex . ||9334325_1025_155871==>The kinase activity of the <PROT1_1025>  PITALRE </PROT1_1025> complex is disrupted by compounds that were identified during an in vitro drug screen as inhibitors of <PROT2_155871>  Tat </PROT2_155871> activity ( TARGET_CITATION ) . ||9334325_1025_155871==>Among the factors which associate with <PROT2_155871>  Tat </PROT2_155871> , <PROT1_1025>  TAK </PROT1_1025> has been shown to interact with <PROT2_155871>  Tat </PROT2_155871> in vitro and in vivo through its activation domain ( CITATION , TARGET_CITATION ) . ||9334325_1025_155871==>While this report was in preparation , we became aware of two recent studies describing the involvement of <PROT1_1025>  PITALRE </PROT1_1025> in <PROT2_155871>  Tat </PROT2_155871> transactivation ( CITATION , TARGET_CITATION ) . ||9334325_1025_155871==><PROT2_155871>  Tat </PROT2_155871> also exists in a second complex , which contains <PROT1_1025>  CDK9 </PROT1_1025> , which , together with cyclin T1 , hyperphosphorylates the CTD ( CITATION , CITATION , TARGET_CITATION ) . ||9334325_1025_155871==>More recently , several studies have pointed to the association of cell cycle regulatory proteins including <PROT1_1025>  PITALRE </PROT1_1025> with HIV - 1 <PROT2_155871>  Tat </PROT2_155871> protein ( CITATION ; TARGET_CITATION ) . ||9334325_1025_155871==>The functional significance of <PROT1_1025>  Cdk9 </PROT1_1025> & # x2013 ; cyclin T was demonstrated by its immunodepletion from transcription extracts , which was found to inhibit activation by <PROT2_155871>  Tat </PROT2_155871> as well as basal transcription ( TARGET_CITATION , CITATION and CITATION ) . ||9334325_1025_155871==>The transactivation domain of <PROT2_155871>  Tat </PROT2_155871> interacts strongly with a nuclear <PROT2_155871>  Tat </PROT2_155871> - associated kinase , called <PROT1_1025>  TAK </PROT1_1025> ( ( CITATION ) ( CITATION ) ) , which was recently shown to be identical to the kinase subunit of P - TEFb ( ( CITATION and TARGET_CITATION ) ) , a positive - acting transcription elongation factor complex that is required for elongation at many genes ( ( CITATION ) ( CITATION ) ) . ||9334325_1025_155871==>Biochemical analysis of purified <PROT1_1025>  TAK </PROT1_1025> / P - TEFb identified the 42 kDa catalytic subunit as a CDC2 - related kinase , <PROT1_1025>  PITALRE </PROT1_1025> ( hereafter called <PROT1_1025>  CDK9 </PROT1_1025> ) , and <PROT1_1025>  CDK9 </PROT1_1025> inhibitors as well as a dominant negative <PROT1_1025>  CDK9 </PROT1_1025> mutant were found to block <PROT2_155871>  Tat </PROT2_155871> transactivation in vivo and in vitro , indicating that <PROT1_1025>  CDK9 </PROT1_1025> kinase activity is critical for <PROT2_155871>  Tat </PROT2_155871> activity ( ( CITATION , CITATION and TARGET_CITATION ) ) . ||9334325_1025_155871==>Previous studies have shown that P - TEFb is critical for DRB - sensitive RNAPII transcription elongation at many promoters ( ( CITATION , CITATION and TARGET_CITATION ) ) , and immunodepletion of <PROT1_1025>  CDK9 </PROT1_1025> from HeLa nuclear extracts has been shown to inhibit RNAPII elongation as well as <PROT2_155871>  Tat </PROT2_155871> transactivation ( ( CITATION and TARGET_CITATION ) ) . ||9334325_1025_155871==>It is interesting to note that the activation domain of <PROT2_155871>  Tat </PROT2_155871> - 1 ( aa 1 & # x2013 ; 48 ) functions as a potent dominant negative inhibitor of the wild - type <PROT2_155871>  Tat </PROT2_155871> protein without affecting basal HIV - 1 transcription or general elongation , even though cyclin T and the <PROT1_1025>  TAK </PROT1_1025> / P - TEFb complex are required for basal transcription from the HIV - 1 promoter ( Figure 3C ; ( CITATION and TARGET_CITATION ) ) . ||9334325_1025_155871==>It was rapidly demonstrated that the kinase that associates with <PROT2_155871>  Tat </PROT2_155871> in vivo was indeed <PROT1_1025>  CDK9 </PROT1_1025> and that <PROT1_1025>  CDK9 </PROT1_1025> mutants lacking kinase activity could selectively inhibit <PROT2_155871>  Tat </PROT2_155871> function ( ( TARGET_CITATION ) ) . ||9334325_1025_155871==>Upon binding the regulatory protein <PROT2_155871>  Tat </PROT2_155871> and the host cellular proteins cyclin T and <PROT1_1025>  CDK9 </PROT1_1025> , the TAR hairpin stabilizes RNA polymerase II to allow viral transcription ( CITATION - TARGET_CITATION ) . ||9334325_1025_155871==><PROT2_155871>  Tat </PROT2_155871> interacts with a positive transcription elongation factor complex ( P - TEFb ; refs. CITATION , TARGET_CITATION , and CITATION ) , which contains the cdc2 - related kinase <PROT1_1025>  CDK9 </PROT1_1025> , the cyclin T1 protein , and other , as yet unidentified subunits . ||9334325_1025_155871==>Two of them , general transcription factor TFIIH ( CITATION , CITATION ) and elongation factor P - TEFb / <PROT1_1025>  TAK </PROT1_1025> ( TARGET_CITATION ) , were found to interact with <PROT2_155871>  Tat </PROT2_155871> ; in the case of TFIIH , <PROT2_155871>  Tat </PROT2_155871> was shown to enhance phosphorylation of the CTD ( CITATION , CITATION , CITATION ) . ||9334325_1025_155871==>This effect of <PROT2_155871>  Tat </PROT2_155871> requires the CTD of RNAP II CITATION , CITATION , CITATION , CITATION , and <PROT2_155871>  Tat </PROT2_155871> can stimulate the hyperphosphorylation of the CTD in vitro CITATION CITATION . The <PROT2_155871>  Tat </PROT2_155871> - induced Ser / Thr phosphorylation of the CTD is likely to require TFIIH as well as another CTD kinase , which has been identified as the <PROT2_155871>  Tat </PROT2_155871> - associated kinase ( <PROT1_1025>  TAK </PROT1_1025> ) , a component of the P - TEFb elongation factor CITATION TARGET_CITATION . Taken together , these results have led to the proposal that TFIIH and P - TEFb may act in a concerted and sequential manner to achieve the hyperphosphorylation of the CTD and the efficient elongation from the HIV promoter CITATION . ||9334325_1025_155871==><PROT2_155871>  Tat </PROT2_155871> has recently been shown to stimulate phosphorylation of the CTD by binding to cyclinT / <PROT1_1025>  cdk9 </PROT1_1025> ( TARGET_CITATION , CITATION ) . ||9334325_1025_155871==>The kinase activity of <PROT1_1025>  CDK9 </PROT1_1025> ( P - TEFb ) is required for <PROT2_155871>  Tat </PROT2_155871> - activity ( CITATION ; TARGET_CITATION ; CITATION ; CITATION ; CITATION ) , and a key role for cyclin T in <PROT2_155871>  Tat </PROT2_155871> function is indicated by the observation that recombinant cyclin T1 , which interacts with <PROT2_155871>  Tat </PROT2_155871> , enhances the affinity and specificity of <PROT2_155871>  Tat </PROT2_155871> & # 150 ; TAR interactions in a loop sequence - dependent manner ( CITATION ; CITATION ; CITATION ) . ||9334325_1025_155871==><PROT1_1025>  CDK9 </PROT1_1025> / P - TEFb was recently identified as a CTD - kinase recruited by <PROT2_155871>  Tat </PROT2_155871> to the HIV promoter ( CITATION ; CITATION ; TARGET_CITATION ) . ||9334325_1025_155871==>There is now general agreement that <PROT2_155871>  Tat </PROT2_155871> activation of elongation is mediated by the <PROT1_1025>  TAK </PROT1_1025> kinase ( CITATION ; TARGET_CITATION ; CITATION ; CITATION ) . ||9334325_1025_155871==>It has been established that the Cyclin T1 / <PROT1_1025>  CDK9 </PROT1_1025> complex is recruited to promoter - proximal RNA sequences via <PROT2_155871>  Tat </PROT2_155871> - mediated binding to TAR ( CITATION ; CITATION ; CITATION ; CITATION ; CITATION ; TARGET_CITATION ) . ||9334325_1025_155871==>Immunodepletion of <PROT1_1025>  Cdk9 </PROT1_1025> from HeLa nuclear extract eliminated basal HIV - 1 transcription elongation and <PROT2_155871>  Tat </PROT2_155871> transactivation ( CITATION , CITATION , TARGET_CITATION ) , and the addition of affinity - purified human P - TEFb complex completely restored these two processes ( CITATION ) . ||9334325_1025_155871==>P - TEFb is comprised minimally of a CTD kinase called <PROT1_1025>  CDK9 </PROT1_1025> , which is part of the <PROT2_155871>  Tat </PROT2_155871> - associated kinase ( CITATION , CITATION , TARGET_CITATION ) , and its cyclin component , termed cyclin T1 , which binds directly to <PROT2_155871>  Tat </PROT2_155871> ( CITATION ) . ||9334325_1025_155871==>Immunodepletion of <PROT1_1025>  CDK9 </PROT1_1025> inhibited basal and <PROT2_155871>  Tat </PROT2_155871> - activated transcription in vitro ( CITATION , TARGET_CITATION ) , and its overexpression abrogated <PROT2_155871>  Tat </PROT2_155871> trans activation in cells ( CITATION ) . ||9334325_1025_155871==>Third , a cellular kinase complex termed <PROT1_1025>  TAK </PROT1_1025> ( <PROT2_155871>  Tat </PROT2_155871> - activated kinase ) that interacts with the activation domain of <PROT2_155871>  Tat </PROT2_155871> and phosphorylates the CTD of Pol II has been identified as P - TEFb ( CITATION , CITATION , CITATION , TARGET_CITATION ) . ||9334325_1025_155871==>The transactivation domain of <PROT2_155871>  Tat </PROT2_155871> interacts with <PROT1_1025>  TAK </PROT1_1025> ( CITATION , CITATION ) , which was recently shown to be identical to the kinase subunit of P - TEFb , a positive - acting transcription elongation factor ( TARGET_CITATION , CITATION ) . ||9334325_1025_155871==>In a second step , <PROT2_155871>  Tat </PROT2_155871> recruits the <PROT1_1025>  CDK9 </PROT1_1025> / <PROT1_1025>  PITALRE </PROT1_1025> CTD kinase ( TARGET_CITATION , CITATION ) . ||9334325_1025_155871==><PROT2_155871>  Tat </PROT2_155871> transactivation of the HIV LTR has been shown to rely on the recruitment of the CDK7 and <PROT1_1025>  CDK9 </PROT1_1025> / <PROT1_1025>  PITALRE </PROT1_1025> kinases onto the transcription complex ( TARGET_CITATION , CITATION , CITATION ) . ||9334325_1025_155871==>Hence , <PROT1_1025>  CDK9 </PROT1_1025> / <PROT1_1025>  PITALRE </PROT1_1025> is likely to be required for actinomycin stimulation as previously reported for the <PROT2_155871>  Tat </PROT2_155871> - activated transcription of the HIV LTR ( TARGET_CITATION , CITATION ) . ||9334325_1025_155871==>More significant to our purpose , <PROT2_155871>  Tat </PROT2_155871> mediates the recruitment of CDK7 ( CITATION ) and <PROT1_1025>  CDK9 </PROT1_1025> / <PROT1_1025>  PITALRE </PROT1_1025> ( TARGET_CITATION , CITATION , CITATION ) which phosphorylate the CTD and promote an efficient transcription elongation throughout the entire viral genome . ||9334325_1025_155871==>These studies have led to a model in which <PROT2_155871>  Tat </PROT2_155871> is believed to activate transcription elongation by stimulating <PROT1_1025>  TAK </PROT1_1025> to phosphorylate the RNA polymerase II carboxyl - terminal domain ( CTD ) ( CITATION , CITATION and TARGET_CITATION ) . ||9334325_1025_155871==>Immunoprecipitated <PROT1_1025>  TAK </PROT1_1025> is able to phosphorylate exogenous substrates ; however , the purified enzyme appears to be constitutively active and there has been no report demonstrating activation of the enzyme by addition of <PROT2_155871>  Tat </PROT2_155871> and TAR RNA ( CITATION and TARGET_CITATION ) . ||9334325_1025_155871==>Furthermore , although immunodepletion of the HeLa nuclear extracts using antibodies directed against <PROT1_1025>  CDK9 </PROT1_1025> has shown that <PROT1_1025>  TAK </PROT1_1025> plays an important role in HIV transcription , these extracts show defects in both basal and <PROT2_155871>  Tat </PROT2_155871> - activated transcription ( CITATION , CITATION and TARGET_CITATION ) . ||9334325_1025_155871==>Since <PROT1_1025>  CDK9 </PROT1_1025> is known to be inhibited by low DRB concentrations and somewhat resistant to H8 ( CITATION , CITATION and TARGET_CITATION ) , it seems likely that the kinase responsible for the CTD hyperphosphorylation in the presence of <PROT2_155871>  Tat </PROT2_155871> is <PROT1_1025>  CDK9 </PROT1_1025> . ||9334325_1025_155871==>Unfortunately , both in our hands , and in the published reports ( CITATION and TARGET_CITATION ) , removal of <PROT1_1025>  CDK9 </PROT1_1025> from the extracts results in a strong block to transcription initiation , as well as <PROT2_155871>  Tat </PROT2_155871> - mediated transcription . ||9334325_1025_155871==>Since <PROT1_1025>  TAK </PROT1_1025> appeared at first to be just another candidate in a long litany of possible co - factors for <PROT2_155871>  Tat </PROT2_155871> , the significance of this important result was largely overlooked until ( TARGET_CITATION ) cloned the kinase subunit of <PROT1_1025>  TAK </PROT1_1025> . ||9334325_1025_155871==>Since <PROT1_1025>  CDK9 </PROT1_1025> is known to be inhibited by low DRB concentrations and somewhat resistant to H8 ( CITATION , CITATION and TARGET_CITATION ) , it seems likely that the kinase responsible for the CTD hyper - phosphorylation in the presence of <PROT2_155871>  Tat </PROT2_155871> is <PROT1_1025>  CDK9 </PROT1_1025> . ||9334325_1025_155871==>Cellular proteins termed cyclin T and cyclin - dependent kinase 9 ( <PROT1_1025>  CDK9 </PROT1_1025> ) have recently been found to be involved in transcription activation of the HIV - 1 LTR by <PROT2_155871>  Tat </PROT2_155871> ( CITATION , TARGET_CITATION ) . ||9334325_1025_155871==>On the other hand , DRB at 5 & # 181 ; M , which inhibits <PROT1_1025>  cdk9 </PROT1_1025> activity in vitro ( 50 % inhibition at 0.65 & # 181 ; M ) and <PROT2_155871>  Tat </PROT2_155871> - dependent transcription activity in vitro ( 50 % inhibition at 1 & # 181 ; M ; ref. TARGET_CITATION ) , only partially inhibited CTD hyperphosphorylation ( fig. 3 , lane 4 ) and did not affect Pol II ubiquitination ( fig. 3 , lane 9 ) . ||9334325_1025_155871==>This conclusion is based both on in vitro biochemical experiments and transient transfection studies , including the observations that immunodepletion of P - TEFb from extracts competent to support <PROT2_155871>  Tat </PROT2_155871> - activated transcription with anti - <PROT1_1025>  CDK9 </PROT1_1025> antibodies abrogates <PROT2_155871>  Tat </PROT2_155871> - dependent transcription in vitro ( CITATION , TARGET_CITATION ) and that transient overexpression of a catalytically inactive <PROT1_1025>  CDK9 </PROT1_1025> mutant inhibits <PROT2_155871>  Tat </PROT2_155871> - dependent reporter gene expression in intact cells ( CITATION , CITATION ) . ||9334325_1025_155871==>Importantly , immunodepletion of <PROT1_1025>  CDK9 </PROT1_1025> from HeLa nuclear extract eliminated basal transcription elongation and <PROT2_155871>  Tat </PROT2_155871> transactivation ( CITATION , CITATION , TARGET_CITATION ) . ||9334325_1025_155871==><PROT2_155871>  Tat </PROT2_155871> regulates an early step in transcription elongation that requires cyclin T1 ( CycT1 ) and <PROT1_1025>  CDK9 </PROT1_1025> ( CITATION , CITATION , CITATION , CITATION , CITATION - TARGET_CITATION ) , which are subunits of the positive transcription elongation factor P - TEFb ( CITATION ) and <PROT2_155871>  Tat </PROT2_155871> - associated kinase ( CITATION , CITATION , CITATION ) complexes . ||9334325_1025_155871==>First , chemical inhibitors and dominant - negative mutants of <PROT1_1025>  CDK9 </PROT1_1025> block <PROT2_155871>  Tat </PROT2_155871> transactivation and HIV - 1 replication in vivo ( CITATION , CITATION , CITATION , TARGET_CITATION ) . ||9334325_1025_155871==><PROT1_1025>  Cdk9 </PROT1_1025> associated with wild - type <PROT2_155871>  Tat </PROT2_155871> but not activation domain mutants of <PROT2_155871>  Tat </PROT2_155871> ( CITATION , TARGET_CITATION ) , and depletion of P - TEFb from HeLa nuclear extract rendered the extract unable to carry out <PROT2_155871>  Tat </PROT2_155871> transactivation ( TARGET_CITATION ) . ||9334325_1025_155871==>Among the cyclin partners of <PROT1_1025>  CDK9 </PROT1_1025> , CycT1 has been the subject of considerable interest because it is a subunit of <PROT1_1025>  TAK </PROT1_1025> , a cellular protein kinase targeted by the <PROT2_155871>  Tat </PROT2_155871> protein of human immunodeficiency virus ( HIV ) to stimulate RNAP II transcriptional elongation of the integrated proviral genome ( CITATION ; CITATION ; CITATION ; CITATION and TARGET_CITATION ) . ||9334325_1025_155871==><PROT1_1025>  CDK9 </PROT1_1025> is also the catalytic subunit of a protein kinase complex termed <PROT1_1025>  TAK </PROT1_1025> ( <PROT2_155871>  Tat </PROT2_155871> - associated kinase ) that binds to the human immunodeficiency virus types 1 and 2 ( HIV - 1 and HIV - 2 ) <PROT2_155871>  Tat </PROT2_155871> proteins ( CITATION ; CITATION ; CITATION and TARGET_CITATION ) . ||9334325_1025_155871==>It has been firmly established that <PROT1_1025>  TAK </PROT1_1025> in the CycT1 / P - TEFb complex mediates <PROT2_155871>  Tat </PROT2_155871> stimulation of RNAP II elongation of the integrated viral genome ( CITATION ; CITATION ; CITATION ; CITATION ; CITATION ; CITATION ; CITATION ; CITATION ; CITATION ; CITATION and TARGET_CITATION ) . ||9334325_1025_155871==>In nuclear extracts , HIV - 1 <PROT2_155871>  Tat </PROT2_155871> associates tightly with the <PROT1_1025>  CDK9 </PROT1_1025> - containing positive transcription elongation factor complex , P - TEFb ( TARGET_CITATION ) . ||9334325_1025_155871==>Within recent years , the identity of such a <PROT2_155871>  Tat </PROT2_155871> - associated kinase ( <PROT1_1025>  TAK </PROT1_1025> ) was described as the positive transcription elongation factor - b ( P - TEFb ) ( CITATION and TARGET_CITATION ) , a finding that has been confirmed by several additional studies ( CITATION and CITATION ) . ||9334325_1025_155871==>PCAF - acetylated <PROT2_155871>  Tat </PROT2_155871> was found to have an increased affinity ( CITATION ) for the <PROT1_1025>  CDK9 </PROT1_1025> / P - TEFb CTD kinase complex ( CITATION , CITATION , TARGET_CITATION ) , suggesting that this acetylation event enhances transcriptional elongation by bringing about hyperphosphorylation of the RNA polymerase II CTD . ||9334325_1025_155871==>In carrying out its transcriptional activation , <PROT2_155871>  Tat </PROT2_155871> interacts with a number of cellular transcription factors , including TFIID ( TBP ) ( CITATION , CITATION , CITATION ) , TFIID - associated TAFs ( CITATION ) , TFIIH ( CITATION ) , <PROT2_155871>  Tat </PROT2_155871> - associated protein ( TAP ) ( CITATION ) , <PROT2_155871>  Tat </PROT2_155871> - associated kinase ( <PROT1_1025>  TAK </PROT1_1025> / pTEFb ) ( CITATION , CITATION , CITATION , CITATION , CITATION , TARGET_CITATION ) , TTK ( CITATION ) , Sp1 ( CITATION ) , and SF - 1 ( CITATION ) , as well as TAR RNA ( CITATION ) . ||9334325_1025_155871==>In humans , the closest kinase to Ctk1 is <PROT1_1025>  Cdk9 </PROT1_1025> ( for review , see ref. CITATION ) , a CTD kinase that was first described as a positive transcription elongation factor ( CITATION ) , and that interacts specifically with the human immunodeficiency virus - 1 transactivator protein <PROT2_155871>  Tat </PROT2_155871> , which acts to enhance the processivity of RNA polymerase II ( TARGET_CITATION , CITATION ) . ||9334325_1025_155871==>The transactivation domain of <PROT2_155871>  Tat </PROT2_155871> interacts with <PROT1_1025>  TAK </PROT1_1025> ( CITATION , CITATION ) , which was recently shown to be identical to the kinase subunit of P - TEFb ( TARGET_CITATION , CITATION ) . ||9334325_1025_155871==>Enhanced phosphorylation of RNA pol II by <PROT2_155871>  Tat </PROT2_155871> and DRB effect on phosphorylation are in agreement with previous reports showing that DRB inhibits the kinase activity of <PROT1_1025>  CDK9 </PROT1_1025> which is responsible for RNA pol II hyperphosphorylation by <PROT2_155871>  Tat </PROT2_155871> ( TARGET_CITATION , CITATION , CITATION ) . ||9334325_1025_155871==>Positive transcription elongation factor ( P - TEFb ) ( CITATION , CITATION ) or <PROT2_155871>  Tat </PROT2_155871> - associated kinase ( CITATION ) , composed of <PROT1_1025>  CDK9 </PROT1_1025> and cyclin T1 ( CycT1 ) ( CITATION , CITATION - TARGET_CITATION ) , regulates <PROT2_155871>  Tat </PROT2_155871> transactivation at an early step in transcription elongation . ||9334325_1025_155871==>Second , dominant - negative mutants of <PROT1_1025>  CDK9 </PROT1_1025> or kinase inhibitors that preferentially block <PROT1_1025>  CDK9 </PROT1_1025> inhibit <PROT2_155871>  Tat </PROT2_155871> transactivation and HIV - 1 replication in vivo ( CITATION , CITATION , TARGET_CITATION , CITATION ) . ||9334325_1025_155871==>Second , dominant - negative mutants of <PROT1_1025>  CDK9 </PROT1_1025> or <PROT1_1025>  CDK9 </PROT1_1025> kinase inhibitors inhibit <PROT2_155871>  Tat </PROT2_155871> transactivation ( CITATION , CITATION , TARGET_CITATION , CITATION ) . ||9334325_1025_155871==>The postintegration restriction of HIV - 1 transcription is mainly regulated by cellular transcription factors ( CITATION ) and enzymatic activities of cellular proteins , such as <PROT1_1025>  cdk9 </PROT1_1025> / cyclin T ( CITATION , TARGET_CITATION , CITATION , CITATION , CITATION ) and cdk7 / cyclin H ( CITATION , CITATION , CITATION , CITATION ) , which play a critical role in <PROT2_155871>  Tat </PROT2_155871> - mediated transactivation . ||9334325_1025_155871==><PROT1_1025>  cdk9 </PROT1_1025> - cyclin T complex is a critical complex in the control of the <PROT2_155871>  Tat </PROT2_155871> protein function ( CITATION , TARGET_CITATION , CITATION ) , and cdk7 and - 2 have also been shown to associate with the <PROT2_155871>  Tat </PROT2_155871> complex ( CITATION ) . ||9334325_1025_155871==>Recent studies have shown that <PROT2_155871>  Tat </PROT2_155871> may target <PROT1_1025>  cdk9 </PROT1_1025> ( CITATION , TARGET_CITATION , CITATION ) , cdk7 ( CITATION , CITATION , CITATION , CITATION ) , and cdk2 ( CITATION ) to transactivate the HIV - 1 promoter . ||9334325_1025_155871==><PROT2_155871>  Tat </PROT2_155871> stimulates transcription elongation because it is able to activate a specific carboxyl - terminal domain ( CTD ) kinase , <PROT1_1025>  CDK9 </PROT1_1025> ( CITATION , CITATION , TARGET_CITATION ) , which phosphorylates RNA Pol II ( CITATION , CITATION ) , as well as the elongation factor SPT5 ( CITATION , CITATION , CITATION , CITATION ) . ||9334325_1025_155871==>Several independent lines of investigation have established that <PROT1_1025>  TAK </PROT1_1025> ( cyclin T1 / P - TEFb ) plays a critical role in <PROT2_155871>  Tat </PROT2_155871> transactivation ( CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , TARGET_CITATION ) . ||9334325_1025_155871==>It has been well established that <PROT1_1025>  TAK </PROT1_1025> , in the cycT1 / P - TEFb complex , mediates <PROT2_155871>  Tat </PROT2_155871> function ( CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , TARGET_CITATION ) . ||9334325_1025_155871==>Recently , the Pol II - associated cyclin - dependent kinase - activating kinase ( CAK ) ( CITATION , CITATION , CITATION , CITATION ) and the <PROT2_155871>  Tat </PROT2_155871> - associated kinase ( <PROT1_1025>  TAK </PROT1_1025> / P - TEFb ) ( CITATION , CITATION , TARGET_CITATION ) have been implicated in the elongation effects of <PROT2_155871>  Tat </PROT2_155871> . ||9334325_1025_155871==>A kinase activity corresponding to <PROT1_1025>  cdk9 </PROT1_1025> has been detected in the HIV <PROT2_155871>  Tat </PROT2_155871> - associated kinase ( <PROT1_1025>  TAK </PROT1_1025> ) complex , where <PROT1_1025>  cdk9 </PROT1_1025> is required for <PROT2_155871>  Tat </PROT2_155871> - dependent stimulation of transcriptional elongation ( CITATION ; TARGET_CITATION ; CITATION ) . ||9334325_1025_155871==>Kinase activity corresponding to <PROT1_1025>  cdk9 </PROT1_1025> has been detected in the HIV <PROT2_155871>  Tat </PROT2_155871> - associated kinase ( <PROT1_1025>  TAK </PROT1_1025> ) complex , where <PROT1_1025>  cdk9 </PROT1_1025> is required for <PROT2_155871>  Tat </PROT2_155871> - dependent stimulation of transcriptional elongation ( CITATION ; TARGET_CITATION ; CITATION ) . ||9334325_1025_155871==>The association of <PROT1_1025>  cdk9 </PROT1_1025> with cyclin T1 is important for activation of the HIV - 1 promoter by the viral transactivator , <PROT2_155871>  Tat </PROT2_155871> , through the TAR RNA sequence ( CITATION ; TARGET_CITATION ) . ||9334325_1025_155871==>The <PROT2_155871>  Tat </PROT2_155871> - activated <PROT1_1025>  CDK9 </PROT1_1025> is then able to extensively phosphorylate the CTD , creating a unique and highly phosphorylated form that we have designated Pol IIo * ( CITATION , CITATION , CITATION , CITATION , TARGET_CITATION ) . ||9334325_1025_155871==>Second , <PROT2_155871>  Tat </PROT2_155871> - activated phosphorylation showed inhibition profiles of DRB and H8 ( data not shown ) on phosphorylation of RNA polymerase CTD similar to those of <PROT1_1025>  CDK9 </PROT1_1025> as reported previously ( CITATION , TARGET_CITATION ) . ||9334325_1025_155871==>The interaction between <PROT2_155871>  Tat </PROT2_155871> and TAR involves not only the binding of <PROT2_155871>  Tat </PROT2_155871> to TAR RNA but also its association with the <PROT2_155871>  Tat </PROT2_155871> - activated kinase ( <PROT1_1025>  TAK </PROT1_1025> ) ( CITATION , CITATION , TARGET_CITATION ) , a protein kinase that is able to phosphorylate the carboxyl - terminal domain ( CTD ) of the large subunit of RNA polymerase II ( CITATION , CITATION , CITATION ) . ||9334325_1025_155871==>Recent findings revealed that P - TEFb , which contains CycT1 and <PROT1_1025>  Cdk9 </PROT1_1025> , is the key cellular factor that supports <PROT2_155871>  Tat </PROT2_155871> transactivation and HIV replication ( CITATION , CITATION , TARGET_CITATION ) . ||9334325_1025_155871==>It was soon discovered that <PROT1_1025>  TAK </PROT1_1025> is in fact P - TEFb , and the interaction of <PROT2_155871>  Tat </PROT2_155871> and cyclin T1 is responsible for the ability of <PROT2_155871>  Tat </PROT2_155871> to activate HIV transcription ( TARGET_CITATION , CITATION ) . ||9334325_1025_155871==>( TARGET_CITATION and CITATION ) Recent studies have shown that <PROT2_155871>  Tat </PROT2_155871> transactivation function is mediated by a kinase <PROT1_1025>  CDK9 </PROT1_1025> that is recruited to the HIV - 1 promoter by cyclinT1 ( CycT1 ) - <PROT2_155871>  Tat </PROT2_155871> interaction . ||9334325_1025_155871==>Those kinases are named <PROT2_155871>  Tat </PROT2_155871> - associated kinases ( TAKs ) and include the RNAPII CTD kinases , TFIIH ( CITATION , CITATION and CITATION ) and <PROT1_1025>  CDK9 </PROT1_1025> / P - TEFb ( ( TARGET_CITATION , CITATION and CITATION ) , reviewed in ( CITATION ) ) . ||9334325_1025_155871==><PROT2_155871>  Tat </PROT2_155871> binding to TAR is facilitated by its association with <PROT2_155871>  Tat </PROT2_155871> - associated kinase ( <PROT1_1025>  TAK </PROT1_1025> ) ( CITATION ) , which is comprised of the <PROT1_1025>  CDK9 </PROT1_1025> kinase subunit and its binding partner cyclin T1 ( CITATION , CITATION , CITATION , CITATION , TARGET_CITATION ) . ||9334325_1025_155871==>Further complicating the diversity of roles played by the CDK family , was the subsequent observation that a molecule that binds to the HIV <PROT2_155871>  tat </PROT2_155871> gene product also facilitates phosphorylation of the RNA pol II CTD by activating a kinase & # x2013 ; <PROT1_1025>  CDK9 </PROT1_1025> & # x2013 ; that has obvious similarity to the CDKs ( CITATION , TARGET_CITATION , CITATION , CITATION , CITATION and CITATION ) . ||9334325_1025_155871==>In nuclear extracts , HIV - 1 <PROT2_155871>  Tat </PROT2_155871> associates tightly with the <PROT1_1025>  CDK9 </PROT1_1025> - containing positive transcription elongation factor complex b , P - TEFb ( TARGET_CITATION ) . ||9334325_1025_155871==><PROT2_155871>  Tat </PROT2_155871> has been originally shown to co - purify with a nuclear <PROT2_155871>  Tat </PROT2_155871> - associated kinase ( <PROT1_1025>  TAK </PROT1_1025> ) ( CITATION ) , which corresponds to the kinase subunit of the P - TEFb complex ( CITATION and TARGET_CITATION ) . ||9334325_1025_155871==>P - TEFb appears to be required for transcription of most class II genes ( CITATION ; CITATION ; CITATION ; CITATION ; CITATION ) and the human <PROT1_1025>  Cdk9 </PROT1_1025> & # 47 ; cyclin T1 complex is a direct cellular target of the HIV - 1 activator , <PROT2_155871>  Tat </PROT2_155871> ( CITATION ; TARGET_CITATION ; CITATION ; CITATION ; CITATION ; CITATION ) . ||9334325_1025_155871==>Moreover , the <PROT1_1025>  Cdk9 </PROT1_1025> & # 47 ; cyclin T1 is the cellular cofactor of the HIV - 1 transactivator <PROT2_155871>  Tat </PROT2_155871> ( CITATION ; TARGET_CITATION ; CITATION ; CITATION ; CITATION ; CITATION ) . ||9334325_1025_155871==>We found that the CC - <PROT1_1025>  Cdk9 </PROT1_1025> effectively inhibits the <PROT2_155871>  Tat </PROT2_155871> activity , which is specifically dependent upon the endogenous cyclin T1 & # 47 ; <PROT1_1025>  Cdk9 </PROT1_1025> activity ( TARGET_CITATION ; CITATION ; CITATION ; CITATION ; CITATION ; CITATION ) . ||9334325_1025_155871==>P - TEFb complexes consisting of cyclin T1 and <PROT1_1025>  CDK9 </PROT1_1025> serve as a human cellular cofactor for the human immunodeficiency virus type 1 ( HIV - 1 ) protein <PROT2_155871>  Tat </PROT2_155871> ( transactivator of transcription ) ( CITATION , CITATION , CITATION , CITATION , TARGET_CITATION ) . ||9334325_1025_155871==><PROT2_155871>  Tat </PROT2_155871> - associated kinase ( <PROT1_1025>  TAK </PROT1_1025> ) assays were described earlier ( TARGET_CITATION ) . ||9334325_1025_155871==>The transactivation domain of <PROT2_155871>  Tat </PROT2_155871> interacts with <PROT1_1025>  TAK </PROT1_1025> ( CITATION , CITATION ) , which was recently shown to be identical to the kinase subunit of the positive transcription elongation factor complex , P - TEFb ( TARGET_CITATION , CITATION ) . ||9334325_1025_155871==>The <PROT2_155871>  Tat </PROT2_155871> - associated CTD - kinase has been shown to comprise cdc2 - related kinase ( <PROT1_1025>  PITALRE </PROT1_1025> / <PROT1_1025>  CDK9 </PROT1_1025> ) ( CITATION , TARGET_CITATION ) and cyclin ( cyclin T1 ) ( CITATION , CITATION ) . ||9334325_1025_155871==>P - TEFb , also known as <PROT1_1025>  TAK </PROT1_1025> ( <PROT2_155871>  Tat </PROT2_155871> - associated kinase ) ( CITATION ) , consists of the cyclin - dependent kinase <PROT1_1025>  CDK9 </PROT1_1025> and cyclin T1 ( CITATION , TARGET_CITATION ) . ||9334325_1025_155871==>However , in the presence of <PROT2_155871>  Tat </PROT2_155871> , the positive transcription elongation factor b ( P - TEFb ) , consisting of the cyclin - dependent kinase 9 ( <PROT1_1025>  Cdk9 </PROT1_1025> ) and cyclin T1 ( CycT1 ) , is recruited to the transactivation response ( TAR ) element , which forms a stable RNA stem - loop at the 5 ' end of all viral transcripts ( CITATION , CITATION , CITATION , TARGET_CITATION ) . ||9334325_1025_155871==>Once P - TEFb was found to be a DRB - sensitive kinase ( CITATION ) , the identification of its catalytic subunit , <PROT1_1025>  CDK9 </PROT1_1025> , allowed the examination of its requirement for <PROT2_155871>  Tat </PROT2_155871> - mediated transactivation ( TARGET_CITATION ) . ||9334325_1025_155871==>Furthermore , it was shown that the depletion of <PROT1_1025>  CDK9 </PROT1_1025> from HeLa nuclear extracts blocked <PROT2_155871>  Tat </PROT2_155871> - dependent transactivation and <PROT1_1025>  TAK </PROT1_1025> activity , indicating that <PROT1_1025>  CDK9 </PROT1_1025> was a subunit of the cellular cofactor required for <PROT2_155871>  Tat </PROT2_155871> to stimulate transcription ( CITATION and TARGET_CITATION ) . ||9334325_1025_155871==>It is now established that the ternary stable complex <PROT2_155871>  Tat </PROT2_155871> / cyclin T1 / <PROT1_1025>  CDK9 </PROT1_1025> recruited by <PROT2_155871>  Tat </PROT2_155871> to TAR stimulates transcriptional elongation by phosphorylating the CTD of RNA pol II and the negative elongation factors DSIF and NELF ( TARGET_CITATION ; CITATION ; CITATION ; CITATION ; CITATION ; CITATION ; CITATION ; CITATION ; CITATION ; CITATION ; CITATION and CITATION ) . ||9334325_1025_155871==>HIV - 1 transcription is also heavily dependent upon the stimulation of elongation by <PROT2_155871>  Tat </PROT2_155871> ( CITATION , CITATION , CITATION , CITATION ) and the associated cyclin T1 - <PROT1_1025>  CDK9 </PROT1_1025> complex ( CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , TARGET_CITATION ) . ||9334325_1025_155871==>Positive transcription elongation factor b ( P - TEFb ) ( CITATION - CITATION ) or <PROT2_155871>  Tat </PROT2_155871> - associated kinase ( CITATION , CITATION , CITATION ) , composed of cyclin - dependent kinase 9 ( <PROT1_1025>  CDK9 </PROT1_1025> ) and cyclin T1 ( CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , TARGET_CITATION ) , is essential for <PROT2_155871>  Tat </PROT2_155871> transactivation . ||9334325_155871_904==><PROT1_155871>  Tat </PROT1_155871> regulates an early step in transcription elongation that requires cyclin T1 ( <PROT2_904>  CycT1 </PROT2_904> ) and CDK9 ( CITATION , CITATION , CITATION , CITATION , CITATION - TARGET_CITATION ) , which are subunits of the positive transcription elongation factor P - TEFb ( CITATION ) and <PROT1_155871>  Tat </PROT1_155871> - associated kinase ( CITATION , CITATION , CITATION ) complexes . ||9334325_155871_904==>Among the cyclin partners of CDK9 , <PROT2_904>  CycT1 </PROT2_904> has been the subject of considerable interest because it is a subunit of TAK , a cellular protein kinase targeted by the <PROT1_155871>  Tat </PROT1_155871> protein of human immunodeficiency virus ( HIV ) to stimulate RNAP II transcriptional elongation of the integrated proviral genome ( CITATION ; CITATION ; CITATION ; CITATION and TARGET_CITATION ) . ||9334325_155871_904==>It has been firmly established that TAK in the <PROT2_904>  CycT1 </PROT2_904> / P - TEFb complex mediates <PROT1_155871>  Tat </PROT1_155871> stimulation of RNAP II elongation of the integrated viral genome ( CITATION ; CITATION ; CITATION ; CITATION ; CITATION ; CITATION ; CITATION ; CITATION ; CITATION ; CITATION and TARGET_CITATION ) . ||9334325_155871_904==>Positive transcription elongation factor ( P - TEFb ) ( CITATION , CITATION ) or <PROT1_155871>  Tat </PROT1_155871> - associated kinase ( CITATION ) , composed of CDK9 and cyclin T1 ( <PROT2_904>  CycT1 </PROT2_904> ) ( CITATION , CITATION - TARGET_CITATION ) , regulates <PROT1_155871>  Tat </PROT1_155871> transactivation at an early step in transcription elongation . ||9334325_155871_904==>Recently , a novel <PROT1_155871>  Tat </PROT1_155871> - interacting protein , human cyclin T1 ( <PROT2_904>  CycT1 </PROT2_904> ) , was discovered ( CITATION ) , which is a component of the pTEFb transcription factor complex ( CITATION , TARGET_CITATION ) . ||9334325_155871_904==>It has been well established that TAK , in the <PROT2_904>  cycT1 </PROT2_904> / P - TEFb complex , mediates <PROT1_155871>  Tat </PROT1_155871> function ( CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , TARGET_CITATION ) . ||9334325_155871_904==>Recent findings revealed that P - TEFb , which contains <PROT2_904>  CycT1 </PROT2_904> and Cdk9 , is the key cellular factor that supports <PROT1_155871>  Tat </PROT1_155871> transactivation and HIV replication ( CITATION , CITATION , TARGET_CITATION ) . ||9334325_155871_904==>( TARGET_CITATION and CITATION ) Recent studies have shown that <PROT1_155871>  Tat </PROT1_155871> transactivation function is mediated by a kinase CDK9 that is recruited to the HIV - 1 promoter by cyclinT1 ( <PROT2_904>  CycT1 </PROT2_904> ) - <PROT1_155871>  Tat </PROT1_155871> interaction . ||9334325_155871_904==>( TARGET_CITATION , CITATION , CITATION , CITATION , CITATION , CITATION and CITATION ) <PROT1_155871>  Tat </PROT1_155871> interacts with the <PROT2_904>  CycT1 </PROT2_904> subunit of P - TEFb and recruits the kinase complex to TAR RNA . ||9334325_155871_904==>However , in the presence of <PROT1_155871>  Tat </PROT1_155871> , the positive transcription elongation factor b ( P - TEFb ) , consisting of the cyclin - dependent kinase 9 ( Cdk9 ) and cyclin T1 ( <PROT2_904>  CycT1 </PROT2_904> ) , is recruited to the transactivation response ( TAR ) element , which forms a stable RNA stem - loop at the 5 ' end of all viral transcripts ( CITATION , CITATION , CITATION , TARGET_CITATION ) . ||9334326_1025_155871==>The <PROT2_155871>  Tat </PROT2_155871> - affinity column also bound several general transcription factors to a lesser extent , in particular , TFIIF and the cyclin - dependent kinase / P - TEFb subunit , <PROT1_1025>  CDK9 </PROT1_1025> , which have both been implicated in <PROT2_155871>  Tat </PROT2_155871> function previously ( Kato et al. 1992 CITATION ; Moncebo et al. 1997 TARGET_CITATION ; Zhu et al. 1997 CITATION ; Gold et al. 1998 CITATION ; Wei et al. 1998 CITATION ) . ||9334326_1025_155871==>The <PROT2_155871>  Tat </PROT2_155871> - associated kinase ( <PROT1_1025>  TAK </PROT1_1025> ) complex in nuclear extracts ( Herrmann and Rice 1993 CITATION , 1995 CITATION ; Yang et al. 1996 CITATION ) has been shown to contain cyclin T1 , <PROT1_1025>  CDK9 </PROT1_1025> , and other as yet unidentified subunits of P - TEFb ( Mancebo et al. 1997 TARGET_CITATION ; Yang et al. 1997 CITATION ; Zhu et al. 1997 CITATION ; Wei et al. 1998 CITATION ) . ||9334326_1025_155871==>Studies using dominant - negative proteins and specific kinase inhibitors have shown that <PROT1_1025>  CDK9 </PROT1_1025> kinase activity is critical for <PROT2_155871>  Tat </PROT2_155871> transactivation ( Mancebo et al. 1997 TARGET_CITATION ; Zhu et al. 1997 CITATION ; Gold et al. 1998 CITATION ) , and the multisubunit P - TEFb complex , but not hCycT1 and <PROT1_1025>  CDK9 </PROT1_1025> alone , can restore <PROT2_155871>  Tat </PROT2_155871> trans - activation to <PROT1_1025>  CDK9 </PROT1_1025> - depleted extracts in vitro ( Mancebo et al. 1997 TARGET_CITATION ; Zhu et al. 1997 CITATION ; Zhou et al. 1998 CITATION ) . ||9334326_1025_155871==>During the purification of P - TEFb activity from HeLa cells by conventional chromatography , TARGET_CITATION have noticed the existence of two different <PROT1_1025>  CDK9 </PROT1_1025> - containing complexes in HeLa nuclear extract , only one of which could support <PROT2_155871>  Tat </PROT2_155871> activation . ||9334326_1025_155871==>DRB has been shown to inhibit various cellular protein kinases including the TFIIH - associated cyclin - dependent kinase ( CITATION ) , the Drosophila - positive transcription elongation factor p - TEFb ( CITATION ) , and a <PROT2_155871>  Tat </PROT2_155871> - associated kinase referred to as <PROT1_1025>  TAK </PROT1_1025> ( CITATION , CITATION ) , which most likely corresponds to the human homolog of p - TEFb ( CITATION , TARGET_CITATION ) . ||9334326_1025_155871==>The TFIIH - associated cyclin dependent kinase , p - TEFb , and <PROT1_1025>  TAK </PROT1_1025> can all associate with <PROT2_155871>  Tat </PROT2_155871> ( CITATION - TARGET_CITATION ) and phosphorylate the carboxyl - terminal domain of RNA polymerase II ( CITATION , CITATION ) , an event that is thought to be required for the transition into the elongation mode of RNA polymerase II . ||9334326_1025_155871==>Another <PROT2_155871>  Tat </PROT2_155871> associated kinase , <PROT1_1025>  PITALRE </PROT1_1025> , is a component of the transcriptional elongation factor P - TEFb ( CITATION , CITATION , TARGET_CITATION and CITATION ) . ||9334326_1025_155871==>Moreover , like TFIIH ( CITATION , CITATION , CITATION and CITATION ) , <PROT1_1025>  PITALRE </PROT1_1025> is also required for <PROT2_155871>  Tat </PROT2_155871> - mediated effects on HIV - 1 trancriptional elongation ( TARGET_CITATION and CITATION ) . ||9334326_1025_155871==>Thus a current model to explain the ability of <PROT2_155871>  Tat </PROT2_155871> to increase HIV - 1 transcriptional elongation is that <PROT2_155871>  Tat </PROT2_155871> association with RNA polymerase II may alter the ability of cellular kinases such as CDK7 or <PROT1_1025>  PITALRE </PROT1_1025> to phosphorylate specific amino residues in the CTD ( CITATION , CITATION , CITATION , TARGET_CITATION and CITATION ) and this may alter the association of transcription factors such as <PROT2_155871>  Tat </PROT2_155871> - CT1 and <PROT2_155871>  Tat </PROT2_155871> - CT2 , with the HIV - 1 transcription complex . ||9334326_1025_155871==>The kinase activity of the <PROT1_1025>  PITALRE </PROT1_1025> complex is disrupted by compounds that were identified during an in vitro drug screen as inhibitors of <PROT2_155871>  Tat </PROT2_155871> activity ( TARGET_CITATION ) . ||9334326_1025_155871==>Second , there is a strong correlation between the ability of protein kinase inhibitors such as the nucleoside analog DRB to inhibit <PROT1_1025>  TAK </PROT1_1025> ( and P - TEFb ) activity and their ability to block <PROT2_155871>  Tat </PROT2_155871> transactivation ( CITATION , TARGET_CITATION ) . ||9334326_1025_155871==>Finally , dominant negative mutants of <PROT1_1025>  Cdk9 </PROT1_1025> selectively inhibit <PROT2_155871>  Tat </PROT2_155871> transactivation in vivo and in vitro ( CITATION , TARGET_CITATION ) . ||9334326_1025_155871==>While this report was in preparation , we became aware of two recent studies describing the involvement of <PROT1_1025>  PITALRE </PROT1_1025> in <PROT2_155871>  Tat </PROT2_155871> transactivation ( TARGET_CITATION , CITATION ) . ||9334326_1025_155871==>Additionally , one study also investigated the effect of transient overexpression of <PROT1_1025>  PITALRE </PROT1_1025> on <PROT2_155871>  Tat </PROT2_155871> transactivation in vivo ( TARGET_CITATION ) ; however , those data , in contrast to the results presented here , indicate activation of <PROT2_155871>  Tat </PROT2_155871> function by overexpression of wild - type <PROT1_1025>  PITALRE </PROT1_1025> . ||9334326_1025_155871==>In agreement with our results , in this recent study it was also observed that a catalytic mutant of <PROT1_1025>  PITALRE </PROT1_1025> does inhibit <PROT2_155871>  Tat </PROT2_155871> transactivation ( TARGET_CITATION ) . ||9334326_1025_155871==><PROT2_155871>  Tat </PROT2_155871> also exists in a second complex , which contains <PROT1_1025>  CDK9 </PROT1_1025> , which , together with cyclin T1 , hyperphosphorylates the CTD ( TARGET_CITATION , CITATION , CITATION ) . ||9334326_1025_155871==>Furthermore , the inhibition of the kinase activity of <PROT1_1025>  CDK9 </PROT1_1025> abolished <PROT2_155871>  Tat </PROT2_155871> transactivation ( TARGET_CITATION ) . ||9334326_1025_155871==>This finding is in agreement with previous studies , where no effect of <PROT2_155871>  Tat </PROT2_155871> on the activity of <PROT1_1025>  CDK9 </PROT1_1025> was reported in vitro ( TARGET_CITATION ) . ||9334326_1025_155871==>The functional significance of <PROT1_1025>  Cdk9 </PROT1_1025> & # x2013 ; cyclin T was demonstrated by its immunodepletion from transcription extracts , which was found to inhibit activation by <PROT2_155871>  Tat </PROT2_155871> as well as basal transcription ( CITATION , CITATION and TARGET_CITATION ) . ||9334326_1025_155871==>The transactivation domain of <PROT2_155871>  Tat </PROT2_155871> interacts strongly with a nuclear <PROT2_155871>  Tat </PROT2_155871> - associated kinase , called <PROT1_1025>  TAK </PROT1_1025> ( ( CITATION ) ( CITATION ) ) , which was recently shown to be identical to the kinase subunit of P - TEFb ( ( TARGET_CITATION and CITATION ) ) , a positive - acting transcription elongation factor complex that is required for elongation at many genes ( ( CITATION ) ( CITATION ) ) . ||9334326_1025_155871==>Biochemical analysis of purified <PROT1_1025>  TAK </PROT1_1025> / P - TEFb identified the 42 kDa catalytic subunit as a CDC2 - related kinase , <PROT1_1025>  PITALRE </PROT1_1025> ( hereafter called <PROT1_1025>  CDK9 </PROT1_1025> ) , and <PROT1_1025>  CDK9 </PROT1_1025> inhibitors as well as a dominant negative <PROT1_1025>  CDK9 </PROT1_1025> mutant were found to block <PROT2_155871>  Tat </PROT2_155871> transactivation in vivo and in vitro , indicating that <PROT1_1025>  CDK9 </PROT1_1025> kinase activity is critical for <PROT2_155871>  Tat </PROT2_155871> activity ( ( TARGET_CITATION , CITATION and CITATION ) ) . ||9334326_1025_155871==>Previous studies have shown that P - TEFb is critical for DRB - sensitive RNAPII transcription elongation at many promoters ( ( CITATION , CITATION and CITATION ) ) , and immunodepletion of <PROT1_1025>  CDK9 </PROT1_1025> from HeLa nuclear extracts has been shown to inhibit RNAPII elongation as well as <PROT2_155871>  Tat </PROT2_155871> transactivation ( ( TARGET_CITATION and CITATION ) ) . ||9334326_1025_155871==>It is interesting to note that the activation domain of <PROT2_155871>  Tat </PROT2_155871> - 1 ( aa 1 & # x2013 ; 48 ) functions as a potent dominant negative inhibitor of the wild - type <PROT2_155871>  Tat </PROT2_155871> protein without affecting basal HIV - 1 transcription or general elongation , even though cyclin T and the <PROT1_1025>  TAK </PROT1_1025> / P - TEFb complex are required for basal transcription from the HIV - 1 promoter ( Figure 3C ; ( TARGET_CITATION and CITATION ) ) . ||9334326_1025_155871==>Reconstituted transcription systems support efficient elongation of RNAPII transcription in the absence of <PROT1_1025>  TAK </PROT1_1025> / P - TEFb ( ( CITATION and CITATION ) ) , however transcription in such systems is not dependent upon the CTD and does not support <PROT2_155871>  Tat </PROT2_155871> activation through TAR ( ( TARGET_CITATION ) ) . ||9334326_1025_155871==>Reconstitution of <PROT2_155871>  Tat </PROT2_155871> activation in a reconstituted system was shown to require <PROT1_1025>  TAK </PROT1_1025> / P - TEFb and a negative inhibitor ( ( TARGET_CITATION ) ) , which suggests that <PROT1_1025>  TAK </PROT1_1025> / P - TEFb may ultimately counteract a negative regulator of transcription elongation . ||9334326_1025_155871==>Upon binding the regulatory protein <PROT2_155871>  Tat </PROT2_155871> and the host cellular proteins cyclin T and <PROT1_1025>  CDK9 </PROT1_1025> , the TAR hairpin stabilizes RNA polymerase II to allow viral transcription ( CITATION - TARGET_CITATION ) . ||9334326_1025_155871==><PROT2_155871>  Tat </PROT2_155871> interacts with a positive transcription elongation factor complex ( P - TEFb ; refs. TARGET_CITATION , CITATION , and CITATION ) , which contains the cdc2 - related kinase <PROT1_1025>  CDK9 </PROT1_1025> , the cyclin T1 protein , and other , as yet unidentified subunits . ||9334326_1025_155871==>Compounds that block the kinase activity of <PROT1_1025>  CDK9 </PROT1_1025> / P - TEFb already have been shown to be highly effective inhibitors of <PROT2_155871>  Tat </PROT2_155871> ( TARGET_CITATION ) , and drugs that prevent the binding of <PROT2_155871>  Tat </PROT2_155871> to cyclin T1 could be even more specific . ||9334326_1025_155871==>This effect of <PROT2_155871>  Tat </PROT2_155871> requires the CTD of RNAP II CITATION , CITATION , CITATION , CITATION , and <PROT2_155871>  Tat </PROT2_155871> can stimulate the hyperphosphorylation of the CTD in vitro CITATION CITATION . The <PROT2_155871>  Tat </PROT2_155871> - induced Ser / Thr phosphorylation of the CTD is likely to require TFIIH as well as another CTD kinase , which has been identified as the <PROT2_155871>  Tat </PROT2_155871> - associated kinase ( <PROT1_1025>  TAK </PROT1_1025> ) , a component of the P - TEFb elongation factor TARGET_CITATION CITATION . Taken together , these results have led to the proposal that TFIIH and P - TEFb may act in a concerted and sequential manner to achieve the hyperphosphorylation of the CTD and the efficient elongation from the HIV promoter CITATION . ||9334326_1025_155871==>The kinase activity of <PROT1_1025>  CDK9 </PROT1_1025> ( P - TEFb ) is required for <PROT2_155871>  Tat </PROT2_155871> - activity ( TARGET_CITATION ; CITATION ; CITATION ; CITATION ; CITATION ) , and a key role for cyclin T in <PROT2_155871>  Tat </PROT2_155871> function is indicated by the observation that recombinant cyclin T1 , which interacts with <PROT2_155871>  Tat </PROT2_155871> , enhances the affinity and specificity of <PROT2_155871>  Tat </PROT2_155871> & # 150 ; TAR interactions in a loop sequence - dependent manner ( CITATION ; CITATION ; CITATION ) . ||9334326_1025_155871==><PROT1_1025>  CDK9 </PROT1_1025> / P - TEFb was recently identified as a CTD - kinase recruited by <PROT2_155871>  Tat </PROT2_155871> to the HIV promoter ( TARGET_CITATION ; CITATION ; CITATION ) . ||9334326_1025_155871==><PROT2_155871>  Tat </PROT2_155871> - dependent transcription from the HIV LTR is strongly inhibited by low concentrations of 5 , 6 - dichloro - 1 - beta - D - ribofuranosylbenzimidazole ( DRB ) ( CITATION ) and a set of related protein kinase inhibitors derived from nucleosides ( TARGET_CITATION ) due to the selective inhibition of <PROT1_1025>  CDK9 </PROT1_1025> by these compounds . ||9334326_1025_155871==>Previous reports have shown that transient overexpression of <PROT1_1025>  CDK9 </PROT1_1025> specifically inhibits <PROT2_155871>  Tat </PROT2_155871> activation of both transfected and integrated HIV LTRs ( TARGET_CITATION ; CITATION ) . ||9334326_1025_155871==>There is now general agreement that <PROT2_155871>  Tat </PROT2_155871> activation of elongation is mediated by the <PROT1_1025>  TAK </PROT1_1025> kinase ( TARGET_CITATION ; CITATION ; CITATION ; CITATION ) . ||9334326_1025_155871==>Similarly , <PROT2_155871>  Tat </PROT2_155871> - activated transcription can be 'squelched ' by overexpressed <PROT1_1025>  CDK9 </PROT1_1025> ( TARGET_CITATION ; CITATION ) . ||9334326_1025_155871==>It has been established that the Cyclin T1 / <PROT1_1025>  CDK9 </PROT1_1025> complex is recruited to promoter - proximal RNA sequences via <PROT2_155871>  Tat </PROT2_155871> - mediated binding to TAR ( CITATION ; CITATION ; TARGET_CITATION ; CITATION ; CITATION ; CITATION ) . ||9334326_1025_155871==>Immunodepletion of <PROT1_1025>  Cdk9 </PROT1_1025> from HeLa nuclear extract eliminated basal HIV - 1 transcription elongation and <PROT2_155871>  Tat </PROT2_155871> transactivation ( TARGET_CITATION , CITATION , CITATION ) , and the addition of affinity - purified human P - TEFb complex completely restored these two processes ( CITATION ) . ||9334326_1025_155871==>In fact , the inhibition profile of a group of kinase inhibitors on the activity of <PROT1_1025>  Cdk9 </PROT1_1025> - P - TEFb , but not on Cdk7 - TFIIH , correlated very well with their abilities to block <PROT2_155871>  Tat </PROT2_155871> function ( TARGET_CITATION ) . ||9334326_1025_155871==>P - TEFb is comprised minimally of a CTD kinase called <PROT1_1025>  CDK9 </PROT1_1025> , which is part of the <PROT2_155871>  Tat </PROT2_155871> - associated kinase ( TARGET_CITATION , CITATION , CITATION ) , and its cyclin component , termed cyclin T1 , which binds directly to <PROT2_155871>  Tat </PROT2_155871> ( CITATION ) . ||9334326_1025_155871==>Second , the sensitivity of <PROT2_155871>  Tat </PROT2_155871> activation to a spectrum of different drugs mirrors those which inhibit <PROT1_1025>  Cdk9 </PROT1_1025> kinase activity in vitro ( TARGET_CITATION ) . ||9334326_1025_155871==>Finally , overexpression of a mutant <PROT1_1025>  Cdk9 </PROT1_1025> kinase blocks <PROT2_155871>  Tat </PROT2_155871> activation of elongation in human cells ( TARGET_CITATION ) . ||9334326_1025_155871==>The transactivation domain of <PROT2_155871>  Tat </PROT2_155871> interacts with <PROT1_1025>  TAK </PROT1_1025> ( CITATION , CITATION ) , which was recently shown to be identical to the kinase subunit of P - TEFb , a positive - acting transcription elongation factor ( CITATION , TARGET_CITATION ) . ||9334326_1025_155871==>The efficiency of kinase inhibitors in blocking the <PROT2_155871>  Tat </PROT2_155871> activation matches their capacity to inhibit <PROT1_1025>  CDK9 </PROT1_1025> / <PROT1_1025>  PITALRE </PROT1_1025> in vitro ( TARGET_CITATION ) . ||9334326_1025_155871==>The inhibition of <PROT2_155871>  Tat </PROT2_155871> - activated HIV LTR expression is correlated with the capacity to inhibit <PROT1_1025>  CDK9 </PROT1_1025> / <PROT1_1025>  PITALRE </PROT1_1025> in vitro ( TARGET_CITATION ) , which suggests that this kinase provides an essential contribution to the average CTD phosphorylation in mammalian cells . ||9334326_1025_155871==>More significant to our purpose , <PROT2_155871>  Tat </PROT2_155871> mediates the recruitment of CDK7 ( CITATION ) and <PROT1_1025>  CDK9 </PROT1_1025> / <PROT1_1025>  PITALRE </PROT1_1025> ( CITATION , CITATION , TARGET_CITATION ) which phosphorylate the CTD and promote an efficient transcription elongation throughout the entire viral genome . ||9334326_1025_155871==>As the actinomycin - activated ( this work ) and the <PROT2_155871>  Tat </PROT2_155871> - activated ( TARGET_CITATION , CITATION ) transcription are very sensitive to <PROT1_1025>  CDK9 </PROT1_1025> / <PROT1_1025>  PITALRE </PROT1_1025> inhibitors in correlation with their inhibition efficiencies in vitro , this kinase is likely to contribute to both the <PROT2_155871>  Tat </PROT2_155871> and the actinomycin D stimulation of the HIV LTR . ||9334326_1025_155871==>Further evidence that <PROT1_1025>  CDK9 </PROT1_1025> is required for <PROT2_155871>  Tat </PROT2_155871> - activated transcription comes from studies using protein kinase inhibitors that selectively inhibit <PROT1_1025>  CDK9 </PROT1_1025> , including DRB ( CITATION ) and a set of novel nucleoside protein kinase inhibitors ( TARGET_CITATION ) . ||9334326_1025_155871==>These studies have led to a model in which <PROT2_155871>  Tat </PROT2_155871> is believed to activate transcription elongation by stimulating <PROT1_1025>  TAK </PROT1_1025> to phosphorylate the RNA polymerase II carboxyl - terminal domain ( CTD ) ( TARGET_CITATION , CITATION and CITATION ) . ||9334326_1025_155871==>Furthermore , although immunodepletion of the HeLa nuclear extracts using antibodies directed against <PROT1_1025>  CDK9 </PROT1_1025> has shown that <PROT1_1025>  TAK </PROT1_1025> plays an important role in HIV transcription , these extracts show defects in both basal and <PROT2_155871>  Tat </PROT2_155871> - activated transcription ( CITATION , TARGET_CITATION and CITATION ) . ||9334326_1025_155871==>Since <PROT1_1025>  CDK9 </PROT1_1025> is known to be inhibited by low DRB concentrations and somewhat resistant to H8 ( TARGET_CITATION , CITATION and CITATION ) , it seems likely that the kinase responsible for the CTD hyperphosphorylation in the presence of <PROT2_155871>  Tat </PROT2_155871> is <PROT1_1025>  CDK9 </PROT1_1025> . ||9334326_1025_155871==>Since <PROT1_1025>  CDK9 </PROT1_1025> is known to be inhibited by low DRB concentrations and somewhat resistant to H8 ( TARGET_CITATION , CITATION and CITATION ) , it seems likely that the kinase responsible for the CTD hyper - phosphorylation in the presence of <PROT2_155871>  Tat </PROT2_155871> is <PROT1_1025>  CDK9 </PROT1_1025> . ||9334326_1025_155871==><PROT2_155871>  Tat </PROT2_155871> directly interacts with at least two cellular kinases , CDK7 ( CITATION , CITATION ) and <PROT1_1025>  CDK9 </PROT1_1025> ( TARGET_CITATION , CITATION ) , to stimulate hyperphosphorylation of the C - terminal domain of RNA polymerase II and increase the processivity of the elongating transcription complexes . ||9334326_1025_155871==>The fact that overexpression of the cyclin - dependent kinases <PROT1_1025>  CDK9 </PROT1_1025> and CDK7 , which are believed to be essential for <PROT2_155871>  Tat </PROT2_155871> - mediated transcriptional activation ( CITATION , CITATION , CITATION , TARGET_CITATION , CITATION ) , had no effect on the process of reverse transcription further serves to distinguish the role of <PROT2_155871>  Tat </PROT2_155871> in reverse transcription from its role in transcription . ||9334326_1025_155871==>This conclusion is based both on in vitro biochemical experiments and transient transfection studies , including the observations that immunodepletion of P - TEFb from extracts competent to support <PROT2_155871>  Tat </PROT2_155871> - activated transcription with anti - <PROT1_1025>  CDK9 </PROT1_1025> antibodies abrogates <PROT2_155871>  Tat </PROT2_155871> - dependent transcription in vitro ( TARGET_CITATION , CITATION ) and that transient overexpression of a catalytically inactive <PROT1_1025>  CDK9 </PROT1_1025> mutant inhibits <PROT2_155871>  Tat </PROT2_155871> - dependent reporter gene expression in intact cells ( TARGET_CITATION , CITATION ) . ||9334326_1025_155871==>Structurally discrete compounds initially identified as inhibitors of <PROT2_155871>  Tat </PROT2_155871> - activated transcription were later shown to inhibit both <PROT1_1025>  CDK9 </PROT1_1025> kinase activity and <PROT2_155871>  Tat </PROT2_155871> activation with a high degree of correlation in vitro ( TARGET_CITATION ) . ||9334326_1025_155871==>The observation that inhibitors of <PROT1_1025>  CDK9 </PROT1_1025> kinase activity can abolish <PROT2_155871>  Tat </PROT2_155871> - dependent transcription from the HIV - 1 LTR promoter ( TARGET_CITATION ) at drug concentrations that do not affect transcription from other pol II promoters suggests that <PROT2_155871>  Tat </PROT2_155871> - dependent gene expression may be critically dependent on <PROT1_1025>  CDK9 </PROT1_1025> . ||9334326_1025_155871==>Structurally discrete compounds previously shown to inhibit <PROT2_155871>  Tat </PROT2_155871> - activated gene expression and <PROT1_1025>  CDK9 </PROT1_1025> kinase activity selectively in vitro ( TARGET_CITATION ) were tested for the ability to block both acute and chronic HIV - 1 replication in cells . ||9334326_1025_155871==>Inhibition of HIV - 1 replication was attained at drug concentrations below those observed to cause cytotoxicity ( IC50s of 9.0 , 2.5 , and 1.0 & # 181 ; M vs. EC50s of 38 , 38 , and 25 & # 181 ; M , respectively ) and within the concentration range previously shown to inhibit <PROT2_155871>  Tat </PROT2_155871> - activation and <PROT1_1025>  CDK9 </PROT1_1025> kinase activity ( TARGET_CITATION ) . ||9334326_1025_155871==>Previously , we showed that transient overexpression of this catalytically inactive <PROT1_1025>  CDK9 </PROT1_1025> mutant inhibits <PROT2_155871>  Tat </PROT2_155871> - transactivated reporter gene expression ( TARGET_CITATION ) . ||9334326_1025_155871==>Furthermore , by using dominant - negative <PROT1_1025>  Cdk9 </PROT1_1025> ( TARGET_CITATION ) as well as a mutant P - TEFb complex with a kinase - defective <PROT1_1025>  Cdk9 </PROT1_1025> ( CITATION ) , the kinase activity of <PROT1_1025>  Cdk9 </PROT1_1025> was shown to be required for P - TEFb to mediate basal as well as <PROT2_155871>  Tat </PROT2_155871> - activated HIV - 1 transcription in vivo and in vitro . ||9334326_1025_155871==>To this end , we used a mutant <PROT1_1025>  CDK9 </PROT1_1025> protein ( DNCDK9 ) that contains a substitution of the aspartate at position 167 to asparagine , which abolishes its kinase activity and blocks <PROT2_155871>  Tat </PROT2_155871> transactivation ( ( TARGET_CITATION ) ) . ||9334326_1025_155871==>Importantly , immunodepletion of <PROT1_1025>  CDK9 </PROT1_1025> from HeLa nuclear extract eliminated basal transcription elongation and <PROT2_155871>  Tat </PROT2_155871> transactivation ( TARGET_CITATION , CITATION , CITATION ) . ||9334326_1025_155871==>By using kinase inhibitors or introduction of dominant negative <PROT1_1025>  Cdk9 </PROT1_1025> mutants into cells , the functional significance of P - TEFb and its kinase activity in <PROT2_155871>  Tat </PROT2_155871> activation has also been demonstrated in vivo ( CITATION , TARGET_CITATION ) . ||9334326_1025_155871==><PROT2_155871>  Tat </PROT2_155871> regulates an early step in transcription elongation that requires cyclin T1 ( CycT1 ) and <PROT1_1025>  CDK9 </PROT1_1025> ( CITATION , TARGET_CITATION , CITATION , CITATION , CITATION ) , which are subunits of the positive transcription elongation factor P - TEFb ( CITATION ) and <PROT2_155871>  Tat </PROT2_155871> - associated kinase ( CITATION , CITATION , CITATION ) complexes . ||9334326_1025_155871==>First , chemical inhibitors and dominant - negative mutants of <PROT1_1025>  CDK9 </PROT1_1025> block <PROT2_155871>  Tat </PROT2_155871> transactivation and HIV - 1 replication in vivo ( CITATION , CITATION , TARGET_CITATION , CITATION ) . ||9334326_1025_155871==>Second , removal of either <PROT1_1025>  CDK9 </PROT1_1025> or CycT1 from HeLa nuclear extracts blocks both transcription elongation and <PROT2_155871>  Tat </PROT2_155871> transactivation ( TARGET_CITATION , CITATION ) . ||9334326_1025_155871==>Concerning dominant - negative inhibitors of <PROT2_155871>  Tat </PROT2_155871> ( TARGET_CITATION ) , our data indicate that the effectiveness of the catalytically inactive <PROT1_1025>  CDK9 </PROT1_1025> - D167N protein as an inhibitor of HIV - 1 <PROT2_155871>  Tat </PROT2_155871> may be affected by its ability to be phosphorylated by endogenous P - TEFb and other kinases . ||9334326_1025_155871==>It has been firmly established that <PROT1_1025>  TAK </PROT1_1025> in the CycT1 / P - TEFb complex mediates <PROT2_155871>  Tat </PROT2_155871> stimulation of RNAP II elongation of the integrated viral genome ( CITATION ; CITATION ; CITATION ; CITATION ; CITATION ; TARGET_CITATION ; CITATION ; CITATION ; CITATION ; CITATION and CITATION ) . ||9334326_1025_155871==>The kinase activity of <PROT1_1025>  Cdk9 </PROT1_1025> was found to be essential for P - TEFb to phosphorylate the C - terminal domain ( CTD ) of RNA polymerase II and to mediate <PROT2_155871>  Tat </PROT2_155871> transactivation ( CITATION - TARGET_CITATION ) . ||9334326_1025_155871==>In nuclear extracts , HIV - 1 <PROT2_155871>  Tat </PROT2_155871> associates tightly with the <PROT1_1025>  CDK9 </PROT1_1025> - containing positive transcription elongation factor complex , P - TEFb ( CITATION - TARGET_CITATION ) . ||9334326_1025_155871==>The general transcription factors TBP / TFIID ( CITATION ) and TFIIH ( CITATION ) and the elongation factors <PROT1_1025>  TAK </PROT1_1025> / PTEFb ( CITATION , TARGET_CITATION ) and cyclin T ( CITATION ) associate with <PROT2_155871>  Tat </PROT2_155871> . ||9334326_1025_155871==>Within recent years , the identity of such a <PROT2_155871>  Tat </PROT2_155871> - associated kinase ( <PROT1_1025>  TAK </PROT1_1025> ) was described as the positive transcription elongation factor - b ( P - TEFb ) ( TARGET_CITATION and CITATION ) , a finding that has been confirmed by several additional studies ( CITATION and CITATION ) . ||9334326_1025_155871==>In carrying out its transcriptional activation , <PROT2_155871>  Tat </PROT2_155871> interacts with a number of cellular transcription factors , including TFIID ( TBP ) ( CITATION , CITATION , CITATION ) , TFIID - associated TAFs ( CITATION ) , TFIIH ( CITATION ) , <PROT2_155871>  Tat </PROT2_155871> - associated protein ( TAP ) ( CITATION ) , <PROT2_155871>  Tat </PROT2_155871> - associated kinase ( <PROT1_1025>  TAK </PROT1_1025> / pTEFb ) ( CITATION , CITATION , CITATION , TARGET_CITATION , CITATION , CITATION ) , TTK ( CITATION ) , Sp1 ( CITATION ) , and SF - 1 ( CITATION ) , as well as TAR RNA ( CITATION ) . ||9334326_1025_155871==>Previous reports documented that basal HIV transcription can also be inhibited by specific <PROT1_1025>  CDK9 </PROT1_1025> inhibitors , suggesting that <PROT1_1025>  CDK9 </PROT1_1025> may be involved in HIV transcription in the absence of <PROT2_155871>  Tat </PROT2_155871> ( TARGET_CITATION ) . ||9334326_1025_155871==>The transactivation domain of <PROT2_155871>  Tat </PROT2_155871> interacts with <PROT1_1025>  TAK </PROT1_1025> ( CITATION , CITATION ) , which was recently shown to be identical to the kinase subunit of P - TEFb ( CITATION , TARGET_CITATION ) . ||9334326_1025_155871==>Enhanced phosphorylation of RNA pol II by <PROT2_155871>  Tat </PROT2_155871> and DRB effect on phosphorylation are in agreement with previous reports showing that DRB inhibits the kinase activity of <PROT1_1025>  CDK9 </PROT1_1025> which is responsible for RNA pol II hyperphosphorylation by <PROT2_155871>  Tat </PROT2_155871> ( CITATION , TARGET_CITATION , CITATION ) . ||9334326_1025_155871==>Positive transcription elongation factor ( P - TEFb ) ( CITATION , CITATION ) or <PROT2_155871>  Tat </PROT2_155871> - associated kinase ( CITATION ) , composed of <PROT1_1025>  CDK9 </PROT1_1025> and cyclin T1 ( CycT1 ) ( CITATION , TARGET_CITATION ) , regulates <PROT2_155871>  Tat </PROT2_155871> transactivation at an early step in transcription elongation . ||9334326_1025_155871==>Second , dominant - negative mutants of <PROT1_1025>  CDK9 </PROT1_1025> or kinase inhibitors that preferentially block <PROT1_1025>  CDK9 </PROT1_1025> inhibit <PROT2_155871>  Tat </PROT2_155871> transactivation and HIV - 1 replication in vivo ( CITATION , TARGET_CITATION , CITATION , CITATION ) . ||9334326_1025_155871==>Second , dominant - negative mutants of <PROT1_1025>  CDK9 </PROT1_1025> or <PROT1_1025>  CDK9 </PROT1_1025> kinase inhibitors inhibit <PROT2_155871>  Tat </PROT2_155871> transactivation ( CITATION , TARGET_CITATION , CITATION , CITATION ) . ||9334326_1025_155871==>The expression of D167N in trans inhibits strongly <PROT1_1025>  CDK9 </PROT1_1025> - dependent <PROT2_155871>  Tat </PROT2_155871> - activated transcription in a wide variety of cell types ( CITATION , TARGET_CITATION ) , but it has only minimal effects on <PROT1_1025>  CDK9 </PROT1_1025> - independent transcriptional elongation stimulated by a cellular enhancer ( CITATION ) . ||9334326_1025_155871==><PROT2_155871>  Tat </PROT2_155871> stimulates transcription elongation because it is able to activate a specific carboxyl - terminal domain ( CTD ) kinase , <PROT1_1025>  CDK9 </PROT1_1025> ( TARGET_CITATION , CITATION , CITATION ) , which phosphorylates RNA Pol II ( CITATION , CITATION ) , as well as the elongation factor SPT5 ( CITATION , CITATION , CITATION , CITATION ) . ||9334326_1025_155871==>Several independent lines of investigation have established that <PROT1_1025>  TAK </PROT1_1025> ( cyclin T1 / P - TEFb ) plays a critical role in <PROT2_155871>  Tat </PROT2_155871> transactivation ( CITATION , CITATION , CITATION , CITATION , CITATION , TARGET_CITATION , CITATION , CITATION , CITATION , CITATION , CITATION ) . ||9334326_1025_155871==>It has been well established that <PROT1_1025>  TAK </PROT1_1025> , in the cycT1 / P - TEFb complex , mediates <PROT2_155871>  Tat </PROT2_155871> function ( CITATION , CITATION , CITATION , CITATION , CITATION , TARGET_CITATION , CITATION , CITATION , CITATION , CITATION , CITATION ) . ||9334326_1025_155871==>Upon overexpression , the <PROT1_1025>  Cdk9 </PROT1_1025> - dn protein inhibits wild - type ( wt ) <PROT1_1025>  Cdk9 </PROT1_1025> function as demonstrated by specific inhibition of <PROT2_155871>  Tat </PROT2_155871> transactivation of the HIV - 1 LTR ( CITATION , TARGET_CITATION ) . ||9334326_1025_155871==>Recently , the Pol II - associated cyclin - dependent kinase - activating kinase ( CAK ) ( CITATION , CITATION , CITATION , CITATION ) and the <PROT2_155871>  Tat </PROT2_155871> - associated kinase ( <PROT1_1025>  TAK </PROT1_1025> / P - TEFb ) ( TARGET_CITATION , CITATION , CITATION ) have been implicated in the elongation effects of <PROT2_155871>  Tat </PROT2_155871> . ||9334326_1025_155871==>A kinase activity corresponding to <PROT1_1025>  cdk9 </PROT1_1025> has been detected in the HIV <PROT2_155871>  Tat </PROT2_155871> - associated kinase ( <PROT1_1025>  TAK </PROT1_1025> ) complex , where <PROT1_1025>  cdk9 </PROT1_1025> is required for <PROT2_155871>  Tat </PROT2_155871> - dependent stimulation of transcriptional elongation ( CITATION ; CITATION ; TARGET_CITATION ) . ||9334326_1025_155871==>Kinase activity corresponding to <PROT1_1025>  cdk9 </PROT1_1025> has been detected in the HIV <PROT2_155871>  Tat </PROT2_155871> - associated kinase ( <PROT1_1025>  TAK </PROT1_1025> ) complex , where <PROT1_1025>  cdk9 </PROT1_1025> is required for <PROT2_155871>  Tat </PROT2_155871> - dependent stimulation of transcriptional elongation ( CITATION ; CITATION ; TARGET_CITATION ) . ||9334326_1025_155871==>The <PROT2_155871>  Tat </PROT2_155871> - activated <PROT1_1025>  CDK9 </PROT1_1025> is then able to extensively phosphorylate the CTD , creating a unique and highly phosphorylated form that we have designated Pol IIo * ( CITATION , CITATION , TARGET_CITATION , CITATION , CITATION ) . ||9334326_1025_155871==><PROT2_155871>  Tat </PROT2_155871> activity can be blocked selectively by inhibitors that block <PROT1_1025>  CDK9 </PROT1_1025> kinase activity ( TARGET_CITATION ) or interfere with CycT1 ( CITATION ) or by immunodepletion of <PROT1_1025>  CDK9 </PROT1_1025> from nuclear extracts ( CITATION ) . ||9334326_1025_155871==>The interaction between <PROT2_155871>  Tat </PROT2_155871> and TAR involves not only the binding of <PROT2_155871>  Tat </PROT2_155871> to TAR RNA but also its association with the <PROT2_155871>  Tat </PROT2_155871> - activated kinase ( <PROT1_1025>  TAK </PROT1_1025> ) ( CITATION , TARGET_CITATION , CITATION ) , a protein kinase that is able to phosphorylate the carboxyl - terminal domain ( CTD ) of the large subunit of RNA polymerase II ( CITATION , CITATION , CITATION ) . ||9334326_1025_155871==>Recent findings revealed that P - TEFb , which contains CycT1 and <PROT1_1025>  Cdk9 </PROT1_1025> , is the key cellular factor that supports <PROT2_155871>  Tat </PROT2_155871> transactivation and HIV replication ( CITATION , TARGET_CITATION , CITATION ) . ||9334326_1025_155871==>Moreover , <PROT2_155871>  Tat </PROT2_155871> binds to cyclin T and recruits <PROT1_1025>  CDK9 </PROT1_1025> to increase the processivity of RNA polymerase II ( CITATION , CITATION - TARGET_CITATION ) . ||9334326_1025_155871==>In particular , the molecular mechanisms underlying the transcriptional function of <PROT2_155871>  Tat </PROT2_155871> have been clarified by showing that <PROT2_155871>  Tat </PROT2_155871> interacts with the p - TEFb complex , which includes the <PROT1_1025>  Cdk9 </PROT1_1025> cyclin - dependent kinase physically associated to members of cyclin T such as T1 , T2a , and T2b ( CITATION , CITATION , TARGET_CITATION , CITATION ) . ||9334326_1025_155871==>( CITATION and TARGET_CITATION ) Recent studies have shown that <PROT2_155871>  Tat </PROT2_155871> transactivation function is mediated by a kinase <PROT1_1025>  CDK9 </PROT1_1025> that is recruited to the HIV - 1 promoter by cyclinT1 ( CycT1 ) - <PROT2_155871>  Tat </PROT2_155871> interaction . ||9334326_1025_155871==><PROT2_155871>  Tat </PROT2_155871> binding to TAR is facilitated by its association with <PROT2_155871>  Tat </PROT2_155871> - associated kinase ( <PROT1_1025>  TAK </PROT1_1025> ) ( CITATION ) , which is comprised of the <PROT1_1025>  CDK9 </PROT1_1025> kinase subunit and its binding partner cyclin T1 ( CITATION , TARGET_CITATION , CITATION , CITATION , CITATION ) . ||9334326_1025_155871==>In nuclear extracts , HIV - 1 <PROT2_155871>  Tat </PROT2_155871> associates tightly with the <PROT1_1025>  CDK9 </PROT1_1025> - containing positive transcription elongation factor complex b , P - TEFb ( CITATION - TARGET_CITATION ) . ||9334326_1025_155871==>The transactivation activity of <PROT2_155871>  Tat </PROT2_155871> is mediated by the interaction with a positive transcription elongation factor , P - TEFb ( CITATION ) , composed of <PROT1_1025>  CDK9 </PROT1_1025> and cyclin T1 ( TARGET_CITATION ) . ||9334326_1025_155871==><PROT2_155871>  Tat </PROT2_155871> has been originally shown to co - purify with a nuclear <PROT2_155871>  Tat </PROT2_155871> - associated kinase ( <PROT1_1025>  TAK </PROT1_1025> ) ( CITATION ) , which corresponds to the kinase subunit of the P - TEFb complex ( TARGET_CITATION and CITATION ) . ||9334326_1025_155871==>P - TEFb appears to be required for transcription of most class II genes ( CITATION ; CITATION ; CITATION ; CITATION ; CITATION ) and the human <PROT1_1025>  Cdk9 </PROT1_1025> & # 47 ; cyclin T1 complex is a direct cellular target of the HIV - 1 activator , <PROT2_155871>  Tat </PROT2_155871> ( TARGET_CITATION ; CITATION ; CITATION ; CITATION ; CITATION ; CITATION ) . ||9334326_1025_155871==>Moreover , the <PROT1_1025>  Cdk9 </PROT1_1025> & # 47 ; cyclin T1 is the cellular cofactor of the HIV - 1 transactivator <PROT2_155871>  Tat </PROT2_155871> ( TARGET_CITATION ; CITATION ; CITATION ; CITATION ; CITATION ; CITATION ) . ||9334326_1025_155871==>We found that the CC - <PROT1_1025>  Cdk9 </PROT1_1025> effectively inhibits the <PROT2_155871>  Tat </PROT2_155871> activity , which is specifically dependent upon the endogenous cyclin T1 & # 47 ; <PROT1_1025>  Cdk9 </PROT1_1025> activity ( CITATION ; TARGET_CITATION ; CITATION ; CITATION ; CITATION ; CITATION ) . ||9334326_1025_155871==>The transactivation domain of <PROT2_155871>  Tat </PROT2_155871> interacts with <PROT1_1025>  TAK </PROT1_1025> ( CITATION , CITATION ) , which was recently shown to be identical to the kinase subunit of the positive transcription elongation factor complex , P - TEFb ( CITATION , TARGET_CITATION ) . ||9334326_1025_155871==>The <PROT2_155871>  Tat </PROT2_155871> transcriptional elongation cofactor <PROT2_155871>  TAT </PROT2_155871> - SF1 ( CITATION and CITATION ) associates both with cyclin T1 ( CITATION and CITATION ) in human positive transcription elongation factor b ( P - TEFb , formerly known as <PROT2_155871>  Tat </PROT2_155871> - associated kinase ( <PROT1_1025>  TAK </PROT1_1025> ) ) ( CITATION , CITATION , TARGET_CITATION , CITATION , CITATION and CITATION ) , a protein kinase heterodimer of cyclin - dependent kinase 9 ( <PROT1_1025>  CDK9 </PROT1_1025> ) and its cyclin T1 ( CYCT1 ) subunit ( CITATION , CITATION , TARGET_CITATION and CITATION ) , and the snRNPs ( CITATION ) essential for mono - splicing of the vpu & # x2013 ; env mRNA . ||9334326_1025_155871==>Recent studies indicate that <PROT2_155871>  Tat </PROT2_155871> induces hyperphosphorylation of the carboxyl - terminal domain of RNA polymerase II , primarily through recruitment of positive transcription elongation factor b ( P - TEFb ) , which contains a cyclin - dependent kinase ( <PROT1_1025>  Cdk9 </PROT1_1025> ) , and perhaps through recruitment of other carboxyl - terminal domain kinases ( CITATION , CITATION , CITATION , CITATION , CITATION , TARGET_CITATION ) . ||9334326_1025_155871==>However , in the presence of <PROT2_155871>  Tat </PROT2_155871> , the positive transcription elongation factor b ( P - TEFb ) , consisting of the cyclin - dependent kinase 9 ( <PROT1_1025>  Cdk9 </PROT1_1025> ) and cyclin T1 ( CycT1 ) , is recruited to the transactivation response ( TAR ) element , which forms a stable RNA stem - loop at the 5 ' end of all viral transcripts ( TARGET_CITATION , CITATION , CITATION , CITATION ) . ||9334326_155871_904==><PROT1_155871>  Tat </PROT1_155871> regulates an early step in transcription elongation that requires cyclin T1 ( <PROT2_904>  CycT1 </PROT2_904> ) and CDK9 ( CITATION , TARGET_CITATION , CITATION , CITATION , CITATION ) , which are subunits of the positive transcription elongation factor P - TEFb ( CITATION ) and <PROT1_155871>  Tat </PROT1_155871> - associated kinase ( CITATION , CITATION , CITATION ) complexes . ||9334326_155871_904==>Second , removal of either CDK9 or <PROT2_904>  CycT1 </PROT2_904> from HeLa nuclear extracts blocks both transcription elongation and <PROT1_155871>  Tat </PROT1_155871> transactivation ( TARGET_CITATION , CITATION ) . ||9334326_155871_904==>It has been firmly established that TAK in the <PROT2_904>  CycT1 </PROT2_904> / P - TEFb complex mediates <PROT1_155871>  Tat </PROT1_155871> stimulation of RNAP II elongation of the integrated viral genome ( CITATION ; CITATION ; CITATION ; CITATION ; CITATION ; TARGET_CITATION ; CITATION ; CITATION ; CITATION ; CITATION and CITATION ) . ||9334326_155871_904==>Positive transcription elongation factor ( P - TEFb ) ( CITATION , CITATION ) or <PROT1_155871>  Tat </PROT1_155871> - associated kinase ( CITATION ) , composed of CDK9 and cyclin T1 ( <PROT2_904>  CycT1 </PROT2_904> ) ( CITATION , TARGET_CITATION ) , regulates <PROT1_155871>  Tat </PROT1_155871> transactivation at an early step in transcription elongation . ||9334326_155871_904==>Recently , a novel <PROT1_155871>  Tat </PROT1_155871> - interacting protein , human cyclin T1 ( <PROT2_904>  CycT1 </PROT2_904> ) , was discovered ( CITATION ) , which is a component of the pTEFb transcription factor complex ( TARGET_CITATION , CITATION ) . ||9334326_155871_904==>It has been well established that TAK , in the <PROT2_904>  cycT1 </PROT2_904> / P - TEFb complex , mediates <PROT1_155871>  Tat </PROT1_155871> function ( CITATION , CITATION , CITATION , CITATION , CITATION , TARGET_CITATION , CITATION , CITATION , CITATION , CITATION , CITATION ) . ||9334326_155871_904==><PROT1_155871>  Tat </PROT1_155871> activity can be blocked selectively by inhibitors that block CDK9 kinase activity ( TARGET_CITATION ) or interfere with <PROT2_904>  CycT1 </PROT2_904> ( CITATION ) or by immunodepletion of CDK9 from nuclear extracts ( CITATION ) . ||9334326_155871_904==>Recent findings revealed that P - TEFb , which contains <PROT2_904>  CycT1 </PROT2_904> and Cdk9 , is the key cellular factor that supports <PROT1_155871>  Tat </PROT1_155871> transactivation and HIV replication ( CITATION , TARGET_CITATION , CITATION ) . ||9334326_155871_904==>( CITATION and TARGET_CITATION ) Recent studies have shown that <PROT1_155871>  Tat </PROT1_155871> transactivation function is mediated by a kinase CDK9 that is recruited to the HIV - 1 promoter by cyclinT1 ( <PROT2_904>  CycT1 </PROT2_904> ) - <PROT1_155871>  Tat </PROT1_155871> interaction . ||9334326_155871_904==>( CITATION , TARGET_CITATION , CITATION , CITATION , CITATION , CITATION and CITATION ) <PROT1_155871>  Tat </PROT1_155871> interacts with the <PROT2_904>  CycT1 </PROT2_904> subunit of P - TEFb and recruits the kinase complex to TAR RNA . ||9334326_155871_904==>The <PROT1_155871>  Tat </PROT1_155871> transcriptional elongation cofactor <PROT1_155871>  TAT </PROT1_155871> - SF1 ( CITATION and CITATION ) associates both with cyclin T1 ( CITATION and CITATION ) in human positive transcription elongation factor b ( P - TEFb , formerly known as <PROT1_155871>  Tat </PROT1_155871> - associated kinase ( TAK ) ) ( CITATION , CITATION , TARGET_CITATION , CITATION , CITATION and CITATION ) , a protein kinase heterodimer of cyclin - dependent kinase 9 ( CDK9 ) and its cyclin T1 ( <PROT2_904>  CYCT1 </PROT2_904> ) subunit ( CITATION , CITATION , TARGET_CITATION and CITATION ) , and the snRNPs ( CITATION ) essential for mono - splicing of the vpu & # x2013 ; env mRNA . ||9334326_155871_904==>However , in the presence of <PROT1_155871>  Tat </PROT1_155871> , the positive transcription elongation factor b ( P - TEFb ) , consisting of the cyclin - dependent kinase 9 ( Cdk9 ) and cyclin T1 ( <PROT2_904>  CycT1 </PROT2_904> ) , is recruited to the transactivation response ( TAR ) element , which forms a stable RNA stem - loop at the 5 ' end of all viral transcripts ( TARGET_CITATION , CITATION , CITATION , CITATION ) . ||9557739_1025_155871==>The <PROT2_155871>  Tat </PROT2_155871> - affinity column also bound several general transcription factors to a lesser extent , in particular , TFIIF and the cyclin - dependent kinase / P - TEFb subunit , <PROT1_1025>  CDK9 </PROT1_1025> , which have both been implicated in <PROT2_155871>  Tat </PROT2_155871> function previously ( Kato et al. 1992 CITATION ; Moncebo et al. 1997 CITATION ; Zhu et al. 1997 CITATION ; Gold et al. 1998 TARGET_CITATION ; Wei et al. 1998 CITATION ) . ||9557739_1025_155871==>Studies using dominant - negative proteins and specific kinase inhibitors have shown that <PROT1_1025>  CDK9 </PROT1_1025> kinase activity is critical for <PROT2_155871>  Tat </PROT2_155871> transactivation ( Mancebo et al. 1997 CITATION ; Zhu et al. 1997 CITATION ; Gold et al. 1998 TARGET_CITATION ) , and the multisubunit P - TEFb complex , but not hCycT1 and <PROT1_1025>  CDK9 </PROT1_1025> alone , can restore <PROT2_155871>  Tat </PROT2_155871> trans - activation to <PROT1_1025>  CDK9 </PROT1_1025> - depleted extracts in vitro ( Mancebo et al. 1997 CITATION ; Zhu et al. 1997 CITATION ; Zhou et al. 1998 CITATION ) . ||9557739_1025_155871==>Recent studies have shown that <PROT2_155871>  Tat </PROT2_155871> transactivation can be mimicked by tethering hCycT1 or <PROT1_1025>  CDK9 </PROT1_1025> to heterologous RNA targets ( Fujinaga et al. 1998 CITATION ; Gold et al. 1998 TARGET_CITATION ) , indicating that binding of P - TEFb to the nascent transcript is sufficient to regulate RNAPII elongation at the HIV - 1 promoter . ||9557739_1025_155871==>Finally , dominant negative mutants of <PROT1_1025>  Cdk9 </PROT1_1025> selectively inhibit <PROT2_155871>  Tat </PROT2_155871> transactivation in vivo and in vitro ( TARGET_CITATION , CITATION ) . ||9557739_1025_155871==>The kinase activity of <PROT1_1025>  CDK9 </PROT1_1025> ( P - TEFb ) is required for <PROT2_155871>  Tat </PROT2_155871> - activity ( CITATION ; CITATION ; CITATION ; TARGET_CITATION ; CITATION ) , and a key role for cyclin T in <PROT2_155871>  Tat </PROT2_155871> function is indicated by the observation that recombinant cyclin T1 , which interacts with <PROT2_155871>  Tat </PROT2_155871> , enhances the affinity and specificity of <PROT2_155871>  Tat </PROT2_155871> & # 150 ; TAR interactions in a loop sequence - dependent manner ( CITATION ; CITATION ; CITATION ) . ||9557739_1025_155871==>Previous reports have shown that transient overexpression of <PROT1_1025>  CDK9 </PROT1_1025> specifically inhibits <PROT2_155871>  Tat </PROT2_155871> activation of both transfected and integrated HIV LTRs ( CITATION ; TARGET_CITATION ) . ||9557739_1025_155871==>A mutation in the catalytic domain of <PROT1_1025>  CDK9 </PROT1_1025> , D167N , is more efficient than the wild - type <PROT1_1025>  CDK9 </PROT1_1025> in inhibiting <PROT2_155871>  Tat </PROT2_155871> - dependent transcription ( TARGET_CITATION ) . ||9557739_1025_155871==>There is now general agreement that <PROT2_155871>  Tat </PROT2_155871> activation of elongation is mediated by the <PROT1_1025>  TAK </PROT1_1025> kinase ( CITATION ; CITATION ; TARGET_CITATION ; CITATION ) . ||9557739_1025_155871==>Similarly , <PROT2_155871>  Tat </PROT2_155871> - activated transcription can be 'squelched ' by overexpressed <PROT1_1025>  CDK9 </PROT1_1025> ( CITATION ; TARGET_CITATION ) . ||9557739_1025_155871==>It has been established that the Cyclin T1 / <PROT1_1025>  CDK9 </PROT1_1025> complex is recruited to promoter - proximal RNA sequences via <PROT2_155871>  Tat </PROT2_155871> - mediated binding to TAR ( CITATION ; TARGET_CITATION ; CITATION ; CITATION ; CITATION ; CITATION ) . ||9557739_1025_155871==>Immunodepletion of <PROT1_1025>  CDK9 </PROT1_1025> inhibited basal and <PROT2_155871>  Tat </PROT2_155871> - activated transcription in vitro ( CITATION , CITATION ) , and its overexpression abrogated <PROT2_155871>  Tat </PROT2_155871> trans activation in cells ( TARGET_CITATION ) . ||9557739_1025_155871==>This conclusion is based both on in vitro biochemical experiments and transient transfection studies , including the observations that immunodepletion of P - TEFb from extracts competent to support <PROT2_155871>  Tat </PROT2_155871> - activated transcription with anti - <PROT1_1025>  CDK9 </PROT1_1025> antibodies abrogates <PROT2_155871>  Tat </PROT2_155871> - dependent transcription in vitro ( CITATION , CITATION ) and that transient overexpression of a catalytically inactive <PROT1_1025>  CDK9 </PROT1_1025> mutant inhibits <PROT2_155871>  Tat </PROT2_155871> - dependent reporter gene expression in intact cells ( CITATION , TARGET_CITATION ) . ||9557739_1025_155871==>Similarly , whereas direct recruitment of either the CDK8 or the <PROT1_1025>  CDK9 </PROT1_1025> kinase to the SLIIB RNA target can also activate the HIV - 1 LTR promoter ( CITATION - TARGET_CITATION ) , this activation was reported to be less than 10 % of the level seen with <PROT2_155871>  Tat </PROT2_155871> - Rev ( CITATION , TARGET_CITATION ) . ||9557739_1025_155871==>By using kinase inhibitors or introduction of dominant negative <PROT1_1025>  Cdk9 </PROT1_1025> mutants into cells , the functional significance of P - TEFb and its kinase activity in <PROT2_155871>  Tat </PROT2_155871> activation has also been demonstrated in vivo ( TARGET_CITATION , CITATION ) . ||9557739_1025_155871==><PROT2_155871>  Tat </PROT2_155871> regulates an early step in transcription elongation that requires cyclin T1 ( CycT1 ) and <PROT1_1025>  CDK9 </PROT1_1025> ( TARGET_CITATION , CITATION , CITATION , CITATION , CITATION ) , which are subunits of the positive transcription elongation factor P - TEFb ( CITATION ) and <PROT2_155871>  Tat </PROT2_155871> - associated kinase ( TARGET_CITATION , CITATION , CITATION ) complexes . ||9557739_1025_155871==>First , chemical inhibitors and dominant - negative mutants of <PROT1_1025>  CDK9 </PROT1_1025> block <PROT2_155871>  Tat </PROT2_155871> transactivation and HIV - 1 replication in vivo ( CITATION , TARGET_CITATION , CITATION , CITATION ) . ||9557739_1025_155871==>Genetic studies have shown that chimeric CycT1 or <PROT1_1025>  CDK9 </PROT1_1025> proteins can activate transcription if tethered directly to nascent RNA ( CITATION , CITATION , TARGET_CITATION ) , indicating that the primary role of <PROT2_155871>  Tat </PROT2_155871> and TAR is to recruit CycT1 - <PROT1_1025>  CDK9 </PROT1_1025> to RNA . ||9557739_1025_155871==>It has been firmly established that <PROT1_1025>  TAK </PROT1_1025> in the CycT1 / P - TEFb complex mediates <PROT2_155871>  Tat </PROT2_155871> stimulation of RNAP II elongation of the integrated viral genome ( CITATION ; CITATION ; CITATION ; TARGET_CITATION ; CITATION ; CITATION ; CITATION ; CITATION ; CITATION ; CITATION and CITATION ) . ||9557739_1025_155871==>The kinase activity of <PROT1_1025>  Cdk9 </PROT1_1025> was found to be essential for P - TEFb to phosphorylate the C - terminal domain ( CTD ) of RNA polymerase II and to mediate <PROT2_155871>  Tat </PROT2_155871> transactivation ( TARGET_CITATION ) . ||9557739_1025_155871==>In nuclear extracts , HIV - 1 <PROT2_155871>  Tat </PROT2_155871> associates tightly with the <PROT1_1025>  CDK9 </PROT1_1025> - containing positive transcription elongation factor complex , P - TEFb ( CITATION - TARGET_CITATION ) . ||9557739_1025_155871==>Positive transcription elongation factor ( P - TEFb ) ( CITATION , CITATION ) or <PROT2_155871>  Tat </PROT2_155871> - associated kinase ( TARGET_CITATION ) , composed of <PROT1_1025>  CDK9 </PROT1_1025> and cyclin T1 ( CycT1 ) ( TARGET_CITATION , CITATION ) , regulates <PROT2_155871>  Tat </PROT2_155871> transactivation at an early step in transcription elongation . ||9557739_1025_155871==>Second , dominant - negative mutants of <PROT1_1025>  CDK9 </PROT1_1025> or kinase inhibitors that preferentially block <PROT1_1025>  CDK9 </PROT1_1025> inhibit <PROT2_155871>  Tat </PROT2_155871> transactivation and HIV - 1 replication in vivo ( TARGET_CITATION , CITATION , CITATION , CITATION ) . ||9557739_1025_155871==>Second , dominant - negative mutants of <PROT1_1025>  CDK9 </PROT1_1025> or <PROT1_1025>  CDK9 </PROT1_1025> kinase inhibitors inhibit <PROT2_155871>  Tat </PROT2_155871> transactivation ( TARGET_CITATION , CITATION , CITATION , CITATION ) . ||9557739_1025_155871==>The expression of D167N in trans inhibits strongly <PROT1_1025>  CDK9 </PROT1_1025> - dependent <PROT2_155871>  Tat </PROT2_155871> - activated transcription in a wide variety of cell types ( TARGET_CITATION , CITATION ) , but it has only minimal effects on <PROT1_1025>  CDK9 </PROT1_1025> - independent transcriptional elongation stimulated by a cellular enhancer ( CITATION ) . ||9557739_1025_155871==>Several independent lines of investigation have established that <PROT1_1025>  TAK </PROT1_1025> ( cyclin T1 / P - TEFb ) plays a critical role in <PROT2_155871>  Tat </PROT2_155871> transactivation ( CITATION , CITATION , CITATION , TARGET_CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , CITATION ) . ||9557739_1025_155871==>It has been well established that <PROT1_1025>  TAK </PROT1_1025> , in the cycT1 / P - TEFb complex , mediates <PROT2_155871>  Tat </PROT2_155871> function ( CITATION , CITATION , CITATION , TARGET_CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , CITATION ) . ||9557739_1025_155871==>Upon overexpression , the <PROT1_1025>  Cdk9 </PROT1_1025> - dn protein inhibits wild - type ( wt ) <PROT1_1025>  Cdk9 </PROT1_1025> function as demonstrated by specific inhibition of <PROT2_155871>  Tat </PROT2_155871> transactivation of the HIV - 1 LTR ( TARGET_CITATION , CITATION ) . ||9557739_1025_155871==><PROT1_1025>  CDK9 </PROT1_1025> in a complex with cyclin T forms pTEFb , a human transcription elongation factor , which also phosphorylates the C - terminal domain of RNA polymerase II , and was found to be required for HIV - 1 <PROT2_155871>  Tat </PROT2_155871> transactivation in vitro and in vivo ( Gold et al. , 1998 TARGET_CITATION ; Wei et al. , 1998 CITATION ) . ||9557739_1025_155871==>In nuclear extracts , HIV - 1 <PROT2_155871>  Tat </PROT2_155871> associates tightly with the <PROT1_1025>  CDK9 </PROT1_1025> - containing positive transcription elongation factor complex b , P - TEFb ( CITATION - TARGET_CITATION ) . ||9557739_1025_155871==>P - TEFb complexes consisting of cyclin T1 and <PROT1_1025>  CDK9 </PROT1_1025> serve as a human cellular cofactor for the human immunodeficiency virus type 1 ( HIV - 1 ) protein <PROT2_155871>  Tat </PROT2_155871> ( transactivator of transcription ) ( TARGET_CITATION , CITATION , CITATION , CITATION , CITATION ) . ||9557739_1025_155871==>Recent studies indicate that <PROT2_155871>  Tat </PROT2_155871> induces hyperphosphorylation of the carboxyl - terminal domain of RNA polymerase II , primarily through recruitment of positive transcription elongation factor b ( P - TEFb ) , which contains a cyclin - dependent kinase ( <PROT1_1025>  Cdk9 </PROT1_1025> ) , and perhaps through recruitment of other carboxyl - terminal domain kinases ( CITATION , CITATION , CITATION , TARGET_CITATION , CITATION , CITATION ) . ||9557739_155871_904==><PROT1_155871>  Tat </PROT1_155871> regulates an early step in transcription elongation that requires cyclin T1 ( <PROT2_904>  CycT1 </PROT2_904> ) and CDK9 ( TARGET_CITATION , CITATION , CITATION , CITATION , CITATION ) , which are subunits of the positive transcription elongation factor P - TEFb ( CITATION ) and <PROT1_155871>  Tat </PROT1_155871> - associated kinase ( TARGET_CITATION , CITATION , CITATION ) complexes . ||9557739_155871_904==>Genetic studies have shown that chimeric <PROT2_904>  CycT1 </PROT2_904> or CDK9 proteins can activate transcription if tethered directly to nascent RNA ( CITATION , CITATION , TARGET_CITATION ) , indicating that the primary role of <PROT1_155871>  Tat </PROT1_155871> and TAR is to recruit <PROT2_904>  CycT1 </PROT2_904> - CDK9 to RNA . ||9557739_155871_904==>It has been firmly established that TAK in the <PROT2_904>  CycT1 </PROT2_904> / P - TEFb complex mediates <PROT1_155871>  Tat </PROT1_155871> stimulation of RNAP II elongation of the integrated viral genome ( CITATION ; CITATION ; CITATION ; TARGET_CITATION ; CITATION ; CITATION ; CITATION ; CITATION ; CITATION ; CITATION and CITATION ) . ||9557739_155871_904==>Positive transcription elongation factor ( P - TEFb ) ( CITATION , CITATION ) or <PROT1_155871>  Tat </PROT1_155871> - associated kinase ( TARGET_CITATION ) , composed of CDK9 and cyclin T1 ( <PROT2_904>  CycT1 </PROT2_904> ) ( TARGET_CITATION , CITATION ) , regulates <PROT1_155871>  Tat </PROT1_155871> transactivation at an early step in transcription elongation . ||9557739_155871_904==>It has been well established that TAK , in the <PROT2_904>  cycT1 </PROT2_904> / P - TEFb complex , mediates <PROT1_155871>  Tat </PROT1_155871> function ( CITATION , CITATION , CITATION , TARGET_CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , CITATION ) . 
stabilizes====14561767_155945_468==>Although the consequences of this transdominant effect of <PROT1_155945>  Vpu </PROT1_155945> have yet to be fully realized , accumulation of the SCF - & # 223 ; TrCP substrates I { kappa } B , & # 223 ; - catenin , and <PROT2_468>  ATF4 </PROT2_468> have all been documented in <PROT1_155945>  Vpu </PROT1_155945> - expressing cells ( TARGET_CITATION , CITATION ) . ||14561767_155945_468==>In this regard , a recent report ( TARGET_CITATION ) has demonstrated that <PROT1_155945>  Vpu </PROT1_155945> , through its ability to function as a competitive inhibitor of & # 223 ; TrCP , allows the accumulation of certain & # 223 ; TrCP substrates , specifically & # 223 ; - catenin , <PROT2_468>  ATF4 </PROT2_468> , and I { kappa } B - { alpha } , in the host cell cytoplasm . ||8849451_155871_2958==>When this fraction was supplemented with TBP , SP1 , <PROT2_2958>  TFIIA </PROT2_2958> , in vitro transcription analysis indicated that it was competent for basal but not <PROT1_155871>  Tat </PROT1_155871> - induced stimulation of the HIV - 1 LTR ( CITATION and TARGET_CITATION ) . 
stimulates====10393184_155871_6829==>In addition , Spt4 / <PROT2_6829>  Spt5 </PROT2_6829> , as well as human Spt6 have been implicated in aiding <PROT1_155871>  Tat </PROT1_155871> activation of HIV transcription ( Wu - Baer et al. 1998 CITATION ; Kim et al. 1999 CITATION ; Parada and Roeder 1999 TARGET_CITATION ; Ivanov et al. 2000 CITATION ) . ||10393184_155871_6829==>Spt4 / <PROT2_6829>  Spt5 </PROT2_6829> and Spt6 stimulate activation by HIV <PROT1_155871>  Tat </PROT1_155871> , again implying a positive role in elongation ( Wu - Baer et al. 1998 CITATION ; Kim et al. 1999 CITATION ; Parada and Roeder 1999 TARGET_CITATION ; Ivanov et al. 2000 CITATION ) . ||10393184_155871_6829==>In addition to P - TEFb ( CITATION ) , multiprotein complexes including NELF ( CITATION ) and <PROT1_155871>  Tat </PROT1_155871> - SF1 ( CITATION , TARGET_CITATION ) modulate the SPT4 - <PROT2_6829>  SPT5 </PROT2_6829> , function suggesting that a number of positive and negative factors may regulate its activity . ||10393184_155871_6829==>Additional factors previously demonstrated to interact with <PROT2_6829>  SPT5 </PROT2_6829> , including <PROT1_155871>  Tat </PROT1_155871> - SF1 ( CITATION and TARGET_CITATION ) and components of the FACT complex ( CITATION ) , were not characterized in this analysis . ||10393184_155871_6829==>Protein complexes that stimulate <PROT1_155871>  Tat </PROT1_155871> activity in vitro have been partially purified from HeLa cells and reported to contain Spt4 - <PROT2_6829>  Spt5 </PROT2_6829> , P - TEFb , <PROT1_155871>  Tat </PROT1_155871> - SF1 , nucleolin , XP - E , Pol II , the small subunit of TFIIF ( equivalent to Tfg2 ) and other novel polypeptides ( CITATION , TARGET_CITATION , CITATION ) . ||10438593_1022_155871==><PROT2_155871>  Tat </PROT2_155871> does not increase phosphorylation of the CTD by <PROT1_1022>  CDK7 </PROT1_1022> in the initiation complex ( TARGET_CITATION , CITATION ) . ||10438593_1022_155871==>However , as previously observed ( TARGET_CITATION ) , <PROT1_1022>  CDK7 </PROT1_1022> was able to induce extensive phosphorylation of the CTD in the absence of <PROT2_155871>  Tat </PROT2_155871> . ||10438593_155871_2071==>First , studies using <PROT1_155871>  Tat </PROT1_155871> - dependent cell - free transcription systems have shown that CTD can be hyper - phosphorylated at the promoter by <PROT2_2071>  TFIIH </PROT2_2071> both in the presence and absence of <PROT1_155871>  Tat </PROT1_155871> ( TARGET_CITATION , CITATION and CITATION ) . ||10438593_155871_2965==>First , studies using <PROT1_155871>  Tat </PROT1_155871> - dependent cell - free transcription systems have shown that CTD can be hyper - phosphorylated at the promoter by <PROT2_2965>  TFIIH </PROT2_2965> both in the presence and absence of <PROT1_155871>  Tat </PROT1_155871> ( TARGET_CITATION , CITATION and CITATION ) . ||10438593_155871_2966==>First , studies using <PROT1_155871>  Tat </PROT1_155871> - dependent cell - free transcription systems have shown that CTD can be hyper - phosphorylated at the promoter by <PROT2_2966>  TFIIH </PROT2_2966> both in the presence and absence of <PROT1_155871>  Tat </PROT1_155871> ( TARGET_CITATION , CITATION and CITATION ) . ||10438593_155871_2968==>First , studies using <PROT1_155871>  Tat </PROT1_155871> - dependent cell - free transcription systems have shown that CTD can be hyper - phosphorylated at the promoter by <PROT2_2968>  TFIIH </PROT2_2968> both in the presence and absence of <PROT1_155871>  Tat </PROT1_155871> ( TARGET_CITATION , CITATION and CITATION ) . ||10454543_155871_6829==>In addition , Spt4 / <PROT2_6829>  Spt5 </PROT2_6829> , as well as human Spt6 have been implicated in aiding <PROT1_155871>  Tat </PROT1_155871> activation of HIV transcription ( Wu - Baer et al. 1998 CITATION ; Kim et al. 1999 TARGET_CITATION ; Parada and Roeder 1999 CITATION ; Ivanov et al. 2000 CITATION ) . ||10454543_155871_6829==>Spt4 / <PROT2_6829>  Spt5 </PROT2_6829> and Spt6 stimulate activation by HIV <PROT1_155871>  Tat </PROT1_155871> , again implying a positive role in elongation ( Wu - Baer et al. 1998 CITATION ; Kim et al. 1999 TARGET_CITATION ; Parada and Roeder 1999 CITATION ; Ivanov et al. 2000 CITATION ) . ||10454543_155871_6829==>Among these are <PROT1_155871>  Tat </PROT1_155871> - SF1 ( TARGET_CITATION , CITATION ) , the human homologue of the Saccharomyces cerevisiae transcription factor <PROT2_6829>  SPT5 </PROT2_6829> ( CITATION ) , TFIIH ( CITATION , CITATION , CITATION ) , TFIIF ( CITATION ) , and a <PROT1_155871>  Tat </PROT1_155871> - associated histone acetyltransferase ( reference CITATION and references therein ) . ||10454543_155871_6829==><PROT1_155871>  Tat </PROT1_155871> - SF1 interacts with <PROT1_155871>  Tat </PROT1_155871> ( CITATION ) and P - TEFb ( CITATION ) and was recently found to interact with human <PROT2_6829>  SPT5 </PROT2_6829> , Pol II , and the RAP30 subunit of TFIIF as well ( TARGET_CITATION ) . ||10454543_155871_6829==>In addition to P - TEFb ( CITATION ) , multiprotein complexes including NELF ( CITATION ) and <PROT1_155871>  Tat </PROT1_155871> - SF1 ( TARGET_CITATION , CITATION ) modulate the SPT4 - <PROT2_6829>  SPT5 </PROT2_6829> , function suggesting that a number of positive and negative factors may regulate its activity . ||10454543_155871_6829==>We were intrigued by the studies reporting that <PROT2_6829>  Spt5 </PROT2_6829> , one of the subunits of DSIF , is involved in <PROT1_155871>  Tat </PROT1_155871> - mediated transactivation and functions as a positive elongation factor in HIV - 1 LTR promoter in the presence of <PROT1_155871>  Tat </PROT1_155871> ( CITATION , TARGET_CITATION ) . ||10454543_155871_6829==>Recent studies also showed that <PROT2_6829>  Spt5 </PROT2_6829> is involved in <PROT1_155871>  Tat </PROT1_155871> transactivation which is a positive elongation activity ( CITATION , TARGET_CITATION ) . ||10454543_155871_6829==>However , CYCT1 - C only partially removed <PROT2_6829>  SPT5 </PROT2_6829> , an elongation factor reported to bind <PROT1_155871>  TAT </PROT1_155871> - SF1 ( ref. TARGET_CITATION ) , and it removed very little Pol II or the TFIIF subunit RAP30 . ||10454543_155871_6829==>Additional factors previously demonstrated to interact with <PROT2_6829>  SPT5 </PROT2_6829> , including <PROT1_155871>  Tat </PROT1_155871> - SF1 ( TARGET_CITATION and CITATION ) and components of the FACT complex ( CITATION ) , were not characterized in this analysis . ||10454543_155871_6829==>DSIF / Spt4 - <PROT2_6829>  Spt5 </PROT2_6829> can inhibit and promote elongation of RNA polymerase II ( Pol II ) on cellular genes and is required for the stimulation of transcription elongation by human immunodeficiency virus type 1 <PROT1_155871>  Tat </PROT1_155871> in vitro ( CITATION , TARGET_CITATION , CITATION , CITATION ) . ||10454543_155871_6829==>Protein complexes that stimulate <PROT1_155871>  Tat </PROT1_155871> activity in vitro have been partially purified from HeLa cells and reported to contain Spt4 - <PROT2_6829>  Spt5 </PROT2_6829> , P - TEFb , <PROT1_155871>  Tat </PROT1_155871> - SF1 , nucleolin , XP - E , Pol II , the small subunit of TFIIF ( equivalent to Tfg2 ) and other novel polypeptides ( TARGET_CITATION , CITATION , CITATION ) . ||10454543_155871_6829==>P - TEFb also phosphorylates human <PROT2_6829>  SPT5 </PROT2_6829> , an elongation factor which is necessary for in vitro <PROT1_155871>  Tat </PROT1_155871> transactivation and can enhance <PROT1_155871>  Tat </PROT1_155871> transactivation in vivo ( CITATION , TARGET_CITATION , CITATION ) . ||10454543_155871_6829==>The level of expression of <PROT2_6829>  SPT5 </PROT2_6829> correlates with lower basal levels of viral gene expression and increased <PROT1_155871>  Tat </PROT1_155871> transactivation CITATION , TARGET_CITATION . ||10536359_1025_155871==>In addition , <PROT1_1025>  CDK9 </PROT1_1025> & # 47 ; Cyclin T binds the <PROT2_155871>  Tat </PROT2_155871> proteins of the HIV virus , suggesting a possible role for this kinase in viral replication ( TARGET_CITATION ) . ||10536359_1025_155871==>In addition to <PROT2_155871>  Tat </PROT2_155871> , other proteins are known to be necessary to achieve efficient transcriptional activation of HIV - 1 ( e.g. , cyclin T1 and <PROT1_1025>  CDK9 </PROT1_1025> ) ( reviewed in references TARGET_CITATION and CITATION ) . ||10757782_1025_155871==><PROT2_155871>  Tat </PROT2_155871> stimulates transcription elongation because it is able to activate a specific carboxyl - terminal domain ( CTD ) kinase , <PROT1_1025>  CDK9 </PROT1_1025> ( CITATION , CITATION , CITATION ) , which phosphorylates RNA Pol II ( CITATION , CITATION ) , as well as the elongation factor SPT5 ( TARGET_CITATION , CITATION , CITATION , CITATION ) . ||10757782_1025_155871==>A further link between <PROT2_155871>  Tat </PROT2_155871> and Spt5 comes from very recent studies demonstrating that Spt5 can be phosphorylated by <PROT1_1025>  CDK9 </PROT1_1025> ( TARGET_CITATION , CITATION , CITATION ) . ||10757782_1025_155871==>As shown in fig. 2 , binding of <PROT2_155871>  Tat </PROT2_155871> complexed with <PROT1_1025>  TAK </PROT1_1025> / P - TEFb to TAR RNA is believed to activate transcriptional elongation through the phosphorylation of the carboxyl terminal domain of RNA polymerase II , the Spt5 subunit of a negative factor termed DSIF , and perhaps other substrates in the polymerase complex ( CITATION and TARGET_CITATION ) . ||10866664_1025_155871==>Recent studies indicate that the association of <PROT2_155871>  Tat </PROT2_155871> with P - TEFb changes the substrate specificity of <PROT1_1025>  CDK9 </PROT1_1025> , the kinase component of P - TEFb ( TARGET_CITATION ) , most likely through a change in protein conformation . ||10866664_1025_155871==><PROT1_1025>  Cdk9 </PROT1_1025> has been shown to phosphorylate Ser 2 , although its substrate specificity is modified to favor Ser 5 by interaction with the viral <PROT2_155871>  Tat </PROT2_155871> protein in vitro ( Zhou et al. 2000 TARGET_CITATION ) . ||10866664_1025_155871==>In addition , two studies on human immunodeficiency virus type 1 gene transcription have reported that the transcriptional activator of this gene , <PROT2_155871>  Tat </PROT2_155871> , stimulates both Ser - 5 phosphorylation of the CTD of Pol II by <PROT1_1025>  Cdk9 </PROT1_1025> and Pol II processivity for transcription elongation ( CITATION , TARGET_CITATION ) . ||10866664_1025_155871==>The transactivator <PROT2_155871>  Tat </PROT2_155871> recruits the <PROT1_1025>  Cdk9 </PROT1_1025> CTD kinase by binding to the RNA TAR element ( for review see ref. CITATION ) and modifies the substrate specificity of the <PROT1_1025>  Cdk9 </PROT1_1025> complex ( TARGET_CITATION ) . ||10866664_1025_155871==>In an independent study , it was demonstrated that <PROT2_155871>  Tat </PROT2_155871> modifies the activity / substrate specificity of the <PROT1_1025>  CDK9 </PROT1_1025> kinase on the RNAP II CTD ( TARGET_CITATION ) . ||10866664_1025_155871==>The postintegration restriction of HIV - 1 transcription is mainly regulated by cellular transcription factors ( CITATION ) and enzymatic activities of cellular proteins , such as <PROT1_1025>  cdk9 </PROT1_1025> / cyclin T ( CITATION , CITATION , CITATION , CITATION , CITATION ) and cdk7 / cyclin H ( CITATION , CITATION , TARGET_CITATION , CITATION ) , which play a critical role in <PROT2_155871>  Tat </PROT2_155871> - mediated transactivation . ||10866664_1025_155871==>Recent studies have shown that <PROT2_155871>  Tat </PROT2_155871> may target <PROT1_1025>  cdk9 </PROT1_1025> ( CITATION , CITATION , CITATION ) , cdk7 ( CITATION , CITATION , TARGET_CITATION , CITATION ) , and cdk2 ( CITATION ) to transactivate the HIV - 1 promoter . ||10866664_1025_155871==>In the absence of <PROT2_155871>  Tat </PROT2_155871> , <PROT1_1025>  cdk9 </PROT1_1025> phosphorylates serine 2 of the CTD , and cdk7 phosphorylates serine 5. However , in the presence of <PROT2_155871>  Tat </PROT2_155871> , <PROT1_1025>  cdk9 </PROT1_1025> can phosphorylate both serines 2 and 5 ( TARGET_CITATION ) . ||10866664_1025_155871==><PROT2_155871>  Tat </PROT2_155871> stimulates transcription elongation because it is able to activate a specific carboxyl - terminal domain ( CTD ) kinase , <PROT1_1025>  CDK9 </PROT1_1025> ( CITATION , CITATION , CITATION ) , which phosphorylates RNA Pol II ( CITATION , TARGET_CITATION ) , as well as the elongation factor SPT5 ( CITATION , CITATION , CITATION , CITATION ) . ||10866664_1025_155871==>The specificity of <PROT1_1025>  Cdk9 </PROT1_1025> is less clear ; one group found that <PROT1_1025>  Cdk9 </PROT1_1025> phosphorylates serine 5 in vitro ( CITATION ) , another study found that <PROT1_1025>  Cdk9 </PROT1_1025> - cyclin T phosphorylates serine 2 and that its specificity is expanded to include serine 5 in the presence of the HIV - 1 <PROT2_155871>  Tat </PROT2_155871> protein ( TARGET_CITATION ) , and RNAi - mediated depletion of <PROT1_1025>  Cdk9 </PROT1_1025> in Caenorhabditis elegans results in specific loss of serine 2 phosphorylation in vivo ( k. Blackwell , unpublished results ) . ||10866664_1025_155871==>The <PROT2_155871>  Tat </PROT2_155871> - activated <PROT1_1025>  CDK9 </PROT1_1025> is then able to extensively phosphorylate the CTD , creating a unique and highly phosphorylated form that we have designated Pol IIo * ( CITATION , CITATION , CITATION , TARGET_CITATION , CITATION ) . ||10866664_1025_155871==>Second , in agreement with a variety of other studies , we found that isolated P - TEFb kinase , which contains the <PROT1_1025>  CDK9 </PROT1_1025> / CycT1 complex , can also be activated by the addition of recombinant <PROT2_155871>  Tat </PROT2_155871> ( CITATION , TARGET_CITATION ) . ||10866664_1025_155871==>However , in the absence of <PROT2_155871>  Tat </PROT2_155871> , there is no increase in Ser2 phosphorylation , a finding consistent with the idea that the substrate specificity of <PROT1_1025>  CDK9 </PROT1_1025> might be influenced by <PROT2_155871>  Tat </PROT2_155871> ( TARGET_CITATION ) . ||10866664_1025_155871==>The interaction between <PROT2_155871>  Tat </PROT2_155871> and TAR involves not only the binding of <PROT2_155871>  Tat </PROT2_155871> to TAR RNA but also its association with the <PROT2_155871>  Tat </PROT2_155871> - activated kinase ( <PROT1_1025>  TAK </PROT1_1025> ) ( CITATION , CITATION , CITATION ) , a protein kinase that is able to phosphorylate the carboxyl - terminal domain ( CTD ) of the large subunit of RNA polymerase II ( CITATION , CITATION , TARGET_CITATION ) . ||10866664_1025_155871==><PROT2_155871>  Tat </PROT2_155871> also modifies the substrate specificity of <PROT1_1025>  CDK9 </PROT1_1025> to achieve additional Ser - 5 phosphorylation ( TARGET_CITATION ) . ||10866664_1025_155871==><PROT2_155871>  Tat </PROT2_155871> has been reported to induce CTD phosphorylation by CDK7 ( CITATION , CITATION , CITATION , CITATION ) and <PROT1_1025>  CDK9 </PROT1_1025> ( TARGET_CITATION , CITATION ) . ||10866664_1025_155871==><PROT2_155871>  Tat </PROT2_155871> induced the TTK and CDK2 to phosphorylate CTD , specifically at serine 2 residues ( CITATION ) , which was in contrast to the reported phosphorylation of Ser - 5 by <PROT1_1025>  CDK9 </PROT1_1025> in the presence of <PROT2_155871>  Tat </PROT2_155871> ( TARGET_CITATION ) . ||10866664_1025_155871==>Interestingly , <PROT2_155871>  Tat </PROT2_155871> stimulated <PROT1_1025>  CDK9 </PROT1_1025> to phosphorylate Ser - 5 in in vitro transcription assays ( TARGET_CITATION , CITATION ) . ||10866664_1025_155871==>In a recent report , Zhou et al. ( TARGET_CITATION ) presented evidence that <PROT1_1025>  CDK9 </PROT1_1025> and CDK7 phosphorylate Ser2 and Ser5 of the RNA Pol II CTD , respectively , and <PROT2_155871>  Tat </PROT2_155871> altered the substrate specificity of <PROT1_1025>  CDK9 </PROT1_1025> in HIV - 1 pre - initiation complexes . ||10866664_1025_155871==>HIV - 1 transcription is also heavily dependent upon the stimulation of elongation by <PROT2_155871>  Tat </PROT2_155871> ( CITATION , CITATION , CITATION , CITATION ) and the associated cyclin T1 - <PROT1_1025>  CDK9 </PROT1_1025> complex ( CITATION , CITATION , CITATION , CITATION , CITATION , TARGET_CITATION , CITATION ) . ||10866664_155871_904==>P - TEFb was reported to associate with the Pol II elongation complex along the HIV - 1 template through position + 529 ( refs CITATION , TARGET_CITATION ) , which may recruit the <PROT1_155871>  TAT </PROT1_155871> - SF1 & # 150 ; snRNP complex to Pol II through the <PROT2_904>  CYCT1 </PROT2_904> & # 150 ; <PROT1_155871>  TAT </PROT1_155871> - SF1 interaction . ||10866664_155871_904==>Recruitment of <PROT1_155871>  TAT </PROT1_155871> - SF1 & # 150 ; snRNP to these templates is probably mediated by the interaction of <PROT1_155871>  TAT </PROT1_155871> - SF1 with <PROT2_904>  CYCT1 </PROT2_904> & # 150 ; P - TEFb , which was shown to travel with the elongating polymerase CITATION , TARGET_CITATION . ||10866664_155871_904==>As P - TEFb is travelling with the elongation complex CITATION , TARGET_CITATION , the <PROT2_904>  CYCT1 </PROT2_904> & # 150 ; <PROT1_155871>  TAT </PROT1_155871> - SF1 interaction may recruit <PROT1_155871>  TAT </PROT1_155871> - SF1 & # 150 ; snRNP to the elongating polymerase , increase the local concentration of splicing factors , and facilitate the assembly of a spliceosome when splice sites emerge from the polymerase . ||10866664_155871_904==>Second , in agreement with a variety of other studies , we found that isolated P - TEFb kinase , which contains the CDK9 / <PROT2_904>  CycT1 </PROT2_904> complex , can also be activated by the addition of recombinant <PROT1_155871>  Tat </PROT1_155871> ( CITATION , TARGET_CITATION ) . ||11547919_1025_155871==><PROT2_155871>  Tat </PROT2_155871> is primarily a transactivator protein that binds in the presence of cofactors cyclin T and cyclin - dependent kinase 9 ( <PROT1_1025>  CDK9 </PROT1_1025> ) , to a short leader RNA , transactivator responsive region ( TAR ) and has the potential to influence the initiation as well as the elongation of transcription ( CITATION , CITATION , TARGET_CITATION and CITATION ) . ||11547919_1025_155871==>Three prominent types of interaction that mediate <PROT2_155871>  Tat </PROT2_155871> transactivation have been biochemically and genetically defined : those with general transcription factors ( including TBP , TAFII250 and RNA polymerase II ) ( reviewed in TARGET_CITATION ) ; those with kinase complexes able to phosphorylate the C - terminal domain of RNA polymerase II ( in particular with cyclin T1 / <PROT1_1025>  CDK9 </PROT1_1025> ) ( see CITATION , CITATION , and references therein ) ; and those with cellular proteins possessing HAT activity . ||9092824_155908_4869==><PROT1_155908>  Rev </PROT1_155908> is often localised to the nucleolus ( CITATION ) , and this may reflect the ability of <PROT1_155908>  Rev </PROT1_155908> to bind the nucleolar shuttling protein , <PROT2_4869>  B23 </PROT2_4869> ( TARGET_CITATION ) . ||9092824_155908_4869==>Although the <PROT2_4869>  B23 </PROT2_4869> protein was found to stimulate import of <PROT1_155908>  Rev </PROT1_155908> , it appears to be neither necessary nor sufficient for <PROT1_155908>  Rev </PROT1_155908> import ( TARGET_CITATION ) . ||9092824_155908_4869==>Our findings do not exclude the possibility that <PROT1_155908>  Rev </PROT1_155908> may also enter the nucleus by additional pathways , for instance through direct binding to other importin - small beta , Greek - related transporter proteins ( CITATION and CITATION ) , or by complexing with other NLS - containing proteins that access the importin pathway , such as <PROT2_4869>  B23 </PROT2_4869> ( TARGET_CITATION ) . ||9092824_155908_4869==>The NLS of <PROT1_155908>  Rev </PROT1_155908> has been reported to associate with importin beta , the large subunit of the conventional importin alpha / importin beta NLS receptor heterodimer , as well as with <PROT2_4869>  B23 </PROT2_4869> , a mammalian protein involved in the nuclear import of ribosomal proteins ( CITATION - TARGET_CITATION ) . ||9092824_155908_4869==>On the other hand , <PROT2_4869>  NPM </PROT2_4869> may function in regulation of UV cell - killing through interaction with various protein ( s ) , such as HIV - 1 <PROT1_155908>  Rev </PROT1_155908> protein ( CITATION , CITATION and TARGET_CITATION ) , protein C23 ( CITATION ) and IRF - 1 ( CITATION ) . ||9092824_155908_4869==><PROT2_4869>  B23 </PROT2_4869> has also been reported to bind the arginine - rich basic region of the human T - cell leukemia virus protein Rex ( CITATION ) , and the HIV proteins <PROT1_155908>  Rev </PROT1_155908> ( CITATION , TARGET_CITATION ) and Tat ( CITATION ) . ||9092824_155908_4869==>To examine the speckle pattern of mutant DD in greater detail , we analyzed DD - transfected cells by confocal microscopy after staining them with the anti - Rex - 2 antibody , an antibody against the nuclear envelope component Nup62 ( added to delineate the nucleus ) , and an antibody recognizing <PROT2_4869>  B23 </PROT2_4869> , a multifunctional nucleolar protein that is proposed to act as a chaperone for nuclear import , nucleolar accumulation , and functional activity of <PROT1_155908>  Rev </PROT1_155908> and Rex ( CITATION , CITATION , CITATION - TARGET_CITATION ) . ||9092824_155908_4869==>During HIV infections , <PROT2_4869>  B23 </PROT2_4869> interacts with <PROT1_155908>  Rev </PROT1_155908> , Rex , and Tat viral proteins and helps to direct them into nucleoli ( ( TARGET_CITATION , CITATION , CITATION and CITATION ) ) . ||9092824_155908_4869==>During HIV infections , for example , <PROT2_4869>  B23 </PROT2_4869> interacts with <PROT1_155908>  Rev </PROT1_155908> , Rex and Tat viral proteins and helps direct them into nucleoli ( ( CITATION , TARGET_CITATION , CITATION and CITATION ) ) . 
suppresses====12167863_155459_60489==><PROT2_60489>  CEM15 </PROT2_60489> / Apobec 3G interferes with the production of infectious virions in the absence of a functional <PROT1_155459>  vif </PROT1_155459> allele ( CITATION , TARGET_CITATION ) ; TSG101 interacts with the PTAPP motif in gag p6 , where it facilitates release of virus from cells ( CITATION ) . ||12167863_155459_60489==>But , central to this infection restriction mechanism is the human protein <PROT2_60489>  CEM15 </PROT2_60489> / <PROT2_60489>  APOBEC3G </PROT2_60489> , which is expressed in human T cells ( the major targets for HIV infection in vivo ) and has been shown to be a potent inhibitor of <PROT1_155459>  Vif </PROT1_155459> - deficient HIV ( TARGET_CITATION ) . ||12167863_155459_60489==><PROT2_60489>  APOBEC3G </PROT2_60489> is encapsidated in & # x394 ; <PROT1_155459>  vif </PROT1_155459> HIV - 1 virions ( TARGET_CITATION ) and upon infection , deaminates newly synthesized viral reverse transcripts . ||12167863_155459_60489==>Differential expression screens flagged a handful of candidates that were selectively expressed in nonpermissive cell lines , and of these a single cDNA , first named <PROT2_60489>  CEM15 </PROT2_60489> , was found to specifically inhibit the replication of <PROT1_155459>  Vif </PROT1_155459> - minus virus ( TARGET_CITATION ) . ||12167863_155459_60489==>Two reports have appeared that disagree as to whether <PROT1_155459>  Vif </PROT1_155459> prevents <PROT2_60489>  APOBEC3G </PROT2_60489> from being incorporated into virions ( CITATION and TARGET_CITATION ) . ||12167863_155459_60489==>However , one group found that <PROT1_155459>  Vif </PROT1_155459> had no effect on virion incorporation of <PROT2_60489>  APOBEC3G </PROT2_60489> ( TARGET_CITATION ) , while the second group reported that <PROT1_155459>  Vif </PROT1_155459> excludes <PROT2_60489>  APOBEC3G </PROT2_60489> from virions ( CITATION ) . ||12167863_155459_60489==>Rather , we note that the <PROT1_155459>  Vif </PROT1_155459> antagonism of the antiretroviral effect of <PROT2_60489>  APOBEC3G </PROT2_60489> appears to be critically sensitive to the <PROT1_155459>  Vif </PROT1_155459> : <PROT2_60489>  APOBEC3G </PROT2_60489> ratio ( TARGET_CITATION , CITATION , CITATION , CITATION , CITATION and CITATION ) . ||12167863_155459_60489==>The inhibitory effect of <PROT2_60489>  CEM15 </PROT2_60489> is counteracted by the viral <PROT1_155459>  Vif </PROT1_155459> protein ( TARGET_CITATION ) . ||12167863_155459_60489==>As shown in fig. 1 , transient expression of His - <PROT2_60489>  CEM15 </PROT2_60489> or EGFP - <PROT2_60489>  CEM15 </PROT2_60489> clearly suppressed the infectivity of { Delta } <PROT1_155459>  vif </PROT1_155459> virions in a dose - dependent manner as reported previously ( TARGET_CITATION ) . ||12167863_155459_60489==>However , recent studies indicate that <PROT1_155459>  Vif </PROT1_155459> is required to counteract the endogenous inhibitor <PROT2_60489>  CEM15 </PROT2_60489> , which is a putative cytidine deaminase ( TARGET_CITATION , CITATION ) . ||12167863_155459_60489==>The HIV genome encodes a protein termed virion infectivity factor ( <PROT1_155459>  vif </PROT1_155459> ) whose function is to overcome <PROT2_60489>  APOBEC3G </PROT2_60489> - mediated immunity ( TARGET_CITATION ) . ||12167863_155459_60489==>Overall , our results indicate that in the absence of <PROT1_155459>  Vif </PROT1_155459> , virions produced in nonpermissive H9 cells contain an activity that results in the introduction of C - > U changes into the minus - strand of viral DNA synthesized during reverse transcription , a feature that is highly compatible with editing of the minus - strand by a cytidine deaminase , such as CEM 15 / <PROT2_60489>  APOBEC3G </PROT2_60489> ( TARGET_CITATION ) . ||12167863_155459_60489==>( TARGET_CITATION ) , <PROT2_60489>  APOBEC3G </PROT2_60489> copurifies in HIV virions produced in the absence of <PROT1_155459>  Vif </PROT1_155459> ( CITATION , CITATION , CITATION ) . ||12167863_155459_60489==>Recently , <PROT2_60489>  CEM15 </PROT2_60489> / <PROT2_60489>  APOBEC3G </PROT2_60489> ( hereafter referred to as <PROT2_60489>  APOBEC3G </PROT2_60489> ) , which is present only in nonpermissive cells , has been identified as a mediator of anti & # 150 ; HIV - 1 activity , and its activity has been shown to be suppressed by <PROT1_155459>  Vif </PROT1_155459> ( TARGET_CITATION ) . ||12167863_155459_60489==>The ability of <PROT1_155459>  Vif </PROT1_155459> to induce ubiquitination and degradation of <PROT2_60489>  APOBEC3G </PROT2_60489> also correlated with its ability to prevent virion incorporation of <PROT2_60489>  APOBEC3G </PROT2_60489> , which is detrimental to HIV - 1 ( TARGET_CITATION & # 150 ; CITATION ) . ||12167863_155459_60489==>As both <PROT2_60489>  CEM15 </PROT2_60489> and <PROT1_155459>  Vif </PROT1_155459> can be packaged into HIV - 1 virions TARGET_CITATION , CITATION , we then investigated whether the newly synthesized HIV - 1 DNA is a substrate of <PROT2_60489>  CEM15 </PROT2_60489> . ||12167863_155459_60489==>Furthermore , we have also examined the effect of <PROT1_155459>  Vif </PROT1_155459> on hypermutation in long - term cell culture by passing the wild - type or Deltavif viruses in C8166 , a T - cell line that is semi - permissive for Deltavif viruses CITATION and that expresses small amounts of <PROT2_60489>  CEM15 </PROT2_60489> ( ref. TARGET_CITATION and data not shown ) . ||12167863_155459_60489==>Recently , we have used gene transfer experiments to show that the <PROT2_60489>  CEM15 </PROT2_60489> protein ( also termed <PROT2_60489>  APOBEC3G </PROT2_60489> ( CITATION ) ) is at least partially responsible for this antiviral activity ( TARGET_CITATION ) ; whether <PROT1_155459>  Vif </PROT1_155459> interacts directly with this protein is unknown . ||12167863_155459_60489==>We have recently reported that an epitope - tagged version of <PROT2_60489>  CEM15 </PROT2_60489> , the T - cell - expressed antiviral factor that is counteracted by <PROT1_155459>  Vif </PROT1_155459> , is incorporated in HIV - 1 particles when overexpressed in virus - producing cells ( TARGET_CITATION ) . ||12167863_155459_60489==>Since <PROT2_60489>  CEM15 </PROT2_60489> shares significant sequence similarity with cytidine deaminases ( TARGET_CITATION ) , it is plausible that viral ( or even cellular ) RNAs ( messenger , genomic , or ) or DNAs are subjected to sequence modification in nonpermissive cells in the absence of <PROT1_155459>  Vif </PROT1_155459> . ||12167863_155459_60489==>Recent work by Sheehy et al. identified a cellular factor , <PROT2_60489>  CEM15 </PROT2_60489> , which was expressed in cell types that are restrictive for the replication of <PROT1_155459>  Vif </PROT1_155459> - defective viruses but was not expressed in permissive cell types ( TARGET_CITATION ) . ||12167863_155459_60489==>Expression of <PROT2_60489>  CEM15 </PROT2_60489> in permissive cell types imposed a restrictive phenotype on these cells , providing intriguing evidence that <PROT2_60489>  CEM15 </PROT2_60489> is indeed a cellular inhibitor whose activity must be overcome by <PROT1_155459>  Vif </PROT1_155459> for HIV replication to proceed ( TARGET_CITATION ) . ||12167863_155459_60489==>Interestingly , <PROT2_60489>  CEM15 </PROT2_60489> , like <PROT1_155459>  Vif </PROT1_155459> , is packaged into virions ( TARGET_CITATION ) . ||12167863_155459_60489==>( TARGET_CITATION ) , <PROT1_155459>  Vif </PROT1_155459> did not affect <PROT2_60489>  APOBEC3G </PROT2_60489> mRNA levels , which remained constant even at high concentrations of <PROT1_155459>  Vif </PROT1_155459> ( fig. 5B , panel <PROT2_60489>  APOBEC3G </PROT2_60489> , and C , RNA ) . ||12167863_155459_60489==>In fact , expression of Hck and <PROT2_60489>  CEM15 </PROT2_60489> appeared to be associated with inhibition of viral infectivity in a <PROT1_155459>  Vif </PROT1_155459> - dependent manner ( CITATION , TARGET_CITATION ) . ||12167863_155459_60489==>Thus , <PROT2_60489>  CEM15 </PROT2_60489> represents to date the most promising factor that fits most , if not all , of the characteristics required of a protein associated with <PROT1_155459>  Vif </PROT1_155459> - dependent host cell restriction : it is expressed exclusively in nonpermissive cells and expression in permissive cells inhibits virus infectivity in the absence but not in the presence of <PROT1_155459>  Vif </PROT1_155459> ( TARGET_CITATION ) . ||12167863_155459_60489==>However , it is possible that after synthesis <PROT1_155459>  Vif </PROT1_155459> gradually associates with cellular factors such as <PROT2_60489>  CEM15 </PROT2_60489> ( TARGET_CITATION ) or vimentin ( CITATION , CITATION ) , thereby preventing packaging of <PROT1_155459>  Vif </PROT1_155459> into virus particles . ||12167863_155459_60489==>In contrast , if expressed at the time of virus production , <PROT2_60489>  CEM15 </PROT2_60489> inhibits postentry steps of HIV - 1 replication ( TARGET_CITATION ) ; the significance of <PROT2_60489>  CEM15 </PROT2_60489> is emphasized by the fact that all primate lentiviruses encode a factor , <PROT1_155459>  Vif </PROT1_155459> , which counteracts its antiviral activity . ||12167863_155459_60489==>In addition , it was reported that <PROT2_60489>  APOBEC3G </PROT2_60489> is incorporated into HIV - 1 virions regardless of whether <PROT1_155459>  Vif </PROT1_155459> is present or absent in the producer cells TARGET_CITATION . ||12167863_155459_60489==>Because <PROT1_155459>  Vif </PROT1_155459> is incorporated in small amounts into HIV - 1 virions CITATION CITATION CITATION , and because it binds to RNA CITATION CITATION , it was suggested that <PROT1_155459>  Vif </PROT1_155459> might bind to the HIV - 1 genomic RNA and shield it from inactivation by <PROT2_60489>  APOBEC3G </PROT2_60489> in the producer cells and the released virions TARGET_CITATION CITATION , thus acting on the target of <PROT2_60489>  APOBEC3G </PROT2_60489> rather than directly on the antiviral protein . ||12167863_155459_60489==>The HIV - 1 <PROT1_155459>  Vif </PROT1_155459> protein reverses these effects by counteracting <PROT2_60489>  APOBEC3G </PROT2_60489> action TARGET_CITATION ; hypothetically , this could occur either by suppressing the enzymatic capabilities of <PROT2_60489>  APOBEC3G </PROT2_60489> or by preventing its incorporation into progeny virions . ||12167863_155459_60489==>Because <PROT1_155459>  Vif </PROT1_155459> does not affect <PROT2_60489>  APOBEC3G </PROT2_60489> mRNA levels TARGET_CITATION , we first addressed this question by examining <PROT2_60489>  APOBEC3G </PROT2_60489> protein expression by pulse - chase analysis ( fig. 2a ) . ||12167863_155459_60489==>Expression vectors for wild - type and <PROT1_155459>  vif </PROT1_155459> - deficient ( Deltavif ) HIV - 1 ( pIIIB and pIIIB / Deltavif , respectively ) , a hemagglutinin epitope & # 150 ; tagged version of CEM - 15 / <PROT2_60489>  APOBEC3G </PROT2_60489> ( renamed here as pAPOBEC3G : HA ) , wild - type <PROT1_155459>  Vif </PROT1_155459> ( pcVIF ) and histidine - tagged ubiquitin ( pRGB4 - 6HisUB ) have been described CITATION TARGET_CITATION CITATION . ||12167863_155459_60489==>Production of HIV - 1 virions lacking <PROT1_155459>  Vif </PROT1_155459> is severely restricted at a postentry , preintegration step of infection , based on an antiviral cellular factor called <PROT2_60489>  CEM15 </PROT2_60489> , which inhibits viral replication CITATION TARGET_CITATION . ||12167863_155459_60489==>Efforts to understand the mechanisms of action of this essential viral gene product led to the demonstration that <PROT1_155459>  Vif </PROT1_155459> functions by blocking the activity of a cellular antiretroviral defense protein termed <PROT2_60489>  APOBEC3G </PROT2_60489> ( TARGET_CITATION ) . ||12167863_155459_60489==>Efforts to elucidate the role of HIV - 1 - encoded viral infectivity factor ( <PROT1_155459>  Vif </PROT1_155459> ) ( CITATION , CITATION ) led to the discovery of the human apolipoprotein B mRNA - editing , enzyme - catalytic , polypeptide - like 3G ( <PROT2_60489>  APOBEC3G </PROT2_60489> ) , first identified as <PROT2_60489>  CEM15 </PROT2_60489> ( TARGET_CITATION ) . ||12167863_155459_60489==>Initially , it was reported that <PROT2_60489>  APOBEC3G </PROT2_60489> is incorporated equally into the HIV - 1 ( & # x394 ; <PROT1_155459>  vif </PROT1_155459> ) and wild - type HIV - 1 virions that were derived from nonpermissive cells ( TARGET_CITATION ) . ||12167863_155459_60489==>Recently , a host protein that interacts with <PROT1_155459>  Vif </PROT1_155459> was identified ( TARGET_CITATION ) , and subsequent reports have revealed that the host factor , now termed <PROT2_60489>  APOBEC3G </PROT2_60489> ( apolipoprotein B mRNA - editing enzyme , catalytic polypeptide - like 3G ) , is a member of the family of enzymes known as cytidine deaminases ( CITATION , CITATION , CITATION , CITATION , CITATION and CITATION ) ( Figure 3 ) . ||12167863_155459_60489==>Under normal circumstances , the viral regulatory protein <PROT1_155459>  Vif </PROT1_155459> protects HIV infection from the inhibitory effects of <PROT2_60489>  APOBEC3G </PROT2_60489> ( CITATION , TARGET_CITATION , CITATION and CITATION ) by inducing its proteasomal degradation and exclusion from virus particles ( CITATION , CITATION , CITATION and CITATION ) . ||12167863_155459_60489==>Non - permissive cells have been found to contain a protein called <PROT2_60489>  APOBEC3G </PROT2_60489> ( also known as CEM - 15 ) , which prevents HIV - 1 replication in the absence of <PROT1_155459>  Vif </PROT1_155459> ( TARGET_CITATION ) . ||12167863_155459_60489==><PROT1_155459>  Vif </PROT1_155459> - negative HIV - 1 produced in non - permissive cells package <PROT2_60489>  APOBEC3G </PROT2_60489> during assembly while <PROT1_155459>  Vif </PROT1_155459> - positive virions do not ( TARGET_CITATION , CITATION ) . ||12167863_155459_60489==>Expression of <PROT2_60489>  APOBEC3G </PROT2_60489> in these permissive cell lines converts them to the nonpermissive phenotype , as determined by their ability to efficiently inactivate HIV - 1 ( { Delta } <PROT1_155459>  vif </PROT1_155459> ) but not wild - type HIV - 1 ( CITATION , CITATION & # 150 ; CITATION , TARGET_CITATION ) . ||12167863_155459_60489==>Recent studies demonstrate that <PROT1_155459>  Vif </PROT1_155459> counteracts the innate antiviral activity of <PROT2_60489>  APOBEC3G </PROT2_60489> , also known as <PROT2_60489>  CEM15 </PROT2_60489> ( TARGET_CITATION & # 150 ; CITATION ) . ||12167863_155459_60489==>Increasing expression of <PROT2_60489>  APOBEC3G </PROT2_60489> dramatically reduced the infectivity of { Delta } <PROT1_155459>  Vif </PROT1_155459> virus in a dose - dependent manner ( fig. 4A ) as previously reported ( TARGET_CITATION ) . ||12167863_155459_60489==>New investigations have shown a requirement for HIV - 1 <PROT1_155459>  Vif </PROT1_155459> protein to counter G - to - A mutations in nascent retroviral DNA induced by the <PROT2_60489>  APOBEC3G </PROT2_60489> protein ( also know as <PROT2_60489>  CEM15 </PROT2_60489> ) expressed in nonpermissive human cells ( CITATION ; CITATION ; CITATION ; TARGET_CITATION and CITATION ) . ||12167863_155459_60489==>Earlier , it had been shown that <PROT2_60489>  Apobec3G </PROT2_60489> was required for the resistance of certain T - cell lines against <PROT1_155459>  vif </PROT1_155459> & # x2212 ; HIV - 1 ( TARGET_CITATION ) and that it is a mutator in e. coli ( CITATION ) . ||12167863_155459_60489==>Malim and colleagues identified <PROT2_60489>  APOBEC3G </PROT2_60489> as a factor expressed by human T cells that failed to support the production of <PROT1_155459>  Vif </PROT1_155459> - deficient HIV - 1 ( ref. TARGET_CITATION ) . ||12167863_155459_60489==>The discovery of <PROT2_60489>  APOBEC3G </PROT2_60489> TARGET_CITATION has now opened the door to molecular studies showing that HIV - 1 <PROT1_155459>  Vif </PROT1_155459> functions by triggering the polyubiquitylation and proteasomal degradation of human <PROT2_60489>  APOBEC3G </PROT2_60489> before virion incorporation CITATION , CITATION , CITATION . ||12167863_155459_60489==><PROT1_155459>  Vif </PROT1_155459> has been shown to interact with and promotes the degradation of the cellular cytidine deaminase , <PROT2_60489>  CEM15 </PROT2_60489> / <PROT2_60489>  APOBEC3G </PROT2_60489> which is expressed only in non - permissive cell lines and inactivates the HIV - 1 genome by introducing multiple mutations during the process of reverse transcription ( TARGET_CITATION and CITATION ) . ||12167863_155459_60489==><PROT1_155459>  Vif </PROT1_155459> counteracts an endogenous cellular factor , <PROT2_60489>  APOBEC3G </PROT2_60489> , that inhibits HIV - 1 replication CITATION and TARGET_CITATION . ||12167863_155459_60489==>Other adaptations , particularly with respect to the <PROT1_155459>  Vif </PROT1_155459> / <PROT2_60489>  APOBEC3G </PROT2_60489> axis ( CITATION , TARGET_CITATION ) , may also be required , but these and other recent findings suggest that the adaptation of HIV - 1 to full replication competence in a practically useful nonhuman species may be possible . ||12167863_155459_60489==>Since it was reported that <PROT1_155459>  Vif </PROT1_155459> degrades <PROT2_60489>  APOBEC3G </PROT2_60489> via the proteasome ( CITATION , CITATION , TARGET_CITATION , CITATION , CITATION ) , we also determined if this pathway was involved in the removal of APOBEC3F . ||12167863_155459_60489==>This high rate of G - to - A transitions suggests a potential role for <PROT2_60489>  Apobec3G </PROT2_60489> / <PROT2_60489>  CEM15 </PROT2_60489> ( CITATION , CITATION ) , the cellular cytosine deaminase whose function is antagonized by viral <PROT1_155459>  Vif </PROT1_155459> expression ( TARGET_CITATION ) . ||12167863_155459_60489==><PROT1_155459>  Vif </PROT1_155459> counteracts an anti - HIV - 1 cellular factor in nonpermissive cells ( CITATION , CITATION ) , referred to as <PROT2_60489>  APOBEC3G </PROT2_60489> , thereby enhancing virion infectivity ( TARGET_CITATION ) . ||12167863_155459_60489==><PROT1_155459>  Vif </PROT1_155459> acts to suppress this inhibitory effect of <PROT2_60489>  APOBEC3G </PROT2_60489> in nonpermissive cells by inducing degradation of <PROT2_60489>  APOBEC3G </PROT2_60489> and preventing it from being packaged into virions ( CITATION , CITATION , CITATION , TARGET_CITATION , CITATION , CITATION ) . ||12167863_155459_60489==>Some cellular antiviral factors can be selectively suppressed by viral proteins , as shown by the interaction between <PROT2_60489>  APOBEC3G </PROT2_60489> , a cytidine deaminase , and <PROT1_155459>  Vif </PROT1_155459> ( CITATION , CITATION - CITATION , TARGET_CITATION ) . ||12167863_155459_60489==>Sheehy et al. identified a cellular factor , <PROT2_60489>  CEM15 </PROT2_60489> , later shown to be the cytidine deaminase <PROT2_60489>  APOBEC3G </PROT2_60489> ( TARGET_CITATION ) , a member of a family of RNA editing enzymes that mediates HIV - 1 { Delta } <PROT1_155459>  vif </PROT1_155459> suppression . ||12167863_155459_60489==><PROT1_155459>  Vif </PROT1_155459> appears to bind <PROT2_60489>  APOBEC3G </PROT2_60489> , thus inhibiting cytidine deamination of the genome ( TARGET_CITATION ) . ||12167863_155459_60489==>Recently , it has been shown that <PROT2_60489>  APOBEC3G </PROT2_60489> has antiviral activity and that its activity is suppressed by <PROT1_155459>  Vif </PROT1_155459> ( TARGET_CITATION ) . ||12167863_155459_60489==><PROT2_60489>  APOBEC3G </PROT2_60489> is a major cellular antiviral factor that needs to be neutralized by HIV - 1 <PROT1_155459>  Vif </PROT1_155459> if successful virus production is to occur ( CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , TARGET_CITATION , CITATION , CITATION , CITATION , CITATION ) . ||12167863_155459_60489==>Recently , <PROT2_60489>  CEM15 </PROT2_60489> ( also called <PROT2_60489>  APOBEC3G </PROT2_60489> , and hereafter referred to by this name ) , which is present only in nonpermissive cells , has been identified as a mediator of anti - HIV - 1 activity , and its activity has been shown to be suppressed by <PROT1_155459>  Vif </PROT1_155459> ( TARGET_CITATION ) . ||12167863_155459_60489==>Although it was shown initially that <PROT2_60489>  APOBEC3G </PROT2_60489> was incorporated efficiently into wild - type and <PROT1_155459>  Vif </PROT1_155459> mutant virions ( TARGET_CITATION ) , recent studies have shown that HIV - 1 <PROT1_155459>  Vif </PROT1_155459> could efficiently prevent virion incorporation of <PROT2_60489>  APOBEC3G </PROT2_60489> ( CITATION , CITATION , CITATION , CITATION , CITATION , CITATION ) . ||12167863_155459_60489==>These results are consistent with the recent observation that <PROT2_60489>  APOBEC3G </PROT2_60489> can convert permissive CEM - SS cells to the nonpermissive phenotype ( TARGET_CITATION ) , suggesting that <PROT2_60489>  APOBEC3G </PROT2_60489> is a major antiviral factor that determines the permissive versus nonpermissive phenotype for HIV - 1 <PROT1_155459>  Vif </PROT1_155459> mutant viruses . ||12167863_155459_60489==>Accumulating evidence indicates that <PROT2_60489>  APOBEC3G </PROT2_60489> is a major cellular antiviral factor that needs to be neutralized by HIV - 1 <PROT1_155459>  Vif </PROT1_155459> if successful virus production is to occur ( CITATION , CITATION , CITATION , CITATION - CITATION , TARGET_CITATION , CITATION , CITATION , CITATION , CITATION ) . ||12719574_155459_60489==>Because <PROT1_155459>  Vif </PROT1_155459> is incorporated in small amounts into HIV - 1 virions TARGET_CITATION CITATION CITATION , and because it binds to RNA CITATION CITATION , it was suggested that <PROT1_155459>  Vif </PROT1_155459> might bind to the HIV - 1 genomic RNA and shield it from inactivation by <PROT2_60489>  APOBEC3G </PROT2_60489> in the producer cells and the released virions CITATION TARGET_CITATION , thus acting on the target of <PROT2_60489>  APOBEC3G </PROT2_60489> rather than directly on the antiviral protein . ||12719574_155459_60489==>Production of HIV - 1 virions lacking <PROT1_155459>  Vif </PROT1_155459> is severely restricted at a postentry , preintegration step of infection , based on an antiviral cellular factor called <PROT2_60489>  CEM15 </PROT2_60489> , which inhibits viral replication TARGET_CITATION CITATION . ||12750511_155459_60489==>The DNA sequences documenting this massive hypermutation of <PROT1_155459>  vif </PROT1_155459> - minus HIV - 1 proviruses are probably the most dramatic demonstration of <PROT2_60489>  APOBEC3G </PROT2_60489> activity ( CITATION ; TARGET_CITATION ; CITATION ; CITATION and CITATION ) . ||12750511_155459_60489==>Another member of the family APOBECG3 / <PROT2_60489>  CEM15 </PROT2_60489> , which deaminates C to U in the minus strand cDNA of several retroviruses ; this activity is inhibited by interaction with the <PROT1_155459>  Vif </PROT1_155459> protein from HIV ( CITATION ; CITATION ; TARGET_CITATION ; CITATION ; CITATION and CITATION ) . ||12750511_155459_60489==>Most recently , a series of papers demonstrated that <PROT2_60489>  APOBEC3G </PROT2_60489> induces hypermutation of viral cDNA in the absence of <PROT1_155459>  Vif </PROT1_155459> ( CITATION , TARGET_CITATION , CITATION , CITATION ) . ||12750511_155459_60489==>After submission of this work , a series of studies was submitted and published that provided convincing evidence that packaging of <PROT2_60489>  APOBEC3G </PROT2_60489> into <PROT1_155459>  Vif </PROT1_155459> - defective virions induces hypermutation of viral cDNA ( CITATION , TARGET_CITATION , CITATION , CITATION ) . ||12750511_155459_60489==>Several recent papers ( TARGET_CITATION , CITATION , CITATION , CITATION and CITATION ) have unambiguously linked the HIV & # x394 ; <PROT1_155459>  vif </PROT1_155459> phenotype to the development of G & # x2192 ; A hypermutated genomes , which relies on the exclusion of <PROT2_60489>  APOBEC3G </PROT2_60489> from the virion . ||12750511_155459_60489==>Indeed , several recent reports have demonstrated that expression of <PROT2_60489>  APOBEC3G </PROT2_60489> in virus producer cells but not target cells results in G - to - A hypermutation of the HIV - 1 genomes in the absence of <PROT1_155459>  Vif </PROT1_155459> ( TARGET_CITATION & # 150 ; CITATION ) . ||12750511_155459_60489==>Therefore , when <PROT1_155459>  Vif </PROT1_155459> is absent from nonpermissive cells , the progeny virions incorporate <PROT2_60489>  APOBEC3G </PROT2_60489> , which attacks the negative strand DNA of HIV - 1 that is made during reverse transcription in the subsequently infected target cells ( TARGET_CITATION , CITATION , CITATION and CITATION ) . ||12750511_155459_60489==>Human <PROT2_60489>  APOBEC3G </PROT2_60489> activity was suppressed by HIV - 1 <PROT1_155459>  Vif </PROT1_155459> , but the AGM and mouse <PROT2_60489>  APOBEC3G </PROT2_60489> were not ( fig. 1b ) , in accord with previous reports CITATION TARGET_CITATION CITATION CITATION CITATION . ||12750511_155459_60489==>The recent identification and characterization of the target of <PROT1_155459>  Vif </PROT1_155459> and its main function in blocking the action of the host antiviral protein <PROT2_60489>  APOBEC3G </PROT2_60489> , as reported by several independent investigators , have greatly contributed to a major breakthrough and took us a giant leap forward in exploring one of the & # x2018 ; final frontiers & # x2019 ; in HIV - 1 molecular virology ( CITATION , CITATION , CITATION , TARGET_CITATION and CITATION ) . ||12750511_155459_60489==>Recent reports on the identification and characterization of HIV - 1 <PROT1_155459>  Vif </PROT1_155459> 's target , <PROT2_60489>  APOBEC3G </PROT2_60489> , shed further light on the field and greatly improved our understanding of viral & # x2013 ; cell interactions ( CITATION , CITATION , CITATION , TARGET_CITATION and CITATION ) ( Figure 1 ) . ||12750511_155459_60489==>Recently , a host protein that interacts with <PROT1_155459>  Vif </PROT1_155459> was identified ( CITATION ) , and subsequent reports have revealed that the host factor , now termed <PROT2_60489>  APOBEC3G </PROT2_60489> ( apolipoprotein B mRNA - editing enzyme , catalytic polypeptide - like 3G ) , is a member of the family of enzymes known as cytidine deaminases ( CITATION , TARGET_CITATION , CITATION , CITATION , CITATION and CITATION ) ( Figure 3 ) . ||12750511_155459_60489==>Under normal circumstances , the viral regulatory protein <PROT1_155459>  Vif </PROT1_155459> protects HIV infection from the inhibitory effects of <PROT2_60489>  APOBEC3G </PROT2_60489> ( CITATION , CITATION , CITATION and TARGET_CITATION ) by inducing its proteasomal degradation and exclusion from virus particles ( CITATION , CITATION , CITATION and CITATION ) . ||12750511_155459_60489==><PROT2_60489>  APOBEC3G </PROT2_60489> is a DNA - editing enzyme with antiviral activity that is overcome by <PROT1_155459>  Vif </PROT1_155459> ( TARGET_CITATION & # 150 ; CITATION , CITATION ) . ||12750511_155459_60489==>In the absence of <PROT1_155459>  Vif </PROT1_155459> , <PROT2_60489>  APOBEC3G </PROT2_60489> induces the deamination of cytosines incorporated in the minus single - strand of the viral DNA during the reverse transcription ( CITATION , TARGET_CITATION , CITATION and CITATION ) . ||12750511_155459_60489==>New investigations have shown a requirement for HIV - 1 <PROT1_155459>  Vif </PROT1_155459> protein to counter G - to - A mutations in nascent retroviral DNA induced by the <PROT2_60489>  APOBEC3G </PROT2_60489> protein ( also know as <PROT2_60489>  CEM15 </PROT2_60489> ) expressed in nonpermissive human cells ( CITATION ; TARGET_CITATION ; CITATION ; CITATION and CITATION ) . ||12750511_155459_60489==><PROT2_60489>  APOBEC3G </PROT2_60489> triggers substantial G - to - A mutations in the nascent DNA in the absence of <PROT1_155459>  Vif </PROT1_155459> ( CITATION ; TARGET_CITATION ; CITATION and CITATION ) . ||12750511_155459_60489==>Analysis of DNA produced by endogenous reverse transcription in dVif - 1 virions provides support for the presence of a cytidine deaminase activity analogous to the <PROT2_60489>  APOBEC3G </PROT2_60489> protein found in <PROT1_155459>  Vif </PROT1_155459> - deficient HIV - 1 virions ( fig. 4 ) , although the cytidine deaminase activity in MVV dVif - 1 grown in FOS and SCP cells appears to be 10 - fold lower than in HIV - 1 grown in nonpermissive cells ( TARGET_CITATION ) . ||12750511_155459_60489==>The main mechanism by which <PROT2_60489>  APOBEC3G </PROT2_60489> restricts retroviral infection became clear when several groups sequenced retroviral cDNA from cells that had been infected with <PROT1_155459>  Vif </PROT1_155459> - deficient retroviruses that were produced in the presence of <PROT2_60489>  APOBEC3G </PROT2_60489> CITATION - TARGET_CITATION . ||12750511_155459_60489==><PROT2_60489>  CEM15 </PROT2_60489> is identical to apolipoprotein B mRNA editing enzyme catalytic polypeptide - like 3G ( <PROT2_60489>  APOBEC3G </PROT2_60489> ) , a cytidine deaminase that introduces C - to - U changes into the minus strand of newly synthesised HIV - 1 viral DNA in infected target cells in the absence of functional <PROT1_155459>  Vif </PROT1_155459> ( CITATION , TARGET_CITATION ) . ||12750511_155459_60489==><PROT1_155459>  Vif </PROT1_155459> acts to suppress this inhibitory effect of <PROT2_60489>  APOBEC3G </PROT2_60489> in nonpermissive cells by inducing degradation of <PROT2_60489>  APOBEC3G </PROT2_60489> and preventing it from being packaged into virions ( TARGET_CITATION , CITATION , CITATION , CITATION , CITATION , CITATION ) . ||12750511_155459_60489==>Some cellular antiviral factors can be selectively suppressed by viral proteins , as shown by the interaction between <PROT2_60489>  APOBEC3G </PROT2_60489> , a cytidine deaminase , and <PROT1_155459>  Vif </PROT1_155459> ( CITATION , TARGET_CITATION - CITATION , CITATION ) . ||12750511_155459_60489==><PROT1_155459>  Vif </PROT1_155459> mutant , but not wild - type , human immunodeficiency virus type 1 ( HIV - 1 ) produced in the presence of <PROT2_60489>  APOBEC3G </PROT2_60489> undergoes hypermutations in newly synthesized viral DNA ( CITATION , TARGET_CITATION , CITATION , CITATION , CITATION ) , presumably due to C - to - U modification during minus - strand viral DNA synthesis ( CITATION , TARGET_CITATION , CITATION , CITATION , CITATION ) . ||12750511_155459_60489==><PROT2_60489>  APOBEC3G </PROT2_60489> is a major cellular antiviral factor that needs to be neutralized by HIV - 1 <PROT1_155459>  Vif </PROT1_155459> if successful virus production is to occur ( CITATION , CITATION , TARGET_CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , CITATION ) . ||12750511_155459_60489==><PROT1_155459>  Vif </PROT1_155459> mutant but not wild - type HIV - 1 viruses produced in the presence of <PROT2_60489>  APOBEC3G </PROT2_60489> undergo hypermutations in newly synthesized viral DNA ( CITATION , TARGET_CITATION , CITATION , CITATION , CITATION ) , presumably due to C - to - U modification during minus - strand viral DNA synthesis ( CITATION , TARGET_CITATION , CITATION , CITATION , CITATION ) . ||12750511_155459_60489==>Accumulating evidence indicates that <PROT2_60489>  APOBEC3G </PROT2_60489> is a major cellular antiviral factor that needs to be neutralized by HIV - 1 <PROT1_155459>  Vif </PROT1_155459> if successful virus production is to occur ( CITATION , CITATION , TARGET_CITATION , CITATION - CITATION , CITATION , CITATION , CITATION , CITATION , CITATION ) . ||12830140_155459_60489==>Rather , we note that the <PROT1_155459>  Vif </PROT1_155459> antagonism of the antiretroviral effect of <PROT2_60489>  APOBEC3G </PROT2_60489> appears to be critically sensitive to the <PROT1_155459>  Vif </PROT1_155459> : <PROT2_60489>  APOBEC3G </PROT2_60489> ratio ( CITATION , CITATION , CITATION , CITATION , TARGET_CITATION and CITATION ) . ||12830140_155459_60489==>Our findings may help to answer a question raised by previous reports regarding how retroviruses without <PROT1_155459>  Vif </PROT1_155459> manage to replicate in the presence of this antiviral enzyme ( CITATION , CITATION , TARGET_CITATION ) ; that is , murine retrovirus replicates in murine target cells expressing <PROT2_60489>  APOBEC3G </PROT2_60489> because it is not a target for murine homologue of this enzyme . ||12840737_155459_60489==>Our findings may help to answer a question raised by previous reports regarding how retroviruses without <PROT1_155459>  Vif </PROT1_155459> manage to replicate in the presence of this antiviral enzyme ( CITATION , TARGET_CITATION , CITATION ) ; that is , murine retrovirus replicates in murine target cells expressing <PROT2_60489>  APOBEC3G </PROT2_60489> because it is not a target for murine homologue of this enzyme . ||12970355_155459_60489==>Therefore , when <PROT1_155459>  Vif </PROT1_155459> is absent from nonpermissive cells , the progeny virions incorporate <PROT2_60489>  APOBEC3G </PROT2_60489> , which attacks the negative strand DNA of HIV - 1 that is made during reverse transcription in the subsequently infected target cells ( CITATION , CITATION , CITATION and TARGET_CITATION ) . ||12970355_155459_60489==>Moreover , replication - competent ( <PROT1_155459>  Vif </PROT1_155459> - deficient ) HIV - 1 produced in the presence of <PROT2_60489>  APOBEC3G </PROT2_60489> has shown a similar strong 5 & # x2032 ; - ImageG preference ( CITATION , CITATION , CITATION , TARGET_CITATION and CITATION ) . ||9846577_155459_60489==><PROT2_60489>  CEM15 </PROT2_60489> / Apobec 3G interferes with the production of infectious virions in the absence of a functional <PROT1_155459>  vif </PROT1_155459> allele ( TARGET_CITATION , CITATION ) ; TSG101 interacts with the PTAPP motif in gag p6 , where it facilitates release of virus from cells ( CITATION ) . ||9846577_155459_60489==>While the mechanism by which <PROT1_155459>  Vif </PROT1_155459> overcomes <PROT2_60489>  APOBEC3G </PROT2_60489> - mediated restriction of viral infection is unknown , it is evident that <PROT1_155459>  Vif </PROT1_155459> ( like <PROT2_60489>  APOBEC3G </PROT2_60489> ) needs to be expressed in the viral - producer cell ( rather than just in the infected cell target ) in order to exhibit an effect on viral replication ( CITATION , CITATION , CITATION and TARGET_CITATION ) . ||9846577_155459_60489==><PROT1_155459>  Vif </PROT1_155459> counteracts an anti - HIV - 1 cellular factor in nonpermissive cells ( CITATION , TARGET_CITATION ) , referred to as <PROT2_60489>  APOBEC3G </PROT2_60489> , thereby enhancing virion infectivity ( CITATION ) . 
synergizes with====10438928_155871_2247==>These results are in agreement with those of other studies indicating that <PROT1_155871>  Tat </PROT1_155871> enhances the angiogenic effects of <PROT2_2247>  bFGF </PROT2_2247> , but not those of VEGF ( Ensoli et al. , 1994a CITATION ; Barillari et al. , 1999a CITATION , b TARGET_CITATION ) , therefore , no further studies were conducted with VEGF . ||10438928_155871_2247==>This is consistent with the finding that <PROT1_155871>  Tat </PROT1_155871> synergizes with <PROT2_2247>  bFGF </PROT2_2247> , but not with VEGF , in promoting endothelial cell growth and in vivo angiogenesis ( Ensoli et al. , 1994a CITATION ; Barillari et al. , 1999b TARGET_CITATION ) , and with the fact that <PROT1_155871>  Tat </PROT1_155871> angiogenic effects correlate with the expression of alpha vbeta 3 , which is induced by <PROT2_2247>  bFGF </PROT2_2247> and binds <PROT1_155871>  Tat </PROT1_155871> RGD region , and not with the expression of alpha vbeta 5 that is promoted by VEGF ( Barillari et al. , 1999b TARGET_CITATION ) . ||10438928_155871_2247==>Consistent with this , the synthetic peptide may act on the anchorage complex formed by MMP - 2 and alpha vbeta 3 integrin ( Brooks et al. , 1996 CITATION ) , which are both induced and activated by <PROT1_155871>  Tat </PROT1_155871> and <PROT2_2247>  bFGF </PROT2_2247> as demonstrated by our present and previous data ( Barillari et al. , 1993 CITATION , 1999a CITATION , b TARGET_CITATION ; Sepp et al. , 1994 CITATION ; Fiorelli et al. , 1995 CITATION , 1999 CITATION ) . ||10438928_155871_2247==>This is sustained by in vivo findings indicating that <PROT1_155871>  Tat </PROT1_155871> increases KS - like lesion formation induced by <PROT2_2247>  bFGF </PROT2_2247> injection into mice ( TARGET_CITATION , CITATION ) and that the KSC growth rate increases in the presence of circulating <PROT1_155871>  Tat </PROT1_155871> protein ( CITATION ) . ||10438928_155871_2247==>However , differently from a true angiogenic molecule such as <PROT2_2247>  bFGF </PROT2_2247> , <PROT1_155871>  Tat </PROT1_155871> acts only on endothelial cells that are activated by inflammatory cytokines ( CITATION , CITATION - TARGET_CITATION , CITATION , CITATION ) ( Table 1 ) . ||10438928_155871_2247==>In contrast , <PROT1_155871>  Tat </PROT1_155871> enhances the mitogenic effect of <PROT2_2247>  bFGF </PROT2_2247> on endothelial cells , consistent with its capability of maintaining exogenously added <PROT2_2247>  bFGF </PROT2_2247> into a soluble form ( CITATION , TARGET_CITATION , CITATION ) . ||10438928_155871_2247==>In contrast , when <PROT1_155871>  Tat </PROT1_155871> is injected in combination with suboptimal ( non - lesion - forming ) amounts of <PROT2_2247>  bFGF </PROT2_2247> it promotes the development of angioproliferative , KS - like lesions in nude mice ( TARGET_CITATION , CITATION ) ( fig. 4 ) . ||10438928_155871_2247==>The angiogenic effect of <PROT1_155871>  Tat </PROT1_155871> and <PROT2_2247>  bFGF </PROT2_2247> is reproduced by the injection of <PROT1_155871>  Tat </PROT1_155871> and combined IL - 1 & # 223 ; , TNF - { alpha } , and IFN - { gamma } , which are the same cytokines that induce endothelial cell responsiveness to <PROT1_155871>  Tat </PROT1_155871> in vitro ( TARGET_CITATION ) ( fig. 4 ) . ||10438928_155871_2247==>Concentrations of IL - 1 & # 223 ; , TNF - { alpha } , and IFN - { gamma } exerting angiogenic synergy with <PROT1_155871>  Tat </PROT1_155871> in vivo induce both <PROT2_2247>  bFGF </PROT2_2247> and VEGF expression in mouse tissues ( TARGET_CITATION ) . ||10438928_155871_2247==>However , as discussed above , differently from <PROT2_2247>  bFGF </PROT2_2247> , VEGF does not synergize with <PROT1_155871>  Tat </PROT1_155871> in promoting in vivo angiogenesis ( TARGET_CITATION ) ( fig. 4 ) . ||10438928_155871_2247==>Consistent with these in vivo results , and as found for inflammatory cytokines ( see `` <PROT1_155871>  Tat </PROT1_155871> Binds Vascular Cells through Multiple Domains '' above ) , exposure to <PROT2_2247>  bFGF </PROT2_2247> induces endothelial cells to adhere to <PROT1_155871>  Tat </PROT1_155871> , while VEGF has no effects ( TARGET_CITATION ) . ||10438928_155871_2247==>In particular , previous studies indicated that <PROT2_2247>  bFGF </PROT2_2247> promotes angiogenesis by inducing the { alpha } 5 & # 223 ; 1 and { alpha } v & # 223 ; 3 integrins ( CITATION , CITATION , CITATION ) , which bind the RGD region of fibronectin , vitronectin ( CITATION ) , and <PROT1_155871>  Tat </PROT1_155871> ( CITATION - TARGET_CITATION ) . ||10438928_155871_2247==>In agreement with these findings , immunohistochemical stainings of tissues from mice injected with inflammatory cytokines , <PROT2_2247>  bFGF </PROT2_2247> , or VEGF indicate that the in vivo angiogenic effect of <PROT1_155871>  Tat </PROT1_155871> correlates with expressions of & # 223 ; 3 and , to a lesser extent , & # 223 ; 1 , which are induced by inflammatory cytokines or <PROT2_2247>  bFGF </PROT2_2247> , but not with & # 223 ; 5 expression , which is promoted by VEGF ( TARGET_CITATION ) ( fig. 4 ) . ||10438928_155871_2247==>In addition , <PROT1_155871>  Tat </PROT1_155871> 's capability of increasing the soluble fraction of <PROT2_2247>  bFGF </PROT2_2247> further upregulates the expression of the { alpha } v & # 223 ; 3 and { alpha } 5 & # 223 ; 1 integrins , which in turn mediate <PROT1_155871>  Tat </PROT1_155871> - promoted endothelial cell invasion , migration , and adhesion ( TARGET_CITATION ) . ||10438928_155871_2247==>Consistent with these in vivo data , exposure to <PROT2_2247>  bFGF </PROT2_2247> induces endothelial cells to migrate and proliferate in response to <PROT1_155871>  Tat </PROT1_155871> , while VEGF has no effect ( TARGET_CITATION ) . ||10438928_155871_2247==>In agreement with this hypothesis , integrin antagonists such as cyclic RGD peptides ( CITATION , CITATION ) specifically block the development of angioproliferative , KS - like lesions induced in nude mice by the injection of combined <PROT1_155871>  Tat </PROT1_155871> and <PROT2_2247>  bFGF </PROT2_2247> ( TARGET_CITATION ) . ||10438928_155871_2247==>Indeed , <PROT1_155871>  Tat </PROT1_155871> enhances <PROT2_2247>  bFGF </PROT2_2247> 's capability of promoting angiogenesis both in vitro and in vivo ( CITATION , TARGET_CITATION , CITATION ) . ||10438928_155871_2247==>This is because , by competing for heparin - binding sites , the highly basic residues of <PROT1_155871>  Tat </PROT1_155871> retrieve extracellularly bound <PROT2_2247>  bFGF </PROT2_2247> into a soluble form which promotes endothelial cell growth and upregulates the expression of { alpha } 5 & # 223 ; 1 and { alpha } v & # 223 ; 3 ( CITATION , TARGET_CITATION ) . ||10438928_155871_2247==>The fact that <PROT1_155871>  Tat </PROT1_155871> and VEGF - A share the same receptor may explain why , as opposed to what occurs with <PROT1_155871>  Tat </PROT1_155871> and <PROT2_2247>  bFGF </PROT2_2247> , <PROT1_155871>  Tat </PROT1_155871> does not enhance VEGF angiogenic effects either in vitro or in vivo ( TARGET_CITATION ) . ||10438928_155871_2247==>Numerous studies have implicated <PROT2_2247>  bFGF </PROT2_2247> in the pathogenesis of kidney disease as a mitogenic factor for renal cells CITATION , CITATION . <PROT2_2247>  bFGF </PROT2_2247> has been also involved in <PROT1_155871>  Tat </PROT1_155871> - induced proliferation of endothelial and Kaposi & # 146 ; s sarcoma cells CITATION , TARGET_CITATION . Thus , we evaluated the possible role of this growth factor in our experimental model . ||10438928_155871_2247==>Our data also indicated that <PROT2_2247>  bFGF </PROT2_2247> was involved , at least in part , in <PROT1_155871>  Tat </PROT1_155871> - induced proliferation of podocytes , as previously seen in other cell types CITATION , TARGET_CITATION . A body of evidence indicates that <PROT2_2247>  bFGF </PROT2_2247> is implicated in the pathogenesis of renal disease , because it is mitogenic for both MCs and proximal tubular epithelial cells and stimulates MC extracellular matrix production CITATION , CITATION . Recent studies demonstrated that <PROT2_2247>  bFGF </PROT2_2247> also has a proliferative effect on podocytes and that it is involved in the development of segmental glomerulosclerotic lesions CITATION , CITATION . Interestingly , increased expression of <PROT2_2247>  bFGF </PROT2_2247> was shown in a transgenic model of HIVAN CITATION , and high levels of <PROT2_2247>  bFGF </PROT2_2247> were detected in the kidneys of HIV - infected children CITATION . Our results suggest that the production of <PROT2_2247>  bFGF </PROT2_2247> in the glomeruli of the HIV - 1 - infected patients may be triggered by <PROT1_155871>  Tat </PROT1_155871> . ||10438928_155871_2247==>( CITATION , CITATION , CITATION , CITATION , TARGET_CITATION and CITATION ) Finally , <PROT2_2247>  bFGF </PROT2_2247> acts synergistically with HIV - 1 <PROT1_155871>  Tat </PROT1_155871> , which is released by HIV - 1 infected cells , to increase the frequency and aggressiveness of KS in HIV - 1 infected individuals . ||10438928_155871_2247==>The basic region that retrieves <PROT2_2247>  bFGF </PROT2_2247> from heparan sulfate proteoglycans of the extracellular matrix ( CITATION and TARGET_CITATION ; CITATION ) and the RGD region that binds to the small alpha , Greek5small beta , Greek1 and small alpha , Greekvsmall beta , Greek3 integrins ( CITATION ) and through this mediates cell adhesion , migration , and cell invasion , which in turn is associated with the induction of matrix metalloproteinase ( MMP ) - 1 by HIV - 1 <PROT1_155871>  Tat </PROT1_155871> ( CITATION and CITATION ) . ||10438928_155871_2247==>Transgenic mice expressing HIV - 1 <PROT1_155871>  Tat </PROT1_155871> develop tumors ( CITATION and CITATION ) , and inoculation into mice of purified HIV - 1 <PROT1_155871>  Tat </PROT1_155871> in combination with IC or <PROT2_2247>  bFGF </PROT2_2247> can lead to the formation of vascular lesions ( CITATION ; TARGET_CITATION ; CITATION and CITATION ) . ||10438928_155871_7124==>The angiogenic effect of <PROT1_155871>  Tat </PROT1_155871> and bFGF is reproduced by the injection of <PROT1_155871>  Tat </PROT1_155871> and combined IL - 1 & # 223 ; , <PROT2_7124>  TNF </PROT2_7124> - { alpha } , and IFN - { gamma } , which are the same cytokines that induce endothelial cell responsiveness to <PROT1_155871>  Tat </PROT1_155871> in vitro ( TARGET_CITATION ) ( fig. 4 ) . ||10438928_155871_7124==>Concentrations of IL - 1 & # 223 ; , <PROT2_7124>  TNF </PROT2_7124> - { alpha } , and IFN - { gamma } exerting angiogenic synergy with <PROT1_155871>  Tat </PROT1_155871> in vivo induce both bFGF and VEGF expression in mouse tissues ( TARGET_CITATION ) . ||10438928_155871_7124==>However , in order to exert its angiogenic effects , <PROT1_155871>  Tat </PROT1_155871> requires the cooperation of inflammatory cytokines , including IL - 1 & # 223 ; , <PROT2_7124>  TNF </PROT2_7124> - { alpha } , and IFN - { gamma } , whose levels are increased in the blood and tissues of individuals at risk for KS or in AIDS - KS patients ( CITATION , CITATION - TARGET_CITATION , CITATION - CITATION , CITATION ) . ||10438928_155871_7124==>Of note , endothelial cells activated by IFN - { gamma } , IL - 1 { beta } , and <PROT2_7124>  TNF </PROT2_7124> - { alpha } , but not nonactivated cells , bind and take up native <PROT1_155871>  Tat </PROT1_155871> very efficiently and in fashion similar to MDDC ( refs. CITATION , CITATION , and TARGET_CITATION , and data not shown ) . ||10454543_155871_2963==>Among these are <PROT1_155871>  Tat </PROT1_155871> - SF1 ( TARGET_CITATION , CITATION ) , the human homologue of the Saccharomyces cerevisiae transcription factor SPT5 ( CITATION ) , TFIIH ( CITATION , CITATION , CITATION ) , <PROT2_2963>  TFIIF </PROT2_2963> ( CITATION ) , and a <PROT1_155871>  Tat </PROT1_155871> - associated histone acetyltransferase ( reference CITATION and references therein ) . ||10454543_155871_2963==><PROT1_155871>  Tat </PROT1_155871> - SF1 interacts with <PROT1_155871>  Tat </PROT1_155871> ( CITATION ) and P - TEFb ( CITATION ) and was recently found to interact with human SPT5 , Pol II , and the <PROT2_2963>  RAP30 </PROT2_2963> subunit of <PROT2_2963>  TFIIF </PROT2_2963> as well ( TARGET_CITATION ) . ||10454543_155871_2963==>However , CYCT1 - C only partially removed SPT5 , an elongation factor reported to bind <PROT1_155871>  TAT </PROT1_155871> - SF1 ( ref. TARGET_CITATION ) , and it removed very little Pol II or the <PROT2_2963>  TFIIF </PROT2_2963> subunit <PROT2_2963>  RAP30 </PROT2_2963> . ||10454543_155871_2963==><PROT1_155871>  Tat </PROT1_155871> - SF1 is found in an interesting complex that also includes the <PROT2_2963>  RAP30 </PROT2_2963> subunit of <PROT2_2963>  TFIIF </PROT2_2963> , P - TEFb , hSPT5 and RNAPIIA ( TARGET_CITATION and CITATION ) . ||10454543_155871_2963==>Protein complexes that stimulate <PROT1_155871>  Tat </PROT1_155871> activity in vitro have been partially purified from HeLa cells and reported to contain Spt4 - Spt5 , P - TEFb , <PROT1_155871>  Tat </PROT1_155871> - SF1 , nucleolin , XP - E , Pol II , the small subunit of <PROT2_2963>  TFIIF </PROT2_2963> ( equivalent to Tfg2 ) and other novel polypeptides ( TARGET_CITATION , CITATION , CITATION ) . ||11909874_155871_1960==>Interaction with <PROT2_1960>  Egr3 </PROT2_1960> was still supported by an amino - acid substitution in <PROT1_155871>  Tat </PROT1_155871> that blocked its transactivation activity ; however , this mutation failed to enhance Egr - dependent regulation of the FasL promoter TARGET_CITATION . ||7935812_155871_2247==>In keeping with these findings , soluble <PROT1_155871>  tat </PROT1_155871> protein can synergize with <PROT2_2247>  bFGF </PROT2_2247> in promoting angiogenic lesions upon subcutaneous inoculation in nude mice , in much in same fashion as fibronectin does ( TARGET_CITATION ) . ||7935812_155871_2247==>In addition , HIV - <PROT1_155871>  tat </PROT1_155871> induces growth , migration , invasion , and adhesion of EC ( Vogel et al CITATION and Albini et al CITATION and references therein ) and synergizes with <PROT2_2247>  bFGF </PROT2_2247> for induction of KS in mice TARGET_CITATION . ||7935812_155871_2247==>The <PROT1_155871>  Tat </PROT1_155871> protein has also been shown to act in synergy with <PROT2_2247>  bFGF </PROT2_2247> in inducing KS - like lesions in mice ( TARGET_CITATION ) . ||7935812_155871_2247==>However , when <PROT1_155871>  Tat </PROT1_155871> is injected in the presence of suboptimal amounts of <PROT2_2247>  bFGF </PROT2_2247> , synergistic angiogenic KS - promoting effects are observed and a higher number of mice develop lesions as compared with injection of <PROT2_2247>  bFGF </PROT2_2247> alone TARGET_CITATION . ||7935812_155871_2247==>These are due to the enhancement by <PROT1_155871>  Tat </PROT1_155871> of endothelial cell growth , migration , and invasion induced by <PROT2_2247>  bFGF </PROT2_2247> and to <PROT2_2247>  bFGF </PROT2_2247> - induced expression of the integrins alpha 5beta 1 and alpha vbeta 3 that function as the receptors for <PROT1_155871>  Tat </PROT1_155871> TARGET_CITATION , CITATION . ||7935812_155871_2247==>Finally , these effects are increased synergistically by <PROT1_155871>  Tat </PROT1_155871> demonstrating its role as a progression factor in AIDS - KS , in fact , <PROT2_2247>  bFGF </PROT2_2247> and <PROT1_155871>  Tat </PROT1_155871> are both present in AIDS - KS lesions and <PROT1_155871>  Tat </PROT1_155871> increases the KS - forming activity of <PROT2_2247>  bFGF </PROT2_2247> TARGET_CITATION . ||7935812_155871_2247==>This is due to both IC induction of the expression of small alpha , Greek5small beta , Greek1 and small alpha , Greekvsmall beta , Greek3 integrins that function as the receptors for <PROT1_155871>  Tat </PROT1_155871> and to the induction of <PROT2_2247>  bFGF </PROT2_2247> expression that , in turn , induces the same integrins ( CITATION , CITATION , CITATION , CITATION ) and it is required for <PROT1_155871>  Tat </PROT1_155871> - angiogenic effect ( TARGET_CITATION ) . ||7935812_155871_2247==>Consistent with these data , inoculation of <PROT1_155871>  Tat </PROT1_155871> alone in nude mice does not lead to angiogenesis , however , when <PROT1_155871>  Tat </PROT1_155871> is inoculated in the presence of suboptimal ( non lesion forming ) amounts of <PROT2_2247>  bFGF </PROT2_2247> or with heparin , it greatly enhances <PROT2_2247>  bFGF </PROT2_2247> - mediated angiogenesis and KS - like lesion formation in terms of both number of mice developing lesions and intensity of the histological alterations including angiogenesis and spindle cell growth ( Table 5 ) ( TARGET_CITATION , CITATION ) . ||7935812_155871_2247==>However , <PROT2_2247>  bFGF </PROT2_2247> released by <PROT1_155871>  Tat </PROT1_155871> - basic region represents the final mediator of <PROT1_155871>  Tat </PROT1_155871> - induced cell growth , whereas <PROT1_155871>  Tat </PROT1_155871> - induced cell adhesion increases the growth response to <PROT2_2247>  bFGF </PROT2_2247> ( TARGET_CITATION ) . ||7935812_155871_2247==>In addition , endothelial and spindle cells of KS lesions express both <PROT2_2247>  bFGF </PROT2_2247> and small alpha , Greek5small beta , Greek1 and small alpha , Greekvsmall beta , Greek3 - <PROT1_155871>  Tat </PROT1_155871> receptors and extracellular <PROT1_155871>  Tat </PROT1_155871> co - stains with these receptors on spindle cells and activated vessels ( TARGET_CITATION ) , suggesting that the mechanisms described here are operative in vivo and that <PROT1_155871>  Tat </PROT1_155871> may explain the higher frequency and aggressiveness of KS in the setting of HIV - 1 infection . ||7935812_155871_2247==>In vivo <PROT1_155871>  Tat </PROT1_155871> enhances angiogenesis triggered by <PROT2_2247>  bFGF </PROT2_2247> and it synergizes with <PROT2_2247>  bFGF </PROT2_2247> to increase endothelial and KS cell growth , invasion , and collagenase IV activation ( TARGET_CITATION , CITATION ) . ||7935812_155871_2247==>These mechanisms are likely to be operative in vivo since <PROT2_2247>  bFGF </PROT2_2247> and <PROT1_155871>  Tat </PROT1_155871> are both present in AIDS - KS lesions and <PROT1_155871>  Tat </PROT1_155871> co - stains with & # 223 ; 1 and & # 223 ; 3 integrins on resident vessels and spindle cells ( TARGET_CITATION ) . ||7935812_155871_2247==>In fact , the addition of <PROT2_2247>  bFGF </PROT2_2247> to endothelial cells plated on <PROT1_155871>  Tat </PROT1_155871> promotes a proliferative response much higher than that observed with cells plated on gelatin or BSA and similar to that observed with FN - coated plates ( TARGET_CITATION ) . ||7935812_155871_2247==>The expression of { alpha } 5 & # 223 ; 1 and { alpha } v & # 223 ; 3 integrins is up - regulated by IC or <PROT2_2247>  bFGF </PROT2_2247> ( CITATION , CITATION ) , and this is simultaneous with the acquisition of the cell responsiveness to <PROT1_155871>  Tat </PROT1_155871> ( CITATION , CITATION , CITATION , TARGET_CITATION ) . ||7935812_155871_2247==>In fact , <PROT1_155871>  Tat </PROT1_155871> promotes the development of angioproliferative KS - like lesions in mice only when injected with suboptimal ( nonlesion forming ) amounts of basic fibroblast growth factor ( <PROT2_2247>  bFGF </PROT2_2247> ) ( TARGET_CITATION ) , an angiogenic molecule highly expressed by KS cells both in vitro and in primary lesions ( TARGET_CITATION , CITATION , CITATION , CITATION ) . ||7935812_155871_2247==>IC and <PROT2_2247>  bFGF </PROT2_2247> are highly expressed in AIDS - KS lesions ( TARGET_CITATION , CITATION , CITATION , CITATION ) , in which extracellular <PROT1_155871>  Tat </PROT1_155871> costains with { alpha } v & # 223 ; 3 on both EC and KS cells ( TARGET_CITATION ) . ||7935812_155871_2247==><PROT1_155871>  Tat </PROT1_155871> in synergy with a basic fibroblast growth factor ( <PROT2_2247>  bFGF </PROT2_2247> ) is involved in the induction of Kaposi 's sarcoma lesions ( TARGET_CITATION ) . ||7935812_155871_2247==>In contrast , when <PROT1_155871>  Tat </PROT1_155871> is injected with IC or with suboptimal ( non - lesion - forming ) amounts of basic fibroblast growth factor ( <PROT2_2247>  bFGF </PROT2_2247> ) , it promotes the development of angioproliferative KS - like lesions in the inoculated animals CITATION , TARGET_CITATION . ||7935812_155871_2247==>These receptors , which bind the RGD sequence of extracellular matrix ( ECM ) proteins , such as fibronectin ( FN ) and vitronectin ( VN ) CITATION , are constitutively expressed by KS cells both in vitro and in primary lesions TARGET_CITATION , CITATION , and their levels are increased in normal endothelial cells by the same IC that induce <PROT2_2247>  bFGF </PROT2_2247> expression and cellular responsiveness to <PROT1_155871>  Tat </PROT1_155871> CITATION , CITATION . ||7935812_155871_2247==>Consistent with this , endothelial cells adhere to immobilized <PROT1_155871>  Tat </PROT1_155871> in a fashion similar to FN or VN CITATION , and under these conditions , the addition of exogenous <PROT2_2247>  bFGF </PROT2_2247> dramatically increases cell growth TARGET_CITATION , as previously described for ECM molecules CITATION , CITATION , CITATION . ||7935812_155871_2247==>Specifically , exogenous <PROT2_2247>  bFGF </PROT2_2247> is required to observe the angiogenic effect of <PROT1_155871>  Tat </PROT1_155871> in vivo TARGET_CITATION . ||7935812_155871_2247==>IC and <PROT2_2247>  bFGF </PROT2_2247> are highly expressed in AIDS - KS lesions , where extracellular <PROT1_155871>  Tat </PROT1_155871> costains with the alpha 5beta 1 and alpha vbeta 3 integrins on both spindle cells and vessels TARGET_CITATION . ||7935812_155871_2247==>It has been demonstrated that the HIV - 1 <PROT1_155871>  Tat </PROT1_155871> protein not only has angiogenic properties ( CITATION ) but also may act synergistically with <PROT2_2247>  bFGF </PROT2_2247> to induce KS - like lesions in mice ( TARGET_CITATION ) . ||7935812_155871_2247==>interleukin - 6 , and interleukin - 6 receptors CITATION , hepatocyte growth factor and its receptors CITATION , CITATION , and thymidine phosphorylase CITATION . The human immunodeficiency virus - 1 <PROT1_155871>  Tat </PROT1_155871> protein has been reported to activate the VPF / VEGF receptor kinase insert domain - containing receptor ( KDR ) CITATION . Synergy between <PROT2_2247>  bFGF </PROT2_2247> and human immunodeficiency virus - 1 <PROT1_155871>  Tat </PROT1_155871> protein in the induction of KS in mice has been reported TARGET_CITATION . Clearly much work remains to be done before one can understand the interactions and relative importance of the various growth factors and their receptors in KS . ||7935812_155871_2247==>Indeed , <PROT1_155871>  Tat </PROT1_155871> has been postulated to have a role in the pathogenesis of Kaposi & # x2019 ; s sarcoma ( KS ) because it is angiogenic and it enhances the proliferative effects of basic fibroblast growth factor ( <PROT2_2247>  bFGF </PROT2_2247> ) ( TARGET_CITATION ) . ||7935812_155871_2247==>In this extracellular form , <PROT1_155871>  Tat </PROT1_155871> promotes the migration , invasion , growth , and adhesion of KS and IC - activated endothelial cells in vitro ( Ensoli et al. , 1990 CITATION , 1993 CITATION ; Barillari et al. , 1992 CITATION , 1993 CITATION ; Albini et al. , 1995 CITATION ; Fiorelli et al. , 1995 CITATION , 1999 CITATION ) and enhances the angiogenic , KS - promoting effect of <PROT2_2247>  bFGF </PROT2_2247> in vivo ( Ensoli et al. , 1994a TARGET_CITATION ) . ||7935812_155871_2247==>Either <PROT1_155871>  Tat </PROT1_155871> or <PROT2_2247>  bFGF </PROT2_2247> induces the mRNA expression of the matrix metalloproteinase - 2 ( MMP - 2 ) , the 72 - kDa type IV collagenase that is involved in tumor growth and angiogenesis ( Ensoli et al. 1994a TARGET_CITATION ; Ray and Stetler - Stevenson , 1994 CITATION ; Barillari et al. , 1999a CITATION ) . ||7935812_155871_2247==>In addition , when <PROT1_155871>  Tat </PROT1_155871> and <PROT2_2247>  bFGF </PROT2_2247> are combined , their effects on MMP - 2 mRNA expression are additive or synergistic ( Ensoli et al. , 1994a TARGET_CITATION ; Barillari et al. , 1999a CITATION ) . ||7935812_155871_2247==><PROT1_155871>  Tat </PROT1_155871> and <PROT2_2247>  bFGF </PROT2_2247> are both capable of promoting MMP - 2 RNA expression in endothelial cells and when combined increase the RNA expression of this collagenase in an additive or synergistic manner ( Ensoli et al. , 1994a TARGET_CITATION ) . ||7935812_155871_2247==>These results are in agreement with those of other studies indicating that <PROT1_155871>  Tat </PROT1_155871> enhances the angiogenic effects of <PROT2_2247>  bFGF </PROT2_2247> , but not those of VEGF ( Ensoli et al. , 1994a TARGET_CITATION ; Barillari et al. , 1999a CITATION , b CITATION ) , therefore , no further studies were conducted with VEGF . ||7935812_155871_2247==>Because <PROT1_155871>  Tat </PROT1_155871> and <PROT2_2247>  bFGF </PROT2_2247> are also highly expressed in these tissues ( Ensoli et al. , 1994a TARGET_CITATION ) , they are likely to be the principal modulators of MMP - 2 in AIDS - KS patients . ||7935812_155871_2247==>Previous studies indicated that <PROT1_155871>  Tat </PROT1_155871> and <PROT2_2247>  bFGF </PROT2_2247> synergize in inducing the mRNA expression of MMP - 2 ( Ensoli et al. , 1994a TARGET_CITATION ) . ||7935812_155871_2247==>This is consistent with the finding that <PROT1_155871>  Tat </PROT1_155871> synergizes with <PROT2_2247>  bFGF </PROT2_2247> , but not with VEGF , in promoting endothelial cell growth and in vivo angiogenesis ( Ensoli et al. , 1994a TARGET_CITATION ; Barillari et al. , 1999b CITATION ) , and with the fact that <PROT1_155871>  Tat </PROT1_155871> angiogenic effects correlate with the expression of alpha vbeta 3 , which is induced by <PROT2_2247>  bFGF </PROT2_2247> and binds <PROT1_155871>  Tat </PROT1_155871> RGD region , and not with the expression of alpha vbeta 5 that is promoted by VEGF ( Barillari et al. , 1999b CITATION ) . ||7935812_155871_2247==>In fact , <PROT2_2247>  bFGF </PROT2_2247> , <PROT1_155871>  Tat </PROT1_155871> , alpha vbeta 3 and alpha 5beta 1 and MMP - 2 are highly expressed in AIDS - KS lesions ( Ensoli et al. , 1994a TARGET_CITATION ) suggesting that these mechanisms are operating in vivo in increasing the angiogenesis and edema present in AIDS - KS patients . ||7935812_155871_2247==>This soluble <PROT2_2247>  bFGF </PROT2_2247> mediates <PROT1_155871>  Tat </PROT1_155871> - induced vascular cell growth ( CITATION ) , explaining why <PROT2_2247>  bFGF </PROT2_2247> is required for the angiogenic effect of <PROT1_155871>  Tat </PROT1_155871> ( TARGET_CITATION ) . ||7935812_155871_2247==>These data provide a molecular basis for previous studies showing that KS lesion formation in nude mice can be triggered in a synergistic way by <PROT2_2247>  bFGF </PROT2_2247> and <PROT1_155871>  Tat </PROT1_155871> ( TARGET_CITATION ) . ||7935812_155871_2247==>Evidence that these mechanisms are operative in vivo comes from data indicating that small alpha , Greek5small beta , Greek1 and small alpha , Greekvsmall beta , Greek3 integrins and <PROT2_2247>  bFGF </PROT2_2247> are highly expressed by KSC and activated EC of primary lesions and , as is the case for extracellular <PROT1_155871>  Tat </PROT1_155871> , are present in AIDS - KS lesions ( fig. 5 ) ( TARGET_CITATION ) . ||7935812_155871_2247==>MMP , and particularly MMP - 2 and MMP - 9 , are highly expressed by KS cells both in vitro and in AIDS - KS lesions ( CITATION and TARGET_CITATION ) ( data not shown ) , and are induced by <PROT2_2247>  bFGF </PROT2_2247> and further increased by <PROT1_155871>  Tat </PROT1_155871> ( Table 1 and Table 2 ) . ||7935812_155871_2247==>This is sustained by in vivo findings indicating that <PROT1_155871>  Tat </PROT1_155871> increases KS - like lesion formation induced by <PROT2_2247>  bFGF </PROT2_2247> injection into mice ( CITATION , TARGET_CITATION ) and that the KSC growth rate increases in the presence of circulating <PROT1_155871>  Tat </PROT1_155871> protein ( CITATION ) . ||7935812_155871_2247==>However , differently from a true angiogenic molecule such as <PROT2_2247>  bFGF </PROT2_2247> , <PROT1_155871>  Tat </PROT1_155871> acts only on endothelial cells that are activated by inflammatory cytokines ( CITATION , CITATION - CITATION , TARGET_CITATION , CITATION ) ( Table 1 ) . ||7935812_155871_2247==>In particular , endothelial cells show an increased proliferative response to <PROT2_2247>  bFGF </PROT2_2247> when they are seeded onto immobilized <PROT1_155871>  Tat </PROT1_155871> protein ( CITATION , TARGET_CITATION ) . ||7935812_155871_2247==>In contrast , <PROT1_155871>  Tat </PROT1_155871> enhances the mitogenic effect of <PROT2_2247>  bFGF </PROT2_2247> on endothelial cells , consistent with its capability of maintaining exogenously added <PROT2_2247>  bFGF </PROT2_2247> into a soluble form ( CITATION , CITATION , TARGET_CITATION ) . ||7935812_155871_2247==>In contrast , when <PROT1_155871>  Tat </PROT1_155871> is injected in combination with suboptimal ( non - lesion - forming ) amounts of <PROT2_2247>  bFGF </PROT2_2247> it promotes the development of angioproliferative , KS - like lesions in nude mice ( CITATION , TARGET_CITATION ) ( fig. 4 ) . ||7935812_155871_2247==>Indeed , <PROT1_155871>  Tat </PROT1_155871> enhances <PROT2_2247>  bFGF </PROT2_2247> 's capability of promoting angiogenesis both in vitro and in vivo ( CITATION , CITATION , TARGET_CITATION ) . ||7935812_155871_2247==>These integrins , in turn , bind the <PROT1_155871>  Tat </PROT1_155871> RGD region and , by this interaction , mediate <PROT1_155871>  Tat </PROT1_155871> - promoted locomotion of KSC and activated endothelial cells and provide these vascular cell types with the adhesion signal they require in order to grow in response to <PROT2_2247>  bFGF </PROT2_2247> ( CITATION , CITATION , TARGET_CITATION ) . ||7935812_155871_2247==>Elevated levels of <PROT2_2247>  bFGF </PROT2_2247> , VEGF , and inflammatory cytokines are detectable in AIDS - KS lesions , where extracellular <PROT1_155871>  Tat </PROT1_155871> costains with { alpha } 5 & # 223 ; 1 and { alpha } v & # 223 ; 3 in both KSC and endothelial cells ( TARGET_CITATION ) . ||7935812_155871_2247==><PROT1_155871>  Tat </PROT1_155871> can also enhance the activity of cellular mediators of angiogenesis , such as basic fibroblast growth factor ( <PROT2_2247>  bFGF </PROT2_2247> ) , but the mechanism by which <PROT1_155871>  Tat </PROT1_155871> functions in this capacity has remained unknown ( TARGET_CITATION ) . ||7935812_155871_2247==>Finally , <PROT2_2247>  bFGF </PROT2_2247> synergizes with the HIV - 1 <PROT1_155871>  Tat </PROT1_155871> protein released by infected cells and increases the frequency and aggressiveness of KS in HIV - 1 - infected individuals TARGET_CITATION , CITATION , CITATION . ||7935812_155871_2247==>The apparently hyperplastic cells forming these often multifocal lesions proliferate over time CITATION , eventuating in locally aggressive sarcoma - like tumors that contain a mixture of FXIIIaDD and true endothelial cells CITATION . Human FXIIIaDD derived from KS lesions have been found to harbor HIV transcripts CITATION and infection of human KS cells in vitro with HIV results in production of IL - 1 & # 223 ; and IL - 6 CITATION , both of which promote growth of human KS spindle cells CITATION . Subcutaneous injection of HIV <PROT1_155871>  Tat </PROT1_155871> protein and / or basic fibroblast growth factor ( <PROT2_2247>  bFGF </PROT2_2247> ) induces local development of KS - like spindle cell proliferation in mouse skin , with the former synergizing with the latter by promoting increased endothelial growth and invasion TARGET_CITATION . Based on these and other data , Ensoli and Gallo CITATION have advanced the central hypothesis that KS pathogenesis involves abnormal production by infected cells of angiogenic cytokines , including , among others , IL - 1 { alpha } , IL - 1 & # 223 ; , IL - 6 , GM - CSF , and TNF - { alpha } . ||7935812_155871_2247==>( TARGET_CITATION , CITATION , CITATION , CITATION , CITATION and CITATION ) Finally , <PROT2_2247>  bFGF </PROT2_2247> acts synergistically with HIV - 1 <PROT1_155871>  Tat </PROT1_155871> , which is released by HIV - 1 infected cells , to increase the frequency and aggressiveness of KS in HIV - 1 infected individuals . ||7935812_155871_2247==>( TARGET_CITATION , CITATION , CITATION , CITATION , CITATION and CITATION ) This happens because <PROT1_155871>  Tat </PROT1_155871> can mimic extracellular matrix proteins by binding to small alpha , Greekvsmall beta , Greek3 and small alpha , Greek5small beta , Greek1 integrins , which are induced by inflammatory cytokines and <PROT2_2247>  bFGF </PROT2_2247> ( figure 2 ) . ||7935812_155871_2247==><PROT1_155871>  Tat </PROT1_155871> induces KS - like lesions when overexpressed alone in transgenic mice ( CITATION ) and synergizes when coinjected with <PROT2_2247>  bFGF </PROT2_2247> in nude mice ( TARGET_CITATION ) . ||7935812_155871_2247==>KS was hypothesized to be mediated by HIV - <PROT1_155871>  Tat </PROT1_155871> , since <PROT1_155871>  Tat </PROT1_155871> binding to the heparan sulfate ( HS ) in the extracellular matrix ( ECM ) was believed to act by the displacement of basic fibroblast growth factor ( <PROT2_2247>  BFGF </PROT2_2247> ) from the matrix ( CITATION , TARGET_CITATION , CITATION ) . ||7935812_155871_2247==>The basic region that retrieves <PROT2_2247>  bFGF </PROT2_2247> from heparan sulfate proteoglycans of the extracellular matrix ( CITATION and CITATION ; CITATION ) and the RGD region that binds to the small alpha , Greek5small beta , Greek1 and small alpha , Greekvsmall beta , Greek3 integrins ( CITATION ) and through this mediates cell adhesion , migration , and cell invasion , which in turn is associated with the induction of matrix metalloproteinase ( MMP ) - 1 by HIV - 1 <PROT1_155871>  Tat </PROT1_155871> ( CITATION and TARGET_CITATION ) . ||7935812_155871_2247==>Transgenic mice expressing HIV - 1 <PROT1_155871>  Tat </PROT1_155871> develop tumors ( CITATION and CITATION ) , and inoculation into mice of purified HIV - 1 <PROT1_155871>  Tat </PROT1_155871> in combination with IC or <PROT2_2247>  bFGF </PROT2_2247> can lead to the formation of vascular lesions ( CITATION ; CITATION ; TARGET_CITATION and CITATION ) . ||8178451_155871_4803==>For instance , <PROT1_155871>  Tat </PROT1_155871> cooperates with <PROT2_4803>  NGF </PROT2_4803> to induce HIV gene expression and replication in neuronal and glial cells with an immature phenotype ( TARGET_CITATION ) , <PROT2_4803>  NGF </PROT2_4803> activates HIV - LTR in a neuronal cell model ( CITATION ) , and neurotrophins , including <PROT2_4803>  NGF </PROT2_4803> , prevent <PROT1_155871>  Tat </PROT1_155871> - induced neuronal apoptosis ( CITATION ) . ||8628270_155871_4782==>The expression plasmids containing the Gal4 DNA binding domain ( amino acids 1 & # 150 ; 147 ) fused at the N terminus of the VP - 16 , <PROT2_4782>  CTF </PROT2_4782> , and <PROT1_155871>  TAT </PROT1_155871> activation domains were described previously ( CITATION , TARGET_CITATION ) . ||8628270_155871_4782==>( TARGET_CITATION ) showed that type I activators such as Sp1 , SW6 , and <PROT2_4782>  CTF </PROT2_4782> specifically synergize with <PROT1_155871>  Tat </PROT1_155871> in transcriptional activation by increasing the efficiency of elongation whereas type IIB activators such as VP16 do not , when present in multiple copies at the promoter . ||8628270_155871_6667==>For example , <PROT2_6667>  SP1 </PROT2_6667> - binding sites in the HIV - 1 LTR increase the level of HIV - 1 <PROT1_155871>  Tat </PROT1_155871> activation to a much greater degree than other upstream binding sites which were inserted in place of the <PROT2_6667>  SP1 </PROT2_6667> - binding sites ( TARGET_CITATION , CITATION ) . ||8628270_155871_6667==>HIV - 1 <PROT1_155871>  Tat </PROT1_155871> has previously been shown to associate with several components comprising the HIV - 1 preinitiation complex including <PROT2_6667>  SP1 </PROT2_6667> ( CITATION ) , TATA - binding protein ( TBP ) and associated factors ( TAFs ) ( CITATION ) , the p62 subunit of TFIIH ( TARGET_CITATION ) in addition to RNA polymerase II ( CITATION ) . ||8628270_155871_6667==>The relative percent CAT conversions plus and minus pSV - HIV - 1 <PROT1_155871>  Tat </PROT1_155871> ( from which the histograms were derived were as follows : <PROT2_6667>  SP1 </PROT2_6667> , 4.9 and 0.13 ; VP16 , 10.0 and 3.2 ; CIITA , 10.9 and 6.0. The 36 - fold activation observed in the presence of HIV - 1 <PROT1_155871>  Tat </PROT1_155871> is lower than the levels reported in the literature ( 50 - fold approximately ) ( TARGET_CITATION ) . ||8628270_155871_6667==>For example , the human immunodeficiency virus <PROT1_155871>  Tat </PROT1_155871> transactivator protein stimulates transcriptional elongation and can synergize with <PROT2_6667>  Sp1 </PROT2_6667> ( which stimulates initiation , but not elongation ) , but not with VP16 , p53 , or E2F1 ( all of which stimulate elongation ) ( TARGET_CITATION ) . ||8628270_155871_6667==>( TARGET_CITATION ) showed that type I activators such as <PROT2_6667>  Sp1 </PROT2_6667> , SW6 , and CTF specifically synergize with <PROT1_155871>  Tat </PROT1_155871> in transcriptional activation by increasing the efficiency of elongation whereas type IIB activators such as VP16 do not , when present in multiple copies at the promoter . ||9075924_155871_4772==><PROT2_4772>  NFATc </PROT2_4772> binds the small kappa , GreekB regulatory elements , synergizes with NF - small kappa , GreekB and <PROT1_155871>  Tat </PROT1_155871> in transcriptional activation of HIV - 1 , and enhances HIV - 1 replication in T cell lines ( ( TARGET_CITATION ) ) . ||9372921_155871_6667==><PROT1_155871>  Tat </PROT1_155871> has been shown to directly interact with transcription factors such as <PROT2_6667>  Sp1 </PROT2_6667> ( CITATION ) and appears to either enhance or permit post - initiation events ( TARGET_CITATION ) . ||9372921_155871_6667==>Thus , it is conceivable that basal <PROT2_6667>  Sp1 </PROT2_6667> activity and <PROT2_6667>  Sp1 </PROT2_6667> - <PROT1_155871>  Tat </PROT1_155871> activity reflect two distinct processes ( TARGET_CITATION ) . ||9372921_155871_6667==>This domain has been shown to serve as the binding site for several cellular factors including <PROT2_6667>  Sp1 </PROT2_6667> and Puralpha ( TARGET_CITATION , CITATION ) and is important for <PROT1_155871>  Tat </PROT1_155871> nuclear localization ( CITATION , CITATION ) . 
ubiquitinated by====11087859_155030_4734==>Although the physiological ligand for p6 has not been definitively identified , a variety of observations suggest that retroviral L domains function by interacting with the host ubiquitination machinery ( reviewed in reference CITATION ) : ( i ) The L domain - containing proteins of MuLV ( p12 ) and HIV ( p6 ) are ubiquitinated ( CITATION ) , ( ii ) depletion of free ubiquitin in virus - producing cells with proteasome inhibitors impairs retrovirus budding ( CITATION , TARGET_CITATION ) , ( iii ) the presence of L domains in HIV - 1 minimal <PROT1_155030>  Gag </PROT1_155030> constructs induces ubiquitination of the minimal <PROT1_155030>  Gag </PROT1_155030> proteins ( CITATION ) , ( iv ) the ubiquitin ligase <PROT2_4734>  Nedd4 </PROT2_4734> interacts with the RSV L domain ( CITATION ) , and ( v ) the cellular protein TSG101 , which contains at its N terminus a ubiquitin - conjugating enzyme - like sequence , interacts with HIV - 1 <PROT1_155030>  Gag </PROT1_155030> in a p6 - dependent fashion ( CITATION ) . ||11087860_155030_4734==>This is because ubiquitylation of retroviral <PROT1_155030>  Gag </PROT1_155030> proteins or some associated cellular component appears to be required for virus budding , and <PROT2_4734>  Nedd4 </PROT2_4734> and its yeast homologue Rsp5 possess the ability to ubiquitylate target proteins containing a PPxY motif as a E3 ubiquitin ligase ( CITATION ; CITATION ; CITATION ; CITATION ; CITATION ; TARGET_CITATION ) . ||11087860_155030_4734==>Although the physiological ligand for p6 has not been definitively identified , a variety of observations suggest that retroviral L domains function by interacting with the host ubiquitination machinery ( reviewed in reference CITATION ) : ( i ) The L domain - containing proteins of MuLV ( p12 ) and HIV ( p6 ) are ubiquitinated ( CITATION ) , ( ii ) depletion of free ubiquitin in virus - producing cells with proteasome inhibitors impairs retrovirus budding ( CITATION , CITATION ) , ( iii ) the presence of L domains in HIV - 1 minimal <PROT1_155030>  Gag </PROT1_155030> constructs induces ubiquitination of the minimal <PROT1_155030>  Gag </PROT1_155030> proteins ( TARGET_CITATION ) , ( iv ) the ubiquitin ligase <PROT2_4734>  Nedd4 </PROT2_4734> interacts with the RSV L domain ( CITATION ) , and ( v ) the cellular protein TSG101 , which contains at its N terminus a ubiquitin - conjugating enzyme - like sequence , interacts with HIV - 1 <PROT1_155030>  Gag </PROT1_155030> in a p6 - dependent fashion ( CITATION ) . ||11087860_155030_4734==>Virion release requires functional interactions between <PROT1_155030>  Gag </PROT1_155030> L domains and cellular proteins involved in ubiquitin ligation , <PROT2_4734>  NEDD4 </PROT2_4734> and BUL1 , or in late endosomal trafficking , TSG101 and Vsp4 ( CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , TARGET_CITATION , CITATION and CITATION ) . ||11087860_155030_4734==>Indeed , the PTAP and PPPY motifs of several <PROT1_155030>  Gag </PROT1_155030> proteins are required for late events in virion release and have been shown to function by interacting with proteins involved in endosomal targeting , such as Tsg101 and <PROT2_4734>  Nedd4 </PROT2_4734> ( CITATION and TARGET_CITATION ) . ||11087860_155030_4734==>There is growing evidence that this process involves the ubiquitin E3 ligase <PROT2_4734>  Nedd4 </PROT2_4734> ( CITATION , TARGET_CITATION , CITATION ) and variant E2 - like protein TSG101 ( CITATION , CITATION , CITATION ) acting through interactions ( CITATION ) with late domain motifs ( CITATION ) in viral <PROT1_155030>  Gag </PROT1_155030> proteins to ligate ubiquitin . ||11087860_155030_4734==>Furthermore , physical interactions between the PPxY motif and <PROT2_4734>  Nedd4 </PROT2_4734> / Rsp5 resulted in the covalent addition of ubiquitin to the viral <PROT1_155030>  matrix </PROT1_155030> proteins ( CITATION , CITATION , CITATION , CITATION , TARGET_CITATION , CITATION ) . ||11087860_155030_4734==>The late assembly domains found in the <PROT1_155030>  Gag </PROT1_155030> polyproteins of retroviruses , such as the Rous sarcoma virus ( TARGET_CITATION ; CITATION ) , Mason - Pfizer monkey virus ( M - PMV ) ( CITATION ) , and Moloney murine leukemia virus ( CITATION ) , contain a PPxY motif that can bind <PROT2_4734>  Nedd4 </PROT2_4734> proteins . ||11087860_155030_4734==>Moreover , PPXY motifs in the L domains of Rous sarcoma virus ( CITATION ) , human T - cell leukemia virus ( CITATION ) , Ebola virus ( CITATION ) , and vesicular stomatitis virus ( CITATION ) have been reported to bind to <PROT2_4734>  Nedd4 </PROT2_4734> - like E3 ubiquitin ligases and can also induce the ubiquitination of a minimal HIV - 1 <PROT1_155030>  Gag </PROT1_155030> protein ( TARGET_CITATION , CITATION ) . ||11112487_155030_4734==>This is because ubiquitylation of retroviral <PROT1_155030>  Gag </PROT1_155030> proteins or some associated cellular component appears to be required for virus budding , and <PROT2_4734>  Nedd4 </PROT2_4734> and its yeast homologue Rsp5 possess the ability to ubiquitylate target proteins containing a PPxY motif as a E3 ubiquitin ligase ( CITATION ; CITATION ; CITATION ; TARGET_CITATION ; CITATION ; CITATION ) . ||11991975_155030_4734==>Moreover , PPXY motifs in the L domains of Rous sarcoma virus ( CITATION ) , human T - cell leukemia virus ( CITATION ) , Ebola virus ( CITATION ) , and vesicular stomatitis virus ( CITATION ) have been reported to bind to <PROT2_4734>  Nedd4 </PROT2_4734> - like E3 ubiquitin ligases and can also induce the ubiquitination of a minimal HIV - 1 <PROT1_155030>  Gag </PROT1_155030> protein ( CITATION , TARGET_CITATION ) . 
upregulates====10068657_155871_6347==>Likewise , <PROT2_6347>  CCL2 </PROT2_6347> secretion was observed in differentiated macrophages stimulated with exogenous <PROT1_155871>  tat </PROT1_155871> ( TARGET_CITATION ) , whereas adenovirus - mediated expression of nef in these cells was sufficient to induce transient production of CCL3 and CCL4 ( CITATION ) . ||10660577_1390_155871==>This hypothesis might be supported further by the finding that HIV - 1 <PROT2_155871>  Tat </PROT2_155871> protein treatment of PC - 12 cells resulted in prolonged increased expression of <PROT1_1390>  ICER </PROT1_1390> mRNA ( Zauli et al. , 2000 TARGET_CITATION ) , and exposure of both PC - 12 and neurons to high doses of this protein was found to induce programmed cell death ( Weeks at al. , 1995 CITATION ; Kruman et al. , 1998 CITATION ) . ||11120849_1232_155871==>Moreover , <PROT2_155871>  Tat </PROT2_155871> protein up - regulates the level of <PROT1_1232>  CCR3 </PROT1_1232> mRNA and surface expression of <PROT1_1232>  CCR3 </PROT1_1232> on human basophils ( TARGET_CITATION ) . ||11120849_1232_155871==>In addition , we found that the HIV - 1 <PROT2_155871>  Tat </PROT2_155871> protein is a potent chemoattractant for human Fc { epsilon } RI + cells and up - regulates the expression of <PROT1_1232>  CCR3 </PROT1_1232> on these cells ( TARGET_CITATION ) . ||11385301_155871_8743==><PROT2_8743>  TRAIL </PROT2_8743> expression in HIV - infected cells might be induced by HIV - 1 <PROT1_155871>  tat </PROT1_155871> , as recently demonstrated ( TARGET_CITATION ) , or by IFN , as previously demonstrated in peripheral blood monocytes in vitro ( CITATION ) . ||11385301_155871_8743==>Following infection with HIV , <PROT2_8743>  TRAIL </PROT2_8743> is released by macrophages and exogenous HIV - encoded <PROT1_155871>  Tat </PROT1_155871> protein upregulates the production of <PROT2_8743>  TRAIL </PROT2_8743> by primary macrophages in vitro TARGET_CITATION , indicating that a <PROT2_8743>  TRAIL </PROT2_8743> - dependent mechanism of destruction of bystander CD4 + T cells occurs in vivo , which might be triggered by <PROT1_155871>  Tat </PROT1_155871> produced by HIV - infected cells . ||11385301_155871_8743==>Although macrophages exposed to <PROT1_155871>  Tat </PROT1_155871> may be protected from apoptosis , <PROT1_155871>  Tat </PROT1_155871> up - regulates macrophage production of TNF - { alpha } ( CITATION ) and <PROT2_8743>  TRAIL </PROT2_8743> ( fig. 4 ) with the potential to induce apoptosis in bystander T cells ( TARGET_CITATION ) . ||11385301_155871_8743==>Using flow cytometry , Zhang et al. ( TARGET_CITATION ) showed that HIV <PROT1_155871>  Tat </PROT1_155871> upregulated <PROT2_8743>  TRAIL </PROT2_8743> on the surface of monocyte - derived macrophages . ||11909874_155871_356==>Interaction with Egr3 was still supported by an amino - acid substitution in <PROT1_155871>  Tat </PROT1_155871> that blocked its transactivation activity ; however , this mutation failed to enhance Egr - dependent regulation of the <PROT2_356>  FasL </PROT2_356> promoter TARGET_CITATION . ||11909874_155871_356==>Recently , a study shows that <PROT1_155871>  Tat </PROT1_155871> itself can bind to Egr - 2 and - 3 and synergizes with these factors to induce expression of a <PROT2_356>  CD95L </PROT2_356> promoter - driven reporter ( TARGET_CITATION ) . ||11909874_155871_356==>This might explain how <PROT1_155871>  Tat </PROT1_155871> induces apoptosis in a wide range of cells through mechanisms that include its action as a transcription factor ( e.g. upregulation of <PROT2_356>  CD95L </PROT2_356> and downregulation of Bcl - 2 ) ( TARGET_CITATION ) , post - transcriptional effects ( e.g. on mitochondrial superoxide dismutase ) and protein synthesis - independent effects , such as an acute Ca2 + overload ( CITATION ) . ||11909874_155871_356==>( TARGET_CITATION ) reported that activation of Egr - 2 and Egr - 3 mediates the <PROT1_155871>  Tat </PROT1_155871> - enhanced <PROT2_356>  FasL </PROT2_356> induction . ||11909874_155871_356==>HIV infection of T cells can induce <PROT2_356>  FasL </PROT2_356> transcription , and the HIV <PROT1_155871>  Tat </PROT1_155871> protein has been found to support EGR - 2 / - 3 - and NF { kappa } B - mediated <PROT2_356>  FasL </PROT2_356> transcription ( TARGET_CITATION , CITATION ) . ||12539042_155871_8743==><PROT1_155871>  Tat </PROT1_155871> facilitates HIV dissemination within the infected body ( TARGET_CITATION ) and causes the death of uninfected bystander T - cells through <PROT2_8743>  TRAIL </PROT2_8743> activation in monocytes ( CITATION ) . ||12767990_155871_8743==><PROT1_155871>  Tat </PROT1_155871> facilitates HIV dissemination within the infected body ( CITATION ) and causes the death of uninfected bystander T - cells through <PROT2_8743>  TRAIL </PROT2_8743> activation in monocytes ( TARGET_CITATION ) . ||1279199_155871_4049==>LT production has been previously described to be positively regulated by <PROT1_155871>  Tat </PROT1_155871> ( CITATION , TARGET_CITATION ) and we have confirmed the spontaneous and activation - induced up - regulation of this <PROT2_4049>  cytokine </PROT2_4049> in Tat72 and Tat101 cells ( m. Ott and e. Verdin , unpublished observation ) . ||1279199_155871_4049==>In this regard , <PROT1_155871>  Tat </PROT1_155871> has been shown to modulate <PROT2_4049>  cytokine </PROT2_4049> expression ( TARGET_CITATION , CITATION , CITATION ) and increase the activation states of T cells and monocytes ( CITATION , CITATION , CITATION ) . ||1279199_155871_4049==>In addition to its role in HIV - 1 transcription , <PROT1_155871>  Tat </PROT1_155871> has also been shown to upregulate the transcription of several host genes such as those encoding tumor necrosis factor alpha ( TARGET_CITATION ) , interleukin 2 ( IL - 2 ) ( CITATION , CITATION ) , and IL - 6 ( CITATION ) by interacting with different cellular factors and contributing to some of the altered <PROT2_4049>  cytokine </PROT2_4049> production which occurs after HIV - 1 infection . ||1279199_155871_4049==>As HIV - 1 <PROT1_155871>  Tat </PROT1_155871> had been shown to play a role in the regulation of numerous <PROT2_4049>  cytokine </PROT2_4049> genes cooperating with different cellular factors ( CITATION , TARGET_CITATION , CITATION , CITATION ) , we asked whether it could affect transactivation by NFAT1 , a transcription factor involved in regulating the expression of a large number of <PROT2_4049>  cytokine </PROT2_4049> genes ( CITATION ) . ||1279199_155871_4049==>Thus , <PROT1_155871>  Tat </PROT1_155871> has been involved in both apoptotic ( CITATION , CITATION , CITATION ) and survival ( CITATION , CITATION ) mechanisms in the alteration of T cell proliferation ( CITATION ) and in the aberrant expression of several <PROT2_4049>  cytokine </PROT2_4049> genes : TNF - { alpha } ( TARGET_CITATION ) , TGF - { beta } ( CITATION ) , IL - 6 ( CITATION ) , IL - 2 ( CITATION , CITATION , CITATION , CITATION ) , IFN - { alpha } ( CITATION ) , and IL - 8 ( CITATION ) . ||1279199_155871_4049==>In this regard we speculated that the high levels of intracellular <PROT1_155871>  Tat </PROT1_155871> expressed in tissues of <PROT1_155871>  tat </PROT1_155871> - transgenic mice ( CITATION ) could transactivate inflammatory <PROT2_4049>  cytokine </PROT2_4049> gene expression , as previously observed in other experimental systems ( fig. 1 ) ( TARGET_CITATION , CITATION ; Lotz et al. , FASEB j. 4 : 2014 , 1990 ) . ||1279199_155871_4049==>Pleiotropic functions have been described for <PROT1_155871>  Tat </PROT1_155871> , including its capacity to enhance CCR5 and CXCR4 coreceptor expression CITATION , apoptosis of T lymphocytes , inhibition of their proliferation CITATION , inhibiton of Th1 - type cytokines ( IL - 12 ) CITATION and stimulation of Th2 - type <PROT2_4049>  cytokine </PROT2_4049> production like IL - 4 , IL - 6 and IL - 10 CITATION , TARGET_CITATION , CITATION and CITATION . ||1279199_155871_7124==><PROT1_155871>  Tat </PROT1_155871> protein can modulate the expression of various cytokines , including tumor necrosis factor alpha ( <PROT2_7124>  TNF </PROT2_7124> - alpha ) , <PROT2_7124>  TNF </PROT2_7124> - beta , interleukin 1 ( IL - 1 ) , IL - 2 , and IL - 6 ( TARGET_CITATION , CITATION , CITATION , CITATION ) . ||1279199_155871_7124==>Overexpression of another HIV - 1 regulatory protein , <PROT1_155871>  Tat </PROT1_155871> , induced both tumor necrosis factor alpha ( <PROT2_7124>  TNF </PROT2_7124> - alpha ) ( TARGET_CITATION ) and gamma interferon ( IFN - gamma ) ( CITATION ) . ||1279199_155871_7124==><PROT1_155871>  Tat </PROT1_155871> can induce <PROT2_7124>  TNF </PROT2_7124> - alpha , IL - 1alpha , IL - 1beta , and interferon ( IFN ) - gamma in a dose - dependent manner TARGET_CITATION , CITATION . ||1279199_155871_7124==>Expression of <PROT1_155871>  Tat </PROT1_155871> in human cells in culture leads to transactivation and overexpression of cellular genes encoding cytokines such as tumor necrosis factor alpha ( <PROT2_7124>  TNF </PROT2_7124> - small alpha , Greek ) , <PROT2_7124>  TNF </PROT2_7124> - small beta , Greek , and interleukin - 2 ( IL - 2 ) ( CITATION ; TARGET_CITATION and CITATION ; CITATION ; CITATION ) , some of which in turn can activate viral transcription through their activation of NF - small kappa , GreekB ( TARGET_CITATION and CITATION ; CITATION ) . ||1279199_155871_7124==><PROT1_155871>  Tat </PROT1_155871> induces MCP - 1 expression in astrocytes and in addition , in patients with acquired immune deficiency syndrome dementia , MCP - 1 is present in the brain and is elevated in the CSF CITATION , CITATION . <PROT1_155871>  Tat </PROT1_155871> has also been shown to up - regulate CXCR4 on the surface of resting CD4 + T CITATION cells and to mimic the & # 223 ; - chemokines MCP - 1 , MCP - 3 , and eotaxin in their interactions with CCR2 and CCR3 CITATION . <PROT1_155871>  Tat </PROT1_155871> can also induce expression of tumor necrosis factor - { alpha } ( <PROT2_7124>  TNF </PROT2_7124> - { alpha } ) in macrophages and astrocytes as well as infected T cells TARGET_CITATION , CITATION . We and others have shown <PROT2_7124>  TNF </PROT2_7124> - { alpha } to induce chemokine expression in various CNS cells CITATION , CITATION . This indicates that the effects of <PROT1_155871>  Tat </PROT1_155871> on chemokine production may be a secondary response of the cell as a result of <PROT1_155871>  Tat </PROT1_155871> induction of <PROT2_7124>  TNF </PROT2_7124> - { alpha } or other cytokines CITATION , CITATION . <PROT1_155871>  Tat </PROT1_155871> also induces apoptotic cell death in primary human neuronal cultures CITATION , CITATION and activates MAP kinase in granular neurons and astrocytes in the rat cerebellum CITATION . It induces interleukin - 6 in human brain endothelial cells as well as increases adhesion molecule expression CITATION , CITATION . In the astrocytic cell line , U - 87MG , treatment with <PROT1_155871>  Tat </PROT1_155871> leads to increases in HIV - 1 gene expression CITATION . Astrocytes , despite their inability to produce significant amounts of virus , do produce functional <PROT1_155871>  Tat </PROT1_155871> and Rev proteins but not several other key proteins CITATION . They suggest that the production of <PROT1_155871>  Tat </PROT1_155871> and Rev by astrocytes could contribute to HIV - 1 neuropathogenesis . ||1279199_155871_7124==>Because it has been shown that HIV - 1 <PROT1_155871>  tat </PROT1_155871> protein induces <PROT2_7124>  TNF </PROT2_7124> - { alpha } , IL - 1 { alpha } , and IFN - { gamma } production ( CITATION , TARGET_CITATION , CITATION ) , it is likely that the undetectable level of IL - 1 { alpha } and <PROT2_7124>  TNF </PROT2_7124> - { alpha } in IFN - & # 223 ; - transduced macrophages reflects resistance to HIV infection . ||1279199_155871_7124==>In addition , <PROT1_155871>  Tat </PROT1_155871> has been shown to play a role in apoptosis ( CITATION ) and to induce the production of several cytokines , including interleukin - 2 ( IL - 2 ) ( CITATION ) , IL - 6 ( CITATION ) , tumor necrosis factor alpha ( <PROT2_7124>  TNF </PROT2_7124> - alpha ) ( TARGET_CITATION ) , and monocyte chemoattractant protein 1 ( MCP - 1 ) ( CITATION ) . ||1279199_155871_7124==>Thus , <PROT1_155871>  Tat </PROT1_155871> has been involved in both apoptotic ( CITATION , CITATION , CITATION ) and survival ( CITATION , CITATION ) mechanisms in the alteration of T cell proliferation ( CITATION ) and in the aberrant expression of several cytokine genes : <PROT2_7124>  TNF </PROT2_7124> - { alpha } ( TARGET_CITATION ) , TGF - { beta } ( CITATION ) , IL - 6 ( CITATION ) , IL - 2 ( CITATION , CITATION , CITATION , CITATION ) , IFN - { alpha } ( CITATION ) , and IL - 8 ( CITATION ) . ||1279199_155871_7124==>For example , in vitro studies based on a variety of non - CNS cell types have demonstrated stimulatory effects of <PROT1_155871>  Tat </PROT1_155871> on the expression of tumor necrosis factor ( <PROT2_7124>  TNF </PROT2_7124> ) - small alpha , Greek , <PROT2_7124>  TNF </PROT2_7124> - small beta , Greek , interleukin ( IL ) - 2 , IL - 6 , transforming growth factor ( TGF ) - small beta , Greek , chemokine receptors , IL - 2 receptors , IL - 4 receptors , and <PROT2_7124>  TNF </PROT2_7124> - small alpha , Greek receptors ( TARGET_CITATION , CITATION , CITATION , CITATION , CITATION , CITATION , CITATION and CITATION ) . ||1318011_155871_4609==>Some experiments suggest that the HIV - encoded transactivator protein , <PROT1_155871>  Tat </PROT1_155871> , can increase c - <PROT2_4609>  Myc </PROT2_4609> expression following addition to cultured B cells TARGET_CITATION . ||7494249_155871_7124==>Among other regulatory proteins , HIV - 1 <PROT1_155871>  Tat </PROT1_155871> induces overexpression of <PROT2_7124>  TNF </PROT2_7124> - alpha and IL - 6 CITATION , TARGET_CITATION Moreover , . ||7494249_155871_7124==>An alternative pathway proposed to explain NF - kappa B activation by <PROT1_155871>  Tat </PROT1_155871> is the induction of expression of the tumor necrosis factor alpha ( <PROT2_7124>  TNF </PROT2_7124> - alpha ) gene and the subsequent activation of NF - kappa B through the <PROT2_7124>  TNF </PROT2_7124> - alpha receptor - mediated signaling mechanism ( TARGET_CITATION ) . ||7494249_155871_7124==>Increased <PROT2_7124>  TNF </PROT2_7124> - alpha production has been described upon productive HIV - 1 infection of several cell types , and it has been ascribed to HIV - 1 <PROT1_155871>  Tat </PROT1_155871> protein ( TARGET_CITATION ) . ||7494249_155871_7124==><PROT1_155871>  Tat </PROT1_155871> sets up positive up - regulatory loops , which greatly superactivate HIV transcription by activating NF - { kappa } B in different cell types ( TARGET_CITATION CITATION CITATION ) and up - regulating several cytokine genes such as <PROT2_7124>  TNF </PROT2_7124> - { alpha } , <PROT2_7124>  TNF </PROT2_7124> - & # 223 ; , IL - 2 , and IL - 6 ( for review , see refs. ( CITATION , CITATION ) ) . ||7526541_155871_7124==>Furthermore , intracerebral injection of a peptide derived from <PROT1_155871>  Tat </PROT1_155871> into rat brain caused increased cytokine production , including <PROT2_7124>  TNF </PROT2_7124> - alpha ( TARGET_CITATION ) . ||7526541_155871_7124==>Also , animal studies in which <PROT1_155871>  Tat </PROT1_155871> was injected intracerebrally , showed elevations of <PROT2_7124>  TNF </PROT2_7124> - small alpha , Greek in the brain ( TARGET_CITATION ) . ||7526541_155871_7124==>Though direct infection of neurons with HIV - 1 likely does not occur ( CITATION and CITATION ) , <PROT1_155871>  Tat </PROT1_155871> appears to induce infected macrophages / microglia to release potentially neurotoxic substances , such as quinolinic acid , tumor necrosis factor small alpha , Greek ( <PROT2_7124>  TNF </PROT2_7124> - small alpha , Greek ) , and transforming growth factor small beta , Greek ( TGF - small beta , Greek ) , among others ( CITATION , TARGET_CITATION and CITATION ) . ||7526541_155871_7124==>It is apparent that one primary mechanism by which <PROT1_155871>  Tat </PROT1_155871> , and possibly other HIV - 1 proteins ( e.g. gp120 , gp160 , and gp41 ) , produce neurotoxicity is the induced over expression in macrophages / microglia of cytokines such as <PROT2_7124>  TNF </PROT2_7124> - small alpha , Greek and TGF - small beta , Greek , both in vivo and in vitro ( CITATION , TARGET_CITATION and CITATION ) . ||7526541_155871_7124==><PROT2_7124>  TNF </PROT2_7124> - & # x3b1 ; expression is increased in macrophages isolated from HIV - 1 patients and <PROT1_155871>  Tat </PROT1_155871> injected in the brains of mice increases <PROT2_7124>  TNF </PROT2_7124> - & # x3b1 ; in cerebrospinal fluid ( CSF ) and increases monocyte / macrophage infiltration into the central nervous system ( CNS ) ( CITATION , TARGET_CITATION , CITATION , CITATION and CITATION ) . ||7539892_155871_355==>Exogenous <PROT1_155871>  Tat </PROT1_155871> accelerates <PROT2_355>  CD95 </PROT2_355> - mediated , activation - induced T cell apoptosis ( TARGET_CITATION ) . ||7539892_155871_355==>Moreover , <PROT1_155871>  Tat </PROT1_155871> has been shown to promote apoptosis by up - regulating <PROT2_355>  CD95 </PROT2_355> ligand expression ( TARGET_CITATION ) or by activating cyclin - dependent kinases ( CITATION ) . ||7539892_155871_355==>Moreover , Westendorp et al. ( TARGET_CITATION ) have recently described that <PROT1_155871>  Tat </PROT1_155871> sensitizes T cells to <PROT2_355>  Fas </PROT2_355> - mediated apoptosis . ||7539892_155871_355==>Interestingly , cross - linking CD4 - bound gp120 with anti - gp120 antibodies enhances susceptibility of CD4 + T cells to TCR - mediated apoptosis , 55 an effect that is enhanced by <PROT1_155871>  Tat </PROT1_155871> and blocked by noncytotoxic anti - <PROT2_355>  Fas </PROT2_355> F ( ab ' ) 2 or soluble <PROT2_355>  Fas </PROT2_355> : Fc fusion molecules TARGET_CITATION . ||7539892_155871_355==>Others have found that CD4 cross - linking in the presence of HIV <PROT1_155871>  tat </PROT1_155871> induces PBMCs to express <PROT2_355>  Fas </PROT2_355> ligand mRNA and presumably <PROT2_355>  Fas </PROT2_355> ligand protein ( TARGET_CITATION ) . ||7539892_155871_355==>Activation of p56lck is clearly not associated with induction of apoptosis , and additional signals may be required , as suggested by the data of Westendorp et al. , showing that gp120 can induce <PROT2_355>  CD95 </PROT2_355> - L expression on activated T cells only after addition of HIV - 1 <PROT1_155871>  Tat </PROT1_155871> protein ( TARGET_CITATION ) . ||7539892_155871_355==>Recent reports also indicate that HIV - <PROT1_155871>  tat </PROT1_155871> protein can induce apoptosis in human T cells both by a <PROT2_355>  Fas </PROT2_355> - <PROT2_355>  Fas </PROT2_355> ligand - dependent mechanism and by down - modulation of Bcl2 expression ( TARGET_CITATION , CITATION ) . ||7539892_155871_355==>In agreement with previous findings ( TARGET_CITATION ) , following stimulation with <PROT2_355>  Fas </PROT2_355> antibody CH11 , cell death was accelerated in HIV - infected H9 ( p & lt ; 0.001 ) or Jurkat - <PROT1_155871>  tat </PROT1_155871> cells ( p & lt ; 0.001 , fig. 4 ) . ||7539892_155871_355==>Other viral genes like <PROT1_155871>  tat </PROT1_155871> may accelerate <PROT2_355>  Fas </PROT2_355> - mediated apoptosis in association with gp120 ( TARGET_CITATION ) . ||7539892_155871_355==>HIV - 1 <PROT1_155871>  Tat </PROT1_155871> and gp120 proteins have been suggested to cause apoptosis in T cells by induction of <PROT2_355>  Fas </PROT2_355> ligand expression , resulting in sensitization to <PROT2_355>  Fas </PROT2_355> - mediated apoptosis ( CITATION , CITATION , CITATION , TARGET_CITATION ) . ||7539892_155871_355==>It has been shown that HIV - 1 - <PROT1_155871>  Tat </PROT1_155871> upregulates the expression of CD95L in human peripheral T cells and accelerates <PROT2_355>  CD95 </PROT2_355> - mediated apoptosis ( TARGET_CITATION ) , and that this viral protein can downregulate not only the bcl - 2 gene expression in a variety of hematopoietic cell types , but also increase the expression of the proapoptotic gene Bax ( CITATION ) . ||7539892_155871_355==>The <PROT1_155871>  tat </PROT1_155871> protein induces oxidative stress CITATION CITATION , and increases surface expression of the <PROT2_355>  Fas </PROT2_355> ligand resulting in accelerated signaling through the <PROT2_355>  Fas </PROT2_355> pathway CITATION TARGET_CITATION . ||7539892_155871_355==>Mechanisms by which <PROT1_155871>  Tat </PROT1_155871> has been reported to induce apoptosis include up - regulation of the apoptosis effector molecule <PROT2_355>  Fas </PROT2_355> ligand ( FasL ) ( TARGET_CITATION ) , inhibition of expression of manganese - dependent superoxide dismutase ( CITATION ) , and activation of cyclin - dependent kinases ( CITATION ) . ||7539892_155871_355==>An earlier study attributed <PROT1_155871>  Tat </PROT1_155871> - induced apoptosis to the up - regulation of the apoptosis effector molecule FasL , which resulted in FasL - <PROT2_355>  Fas </PROT2_355> interactions in the culture and apoptosis ( TARGET_CITATION ) . ||7539892_155871_355==>The HIV gp120 and <PROT1_155871>  tat </PROT1_155871> proteins have been shown to induce <PROT2_355>  fas </PROT2_355> expression on CD4 + T cells in vitro and elevated levels of <PROT2_355>  fas </PROT2_355> have been reported on CD4 + T cells in HIV - infected individuals CITATION - TARGET_CITATION . ||7539892_155871_355==>Importantly , <PROT1_155871>  Tat </PROT1_155871> has also been implicated as an inducer of apoptosis in uninfected T cells , potentially by <PROT2_355>  Fas </PROT2_355> - dependent mechanisms , superoxide dismutase inhibition , or activation of cyclin - dependent kinases CITATION - TARGET_CITATION . ||7539892_155871_355==>Extracellular <PROT1_155871>  Tat </PROT1_155871> also promotes T cell destruction by increasing expression of CD95L / <PROT2_355>  Fas </PROT2_355> ligand on monocyte / macrophages ( CITATION ) and sensitizing cells to the effects of this molecule ( CITATION , TARGET_CITATION ) . ||7539892_155871_355==>Although the mechanism by which this occurs is still under extensive investigation , some reports indicate that it is the envelope protein of HIV - 1 , gp120 , perhaps in conjunction with the viral - encoded protein <PROT1_155871>  Tat </PROT1_155871> , that facilitates the activation of the <PROT2_355>  Fas </PROT2_355> death pathway by inducing the expression of FasL ( CITATION , TARGET_CITATION , CITATION ) . ||7539892_155871_355==>The viral proteins <PROT1_155871>  Tat </PROT1_155871> or gp120 are reported to sensitize T lymphocytes to undergo apoptosis in response to Abs to <PROT2_355>  CD95 </PROT2_355> in culture ( TARGET_CITATION , CITATION ) . ||7539892_155871_355==>Another viral protein , <PROT1_155871>  Tat </PROT1_155871> , has been shown to sensitize T cells to TCR - and CD4 - induced apoptosis by upregulation of FasL expression TARGET_CITATION , and to increase the sensitivity to <PROT2_355>  Fas </PROT2_355> mediated apoptosis by upregulation of caspase - 8 CITATION . ||7539892_155871_355==>The pleiotropic activities that <PROT1_155871>  Tat </PROT1_155871> displays on cell survival and growth are likely mediated by <PROT1_155871>  Tat </PROT1_155871> capability of modulating the expression and the activity of genes relevant for cell survival , including p53 ( CITATION ) , bcl - 2 ( CITATION ) , and the gene coding for <PROT2_355>  CD95 </PROT2_355> ligand ( TARGET_CITATION ) . ||7539892_155871_355==>Given the evidence that FasL - <PROT2_355>  Fas </PROT2_355> interactions may account for bystander killing of T cells in patients infected with HIV ( CITATION ) , one of the more intriguing activities ascribed to <PROT1_155871>  Tat </PROT1_155871> is that it synergizes with T cell activating stimuli in the up - regulation of FasL expression ( TARGET_CITATION ) . ||7539892_155871_355==>Similarly , <PROT1_155871>  Tat </PROT1_155871> , which is secreted by infected cells , upregulates <PROT2_355>  CD95 </PROT2_355> and CD95L on uninfected cells and enhances their susceptibility to <PROT2_355>  CD95 </PROT2_355> - induced apoptosis TARGET_CITATION , CITATION . ||7539892_155871_355==>Extracellular <PROT1_155871>  Tat </PROT1_155871> can up - regulate the expression of <PROT2_355>  Fas </PROT2_355> ligand ( FasL ) mRNA in macrophages and increase the susceptibility of <PROT1_155871>  Tat </PROT1_155871> - expressing cells to CD4 cross - linking - induced death ( TARGET_CITATION , CITATION , CITATION ) . ||7539892_155871_356==>HIV <PROT1_155871>  Tat </PROT1_155871> has been shown to upregulate <PROT2_356>  FasL </PROT2_356> expression independent of Nef ( TARGET_CITATION ) . ||7539892_155871_356==>HIV has been shown to upregulate <PROT2_356>  FasL </PROT2_356> expression on macrophages ( CITATION ) and HIV <PROT1_155871>  tat </PROT1_155871> / gp120 can enhance anti - CD3 - induced apoptosis by increasing the expression of <PROT2_356>  FasL </PROT2_356> on CD4 + cells ( TARGET_CITATION ) . ||7539892_155871_356==>Furthermore , <PROT1_155871>  Tat </PROT1_155871> upregulates expression of <PROT2_356>  FasL </PROT2_356> and sensitizes uninfected T cells to killing by TCR - induced apoptosis TARGET_CITATION . ||7539892_155871_356==>Accordingly , Westendorp et al TARGET_CITATION showed that stimulation of Jurkat cells with 10 & # 181 ; g / mL of anti - CD3 MoAb for 1 hour ( a suboptimal concentration chosen to give only minor apoptosis in the absence of HIV <PROT1_155871>  tat </PROT1_155871> or gp120 ) did not induce <PROT2_356>  CD95L </PROT2_356> mRNA ; on the contrary , the same dose of anti - CD3 MoAb induced <PROT2_356>  CD95L </PROT2_356> mRNA when combined with HIV - <PROT1_155871>  tat </PROT1_155871> . ||7539892_155871_356==>Soluble exogenous <PROT1_155871>  Tat </PROT1_155871> combined with CD4 cross - linking by antibody has been shown to increase <PROT2_356>  FasL </PROT2_356> mRNA expression in uninfected PBMCs ( TARGET_CITATION ) . ||7539892_155871_356==><PROT1_155871>  Tat </PROT1_155871> , a potent inducer of <PROT2_356>  CD95L </PROT2_356> , has been reported to induce oxidative stress and subsequent NF - kappa B activation through the down - regulation of the antioxidant enzyme manganese superoxide dismutase ( CITATION , TARGET_CITATION ) . ||7539892_155871_356==>Previous reports have indicated that CD4 cross - linking in T cells induces a transient up - regulation of <PROT2_356>  FasL </PROT2_356> mRNA , which is further enhanced by the presence of HIV - <PROT1_155871>  tat </PROT1_155871> ( TARGET_CITATION ) . ||7539892_155871_356==>We next investigated whether HIV - <PROT1_155871>  tat </PROT1_155871> , previously shown to synergize with CD4 cross - linking in increasing <PROT2_356>  FasL </PROT2_356> mRNA levels and to induce the transcription of a variety of cellular genes ( TARGET_CITATION ) , participated in directing the transcription of the <PROT2_356>  FasL </PROT2_356> enhancer - promoter gene . ||7539892_155871_356==>Although <PROT1_155871>  tat </PROT1_155871> has been shown to enhance and increase the level of <PROT2_356>  FasL </PROT2_356> mRNA induced solely by CD4 activation in primary T cells ( TARGET_CITATION ) , <PROT1_155871>  tat </PROT1_155871> alone did not induce de novo <PROT2_356>  FasL </PROT2_356> mRNA , and as shown here , did not directly increase <PROT2_356>  FasL </PROT2_356> gene transcription . ||7539892_155871_356==>Cross - linking of CD4 by HIVgp120 in the presence of <PROT1_155871>  Tat </PROT1_155871> protein can induce <PROT2_356>  FasL </PROT2_356> expression and apoptosis of uninfected T cells ( TARGET_CITATION ) . ||7539892_155871_356==>It has been shown that HIV - 1 - <PROT1_155871>  Tat </PROT1_155871> upregulates the expression of <PROT2_356>  CD95L </PROT2_356> in human peripheral T cells and accelerates CD95 - mediated apoptosis ( TARGET_CITATION ) , and that this viral protein can downregulate not only the bcl - 2 gene expression in a variety of hematopoietic cell types , but also increase the expression of the proapoptotic gene Bax ( CITATION ) . ||7539892_155871_356==>Moreover , HIV - 1 & # 150 ; derived proteins ( <PROT1_155871>  Tat </PROT1_155871> , gp120 ) can enhance <PROT2_356>  CD95L </PROT2_356> expression ( TARGET_CITATION ) ( CITATION ) . ||7539892_155871_356==>Mechanisms by which <PROT1_155871>  Tat </PROT1_155871> has been reported to induce apoptosis include up - regulation of the apoptosis effector molecule Fas ligand ( <PROT2_356>  FasL </PROT2_356> ) ( TARGET_CITATION ) , inhibition of expression of manganese - dependent superoxide dismutase ( CITATION ) , and activation of cyclin - dependent kinases ( CITATION ) . ||7539892_155871_356==><PROT1_155871>  Tat </PROT1_155871> has previously been reported to induce the expression of <PROT2_356>  FasL </PROT2_356> ( TARGET_CITATION ) . ||7539892_155871_356==>An earlier study attributed <PROT1_155871>  Tat </PROT1_155871> - induced apoptosis to the up - regulation of the apoptosis effector molecule <PROT2_356>  FasL </PROT2_356> , which resulted in <PROT2_356>  FasL </PROT2_356> - Fas interactions in the culture and apoptosis ( TARGET_CITATION ) . ||7539892_155871_356==>Other HIV - 1 gene products such as extracellular HIV - 1 <PROT1_155871>  Tat </PROT1_155871> and gp120 have been shown to increase the expression of <PROT2_356>  CD95L </PROT2_356> on activated T cells TARGET_CITATION . ||7539892_155871_356==>Extracellular <PROT1_155871>  Tat </PROT1_155871> also promotes T cell destruction by increasing expression of <PROT2_356>  CD95L </PROT2_356> / Fas ligand on monocyte / macrophages ( CITATION ) and sensitizing cells to the effects of this molecule ( CITATION , TARGET_CITATION ) . ||7539892_155871_356==>Although the mechanism by which this occurs is still under extensive investigation , some reports indicate that it is the envelope protein of HIV - 1 , gp120 , perhaps in conjunction with the viral - encoded protein <PROT1_155871>  Tat </PROT1_155871> , that facilitates the activation of the Fas death pathway by inducing the expression of <PROT2_356>  FasL </PROT2_356> ( CITATION , TARGET_CITATION , CITATION ) . ||7539892_155871_356==><PROT1_155871>  Tat </PROT1_155871> , the transcriptional activator of human immunodeficiency virus type 1 ( HIV - 1 ) , stimulates Sp1 phosphorylation ( CITATION ) , activates <PROT2_356>  FasL </PROT2_356> expression ( TARGET_CITATION ) , and can induce apoptosis ( TARGET_CITATION , CITATION ) . ||7539892_155871_356==>Another viral protein , <PROT1_155871>  Tat </PROT1_155871> , has been shown to sensitize T cells to TCR - and CD4 - induced apoptosis by upregulation of <PROT2_356>  FasL </PROT2_356> expression TARGET_CITATION , and to increase the sensitivity to Fas mediated apoptosis by upregulation of caspase - 8 CITATION . ||7539892_155871_356==><PROT1_155871>  Tat </PROT1_155871> is secreted from infected cells CITATION and may upregulate <PROT2_356>  FasL </PROT2_356> on uninfected cells TARGET_CITATION . ||7539892_155871_356==>Given the evidence that <PROT2_356>  FasL </PROT2_356> - Fas interactions may account for bystander killing of T cells in patients infected with HIV ( CITATION ) , one of the more intriguing activities ascribed to <PROT1_155871>  Tat </PROT1_155871> is that it synergizes with T cell activating stimuli in the up - regulation of <PROT2_356>  FasL </PROT2_356> expression ( TARGET_CITATION ) . ||7539892_155871_356==>We and others have previously shown that the HIV - 1 <PROT1_155871>  Tat </PROT1_155871> protein essential for efficient HIV - 1 gene expression and replication enhances apoptosis via up - regulation of <PROT2_356>  CD95L </PROT2_356> expression CITATION , TARGET_CITATION . ||7539892_155871_356==>The mechanisms responsible for the induction of <PROT2_356>  CD95L </PROT2_356> are unclear , but it has been suggested that the increase in <PROT2_356>  CD95L </PROT2_356> levels observed in T cells from HIV - infected humans result from immune activation and & # 47 ; or the interaction of T cells with viral proteins , including gp120 and <PROT1_155871>  tat </PROT1_155871> TARGET_CITATION , CITATION . ||7539892_155871_356==>For example , crosslinking of CD4 by the HIV envelope in the presence of soluble <PROT1_155871>  Tat </PROT1_155871> can induce the expression of <PROT2_356>  FASL </PROT2_356> and the apoptosis of uninfected cells TARGET_CITATION . ||7539892_155871_356==>Similarly , <PROT1_155871>  Tat </PROT1_155871> , which is secreted by infected cells , upregulates CD95 and <PROT2_356>  CD95L </PROT2_356> on uninfected cells and enhances their susceptibility to CD95 - induced apoptosis TARGET_CITATION , CITATION . ||7539892_155871_356==>Extracellular <PROT1_155871>  Tat </PROT1_155871> can up - regulate the expression of Fas ligand ( <PROT2_356>  FasL </PROT2_356> ) mRNA in macrophages and increase the susceptibility of <PROT1_155871>  Tat </PROT1_155871> - expressing cells to CD4 cross - linking - induced death ( TARGET_CITATION , CITATION , CITATION ) . ||7539892_155871_356==>Ug05RP Induces Greater Up - regulation of <PROT2_356>  FasL </PROT2_356> mRNA Compared with Ug11LTS & # 151 ; Previous reports have suggested that <PROT1_155871>  Tat </PROT1_155871> may induce apoptosis by up - regulating the expression of <PROT2_356>  FasL </PROT2_356> ( TARGET_CITATION ) . ||7539892_155871_356==>Another pathway by which <PROT1_155871>  Tat </PROT1_155871> has been shown to induce apoptosis is by up - regulating <PROT2_356>  FasL </PROT2_356> expression ( TARGET_CITATION ) . ||9269771_155871_6401==><PROT1_155871>  Tat </PROT1_155871> is secreted by infected cells and contributes to the activation of ECs and expression of EC adhesion molecules ( ELAMs ; <PROT2_6401>  ELAM </PROT2_6401> - 1 , VCAM - 1 , and ICAM - 1 ) . ( CITATION , TARGET_CITATION and CITATION ) Extracellular HIV - 1 <PROT1_155871>  tat </PROT1_155871> protein promotes the growth of spindle cells derived from KS and normal vascular cells. ( CITATION ) Basic fibroblast growth factor and HIV - 1 <PROT1_155871>  tat </PROT1_155871> acted synergistically to induce KS - like lesions in nude mice. ( CITATION ) HIV - 1 <PROT1_155871>  tat </PROT1_155871> protein transactivates HIV - 1 , viral , and some host genes. ( CITATION ) HIV - 1 - infected cells release <PROT1_155871>  tat </PROT1_155871> . ( CITATION ) <PROT1_155871>  Tat </PROT1_155871> acts extracellularly and regulates the functions of immunocompetent and mesenchymal cells. ( CITATION ) ||9269771_155871_6401==>HIV - <PROT1_155871>  Tat </PROT1_155871> protein was shown to induce the expression of <PROT2_6401>  ELAM </PROT2_6401> - 1 , VCAM - 1 and ICAM - 1 in human endothelial cells , and this induction , which required protein synthesis , allowed an enhanced adhesion of human promyelomonocytic HL - 60 cells to endothelial cells ( TARGET_CITATION ) . ||9708406_155871_5747==>Moreover , <PROT1_155871>  Tat </PROT1_155871> was found capable of inducing the expression of p125 focal adhesion kinase ( p125 <PROT2_5747>  FAK </PROT2_5747> ) that is activated by integrin triggering ( CITATION , TARGET_CITATION and data not shown ) . ||9708406_155871_5747==>This effect is specifically mediated by the RGD region of <PROT1_155871>  Tat </PROT1_155871> , because mutations of this <PROT1_155871>  Tat </PROT1_155871> region , but not of other <PROT1_155871>  Tat </PROT1_155871> sequences , strongly decrease <PROT1_155871>  Tat </PROT1_155871> - induced p125 <PROT2_5747>  FAK </PROT2_5747> tyrosine phosphorylation ( TARGET_CITATION ) .