tat, Protein Tat
Also Known as
Nuclear transcriptional activator of viral gene expression, that is essential for viral transcription from the LTR promoter and replication. Acts as a sequence-specific molecular adapter, directing components of the cellular transcription machinery to the viral RNA to promote processive transcription elongation by the RNA polymerase II (RNA pol II) complex, thereby increasing the level of full-length transcripts. In the absence of Tat, the RNA Pol II generates short or non-processive transcripts that terminate at approximately 60 bp from the initiation site. Tat associates with the CCNT1/cyclin-T1 component of the P-TEFb complex (CDK9 and CCNT1), which promotes RNA chain elongation. This binding increases Tat's affinity for a hairpin structure at the 5'-end of all nascent viral mRNAs referred to as the transactivation responsive RNA element (TAR RNA) and allows Tat/P-TEFb complex to bind cooperatively to TAR RNA. The CDK9 component of P-TEFb and other Tat-activated kinases hyperphosphorylate the C-terminus of RNA Pol II that becomes stabilized and much more processive. Other factors such as HTATSF1/Tat-SF1, SUPT5H/SPT5, and HTATIP2 are also important for Tat's function. Besides its effect on RNA Pol II processivity, Tat induces chromatin remodeling of proviral genes by recruiting the histone acetyltransferases (HATs) CREBBP, EP300 and PCAF to the chromatin. This also contributes to the increase in proviral transcription rate, especially when the provirus integrates in transcriptionally silent region of the host genome. To ensure maximal activation of the LTR, Tat mediates nuclear translocation of NF-kappa-B by interacting with host RELA. Through its interaction with host TBP, Tat may also modulate transcription initiation. Tat can reactivate a latently infected cell by penetrating in it and transactivating its LTR promoter. In the cytoplasm, Tat is thought to act as a translational activator of HIV-1 mRNAs., Extracellular circulating Tat can be endocytosed by surrounding uninfected cells via the binding to several surface receptors such as CD26, CXCR4, heparan sulfate proteoglycans (HSPG) or LDLR. Neurons are rarely infected, but they internalize Tat via their LDLR. Through its interaction with nuclear HATs, Tat is potentially able to control the acetylation-dependent cellular gene expression. Modulates the expression of many cellular genes involved in cell survival, proliferation or in coding for cytokines or cytokine receptors. Tat plays a role in T-cell and neurons apoptosis. Tat induced neurotoxicity and apoptosis probably contribute to neuroAIDS. Circulating Tat also acts as a chemokine-like and/or growth factor-like molecule that binds to specific receptors on the surface of the cells, affecting many cellular pathways. In the vascular system, Tat binds to ITGAV/ITGB3 and ITGA5/ITGB1 integrins dimers at the surface of endothelial cells and competes with bFGF for heparin-binding sites, leading to an excess of soluble bFGF. Interacts with host CCNT1. Associates with the P-TEFb complex composed at least of Tat, P-TEFb (CDK9 and CCNT1), TAR RNA, RNA Pol II. Recruits the HATs CREBBP, TAF1/TFIID, EP300, PCAF and GCN5L2. Interacts with host KAT5/Tip60; this interaction targets the latter to degradation. Interacts with the host deacetylase SIRT1. Interacts with host capping enzyme RNGTT; this interaction stimulates RNGTT. Binds to host KDR, and to the host integrins ITGAV/ITGB3 and ITGA5/ITGB1. Interacts with host KPNB1/importin beta-1 without previous binding to KPNA1/importin alpha-1. Interacts with EIF2AK2. Interacts with host nucleosome assembly protein NAP1L1; this interaction may be required for the transport of Tat within the nucleus, since the two proteins interact at the nuclear rim. Interacts with host C1QBP/SF2P32; this interaction involves lysine-acetylated Tat. Interacts with the host chemokine receptors CCR2, CCR3 and CXCR4. Interacts with host DPP4/CD26; this interaction may trigger an anti-proliferative effect. Interacts with host LDLR. Interacts with the host extracellular matrix metalloproteinase MMP1. Interacts with host PRMT6; this interaction mediates Tat's methylation. Interacts with, and is ubiquitinated by MDM2/Hdm2. Interacts with host PSMC3 and HTATIP2. Interacts with STAB1; this interaction may overcome SATB1-mediated repression of IL2 and IL2RA (interleukin) in T cells by binding to the same domain than HDAC1. Interacts (when acetylated) with human CDK13, thereby increasing HIV-1 mRNA splicing and promoting the production of the doubly spliced HIV-1 protein Nef.Interacts with host TBP; this interaction modulates the activity of transcriptional pre-initiation complex. Interacts with host RELA.