Structural highlights
Function
[TAT_HV1Z2] 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. In this purpose, it activates EIF2AK2/PKR which, in turns, may phosphorylate and target to degradation the inhibitor IkappaB-alpha which normally retains NF-kappa-B in the cytoplasm of unstimulated cells. Through its interaction with TBP, Tat may be involved in transcription initiation as well. Interacts with the cellular capping enzyme RNGTT to mediate co-transcriptional capping of viral mRNAs. Tat protein exerts as well a positive feedback on the translation of its cognate mRNA. 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 (By similarity). 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. Endosomal low pH allows Tat to cross the endosome membrane to enter the cytosol and eventually further translocate into the nucleus, thereby inducing severe cell dysfunctions ranging from cell activation to cell death. Through its interaction with nuclear HATs, Tat is potentially able to control the acetylation-dependent cellular gene expression. Tat seems to inhibit the HAT activity of KAT5/Tip60 and TAF1, and consequently modify the expression of specific cellular genes. Modulates the expression of many cellular genes involved in cell survival, proliferation or in coding for cytokines (such as IL10) or cytokine receptors. May be involved in the derepression of host interleukin IL2 expression. Mediates the activation of cyclin-dependent kinases and dysregulation of microtubule network. Tat plays a role in T-cell and neurons apoptosis. Tat induced neurotoxicity and apoptosis probably contribute to neuroAIDS. Host extracellular matrix metalloproteinase MMP1 cleaves Tat and decreases Tat's mediated neurotoxicity. 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. Binds to KDR/VEGFR-2. All these Tat-mediated effects enhance angiogenesis in Kaposi's sarcoma lesions (By similarity).
Evolutionary Conservation
Check, as determined by ConSurfDB. You may read the explanation of the method and the full data available from ConSurf.
Publication Abstract from PubMed
Tat (trans-activator) proteins are early RNA binding proteins regulating lentiviral transcription. These proteins are necessary components in the life cycle of all known lentiviruses, such as the human immunodeficiency viruses (HIV) or the equine infectious anemia virus (EIAV). Tat proteins are thus ideal targets for drugs intervening with lentiviral growth. The consensus RNA binding motif (TAR, trans-activation responsive element) of HIV-1 is well characterized. Structural features of the 86 amino acid HIV-1, Zaire 2 isolate (HV1Z2) Tat protein in solution were determined by two dimensional (2D) nuclear magnetic resonance (NMR) methods and molecular dynamics (MD) calculations. In general, sequence regions corresponded to structural domains of the protein. It exhibited a hydrophobic core of 16 amino acids and a glutamine-rich domain of 17 amino acids. Part of the NH2 terminus, Val4 to Pro14, was sandwiched between these domains. Two highly flexible domains corresponded to a cysteine-rich and a basic sequence region. The 16 amino acid sequence of the core region is strictly conserved among the known Tat proteins, and the three-dimensional fold of these amino acids of HV1Z2 Tat protein was highly similar to the structure of the corresponding EIAV Tat domain. HV1Z2 Tat protein contained a well defined COOH-terminal Arg-Gly-Asp (RGD) loop similar to the recently determined decorsin RGD loop.
Structural studies of HIV-1 Tat protein.,Bayer P, Kraft M, Ejchart A, Westendorp M, Frank R, Rosch P J Mol Biol. 1995 Apr 7;247(4):529-35. PMID:7723010[1]
From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.
See Also
References
- ↑ Bayer P, Kraft M, Ejchart A, Westendorp M, Frank R, Rosch P. Structural studies of HIV-1 Tat protein. J Mol Biol. 1995 Apr 7;247(4):529-35. PMID:7723010 doi:http://dx.doi.org/10.1006/jmbi.1995.0158
