In molecular biology, Tat is a protein that is encoded for by the tatgene in HIV-1. Tat is a regulatory protein that drastically enhances the efficiency of viral transcription. Tat stands for "Trans-Activator of Transcription". The protein consists of between 86 and 101 amino acids depending on the subtype. Tat vastly increases the level of transcription of the HIVdsDNA. Before Tat is present, a small number of RNA transcripts will be made, which allow the Tat protein to be produced. Tat then binds to cellular factors and mediates their phosphorylation, resulting in increased transcription of all HIV genes, providing a positive feedback cycle. This in turn allows HIV to have an explosive response once a threshold amount of Tat is produced, a useful tool for defeating the body's response. Tat also appears to play a more direct role in the HIV disease process. The protein is released by infected cells in culture, and is found in the blood of HIV-1 infected patients. It can be absorbed by cells that are not infected with HIV, and can act directly as a toxin producing cell death via apoptosis in uninfected "bystander" T cells, assisting in progression toward AIDS. By antagonizing the CXCR4 receptor, Tat also appears to selectively encourage the reproduction of less virulent M-tropic strains of HIV early in the course of infection, allowing the more rapidly pathogenic T-tropic strains to emerge later after mutating from M-tropic strains.
Function and mechanism
Like other lentiviruses, HIV-1 encodes a trans-activating regulatory protein, which is essential for efficient transcription of the viral genome. Tat acts by binding to an RNA stem-loop structure, the trans-activating response element, found at the 5′ ends of nascent HIV-1 transcripts. In binding to TAR, Tat alters the properties of the transcription complex, recruits the positive transcription elongation complex of cellular CDK9 and cyclin T1, and hence increases the production of full-length viral RNA. Tat protein also associates with RNA polymerase II complexes during early transcription elongation after the promoter clearance and before the synthesis of full-length TAR RNA transcript. This interaction of Tat with RNA polymerase II elongation complexes is P-TEFb-independent. There are two Tat binding sites on each transcription elongation complex; one is located on TAR RNA and the other one on RNA polymerase II near the exit site for nascent mRNA transcripts which suggests that two Tat molecules are involved in performing various functions during a single round of HIV-1 mRNA synthesis. The minimum Tat sequence that can mediate specific TAR binding in vitro has been mapped to a basic domain of 10 amino acids, comprising mostly Arg and Lys residues. Regulatory activity, however, also requires the 47 N-terminal residues, which interact with components of the transcription complex and function as a transcriptional activation domain. Tat also uses an unusual transcellular transport pathway. Firstly, it binds with high affinity to Phosphatidylinositol 4,5-bisphosphate, found on the inner surface of the cell membrane, this enables Tat recruitment at this level. Tat then crosses the plasma membrane to reach the extracellular space. Tat secretion by infected cells is highly active, and export is the major destination for HIV-1 Tat.
Structure
The basic region of HIV-Tat protein is suggested to form an alpha helix. The basic region is involved in RNA binding and Tat proteins thus belong to the family of arginine-rich motif RNA binding proteins.
Tat contains a protein transduction domain, which is therefore known as a cell-penetrating peptide. Originally characterised by Frankel and Pabo and Green and Loewenstein, this domain allows Tat to enter cells by crossing the cell membrane. The amino acid sequence of the protein transduction domain is YGRKKRRQRRR. The nuclear localisation signal found within the domain, GRKKR, mediates further translocation of Tat into the cell nucleus. The biological role of this domain and exact mechanism of transfer is unknown.
Clinical significance
Inhibition of Tat has been investigated. It has been suggested that Tat antagonists may be of use in the treatment of HIV infections. Biosantech has developed a novel vaccine called Tat Oyi, which aims at the Tat protein. The company’s HIV vaccine candidate is not toxic to 48 HIV-positive patients enrolled in a double-blind study taking place in France. A Phase I/IIa study, published in 2016, shows a reduction in viral RNA for one of three doses tested. A dose-dependent response was not observed, raising questions about the robustness of the findings.
