Bovine immunodeficiency virus tat–tar complex, nmr, 5 structures
Tat vastly increases the level of transcription of the HIV dsDNA. 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 interacting with the CXCR4 receptor, Tat also appears to encourage the reproduction of less virulent M-tropic (macrophage-tropic) strains of HIV (which use the CCR5 receptor) early in the course of infection, allowing the more rapidly pathogenic T-tropic (T-cell-tropic) strains (which use the CXCR4 receptor) to emerge later.
Function and mechanism
Like other lentiviruses, HIV-1 encodes a trans-activating regulatory protein (Tat), 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 (TAR), found at the 5′ ends of nascent HIV-1 transcripts. In binding to TAR, Tat alters the properties of the transcription complex, recruits a positive transcription elongation complex (P-TEFb) 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 (PI(4,5)P2), 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.
The basic region of HIV-Tat protein is suggested to form an alpha helix. The basic region is involved in RNA (TAR, trans-activation response element) binding and Tat proteins thus belong to the family of arginine-rich motif (ARM) RNA binding proteins.
Protein transduction domain
Tat contains a protein transduction domain, and is therefore known as a cell-penetrating peptide. Originally characterised by Frankel and Pabo (1988) and Green and Loewenstein (1988), 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. As of 2000[update] The biological role of this domain and exact mechanism of transfer is unknown.
- tat Gene Products, Human Immunodeficiency Virus at the US National Library of Medicine Medical Subject Headings (MeSH)
- Genes, tat at the US National Library of Medicine Medical Subject Headings (MeSH)
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