Tat (HIV)

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Tat is a small HIV-1 protein that acts as a key regulator to boost the virus’s transcription.

- What it does: Tat greatly increases the amount of viral RNA made from the HIV genome. It helps the virus switch from making a few transcripts to producing lots of viral RNA, creating a fast, self‑reinforcing cycle of replication.

- How it works: Tat binds to TAR RNA, a structure at the start of new HIV transcripts. This binding helps recruit cellular factors (the P-TEFb complex with CDK9 and cyclin T1) and RNA polymerase II to efficiently elongate transcription, producing full‑length viral RNA. There are two Tat binding sites on each transcription elongation complex, suggesting two Tat molecules can help one round of RNA synthesis.

- Important parts of Tat: The basic region (rich in arginine and lysine) is crucial for binding TAR and activating transcription. A minimal 10‑amino‑acid basic segment is enough to bind TAR, but full activity needs the surrounding 47 N‑terminal residues. Tat’s basic region likely forms an alpha helix and is part of the arginine‑rich motif (ARM) RNA‑binding family. Tat also has a trans‑activation domain in its N‑terminal region.

- How Tat gets into cells: Tat contains a cell‑penetrating domain (transcellular transport domain) with the sequence YGRKKRRQRRR. This helps Tat cross cell membranes and enter cells. Its entry can involve interactions with heparan sulfate on the cell surface and may use a potocytosis pathway. The nuclear localization signal GRKKR helps Tat move into the nucleus.

- Tat outside the cell and its effects: Infected cells release Tat, and it can be found in the blood of HIV‑1–infected people. Tat can be taken up by uninfected cells and can promote death of bystander T cells via apoptosis, contributing to AIDS progression. It may also influence which HIV strains rise early in infection by affecting receptors on cells.

- Tat in therapy and vaccines: Because Tat is crucial for HIV replication, scientists have looked for Tat inhibitors. A vaccine candidate called Tat Oyi has been tested in humans. In a 2016 early study, one dose showed a reduction in viral RNA in some participants, but results were not consistently dose‑dependent, so more evidence is needed.