HIV-1 Tat and Host AFF4 Recruit Two Transcription Elongation Factors into a Bifunctional Complex for Coordinated Activation of HIV-1 Transcription

SummaryRecruitment of the P-TEFb kinase by HIV-1 Tat to the viral promoter triggers the phosphorylation and escape of RNA polymerase II from promoter-proximal pausing. It is unclear, however, if Tat recruits additional host factors that further stimulate HIV-1 transcription. Using a sequential affinity-purification scheme, we have identified human transcription factors/coactivators AFF4, ENL, AF9, and elongation factor ELL2 as components of the Tat-P-TEFb complex. Through the bridging functions of Tat and AFF4, P-TEFb and ELL2 combine to form a bifunctional elongation complex that greatly activates HIV-1 transcription. Without Tat, AFF4 can mediate the ELL2-P-TEFb interaction, albeit inefficiently. Tat overcomes this limitation by bringing more ELL2 to P-TEFb and stabilizing ELL2 in a process that requires active P-TEFb. The ability of Tat to enable two different classes of elongation factors to cooperate and coordinate their actions on the same polymerase enzyme explains why Tat is such a powerful activator of HIV-1 transcription.

► ELL2, AFF4, ENL, and AF9 join P-TEFb as Tat partners for activated HIV transcription
► AFF4 recruits ELL2 and P-TEFb into a bifunctional complex for general elongation
► Tat further promotes the formation of this complex to activate HIV-1 transcription
► The Tat/AFF4-promoted interaction of ELL2 with active P-TEFb stabilizes ELL2