Mol Cell. 2010 May 14;38(3):428-38. doi: 10.1016/j.molcel.2010.04.013.

HIV-1 Tat and host AFF4 recruit two transcription elongation factors into a bifunctional complex for coordinated activation of HIV-1 transcription.

He N, Liu M, Hsu J, Xue Y, Chou S, Burlingame A, Krogan NJ, Alber T, Zhou Q.

Source

Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, CA 94720, USA.

Abstract

Recruitment 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.