The HIV-1 Tat protein recruits a ubiquitin ligase to reorganize the 7SK snRNP for transcriptional activation

Abstract

The HIV-1 Tat protein hijacks P-TEFb kinase to activate paused RNA polymerase II (RNAP II) at the viral promoter. Tat binds additional host factors, but it is unclear how they regulate RNAP II elongation. Here, we identify the cytoplasmic ubiquitin ligase UBE2O as critical for Tat transcriptional activity. Tat hijacks UBE2O to ubiquitinate the P-TEFb kinase inhibitor HEXIM1 of the 7SK snRNP, a fraction of which also resides in the cytoplasm bound to P-TEFb. HEXIM1 ubiquitination sequesters it in the cytoplasm and releases P-TEFb from the inhibitory 7SK complex. Free P-TEFb then becomes enriched in chromatin, a process that is also stimulated by treating cells with a CDK9 inhibitor. Finally, we demonstrate that UBE2O is critical for P-TEFb recruitment to the HIV-1 promoter. Together, the data support a unique model of elongation control where non-degradative ubiquitination of nuclear and cytoplasmic 7SK snRNP pools increases P-TEFb levels for transcriptional activation.

CDK9: a signaling hub for transcriptional control

“…While the majority of 7SK-bound CDK9 is nuclear, a small fraction appears to be located in the cytoplasm [41]. However, some controversy still exists in the field regarding 7SK snRNP complex subcellular distribution.…”

“…While 7SK RNA is predominantly, if not exclusively, nuclear, this data is derived from RNA fluorescence in situ hybridization in which the probes used could, potentially, only detect the nuclear fraction of 7SK RNA and 7SK snRNP complex due to probe accessibility [42]. Similarly, various studies using immunofluorescence to detect the protein components of the snRNP have provided conflictive results (nuclear vs nuclear-cytoplasmic) potentially due to antibodies recognizing different 7SK component forms [30,[41][42][43]. Thus, rigorous studies examining the localization of the complex (and not its components in isolation) would be required to precisely address this conundrum and to further evaluate functional roles for the nuclear and cytoplasmic 7SK snRNP pools.…”

“…Phosphorylation is not the only post-translational modification purposed to facilitate CDK9 release from the 7SK snRNP complex. The HIV trans-activator of transcription (Tat) protein was recently shown to recruit the UBE2O ubiquitin ligase to cytoplasmic and nuclear 7SK snRNP pools, leading to the non-degradative ubiquitination of HEXIM [41]. This ubiquitination event promotes release of HEXIM from the 7SK snRNP and its redistribution from the nucleus to the cytoplasm thereby promoting an enrichment of free, active CDK9 in the nucleus for HIV gene activation [41].…”

“…The HIV trans-activator of transcription (Tat) protein was recently shown to recruit the UBE2O ubiquitin ligase to cytoplasmic and nuclear 7SK snRNP pools, leading to the non-degradative ubiquitination of HEXIM [41]. This ubiquitination event promotes release of HEXIM from the 7SK snRNP and its redistribution from the nucleus to the cytoplasm thereby promoting an enrichment of free, active CDK9 in the nucleus for HIV gene activation [41]. Besides ubiquitination, factor acetylation has also been implicated in CDK9 activation [50].…”

“…The HIV transcription activator Tat tethers CDK9 to the 5ʹ-end of viral nascent pre-mRNAs (Trans-activation response element: TAR RNA) at the HIV promoter through direct interaction with CycT to promote viral transcription elongation (Figure 4(c), left) [8,25,122]. Additionally, Tat has been implicated in the release and activation of CDK9 from the 7SK snRNP by both competing with HEXIM for CDK9 binding and recruiting CDK9 RFs (Figure 1(d,e)) to the 7SK snRNP complex [28,40,41].…”