HUSH-mediated HIV silencing is independent of TASOR phosphorylation on threonine 819.

TASOR, a component of the HUSH repressor epigenetic complex, and SAMHD1, a cellular triphosphohydrolase (dNTPase), are both anti-HIV proteins antagonized by HIV-2/SIVsmm Viral protein X. As a result, the same viral protein is able to relieve two different blocks along the viral life cell cycle, one at the level of reverse transcription, by degrading SAMHD1, the other one at the level of proviral expression, by degrading TASOR. Phosphorylation of SAMHD1 at T592 has been shown to downregulate its antiviral activity. The discovery that T819 in TASOR was lying within a SAMHD1 T592-like motif led us to ask whether TASOR is phosphorylated on this residue and whether this post-translational modification could regulate its repressive activity. Using a specific anti-phospho-antibody, we found that TASOR is phosphorylated at T819, especially in cells arrested in early mitosis by nocodazole. We provide evidence that the phosphorylation is conducted by a Cyclin/CDK1 complex, like that of SAMHD1 at T592. While we could not detect TASOR in quiescent CD4 + T cells, TASOR and its phosphorylated form are present in activated primary CD4 + T lymphocytes. In addition, TASOR phosphorylation appears to be independent from TASOR repressive activity. Indeed, on the one hand, nocodazole barely reactivates HIV-1 in the J-Lat A1 HIV-1 latency model despite TASOR T819 phosphorylation. On the other hand, etoposide, a second cell cycle arresting drug, reactivates latent HIV-1, without concomitant TASOR phosphorylation. Furthermore, overexpression of wt TASOR or T819A or T819E similarly represses gene expression driven by an HIV-1-derived LTR promoter. Finally, while TASOR is degraded by HIV-2 Vpx, TASOR phosphorylation is prevented by HIV-1 Vpr, likely as a consequence of HIV-1 Vpr-mediated-G2 arrest. Altogether, we show that TASOR phosphorylation occurs in vivo on T819. This event does not appear to correlate with TASOR-mediated HIV-1 silencing. We speculate that TASOR phosphorylation is related to a role of TASOR during cell cycle progression.