TASOR epigenetic repressor cooperates with a CNOT1 RNA degradation pathway to repress HIV

Abstract

The Human Silencing Hub (HUSH) complex constituted of TASOR, MPP8 and Periphilin is involved in the spreading of H3K9me3 repressive marks across genes and transgenes such as ZNF encoding genes, ribosomal DNAs, LINE-1, Retrotransposons and Retroelements or the integrated HIV provirus1-5. The deposit of these repressive marks leads to heterochromatin formation and inhibits gene expression. The precise mechanisms of silencing mediated by HUSH is still poorly understood. Here, we show that TASOR depletion increases the accumulation of transcripts derived from the HIV-1 LTR promoter at a post-transcriptional level. By counteracting HUSH, Vpx from HIV-2 mimics TASOR depletion. With the use of a Yeast-Two-Hybrid screen, we identified new TASOR partners involved in RNA metabolism including the RNA deadenylase CCR4-NOT complex scaffold CNOT1. TASOR and CNOT1 interact in vivo and synergistically repress HIV expression from its LTR. In fission yeast, the RNA-induced transcriptional silencing (RITS) complex presents structural homology with HUSH. During transcription elongation by RNA polymerase II, RITS recruits a TRAMP-like RNA degradation complex composed of CNOT1 partners, MTR4 and the exosome, to ultimately repress gene expression via H3K9me3 deposit. Similarly, we show that TASOR interacts and cooperates with MTR4 and the exosome, in addition to CNOT1. We also highlight an interaction between TASOR and RNA Polymerase II, predominantly under its elongating state, and between TASOR and some HUSH-targeted nascent transcripts. Furthermore, we show that TASOR overexpression facilitates the association of the aforementioned RNA degradation proteins with RNA polymerase II. Altogether, we propose that HUSH operates at the transcriptional and post-transcriptional levels to repress HIV proviral gene expression.