Structure and methyl-lysine binding selectivity of the HUSH complex subunit MPP8

Abstract

ABSTRACTThe Human Silencing Hub (HUSH) guards the genome from the pathogenic effects of retroelement expression. Composed of MPP8, TASOR, and Periphilin-1, HUSH recognizes actively transcribed retrotransposed sequences by the presence of long (>1.5-kb) nascent transcripts without introns. HUSH recruits effectors that alter chromatin structure, degrade transcripts, and deposit transcriptionally repressive epigenetic marks. Structural and biochemical data for the three HUSH subunits are sparse. Here, we report the crystal structure of the C-terminal domain (CTD) of MPP8 necessary for HUSH activity. The MPP8 CTD consists of five ankyrin repeats followed by a domain with structural homology to the PINIT domains of Siz/PIAS-family SUMO E3 ligases. AlphaFold-Multimer modeling predicts that the MPP8 CTD forms extended interaction interfaces with a SPOC domain and a domain with a novel fold, DomI, in TASOR. The chromodomain of MPP8, known to bind the repressive mark H3K9me3, binds with similar or higher affinity to H3K9-like sequences in the mammalian H3K9 methyltransferase subunits SETDB1, ATF7IP, G9a, and GLP. Hence, MPP8 promotes heterochromatinization by recruiting H3K9 methyltransferases. Our work identifies novel structural elements in MPP8 that are required for HUSH complex assembly and silencing, thereby fulfilling vital functions in controlling the transcription of retrotransposons of endogenous and viral origin.Key PointsStructure of MPP8 CTD reveals ankyrin repeats and PINIT-like domain required for HUSH activityAlphaFold predicts a large interaction interface between MPP8 CTD and TASOR SPOC and DomI domainsThe MPP8 chromodomain binds trimethyl-lysines in ATF7IP and GLP with higher affinity than H3K9me3