RNA binding by Periphilin plays an essential role in initiating silencing by the HUSH complex

The human silencing hub (HUSH) complex is a transcription-dependent, epigenetic repressor complex that provides a genome-wide immunosurveillance system for the recognition and silencing of newly-integrated retroelements. The core HUSH complex of TASOR, MPP8 and Periphilin, represses these retroelements through SETDB1-mediated H3K9me3 deposition and MORC2-dependent chromatin compaction. HUSH-dependent silencing is RNA-mediated, yet no HUSH components encode any RNA-binding domain. Here we used an unbiased approach to identify which HUSH component was able to bind RNA and determine whether RNA-binding was essential for HUSH function. We identify Periphilin as the major RNA-binding component of the HUSH complex and show that Periphilin's N-terminal domain is essential for both RNA binding and HUSH function. Periphilin binding to RNA was independent of its interaction with TASOR or MPP8, as its N-terminal domain was sufficient for RNA targeting. The artificial tethering of Periphilin, to a HUSH-insensitive, nascent transcript, enabled the HUSH-dependent silencing of the transcript. This tethering of Periphilin allowed the RNA-binding region of Periphilin to be removed such that only its C-terminal domain was required, for oligomerisation and interaction with TASOR. We therefore show that Periphilin is the predominant RNA-binding protein of the HUSH complex and this RNA-binding is essential for HUSH activity. Overall design: To characterize the endogenous targets of PPHLN1 binding we initially used UV-crosslinked RNA immunoprecipitation sequencing (UV-RIPseq) in 293T PPHLN1 and SETDB1 knockout cells, expressing a panel of HA tagged PPHLN1 constructs