Correlated protein-RNA associations and a requirement for HNRNPU in the long-range recruitment of Polycomb Repressive Complexes by the lncRNAs Airn and Kcnq1ot1

The lncRNAs Airn and Kcnq1ot1 recruit Polycomb Repressive Complexes (PRCs) and repress genes over multi-megabase genomic intervals, but how they interact with proteins to direct repression remains poorly understood. We conducted formaldehyde-linked RNA-immunoprecipitations (RIPs) of 27 proteins from mouse trophoblast stem cells (TSCs), using a protocol exhibiting similar signal-to-non-specific signal and post-lysis reassociation ratios as crosslinking immunoprecipitation (CLIP) and crosslinking affinity purification (CLAP). Patterns of protein associations across Airn and Kcnq1ot1 were more similar to each other than to nearly all other transcripts and partitioned to extents that mirrored the degree of repression each lncRNA induced, implying connections to mechanism. Indeed, HNRNPU, a factor essential for Xist's localization to chromatin, was enriched over Airn and Kcnq1ot1 and required to maintain normal levels of PRC1- and PRC2-directed chromatin modifications across the Airn and Kcnq1ot1 target domains, yet was dispensable for both lncRNAs’ localization to chromatin and for their association with PRC1. HNRNPU depletion caused a greater reduction in PRC-directed chromatin modifications and gene repression across the inactive X chromosome and the ~15 Mb Airntarget domain than across the ~3 Mb Kcnq1ot1 domain. Perhaps relatedly, HNRNPU depletion significantly reduced overall levels of the Xist and Airn but not the Kcnq1ot1 lncRNAs. Our study reports architectures of protein association along Airn and Kcnq1ot1 compared to the transcriptome at large, highlights shared and distinct features between the two lncRNAs, and provides new perspective on the role of HNRNPU in long-range chromatin regulation by lncRNAs.