Combinatorial Control of Recruitment of a Variant PRC1.6 Complex in Embryonic Stem Cells

Abstract

Though genetic data suggest that Polycomb group proteins (PcGs) are central chromatin modifiers and repressors that have been implicated in control of embryonic stem cell (ESC) pluripotency, the precise mechanism of PcG complex recruitment remains elusive, especially in mammals. We now report that the first and second MBT repeats of L3mbtl2 are important structural and functional features that are necessary and sufficient for L3mbtl2-mediated recruitment of PRC1.6 complex to target promoters. Interestingly, this region of L3mbtl2 harbors the evolutionarily conserved Pho-binding pocket also present in Drosophila Sfmbt, and mutation of the critical residues within this pocket completely abolishes its interaction with target promoters. Additionally, decreased PRC1.6 chromatin occupancy was observed following loss of individual components (L3mbtl2, Pcgf6, and Max) of the complex. Our findings suggest that the recruitment of noncanonical PRC1.6 complex in ESCs might be the result of L3mbtl2's interaction with multiple components of the complex.

The polycomb group protein Yaf2 regulates the pluripotency of embryonic stem cells in a phosphorylation-dependent manner

“…in Rybp Ϫ/Ϫ mESCs (Table S4). Of the 48 up-regulated genes with Ͼ10-fold increase in expression, 16 were strongly associated with germline development (meiosis and spermatogenesis), consistent with a role of Rybp in PRC1.6 activity (26,27). Further analysis revealed no significant overlap between the genes regulated by Yaf2 and Rybp ( Fig.…”

Functional Landscape of PCGF Proteins Reveals Both RING1A/B-Dependent-and RING1A/B-Independent-Specific Activities

“…It is important to highlight that PCGF6 binding was clearly affected but not completely abolished by E2F6 shRNA (Figures 6D, 6E, and 6H). Although this could be a consequence of an incomplete loss of E2F6 expression (Figure S9F), this result may also suggest that additional recruitment mechanisms (i.e., via L3MBTL2; Huang et al., 2018, Trojer et al., 2011) contribute to recruiting the PCGF6 complex to its specific target loci.…”

Histone H2AK119 Mono-Ubiquitination Is Essential for Polycomb-Mediated Transcriptional Repression

“…These PRC1 forms are defined as variant PRC1 (vPRC1) and are tethered to target loci by intrinsic DNA binding activities. This includes PRC1.1 recognition of unmethylated CpG di-nucleotides by the KDM2B subunit (Farcas et al, 2012); PRC1.6 recognition of E-BOX and E2F DNA elements by the MAX/MGA and E2F6/DP dimers stably associated with the complex (Huang et al, 2018;Scelfo et al, 2019;Stielow et al, 2018); and PRC1.3 (and likely PRC1.5) by the recognition of an E-BOX variant directly bound by the USF1/2 transcription factors that can interact with and recruit the PRC1.3 complex to chromatin (Scelfo et al, 2019). Overall, this involves the cooperative activity of both cPRC1 and vPRC1 forms at repressed sites together with the exclusive presence of vPRC1 forms (PRC1.6 and PRC1.3) at several highly expressed genes.…”