CONTROL OF TRANSCRIPTIONAL IDENTITY BY POLYCOMB GROUP PROTEINS

Establishment and maintenance of transcriptional identity is the cornerstone of tissue organization and normal development in multicellular organisms. By establishing facultative heterochromatin, Polycomb Repressive Complex 1 (PRC1) and Polycomb Repressive Complex 2 (PRC2) constitute the main dynamic and plastic mechanism to maintain gene repression in eukaryotic cells. PRC1 and PRC2 modify histones by depositing H2AK119Ub1 and H3K27me3, respectively, and both can be further subdivided in multiple sub-complexes based on their biochemical composition. In particular, PRC1 can be subdivided in 6 different sub-complexes depending on which PCGF interacts with its catalytic subunits RING1A/B (PCGF1-6). It is not clear if these sub-complexes act redundantly to maintain PRC1 function or if instead they have specific activities. It is also under debate which is the role of its catalytic product, H2AK119Ub1, in establishing PcG-mediated gene repression. Here, by combining the use of engineered mouse embryonic stem cell lines (mESC) and transgenic mice, we have been able to delete individual PRC1 sub-complexes in vitro and in vivo to assess their roles in orchestrating PRC1 function. Our data show that individual PRC1 sub-complexes regulate distinct biological processes and are able to maintain binding specificity both in vitro and in vivo. We also demonstrate that PRC1.1 and PRC1.2/4 are responsible for the maintenance of most of H2AK119Ub1 levels associated with promoter regions, however, the residual PRC1 activity from PRC1.3/5 and PRC1.6 is enough to maintain low H2AK119Ub1 levels and PcG-mediated gene repression. Finally, by using a fully catalytic inactive PRC1 in mESC, we demonstrate that H2AK119Ub1 constitutes the central hub of PcG-mediated gene repression. Complete loss of H2AK119Ub1 abolishes PcG-mediated gene repression and induces the disassembly of PcG chromatin domains by displacing PRC2.2 and cPRC1 from chromatin. Importantly, removal of PRC2 and H3K27me3 does not phenocopy H2AK119Ub1 loss, demonstrating that H2AK119Ub1 is central to PcG system function.