| Polycomb group (PcG) proteins are chromatin modulators that exert essential functions during Drosophila development by maintaining the spatial and temporal expression of Hox genes through transcriptional repression. Several PcG protein complexes have been identified in Drosophila, including Pleiohomeotic (PHO) repressive complex (PhoRC) and Polycomb repressive complex 1 (PRC1). PhoRC consists of PHO and SFMBT, an MBT-domain containing protein that binds to mono- and di-methylated histone lysines. Drosophila PRC1 contains four core components: RING, a histone H2A mono-ubiquitin ligase, Posterior Sex Combs (PSC), Polyhomeotic (PH), and Polycomb (PC), which binds to tri-methylated histone H3 lysine 27 (H3K27me3). The key role of these complexes as epigenetic regulators is conserved in mammals, but our understanding of their roles has been hindered by the existence of multiple homologues for each of the core components, which assemble in a combinatorial fashion, giving rise to a variety of protein complexes, likely with different functions. To shed some light on this complexity, we have undertaken a comprehensive characterization of mammalian PRC1 family and SFMBT-containing complexes using biochemical and genomic approaches. We show that SFMBT1 forms a stable complex with LSD1 and CoREST that represses transcription in somatic cells and during spermatogenesis. The interplay between the repressive SFMBT1-LSD1-CoREST complex and RNA polymerase II contributes to the timely transcriptional regulation of histone genes in human cells. We also show that PCGFs, CBXs and RYBP, the mammalian homologues of Drosophila PSC, PC, and RYBP, define PRC1 family of complexes. Six major groups of PRC1 all contain the RING1A/B ubiquitin ligase, but each group is composed of unique sets of associated polypeptides and displaying distinctive genomic localization. Importantly, our data reveals that only a small subset of mammalian PRC1 co-localize with the H3K27me3 mark on chromatin, which was previously thought to be an essential requirement for PRC1 recruitment. Moreover, the association of PRC1 complexes with RYBP stimulates the enzymatic activity of RING1B while preventing the incorporation of other subunits such as CBX, PHC and SCM, suggesting further functional specialization. These studies have broadened our understanding of the biochemical organization, genomic localization, and molecular functions of mammalian PcG complexes. |
