Institut de recherches cliniques de Montréal, 110 Avenue des Pins Ouest, Montréal, QC, H2W 1R7, Canada.
Division of Experimental Medicine, McGill University, 1001 Decarie Boulevard, Montreal, QC, H4A 3J1, Canada.
Nat Commun. 2020 Nov 5;11(1):5609. doi: 10.1038/s41467-020-19435-z.
Polycomb Group (PcG) proteins organize chromatin at multiple scales to regulate gene expression. A conserved Sterile Alpha Motif (SAM) in the Polycomb Repressive Complex 1 (PRC1) subunit Polyhomeotic (Ph) has been shown to play an important role in chromatin compaction and large-scale chromatin organization. Ph SAM forms helical head to tail polymers, and SAM-SAM interactions between chromatin-bound Ph/PRC1 are believed to compact chromatin and mediate long-range interactions. To understand the underlying mechanism, here we analyze the effects of Ph SAM on chromatin in vitro. We find that incubation of chromatin or DNA with a truncated Ph protein containing the SAM results in formation of concentrated, phase-separated condensates. Ph SAM-dependent condensates can recruit PRC1 from extracts and enhance PRC1 ubiquitin ligase activity towards histone H2A. We show that overexpression of Ph with an intact SAM increases ubiquitylated H2A in cells. Thus, SAM-induced phase separation, in the context of Ph, can mediate large-scale compaction of chromatin into biochemical compartments that facilitate histone modification.
多梳抑制复合物 1（PRC1）亚基 Polyhomeotic（Ph）中的保守无菌α基序（SAM）已被证明在染色质紧缩和大规模染色质组织中发挥重要作用。Ph SAM 形成螺旋头对头聚合物，并且结合 Ph/PRC1 的染色质之间的 SAM-SAM 相互作用被认为可紧缩染色质并介导长距离相互作用。为了了解潜在的机制，我们在此在体外分析 Ph SAM 对染色质的影响。我们发现，用包含 SAM 的截断 Ph 蛋白孵育染色质或 DNA 会导致浓缩的、相分离的凝聚物的形成。Ph SAM 依赖性凝聚物可以从提取物中募集 PRC1，并增强 PRC1 对组蛋白 H2A 的泛素连接酶活性。我们表明，具有完整 SAM 的 Ph 的过表达会增加细胞中泛素化的 H2A。因此，在 Ph 的情况下，SAM 诱导的相分离可以介导染色质的大规模紧缩成生化隔室，从而促进组蛋白修饰。
