Polycomb Group proteins regulate gene expression by modifying chromatin. Polycomb Repressive Complex 1 (PRC1) has two activities: a ubiquitin ligase activity for histone H2A and a chromatin compacting activity. In Drosophila, the Posterior Sex Combs (PSC) subunit of PRC1 is central to both activities. The N-terminal of PSC assembles into PRC1, including partnering with dRING to form the ubiquitin ligase. The intrinsically disordered C-terminal region of PSC compacts chromatin and inhibits chromatin remodeling and transcription in vitro. Both regions of PSC are essential in vivo. To understand how these two activities may be coordinated in PRC1, we used crosslinking mass spectrometry to analyze the conformations of the C-terminal region of PSC in PRC1 and how they change on binding DNA. Crosslinking identifies interactions between the C-terminal region of PSC and the core of PRC1, including between N and C-terminal regions of PSC. New contacts and overall more compacted PSC C-terminal region conformations are induced by DNA binding. Protein footprinting of accessible lysine residues reveals an extended, bipartite candidate DNA/chromatin binding surface in the C-terminal region of PSC. Our data suggest a model in which DNA (or chromatin) follows a long path on the flexible disordered region of PSC. Intramolecular interactions of PSC detected by crosslinking can bring the high-affinity DNA/chromatin binding region close to the core of PRC1 without disrupting the interface between the ubiquitin ligase and the nucleosome. Our approach may be applicable to understanding the global organization of other large intrinsically disordered regions that bind nucleic acids.

Polycomb Group 蛋白通过修饰染色质来调节基因表达。Polycomb Repressive Complex 1 (PRC1) 具有两种活性：组蛋白 H2A 的泛素连接酶活性和染色质压缩活性。在果蝇中，PRC1 的后性梳 (PSC) 亚基是这两种活动的核心。PSC 的 N 端组装成 PRC1，包括与 dRING 合作形成泛素连接酶。PSC 本质上无序的 C 端区域会压缩染色质并抑制体外染色质重塑和转录。PSC 的两个区域在体内都是必不可少的. 为了了解这两种活动如何在 PRC1 中协调，我们使用交联质谱分析了 PRC1 中 PSC C 端区域的构象以及它们如何在结合 DNA 上发生变化。交联识别 PSC 的 C 端区域与 PRC1 核心之间的相互作用，包括 PSC 的 N 和 C 端区域之间的相互作用。DNA 结合诱导了新的接触和整体更紧凑的 PSC C 端区域构象。可接近的赖氨酸残基的蛋白质足迹揭示了 PSC C 末端区域中扩展的二分候选 DNA/染色质结合表面。我们的数据表明了一个模型，其中 DNA（或染色质）在 PSC 的柔性无序区域上遵循一条长路径。通过交联检测到的 PSC 的分子内相互作用可以使高亲和力 DNA/染色质结合区域靠近 PRC1 的核心，而不会破坏泛素连接酶和核小体之间的界面。我们的方法可能适用于理解结合核酸的其他大的内在无序区域的全局组织。