Polycomb Group (PcG) proteins regulate gene expression by modifying chromatin. A key PcG complex, 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 homology region (HR) of PSC assembles into PRC1, including partnering with dRING to form the ubiquitin ligase for H2A. The intrinsically disordered C-terminal region of PSC (PSC-CTR) compacts chromatin, and inhibits chromatin remodeling and transcription in vitro. Both the PSC-HR and the PSC-CTR are essential in vivo. To understand how these two activities may be coordinated in PRC1, we used cross-linking mass spectrometry (XL-MS) to analyze the conformations of the PSC-CTR in PRC1 and how they change on binding DNA. XL-MS identifies interactions between the PSC-CTR and the core of PRC1, including between the PSC-CTR and PSC-HR. New contacts and overall more compacted PSC-CTR conformations are induced by DNA binding. Protein footprinting of accessible lysine residues in the PSC-CTR reveals an extended, bipartite candidate DNA/chromatin binding surface. Our data suggest a model in which DNA (or chromatin) follows a long path on the flexible PSC-CTR. Intramolecular interactions of the PSC-CTR detected by XL-MS 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 IDRs that bind nucleic acids.

中文翻译：

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DNA结合重组果蝇PRC1中PSC的固有无序C末端区域

Polycomb Group（PcG）蛋白通过修饰染色质来调节基因表达。关键的PcG复合物，Polycomb Repressive Complex 1（PRC1），具有两个活性：组蛋白H2A的泛素连接酶活性和染色质紧实活性。在果蝇中，PRC1的后性梳（PSC）亚基是这两种活动的中心。PSC的N端同源区域（HR）组装成PRC1，包括与dRING结合形成H2A的泛素连接酶。PSC（PSC-CTR）的固有无序C末端区域压实染色质，并在体外抑制染色质重塑和转录。PSC-HR和PSC-CTR在体内都是必不可少的。要了解在PRC1中如何协调这两项活动，我们使用交联质谱（XL-MS）分析PRC1中PSC-CTR的构象，以及它们在结合DNA上如何变化。XL-MS识别PSC-CTR和PRC1核心之间的交互，包括PSC-CTR和PSC-HR之间的交互。DNA结合可以诱导新的接触和更紧密的PSC-CTR构象。PSC-CTR中可接近的赖氨酸残基的蛋白质印迹显示了扩展的二分候选DNA /染色质结合表面。我们的数据提出了一个模型，其中DNA（或染色质）在柔性PSC-CTR上遵循很长的路径。通过XL-MS检测到的PSC-CTR的分子内相互作用可以使高亲和力DNA /染色质结合区靠近PRC1的核心，而不会破坏泛素连接酶和核小体之间的界面。