Dissecting molecular mechanisms mediated by Polycomb group proteins in maintenance and differentiation of ES cells

两个不同的多梳复合体，PRC1和PRC2，合作维持对胚胎干细胞(ESCs)关键发育位点的表观遗传抑制。尽管各自的络合物催化组蛋白H2A的单一泛素化(H2Aub1)和H3赖氨酸27的三甲基化(H3K27me3)，但这些修饰对Polycomb抑制的影响程度仍然存在争议。为了解决这个问题，我们对缺乏催化H2Aub1的能力或无效结合H3K27me3的PrC1突变体进行了表征。我们发现，无催化活性的prc1突变体保持了在Hox基因座压缩染色质的能力，但对于促进靶基因的抑制和维持ESCs处于未分化状态是必不可少的。我们进一步证明，PRC1的定位和H_2Aub1的沉积最多不是所有的靶点，而是取决于核心的PRC1亚基CBX2对H3K27me3的物理识别。我们的数据表明，异质性机制，包括染色质紧凑和泛素化，以及依赖于PRC2和不依赖于PRC2的招募模式是PRC1功能的基础。在ES细胞的自我更新和分化过程中，这种不同的感知机制和效应器可能需要适当地将PRC1装载到不同的靶基因上，和/或保持一种抑制状态，这种状态对发育信号具有很强的响应性。

Two distinct Polycomb complexes, PRC1 and PRC2, collaborate to maintain epigenetic repression of key developmental loci in embryonic stem cells (ESCs). Although the respective complexes catalyze mono-ubiquitination of histone H2A (H2Aub1) and trimethylation of H3 lysine 27 (H3K27me3), the extent which these modifications contribute to Polycomb repression remains controversial. To address this issue, we characterized PRC1 mutants that either lack the ability to catalyze H2Aub1 or ineffectively bind H3K27me3. We find that the catalytically-inactive PRC1 mutant maintains the ability to compact chromatin at the Hox loci, but is nonetheless essential to facilitate repression of target genes and to maintain ESCs in an undifferentiated state. We further show that PRC1 localization and H2Aub1 deposition at most but not all target loci depends on physical recognition of H3K27me3 by CBX2, a core PRC1 subunit. Our data suggest that heterogeneous mechanisms, including chromatin compaction and ubiquitination, as well as PRC2-dependent and PRC2-independent recruitment modes underlie PRC1 function. Such diverse sensing mechanisms and effectors might be required for appropriate loading of PRC1 onto heterogeneous target loci, and/or to maintain a repressive state that is robust yet highly responsive to developmental cues during ES cell self-renewal and differentiation.