Wang Xueyin, Paucek Richard D, Gooding Anne R, Brown Zachary Z, Ge Eva J, Muir Tom W, Cech Thomas R
Department of Chemistry & Biochemistry, BioFrontiers Institute, University of Colorado Boulder, Boulder, Colorado, USA.
Howard Hughes Medical Institute, University of Colorado Boulder, Boulder, Colorado, USA.
Nat Struct Mol Biol. 2017 Dec;24(12):1028-1038. doi: 10.1038/nsmb.3487. Epub 2017 Oct 23.
Many studies have revealed pathways of epigenetic gene silencing by Polycomb repressive complex 2 (PRC2) in vivo, but understanding the underlying molecular mechanisms requires biochemistry. Here we analyze interactions of reconstituted human PRC2 with nucleosome complexes. Histone modifications, the H3K27M cancer mutation, and inclusion of JARID2 or EZH1 in the PRC2 complex have unexpectedly minor effects on PRC2-nucleosome binding. Instead, protein-free linker DNA dominates the PRC2-nucleosome interaction. Specificity for CG-rich sequences is consistent with PRC2 occupying CG-rich DNA in vivo. PRC2 preferentially binds methylated DNA regulated by its AEBP2 subunit, suggesting how DNA and histone methylation collaborate to repress chromatin. We find that RNA, known to inhibit PRC2 activity, is not a methyltransferase inhibitor per se. Instead, RNA sequesters PRC2 from nucleosome substrates, because PRC2 binding requires linker DNA, and RNA and DNA binding are mutually exclusive. Together, we provide a model for PRC2 recruitment and an explanation for how actively transcribed genomic regions bind PRC2 but escape silencing.
许多研究揭示了体内多梳抑制复合物2（PRC2）介导的表观遗传基因沉默途径，但要了解其潜在的分子机制还需要生物化学研究。在此，我们分析了重组人PRC2与核小体复合物的相互作用。组蛋白修饰、H3K27M癌症突变以及在PRC2复合物中加入JARID2或EZH1对PRC2与核小体的结合产生的影响出人意料地小。相反，无蛋白的连接DNA在PRC2与核小体的相互作用中起主导作用。对富含CG序列的特异性与PRC2在体内占据富含CG的DNA一致。PRC2优先结合由其AEBP2亚基调控的甲基化DNA，这表明了DNA和组蛋白甲基化如何协同抑制染色质。我们发现，已知能抑制PRC2活性的RNA本身并不是甲基转移酶抑制剂。相反，RNA将PRC2从核小体底物中隔离出来，因为PRC2的结合需要连接DNA，而RNA和DNA的结合是相互排斥的。我们共同提供了一个PRC2募集模型，并解释了活跃转录的基因组区域如何结合PRC2但逃脱沉默。
