西亚试剂：Xist的基因沉默机制

Many long non-coding RNAs (lncRNAs) affect gene expression1, but the mechanisms by which they act are still largely unknown2. One of the best-studied lncRNAs is Xist, which is required for transcriptional silencing of one X chromosome during development in female mammals3, 4. Despite extensive efforts to define the mechanism of Xist-mediated transcriptional silencing, we still do not know any proteins required for this role3. The main challenge is that there are currently no methods to comprehensively define the proteins that directly interact with a lncRNA in the cell5. Here we develop a method to purify a lncRNA from cells and identify proteins interacting with it directly using quantitative mass spectrometry. We identify ten proteins that specifically associate with Xist, three of these proteins—SHARP, SAF-A and LBR—are required for Xist-mediated transcriptional silencing. We show that SHARP, which interacts with the SMRT co-repressor6 that activates HDAC37, is not only essential for silencing, but is also required for the exclusion of RNA polymerase II (Pol II) from the inactive X. Both SMRT and HDAC3 are also required for silencing and Pol II exclusion. In addition to silencing transcription, SHARP and HDAC3 are required for Xist-mediated recruitment of the polycomb repressive complex 2 (PRC2) across the X chromosome. Our results suggest that Xist silences transcription by directly interacting with SHARP, recruiting SMRT, activating HDAC3, and deacetylating histones to exclude Pol II across the X chromosome.

Xist是雌性哺乳动物一个X染色体的转录沉默所需的一个长非编码RNA。Mitchell Guttman及同事采用基于质谱的方法识别出与Xist直接发生相互作用的几种蛋白，其中包括转录抑制因子SHARP。SHARP和另一个抑制因子HDAC3是胚胎干细胞中整个X染色体上“多梳”(polycomb)的转录沉默和由Xist介导的招募所必需的。

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