Klymenko Tetyana, Papp Bernadett, Fischle Wolfgang, Köcher Thomas, Schelder Malgorzata, Fritsch Cornelia, Wild Brigitte, Wilm Matthias, Müller Jürg
Gene Expression Programme, European Molecular Biology Laboratory, 69117 Heidelberg, Germany.
Genes Dev. 2006 May 1;20(9):1110-22. doi: 10.1101/gad.377406. Epub 2006 Apr 17.
Polycomb response elements (PREs) are specific cis-regulatory sequences needed for transcriptional repression of HOX and other target genes by Polycomb group (PcG) proteins. Among the many PcG proteins known in Drosophila, Pho is the only sequence-specific DNA-binding protein. To gain insight into the function of Pho, we purified Pho protein complexes from Drosophila embryos and found that Pho exists in two distinct protein assemblies: a Pho-dINO80 complex containing the Drosophila INO80 nucleosome-remodeling complex, and a Pho-repressive complex (PhoRC) containing the uncharacterized gene product dSfmbt. Analysis of PhoRC reveals that dSfmbt is a novel PcG protein that is essential for HOX gene repression in Drosophila. PhoRC is bound at HOX gene PREs in vivo, and this targeting strictly depends on Pho-binding sites. Characterization of dSfmbt protein shows that its MBT repeats have unique discriminatory binding activity for methylated lysine residues in histones H3 and H4; the MBT repeats bind mono- and di-methylated H3-K9 and H4-K20 but fail to interact with these residues if they are unmodified or tri-methylated. Our results establish PhoRC as a novel Drosophila PcG protein complex that combines DNA-targeting activity (Pho) with a unique modified histone-binding activity (dSfmbt). We propose that PRE-tethered PhoRC selectively interacts with methylated histones in the chromatin flanking PREs to maintain a Polycomb-repressed chromatin state.
多梳反应元件（PREs）是多梳蛋白家族（PcG）蛋白对HOX及其他靶基因进行转录抑制所必需的特定顺式调控序列。在果蝇中已知的众多PcG蛋白中，Pho是唯一具有序列特异性的DNA结合蛋白。为深入了解Pho的功能，我们从果蝇胚胎中纯化了Pho蛋白复合物，发现Pho存在于两种不同的蛋白组装体中：一种是包含果蝇INO80核小体重塑复合物的Pho-dINO80复合物，另一种是包含未鉴定基因产物dSfmbt的Pho抑制复合物（PhoRC）。对PhoRC的分析表明，dSfmbt是一种新型的PcG蛋白，对果蝇中HOX基因的抑制至关重要。PhoRC在体内与HOX基因的PREs结合，且这种靶向作用严格依赖于Pho结合位点。对dSfmbt蛋白的特性分析表明，其MBT重复序列对组蛋白H3和H4中的甲基化赖氨酸残基具有独特的识别结合活性；MBT重复序列可结合单甲基化和二甲基化的H3-K9和H4-K20，但如果这些残基未被修饰或三甲基化，则无法与之相互作用。我们的研究结果表明，PhoRC是一种新型的果蝇PcG蛋白复合物，它将DNA靶向活性（Pho）与独特的修饰组蛋白结合活性（dSfmbt）结合在一起。我们提出，与PRE相连的PhoRC与PRE侧翼染色质中的甲基化组蛋白选择性相互作用，以维持多梳抑制的染色质状态。
