近日,，國際學術(shù)期刊《生物化學雜志》JBC在線發(fā)表了中科院上海巴斯德所張巖研究組的最新研究成果：MLL5 Protein Regulates Cell Cycle Progression and E2F1-responsive Gene Expression via Association with HCF-1。該成果揭示了一種組蛋白H3K4甲基轉(zhuǎn)移酶MLL5蛋白調(diào)節(jié)細胞周期的新機制,，為探討MLL5調(diào)節(jié)造血干細胞“自我更新”分子作用機理做了前期研究,。
Mixed Lineage Leukemia 5 (MLL5)蛋白是Trithorax (TrxG)蛋白家族的重要成員，MLL5編碼基因定位在人染色體7q22區(qū)域,，是人類急性髓性白血?。ˋcute Myeloid Leukemia， AML）的重要致病基因,。該研究組前期通過對Mll5“基因敲除”的小鼠的研究發(fā)現(xiàn),，Mll5基因?qū)υ煅庖呒毎陌l(fā)生發(fā)育起著重要的調(diào)節(jié)作用，但其作用的具體分子機制并未被完全闡明,。
本研究中,，碩士研究生周培培和助理研究員王之龍等在張巖研究員的指導下，通過蛋白質(zhì)譜測序,，發(fā)現(xiàn)MLL5蛋白可以和細胞周期調(diào)節(jié)因子“Host Cell Factor 1”（HCF-1）相互結(jié)合,， MLL5蛋白中一個新發(fā)現(xiàn)的HBM結(jié)構(gòu)域和HCF-1蛋白的Kelch結(jié)構(gòu)域介導了二者的結(jié)合。激光共聚焦顯微鏡實驗發(fā)現(xiàn),，MLL5蛋白與HCF-1蛋白共定位在細胞核內(nèi)核仁外的區(qū)域,。
研究人員敲低MLL5蛋白后，發(fā)現(xiàn)細胞增殖變慢,，細胞周期阻滯在G1時期,，受E2F1調(diào)節(jié)的靶基因的表達水平與其啟動子區(qū)域組蛋白H3K4的三甲基化（H3K4me3）水平也顯著降低；敲低HCF-1蛋白后,，發(fā)現(xiàn)MLL5蛋白結(jié)合到E2F1靶基因啟動子區(qū)的能力減弱,，與其啟動子區(qū)域組蛋白H3K4的三甲基化（H3K4me3）水平也顯著降低。
此外,，與TrxG蛋白家族其它成員不同,，MLL5蛋白在行使H3K4三甲基化功能時，不依賴于包括ASH2L,、RBBP5和WDR5等核心蛋白組分形成的支架結(jié)構(gòu),，但卻包含氧連N-乙酰葡萄糖胺轉(zhuǎn)移酶（O-GlcNAc Transferase，OGT）,，提示MLL5使用了迥異于其它TrxG蛋白家族成員的分子機制來發(fā)揮其H3K4三甲基化酶的活性,。
研究人員由此推斷出，MLL5蛋白采用了一種全新的分子作用機制參與調(diào)節(jié)細胞周期,，即：MLL5蛋白通過與HCF-1蛋白相互結(jié)合,，被招募到E2F1靶基因的啟動子區(qū)域促進H3K4的三甲基化并促進E2F1靶基因的轉(zhuǎn)錄激活，從而促進細胞周期由G1期向S期的轉(zhuǎn)換,。該成果為進一步研究MLL5蛋白調(diào)節(jié)造血干細胞“自我更新”的分子作用機制打下了重要基礎,，并且為研究TrxG蛋白家族其它成員的功能提供了新的思路,。
該研究工作得到國家自然科學基金委、科技部重大研究計劃與中科院等項目經(jīng)費支持,。（生物谷Bioon.com）
doi:10.1074/jbc.M112.439729
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Mixed Lineage Leukemia 5 (MLL5) Regulates Cell Cycle Progression and E2F1-responsive Gene Expression via Association with Host Cell Factor-1 (HCF-1)
Peipei Zhou1, Zhilong Wang1, Xiujie Yuan1, Cuihong Zhou1, Lulu Liu1, Xiaoling Wan1, Feng Zhang1, Xiaodan Ding1, Chuangui Wang2, Sidong Xiong3, Zhen Wang4, Jinduo Yuan4, Qiang Li5 and Yan Zhang1*
Trithorax group proteins methylate lysine 4 of histone3 (H3K4) at active gene promoters. MLL5 protein, a member of the Trithorax protein family, has been implicated in the control of the cell cycle progression; however, the underlying molecular mechanism(s) have not been fully determined. In this study, we found that the MLL5 protein can associate with the cell cycle regulator host cell factor (HCF-1). The interaction between MLL5 and HCF-1 is mediated by the HCF-1 Binding Motif (HBM) of the MLL5 protein and the Kelch domain of the HCF-1 protein. Confocal microscopy showed that MLL5 protein largely colocalized with HCF-1 in the nucleus. Knockdown of MLL5 resulted in reduced cell proliferation and cell cycle arrest in the G1 phase. Moreover, down-regulation of the E2F1 target gene expression and decreased H3K4me3 levels at E2F1-responsive promoters were observed in MLL5 knockdown cells. Additionally, the core subunits, including ASH2L, RBBP5, and WDR5, that are necessary for effective H3K4 methyltransferase activities of the Trithorax protein complexes, were absent in the MLL5 complex, suggesting that a distinct mechanism may be used by MLL5 for exerting its H3K4 methyltransferase activity. Together, our findings demonstrate that MLL5 could associate with HCF-1 and then be recruited to E2F1-responsive promoters to stimulate H3K4 trimethylation and transcriptional activation, thereby facilitating the cell cycle G1-to-S phase transition.
