近日，国际学术期刊《生物化学杂志》JBC在线发表了中科院上海巴斯德所张岩研究组的最新研究成果：MLL5 Protein Regulates Cell Cycle Progression and E2F1-responsive Gene Expression via Association with HCF-1。该成果揭示了一种组蛋白H3K4甲基转移酶MLL5蛋白调节细胞周期的新机制，为探讨MLL5调节造血干细胞“自我更新”分子作用机理做了前期研究。
Mixed Lineage Leukemia 5 (MLL5)蛋白是Trithorax (TrxG)蛋白家族的重要成员，MLL5编码基因定位在人染色体7q22区域，是人类急性髓性白血病（Acute Myeloid Leukemia， AML）的重要致病基因。该研究组前期通过对Mll5“基因敲除”的小鼠的研究发现，Mll5基因对造血免疫细胞的发生发育起着重要的调节作用，但其作用的具体分子机制并未被完全阐明。
本研究中，硕士研究生周培培和助理研究员王之龙等在张岩研究员的指导下，通过蛋白质谱测序，发现MLL5蛋白可以和细胞周期调节因子“Host Cell Factor 1”（HCF-1）相互结合， MLL5蛋白中一个新发现的HBM结构域和HCF-1蛋白的Kelch结构域介导了二者的结合。激光共聚焦显微镜实验发现，MLL5蛋白与HCF-1蛋白共定位在细胞核内核仁外的区域。
研究人员敲低MLL5蛋白后，发现细胞增殖变慢，细胞周期阻滞在G1时期，受E2F1调节的靶基因的表达水平与其启动子区域组蛋白H3K4的三甲基化（H3K4me3）水平也显著降低；敲低HCF-1蛋白后，发现MLL5蛋白结合到E2F1靶基因启动子区的能力减弱，与其启动子区域组蛋白H3K4的三甲基化（H3K4me3）水平也显著降低。
此外，与TrxG蛋白家族其它成员不同，MLL5蛋白在行使H3K4三甲基化功能时，不依赖于包括ASH2L、RBBP5和WDR5等核心蛋白组分形成的支架结构，但却包含氧连N-乙酰葡萄糖胺转移酶（O-GlcNAc Transferase，OGT），提示MLL5使用了迥异于其它TrxG蛋白家族成员的分子机制来发挥其H3K4三甲基化酶的活性。
研究人员由此推断出，MLL5蛋白采用了一种全新的分子作用机制参与调节细胞周期，即：MLL5蛋白通过与HCF-1蛋白相互结合，被招募到E2F1靶基因的启动子区域促进H3K4的三甲基化并促进E2F1靶基因的转录激活，从而促进细胞周期由G1期向S期的转换。该成果为进一步研究MLL5蛋白调节造血干细胞“自我更新”的分子作用机制打下了重要基础，并且为研究TrxG蛋白家族其它成员的功能提供了新的思路。
该研究工作得到国家自然科学基金委、科技部重大研究计划与中科院等项目经费支持。（生物谷Bioon.com）
doi:10.1074/jbc.M112.439729
PMC:
PMID:
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.
