近日，国际杂志Nucleic Acids Research刊登了北京生命科学研究所叶克穷实验室和首都师范大学许兴智实验室合作的最新研究成果“Structural mechanism of the phosphorylation-dependent dimerization of the MDC1 forkhead-associated domain，”文章中，作者揭示了哺乳动物修复因子MDC1自身二聚化调节新的分子机制。
编码基因的DNA分子时刻会受各种因子(如紫外线、自由基)的破坏发生损伤，而细胞能监视和修复各种DNA损伤。DNA双链断裂是最严重的损伤，在断裂发生后，ATM蛋白激酶得到激活，磷酸化一系列底物，包括断裂位点附近的组蛋白H2AX。MDC1是哺乳动物修复DNA双链断裂的重要因子，它通过其羰端BRCT结构域识别磷酸化的H2AX，从而定位在断裂位点，它同时招募多个修复因子参与修复过程。MDC1氮端含有一个能结合磷酸化苏氨酸的FHA结构域，但其功能仍不清楚。
该论文通过蛋白晶体学、生物化学和细胞生物学等多种手段发现，在DNA损伤发生后，FHA结构域介导依赖于MDC1磷酸化的二聚化。作者发现MDC1 FHA结构域自身能形成不太稳定的二聚体。在DNA损伤后，ATM激酶磷酸化MDC1第4位的苏氨酸，然后FHA结构域反式结合另一个MDC1分子上磷酸化的第4位苏氨酸，从而形成稳定的二聚体。作者还通过细胞学实验发现，MDC1二聚化能促进其定位于DNA损伤位点。该工作揭示了MDC1 功能受其自身二聚化调节的新的分子机制。
首都师范大学的刘金平和我所研究生罗树坤为本文的共同第一作者。叶克穷博士和首都师范大学的许兴智教授为本文的通讯作者。在北京生命科学研究所的研究工作受到科技部和北京市政府的支持。（生物谷Bioon.com）
doi:10.1093/nar/gkr1296
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Structural mechanism of the phosphorylation-dependent dimerization of the MDC1 forkhead-associated domain
Jinping Liu1, Shukun Luo2,3, Hongchang Zhao1, Ji Liao1, Jing Li1, Chunying Yang4, Bo Xu4, David F. Stern5, Xingzhi Xu1,* and Keqiong Ye3,*
MDC1 is a key mediator of the DNA-damage response in mammals with several phosphorylation-dependent protein interaction domains. The function of its N-terminal forkhead-associated (FHA) domain remains elusive. Here, we show with structural, biochemical and cellular data that the FHA domain mediates phosphorylation-dependent dimerization of MDC1 in response to DNA damage. Crystal structures of the FHA domain reveal a face-to-face dimer with pseudo-dyad symmetry. We found that the FHA domain recognizes phosphothreonine 4 (pT4) at the N-terminus of MDC1 and determined its crystal structure in complex with a pT4 peptide. Biochemical analysis further revealed that in the dimer, the FHA domain binds in trans to pT4 from the other subunit, which greatly stabilizes the otherwise unstable dimer. We show that T4 is phosphorylated primarily by ATM upon DNA damage. MDC1 mutants with the FHA domain deleted or impaired in its ability to dimerize formed fewer foci at DNA-damage sites, but the localization defect was largely rescued by an artificial dimerization module, suggesting that dimerization is the primary function of the MDC1 FHA domain. Our results suggest a novel mechanism for the regulation of MDC1 function through T4 phosphorylation and FHA-mediated dimerization.
