Uncovering Global Non-Covalent SUMO Binder Networks Reveals that Sumoylation enhances XRCC4 activitythe stabilization of the classical Non Homologous End Joining complex.

In contrast to our extensive knowledge on covalent SUMO target proteins, we are limited in our understanding of proteins that bind SUMO family members in a non-covalent manner. We have identified interactors of different SUMO isoforms: monomeric SUMO1, monomeric SUMO2 or linear trimeric SUMO2 chains, using a mass spectrometry-based proteomics approach. We identified 382 proteins that bind to different SUMO isoforms mainly in a preferential manner. Interestingly, XRCC4 was the only DNA repair protein in our screen with a preference for SUMO2 trimers over mono-SUMO2 as well as the only protein in our screen that belongs to the Non-Homologous End Joining (NHEJ) DNA double-strand break repair pathway. A functional SIM in XRCC4 regulated its recruitment to local sites of DNA damage and its phosphorylation in S320 by DNA-PKcs. Combined, our data highlight the importance of non-covalent and covalent sumoylation for DNA double-strand break repair via the NHEJ pathway and provides a resource of SUMO isoform interactors.

与我们对共价结合型小泛素样修饰蛋白（Small Ubiquitin-like Modifier, SUMO）靶蛋白的深入认知形成鲜明对比的是，学界对以非共价方式结合SUMO家族成员的蛋白质的认知仍较为匮乏。本研究采用基于质谱的蛋白质组学方法，筛选鉴定了三类SUMO亚型的互作蛋白：单体SUMO1、单体SUMO2以及线性三聚体SUMO2链。本研究共鉴定得到382种可优先结合不同SUMO亚型的蛋白质。值得注意的是，X射线修复交叉互补蛋白4（X-ray repair cross-complementing protein 4, XRCC4）是本次筛选中唯一偏好结合SUMO2三聚体而非单体SUMO2的DNA修复蛋白，同时也是本次筛选中唯一属于非同源末端连接（Non-Homologous End Joining, NHEJ）DNA双链断裂修复通路的蛋白质。XRCC4中存在功能型SUMO相互作用基序（SUMO-interacting motif, SIM），该基序可调控其向DNA损伤局部位点的招募过程，以及其在S320位点被DNA依赖性蛋白激酶催化亚基（DNA-dependent protein kinase catalytic subunit, DNA-PKcs）的磷酸化修饰。综上，本研究数据揭示了非共价与共价SUMO化修饰在通过NHEJ通路完成DNA双链断裂修复中的重要作用，并为SUMO亚型互作蛋白提供了数据集资源。