Post-translational modification of proteins by ubiquitin and ubiquitin-like modifiers, such as SUMO, are key events in protein homeostasis or DNA damage response. Smc5/6 is a nuclear multi-subunit complex that participates in the recombinational DNA repair processes and is required in the maintenance of chromosome integrity. Nse2 is a subunit of the Smc5/6 complex that possesses SUMO E3 ligase activity by the presence of a SP-RING domain that activates the E2~SUMO thioester for discharge on the substrate. Here we present the crystal structure of the SUMO E3 ligase Nse2 in complex with an E2-SUMO thioester mimetic. In addition to the interface between the SP-RING domain and the E2, the complex reveals how two SIM (SUMO-Interacting Motif) -like motifs in Nse2 are restructured upon binding the donor and E2-backside SUMO during the E3-dependent discharge reaction. Both SIM interfaces are essential in the activity of Nse2 and are required to cope with DNA damage.

泛素和泛素样修饰剂（如 SUMO）对蛋白质的翻译后修饰是蛋白质稳态或 DNA 损伤反应的关键事件。Smc5/6 是一种核多亚基复合体，参与重组 DNA 修复过程，是维持染色体完整性所必需的。Nse2 是 Smc5/6 复合物的一个亚基，它通过存在激活 E2~SUMO 硫酯以在底物上放电的 SP-RING 域而具有 SUMO E3 连接酶活性。在这里，我们展示了与 E2-SUMO 硫酯模拟物复合的 SUMO E3 连接酶 Nse2 的晶体结构。除了SP-RING域和E2之间的接口，该复合物揭示了 Nse2 中的两个 SIM（相扑相互作用基序）样基序如何在 E3 依赖性放电反应期间结合供体和 E2 背面相扑而重构。两个 SIM 接口对于 Nse2 的活动都是必不可少的，并且是应对 DNA 损伤所必需的。