A family of Structural Maintenance of Chromosome (SMC) complexes is essential for key cellular processes ensuring proper cohesion, condensation and replication. They share a common SMC-kleisin architecture allowing them to embrace DNA. In SMC5/6, the NSE1 and NSE3 KITE and NSE4 kleisin subunits form a stable subcomplex that binds DNA and regulates essential processes. In addition, NSE5 and NSE6 subunits associate with the core SMC5/6 complex and recruit it to DNA repair sites.

The architecture of the SMC5/6 complex is crucial for its proper functioning, and mutations within the human SMC5/6 subunits result in severe syndromes. Therefore, we aimed to analyze interactions within the human SMC5/6 complex and determine its detailed architecture. Firstly, we analyzed different parts of SMC5/6 by crosslinking and MS/MS analysis. Our data suggested domain arrangements of hNSE1–hNSE3 and orientation of hNSE4 within the hNSE1–hNSE3–hNSE4 subcomplex. The crosslinking and electron microscopic analysis of the SMC5/6 core complex showed its rod-like architecture with juxtaposed hSMC5–hSMC6 arms. Additionally, we observed fully or partially opened hSMC5–hSMC6 shapes with the hNSE1–hNSE3–hNSE4 trimer localized in the SMC head domains. To complete mapping of the human SMC5/6 complex architecture, we analyzed positions of hNSE5-hNSE6 at the hSMC5–hSMC6 arms. We showed that hNSE6 binding to hNSE5 and the coiled-coil arm of hSMC6 is mediated by a conserved FAM178 domain, which we therefore renamed CANIN (Coiled-coil SMC6 And NSE5 INteracting) domain. Interestingly, hNSE6 bound both hSMC5 and hSMC6 arms, suggesting that hNSE6 may lock the arms and regulate the dynamics of the human SMC5/6 complex.

染色体（SMC）复合物的结构维护家族对于关键的细胞过程至关重要，以确保适当的凝聚，凝结和复制。它们共享一个通用的SMC-kleisin体系结构，从而可以拥抱DNA。在SMC5 / 6中，NSE1和NSE3 KITE以及NSE4 kleisin亚基形成一个稳定的亚复合物，该复合物结合DNA并调节基本过程。此外，NSE5和NSE6亚基与核心SMC5 / 6复合体结合并募集到DNA修复位点。

SMC5 / 6复合体的结构对其正常运行至关重要，而人SMC5 / 6亚基内的突变会导致严重的综合症。因此，我们旨在分析人SMC5 / 6复合体内的相互作用并确定其详细架构。首先，我们通过交联和MS / MS分析来分析SMC5 / 6的不同部分。我们的数据表明hNSE1–hNSE3的域排列和hNSE1–hNSE3–hNSE4子复合体中hNSE4的方向。SMC5 / 6核复合物的交联和电子显微镜分析表明，其棒状结构与并列的hSMC5-hSMC6臂。此外，我们观察到hSMC5–hSMC6形状完全或部分打开，其中hNSE1–hNSE3–hNSE4三聚体位于SMC头域中。要完成人类SMC5 / 6复杂体系结构的映射，我们分析了hNSE5-hNSE6在hSMC5-hSMC6臂上的位置。我们发现hNSE6与hNSE5和hSMC6的卷曲螺旋臂结合是由保守的FAM178域介导的，因此我们将其重命名为CANIN（Ç上油线圈SMC6甲ND Ñ SE5 IN teracting）结构域。有趣的是，hNSE6绑定了hSMC5和hSMC6臂，这表明hNSE6可以锁定臂并调节人类SMC5 / 6复合体的动力学。