Complexes containing a pair of structural maintenance of chromosomes (SMC) family proteins are fundamental for the three-dimensional (3D) organization of genomes in all domains of life. The eukaryotic SMC complexes cohesin and condensin are thought to fold interphase and mitotic chromosomes, respectively, into large loop domains, although the underlying molecular mechanisms have remained unknown. We used cryo-EM to investigate the nucleotide-driven reaction cycle of condensin from the budding yeast Saccharomyces cerevisiae. Our structures of the five-subunit condensin holo complex at different functional stages suggest that ATP binding induces the transition of the SMC coiled coils from a folded-rod conformation into a more open architecture. ATP binding simultaneously triggers the exchange of the two HEAT-repeat subunits bound to the SMC ATPase head domains. We propose that these steps result in the interconversion of DNA-binding sites in the catalytic core of condensin, forming the basis of the DNA translocation and loop-extrusion activities.

含有一对染色体结构维持 (SMC) 家族蛋白的复合物是所有生命领域中基因组三维 (3D) 组织的基础。真核 SMC 复合物 cohesin 和 condensin 被认为分别将间期和有丝分裂染色体折叠成大环结构域，尽管潜在的分子机制仍然未知。我们使用cryo-EM研究了出芽酵母酿酒酵母中凝聚素的核苷酸驱动反应循环. 我们在不同功能阶段的五亚基凝聚素全息复合物的结构表明，ATP 结合诱导 SMC 卷曲线圈从折叠杆构象转变为更开放的结构。ATP 结合同时触发与 SMC ATPase 头部结构域结合的两个 HEAT 重复亚基的交换。我们建议这些步骤导致凝聚素催化核心中 DNA 结合位点的相互转化，形成 DNA 易位和环挤出活性的基础。