Structural maintenance of chromosomes (SMC) protein complexes are essential for the spatial organization of chromosomes¹. Whereas cohesin and condensin organize chromosomes by extrusion of DNA loops, the molecular functions of the third eukaryotic SMC complex, Smc5/6, remain largely unknown². Using single-molecule imaging, we show that Smc5/6 forms DNA loops by extrusion. Upon ATP hydrolysis, Smc5/6 reels DNA symmetrically into loops at a force-dependent rate of one kilobase pair per second. Smc5/6 extrudes loops in the form of dimers, whereas monomeric Smc5/6 unidirectionally translocates along DNA. We also find that the subunits Nse5 and Nse6 (Nse5/6) act as negative regulators of loop extrusion. Nse5/6 inhibits loop-extrusion initiation by hindering Smc5/6 dimerization but has no influence on ongoing loop extrusion. Our findings reveal functions of Smc5/6 at the molecular level and establish DNA loop extrusion as a conserved mechanism among eukaryotic SMC complexes.

染色体结构维持 (SMC) 蛋白复合物对于染色体的空间组织至关重要¹ 。虽然粘连蛋白和凝缩蛋白通过挤压 DNA 环来组织染色体，但第三个真核 SMC 复合物 Smc5/6 的分子功能仍然很大程度上未知² 。使用单分子成像，我们表明 Smc5/6 通过挤压形成 DNA 环。 ATP 水解后，Smc5/6 以每秒 1 个千碱基对的力依赖性速率将 DNA 对称地卷入环中。 Smc5/6 以二聚体形式挤出环，而单体 Smc5/6 沿着 DNA 单向易位。我们还发现亚基 Nse5 和 Nse6 (Nse5/6) 充当环挤出的负调节因子。 Nse5/6 通过阻碍 Smc5/6 二聚化来抑制环挤出起始，但对正在进行的环挤出没有影响。我们的研究结果揭示了 Smc5/6 在分子水平上的功能，并将 DNA 环挤出确立为真核 SMC 复合物中的保守机制。