Structural maintenance of chromosome (SMC) protein complexes are the key organizers of the spatiotemporal structure of chromosomes. The condensin SMC complex has recently been shown to be a molecular motor that extrudes large loops of DNA, but the mechanism of this unique motor remains elusive. Using atomic force microscopy, we show that budding yeast condensin exhibits mainly open ‘O’ shapes and collapsed ‘B’ shapes, and it cycles dynamically between these two states over time, with ATP binding inducing the O to B transition. Condensin binds DNA via its globular domain and also via the hinge domain. We observe a single condensin complex at the stem of extruded DNA loops, where the neck size of the DNA loop correlates with the width of the condensin complex. The results are indicative of a type of scrunching model in which condensin extrudes DNA by a cyclic switching of its conformation between O and B shapes.

染色体结构维持 (SMC) 蛋白复合物是染色体时空结构的关键组织者。凝聚素 SMC 复合物最近被证明是一种分子马达，可以挤出大的 DNA 环，但这种独特马达的机制仍然难以捉摸。使用原子力显微镜，我们显示出芽酵母凝聚素主要表现出开放的“O”形和折叠的“B”形，并且随着时间的推移，它在这两种状态之间动态循环，ATP 结合诱导 O 到 B 的转变。凝聚素通过其球状结构域和铰链结构域结合 DNA。我们在挤出的 DNA 环的茎上观察到一个单一的凝聚素复合物，其中 DNA 环的颈部大小与凝聚素复合物的宽度相关。