The cohesin complex mediates DNA-DNA interactions both between (sister chromatid cohesion) and within chromosomes (DNA looping). It has been suggested that intra-chromosome loops are generated by extrusion of DNAs through the lumen of cohesin’s ring. Scc2 (Nipbl) stimulates cohesin’s ABC-like ATPase and is essential for loading cohesin onto chromosomes. However, it is possible that the stimulation of cohesin’s ATPase by Scc2 also has a post-loading function, for example driving loop extrusion. Using fluorescence recovery after photobleaching (FRAP) and single-molecule tracking in human cells, we show that Scc2 binds dynamically to chromatin, principally through an association with cohesin. Scc2’s movement within chromatin is consistent with a 'stop-and-go' or 'hopping' motion. We suggest that a low diffusion coefficient, a low stoichiometry relative to cohesin, and a high affinity for chromosomal cohesin enables Scc2 to move rapidly from one chromosomal cohesin complex to another, performing a function distinct from loading.

中文翻译：

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加载后 Scc2/Nipbl 在染色体粘连蛋白环之间跳跃


粘连蛋白复合物介导染色体之间（姐妹染色单体粘连）和染色体内部（DNA 环）的 DNA-DNA 相互作用。有人提出，染色体内环是通过将 DNA 挤出粘连蛋白环的内腔而产生的。 Scc2 (Nipbl) 刺激粘连蛋白的 ABC 样 ATP 酶，对于将粘连蛋白加载到染色体上至关重要。然而，Scc2 对粘连蛋白 ATP 酶的刺激也可能具有后加载功能，例如驱动环挤出。利用人类细胞中的光漂白后荧光恢复 (FRAP) 和单分子追踪，我们发现 Scc2 主要通过与粘连蛋白的结合动态地与染色质结合。 Scc2 在染色质内的运动与“走走停停”或“跳跃”运动一致。我们认为，低扩散系数、相对于粘连蛋白的低化学计量以及对染色体粘连蛋白的高亲和力使得 Scc2 能够快速从一个染色体粘连蛋白复合物移动到另一个染色体粘连蛋白复合物，执行与加载不同的功能。