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, 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.

粘着蛋白复合物介导（姐妹染色单体内聚）之间和染色体内（DNA环化）两者之间的DNA-DNA相互作用。已经提出，染色体内环是通过DNA通过粘着剂环的内腔挤出而产生的。Scc2（Nipbl）刺激黏附素的ABC样ATPase，对于将黏附素加载到染色体上是必不可少的。但是，Scc2对粘着蛋白ATPase的刺激也可能具有后加载功能，例如驱动环挤出。使用光漂白（FRAP）和单分子跟踪后的荧光恢复，我们显示Scc2主要通过与黏附素的缔合而动态结合至染色质。Scc2在染色质中的运动与“走走停停”或“跳跃”运动一致。我们建议扩散系数低，