Department of Biochemistry, Oxford University, Oxford, United Kingdom.
Istituto Scientifico Ospedale San Raffaele, Centro di Imaging Sperimentale, Milano, Italy.
Elife. 2017 Sep 15;6:e30000. doi: 10.7554/eLife.30000.
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.
着丝粒复合物介导 DNA-DNA 相互作用，包括（姐妹染色单体黏合）和染色体内（DNA 环化）。有人提出，染色体内环是通过将 DNA 挤出着丝粒环的腔而产生的。Scc2（Nipbl）刺激着丝粒复合物的 ABC 样 ATP 酶，对于将着丝粒加载到染色体上是必不可少的。然而，Scc2 对着丝粒 ATP 酶的刺激也可能具有加载后的功能，例如驱动环化。我们使用人细胞中的光漂白后荧光恢复（FRAP）和单分子追踪技术表明，Scc2 通过与着丝粒复合物的结合，在染色质上动态结合。Scc2 在染色质内的运动与“停走”或“跳跃”运动一致。我们认为，低扩散系数、与着丝粒复合物的低比例以及对染色体着丝粒复合物的高亲和力使 Scc2 能够快速从一个染色体着丝粒复合物转移到另一个复合物，从而发挥不同于加载的功能。
