Depletion or cleavage of cohesin during anaphase differentially affects chromatin structure and segregation

Chromosome segregation requires both the separation of sister chromatids and the sustained condensation of chromatids during anaphase. In yeast cells, cohesin is not only required for sister chromatid cohesion but also plays a major role in determining the structure of individual chromatids in metaphase. Separase cleavage is thought to remove all cohesin complexes from chromosomes to initiate anaphase. It is thus not clear how the length and organisation of segregating chromatids are maintained during anaphase in the absence of cohesin. Here we show that degradation of cohesin at the anaphase onset causes aberrant chromatid segregation. Hi-C analysis on segregating chromatids demonstrates that cohesin depletion causes loss of intrachromatid organisation. Surprisingly, TEV-mediated cleavage of cohesin does not dramatically disrupt chromatid organisation in anaphase, explaining why bulk segregation is achieved. In addition, we identified a small pool of cohesin complexes bound to telophase chromosomes in wildtype cells and show that they play a role in the organisation of centromeric regions. Our data demonstrate that in yeast cells, cohesin function is not over in metaphase, but extends to the anaphase period when chromatids are segregating.染色体分离既需要姐妹染色单体的分离，也依赖分裂后期中染色单体的持续凝缩。在酵母细胞中，黏连蛋白（cohesin）不仅是姐妹染色单体黏连所必需的分子，还在决定中期单个染色单体的结构方面发挥核心作用。既往研究认为，分离酶（Separase）的切割会将染色体上的所有黏连蛋白复合物移除，从而启动分裂后期。因此，在黏连蛋白缺失的情况下，分裂后期中分离中的染色单体的长度与组织结构如何维持，这一问题尚未明确。本研究证实，在分裂后期起始阶段降解黏连蛋白会导致染色单体分离异常。对分离中染色单体开展的Hi-C分析显示，黏连蛋白的缺失会导致染色单体内部组织结构的丧失。令人意外的是，经由烟草蚀刻病毒（TEV）介导的黏连蛋白切割，并未显著破坏分裂后期的染色单体组织结构，这解释了为何细胞能够完成整体染色体分离。此外，我们在野生型细胞的末期染色体上鉴定到了少量结合的黏连蛋白复合物，并证实其在着丝粒区域的组织构建中发挥作用。本研究数据表明，在酵母细胞中，黏连蛋白的功能并未在中期终止，而是延伸至染色单体正在分离的分裂后期阶段。