Live-cell imaging reveals a stable cohesin-chromatin interaction after but not before DNA replication

Cohesin is a multisubunit protein complex that links sister chromatids from replication until segregation. The lack of obvious cohesin-targeting-specific sequences on DNA, as well as cohesin's molecular arrangement as a large ring, has led to the working hypothesis that cohesin acts as a direct topological linker. To preserve the identity of sister chromatids, such a linkage would need to stably persist throughout the entire S and G2 phases of the cell cycle. Unexpectedly, cohesin binds chromatin already in telophase, and a large fraction dissociates from chromosomes during prophase in a phosphorylation-dependent manner, whereas initiation of anaphase requires proteolytic cleavage of only a small fraction of cohesin. These observations raised the question of how and when cohesin interacts with chromatin during the cell cycle. Here, we report a cell-cycle dependence in the stability of cohesin binding to chromatin. Using photobleaching and quantitative live-cell imaging, we identified several cohesin pools with different chromatin binding stabilities. Although all chromatin bound cohesin dissociated after a mean residence time of less than 25 min before replication, about one-third of cohesin was bound much more stably after S phase and persisted until metaphase, consistent with long-lived links mediating cohesion between sister chromatids.