Rapid movement and transcriptional re‐localization of human cohesin on DNA

Abstract

The spatial organization, correct expression, repair, and segregation of eukaryotic genomes depend on cohesin, ring‐shaped protein complexes that are thought to function by entrapping DNA. It has been proposed that cohesin is recruited to specific genomic locations from distal loading sites by an unknown mechanism, which depends on transcription, and it has been speculated that cohesin movements along DNA could create three‐dimensional genomic organization by loop extrusion. However, whether cohesin can translocate along DNA is unknown. Here, we used single‐molecule imaging to show that cohesin can diffuse rapidly on DNA in a manner consistent with topological entrapment and can pass over some DNA‐bound proteins and nucleosomes but is constrained in its movement by transcription and DNA‐bound CCCTC‐binding factor (CTCF). These results indicate that cohesin can be positioned in the genome by moving along DNA, that transcription can provide directionality to these movements, that CTCF functions as a boundary element for moving cohesin, and they are consistent with the hypothesis that cohesin spatially organizes the genome via loop extrusion.

DNA entry, exit and second DNA capture by cohesin: insights from biochemical experiments

“…Indeed, purified fission yeast and human cohesin have been demonstrated to topologically entrap DNA in an ATP-dependent manner [18–20]. Topological DNA binding has been suggested to be the common characteristic of other essential SMC complexes [21–23].…”

“…Protein crosslinking experiments at the interfaces of SMC and Scc1 subunits have also suggested that a single cohesin complex tethers replicated sister chromosomes in budding yeast cells [16,17]. In addition, single molecule observations have revealed that purified cohesin binds and diffuses along DNA as a single complex [19,20,50]. Another mechanistic explanation involves protein-protein interactions.…”

“…This suggests that cohesin’s ATPase has other roles in addition to facilitating DNA loading [83]. However, in contrast to budding yeast condensin, ATP-dependent, unidirectional translocations have not been seen for both the fission yeast and human cohesins [19,20,50]. It should also be noted that the purified budding yeast condensin can tether two intermolecular DNA strands [82].…”

The torments of the cohesin ring

“…The recent development of an in vitro DNA loading assay with purified cohesin and cohesin loader should make it possible to address these hypotheses. 19 Interestingly, small quantities of fission yeast 19 and human 22 cohesin can both be loaded onto DNA in the absence of ATP and the cohesin loader in vitro. The low level of DNA binding of fission yeast cohesin observed in the absence of the cohesin loader is sensitive to DNA linearization, suggesting that cohesin can load topologically without requirement for the cohesin loader in vitro.…”

“…19,22,23 Attempts to load budding yeast cohesin have been unsuccessful. It is conceivable that cohesin preparations from different species, which are used for the experiments in vitro, differ in relative amounts of different cohesin conformations, e.g., O-shaped vs rod-like.…”