Cohesin is a conserved ring-shaped protein complex that tethers DNA molecules together and, in doing so, plays central roles in the organisation and segregation of chromosomes. 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 organisation by loop extrusion. However, whether cohesin can actually translocate along DNA is unknown. Using single-molecule imaging, I showed 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.
I will present this data and provide an overview of my recent work to reconstitute other aspects of cohesin – DNA interactions. I will also include a summary of my future plans to use single-molecule imaging to determine the mechanism by which cohesin tethers replicated sister chromatids, an activity essential for chromosome segregation.