Cohesin in motion: single molecule imaging of the human cohesin complex on DNA

The spatial organisation, expression, repair and segregation of eukaryotic genomes depend on cohesin. These are ring-shaped protein complexes which are thought to generate sister chromatid cohesion, chromatin loops and topologically associated domains (TADs) by entrapping DNA. To mediate long-range chromosomal cis interactions, cohesin is enriched at specific genomic sites. It has been proposed that cohesin is recruited to these from distal loading sites by an unknown mechanism which depends on transcription and that this movement creates three-dimensional genomic organisation. However, it is unknown if cohesin can translocate along DNA and if so by which mechanism.

Using a system in which we visualise the binding of fluorescently-labelled human cohesin complexes to single stretched DNA molecules and monitor these interactions in real-time, we find that cohesin diffuses rapidly on DNA in a salt-resistant manner. Consistent with topological entrapment, recombinant human cohesin associates with DNA in a manner that depends on integrity of DNA and the cohesin ring and can pass over some DNA bound proteins and nucleosomes. We are currently investigating how CCCTC-binding factor (CTCF) and transcription affect DNA bound cohesin, with an aim to provide insight into how cohesin is positioned in vivo and how this might contribute to genomic organisation