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

Abstract

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 that 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.

To investigate this, we developed a single-molecule microscopy assay to monitor the binding of fluorescently-labelled human cohesin complexes to single stretched DNA molecules in real-time. We found that cohesin diffused rapidly on DNA in a salt-resistant manner. Consistent with topological entrapment, recombinant human cohesin associated with DNA in a manner that was dependent on the integrity of DNA and the cohesin ring and could pass over some DNA bound proteins and nucleosomes but was 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.

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I did my PhD in the lab of Julian Blow in Dundee from 2003 – 2008. The title of my thesis was ‘Consequences of chromosomal re-replication in Xenopus’. I then moved to EMBL Heidelberg to study nuclear envelope disassembly and reassembly in mitosis in the lab of Iain Mattaj (2008 – 2012). After that I moved to the Research Institute of Molecular Pathology (IMP) in Vienna where I’m currently a postdoc in the lab of Jan-Michael Peters (I’ll describe the work I did in Jan’s lab in my talk).