Biochemical reconstitution of topological DNA binding by the cohesin ring

Cohesion between sister chromatids, mediated by the chromosomal cohesin complex, is a prerequisite for faithful chromosome segregation in mitosis. Cohesin plays vital roles also in DNA repair and transcriptional regulation. The ring-shaped cohesin complex is thought to encircle sister DNA strands, but its molecular mechanism of action is poorly understood and the biochemical reconstitution of cohesin activity in vitro has remained an unattained goal. We have now reconstituted cohesin loading onto DNA using purified fission yeast cohesin and its loader complex, Mis4(Scc2)/Ssl3(Scc4). Incubation of cohesin with DNA leads to spontaneous topological loading, but that remains inefficient. The loader contacts cohesin at multiple sites around the ring circumference, including the hitherto enigmatic Psc3(Scc3) subunit, and stimulates cohesin’s ATPase, resulting in efficient topological loading. The in vitro reconstitution of cohesin loading onto DNA provides mechanistic insight into the initial steps of establishing sister chromatid cohesion and other chromosomal processes mediated by cohesin.