A Topological Interaction between Cohesin Rings and a Circular Minichromosome

Sister chromatid cohesion depends on a multiprotein cohesin complex containing two SMC subunits, Smc1 and Smc3, that dimerize to form V-shaped molecules with ABC-like ATPase heads at the tips of their two arms. Cohesinʹs Smc1 and Smc3 "heads" are connected by an (alpha) kleisin subunit called Scc1, forming a tripartite ring with a diameter around 40 nm. We show here that some cohesin remains tightly bound to circular minichromosomes after their purification from yeast cells and that cleavage either of cohesinʹs ring or of the minichromosomeʹs DNA destroys their association. This suggests that the stable association between cohesin and chromatin detected here is topological rather than physical, which is consistent with the notion that DNA is trapped inside cohesin rings.