How are cohesin rings opened and closed?

The cohesin complex is proposed to embrace sister chromatids within its ring-like structure, in which two ATP-binding ‘head’ domains of an SMC (structural maintenance of chromosomes) heterodimer are linked by a kleisin subunit. Recent studies shed new light on the crucial functions of the ‘hinge’ domain of the SMC dimer, which is located ∼50 nm from the head domains. An emerging idea is that the hinge and head domains cooperatively modulate cohesin–DNA interactions by opening and closing the ring in a highly regulated manner.