Cohesin is made up of four subunits, Scc1, Scc3, Smc1 and Smc3. Smc1 and Smc3 are members of the Structural Maintenance of Chromosomes (SMC) family. SMC proteins have two main structural characteristics: an ATPase activity domain (formed when the amino and carboxy terminals interact) and a hinge region that allows dimerization of SMCs. The ATPase domain and the hinge are connected to each other via long anti-parallel coiled coils. The overall structure of a dimer has, therefore, an ATPase domain at each end and a hinge at the center. When ATP is present, the two ATPase domains are able to bind, forming a ring structure. ATP hydrolysis can therefore trigger opening and closing of the ring.
Scc1 and Scc3 bind the ATPase domains of Smc1 and Smc3 stabilizing the ring structure. Scc1 is a member of the kleisin protein family and it controls sister-chromatid separation. The amino and carboxy terminus of Scc1 bind Smc1 and Smc3. Once Scc1 binds on the SMC proteins, Scc3 can also associate by binding with the C-terminal region of Scc1. When Scc1 binds on both Smc1 and Smc3, the cohesin complex forms a closed ring structure. When it binds to only one of the SMC proteins, the complex forms an open ring. However more recently cohesin rings were found to dimerise, with two rings held together by the Scc3 subunit in a handcuff shape, one strand of DNA in each cohesin ring.
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