The structure of SMC proteins. (a) SMC proteins have five domains. (b) Each SMC polypeptide is folded so that the two coiled-coil domains wrap around each other and the N and C domains come together to form a complete ATP-binding site. Two polypeptides are linked at the hinge region to form the dimeric V-shaped SMC molecule. (c) Bacterial SMC proteins form a homodimer. The three different eukaryotic SMC proteins form heterodimers. Cohesins are made up of SMC1-SMC3 pairs, and condensins consist of SMC2-SMC4 pairs. (d) Electron micrographs of different bacterial SMC dimers show the variety of shapes these dimers can take.