Figure 1

Sliding clamp rings of different organisms. Clamps are constructed from either two or three monomers to yield a ring composed of six domains. (a) The prokaryotic β subunit contains three domains, whereas PCNA and T4 gp45 are about two thirds the size of β and comprise only two domains each. The crystal structures of the oligomeric rings: (b) E. coli β; (c) human PCNA; (d) T4 phage gp45. In (b-d), the interfaces between protomers are indicated by the arrows, and the domains within each monomer unit are numbered (1-3 for β and 1,2 for PCNA and gp45).