Sliding clamp subunit that acts as a moving platform for DNA processing. Responsible for tethering the catalytic subunit of DNA polymerase and other proteins to DNA during high-speed replication (By similarity). Both trimeric complexes inhibit DNA ligase and both 3'-5' and 5'-3' activity of Hel308 (Hjm) helicase, but stimulate Hjc, the Holliday junction cleavage enzyme. The subunits circularize to form a toroid; DNA passes through its center. Replication factor C (RFC) is required to load the toroid on the DNA (By similarity). Forms dimeric complexes with PCNA1 and PCNA2, and trimeric complexes with PCNA123 and PCNA323; does not form homotrimers (PubMed:18782564). Crystal structures show a heterotetramer of 2 PCNA2 and 2 PCNA3, which would be large enough to clamp a Holliday junction (PubMed:21352919). Belongs to the PCNA family.