One of the sliding clamp subunits that acts as a moving platform for DNA processing. Responsible for tethering the catalytic subunit of DNA polymerase to DNA during high-speed replication. DNA polymerase I, DNA ligase and the flap endonuclease may be constitutively associated with the PCNA heterotrimer forming a scanning complex able to couple DNA synthesis and Okazaki fragment maturation. Heterotrimer stimulates the Holliday junction resolvase Hjc. Does not individually interact with either of the other PCNA subunits, but is recruited to a PCNA1-PCNA2 heterodimer; does not form homotrimers (PubMed:12535540, PubMed:18703842). Heterotrimer which circularizes head-to-tail (head is at N-terminus, tail is at C-terminus) to form a toroid; DNA passes through its center. Replication factor C (RFC) is required to load the toroid on the DNA. This subunit interacts with DNA ligase, RfcL, XPF, and weakly with DNA polymerase I (PubMed:12535540). Belongs to the PCNA family. Extended N-terminus.