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. Homotrimer. 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. Belongs to the PCNA family. Extended N-terminus.