In eukaryotes, the sliding clamp for DNA polymerase is PCNA, short for proliferating cell nuclear antigen. PCNA forms a ring around DNA and helps replicative polymerases remain attached to the template for long stretches of synthesis.
PCNA is a homotrimer: three identical protein molecules assemble into a closed ring. The ring does not itself synthesize DNA. Instead, it acts as a processivity factor and a docking platform for DNA polymerases, nucleases, ligases, chromatin proteins, and repair factors. Its interaction with DNA polymerase delta is especially important during replication.
PCNA must be opened and placed around DNA by a clamp loader, replication factor C, or RFC. RFC uses ATP to position the clamp at a primer-template junction. Topoisomerase relieves DNA twisting, RPA coats exposed single-stranded DNA, and RFC loads PCNA; none of those proteins is the eukaryotic sliding clamp. PCNA also participates in DNA repair and becomes ubiquitinated in some damage-response pathways.