PCNA (proliferating cell nuclear antigen) is a member of the structurally and functionally conserved family of DNA sliding clamps (β clamps). They exist as ring-shaped complexes- homotrimers in eukaryotes, having pseudohexameric symmetry. A monomer of PCNA is composed of two similar globular domains, connected by an interdomain connecting loop, which is a long, and probably a flexible loop. These monomers organize themselves in a head-to-tail manner to form a ring, which has an inner positively charged surface composed of α helices and an outer surface of β sheets.
