Electron Microscopic Single Particle Analysis of the Clamp Loading Complex from Pyrococcus furiosus

Ring-shaped sliding clamps and clamp loader ATPases are essential factors for rapid and accurate DNA replication. The clamp ring is once opened and resealed at the primer-template junctions by ATP-fueled clamp loader function. Processivity of DNA polymerase is conferred by attachment to the clamp loaded onto DNA. In eukarya and archaea, the hetero-pentemeric replication factor C (RFC) and the proliferating cell nuclear antigen (PCNA) trimer play crucial roles as the clamp loader and the sliding clamp, respectively [1]. Here we report an EM structure of an archaeal RFC–PCNA–DNA complex at 12 Å resolution. This complex exhibits excellent fitting of each atomic structure of RFC, PCNA, and a primed DNA with the convincing positions of 3’ and 5’ termini into the map. The PCNA ring is opened by extensive interactions with RFC, with the distorted structural view of a washer-like conformation. The RFC–PCNA contact mode is distinct from that in the yeast RFC–PCNA crystal structure [2]. Thus, the complex appears to represent a scene, where the PCNA ring is kept open before ATP hydrolysis by RFC.