Structural analysis of a eukaryotic sliding DNA clamp-clamp loader complex

Sliding clamps are ring-shaped proteins that encircle DNA and confer high processivity on DNA polymerases. Here we report the crystal structure of the five-protein clamp loader complex ( replication factor-C, RFC) of the yeast Saccharomyces cerevisiae, bound to the sliding clamp ( proliferating cell nuclear antigen, PCNA). Tight interfacial coordination of the ATP analogue ATP-gamma S by RFC results in a spiral arrangement of the ATPase domains of the clamp loader above the PCNA ring. Placement of a model for primed DNA within the central hole of PCNA reveals a striking correspondence between the RFC spiral and the grooves of the DNA double helix. This model, in which the clamp loader complex locks onto primed DNA in a screw-cap-like arrangement, provides a simple explanation for the process by which the engagement of primer-template junctions by the RFC: PCNA complex results in ATP hydrolysis and release of the sliding clamp on DNA.