Crystal structure of the processivity clamp loader gamma (γ) complex of E-coli DNA polymerase III

The gamma complex, an AAA+ ATPase, is the bacterial homolog of eukaryotic replication factor C (RFC) that loads the sliding clamp (beta, homologous to PCNA) onto DNA. The 2.7/3.0 Angstrom crystal structure of gamma complex reveals a pentameric arrangement of subunits, with stoichiometry delta':gamma (3):delta. The C-terminal domains of the subunits form a circular collar that supports an asymmetric arrangement of the N-terminal ATP binding domains of the gamma motor and the structurally related domains of the delta' stator and the delta wrench. The structure suggests a mechanism by which the gamma complex switches between a closed state, in which the beta -interacting element of delta is hidden by delta', and an open form similar to the crystal structure, in which delta is free to bind to beta.