Crystal Structure of the Processivity Clamp Loader Gamma (γ) Complex of E. coli DNA Polymerase III

The γ complex, an AAA+ ATPase, is the bacterial homolog of eukaryotic replication factor C (RFC) that loads the sliding clamp (β, homologous to PCNA) onto DNA. The 2.7/3.0 Å crystal structure of γ complex reveals a pentameric arrangement of subunits, with stoichiometry δ′:γ3:δ. 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 γ motor and the structurally related domains of the δ′ stator and the δ wrench. The structure suggests a mechanism by which the γ complex switches between a closed state, in which the β-interacting element of δ is hidden by δ′, and an open form similar to the crystal structure, in which δ is free to bind to β.