Communication between subunits within an archaeal clamp-loader complex

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Abstract
We have investigated the communication between subunits in replication factor C (RFC) from Archaeoglobus fulgidus. Mutation of the proposed arginine finger in the small subunits results in a complex that can still bind ATP but has impaired clamp‐loading activity, a process that normally only requires binding of nucleotide. The small subunit alone forms a hexameric ring that is six‐fold symmetric in the absence of ATP. However, this symmetry is broken when the nucleotide is bound to the complex. A conformational change associated with nucleotide binding may relate to the opening of PCNA rings by RFC during the loading reaction. The structures also reveal the importance of the N‐terminal helix of each subunit at the ATP‐binding site. Analysis of mutant protein complexes containing subunits lacking this N‐terminal helix reveals key distinct regulatory roles during clamp loading that are different for the large and small subunits in the RFC complex.