Composition of clamp loaders from the different branches of life. (a) Bacterial clamp loaders consist of three different proteins: δ, δ', and the τ or γ protein (γ, a truncation of the τ protein, is shown here). The δ protein is at the A position, with three copies of the ATPase subunits τ or γ at the B, C and D positions. The δ' protein sits at the E position. (b) Eukaryotic clamp loaders consist of five distinct proteins, RFC1 through RFC5. RFC1 lies at the A position, RFC4 at B, RFC3 at C, RFC2 at D and RFC5 at the E position. Eukaryotic A subunits contain an additional domain that bridges the gap between the A and E positions. (c) Bacteriophage clamp loaders consist of two distinct proteins. The gp62 protein, which lacks a AAA+ module but has an A' domain similar to that of eukaryotic clamp loaders, lies at the A position. The ATPase gp44 protein lies at the B, C, D, and E positions. Archaeal clamp loaders have a similar composition (see text for details). Figure adapted from .
Kelch et al. BMC Biology 2012 10:34 doi:10.1186/1741-7007-10-34