Crystal Structure of the δ′ Subunit of the Clamp-Loader Complex of E. coli DNA Polymerase III

by genes 62 and 44 (Jarvis et al., 1989). The bacterial, eukaryotic, and T4 bacteriophage proSummary cessivity factors are all structurally similar, even though they do not share significant sequence similarity (Kong The crystal structure of the d9 subunit of the clampet al., 1992; Krishna et al., 1994; I. Moarefi and J. K., loader complex of E. coli DNA polymerase III has been unpublished data). Each of the proteins forms a stable determined. Three consecutive domains in the strucclosed ring insolution, in the absence of DNA. The mechture are arranged in a C-shaped architecture. The anism of action of clamp-loader complexes is therefore N-terminal domain contains a nonfunctional nucleoexpected to be similar in each case, and is likely to tide binding site. The catalytic component of the involve a mechanical opening of the ring-shaped proclamp-loader complex is the g subunit, which is hocessivity factors since there is no evidence for DNA mologous to d9. A sequence-structure alignment sugcleavage during loading of the clamps. A common gests that nucleotides bind to g at an interdomain mechanism is also suggested by the fact that subunits interface within the inner surface of the “C.” The alignof theclamp-loader complexes from different organisms ment is extended to other clamp-loader complexes share some sequence similarity (Carter etal., 1993; Dong and to the RuvB family of DNA helicases, and suggests et al., 1993; O’Donnell et al., 1993). that each of these is assembled from C-shaped comThe clamp-loader complex of E. coli binds to the b ponents that can open and close the jaws of the “C” ring and to primed template (single-strand/doublein response to ATP binding and hydrolysis. strand junctions or nicked duplex DNA) and then places the b ring upon DNA in an ATP-dependent manner. The