Sliding clamps are loaded onto DNA by ATP-driven clamp loader complexes. The structure of the E. coli clamp loader in a nucleotide-free state has been determined previously. We now report crystal structures of a truncated form of the isolated γ-ATPase subunit, γ1-243, of the E. coli clamp loader, in nucleotide-free and bound forms. The γ subunit adopts a defined conformation when empty, in which the nucleotide binding site is blocked. The binding of either ATPγS or ADP, which are shown to bind with equal affinity to γ1-243, induces a change in the relative orientation of the two domains such that nucleotides can be accommodated. This change would break one of the γ:γ interfaces seen in the empty clamp loader complex, and may represent one step in the activation process.
Bibliographical noteFunding Information:
We thank David Jeruzalmi, Gregory D. Bowman, Holger Sondermann, and Bhushan Nagar for valuable assistance and discussions. We thank the staff at the 5.0.2 and 8.3.1 beamlines at the Advanced Light Source, Berkeley for assistance with synchrotron data collection. This work was supported by NIH grants GM45547 (J.K.) and GM38839 (M.O.D).
- AAA+ ATPase
- Clamp loader
- DNA polymerase III
- Replication factor C (RFC)
- γ subunit