| Abstract: |
Replicative DNA polymerases are multiprotein machines that are tethered to DNA during chain extension by sliding clamp proteins. The clamps are designed to encircle DNA completely, and they are manipulated rapidly onto DNA by the ATP-dependent activity of a clamp loader. We outline the detailed mechanism of γ complex, a five-protein clamp loader that is part of the Escherichia coli replicase, DNA polymerase III holoenzgme. The γ complex uses ATP to open the β clamp and assemble it onto DNA. Surprisingly, ATP is not needed for γ complex to crack open the β clamp. The function of ATP is to regulate the activity of one subunit, δ, which opens the clamp simply by binding to it. The δ' subunit acts as a modulator of the interaction between δ and β. On binding ATP, the γ complex is activated such that the δ' subunit permits δ to bind β and crack open the ring at one interface. The clamp loader-open clamp protein complex is now ready for an encounter with primed DNA to complete assembly of the clamp around DNA. Interaction with DNA stimulates ATP hydrolysis which ejects the γ complex from DNA, leaving the ring to close around the duplex. |
