Binding, activation and dissociation of the dimeric SecA ATPase at the dimeric SecYEG translocase

The bacterial preprotein translocase is comprised of a membrane-embedded oligomeric SecYEG structure and a cytosolic dimeric SecA ATPase. The associations within SecYEG oligomers and SecA dimers, as well as between these two domains are dynamic and reversible. Here, it is shown that a covalently linked SecYEG dimer forms a functional translocase and a high affinity binding site for monomeric and dimeric SecA in solution. The interaction between these two domains stimulates the SecA ATPase, and nucleotides modulate the affinity and ratio of SecA monomers and dimers bound to the linked SecYEG complex. During the translocation reaction, the SecA monomer remains in stable association with a SecYEG protomer and the translocating preprotein. The nucleotides and translocation-dependent changes of SecA-SecYEG associations and the SecA dimeric state may reflect important facets of the preprotein translocation reaction.