ATP is essential for protein translocation into Escherichia coli membrane vesicles

The energy requirement for translocation of alkaline phosphatase and the outer membrane protein OmpA into Escherichia coli membrane vesicles was studied under conditions that permit posttranslational translocation and, hence, prior removal of various components necessary for protein synthesis. Translocation could be supported by an ATP-generating system or, less well, by the protonmotive force generated by D-lactate oxidation; the latter might act by generating ATP from residual bound nucleotides. However, when protonmotive force inhibitors were used or when ATP was further depleted by E. coli glycerol kinase, D-lactate no longer supported the translocation. Furthermore, ATP could still support protein translocation in the presence of proton uncouplers or with membranes defective in the F1 fraction of the H+-ATPase. We conclude that ATP is required for protein translocation in this posttranslational system (and probably also in cotranslational translocation); the protonmotive force may contribute but does not appear to be essential.