SecG is an integral membrane component of E. coli protein translocase. However, a discrepancy exists as to the importance of SecG for protein translocation at 37°C between cells and reconstituted proteoliposomes; protein translocation in ΔsecG cells is defective at 20°C but normal at 37°C, indicating that SecG is dispensable at 37°C, whereas SecG remarkably stimulates protein translocation into reconstituted proteoliposomes at 37°C. In this study, protein translocation into membrane vesicles containing or not containing SecG was examined in the presence and absence of the proton motive force at 37°C and 20°C. We found that the absence of the proton motive force renders protein translocation strongly dependent on SecG even at 37°C. Protein translocation into proteoliposomes in the absence of the proton motive force thus required SecG whereas that in cells, which always generate the proton motive force, did not.
