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 degrees C between cells and reconstituted proteoliposomes; protein translocation in deltasecG cells is defective at 20 degrees C but normal at 37 degrees C, indicating that SecG is dispensable at 37 degrees C, whereas SecG remarkably stimulates protein translocation into reconstituted proteoliposomes at 37 degrees 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 degrees C and 20 degrees C. We found that the absence of the proton motive force renders protein translocation strongly dependent on SecG even at 37 degrees 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.