SecDFyajC is not required for the maintenance of the proton motive force

SecDFyajC of Escherichia coli is required for efficient export of proteins in vivo. However, the functional role of SecDFyajC in protein translocation is unclear. We evaluated the postulated function of SecDFyajC in the maintenance of the proton motive force. As previously reported, inner membrane vesicles (IMVs) lacking SecDFyajC are defective in the generation of a stable proton motive force when energized with succinate. This phenomenon is, however, not observed when NADH is used as an electron donor. Moreover, the proton motive force generated in SecDFyajC-depleted vesicles stimulated translocation to the same extent as seen with IMVs containing SecDFyajC. Further analysis demonstrates that the reduced proton motive force with succinate in IMVs lacking SecDFyajC is due to a lower amount of the enzyme succinate dehydrogenase. The expression of this enzyme complex is repressed by growth on glucose media, the condition used to deplete SecDFyajC. These results demonstrate that SecDFyajC is not required for proton motive force-driven protein translocation.