Proton motive force drives the interaction of the inner membrane TolA and outer membrane Pal proteins in Escherichia coli

The Tol-Pal system of the Escherichia coil envelope is formed from the inne r membrane TolQ, TolR and TolA proteins, the periplasmic TolB protein and t he outer membrane Pal lipoprotein. Any defect in the Tol-Pal proteins or in the major lipoprotein (Lpp) results in the loss of outer membrane integrit y giving hypersensitivity to drugs and detergents, periplasmic leakage and outer membrane Vesicle formation. We found that multicopy plasmid overprodu ction of TolA was able to complement the membrane defects of an lpp strain but not those of a pal strain, This result indicated that overproduced TolA has an envelope-stabilizing effect when Pal is present. We demonstrate tha t Pal and TolA formed a complex using in vivo cross-linking and immunopreci pitation experiments. These results, together with in vitro experiments wit h purified Pal and TolA derivatives, allowed us to show that Pal interacts with the TolA C-terminal domain. We also demonstrate using protonophore, K carrier valinomycin, nigericin, arsenate and fermentative conditions that the proton motive force was coupled to this interaction.