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

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Abstract
The Tol–Pal system of the Escherichia coli envelope is formed from the inner membrane TolQ, TolR and TolA proteins, the periplasmic TolB protein and the outer membrane Pal lipoprotein. Any defect in the Tol–Pal proteins or in the major lipoprotein (Lpp) results in the loss of outer membrane integrity giving hypersensitivity to drugs and detergents, periplasmic leakage and outer membrane vesicle formation. We found that multicopy plasmid overproduction 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 that Pal and TolA formed a complex using in vivo cross‐linking and immunoprecipitation experiments. These results, together with in vitro experiments with 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.