Defective lipoprotein sorting induces lolA expression through the Rcs stress response phosphorelay system

The Escherichia coli LolA protein is a lipoprotein-specific chaperone that carries lipoproteins from the inner to the outer membrane. A dominant negative LolA mutant, LolA(I93C/F140C), in which both (93)Ile and (140)Phe are replaced by Cys, binds tightly to the lipoprotein-dedicated ABC transporter LolCDE complex on the inner membrane and therefore inhibits the detachment of outer membrane-specific lipoproteins from the inner membrane. We found that the expression of this mutant strongly induced lolA gene transcription. The depletion of the LolA or LolB protein also triggered lolA gene transcription, indicating that the inhibition of outer membrane lipoprotein transport triggers lolA transcription. To elucidate the mechanism, we isolated mutants that are unable to induce lolA transcription using the lacZ gene fused to the lolA promoter as a reporter and found that the Rcs phosphorelay system directly regulates lolA transcription. An outer membrane lipoprotein, RcsF, was essential for this activation, while the coactivator RcsA was dispensable. Taking the observation that an RcsF mutant localized in the inner membrane constitutively activated the Rcs phosphorelay system into consideration, the results shown here strongly suggest that correct lipoprotein sorting to the outer membrane is monitored by RcsF, which plays a key role in the Rcs stress response system.