Overproduction of SecA suppresses the export defect caused by a mutation in the gene encoding the Escherichia coli export chaperone SecB

SecB is a cytosolic protein required for rapid and efficient export of part icular periplasmic and outer membrane proteins in Escherichia coli, SecB pr omotes export by stabilizing newly synthesized precursor proteins in a nonn ative conformation and by targeting the precursors to the inner membrane. B iochemical studies suggest that SecB facilitates precursor targeting by bin ding to the SecA protein, a component of the membrane-embedded translocatio n apparatus. To gain more insight into the functional interaction of SecB a nd SecA, in vivo, mutations in the secA locus that compensate for the expor t defect caused by the secB missense mutation secBL75Q were isolated. Two s uppressors were isolated, both of which led to the overproduction of wild-t ype SecA protein. In vivo studies demonstrated that the SecBL75Q mutant pro tein releases precursor proteins at a lower rate than does wild-type SecB, Increasing the level of SecA protein in the cell was found to reverse this slow-release defect, indicating that overproduction of SecA stimulates the turnover of SecBL75Q-precursor complexes. These findings lend additional su pport to the proposed pathway for precursor targeting in which SecB promote s targeting to the translocation apparatus by binding to the SecA protein.