SecA is required for the insertion of inner membrane proteins targeted by the Escherichia coli signal recognition particle

Recent work has demonstrated that the signal recognition particle (SRP) is required for the efficient insertion of many proteins into the Escherichia coli inner membrane (IM), Based on an analogy to eukaryotic SRP, it is likely that bacterial SRP binds to inner membrane proteins (IMPs) co-translationally and then targets them to protein transport channels ("translocons"), Here we present evidence that SecA, which has previously been shown to facilitate the export of proteins targeted in a post-translational fashion, is also required for the membrane insertion of proteins targeted by SRP. The introduction of SecA mutations into strains that have modest SRP deficiencies produced a synthetic lethal effect, suggesting that SecA and SRP might function in the same biochemical pathway. Consistent with this explanation, depletion of SecA by inactivating a temperature-sensitive amber suppressor in a secA(am) strain completely blocked the membrane insertion of AcrB, a protein that is targeted by SRP. In the absence of substantial SecA, pulse-labeled AcrB was retained in the cytoplasm even after a prolonged chase period and was eventually degraded. Although protein export was also severely impaired by SecA depletion, the observation that more than 20% of the OmpA molecules were translocated properly showed that translocons were still active. Taken together, these results imply that SecA plays a much broader role in the transport of proteins across the E. coli IM than has been previously recognized.