The structure of multiple polypeptide domains determines the signal recognition particle targeting requirement of Escherichia coli inner membrane proteins

The signal recognition particle (SRP) targeting pathway is required for the efficient insertion of many polytopic inner membrane proteins (IMPs) into the Escherichia coli inner membrane, but in the absence of SRP protein expo rt proceeds normally. To define the properties of IMPs that impose SRP depe ndence, we analyzed the targeting requirements of bitopic IMPs that are str ucturally intermediate between exported proteins and polytopic IMPs. We fou nd that disruption of the SRP pathway inhibited the insertion of only a sub set of bitopic IMPs. Studies on a model bitopic AcrB-alkaline phosphatase f usion protein (AcrB 265-AP) showed that the SRP requirement for efficient i nsertion correlated with the presence of a large periplasmic domain (P1). A s previously reported, perturbation of the SRP pathway also affected the in sertion of a polytopic AcrB-AP fusion. Even exhaustive SRP depletion, howev er, failed to block the insertion of any AcrB derivative by more than 50%. Taken together, these data suggest that many proteins that are normally tar geted by SRP can utilize alternative targeting pathways and that the struct ure of both hydrophilic and membrane-spanning domains determines the degree to which the biogenesis of a protein is SRP dependent.