Dissecting the translocase and integrase functions of the Escherichia coliSecYEG translocon

Recent evidence suggests that in Escherichia coli, SecA/SecB and signal rec ognition particle (SRP) are constituents of two different pathways targetin g secretory and inner membrane proteins to the SecYEG translocon of the pla sma membrane. We now show that a secY mutation, which compromises a functio nal SecY-SecA interaction, does not impair the SRP-mediated integration of polytopic inner membrane proteins. Furthermore, under conditions in which t he translocation of secretory proteins is strictly dependent on SecG for as sisting SecA, the absence of SecG still allows polytopic membrane proteins to integrate at the wildtype level. These results indicate that SRP-depende nt integration and SecA/SecB-mediated translocation do not only represent t wo independent protein delivery systems, but also remain mechanistically di stinct processes even at the level of the membrane where they engage differ ent domains of SecY and different components of the translocon. In addition , the experimental setup used here enabled us to demonstrate that SRP-depen dent integration of a multispanning protein into membrane vesicles leads to a biologically active enzyme.