THE ESCHERICHIA-COLI SRP AND SECB TARGETING PATHWAYS CONVERGE AT THE TRANSLOCON

Two distinct protein targeting pathways can direct proteins to the Esc herichia coli inner membrane, The Sec pathway involves the cytosolic c haperone SecB that binds to the mature region of pre-proteins. SecB ta rgets the pre-protein to SecA that mediates preprotein translocation t hrough the SecYEG translocon, The SRP pathway is probably used primari ly for the targeting and assembly of inner membrane proteins, It invol ves the signal recognition particle (SRP) that interacts with the hydr ophobic targeting signal of nascent proteins, By using a protein cross -linking approach, we demonstrate here that the SRP pathway delivers n ascent inner membrane proteins at the membrane. The SRP receptor FtsY, GTP and inner membranes are required for release of the nascent prote ins from the SRP, Upon release of the SRP at the membrane, the targete d nascent proteins insert into a translocon that contains at least Sec A, SecY and SecG, Hence, as appears to be the case for several other t ranslocation systems, multiple targeting mechanisms deliver a variety of precursor proteins to a common membrane translocation complex of th e E.coli inner membrane.