THE BINDING CASCADE OF SECB TO SECA TO SECY/E MEDIATES PREPROTEIN TARGETING TO THE ESCHERICHIA-COLI PLASMA-MEMBRANE

The export of many E. coli proteins such as proOmpA requires the cytosolic chaperone SecB and the membrane-bound preprotein translocase. Translocase is a multisubunit enzyme with the SecA protein as its peripheral membrane domain and the SecY E protein as its integral domain. SecB, by binding to proOmpA in the cytosol, prevents its aggregation or association with membranes at nonproductive sites. The SecA receptor binds the proOmpA-SecB complex (Kd ≈ 6 × 10-8 M) through direct recognition of both the SecB (Kd ≈ 2 × 10-7 M) as well as the leader and mature domains of the precursor protein. SecB has a dual function in stabilizing the precursor and in passing it on to membrane-bound SecA, the next step in the pathway. SecA itself is bound to the membrane by its affinity (Kd ≈ 4 × 10-8 M) for SecY E and for acidic lipids. The functions of SecB and SecA as a two-stage receptor system are linked by their affinity for each other. © 1990.