CHARGED RESIDUES RENDER PRO-OMPA POTENTIAL-DEPENDENT FOR INITIATION OF MEMBRANE TRANSLOCATION

We have examined the effects of positively and negatively charged resi dues on the translocation of outer membrane protein A precursor (pro-O mpA) across the bacterial inner membrane. Pro-OmpA does not translocat e across the membrane when 2 positively charged residues are inserted immediately after the leader peptide, whereas it does insert when 2 ne utral or negatively charged residues are introduced. Using a cell-free translocation system, we show that the membrane potential stimulated the rate of initial insertion of pro-OmpA with negatively charged resi dues, inhibited pro-OmpA with positively charged residues, and had no effect on neutral pro-OmpA. Thus, acidic residues render pro-OmpA pote ntial-dependent for loop formation, which then initiates the transloca tion process.