SECA PROTEIN IS EXPOSED TO THE PERIPLASMIC SURFACE OF THE ESCHERICHIA-COLI INNER MEMBRANE IN ITS ACTIVE STATE

E. coli cells harboring pCG169 containing the secD secF locus possesse d SecA protein almost entirely in an integral membrane form in which i t displayed normal protein translocation activity. These results imply that integral membrane SecA is the catalytically active form of this enzyme and that products of the secD secF locus regulate SecA associat ion with the inner membrane. Protease and biotinylation accessibility studies of right side-out and inside-out membrane vesicles derived fro m this strain revealed that SecA was exposed to the periplasmic surfac e of the inner membrane. These studies suggest a model of bacterial pr otein secretion, whereby insertion of SecA into the inner membrane and its association with SecY/E/G promotes assembly of active protein-con ducting channels comprised in part of integral membrane SecA protein, and products of the secD secF locus regulate the channel assembly-disa ssembly reaction by modulating the SecA insertion-deinsertion step.