NEED FOR TOLC, AN ESCHERICHIA-COLI OUTER-MEMBRANE PROTEIN, IN THE SECRETION OF HEAT-STABLE ENTEROTOXIN-I ACROSS THE OUTER-MEMBRANE

Escherichia coli heat-stable enterotoxin Ip (STIp) is a typical extrac ellular toxin consisting of 18 amino acid residues synthesized as a pr ecursor of pre (amino acid residues 1 to 19), pro (amino acid residues 20 to 54), and mature (amino acid residues 55 to 72) regions. STIp sy nthesized in the cytoplasm must cross the inner and outer membranes to migrate into the extracellular environment. Previous studies showed t hat the precursor translocates across the inner membrane utilizing the general export pathway consisting of Sec proteins. However, it remain s unclear how it crosses the outer membrane. In this study, we examine d the effects of mutation of the tolC gene which encodes an E. coli ou ter membrane protein, TolC, on the release of STIp into the extracellu lar environment. The mutation reduced the amount of STIp released into culture supernatant and increased the amount of STIp accumulated in t he periplasm. This indicates that TolC mediates the translocation of S TIp across the outer membrane. The inability to transfer STIp in the p eriplasm into the culture supernatant was restored by introduction of the tolC gene into the mutant cells. In the mouse intestinal loop assa y, living cells of the mutants did not show a positive response, but w ild-type cells did. These results showed that TolC is involved in the translocation of STIp across the outer membrane. (C) 1998 Academic Pre ss