Colicin pore-forming domains bind to Escherichia coli trimeric porins

Colicin N kills sensitive Escherichia coli cells by first binding to its tr imeric receptor (OmpF) via its receptor binding domain. It then uses OmpF t o translocate across the outer membrane and in the process it also needs do mains II and III of the protein TolA. Recent studies have demonstrated sodi um dodecyl sulfate- (SDS) dependent complex formation between trimeric pori ns and TolA-II. Here we demonstrate that colicin N forms similar complexes with the same trimeric porins and that this association is unexpectedly sol ely dependent upon the pore-forming domain (P-domain). No binding was seen with the monomeric porin OmpA. In mixtures of P-domain and TolA with OmpF p orin, only binary and no ternary complexes were observed, suggesting that b inding of these proteins to the porin is mutually exclusive. Pull-down assa ys in solution show that porin-P-domain complexes also form in the presence of outer membrane lipopolysaccharide. This indicates that an additional co licin-porin interaction may occur within the outer membrane, one that invol ves the colicin pore domain rather than the receptor-binding domain. This m ay help to explain the role of porins and TolA-II in the later stages of co licin translocation.