OmpF enhances the ability of BtuB to protect susceptible Escherichia coli cells from colicin E9 cytotoxicity

The outer membrane (OM) vitamin B12 receptor, BtuB, is the primary receptor for E group colicin adsorption to Escherichia coli. Cell death by this family of toxins requires the OM porin OmpF but its role remains elusive. We show that OmpF enhances the ability of purified BtuB to protect bacteria against the endonuclease colicin E9, demonstrating either that the two OM proteins form the functional receptor or that OmpF is recruited for subsequent translocation of the bacteriocin. While stable binary colicin E9–BtuB complexes could be readily shown in vitro, OmpF-containing complexes could not be detected, implying that OmpF association with the BtuB–colicin complex, while necessary, must be weak and/or transient in nature.