Crystal structures of the OmpF porin: function in a colicin translocon

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Abstract
The OmpF porin in the Escherichia coli outer membrane (OM) is required for the cytotoxic action of group A colicins, which are proposed to insert their translocation and active domains through OmpF pores. A crystal structure was sought of OmpF with an inserted colicin segment. A 1.6 Å OmpF structure, obtained from crystals formed in 1 M Mg2+, has one Mg2+ bound in the selectivity filter between Asp113 and Glu117 of loop 3. Co‐crystallization of OmpF with the unfolded 83 residue glycine‐rich N‐terminal segment of colicin E3 (T83) that occludes OmpF ion channels yielded a 3.0 Å structure with inserted T83, which was obtained without Mg2+ as was T83 binding to OmpF. The incremental electron density could be modelled as an extended poly‐glycine peptide of at least seven residues. It overlapped the Mg2+ binding site obtained without T83, explaining the absence of peptide binding in the presence of Mg2+. Involvement of OmpF in colicin passage through the OM was further documented by immuno‐extraction of an OM complex, the colicin translocon, consisting of colicin E3, BtuB and OmpF.