Mechanism of ferric enterobactin transport through Escherichia coli FepA: The evolution of a bacterial venus flytrap.

Abstract

The TonB-dependent, energy dependent ferric siderophore transporters of the gram-negative bacterial outer membrane possess a unique architecture: Their N-termini fold into a globular domain that lodges within, and physically obstructs, a transmembrane porin beta-barrel formed by their C-termini. I exchanged and deleted the N-domains of two such receptors, FepA and FhuA, which recognize and transport ferric enterobactin and ferrichrome, respectively. The resultant chimeric and beta-barrel proteins avidly bound appropriate ligands, including iron-complexes, protein toxins, and bacteriophages. Thus, the ability of these receptors to discriminate these molecules fully originates in the transmembrane beta-barrel domain. Both the chimeric and deletion constructs transported the siderophore they bound. The FepA constructs showed less activity than the wild type receptor protein, but constructs of FhuA functioned with turnover numbers equivalent to wild type. The mutant proteins displayed the full range of functionality indicating the globular domain within the pore is dispensable to the internalization reaction, and when present, acts without specificity during solute uptake. These and other data suggest siderophore receptors undergo multiple conformational states that ultimately expel the N-terminus from the channel concomitant with internalization.