Identification and mutational studies of conserved amino acids in the outer membrane receptor protein, FepA, which affect transport but not binding of ferric-enterobactin in Escherichia coli

Many gram-negative bacteria produce and excrete siderophores, which complex iron with high affinity in the environment. The ferric siderophore complexes are transported across the outer membrane by receptor proteins. This process requires energy and is TonB dependent and must involve conformational changes in the receptor proteins to allow the transport of the ferric siderophores from the extracellular binding site to the periplasm. There is a large variety in the structures, molecular weights and charges among the siderophores. It was therefore realized that when the sequences of the many different receptor proteins were compared, simultaneously, all identities and close similarities, found in this manner, could only be due to residues involved in the conformational changes and transport mechanism, common to all the proteins, and not be due to the specificity of ligand recognition. Once the crystal structures of FepA, FhuA and FecA became available, it was immediately clear that the sequence similarities which were found in the simultaneous alignment, were all localized in a few structural domains, which are identical in the three structures and can therefore be expected to be maintained in all the proteins in this family. One of these domains, tentatively named the lock region, consists of 10 residues with a central quadrupole formed by two arginines and two glutamates, from the plug region and the beta barrel. We mutated several of these residues in FepA. All showed normal binding in quantitative binding studies. Some showed normal transport as well, however, the majority showed moderate to severe defective transport with ferric enterobactin. The results therefore show the validity of the hypothesis that the simultaneous sequence alignment will select the residues involved in the transport function of the receptor proteins. In addition the results allow to relate the severity of the transport deficiency to be correlated with the structure of the lock region while it is also possible to propose a function of this region in the conformational changes of the protein during the transport of the ligand from the binding site to the periplasm.