title: The role of the synergistic phosphate anion in iron transport by the periplasmic iron-binding protein from Haemophilus influenzae.
abstract: The acquisition of iron from transferrin by Gram-negative bacterial pathogens is dependent on a periplasmic ferric-ion-binding protein, FbpA. FbpA shuttles iron from the outer membrane to an inner membrane transport complex. A bound phosphate anion completes the iron co-ordination shell of FbpA and kinetic studies demonstrate that the anion plays a critical role in iron binding and release in vitro. The present study was initiated to directly address the hypothesis that the synergistic anion is required for transport of iron in intact cells. A series of site-directed mutants in the anion-binding amino acids of the Haemophilus influenzae FbpA (Gln-58, Asn-175 and Asn-193) were prepared to provide proteins defective in binding of the phosphate anion. Crystal structures of various mutants have revealed that alteration of the C-terminal domain ligands (Asn-175 or Asn-193) but not the N-terminal domain ligand (Gln-58) abrogated binding of the phosphate anion. The mutant proteins were introduced into H. influenzae to evaluate their ability to mediate iron transport. All of the single site-directed mutants (Q58L, N175L and N193L) were capable of mediating iron acquisition from transferrin and from limiting concentrations of ferric citrate. The results suggest that the transport of iron by FbpA is not dependent on binding of phosphate in the synergistic anion-binding site.
authors.affiliation: Department of Microbiology and Infectious Diseases, University of Calgary, Calgary, AB, Canada T2N 4N1.
authors.forename: Ali G
authors.initials: AG
authors.lastname: Khan
authors.forename: Stephen R
authors.initials: SR
authors.lastname: Shouldice
authors.forename: Leslie W
authors.initials: LW
authors.lastname: Tari
authors.forename: Anthony B
authors.initials: AB
authors.lastname: Schryvers
mesh_terms: D019943:Amino Acid Substitution; D000693:Anaerobiosis; D000838:Anions / Q000378:metabolism; D001692:Biological Transport; D003001:Cloning, Molecular; D003460:Crystallization; D004926:Escherichia coli / Q000235:genetics / Q000378:metabolism; D006193:Haemophilus influenzae / Q000254:growth & development / Q000378:metabolism*; D007501:Iron / Q000378:metabolism*; D033862:Iron-Binding Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D008958:Models, Molecular; D016297:Mutagenesis, Site-Directed; D033902:Periplasmic Binding Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D010710:Phosphates / Q000378:metabolism*; D011487:Protein Conformation; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D014961:X-Ray Diffraction
chemical_list: D000838:Anions; D033862:Iron-Binding Proteins; D033902:Periplasmic Binding Proteins; D010710:Phosphates; D011994:Recombinant Proteins; D007501:Iron