The Role of FhuD2 in Iron(III)-Hydroxamate Transport in Staphylococcus aureus

The fhuD2 gene encodes a lipoprotein that has previously been shown to be important for the utilization of iron(III)-hydroxamates by Staphylococcus aureus. We have studied the function of the FhuD2 protein in greater detail, and demonstrate here that the protein binds several iron(III)-hydroxamates. Mutagenesis of FhuD2 identified several residues that were important for the ability of the protein to function in iron(III)-hydroxamate transport. Several residues, notably Tyr-191, Trp-197, and Glu-202, were found to be critical for ligand binding. Moreover, mutation of two highly conserved glutamate residues, Glu-97 and Glu-231, had no affect on ligand binding, but did impair iron(III)-hydroxamate transport. Interestingly, the transport defect was not equivalent for all iron(III)-hydroxamates. We modeled FhuD2 against the high resolution structures of Escherichia coli FhuD and BtuF, two structurally related proteins, and showed that the three proteins share a similar overall structure. FhuD2 Glu-97 and Glu-231 were positioned on the surface of the N and C domains, respectively. Characterization of E97A, E231A, or E97A/E231A mutants suggests that these residues, along with the ligand itself, play a cumulative role in recognition by the ABC transporter FhuBGC₂. In addition, small angle x-ray scattering was used to demonstrate that, in solution, FhuD2 does not undergo a detectable change in conformation upon binding iron(III)-hydroxamates. Therefore, the mechanism of binding and transport of ligands for binding proteins within this family is significantly different from that of other well studied binding protein families, such as that represented by maltose-binding protein.