Structural and functional characterization of the Staphylococcus aureus virulence factor and vaccine candidate FhuD2.

Biochemical Journal (Impact Factor: 4.65). 10/2012; DOI: 10.1042/BJ20121426
Source: PubMed

ABSTRACT Staphylococcus aureus is a human pathogen causing globally significant morbidity and mortality. The development of antibiotic resistance in S. aureus highlights the need for a preventive vaccine. Here we explore the structure and function of FhuD2, a staphylococcal surface lipoprotein mediating iron-uptake during invasive infection, recently described as a promising vaccine candidate. Differential scanning fluorimetry and calorimetry studies revealed that FhuD2 is stabilized by hydroxamate siderophores. The FhuD2-ferrichrome interaction was of nanomolar affinity in surface plasmon resonance experiments and fully Fe3+-dependent. We determined the x-ray crystallographic structure of ligand-bound FhuD2 at 1.9Å resolution, revealing the bilobate fold of class III solute-binding proteins (SBPs). The ligand, ferrichrome, occupies a cleft between the FhuD2 N- and C-terminal lobes. Many FhuD2-siderophore interactions enable the specific recognition of ferrichrome. Biochemical data suggest that FhuD2 does not undergo significant conformational changes upon siderophore binding, supporting the hypothesis that the ligand-bound complex is essential for receptor engagement and uptake. Finally, immunizations with FhuD2 alone or FhuD2 formulated with hydroxamate siderophores were equally protective in a murine staphylococcal infection model, confirming the suitability and efficacy of apo-FhuD2 as a protective antigen, and suggesting that other Class III SBPs might also be exploited as vaccine candidates.

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    ABSTRACT: The features that govern the interaction of ligand binding proteins with membrane permeases of cognate ABC transporters are largely unknown. Using sequence alignments and structural modeling based on the structure of the Escherichia coli BtuCD vitamin B12 transporter, we identified six conserved basic residues in the permease, comprised of FhuB and FhuG proteins, in the ferrichrome transporter of Staphylococcus aureus. Using alanine-scanning mutagenesis we demonstrate that two of these residues, FhuB Arg-71 and FhuG Arg-61, play a more dominant role in transporter function than FhuB Arg-74 and Arg-311, and FhuG Arg-64 and Lys-306. Moreover, we show that at positions 71 and 61 in FhuB and FhuG, respectively, arginine cannot be substituted for lysine without loss of transporter function. Previously, our laboratory demonstrated the importance of conserved acidic residues in the ferrichrome binding protein, FhuD2. Taken together, these results support the hypothesis that Glu-Arg salt bridges are critical for the interaction of the ligand binding protein with the transmembrane domains FhuB and FhuG. This hypothesis was further studied by "charge swapping" experiments whereby we constructed a S. aureus strain expressing FhuD2 with conserved residues Glu-97 and Glu-231 replaced by Arg and FhuB and FhuG with conserved basic residues Arg-71 and Arg-61, respectively, replaced by Glu. A strain containing this combination of substitutions restored partial function to the ferrichrome transporter. The results provide a direct demonstration of the functional importance of conserved basic residues on the extracellular surface of the ferrichrome permease in the Gram-positive bacterium S. aureus.
    Biology of Metals 12/2013; · 3.17 Impact Factor


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May 19, 2014