title: Structural and functional characterization of the Staphylococcus aureus virulence factor and vaccine candidate FhuD2.
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. In the present paper we explore the structure and function of FhuD2 (ferric-hydroxamate uptake D2), 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 iron(III)-dependent. We determined the X-ray crystallographic structure of ligand-bound FhuD2 at 1.9 Å (1 Å=0.1 nm) resolution, revealing the bilobate fold of class III SBPs (solute-binding proteins). 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.
authors.affiliation: Novartis Vaccines and Diagnostics srl, Via Fiorentina 1, 53100 Siena, Italy.
authors.forename: Paolo
authors.initials: P
authors.lastname: Mariotti
authors.forename: Enrico
authors.initials: E
authors.lastname: Malito
authors.forename: Marco
authors.initials: M
authors.lastname: Biancucci
authors.forename: Paola
authors.initials: P
authors.lastname: Lo Surdo
authors.forename: Ravi P N
authors.initials: RP
authors.lastname: Mishra
authors.forename: Vincenzo
authors.initials: V
authors.lastname: Nardi-Dei
authors.forename: Silvana
authors.initials: S
authors.lastname: Savino
authors.forename: Mikkel
authors.initials: M
authors.lastname: Nissum
authors.forename: Glen
authors.initials: G
authors.lastname: Spraggon
authors.forename: Guido
authors.initials: G
authors.lastname: Grandi
authors.forename: Fabio
authors.initials: F
authors.lastname: Bagnoli
authors.forename: Matthew J
authors.initials: MJ
authors.lastname: Bottomley
mesh_terms: D000818:Animals; D000942:Antigens, Bacterial / Q000737:chemistry / Q000235:genetics; D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics / Q000276:immunology / Q000378:metabolism; D018360:Crystallography, X-Ray; D005290:Ferric Compounds / Q000378:metabolism; D005291:Ferrichrome / Q000378:metabolism; D005798:Genes, Bacterial; D006801:Humans; D006877:Hydroxamic Acids / Q000378:metabolism; D026901:Membrane Transport Proteins / Q000737:chemistry* / Q000235:genetics / Q000276:immunology / Q000378:metabolism; D051379:Mice; D008958:Models, Molecular; D033902:Periplasmic Binding Proteins / Q000737:chemistry* / Q000235:genetics / Q000276:immunology / Q000378:metabolism; D055550:Protein Stability; D017262:Siderophores / Q000378:metabolism; D013209:Staphylococcal Vaccines / Q000737:chemistry; D013211:Staphylococcus aureus / Q000235:genetics / Q000276:immunology / Q000378:metabolism* / Q000472:pathogenicity; D055672:Static Electricity; D014168:Transferrin / Q000378:metabolism; D014774:Virulence; D037521:Virulence Factors / Q000737:chemistry* / Q000235:genetics / Q000276:immunology / Q000378:metabolism
chemical_list: D000942:Antigens, Bacterial; D001426:Bacterial Proteins; D005290:Ferric Compounds; D006877:Hydroxamic Acids; D026901:Membrane Transport Proteins; D033902:Periplasmic Binding Proteins; D017262:Siderophores; D013209:Staphylococcal Vaccines; D014168:Transferrin; D037521:Virulence Factors; C074579:iron (III) hydroxamate; D005291:Ferrichrome