Fibronectin-binding protein B variation in Staphylococcus aureus

Abstract

BackgroundFibronectin binding proteins A and B (FnBPA and FnBPB) mediate adhesion of S. aureus to fibrinogen, elastin and fibronectin. We previously identified seven different isotypes of FnBPA based on divergence in the fibrinogen- and elastin-binding A domains. The variation created differences in antigenicity while ligand binding functions were retained. Here, FnBPB variation was examined in both human and bovine isolates and compared to that of FnBPA.ResultsSeven different fnbB allelic variants were identified. Some strains that cluster by phylogenetic analysis contain different fnbB variants, whereas more divergent strains contain the same fnbB variant. The phylogeny of fnbB alleles does not match the phylogeny of fnbA alleles. Some FnBPA and FnBPB isotypes that are specified by human S. aureus strains are also found in bovine strains. The seven fnbB allelic variants encode seven distinct isotypes of the FnBPB A domain that are 61 to 85% identical in amino acid sequence. Variant amino acid residues were mapped on a three-dimensional model of the FnBPB A domain and were predicted to be surface-exposed. They are responsible for the antigenic diversity detected with polyclonal antibody and a monoclonal antibody raised against isotype I. Ligand binding by recombinant FnBPB N23 isotypes was compared by ELISA-based solid phase assays and surface plasmon resonance. Each bound to immobilized fibrinogen, elastin and fibronectin dose-dependently and saturably with similar affinities. Binding to fibronectin was surprising because the A domains do not contain any known motifs that mediate binding to fibronectin. This raises the possibility that the A domain of FnBPB contains a novel fibronectin binding motif that binds fibronectin by a novel mechanism.ConclusionsSeven different isoforms of FnBPB A domain retain ligand-binding functions but are antigenically distinct. The variation in FnBPA and FnBPB occurs in human and bovine S. aureus strains and may act as an immune evasion mechanism. All seven isotypes of FnBPB are capable of binding fibronectin though none contain any known fibronectin-binding motifs. These results have implications for the development of vaccines or immunotherapeutics that target FnBPB

Molecular Interactions of Human Plasminogen with Fibronectin-binding Protein B (FnBPB), a Fibrinogen/Fibronectin-binding Protein from Staphylococcus aureus

“…9). The K D for the interaction was 0.532 Ϯ 0.028 M, an affinity that is nearly 4-fold higher than the interaction between FnBPB and FBG (33).…”

“…There are seven isoforms of FnBPB N2N3 subdomains with amino acid sequence identities ranging from 61 to 85% (33). FnBPBs from strains 8325-4, LAC, and BH1CC have identical amino acid sequences and belong to isoform I.…”

“…FnBPBs from strains 8325-4, LAC, and BH1CC have identical amino acid sequences and belong to isoform I. Each of the seven isoforms retains the ability to bind FBG, fibronectin, and elastin (33). The recombinant N2N3 subdomains of isoforms II-VII were purified and tested for their ability to bind to PLG in an ELISA-type assay.…”

Subdomains N2N3 of Fibronectin Binding Protein A Mediate Staphylococcus aureus Biofilm Formation and Adherence to Fibrinogen Using Distinct Mechanisms

“…The sequences of the fnbA and fnbB genes of HA-MRSA strain BH1CC (CC8) were determined by DNA sequencing of PCR products amplified from genomic DNA. BH1CC expresses fibronectin binding proteins with A domain amino acid sequences identical to those of FnBPA and FnBPB from strain 8325 (isotype I) (20,21). Ligand affinity blotting analysis of proteins solubilized from the cell wall and probed with biotinylated fibronectin revealed that FnBPs were expressed at much higher levels by BH1CC than by laboratory strain SH1000 in the exponential phase of growth ( Fig.…”

“…The N2 and N3 subdomains of FnBPA and FnBPB have undergone considerable amino acid sequence divergence. At least seven different isotypes of FnBPA and FnBPB exist, and all bind to ligands with similar affinities despite considerable antigenic differences (20,21).…”