Molecular characterization of the interaction of staphylococcal microbial surface components recognizing adhesive matrix molecules (MSCRAMMS) ClfA and Fbl with fibrinogen

Abstract

The ligand binding domain of the fibrinogen binding protein from Staphylococcus lugdunensis (Fbl) shares 60% sequence identity with clumping factor A (ClfA) of Staphylococcus aureus. Recombinant Fbl corresponding to the minimum fibrinogen-binding region (subdomains N2N3) was compared to ClfA for binding to fibrinogen. Fbl and ClfA had very similar affinities for fibrinogen by surface plasmon resonance. The binding site for Fbl in fibrinogen was localised to the extreme C-terminus of the fibrinogen gamma-chain at the same site recognised by ClfA. Isothermal titration calorimetry (ITC) showed that Fbl and ClfA had very similar affinities for a peptide mimicking the C-terminal segment of the fibrinogen gamma-chain. The peptide also inhibited binding of Fbl and ClfA to fibrinogen. A series of substituted gamma-chain variant peptides behaved very similarly when used to inhibit ClfA and Fbl binding to immobilized fibrinogen. Both ClfA and Fbl bound to bovine fibrinogen with a lower affinity compared to human fibrinogen and did not bind detectably to ovine fibrinogen. The structure of the N2N3 subdomains of Fbl in complex with the fibrinogen gamma-chain peptide was modelled based on the crystal structure of the N2N3 subdomains of ClfA:gamma-chain peptide complex. Residues in the putative binding trench likely to be involved in fibrinogen binding were identified. Fbl variant proteins with alanine substitutions in key residues had reduced affinities for fibrinogen. Thus Fbl and ClfA bind the same site in fibrinogen by similar mechanisms.