SdrG, a Fibrinogen-binding Bacterial Adhesin of the Microbial Surface Components Recognizing Adhesive Matrix Molecules Subfamily from Staphylococcus epidermidis, Targets the Thrombin Cleavage Site in the Bβ Chain

Staphylococcus epidermidis is an important opportunistic pathogen and is a major cause of foreign body infections. We have characterized the ligand binding activity of SdrG, a fibrinogen-binding microbial surface component recognizing adhesive matrix molecules from S. epidermidis. Western ligand blot analysis showed that a recombinant form of the N-terminal A region of SdrG bound to the native Bβ chain of fibrinogen (Fg) and to a recombinant form of the Bβ chain expressed in Escherichia coli. By analyzing recombinant truncates and synthetic peptide mimetics of the Fg Bβ chain, the binding site for SdrG was localized to residues 6-20 of this polypeptide. Recombinant SdrG bound to a synthetic 25-amino acid peptide (β1-25) representing the N terminus of the Fg Bβ chain with a K_D of 1.4 × 10 ^-7 M as determined by fluorescence polarization experiments. This was similar to the apparent K_D (0.9 × 10^-7 M) calculated from an enzyme-linked immunosorbent assay where SdrG bound immobilized Fg in a concentration-dependent manner. SdrG could recognize fibrinopeptide B (residues 1-14), but with a substantially lower affinity than that observed for SdrG binding to synthetic peptides >1-25 and >6-20. However, SdrG does not bind to thrombin-digested Fg. Thus, SdrG appears to target the thrombin cleavage site in the Fg B> chain. In fact, SdrG was found to inhibit thrombin-induced fibrinogen clotting by interfering with fibrinopeptide B release.