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 B beta chain

Staphylococcus epidermidis is an important opportunistic pathogen and is a major cause of foreign body infections. We have characterized the ligand bi nding activity of SdrG, a fibrinogen-binding microbial surface component re cognizing adhesive matrix molecules from S. epidermidis. Western ligand blo t analysis showed that a recombinant form of the N-terminal A region of Sdr G bound to the native B beta chain of fibrinogen (Fg) and to a recombinant form of the B beta chain expressed in Escherichia coli. By analyzing recomb inant truncates and synthetic peptide mimetics of the Fg B beta chain, the binding site for SdrG was localized to residues 6-20 of this polypeptide. R ecombinant SdrG bound to a synthetic 25-amino acid peptide (beta1-25) repre senting the N terminus of the Fg B beta chain with a K-D of 1.4 x 10(-7) M as determined by fluorescence polarization experiments. This was similar to the apparent K-D (0.9 x 10(-7) M) calculated from an enzyme-linked immunos orbent assay where SdrG bound immobilized Fg in a concentration-dependent m anner. SdrG could recognize fibrinopeptide B (residues 1-14), but with a su bstantially lower affinity than that observed for SdrG binding to synthetic peptides beta1-25 and beta6-20. However, SdrG does not bind to thrombin-di gested Fg. Thus, SdrG appears to target the thrombin cleavage site in the F g B beta chain. In fact, SdrG was found to inhibit thrombin-induced fibrino gen clotting by interfering with fibrinopeptide B release.