Identification of a fibronectin binding protein from Streptococcus mutans.

The interaction of viridans streptococci with components of the extracellular matrix (ECM) plays an important role in the pathogenesis of infective endocarditis. We have identified a surface protein of Streptococcus mutans which binds the ECM constituent fibronectin (Fn). Initially, we found that S. mutans could adsorb soluble Fn in plasma, but with lower efficiency than Streptococcus pyogenes. In addition, S. mutans could bind immobilized Fn in a dose-dependent manner when tested using an enzyme-linked immunosorbent assay. Crude extracts of cell wall-associated proteins or extracellular proteins from S. mutans MT8148 specifically bound Fn through a protein with the molecular mass of ca. 130 kDa, as detected by far-Western immunoblotting. The candidate Fn binding protein (FBP-130) was purified to near homogeneity by using Fn coupled Sepharose 4B affinity column chromatography. A rabbit polyclonal antibody against FBP-130 reacted specifically with a protein of molecular mass of ca. 130 kDa in both cell wall and extracellular fractions, and the abundance of FBP was higher in the former than in the latter fractions. The purified FBP bound specifically to immobilized Fn, whereas the binding of soluble Fn to coated FBP could only be detected in the presence of high concentrations of Fn. The purified FBP, as well as anti-FBP immunoglobulin G, inhibited the adherence of S. mutans to immobilized Fn and endothelial cells (ECV304) in a dose-dependent manner. These results demonstrated that FBP-130 mediated the adherence of S. mutans specifically to Fn and endothelial cells in vitro. The characteristics of S. mutans and FBP-130 in binding Fn confirmed that viridans streptococci adopt different strategies in their interaction with ECM.