Heterogeneity of fibronectin reactivity among streptococci as revealed by binding of fibronectin fragments.

Streptococci belonging to serological groups A, B, C, G, L and U were studied for their interaction with 125I-labelled fibronectin and its fragments. Fibronectin purified from humans plasma by affinity chromatography on gelatin-agarose and heparin-agarose was cleaved by thrombin into a 29,000 Dalton and a 210,000 Dalton fragments. Terminal analysis of purified fragments indicated that 29,000 fragment was from amino-terminal and 210,000 fragment from carboxyl-terminal domain of fibronectin. Binding of fibronectin was observed in all streptococci except those of group B. Streptococci of groups A, G, L, U and S. equisimilis reacted only with 29,000 Dalton fragment whereas S. dysgalactiae, S. zooepidermicus and S. equi reacted only with 210,000 Dalton fragment. The streptococcal binding sites for these two fragments were distinct from each other. Fibrinogen blocked the binding of 210,000 Dalton fragment but not of 29,000 Dalton fragment. Trypsinization of streptococci did not affect their binding sites for 210,000 Dalton fragment but destroyed those for 29,000 Dalton fragment. The results indicate that the streptococci of group A and G as well as S. equisimilis which are mainly pathogenic in humans bind amino-terminal fragment of fibronectin. This may facilitate the adherence of these pathogens. On the other hand, the streptococci isolated from animal infections had different binding sites recognized only by carboxyl-terminal part of fibronectin.