Fibronectin binding to a Streptococcus pyogenes strain

In previous studies, Staphylococcus aureus has been shown to bind fibronectin (P. Kuusela, Nature (London) 276:718-720, 1978), an interaction that may be important in bacterial attachment and opsonization. Recently some strains of streptococci of serological groups A, C, and G were also found to bind fibronectin. The binding to one selected strain of Streptococcus pyogenes has been characterized here. The binding of [¹²⁵I]fibronectin to streptococcal cells resembles that to staphylococcal cells and was found to be time dependent, functionally irreversible, and specific in the sense that unlabeled proteins other than fibronectin did not block binding. Bacteria incubated with proteases largely lost their ability to bind fibronectin, and material released from the streptococci by a brief trypsin digestion contained active fibronectin receptors. This material inhibited the binding of [¹²⁵I]fibronectin to the streptococci. The inhibitory activity was adsorbed on a column of fibronectin-Sepharose but not on a column of unsubstituted Sepharose 4B or egg albumin Sepharose. The receptor appeared to be a protein nature since the inhibitory activity of the trypsinate was destroyed by papain and was not adsorbed on a column containing monoclonal antibodies direct against lipoteichoic acid bound to protein A-Sepharose. Binding sites in fibronectin for streptococci and staphylococci, respectively, were localized by analyzing the ability of isolated fragments to inhibit [¹²⁵I]fibronectin binding to bacteria and by adsorbing ¹²⁵I-labeled tryptic fragments with staphylococcal and streptococcal cells. Both species of bacteria appeared to preferentially bind a fragment (M(r) = ~25,000) originating from the N-terminal region of the protein. In addition, streptococci also bound a slightly smaller fragment (M(r) = ~23,000). Fibronectin receptors solubilized from either streptococci or staphylococci inhibited the binding of fibronectin to both species of bacteria.