Co-operative binding of human fibronectin to SfbI protein triggers streptococcal invasion into respiratory epithelial cells

Streptococcal fibronectin binding protein I (SfbI) mediates adherence to an d invasion of Streptococcus pyogenes into human epithelial cells. In this s tudy, we analysed the binding activity of distinct domains of SfbI protein towards its ligand, the extracellular matrix component fibronectin, as well as the biological implication of the binding events during the infection p rocess, By using purified recombinant SfbI derivatives as well as in vivo e xpressed SfbI domains on the surface of heterologous organism Streptococcus gordonii, we were able to dissociate the two major streptococcal target do mains on the human fibronectin molecule. The SfbI repeat region exclusively bound to the 30 kDa N-terminal fragment of fibronectin, whereas the SfbI s pacer region exclusively bound to the 45 kDa collagen-binding fragment of f ibronectin, In the case of native surface-expressed SfbI protein, an induce d fit mode of bacteria-fibronectin interaction was identified. We demonstra te that binding of the 30 kDa fibronectin fragment to the repeat region of SfbI protein co-operatively activates the adjacent SfbI spacer domain to bi nd the 45 kDa fibronectin fragment. The biological consequence arising from this novel mode of fibronectin targeting was analysed in eukaryotic cell i nvasion assays. The repeat region of SfbI protein is mediating adherence an d constitutes a prerequisite for subsequent invasion, whereas the SfbI spac er domain efficiently triggers the invasion process of streptococci into th e eukaryotic cell. Thus, we were able to dissect bacterial adhesion from in vasion by manipulating one protein. SfbI protein therefore represents a hig hly evolved prokaryotic molecule that exploits the host factor fibronectin not only for extracellular targeting but also for its subsequent activation that leads to efficient cellular invasion.