Streptococcus pyogenes fibronectin-binding protein F2: expression profile, binding characteristics, and impact on eukaryotic cell interactions.

Some Streptococcus pyogenes (group A streptococci, GAS) strains have previously been shown to express the fibronectin-binding protein F2 instead of the functionally related but structurally dissimilar protein F1/SfbI. In this study, recombinant N-terminal and C-terminal portions and the two fibronectin-binding domains of protein F2 were used to assess affinity parameters of the interaction with fibronectin and its N-terminal 70-, 30-, and 45-kDa fragments. The association and dissociation equilibrium constants for both binding domains were in the nanomolar range, although the repeat domain of protein F2 exceeded the affinity of the unique domain by up to one order magnitude. Both domains primarily interacted with the 30-kDa fibronectin fragment. Using a prtF2 gene isogenic mutant of a serotype M49 GAS strain that does not harbor the protein F1/SfbI gene, the attachment values of whole bacteria to immobilized fibronectin and to HEp-2 epithelial cells were found to be 6- and 2-fold decreased, respectively. Reduction of prtF2 mutant internalization rates for eukaryotic cells exceeded the reduction of attachment rates, indicating an independent contribution of protein F2 to both processes. The prtF2 transcription and protein F2 expression profiles documented maximum expression at the transition to the stationary phase especially under aerobic growth condition. The protein F2 function as the major fibronectin-binding adhesin in a subset of GAS strains, its expression pattern, and highly specific interaction with fibronectin would be consistent with a status as an indispensable virulence factor for both earlier and later pathogenetic stages of GAS superficial infections.