Protein F, a fibronectin-binding protein of Streptococcus pyogenes, also binds human fibrinogen: isolation of the protein and mapping of the binding region.

During screening of a gene library of Streptococcus pyogenes type M15 for fibrinogen-binding material, a protein of approximately 100 kDa, encoded outside the vir region, was found. DNA sequencing revealed this component to be identical to protein F, a fibronectin-binding protein. Isolation of the recombinant protein, termed F15, was performed by the use of fibrinogen affinity chromatography. The affinity constant (Ka) of protein F15 for fibrinogen, 1.25 x 10(7) mol-1, was lower than that for fibronectin, 1.8 x 10(8) mol-1. The fibrinogen-binding domain was located in the N-terminal part of the molecule, while the fibronectin-binding domains, as previously determined, were in the C-terminal portion of protein F. To examine the amino acid sequence heterogeneity of protein F, the 5' part of the prtF gene, corresponding to the N-terminal variable region of the protein, was amplified by PCR from 12 strains of S. pyogenes belonging to six different M-types. Alignment of these nucleotide sequences indicated that the 5' portion of the prtF gene had probably undergone a number of intragenic recombination and horizontal gene transfer events, allowing a pattern of structural diversity of protein F observed earlier for some other streptococcal virulence factors. There was no strict correlation between M-type and nucleotide sequence of the variable region of the prtF gene and, compared to streptococcal M protein, the overall variation observed for protein F appeared more limited.