CHARACTERIZATION OF A NOVEL FIBRONECTIN-BINDING SURFACE PROTEIN IN GROUP-A STREPTOCOCCI

Streptococcus pyogenes interacts with host fibronectin via distinct surface components. One of these components is the SfbI protein (streptococcal fibronectin-binding protein, now specified as class I), an adhesin that represents a protein family with characteristic features. Here we present the complete structure of a novel fibronectin-binding protein of S. pyogenes, designated SfbII, which is distinct from the previously described SfbI proteins. The sfbII gene originated from a lambda EMBL3 library of chromosomal DNA from group A streptococcal strain A75 and coded for a 113 kDa protein exhibiting features of membrane-anchored surface proteins of Gram-positive cocci. The expression of biologically active fusion proteins allowed the determination of the location of the fibronectin-binding domain within the C-terminal part of the protein. It consisted of two and a half repeats which share common motifs with fibronectin-binding repeats of other streptococcal and staphylococcal proteins. Purified recombinant fusion protein containing this domain competitively inhibited the binding of fibronectin to the parental S. pyogenes strain. Furthermore, polyclonal antibodies against the binding domain specifically blocked the SfbII receptor site on the streptococcal surface. No cross-reactivity could be detected between anti-SfbII antibodies and the sfbI gene product, and vice versa, indicating that the two proteins do not share common immunogenic epitopes. Southern hybridization experiments performed with specific sfbII gene probes revealed the presence of the sfbII gene in more than 55% of 93 streptococcal isolates tested. The majority of the strains also harboured the sfbI gene, and 86% carried at least one of the two sfb genes.