Multiple specificities of the staphylococcal and streptococcal fibronectin-binding microbial surface components recognizing adhesive matrix molecules

Many pathogenic Gram-positive bacteria express fibronectin (Fn)-binding mic robial surface components recognizing adhesive matrix molecules (MSCRAMMs), most of which have a similar structural organization with a primary ligand -binding domain consisting of 3-6 repeats of 40-50 amino-acid-residue motif s. The MSCRAMMs appear to preferentially bind to the N-terminal region of F n, which is composed of five type-I modules. Here we report that the Fn-bin ding MSCRAMM FnbpA of Staphylococcus aureus contains a second ligand-bindin g domain located outside the repeat units. In addition, several sites in th e Fn N-terminus presented as recombinant type-I module pairs bind to the re peat domain of the MSCRAMM. All of the MSCRAMMs analyzed, which include Fnb pA of Staphylococcus aureus, Sfb of Streptococcus pyogenes, and FnbA and Fn bB of Streptococcus dysgalactiae, were shown to bind to multiple sites in t he N-terminal domain of Fn. By dissecting the repeat domain of FnbpA using synthetic peptides and recombinant fragments, we show that discrete, differ ent motifs are responsible for the binding to individual sites in Fn, rathe r than a common motif being able to bind to several pairs of type-I Fn modu les. The C-terminal half of many of the MSCRAMM repeat units contain a comm on motif. which is shown here to bind to the type-I module pair 4 and 5. In addition, some of the repeat units of FnbpA contain N-terminal motifs whic h bound to the type-I module pairs 1-2 and 2-3, respectively. These latter binding motifs appear to be partly overlapping and dependent on flanking se quences. Fluorescence polarization experiments using fluorescein-labeled MS CRAMM peptides and recombinant type-I Fn module pairs revealed dissociation constants of 1-13 mu M. It was also shown that the fluorescein-labeled pep tides differed in their primary binding sites on Fn.