Staphylococcus aureus experimental
endocarditis relies on sequential
fibrinogen binding (for valve colonization) and
fibronectin binding (for endothelial invasion) conferred by
peptidoglycan-attached adhesins.
Fibronectin-binding protein A (FnBPA) reconciles these two properties--as well as
elastin binding--and promotes experimental
endocarditis by itself. Here we attempted to delineate the minimal subdomain of FnBPA responsible for
fibrinogen and
fibronectin binding, cell invasion, and in vivo
endocarditis. A large library of truncated constructs of FnBPA was expressed in Lactococcus lactis and tested in vitro and in animals. A 127-amino-acid subdomain spanning the hinge of the FnBPA
fibrinogen-binding and
fibronectin-binding regions appeared necessary and sufficient to confer the sum of these properties. Competition with synthetic
peptides could not delineate specific
fibrinogen- and
fibronectin-binding sites, suggesting that dual binding arose from protein folding, irrespective of clearly defined binding domains. Moreover, coexpressing the 127-amino-acid subdomain with remote domains of FnBPA further increased
fibrinogen binding by > or =10 times, confirming the importance of domain interactions for binding efficacy. In animals,
fibrinogen binding (but not
fibronectin binding) was significantly associated with
endocarditis induction, whereas both
fibrinogen binding and
fibronectin binding were associated with disease severity. Moreover,
fibrinogen binding also combined with
fibronectin binding to synergize the invasion of cultured cell lines significantly, a feature correlating with
endocarditis severity. Thus, while
fibrinogen binding and
fibronectin binding were believed to act sequentially in colonization and invasion, they appeared unexpectedly intertwined in terms of both functional anatomy and pathogenicity (in
endocarditis). This unforeseen FnBPA subtlety might bear importance for the development of antiadhesin strategies.