The fibronectin-binding MSCRAMM FnbpA of Staphylococcus aureus is a bifunctional protein that also binds to fibrinogen

Staphylococcus aureus is an important pathogen capable of causing a wide sp ectrum of diseases in humans and animals, This bacterium expresses a variet y of virulence factors that participate in the process of infection. These include MSCRAMMs ((m) under bar icrobial (s) under bar urface (c) under bar omponents (r) under bar ecognizing (a) under bar dhesive (m) under bar atr ix (m) under bar olecules) that mediate the adherence of the bacteria to ho st extracellular matrix components, such as collagen, fibronectin (Fn), and fibrinogen (Fg), Two Fn-binding MSCRAMMs, FnbpA and FnbpB, have been previ ously identified. The Fn binding activity has been localized to the similar to 40-amino acid residue D repeats in the C-terminal part of these protein s. However, no biological activity has yet been attributed to the N-termina l A regions of these proteins. These regions exhibit substantial amino acid sequence identity to the A regions of other staphylococcal MSCRAMMs, inclu ding ClfA, ClfB, and SdrG (Fbe), all of which bind Fg. This raises the ques tion of whether the Fn-binding MSCRAMMs can also bind specifically to Fg, I n this report, we show that a recombinant form of the A region of FnbpA doe s specifically recognize Fg. We localize the binding site in Fg for recombi nant FnbpA to the gamma-chain, in particular to the C-terminal residues of this polypeptide, the site also recognized by ClfA. In addition, we demonst rate that recombinant FnbpA can compete with ClfA for binding to both immob ilized and soluble Fg. By the use of surface plasmon resonance spectroscopy and fluorescence polarization, we determine the dissociation equilibrium c onstant for the interaction of recombinant FnbpA with intact immobilized Fg and with a synthetic C-terminal gamma-chain peptide, respectively. Finally , by overexpressing FnbpA in a mutant strain of S. aureus that lacks the ex pression of both ClfA and ClfB, we show that native FnbpA can mediate the i nteraction of S. aureus with soluble Fg.