PROTEIN-F - AN ADHESIN OF STREPTOCOCCUS-PYOGENES BINDS FIBRONECTIN VIA 2 DISTINCT DOMAINS

The binding of Streptococcus pyogenes to fibronectin (FN) enables the adherence of this pathogen to target epithelial cells, which is the fi rst necessary step for initiation of infection. Binding is mediated by a bacterial surface protein termed protein F. Here we provide the com plete structure of protein F and identify two domains responsible for binding to fibronectin. The first domain is located towards the C-term inal end of the molecule and is composed of five repeats of 37 amino a cids that are completely repeated four times and a fifth time partiall y. The second domain is adjacent to the first domain and is located on the N-terminal side of it. It is composed of a single stretch of 43 a mino acids. Protein F expressed in Escherichia coli completely blocked the binding of fibronectin to S. pyogenes. However, mutant proteins t hat contained only one or the other of the two domains were only capab le of partial blockage of binding. Complete blockage of binding of fib ronectin could be achieved when a protein extract containing the N-ter minal domain was mixed in a binding reaction with a protein extract co ntaining the C-terminal domain. Similarly, a purified recombinant prot ein containing the two domains only, blocked the binding completely. I n contrast, a purified recombinant protein containing just the C-termi nal domain, blocked the binding partially. A clone exclusively express ing the C-terminal domain, completely blocked the binding of the 30 kD a N-terminal fragment of fibronectin to S. pyogenes, whereas a clone e xpressing the N-terminal domain failed to block the binding of this FN fragment. Thus, the two FN-binding domains of protein F are necessary for maximal bacterial binding and act in concert to enhance the bindi ng to fibronectin. The Possibility that the two domains bind to two di fferent regions on the fibronectin molecule is discussed.