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The Yersinia pestis adhesin molecule Ail interacts with the extracellular matrix protein fibronectin (Fn) on host cells to facilitate efficient delivery of cytotoxic Yop proteins, a process essential for plague virulence. A number of bacterial pathogens are known to bind to the N-terminal region of Fn, comprising type I Fn (FNI) repeats. Using proteolytically generated Fn fragments and purified recombinant Fn fragments, we demonstrated that Ail binds the centrally located 120-kDa fragment containing type III Fn (FNIII) repeats. A panel of monoclonal antibodies (mAbs) that recognize specific epitopes within the 120-kDa fragment demonstrated that mAb binding to ⁹FNIII blocks Ail-mediated bacterial binding to Fn. Epitopes of three mAbs that blocked Ail binding to Fn were mapped to a similar face of ⁹FNIII. Antibodies directed against ⁹FNIII also inhibited Ail-dependent cell binding activity, thus demonstrating the biological …