Ail Protein Binds Ninth Type III Fibronectin Repeat (9FNIII) within Central 120-kDa Region of Fibronectin to Facilitate Cell Binding by Yersinia pestis

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 (FNIII)-F-9 blocks Ail-mediated bacterial binding to Fn. Epitopes of three mAbs that blocked Ail binding to Fn were mapped to a similar face of (FNIII)-F-9. Antibodies directed against (FNIII)-F-9 also inhibited Ail-dependent cell binding activity, thus demonstrating the biological relevance of this Ail binding region on Fn. Bacteria expressing Ail on their surface could also bind a minimal fragment of Fn containing repeats 9-10FNIII, and this binding was blocked by a mAb specific for 9FNIII. These data demonstrate that Ail binds to (FNIII)-F-9 of Fn and presents Fn to host cells to facilitate cell binding and delivery of Yops (cytotoxins of Y. pestis), a novel interaction, distinct from other bacterial Fn-binding proteins.