host tissue. In this report we have analyzed the binding of a strain of Streptococcus dysgalactiae to fibronectin. The cells bind to a site in the NH2-terminal domain of the protein via trypsin-sensitive cell surface components. Furthermore, a lysate prepared by sonication of streptococcal cells contained fibronectin-binding proteins that inhibit the binding of the ligand to intact bacteria. When the proteins were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, blotted to an Immobilon-P filter, and probed with 125I-labeled fibronectin, a 140-kDa fibronectin-binding protein was identified along with a number of smaller binding proteins. A genomic DNA library was constructed and screened for the expression of fibronectin-binding proteins. Two clones were isolated and shown to contain unrelated inserts by restriction mapping and cross-hybridization experiments. The two encoded proteins were also immunologically distinct although both bound to the same region of the fibronectin molecule, and both effectively inhibited the binding of 125I-fibronectin to bacterial cells. Immunological analyses showed that only one of the two proteins tentatively identified as fibronectin receptors was expressed in detectable quantities in the Streptococcus dysgalactiae strain under the culture conditions employed.
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