A POINT MUTATION IN THE INTEGRIN BETA-6 SUBUNIT ABOLISHES BOTH ALPHA-V-BETA-6 BINDING TO FIBRONECTIN AND RECEPTOR LOCALIZATION TO FOCAL CONTACTS

The integrin alpha V beta 6 was initially identified from primary cult ures of airway epithelial cells. This integrin is expressed in bronchi olar and alveolar epithelium during development and in settings of inj ury and/or inflammation and mediates attachment of epithelial cells to fibronectin and tenascin. Like other integrins, this receptor localiz es to structures called focal contacts in cells plated on appropriate ligands. In the present study, we produced a mutant beta 6 cDNA (beta 6m) containing a single substitution of Asp(140) With Ala and transfec ted mutant (or wild-type) beta 6 cDNA into the human colon carcinoma c ell line SW480. In parallel, we used cDNAs truncated just proximal to the transmembrane domain to generate secreted forms of mutant alpha V beta 6 in Chinese hamster ovary (CHO) cells. The mutant beta 6, like t he wild type, formed heterodimers with human alpha V that were express ed on the cell surface of SW480 cells and secreted by CHO cells. Secre ted alpha V beta 6 containing this point mutation did not bind to fibr onectin-Sepharose. Furthermore, in contrast to wild-type beta 6, the m utant form did not allow SW480 cells to bind to fibronectin in the pre sence of beta 1-blocking antibody and did not localize to focal contac ts. Our results confirm that the Asp(140) of beta 6, like the correspo nding residues in beta 1 (Asp(130)) and beta 3 (Asp(119)), is critical for interactions of alpha V beta 6 with ligand, and also suggest that ligand binding to alpha V beta 6 is necessary for localization of thi s receptor to focal contacts.