BIOCHEMICAL-EVIDENCE FOR A HOMOPHILIC INTERACTION OF THE ALPHA-3-BETA-1 INTEGRIN

The purpose of this study was to determine if the alpha3beta1 integrin could interact in a homophilic manner. Several earlier reports have s hown that certain integrin adhesion receptors, namely alpha2beta1, alp ha3beta1, and alpha6beta4 localize to intercellular adhesion structure s and, therefore, may participate in cell-cell interactions (Carter, W . G., Wayner, E. A., Bouchard, T. S., and Kaur, P. (1990) J. Cell Biol . 110, 1387-1404; Kaufmann, R., Frosch, D., Westphal, C., Weber, L., a nd Klein, C. E. (1989) J. Cell Biol. 109, 1807-1815; Hynes, R. O. (198 7) Cell 48, 549-554; Symington, B. E., Takada, Y., and Carter, W. G. ( 1993) J. Cell Biol. 120, 523-535). We present data herein suggesting t hat the integrin alpha3beta1 may interact homophilically in such cell- cell adhesion structures which contain this specific receptor or, alte rnatively, in receptor aggregates found in focal adhesions. The alpha3 beta1 receptor was purified by affinity chromatography on either human laminin or peptide GD-6-Sepharose and subsequently used as a substrat e in cell adhesion assays. The immobilized alpha3beta1 supported the a dhesion of cells containing alpha3beta1, and this attachment was speci fically inhibited by monoclonal antibodies to both beta1 and alpha3 su bunits. In addition, an affinity matrix containing purified alpha3beta 1 showed specific binding of only alpha3beta1 from detergent extracts of cell surface proteins and such binding was cation-dependent. Finall y, using biosensor technology involving the principle of surface plasm on resonance (BIAcore(TM), Pharmacia Biosensor), alpha3beta1, when bou nd to a carboxymethyl dextran-modified gold surface, was found to bind only other soluble alpha3beta1 receptors and did not bind other purif ied integrins, including alpha5beta1 and alphavbeta3. These data stron gly suggest that alpha3beta1 likely interacts in a homophilic manner u nder our experimental conditions.