Biochemical evidence for a homophilic interaction of the α3β1 integrin

The purpose of this study was to determine if the α3β1 integrin could interact in a homophilic manner. Several earlier reports have shown that certain integrin adhesion receptors, namely α2β1, α3β1, and α6β4 localize to intercellular adhesion structures 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., and Klein, C. E. (1989) J. Cell Biol. 109, 1807- 1815; Hynes, R. O. (1987) 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 that the integrin α3β1 may interact homophilically in such cell- cell adhesion structures which contain this specific receptor or, alternatively, in receptor aggregates found in focal adhesions. The α3β1 receptor was purified by affinity chromatography on either human laminin or peptide GD-6-Sepharose and subsequently used as a substrate in cell adhesion assays. The immobilized α3β1 supported the adhesion of cells containing α3β1, and this attachment was specifically inhibited by monoclonal antibodies to both β1 and α3 subunits. In addition, an affinity matrix containing purified α3β1 showed specific binding of only α3β1 from detergent extracts of cell surface proteins and such binding was cation- dependent. Finally, using biosensor technology involving the principle of surface plasmon resonance (BIAcore(TM), Pharmacia Biosensor), α3β1, when bound to a carboxymethyl dextran-modified gold surface, was found to bind only other soluble α3β1 receptors and did not bind other purified integrins, including α5β1 and αvβ3. These data strongly suggest that α3β1 likely interacts in a homophilic manner under our experimental conditions.