The EIIIA Segment of Fibronectin Is a Ligand for Integrins α₉β₁ and α₄β₁Providing a Novel Mechanism for Regulating Cell Adhesion by Alternative Splicing*

Alternative splicing of the fibronectin gene transcript gives rise to forms that include the EIIIA (or ED-A) segment. EIIIA-containing fibronectins are prominently expressed during embryogenesis and wound healing and appear to mediate changes in cell adhesion and gene expression. Nonetheless, integrins that bind the EIIIA segment have not been identified. We previously mapped the epitope for two function-blocking monoclonal antibodies to the C-C′ loop region of the EIIIA segment (Liao, Y.-F., Wieder, K. G., Classen, J. M., and Van De Water, L. (1999) J. Biol. Chem. 274, 17876–17884). The sequence of this epitope (³⁹PEDGIHELFP⁴⁸) resembles the sequence within tenascin-C to which the integrin α₉β₁binds. We now report that either integrin α₉β₁ or α₄β₁can mediate cell adhesion to the EIIIA segment. Moreover, this interaction is blocked both by epitope-mapped EIIIA antibodies as well as by the respective anti-integrins. Deletion mutants of the EIIIA segment that include the C-C′ loop and flanking sequence bind cells expressing either α₉β₁ or α₄β₁. Adhesion of α₄β₁-containing MOLT-3 cells to the EIIIA segment stimulates phosphorylation of p44/42 MAP kinase. Our observation that two integrins bind the EIIIA segment establishes a novel mechanism by which cell adhesion to fibronectin is regulated by alternative splicing.