Transmembrane signal transduction by integrin cytoplasmic domains expressed in single-subunit chimeras

Integrins are heterodimeric, transmembrane cell adhesion receptors that have recently been shown to function in transmembrane signal transduction. To examine the specific role of integrin intracellular domains in signal transduction, chimeric receptors containing various integrin intracellular domains coupled to a reporter consisting of the transmembrane and extracellular domains of the small, non-signaling subunit of the interleukin-2 receptor were expressed in cultured human fibroblasts and assayed for their ability to trigger tyrosine phosphorylation of the 125-kDa cytoplasmic tyrosine kinase, pp125FAK. Tyrosine phosphorylation of pp125FAK was induced in cultured fibroblasts that transiently expressed chimeric receptors containing either the beta 1, beta 3, or beta 5 integrin intracellular domain and were selected by magnetic bead sorting. However, expression of chimeric receptors containing either the alpha 5 or an alternatively spliced form of the beta 3 intracellular domain (beta 3B), as well as those lacking an intracellular domain, failed to induce tyrosine phosphorylation of pp125FAK. These results indicate that information contained in the beta 1, beta 3, or beta 5 integrin intracellular domain is sufficient to stimulate integrin-mediated tyrosine phosphorylation of specific intracellular proteins and that integrin extracellular and transmembrane domains are not required for inducing tyrosine phosphorylation. Our results also indicate that alternative splicing can regulate the ability of beta integrin intracellular domains to participate in signal transduction, and they further suggest that the carboxyl-terminal region of specific beta integrins may play a role in the signal transduction pathway involving extracellular matrix molecules.