α₄β₁ Integrin-Mediated Tyrosine Phosphorylation in Human T Cells: Characterization of Crk- and Fyn-Associated Substrates (pp105, pp115, and Human Enhancer of Filamentation-1) and Integrin-Dependent Activation of p59^fyn1

Integrin adhesion receptors transduce signals that transmit information from the extracellular environment to the cell interior. Although integrins lack intrinsic tyrosine kinase activity, stimulation of the α₄β₁ integrin on human H9 T cells results in rapid tyrosine phosphorylation of proteins in the 105 to 115 kDa range. In this study, we report that α₄ integrin stimulation of H9 T cells results in tyrosine phosphorylation of three distinct proteins: pp105, pp115, and human enhancer of filamentation 1 (HEF1), all of which associate with the adapter protein c-Crk. However, pp115 can be distinguished from pp105 and HEF1 by its ability to associate with the SH2 domain of the tyrosine kinase p59^fyn. Both pp105 and pp115 are antigenically distinct from HEF1, p130^Cas, pp125^FAK, Pyk2, p120^cbl, and the p110 subunit of phosphatidylinositol 3-kinase. The functional significance of pp115 association with p59^fyn is suggested by the ability of α₄ integrin stimulation to activate Fyn tyrosine kinase activity. These studies show that α₄ integrin stimulation of T cells results in the tyrosine phosphorylation of several distinct substrates. The association of these substrates with intracellular signaling intermediates, such as Crk and Fyn, may play a critical role in integrin-mediated regulation of T cell function.