Structural Requirements for β₁ Integrin-Mediated Tyrosine Phosphorylation in Human T Cells

The β₁ integrin adhesion receptors activate signal transduction pathways that induce tyrosine phosphorylation of a variety of substrates. Increased tyrosine phosphorylation is mediated by the β₁, subunit cytoplasmic domain, which consists of 46 amino acids and contains no intrinsic kinase activity. In the H9 T cell line, β₁ integrin engagement leads to the increased tyrosine phosphorylation of three 105 to 115-kDa substrates that are distinct from focal adhesion kinase (FAK): HEF1 (human enhancer of filamentation 1), a protein with structural homology to p130^Cas, and two novel substrates, pp105 and pp115. DNA-mediated gene transfer was used to explore the role of the β₁ cytoplasmic domain in integrin-mediated tyrosine phosphorylation of HEF1, pp105, and pp115 in human T cells. Using a chimeric receptor composed of the cytoplasmic domain of the β₁ integrin subunit and the extracellular and transmembrane domains of the CD2 Ag, we demonstrate that the β₁ cytoplasmic domain is necessary and sufficient for inducing tyrosine phosphorylation of each of these three substrates in H9 T cells. Analysis of a series of β₁ cytoplasmic domain truncations reveals that a truncation of only five amino acids from the carboxyl-terminal end of the β₁ cytoplasmic domain abrogates the ability of the CD2/β₁ chimera to activate tyrosine phosphorylation of HEF1, pp105, or pp115. Thus, the carboxyl-terminal five amino acids, Lys-Tyr-Glu-Gly-Lys (KYEGK), of the β₁ integrin cytoplasmic domain are critical for the coordinate tyrosine phosphorylation of three non-FAK substrates in human T cells.