Identification of the minimal tyrosine residues required for linker for activation of T cell function

The linker for activation of T cells (LAT) is essential for signaling through the T cell receptor (TOR). Following TCR stimulation, LAT becomes tyrosine-phosphorylated, creating docking sites for other signaling proteins such as phospholipase C-gamma (1) (PLC-gamma (1)), Grb2, and Gads. In this study, we have attempted to identify the critical tyrosine residues in LAT that mediate TCR activation-induced mobilization of intracellular Ca2+ and activation of the MAP kinase Erk2. By using the LAT-deficient Jurkat derivative, J.CaM2, stable cell lines were established expressing various tyrosine mutants of LAT. We show that three spe. cific tyrosine residues (Tyr(132), Tyr(171), and Tyr(191)) are necessary and sufficient to achieve a Ca2+ flux following TOR stimulation. These tyrosine residues function by reconstituting PLC-gamma (1) phosphorylation and recruitment to LAT. However, these same tyrosines can only partially reconstitute Erk activation. Full reconstitution of Erk requires two additional tyrosine residues (Tyr(110) and Tyr(226)), both of which have the Grb2-binding motif YXN. This reconstitution of Erk activation requires that the critical tyrosine residues be on the same molecule of LAT, suggesting that a single LAT molecule nucleates multiple protein-protein interactions required for optimal signal transduction.