Phospho-LAT-independent activation of the ras-mitogen-activated protein kinase pathway: a differential recruitment model of TCR partial agonist signaling.

Stimulation of mature T cells with agonist ligands of the Ag receptor (TCR) causes rapid phosphorylation of tyrosine-based activation motifs in the intracellular portion of TCR-zeta and CD3 and activation of several intracellular signaling cascades. Coordinate activation of these pathways is dependent on Lck- and ZAP-70-mediated tyrosine phosphorylation of a 36-kDa linker for activation of T cells and subsequent recruitment of phospholipase C-gamma1, Grb2-SOS, and SLP-76-vav. Here, we show that TCR partial agonist ligands can selectively activate one of these pathways, the Ras-mitogen-activated protein kinase pathway, by inducing recruitment of Grb2-SOS complexes to incompletely phosphorylated p21 phospho-TCR-zeta. This bypasses the need for activation of Lck and ZAP-70, and for phosphorylation of the linker for activation of T cells to activate Ras. We propose a general model in which differential recruitment of activating complexes away from transmembrane linker proteins may determine selective activation of a given signaling pathway.