Lck promotes Zap70-dependent LAT phosphorylation by bridging Zap70 to LAT

T cell antigen receptor (TCR) signaling depends upon the kinases Lck and Zap70. Lck phosphorylates the TCR, facilitating Zap70 recruitment to the stimulated TCR. Lck also phosphorylates Zap70, relieving its auto-inhibition and activating its catalytic domain. Zap70 then phosphorylates the critical adaptors LAT and SLP76 which serve to nucleate key effector molecules required for downstream responses. However, mechanisms facilitating the interaction of Zap70 with its substrates have not been described. We report an evolutionarily conserved proline-rich motif in LAT is important for Zap70-induced phosphorylation of LAT and downstream signaling. This LAT proline-rich motif associated with the Lck SH3 domain, thereby facilitating Zap70-mediated phosphorylation of LAT and downstream functions. Our results suggest Lck orchestrates multiple steps in TCR signaling including the newly described facilitation of the interaction of Zap70 with its substrate LAT. This previously unrecognized feature of TCR proximal signaling may contribute to the development of more immunomodulatory therapies.