The Lck SH3 domain is required for activation of the mitogen-activated protein kinase pathway but not the initiation of T-cell antigen receptor signaling

Initiation of T-cell antigen receptor (TCR) signaling is dependent upon the activity of protein tyrosine kinases, The Src family kinase Lck is require d for the initial events in TCR signaling, such as the phosphorylation of t he TCR complex and the activation of ZAP-70, but little is known of its rol e in downstream signaling. Expression of a mutated form of Lck lacking SH3 domain function (LckW97A) in the Lck-deficient T-cell line JCaM1 revealed a requirement for Lck beyond the initiation of TCR signaling. In cells expre ssing LckW97A, stimulation of the TCR failed to activate the mitogen-activa ted protein kinase (MAPK) pathway, despite normal TCR zeta chain phosphoryl ation, ZAP-70 recruitment, and ZAP-70 activation. Activation of extracellul ar signal-regulated kinase (ERK) and MAPK kinase (MEK), as well as the indu ction of CD69 expression, was greatly impaired in JCaM1/LckW97A cells. In c ontrast, the phosphorylation of phospholipase C gamma 1 (PLC gamma 1) and c orresponding elevations in intracellular calcium concentration ([Ca2+](i)) were intact. Thus, cells expressing LckW97A exhibit selective defect in the activation of the MAPK pathway. These results demonstrate that Lck has a r ole in the activation of signaling pathways beyond the initiation of TCR si gnaling and suggest that the MAPK pathway may be selectively controlled by regulating the function of Lck.