RECONSTITUTION OF T-CELL ANTIGEN RECEPTOR-INDUCED ERK2 KINASE ACTIVATION IN LCK-NEGATIVE JCAM1 CELLS BY SYK

The two related protein-tyrosine kinases Syk and Zap are rapidly phosp horylated on tyrosine residues and enzymatically activated upon crossl inking of the T cell antigen receptor. We have previously reported tha t the activation of Syk is less dependent on the Src family kinase Lck than the activation of Zap. Here we report that overexpression of Syk in the Lck-negative JCaM1 cells enabled the T cell antigen receptor/C D3 complex to induce a normal activation of the mitogen-activated prot ein kinase (MAPK) pathway and expression of a nuclear factor of activa ted T cells reporter construct. In contrast, Zap and other protein-tyr osine kinases were unable to reconstitute these signaling pathways whe n expressed at the same levels. In parallel, Syk was phosphorylated on tyrosine, while Zap was not. The Syk-mediated T cell antigen receptor -induced MAPK activation was detectable within 1 min of receptor stimu lation and peaked at 3-5 min. The capacity of Syk to reconstitute the MAPK response required the catalytic activity of Syk, an intact autoph osphorylation site (Y518 and Y519), both Src homology 2 domains and it was blocked by the inhibitory N17-mutated dominant-negative Ras const ruct. A Y341-->F mutant of Syk, which is deficient in its interaction with phospholipase C gamma 1 and Vav, was less efficient than wild-typ e Syk. Our results suggest that Syk, in contrast to Zap, can transduce signals from the T cell antigen receptor independently of Lck.