T-CELL ACTIVATION-INDUCED BY NOVEL GAIN-OF-FUNCTION MUTANTS OF SYK AND ZAP-70

The Syk family tyrosine kinases play a crucial role in antigen recepto r-mediated signal transduction, but their regulation and cellular targ ets remain incompletely defined. Following receptor engagement, phosph orylation of tyrosine residues within ZAP-70 and Syk is thought to con trol both kinase activity and recruitment of modulatory factors. We re port here the characterization of novel mutants of ZAP-70 and Syk, in which conserved C-terminal tyrosine residues have been replaced by phe nylalanines (ZAP YF-C, Syk YF-C), Both mutant kinases display a promin ent gain-of-function phenotype in Jurkat T cells, as demonstrated by l ymphokine promoter activation, tyrosine phosphorylation of potential t argets in vivo, and elevated intracellular calcium mobilization. While the presence of p56-Lck was required for ZAP YF-C-induced signaling, Syk YF-C showed enhanced functional activity in Lck-deficient JCaM1 Ju rkat cells. Our results implicate the C terminus of Syk family kinases as an important regulatory region modulating T cell activation.