FUNCTIONAL AND PHYSICAL INTERACTION OF PROTEIN-TYROSINE KINASES FYN AND CSK IN THE T-CELL SIGNALING SYSTEM

The Src-like protein-tyrosine kinase Fyn is associated with T-cell ant igen receptor. Transient expression of actively mutated Fyn, having Ph e-528 instead of Tyr-528 or Thr-338 instead of Ile-338, in Jurkat T-ce lls stimulated the serum response element (SRE), 12-O-tetradecanoyl-ph orbol-13-acetate response element, cyclic AMP response element, and c- fos promoter. The stimulation of SRE was particularly prominent not on ly with active Fyn but also with normal (wild-type) Fyn. SRE was also stimulated by both normal and active Lck. Furthermore, normal and acti ve Fyn stimulated transcription from the IL-2 gene promoter when trans fected cells were stimulated by concanavalin A plus 12-O-tetradecanoyl -phorbol-13-acetate Under the same conditions, Lck did not stimulate I L-2 promoter unless it was activated by mutation. Interestingly, a mut ant Fyn, which has deletions within the SH2 region and so is able to t ransform chicken embryo fibroblasts, did not stimulate either the c-fo s or IL-2 promoter, suggesting the importance of this region in T-cell signaling. Csk, which phosphorylates tyrosine residues in the negativ e regulatory sites of Src family kinases, down-regulated Fyn- and Lck- mediated stimulation of the serum response element and Fyn-mediated en hancement of IL-2 promoter activity. These data suggest that Fyn and L ck, whose activities are regulated by Csk, are involved in different p hases of T-cell activation.