THE SRC-FAMILY KINASE, FYN, REGULATES THE ACTIVATION OF PHOSPHATIDYLINOSITOL 3-KINASE IN AN INTERLEUKIN 2-RESPONSIVE T-CELL LINE

The proliferation of antigen-activated T cells is mediated by the T ce ll-derived growth factor, interleukin 2 (IL-2). The biochemical signal ing cascades initiating IL-2-induced growth are dependent upon protein tyrosine kinase (PTK) activity. One IL-2-regulated PTK implicated in this cascade is the Src-family kinase, Fyn. Previous studies have desc ribed a physical association between Fyn and a potential downstream su bstrate, phosphatidylinositol 3-kinase (PI3-kinase) as well as the IL- 2-dependent activation of PI3-kinase in T cells; however, the role of Fyn in IL-2-induced PI3-kinase activation remains unclear. In this rep ort, we demonstrate that IL-2 stimulation triggers tyrosine phosphoryl ation of the p85 subunit of PI3-kinase in the murine T cell line, CTLL -2. Lysates prepared from growth factor-deprived and IL-2-stimulated T cells reconstituted both the binding of CTLL-2 cell-derived Fyn to an d the IL-2-inducible tyrosine phosphorylation of exogenously added rec ombinant p85. Furthermore, overexpression of wild-type Fyn in these ce lls enhanced both the basal and IL-2-mediated activation of PI3-kinase . Additional studies of the Fyn-PI3-kinase interaction demonstrated th at the Src homology 3 (SH3) domain of Fyn constitutes a direct binding site for the p85 subunit of PI3-kinase. These results support the not ion that Fyn may be directly involved in the activation of the downstr eam signaling enzyme, PI3-kinase, in IL-2-stimulated T cells.