Multiple roles played by Src family tyrosine kinases in TCR signal transduction

Research and Development Corporation and Pharmaceuticals Laboratory II, Life Science Research Sector, Mitsubishi Chemical Corporation Antigenic stimulation on the T cell antigen receptor (TCR) initiates signal transduction through the immunoreceptor tyrosine-based activation motifs (ITAMs). When its two tyrosines are phosphorylated, ITAM forms a binding site for Z:\P-70, one of the cytoplasmic protein tyrosine kinases essential for T cell activation. Lck appears to be a dominant kinase that phosphorylates tyrosines in ITAMs but Fyn also plays the equivalent role to Lck when T cells are stimulated by super antigens. The signaling process that follows ZAP-70 binding to ITAM has been analyzed by the construction of fusion proteins that localize ZAP-70 to the plasma membrane. We found that membrane-localized forms of chimeric ZAP-70 proteins induce late signaling events such as activation of nuclear factor of activated T-cells (NF-AT) without any stimulation. This activity was observed only when Lck was expressed and functional. Each mutation that affects the function of Lck in the kinase, Src-homology 2 (SH2) and Src-homology 3 (SII3) domains greatly impaired the signaling ability of the chimeric protein. Among the SH3 domain binding proteins, we identified a molecule that is a substrate for ZAP-70. TCR stimulation also induces tyrosino phosphorylation of the molecule. This indicates that the Stt3 domain of Lck may flmction as an adaptor to bring the substrate for ZAP-70 into the TCR complex for tyrosine phosphorylation by ZAP-70 and induction of downstream signaling events.