THE KINASE, SH3, AND SH2 DOMAINS OF LCK PLAY CRITICAL ROLES IN T-CELLACTIVATION AFTER ZAP-70 MEMBRANE LOCALIZATION

Antigenic stimulation of the T-cell antigen receptor initiates signal transduction through the immunoreceptor tyrosine-based activation moti fs (ITAMs). When its two tyrosines are phosphorylated, ITAM forms a bi nding site for ZAP-70, one of the cytoplasmic protein tyrosine kinases essential for T-cell activation. The signaling process that follows Z AP-70 binding to ITAM has been analyzed by the construction of fusion proteins that localize ZAP-70 to the plasma membrane. We found that me mbrane-localized forms of ZAP-70 induce late signaling events such as activation of nuclear factor of activated T cells without any stimulat ion. This activity was observed only when Lck was expressed and functi onal, In addition, each mutation that affects the function of Lck in t he kinase, Src homology 2 (SH2), and SH3 domains greatly impaired the signaling ability of the chimeric protein. Therefore, Lck functions in multiple manners in T-cell activation for the steps following ZAP-70 binding to ITAM.