GENETIC-EVIDENCE OF A ROLE FOR LCK IN T-CELL RECEPTOR FUNCTION INDEPENDENT OR DOWNSTREAM OF ZAP-70 SYK PROTEIN-TYROSINE KINASES/

T-cell antigen receptor (TCR) engagement results in sequential activat ion of the Src protein tyrosine kinases (PTKs) Lck and Fyn and the Syk PTKs, ZAP-70 and Syk. While the Src PTKs mediate the phosphorylation of TCR-associated signaling subunits and the phosphorylation and activ ation of the Syk PTKs, the lack of a constitutively active Syk PTK has prohibited the analysis of Lck function downstream of these initiatin g signaling events. We describe here the generation of an activated Sy k family PTK by substituting the kinase domain of Syk for the homologo us region in ZAP-70 (designated as KS for kinase swap). Expression of the KS chimera resulted in its autophosphorylation, the phosphorylatio n of cellular proteins, the upregulation of T-cell activation markers, and the induction of interleukin-2 gene synthesis in a TCR-independen t fashion. The KS chimera and downstream ZAP-70 or Syk substrates, suc h as SLP-76, were still phosphorylated when expressed in Lck-deficient JCaM1.6 T cells. However, expression of the KS chimera in JCaM1.6 cel ls failed to rescue downstream signaling events, demonstrating a funct ional role for Lck beyond the activation of the ZAP-70 and Syk PTKs. T hese results indicate that downstream TCR signaling pathways may be di fferentially regulated by ZAP-70 and Lck PTKs and provide a mechanism by which effector functions may be selectively activated in response t o TCR stimulation.