Fyn and Lck tyrosine kinases regulate tyrosine phosphorylation of p105(CasL), a member of the p130(Cas) docking protein family, in T-cell receptor-mediated signalling

We have previously shown that engagement of the T-cell receptor (TCR)/CD3 c omplex with anti-CD3 antibody induces tyrosine phosphorylation of p105(CasL ) (CasL), a member of the p130(Cas) docking protein family. In the present work, we attempted to determine which protein tyrosine kinases (PTKs) regul ate TCR-mediated phosphorylation of Cast. We show here that an association between Cast and two types of Src family PTKs, Fyn and Lck, is induced by a nti-CD3 cross-linking of human H9 T cells. In contrast, ZAP-70, another PTK that also plays a critical role in the TCR signalling, failed to bind Cast , even after anti-CD3 stimulation. In vitro kinase assays revealed that Fyn and Lck, but not ZAP-70, were capable of phosphorylating Cast. Moreover, w e found that Cast was constitutively hyperphospborylated in vivo in splenoc ytes of MRL-MP-lpr/lpr mice, in which overproduction and excessive activati on of Fyn and Lck have previously been shown to occur. Constitutive ill viv o binding of Cast to both kinases was also demonstrated in lpr splenocytes. These results strongly suggest that Cast is a substrate far Fyn and Lck PT Ks in TCR signal transduction.