CD45-ASSOCIATED KINASE-ACTIVITY REQUIRES LCK BUT NOT T-CELL RECEPTOR EXPRESSION IN THE JURKAT T-CELL LINE

The earliest biochemical event known to be associated with T cell rece ptor (TCR) engagement is the activation of a protein tyrosine kinase t hought to be a member of the src family. Expression of CD45, the major receptor tyrosine phosphatase present on T cells, is required for eff icient coupling between the TCR and its signaling machinery. One model to explain the role of CD45 in regulating TCR signaling is that the p hosphatase dephosphorylates the regulatory C-terminal tyrosine of lck. In the present report we confirm the finding of a trimolecular comple x containing CD45, lck, and a 34-kDa protein (p34) in the Jurkat T cel l line. Additionally, we extend this work with the observation that sp ecific in vitro kinase activity associated with CD45 requires the expr ession of lck in Jurkat-derived clones as does the in vivo phosphoryla tion of p34. The association between CD45 and lck is shown not to requ ire the expression of or activation of the TCR. Finally, we demonstrat e that even in the absence of lck p34 associates with CD45, implying a direct association between this molecule and the phosphatase. These d ata suggest strongly that lck is the relevant protein tyrosine kinase found in CD45 immunoprecipitates in the Jurkat T cell line and that th ere is an additional association between CD45 and p34 which does not r equire the presence of the protein tyrosine kinase.