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 receptor (TCR) engagement is the activation of a protein tyrosine kinase thought to be a member of the src family. Expression of CD45, the major receptor tyrosine phosphatase present on T cells, is required for efficient coupling between the TCR and its signaling machinery. One model to explain the role of CD45 in regulating TCR signaling is that the phosphatase dephosphorylates the regulatory C-terminal tyrosine of lck. In the present report we confirm the finding of a trimolecular complex containing CD45, lck, and a 34-kDa protein (p34) in the Jurkat T cell line. Additionally, we extend this work with the observation that specific in vitro kinase activity associated with CD45 requires the expression of lck in Jurkat-derived clones as does the in vivo phosphorylation of p34. The association between CD45 and lck is shown not to require the expression of or activation of the TCR. Finally, we demonstrate that even in the absence of lck p34 associates with CD45, implying a direct association between this molecule and the phosphatase. These data suggest strongly that lck is the relevant protein tyrosine kinase found in CD45 immunoprecipitates in the Jurkat T cell line and that there is an additional association between CD45 and p34 which does not require the presence of the protein tyrosine kinase.