Analysis of the activity and phosphorylation of the lck protein in lymphoid cells

p56lck, the tyrosine protein kinase encoded by the lck gene, is expressed at a 40-fold elevated level in the LSTRA cell line. This is associated with increased tyrosine phosphorylation of cellular proteins. We have asked here whether the increased tyrosine protein phosphorylation is due to an altered activity of the protein or to the unusually high level of p56lck. In vitro protein kinase assays showed that neither the specific activity nor the affinity of p56lck for two different substrates was abnormal in LSTRA cells. Additionally, analysis of the phosphorylation of p56lck in LSTRA and other cell lines showed that the protein was phosphorylated extensively at a negative-regulatory site, Tyr 505, in all of the cells examined. Since the primary structure of the p56lck expressed at a high level in LSTRA cells is the same as that found in normal thymus and we found no evidence of activation of the protein by dephosphorylation, it appears that high levels of p56lck can induce increased tyrosine protein phosphorylation in lymphoid cells. In contrast, high levels of the closely related protein, p60c-src have no significant effect on tyrosine protein phosphorylation in fibroblasts. The regulation of the protein kinase activity of p56lck in lymphoid cells may therefore differ from the regulation of p60c-src in fibroblasts.