Sharon, M., Cell Biology and Metabolism Branch, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892, United States
Gnarra, J.R., Cell Biology and Metabolism Branch, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892, United States
Leonard, W.J., Cell Biology and Metabolism Branch, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892, United States

IL-2 has previously been shown to rapidly induce activity of a tyrosine kinase. High-affinity IL-2 receptors that mediate the major mitogenic signals of IL-2 contain both p70 and p55 chains. p55 has no potential tyrosine phosphorylation sites and lacks consensus sequences found in protein tyrosine kinases. Inasmuch as the phosphorylation of hormone receptors is generally an important mechanism for regulating receptor function, we have now investigated the phosphorylation status of p70. By using anti-phosphotyrosine antibodies to immuno-precipitate affinity-labeled IL-2 receptors and to probe Western blots, we provide data suggesting that p70, but not p55, is constitutively tyrosine-phosphorylated on the leukemic cell lines studied.