Activation of a cellular tyrosine-specific protein kinase by phosphorylation.

A tyrosine-specific protein kinase of Mr 56 000 was purified over 200-fold from rat spleen. Incubation of this kinase preparation with ATP and Mg2+ results in about 10-fold increase in the protein kinase activity. The activation of the kinase was unaffected in the presence of soybean trypsin inhibitor. Polyacrylamide gel electrophoresis of the enzyme preparation after phosphorylation with ATP showed one phosphoprotein band of Mr 56 000. During purification of this kinase a large decrease in enzyme activity was observed which could be prevented by adding 10 microM vanadate, as inhibitor of tyrosine-specific protein phosphatases. These results suggest that the activation of the protein kinase by ATP is due to phosphorylation of the enzyme.