A plasmocytoma ribosome-associated protein kinase which phosphorylates a specific protein of the ribosomal KCl wash.

Abstract

One ribosomal protein kinase activity and 3 soluble protein kinase activities have been identified in plasma cell tumors by DEAE-cellulose chromatography. We have shown phosphorylation in vivo and in vitro of a protein fraction from the ribosomal KCl wash which we have termed 'PPx fraction'. Phosphorylation of this protein fraction has been obtained in vitro with the ribosome-associated protein kinase. We have determined for the ribosomal protein kinase the following characteristics. 1. It is an Mg2+-dependent enzyme that transfers the gamma-phosphate from ATP into phosphoseryl and phosphothreonyl residues of the substrate. 2. It has a wide substrate specificity. Like the soluble protein kinases it catalyses the phosphorylation of several proteins like histone, phosvitin, casein and ribosomal proteins but it differs from the main soluble kinases (I, II) by the fact that it catalyses specifically the phosphorylation at least of one of the ribosomal KCl wash proteins. On dodecylsulfate-polyacrylamide gels this protein has a molecular weight of approximately 90000 and it is released from ribosomes under conditions commonly employed for extraction of initiation factors. 3. The ribosome-associated protein kinase is not stimulated by the addition of cyclic adenosine 3':5'-monophosphate. 4. KCl has no effect, NaCl has a weak effect on the phosphorylation, Mn2+ and Ca2+ are inhibitors. 5. ADP has been found to be a competitive inhibitor. 6. The maximum velocity of the ribosomal protein-kinase-catalysed reaction is 0.65 nmol of 32P incorporated in the KCl wash protein per min and per mg protein. 7. The apparent Km for the ribosomal KCl wash protein as substrate is 0.71 mg/ml and the Km for ATP is 94 muM. 8. The molecular weight of the ribosomal protein kinase, estimated by electrophoresis in polyacrylamide-dodecylsulfate gels, is 60000 and corresponds probably to a catalytic subunit.