Rabbit reticulocyte ribosomes isolated from low ionic strength solutions are contaminated with a ribosomal protein kinase. Washing the ribosomes in a high ionic strength solution elutes the kinase. The same ribosomal proteins previously shown to be phosphorylated within reticulocytes incubated with [32P]orthophosphate are labeled in the cell-free system with [γ-32P]ATP. Radioactivity in both cases is incorporated into o-phosphoserine and o-phosphothreonine residues of the polypeptide chains. The rate of ribosomal protein phosphorylation declines as ATP becomes depleted from the cell-free system. The phosphorylation kinetics are accordingly very strongly influenced by ATPase and also by phosphoprotein phosphatase which are present in reticulocytes. The kinase may have a broad substrate specificity since it appears to be active in phosphorylation of chicken erythrocyte histones.
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