55] Phosphorylation of ribosomal proteins: preparation of rat liver ribosomal protein kinases. Assay of the phosphorylation of ribosomal subunit proteins. Assay of the function of phosphorylated ribosomes

Abstract

The proteins of eukaryotic ribosomes are phosphorylated in vivo and in vitro . In the latter case the reaction is catalyzed by protein kinases present in the cytosol or associated with ribosomes. Two such enzymes, both stimulated by cyclic adenosine 3', 5’-monophosphate (cyclic AMP), can be isolated from rat liver cytosol; the protein kinases transfer the terminal phosphate from ATP to serine and threonine residues of specific proteins of rat liver ribosomal subunits. The chapter describes techniques devised (or adapted) for the preparation of rat liver ribosomal protein kinases, for the assay of the phosphorylation of the proteins of ribosomal subunits, for analysis of the product of the reaction, for assay of ribosomal phosphoprotein phosphatase, and for the comparison of the function of phosphorylated and nonphosphorylated ribosomes.