Protein Modification Enzymes Associated with the Protein- synthesizing Complex from Rabbit Reticulocytes

A number of protein modification activities are present in the protein-synthesizing complex isolated from rabbit reticulocytes. These enzymes are solubilized by sedimentation of the ribosomes through buffered sucrose containing 0.5 M KCI, and have been partially purified from the high salt wash fraction by chromatography on DEAE-cellulose and phosphocellulose. The ribosomal-associated enzymatic activities include cyclic AMP-regulated and cyclic nucleotideindependent protein kinases, phosphoprotein phosphatase, and acetyltransferase activities. These enzymatic activities have been shown to modify specific ribosomal and ribosomal-associated proteins. The cyclic AMP-regulated protein kinases phosphorylate the 40 S ribosomal subunit from rabbit reticulocytes. One of the cyclic nucleotide-independent protein kinases catalyzes the phosphorylation of two different factors involved in the initiation of hemoglobin synthesis. A single phosphoprotein phosphatase activity is shown to remove phosphate from 40 S ribosomal subunits. The major acetyltransferase activity associated with ribosomes acetylates a 60 S ribosomal protein.