Effect of Cyclic Adenosine 3′ : 5′-Monophosphate on Ribosomal Protein Phosphorylation in Reticulocytes

We have studied the effects of cyclic adenosine 3' : 5'-monophosphate (cyclic AMP) on ribosomal protein phosphorylation in rabbit reticulocytes. The phosphoryl groups in the five different ribosomal phosphoproteins turn over intracellularly and become radioactive when the cells are incubated with [32P]orthophosphate. These phosphoproteins are present in highly purified ribosomal subunits which lack messenger ribonucleic acid. Cyclic AMP and N6, O2'-dibutyryl adenosine 3' : 5'-monophosphate stimulate the incorporation of radioactivity into the ribosomal phosphoproteins. This increase in labeling occurs rapidly and is caused by two separable influences. First, cyclic AMP causes an increase in the specific activity of the incorporated phosphoryl groups. Second, cyclic AMP specifically causes increased phosphorylation of one among the ribosomal phosphoproteins. This protein (which we have termed Protein II) has a molecular weight of approximately 27,500 and is located on the smaller ribosomal subunits.