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Turnover of Phosphoryl Groups in Reticulocyte Ribosomal Phosphoproteins

From the ¹ From the Department of Biochemistry, University of Oregon Medical School, Portland, Oregon 97201

A quantitative study is made of the number and turnover rate of phosphoryl groups in rabbit reticulocyte ribosomal phosphoproteins. Reticulocytes are incubated with [³²P]-orthophosphate at 37° in a nutrient medium, and the labeling of ribosomal proteins is compared with the labeling of the precursor -phosphoryl group of ATP. Whereas ATP attains a constant specific activity within 30 min, the specific activities of the five most highly labeled ribosomal phosphoproteins continue to increase for 2 hours in an approximately constant ratio. The data for kinetics of specific activity change suggest that ribosomes contain a steady state average of 7 to 11 protein phosphoryl groups, which are distributed among at least five different polypeptide chains, and which turn over at the rate of approximately 3% per min.

Polyribosomes and single ribosomes have a strikingly different pattern of protein phosphorylation. At least three of the phosphoproteins in single ribosomes are phosphorylated to a different steady state level than the same protein in polyribosomes. These data suggest that there is no rapid mixing of these classes of ribosomes or of their phosphoproteins and supports previous studies which have shown that single ribosomes in eukaryotes are not participating in the ribosomal subunit-polyribosome cycle of protein synthesis. Furthermore, it is concluded from the kinetic data that polysomal and single ribosomes differ in their conformations, and that their ³²P-labeling differences result from this difference in ribosomal conformation.

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