Phosphorylation of Ribosomal Proteins in Higher Organisms

Abstract

Highly purified ribosomes from rabbit reticulocytes and from mouse sarcoma 180 tumor cells contain at least five different phosphoproteins which become radioactive when the cells are incubated with [³²P]-orthophosphate. That the phosphoproteins from these two sources may be homologous proteins is suggested by their identical molecular weights and ribosomal subunit localizations.

Certain of the ribosomal phosphoproteins occur in the larger ribosomal subunits, whereas one of the most heavily phosphorylated proteins occurs in the smaller subunits. The components in larger subunits are more extensively phosphorylated in single ribosomes (which are inactive in protein synthesis) than in polysomal ribosomes, whereas the converse is true for the phosphoprotein located in smaller subunits. These differences reflect a known configurational difference between these two classes of ribosomes. Cyclic AMP specifically enhances the phosphorylation only of the protein in the smaller subunits. A cell-free system has been developed for assaying the protein synthetic activity of mammalian ribosomal subunits. Ribosomal subunits derived from single ribosomes are as active in this system as those derived from polysomes. Apparently, the phosphorylation differences between subunits from these sources do not affect their activities in this assay system.

Supported by U.S. Public Health Service Grant HL-CA-14960-04. A. Krystosek, and L. Bitte were supported by a predoctoral training grant from the U.S. Public Health Service. The paper was presented by D. Kabat.