Phosphorylation of Ribosomal Proteins in Eukaryotes

Abstract

INTRODUCTION
Approximately four years ago, we first obtained evidence that several of the proteins in rabbit reticulocyte ribosomes are phosphoproteins rather than simple proteins. Their phosphoryl groups turn over intracellularly with a half-life of approximately 25 min, they become radioactive when the cells are incubated with [³²P]orthophosphate, and they occur in o-phosphoserine and in o-phosphothreonine residues (Kabat 1970Kabat 1972). Thus the phosphoproteins are sites of an active and continuous metabolism. In addition, phosphorylation of ribosomal proteins occurs in other tissues (Blat and Loeb 1971; Loeb and Blat 1970; Majumder and Turkington 1972; Barden and Labrie 1973; Correze, Pinell and Nunez 1972; Bitte and Kabat 1972), in higher plants (Trewavas 1973) and in yeasts (J. Warner, personal communication), and it may therefore be ubiquitous in eukaryotes. However, ribosome phosphorylation is apparently absent from bacteria (Gordon 1971).

In this paper we review the evidence which indicates that ribosomal phosphoproteins exist intracellularly and describe some of the physiological variables, including cyclic AMP, that influence this metabolism. Also described are some of our preliminary attempts to analyze the functions of ribosome phosphorylation. In addition, we have briefly reviewed the studies of ribosomal protein phosphorylation in vitro which have employed [γ-³²P]ATP and several different kinds of protein kinase.

Evidence for Ribosomal Phosphoproteins
When rabbit reticulocytes are incubated in a nutritionally rich medium with [³²P]orthophosphate at 37°C, several of their ribosomal proteins become highly radioactive. Furthermore, acid hydrolysates of these proteins contain radioactive o-phosphoserine and o-phosphothreonine (Kabat 1970Kabat 1972; Bitte and Kabat 1974). Typical electrophoretic fractionations...