The specific phosphorylation of a 40S ribosomal protein in growth‐arrested tetrahymena is induced by sodium

Abstract

In the past few years, in vivo phosphorylation of ribosomal proteins has been the subject of extensive studies and the results have shown that reversible phosphorylation of small subunit ribosomal protein S6, ubiquitous in eukaryotic cells, is apparently related to regulation of protein synthesis initiation. Thus the level of protein synthesis under various conditions is correlated with the level of S6 phosphorylation. In exponentially growing Tetrahymena, however, such phosphorylation does not occur, but when these cells are transferred to starvation buffers, the rate of protein synthesis is drastically reduced and a 40S ribosomal protein analogous to S6 of higher eukaryotic cells is fully and rapidly phosphorylated in all the ribosomes. We have studied the conditions which lead to this phosphorylation in growth-arrested Tetrahymena, in order to understand the physiological significance of this process. Our results show that there is no obvious correlation between this phosphorylation and starvation. Moreover, it is not a developmentally regulated process related to the conjugation cycle, but a modification induced by the presence of sodium ions or high concentration of Tris in the starvation buffer. The physiological significance of this process is discussed in terms of accumulation of negative charge density probably required for initiation of protein synthesis in the growth-arrested cells starving in Na+-containing buffers.

Monovalent cation‐dependent reversible phosphorylation of a 40 S ribosomal subunit protein in growth‐arrested Tetrahymena: correlation with changes in intracellular pH

“…It is therefore possible that in hormone or mitogen stimulated mammalian cells, increase in pHi and phosphorylation of S6 may be independent manifestations of the effect of these stimuli on cell metabolism. The present study shows that, in T. thermoph~la, phosphorylation and dephosphorylation of ribosomal protein S8 can be induced in starved cells which, as we have previously shown [8,24], do not divide and display a slow net decline in their total RNA content, and a rapid reduction in their rate of protein synthesis. Under these conditions, which differ greatly from those pertaining in growth-stimulated cells, phosphorylation of protein S8 and an increase in pHi are induced together by addition of Na+ to the cell suspension medium, and the two effects are reversed together by subsequent addition of K+.…”

“…Na+ on the phosphorylation of protein S8 in starved Tetrahymena thermophila Na+ but not K+ induces phosphorylation of S8 in starved Tetrahymena and we have proposed that it may do so by provoking intracellular alkalinisation [8]. Results in table 1 compare the effects of Na+ with those of other cations including organic amines which can increase pHi by passive diffusion into the cell [ 111.…”

“…In growth-stimulated cells phosphorylation of this protein often shows evident temporal correlations with increases in the rate of protein synthesis and in intracellular pH [4,5]. Phosphorylation of 58 does not occur in growing Tefrahymena [6,7] but can be induced in starved cells (conditions not involving growth factors, hormones, and their receptors and ionic signals) by addition of appropriate cations (Na* but not K+) to the starvation medium [8].…”

Effects of environmental hypercapnia on animal physiology: A 13C NMR study of protein synthesis rates in the marine invertebrate Sipunculus nudus

“…Oxygen limitation, for instance, has been demonstrated to depress protein synthesis in rat liver (Surks and Berkowitz, 1971). Protozoan cells like Tetrahymena decrease protein synthesis by up to 70% during starvation (Cuny et al, 1985).…”

Effect of essential amino acids on the phosphorylation of a 40S ribosomal protein and protein synthesis in Acanthamoeba castellanii

“…In exponentially growing Tetrahymena (Cuny et al, 1985), Dictyostelium (Juliani et al, 19831, and Acanthamoeba (Jantzen, 1981a;Jantzen et al, 1983) phosphorylation of ribosomal protein is not observed, while transfer of these cells to nutrient-depleted media leads to effective phosphorylation of a 40s ribosomal protein. The results shown here support our earlier finding that log phase cells of Acanthamoeba, cultivated in a nondefined nutrient medium (ND cells), contain low levels of the phosphorylated 40s ribosomal protein S3.…”