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

The effect of trifluoperazine on the phosphorylation of ribosomal small subunit protein S6 in D-galactosamine-injured and normal rat liver

The effect of the Ca2+-calmodulin antagonist trifluoperazine on the elevation of phosphorylation of rat liver small ribosomal subunit protein S6 induced by the hepatotoxic agent D-galactosamine has been studied. Trifluoperazine applied in various doses (1-100 mg/kg body wt) before injection of D-galactosamine into the rat does not reverse the strong increase in phosphorylation promoted by D-galactosamine. Instead, trifluoperazine has been identified as a potent stimulator of S6 phosphorylation in normal rat liver in vivo without causing significant changes in the cyclic AMP content of liver and the overall rate of liver protein synthesis. Both drugs, however, were not effective in stimulating the incorporation of [32P]phosphate into microsomes or crude ribosomes in liver slices in vitro. The results suggest that a calmodulin-activated protein kinase probably is not primarily engaged in S6 phosphorylation produced by D-galactosamine. However, further in vitro studies are needed to reach a definite conclusion.

Phosphorylation of 40-S ribosomal subunits by cAMP-dependent, cGMP-dependent and protease-activated protein kinases

The phosphorylation of 40-S ribosomal subunits by cyclic-nucleotide-dependent and protease-activated protein kinases from rabbit reticulocytes was studied in vitro. Under optimal conditions the cAMP-dependent protein kinases incorporated up to 2 mol phosphate/mol S6. The electrophoretic mobility of S6 following phosphorylation indicated that this value was not an average for a population of maximally phosphorylated and non-phosphorylated S6 but represented a uniform population of diphosphorylated 40-S ribosomal subunits. Tryptic digests of S6 were analyzed by two-dimensional fingerprinting following phosphorylation with the cAMP-dependent protein kinase; two phosphopeptides, A and B, were observed. When 40-S ribosomal subunits were examined with the cGMP-dependent protein kinase, 1 mol phosphate was incorporated/mol S6. Upon analysis of the phosphopeptides obtained with the cGMP-dependent protein kinase, only peptide A was observed. S6 was also modified by a cyclic-nucleotide-independent protein kinase, protease-activated kinase II, following activation of the enzyme by limited proteolytic digestion. These findings suggest that a multiple protein kinase system may regulate the phosphorylation state of S6. A second ribosomal protein, S10, was phosphorylated by a different cyclic-nucleotide-independent protein kinase, protease-activated kinase I, and up to 1 mol phosphate was incorporated.

Phosphorylation of ribosomal proteins S3, L1 and L24 during spherulation in Physarum polycephalum

The phosphate content of ribosomal proteins S3, L1 and L24 has been determined in the course of spherulation of Physarum polycephalum. The major phosphoprotein, S3, was completely dephosphorylated after 4 h of differentiation. The phosphate content of L1 and L24 was not altered during the differentiation. The cellular level of ATP remained constant for at least 5 h. A 3-fold reduction of cyclic AMP concentration occurred in the first hour, followed by a slow increase to a final value of twice the level observed in growing cells. The results showed that the phosphorylation of ribosomal proteins is regulated by at least two different mechanisms and that the dephosphorylation of S3 is not induced by a lack of cellular ATP. Although cyclic AMP might trigger the dephosphorylation of S3, the phosphate content of this protein remained at a very low value even when the cellular concentration of cyclic AMP rose significantly. Since the polysome level remains constant during the first 24 h of spherulation, the phosphorylation of S3 is not necessary for active protein synthesis and the phosphorylation of L1 and L24 is not involved in ribosome inactivation, which occurs after 24 h.

Differences in the distribution of phosphate content in the ribosomal subunit proteins of free and membrane-bound ribosomes from normal and regenerating rat liver

Proteins of membrane-bound ribosomes from normal liver contained 60-70% more phosphate than did proteins from free ribosomes. This difference was not a reflection of the phosphate contents of respective 40 S subunits. Instead, it was owing to the presence of high levels of phosphorylated proteins in the 60 S subunits, i.e., phosphate contents equal to or greater than those for 40 S subunits. The proteins of membrane-bound 60 S subunits contained twice the phosphate as free 60 S subunits. In regenerating rat liver, membrane-bound ribosomes had increased phosphate in the proteins of the 40 S subunits and decreased phosphate in proteins of the 60 S subunit when compared to controls for normal rat liver. No significant changes occurred in the proteins of free ribosomes from regenerating rat liver. These findings are discussed with respect to (a) the importance of assessing total phosphate contents of proteins in the study of ribosomal protein phosphorylation, and (b) the possible involvement of ribosomal protein phosphorylation in the segregation of ribosomes into free and membrane-bound populations and the regulation of these distributions to meet changes in the translational demands of the cell.

Turnover rates of phosphoryl groups in ribosomal proteins of Physarum polycephalum. Evidence for two different mechanisms

The rate of phosphate exchange in individual ribosomal proteins of Physarum polycephalum was determined in vivo. It was observed that the phosphoryl groups of S3, the major phosphoprotein, had a turnover rate of 1.5% per minute. The phosphoryl groups of proteins L1, L20 and L24 were stable. These results show that the phosphorylation of ribosomal proteins is regulated by at least two different mechanisms. The rapid turnover of phosphoryl groups of the major phosphoprotein is in agreement with the general observation that the phosphate content of this protein is modulated by the physiological state of the cells and possibly involved in the regulation of ribosome activity. The absence of phosphate exchange in acidic proteins suggests that these groups could play a structural role in the ribosome functions.

GTP-sensitive phosphorylation of proteins in a postmitochondrial supernatant from rat brainstem affected by ACTH1-24

The effect of ACTH1-24 and cyclic nucleotides on the endogenous phosphorylation of proteins from a postmitochondrial supernatant from rat brainstem was investigated in the presence and absence of GTP. Phosphorylation and its modulation by these compounds were studied in vitro by incorporation of labeled phosphate from [gamma-32P]ATP added to the incubation mixture. Phosphoproteins were subsequently analyzed by autoradiography after one- and two-dimensional separation. Eight ACTH-sensitive phosphoproteins of molecular weights 75 (IEP 4.0), 67, 64, 50 (IEP 4.7), 47 (IEP 4.8), 38, 34, and 24K were found. The effects of ACTH on phosphorylation were mainly inhibitory, and the affected protein bands did not coincide with the phosphoproteins sensitive to cyclic AMP and cyclic GMP. Phosphorylation of those phosphoprotein bands and its ACTH sensitivity appeared to be highly sensitive to GTP. It is suggested that the activity of protein kinases involved in hormone-sensitive phosphorylation in a postmitochondrial rat brainstem fraction is regulated by GTP-dependent mechanisms.

Phosphorylation of ribosomal proteins in hamster fibroblasts infected with pseudorabies virus or herpes simplex virus

In BHK cells infected with pseudorabies virus, there was a substantial increase in the phosphorylation of ribosomal protein S6. This increase occurred between 2 and 4 h after infection and persisted at least until 9 h. We estimated that in mock-infected cells S6 contained, on an average, one phosphate group per protein chain, whereas in infected cells this rose to between four and five phosphate groups per protein chain. A second ribosomal protein, either S16 or S18, was also phosphorylated after infection. No increase in cyclic AMP was found at the time of phosphorylation. We also found an increased phosphorylation of S6 in herpes simplex virus-infected BHK cells.

Effects of cyclic AMP and fluoride on phosphorylation of ribosomal protein S6 and on protein synthesis in rabbit reticulocytes

The effects of N6,O2-dibutyryl-adenosine 3',5'-monophosphate (Bt2cAMP) and sodium fluoride on the phosphorylation of ribosomal proteins S6 and on protein synthesis were examined. Rabbit reticulocytes were incubated in a nutritional medium containing 32Pi in the presence and absence of Bt2cAMP (1mM) and 3-isobutyl-1-methyl-xanthine (1mM). In the control cells, four phosphorylated derivatives of S6 were observed, with most of the radioactivity in the monophosphorylated form. Upon addition of cyclic nucleotide, a twofold increase in the phosphorylation of ribosomal protein S6 was observed. This was accompanied by an increase of radioactive phosphate in the diphosphorylated derivative. No alteration in protein synthesis was observed upon addition of cAMP and analogues of cAMP in conjunction with 3-isobutyl-1-methyl-xanthine or theophylline. The effects of sodium fluoride on phosphorylation of S6 and on protein synthesis were examined also. At 5 mM sodium fluoride, protein synthesis was inhibited by 85%. A 2.5-fold increase in the phosphorylation of ribosomal protein S6 was observed with an accumulation of 32Pi in the diphosphorylated, triphosphorylated and tetraphosphorylated derivatives. Inhibition of protein synthesis coincided with an increase in the more highly phosphorylated derivatives, whereas an increase of radioactive phosphate in the diphosphorylated derivative could not be correlated with an alteration in globin synthesis.

The phosphorylation of protein S6 in the newly-synthesized cytoplasmic ribosomes of hamster fibroblasts

Ribosomes were isolated from baby hamster kidney fibroblasts, either 20 min or 2 days after labelling with radioactive amino acids, and their proteins subjected to two-dimensional polyacrylamide gel electrophoresis. No significant differences were observed between the amounts of radioactivity associated with the position of the phosphorylated derivatives of protein S6. This suggests that the phosphorylation is unlikely to be important in ribosomal biogenesis or extranuclear transport.

Interaction of synthetic polynucleotides with small rat liver ribosomal subunits possessing low and highly phosphorylated protein S6

The interaction of some synthetic mRNAs (polyuridylate, polyadenylate, polycytidylate) with small rat liver ribosomal subunits which have protein S6 in different states of phosphorylation was studied by BioGel column chromatography, affinity chromatography on poly(U)-Sepharose 4B, and continuous diafiltration at 4 degrees C. 40-S subunits with low phosphorylated protein S6 (isolated from normal liver) and small subunits with highly phosphorylated protein S6 (from galactosamine-, thioacetamide-, dimethylnitrosamine-, puromycin-, and cycloheximide-treated livers) bind initially equal amounts of poly(U) but the dissociation of the radioactive polyuridylate occurs much more rapidly and to a greater extent from the low than from the highly phosphorylated type of subunits. From control- and galactosamine-4-S subunits 62% and 22%, respectively, of originally bound [3H]poly(U) was removed. The release of initially bound poly(A) from 40-S subunits of galactosamine-treated liver ws retarded but reached finally the same level as with control liver ribosomal subunits (removal of 40% of bound [3H]poly(A)). No differences between low and highly phosphorylated subunits were observed with poly(C). If the dissociation reaction was performed at 22 degrees C instead of 4 degrees C the differences in the release of poly(U) described above disappeared.

Stimulation of protein synthesis and Met-tRNAf binding by phosphorylated sugars: studies on their mechanism of action

It has been previously reported by J. R. Lenz et al. [(1978) Biochemistry 17, 80--87] that certain phosphorylated sugars stimulate protein synthesis in extracts of mammalian cells. This effect was found to be due to a stimulation of Met-tRNAf binding to 40S ribosomal subunits, both in whole extracts and with isolated ribosomes. However, formation of a ternary complex of Met-tRNAf, initiation factor eIF-2, and GTP was not stimulated. It was also shown that the stimulation is not due solely to metabolism of the sugars. The present communication further characterizes the stimulatory effect of the sugars. They were found to prevent the inactivation of ribosomes that occurs during protein synthesis incubations. The sugars were also found to inhibit cAMP-dependent protein kinases noncompetitively. However, they stimulate Met-tRNAf binding to 40S ribosomal subunits even under conditions in which an inhibition of protein kinase has no effect. Although it has bot been possible to demonstrate a direct association of the sugars with the 40S initiation complex, the evidence suggests that their effect is mediated by an interaction with one of the components involved in the formation of this complex.

Phosphorylation in vivo of Ribosomes in Tetrahymena pyriformis

Phosphorylation of ribosomal proteins in vivo was studied in exponentially growing and starved cells of the ciliated protozoan, Tetrahymena pyriformis. No phosphorylation of ribosomal proteins could be demonstrated in cells growing exponentially in complex nutrient media. However, when Tetrahymena cells were transferred into a non-nutrient medium, pronounced phosphorylation of a single ribosomal protein was observed. During two-dimensional polyacrylamide gel electrophoresis the phosphorylated ribosomal protein migrated in a manner virtually identical to that of the phosphorylated ribosomal protein S6 of rat liver. The phosphorylated ribosomal protein has a molecular weight of 38000 as estimated by dodecylsulfate polyacrylamide gel electrophoresis. Thus, the phosphorylated ribosomal protein found in starved Tetrahymena is apparently homologous with the ribosomal protein which is predominantly phosphorylated in higher eukaryotes. When phosphorylated ribosomes were dissociated by treatment with high concentration of KCl, the phosphorylated protein was found only on the small subunit. If dissociation was achieved by dialysis against a buffer low in MgCl2, the phosphorylated protein was distributed almost equally between the two subunits. This indicates that the phosphorylated ribosomal protein is located at the interface between the two subunits.

Phosphorylation of ribosomal protein S6 in suspension cultured HeLa cells

HeLa cell ribosomal protein S6, and the increase in its phosphorylation level that occurs after resuspending cells in fresh medium plus serum, were studied using two-dimensional gel electrophoresis. The maximum level of S6 phosphorylation occurs about 2 h after adding fresh medium and seum to cells that have been allowed to grow to high density; this results in an almost complete shift of the spot representing S6 in two-dimensional polacrylamide gels to a new location. Mixing experiments showed that the differences in the level of phosphorylation occur in vivo and are not an artifact of in vitro sample preparation. This method of stimulating S6 phosphorylation provides a convenient system for studying the functional significance of the phenomenon. Only one other ribosomal protein was detectably phosphorylated using [32P]-labeling and autoradiography of dried two-dimensional gels. The level of phosphorylation of this protein, L14, does not change after serum stimulation.

Phosphorylation in vitro and in vivo of ribosomal proteins from Saccharomyces cerevisia

Crude ribosomes from Saccharomyces cerevisiae cultures were phosphorylated in vitro when incubated in the presence of [gamma-32P]ATP. Analysis of the ribosomal proteins with two-dimensional electrophoresis revealed that of the 29 proteins identified in the small subunit, only protein S6 was phosphorylated. Of the 37 proteins identified in the large subunit, one was highly phosphorylated (L3) and two only slightly phosphorylated (L11 and L14). The protein kinase activity associated with the ribosomes was extracted with 1 M KCl and was not dependent on adenosine 3':5'-monophosphate; it preferentially phosphorylated casein and phosvitin, but was less active on histones. Structural ribosomal proteins were also phosphorylated in vivo when the yeast cultures were incubated with [32P]orthophosphate; the radioactivity resistant to hydrolysis by hot perchloric acid was incorporated into the proteins of the two subunits. Radioactive phosphoserine was found by subjecting hydrolysates of ribosomal proteins to high-voltage electrophoresis. After two-dimensional electrophoresis, one poorly phosphorylated protein (S10) was identified in the small subunit. In the large subunit, one protein (L3) was highly labelled, and two proteins (L11 and L24) only slightly labelled.

Widespread occurrence of a specific protein in vertebrate tissues and regulation by cyclic AMP of its endogenous phosphorylation and dephosphorylation

A protein whose endogenous phosphorylation and dephosphorylation are affected by cAMP has been found in the soluble and particulate fractions of all vertebrate tissues studied. This phosphoprotein, which contained a substantial proportion of the radioactive phosphate observed on SDS-polyacrylamide gels, was estimated to have an apparent molecular weight of 49,000. In the presence of Zn++, cAMP inhibited the endogenous phosphorylation of this protein (protein 49) in the cytosol and microsomal fractions. In the presence of Mg++, cAMP stimulated the phosphorylation of protein 49 in the cytosol fractions, but had only slight effects in the microsomal fractions. The dephosphorylation of protein 49 by an endogenous protein phosphatase was markedly stimulated by cAMP in the cytosol and microsomal fractions of all tissues studied. The binding of 8-azido-cAMP (a photoaffinity analog of cAMP, which reacts specifically with cAMP-binding sites) to subcellular fractions was also studied. This binding was principally to a protein of molecular weight 49,000. These and other data suggest that a cAMP-binding protein with a molecular weight of 49,000 capable of undergoing cAMP-dependent phosphorylation and dephosphorylation, occurs in a variety of tissues.