Phosphorylation of Ribosomal Proteins in Sarcoma 180 Tumor Cells

The phosphorylation of eukaryotic ribosomal protein S6 by protein kinase C

Purified Ca2+-dependent and phospholipid-dependent protein kinase (protein kinase C) from bovine brain catalysed the phosphorylation of ribosomal protein S6 when incubated with 40S ribosomal subunits from rat liver or from hamster fibroblasts. The phosphorylation was dependent on Ca2+ and phospholipid, and occurred under ionic conditions similar to those which support protein biosynthesis in vitro. Protein kinase C phosphorylated at least three sites on ribosomal protein S6 when incubated with unphosphorylated ribosomes, and increased the extent of phosphorylation of ribosomes previously phosphorylated predominantly on two sites by cyclic-AMP-dependent protein kinase, converting some molecules to the tetraphosphorylated or pentaphosphorylated form. This indicates that protein kinase C can phosphorylate sites on ribosomal protein S6 other than those phosphorylated by the cyclic-AMP-dependent protein kinase, and this conclusion was confirmed by analysis of tryptic phosphopeptides. These results strengthen the possibility that protein kinase C might be involved in catalysing the multisite phosphorylation of ribosomal protein S6 in certain circumstances in vivo.

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.

Characterization of the acidic phosphorprotein of eukaryotic ribosomes using a new system of two-dimensional gel-electrophoresis

1. A modified method of two-dimensional gel electrophoresis has been developed for detecting acidic eukaryotic ribosomal proteins. 2. Using this method it has been demonstrated that the major phosphoprotein (Lgamma) of mouse ascites and hamster fibroblast 60-S ribosomal subunits is an acidic protein, apparently analagous to L7/12 of Escherichia coli, and not a neutral protein as previously thought. Electrophoretic resolution of phosphorylated and non-phosphorylated forms of Lgamma has enabled the stoichiometric nature of the phosphorylation (previously deduced from measurement of the specific radioactivity of the ATP pool) to be confirmed. 3. When ascites cells were incubated for 3 h there appeared an altered form of Lgamma which is most likely produced by proteolytic cleavage of the original form. The extent of phosphorylation of Lgamma was decreased by one-half or more in these circumstances. 4. Phosphoprotein Lgamma was found to be almost completely phosphorylated in both polyribosomes and monoribosomes isolated from hamster fibroblasts. Thus the function of the phosphorylation of Lgamma appears not to be concerned with the inactivation of ribosomes.

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 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.

Light-induced enzyme synthesis in cell suspension cultures of Petroselinum hortense. Demonstration in a heterologous cell-free system of rapid changes in the rate of phenylalanine ammonia-lyase synthesis

The conditions for protein synthesis in vitro with polyribosomes from cell suspension cultures of parsel (Petroselinum hortense) and a wheat-germ extract were investigated. Two different criteria were used as estimated of the translational activity: (a) the total rate of incorporation of [35S]methionine into acid-insoluble material; (b) the ratio of large (molecular weight greater than 25000) to small (molecular weight less than 25000) peptide products. Depending on which of the criteria was employed, the pH optimum and the optimal concentrations for Tris=acetate, magnesium acetate, KCL, methionine and the wheat-germ extract differed considerably. The translational activity of the polyribosomes (both criteria) was effciently protected by 0.1 M Mg2+ against degradation during the isolation procedure. The rate of synthesis of phenylalanine ammonia-lyase in vitro with the polyribosomes was determined by measuring the incorporation rate of L-[35S]methionine into protein which was precipitable by a rabbit antiserum prepared for the purified enzyme. The immunoprecipitate was analyzed by disc gel electrophoresis in the presence of dodecylsulfate and was shown to contain small amounts of the complete enzyme subunits and relatively large amounts of shorter peptides which were also characteristic for the enzyme. The time course of light-induced changes in the rate of phenylalanine ammonia-lyase synthesis in vitro were investigated during a period of 15 h under two different conditions of induction: the cell cultures were irradiated with ultraviolet light eith (A) continuously or (B) for 2.5 h and then returned to darkness. Although the highest rate of enzyme synthesis was observed somewhat later inexperiment A than in experiment B, the periods of time during which the rate of synthesis increased rapidly were limited in both cases to only a few hours. The results obtained in vitro were identical within the limits of the experimental error with theoretical calculations of the changes in the rate constant of phenylalanine ammonia-lyase synthesis in vivo. These changes were calculated from the corresponding curves for the changes in the enzyme activity under the conditions of induction. The results are in agreement with previous observations suggesting that the induction of phenylalanine ammonia-lyase by light in the parsley cells was a short-term effect whose efficiency was greatly reduced within the 15 h of experimentation, even under continuous irradiation.

In vivo and in vitro phosphorylation of ribosomal proteins by protein kinases from Saccharomyces cerevisiae

From the high salt wash of the ribosomes of the yeast Saccharomyces cerevisiae, three protein kinases have been isolated and separated by DEAE-cellulose chromatography. The three kinases differ in their abilities to phosphorylate substrates such as histones (calf thymus), casein, and S. cerevisiae ribosomes; two of the kinases showed increased activity in the presence of cyclic adenosine 3',5'-monophosphate when histones and 40S ribosomal subunits were used as substrates. The protein kinases catalyzed phosphorylation of certain proteins of the 40S and 60S ribosomal subunits, and 80S ribosomes in vitro. Nine proteins of the 80S ribosome, seven proteins of the 40S subunit, and eleven of the 60S subunit were phosphorylated; different proteins were modified to various extents when different kinases were used. We have identified several proteins of 40S and 60S ribosomal subunits which are not available to the kinases in the 80S particles. Ribosomes isolated from S. cerevisiae cells growing in logarithmic phase of growth were found to contain a number of phosphorylated proteins. Studies by two-dimensional polyacrylamide gel electrophoresis indicated that the ribosomal proteins phosphorylated in vivo correspond with those phosphorylated in vitro. The relationship of in vivo phsophorylation of ribosomes to the growth and physiology of S. cerevisiae is not known.

A plasmocytoma ribosome-associated protein kinase which phosphorylates a specific protein of the ribosomal KCl wash

One ribosomal protein kinase activity and 3 soluble protein kinase activities have been identified in plasma cell tumors by DEAE-cellulose chromatography. We have shown phosphorylation in vivo and in vitro of a protein fraction from the ribosomal KCl wash which we have termed 'PPx fraction'. Phosphorylation of this protein fraction has been obtained in vitro with the ribosome-associated protein kinase. We have determined for the ribosomal protein kinase the following characteristics. 1. It is an Mg2+-dependent enzyme that transfers the gamma-phosphate from ATP into phosphoseryl and phosphothreonyl residues of the substrate. 2. It has a wide substrate specificity. Like the soluble protein kinases it catalyses the phosphorylation of several proteins like histone, phosvitin, casein and ribosomal proteins but it differs from the main soluble kinases (I, II) by the fact that it catalyses specifically the phosphorylation at least of one of the ribosomal KCl wash proteins. On dodecylsulfate-polyacrylamide gels this protein has a molecular weight of approximately 90000 and it is released from ribosomes under conditions commonly employed for extraction of initiation factors. 3. The ribosome-associated protein kinase is not stimulated by the addition of cyclic adenosine 3':5'-monophosphate. 4. KCl has no effect, NaCl has a weak effect on the phosphorylation, Mn2+ and Ca2+ are inhibitors. 5. ADP has been found to be a competitive inhibitor. 6. The maximum velocity of the ribosomal protein-kinase-catalysed reaction is 0.65 nmol of 32P incorporated in the KCl wash protein per min and per mg protein. 7. The apparent Km for the ribosomal KCl wash protein as substrate is 0.71 mg/ml and the Km for ATP is 94 muM. 8. The molecular weight of the ribosomal protein kinase, estimated by electrophoresis in polyacrylamide-dodecylsulfate gels, is 60000 and corresponds probably to a catalytic subunit.

Phosphorylation of ribosome-associated proteins during the mammalian cell cycle. Unique phosphorylation of a specific protein during mitosis

Chinese hamster ovary cells in monolayer culture were incubated with [32P] phosphate. Ribosome-associated proteins, including both structural proteins and those tightly bound to washed, centrifuged ribosomes, were isolated and separated by electrophoresis on sodium dodecyl sulfate-polyacrylamide gels. The electrophoretic pattern showed five major regions or peaks of 32P radioactivity which represented phosphorylated ribosome-associated proteins with molecular weights of 17 500, 23 000, 30 000, 38 000, and 57 000. When asynchronous cells were pulse-labeled with [32P] phosphate, the predominant peak of 32P radioactivity was associated with the protein of 38 000 daltons. Similar results were obtained with cells synchronized in the G1, S, or G2 phase of the mammalian cell cycle. Conversely, proteins isolated from the ribosomes of mitotic cells, collected and labeled with [32P] phosphate in the presence of colcemid, showed a new and predominant peak of 32P radioactivity migrating with a protein of 45 000 daltons. When cells labeled in mitosis were allowed to progress into G1 phase, this peak of 32P radioactivity rapidly disappeared from the electrophoretic pattern. These results suggest that a specific protein associated with the ribosomes was phosphorylated uniquely during the mitotic phase of the cell cycle.

Analysis by two-dimensional polyacrylamide gel electrophoresis of the in vivo phosphorylation of ribosomal proteins derived from free and membrane-bound polysomes

Analysis of in vivo phosphorylation of mouse liver ribosomal proteins was performed by two-dimensional polyacrylamide gel electrophoresis following 32P-injection. Our method is special and differs from other eukaryotic systems reported in that all proteins separated on the first dimension gel are completely solubilized, moving quantitatively to the second dimension gel. Only ribosomes from polysomes were used, ensuring analysis of ribosomes actively engaged in protein synthesis. We resolved sixty-five distinct proteins from ribosomes from membrane bound or free polysomes. In both cases radioautography revealed similar labeled patterns with one highly phosphorylated ribosomal protein and five marginally labeled spots.