Increased phosphorylation of ribosomal protein S6 during meiotic maturation of Xenopus oocytes

Insulin and progesterone activate a common synthetic ribosomal protein S6 peptide kinase in Xenopus oocytes

A synthetic peptide Arg-Arg-Leu-Ser-Ser-Leu-Arg-Ala, the structure of which is based on that of a phosphorylated sequence in ribosomal protein S6, was employed as a probe for stimulated kinase activity in Xenopus laevis oocytes induced to mature with insulin or progesterone. Insulin elicited an early (20-30 min) 3-fold stimulation of S6 peptide phosphorylating activity that was not evident with progesterone. However, both hormones produced a delayed 7-12-fold stimulation of S6 peptide phosphorylating activity at the time of germinal vesicle breakdown. The results of DEAE-Sephacel, Sephacryl S-200, TSK-400, and heparin-Sepharose chromatographic fractionation experiments imply that a common S6 peptide kinase is activated as a consequence of short and long term insulin exposure, as well as in long term progesterone treatment of oocytes. Omission of potassium from the oocyte culture medium greatly facilitated insulin-induced meiotic maturation.

Polyomavirus middle T antigen induces ribosomal protein S6 phosphorylation through pp60c-src-dependent and -independent pathways

Phosphorylation of ribosomal protein S6 is elevated in polyomavirus-infected cells. This elevation results only in part from activation of S6 kinase activity. These effects appear to reflect independent activities of wild-type middle T antigen. Hr-t mutant NG59, encoding a defective middle T protein, and mutant Py808A, encoding no middle T protein, were unable to induce S6 kinase activity or elevate S6 phosphorylation. Two other site-directed mutants encoding altered middle T proteins did elevate S6 phosphorylation while only weakly stimulating S6 kinase activity. These results suggest at least two independent pathways leading to elevation of S6 phosphorylation. One pathway leads to induction of S6 kinase activity following activation of pp60c-src by transformation-competent middle T antigen. Another pathway operates independently of S6 kinase induction and can be regulated by transformation-defective middle T mutants such as Py1387T. This mutant, encoding a truncated middle T protein that failed to associate with the plasma membrane and to activate pp60c-src, caused increased levels of S6 phosphorylation without detectably increasing S6 kinase activity. The ability of mutants such as Py1387T to induce S6 phosphorylation correlated with their ability to increase phosphorylation of VP1, an event linked to maturation of infectious virions.

S6 phosphorylation is regulated at multiple levels in Xenopus laevis oocytes

It is known that the 40s ribosomal protein S6 undergoes a dramatic increase in its level of phosphorylation during Xenopus oocyte meiotic maturation in response to progesterone stimulation. During prophase arrest, the majority of S6 has 0 moles phosphate per mole protein; this increases to 4-5 moles phosphate per mole protein by the time of germinal vesicle breakdown (GVBD). Our in vitro and in vivo studies indicate that the accumulation of phosphate on S6 is the net result of a 4-5-fold increase in S6 kinase activity and a 30-50% decrease in the rate of dephosphorylation and/or turnover of phosphate groups on S6 in maturing oocytes. In addition, the level of phosphorylation of S6 on 80s monosomes injected into non-hormone-stimulated oocytes was unexpectedly high. This indicates that the S6 kinase/phosphatase ratio in prophase arrested oocytes is higher than anticipated from previous studies. This observation implies that the majority of the oocyte ribosomes may be sequestered from any S6 kinase during meiotic prophase. Furthermore, these observations suggest that a portion of the increased accumulation of phosphate on S6 may be the result of increased accessibility of the ribosomes to S6 kinase during oocyte meiotic maturation.

Oncogenic ras protein induces meiotic maturation of amphibian oocytes in the presence of protein synthesis inhibitors

Microinjection of the activated ras oncogenic protein can induce the meiotic maturation of Xenopus laevis oocytes, a process that can also be triggered by progesterone or high concentrations of insulin. Cycloheximide and puromycin, well-known inhibitors of protein synthesis, block the maturation process induced by progesterone and insulin but do not affect the maturation caused by H-raslys12 protein microinjection. Theophylline, an inhibitor of cAMP phosphodiesterase that also affects oocyte protein synthesis, does cause a partial inhibition of ras protein-induced maturation. These findings indicate that ras protein acts on the oocyte maturation process at a point that is downstream of the protein synthesis requirement, a characteristic shared with the maturation promoting factor, an activity that appears in oocytes and mitotic cells at the onset of cell division.

Polyomavirus middle T antigen induces ribosomal protein S6 phosphorylation through pp60c-src-dependent and -independent pathways

Phosphorylation of ribosomal protein S6 is elevated in polyomavirus-infected cells. This elevation results only in part from activation of S6 kinase activity. These effects appear to reflect independent activities of wild-type middle T antigen. Hr-t mutant NG59, encoding a defective middle T protein, and mutant Py808A, encoding no middle T protein, were unable to induce S6 kinase activity or elevate S6 phosphorylation. Two other site-directed mutants encoding altered middle T proteins did elevate S6 phosphorylation while only weakly stimulating S6 kinase activity. These results suggest at least two independent pathways leading to elevation of S6 phosphorylation. One pathway leads to induction of S6 kinase activity following activation of pp60c-src by transformation-competent middle T antigen. Another pathway operates independently of S6 kinase induction and can be regulated by transformation-defective middle T mutants such as Py1387T. This mutant, encoding a truncated middle T protein that failed to associate with the plasma membrane and to activate pp60c-src, caused increased levels of S6 phosphorylation without detectably increasing S6 kinase activity. The ability of mutants such as Py1387T to induce S6 phosphorylation correlated with their ability to increase phosphorylation of VP1, an event linked to maturation of infectious virions.

A Xenopus ribosomal protein S6 kinase has two apparent kinase domains that are each similar to distinct protein kinases

We report the molecular cloning of cDNAs for S6 kinase II (S6KII) mRNAs present in Xenopus ovarian tissue. Two cDNAs were isolated by hybridization to oligonucleotide probes designed to encode tryptic peptides isolated from S6KII. The two cDNAs show 91% sequence similarity to each other. These two cDNAs predict proteins of 733 (S6KII alpha) and 629 (S6KII beta) amino acids that show 95% sequence similarity over the 629 amino acids where they are colinear. Amino acids 44-733 of S6KII alpha were expressed in Escherichia coli and the recombinant protein was used to raise antiserum in rabbits. This antiserum reacted with authentic S6KII prepared from Xenopus eggs. This interaction was specifically blocked by the recombinant protein from E. coli. The sequences of S6KII alpha and -beta predict four tryptic peptides whose sequences are identical to four peptides isolated from a tryptic digest of S6KII. The S6KII proteins have a very unusual structure when compared with previously studied protein kinases. They contain two apparent kinase domains, each similar to distinct protein kinases. The amino-terminal 366 amino acids show high sequence similarity to the regions of protein kinase C, the catalytic subunit of cAMP-dependent protein kinase, and cGMP-dependent protein kinase that contain the sites for ATP binding and are believed to be the catalytic centers for phosphotransferase activity. The remainder of the S6 kinase molecule shows high sequence similarity to the ATP-binding and presumed catalytic domain of the catalytic subunit of phosphorylase b kinase.

Identification and characterization of a mitogen-activated S6 kinase

Treatment of Swiss mouse 3T3 cells with epidermal growth factor, orthovanadate, or serum results in the activation of a kinase that phosphorylates protein S6 of the 40S ribosomal subunit in vitro. This kinase is eluted as a single peak of activity from either a Mono Q anion-exchange column at 0.34 M NaCl or a Mono S cation-exchange column at 0.20 M NaCl. Treatment of the peak fraction from the Mono S column with phosphatase 2A completely abolishes the activity of the enzyme. The kinase appears to be distinct from protein kinase C, cAMP-dependent protein kinase, and two protease-activated kinases, PAK II and H4P. The kinase has been purified to apparent homogeneity and migrates as a single band at Mr 70,000 in NaDodSO4/polyacrylamide gels. The kinase exhibits the ability to autophosphorylate, and this activity directly parallels S6 phosphorylation activity on the final step of purification. In vitro, the kinase incorporates up to 5 mol of phosphate into S6, and the tryptic phosphopeptide maps obtained are equivalent to those from S6 phosphorylated in vivo. Most important, treatment of the purified kinase with phosphatase 2A results in complete inactivation of the enzyme, arguing that the activity of the kinase is directly controlled by phosphorylation.

Antibodies to Xenopus egg S6 kinase II recognize S6 kinase from progesterone- and insulin-stimulated Xenopus oocytes and from proliferating chicken embryo fibroblasts

Ribosomal protein S6 becomes highly phosphorylated during progesterone- or insulin-induced maturation of Xenopus laevis oocytes. We have previously purified an Mr 92,000 protein as one of the major S6 kinases from Xenopus unfertilized eggs. In this paper we confirm by renaturation of activity from a sodium dodecyl sulfate-polyacrylamide gel that this protein is an S6 kinase. This enzyme, termed S6 kinase II (S6 K II), was used for the preparation of polyclonal antiserum. Immunocomplexes formed with the antiserum and purified S6 K II were able to express kinase activity with the same substrate specificity as that of the purified enzyme, including autophosphorylation of S6 K II itself. The antiserum did not react with S6 kinase I, another major S6 kinase present in Xenopus eggs, which is chromatographically distinct from S6 K II. The administration of progesterone to oocytes resulted in a 20- to 25-fold increase in S6 kinase activity in extracts of these cells. Immunocomplex kinase assays done on extracts revealed that anti-S6 K II serum reacted with S6 kinase from progesterone-treated oocytes. This antiserum also reacted with the activated S6 kinase from insulin-stimulated oocytes. In addition, anti-S6 K II serum reacted with activated S6 kinase from chicken embryo fibroblasts stimulated with serum or transformed by Rous sarcoma virus. These results indicate that S6 K II or an antigenically related S6 kinase(s) is subject to regulation by mitogenic stimuli in various cell types.

Antibodies to Xenopus egg S6 kinase II recognize S6 kinase from progesterone- and insulin-stimulated Xenopus oocytes and from proliferating chicken embryo fibroblasts

Ribosomal protein S6 becomes highly phosphorylated during progesterone- or insulin-induced maturation of Xenopus laevis oocytes. We have previously purified an Mr 92,000 protein as one of the major S6 kinases from Xenopus unfertilized eggs. In this paper we confirm by renaturation of activity from a sodium dodecyl sulfate-polyacrylamide gel that this protein is an S6 kinase. This enzyme, termed S6 kinase II (S6 K II), was used for the preparation of polyclonal antiserum. Immunocomplexes formed with the antiserum and purified S6 K II were able to express kinase activity with the same substrate specificity as that of the purified enzyme, including autophosphorylation of S6 K II itself. The antiserum did not react with S6 kinase I, another major S6 kinase present in Xenopus eggs, which is chromatographically distinct from S6 K II. The administration of progesterone to oocytes resulted in a 20- to 25-fold increase in S6 kinase activity in extracts of these cells. Immunocomplex kinase assays done on extracts revealed that anti-S6 K II serum reacted with S6 kinase from progesterone-treated oocytes. This antiserum also reacted with the activated S6 kinase from insulin-stimulated oocytes. In addition, anti-S6 K II serum reacted with activated S6 kinase from chicken embryo fibroblasts stimulated with serum or transformed by Rous sarcoma virus. These results indicate that S6 K II or an antigenically related S6 kinase(s) is subject to regulation by mitogenic stimuli in various cell types.

Phosphorylation of the Saccharomyces cerevisiae equivalent of ribosomal protein S6 has no detectable effect on growth

The phosphorylation of mammalian ribosomal protein S6 is affected by a variety of agents, including growth factors and tumor promoters, as well as by expressed oncogenes. Its potential role in the regulation of protein synthesis has been the object of much study. We have developed strains of Saccharomyces cerevisiae in which the phosphorylatable serines of the equivalent ribosomal protein (S10) were converted to alanines by site-directed mutagenesis. The S10 of such cells is not phosphorylated. Comparison of these cells with the parental cells, whose genomes differ by only six nucleotides, revealed no differences in the lag phase or logarithmic phase of a growth cycle, in growth on different carbon sources, in sporulation, or in sensitivity to heat shock. We conclude that in S. cerevisiae the phosphorylation of ribosomal protein S10 may play no role in regulating the synthesis of proteins. This conclusion leads one to ask whether certain protein phosphorylations are simply the adventitious, if easily observable, result of the imperfect specificity of one or another protein kinase.

Phosphorylation of ribosomal protein S6 during lens cell differentiation: correlation with translational efficiency

In vitro differentiation of embryonic chicken lens epithelial explants to form lens fiber cells is accompanied by an increase in protein synthesis without a corresponding increase in mRNA levels. This apparent increase in translational efficiency is correlated with a specific enhancement of phosphorylation of a 32K protein, which we identify as ribosomal protein S6 by two dimensional gel electrophoresis of purified ribosomal proteins. Serum, insulin, and chicken vitreous humor, three agents known to initiate differentiation in this system, all lead to enhanced S6 phosphorylation. Maximal enhancement of phosphorylation is reached within the first hour after the onset of differentiation, and is not blocked by inhibitors of RNA and protein synthesis.

Estrogen stimulates growth of mammary tumor cells ZR-75 without activation of S6 kinase and S6 phosphorylation. Difference from epidermal growth factor and alpha-transforming growth-factor-induced proliferation

Growth of the human mammary tumor cell line ZR-75-1 is stimulated by epidermal growth factor (EGF) and alpha-type transforming growth factor (alpha TGF), as well as by estradiol (E2). The role of activation of S6 kinase and S6 phosphorylation in the EGF(alpha TGF)-induced and E2-induced growth was investigated. Maximal effects on growth are observed at 10 nM EGF or alpha TGF. EGF as well as alpha TGF treatment of serum-starved cells leads to rapid activation of S6 kinase; the activity is increased about tenfold after 30 min of EGF treatment and declines with the time reaching about 25% of the maximal activity after 2 h of EGF treatment. Similar to the growth response, S6 kinase is activated at lower doses of EGF than alpha TGF and shows a maximal response at 10 nM for both growth factors. In contrast to this finding the incubation of serum-starved cells with E2 over a concentration range between 1 pM and 10 nM and times from 30 min to 4 h does not lead to increased S6 kinase activity. On investigating whether this lack of response to E2 is due to desensitization of the system by induction of alpha TGF it was found that preincubation of cells with alpha TGF for 2-6 h desensitizes them to reactivation of S6 kinase by alpha TGF, whereas preincubation with E2 does not. When S6 phosphorylation is monitored over times from 1 h to 6 h, it is observed that EGF leads to increased S6 phosphorylation, whereas E2 does not. The rate of onset of protein synthesis in the first 2 h of stimulation, when EGF-induced S6 phosphorylation is maximal, is more rapid with EGF than with E2. The results suggest that different pathway are involved in E2-induced and EGF(alpha TGF)-induced proliferation.

Phosphorylation of the Saccharomyces cerevisiae equivalent of ribosomal protein S6 has no detectable effect on growth

The phosphorylation of mammalian ribosomal protein S6 is affected by a variety of agents, including growth factors and tumor promoters, as well as by expressed oncogenes. Its potential role in the regulation of protein synthesis has been the object of much study. We have developed strains of Saccharomyces cerevisiae in which the phosphorylatable serines of the equivalent ribosomal protein (S10) were converted to alanines by site-directed mutagenesis. The S10 of such cells is not phosphorylated. Comparison of these cells with the parental cells, whose genomes differ by only six nucleotides, revealed no differences in the lag phase or logarithmic phase of a growth cycle, in growth on different carbon sources, in sporulation, or in sensitivity to heat shock. We conclude that in S. cerevisiae the phosphorylation of ribosomal protein S10 may play no role in regulating the synthesis of proteins. This conclusion leads one to ask whether certain protein phosphorylations are simply the adventitious, if easily observable, result of the imperfect specificity of one or another protein kinase.

Regulation of ribosomal protein S6 phosphorylation in heat-shocked HeLa cells

Decreases in energy charge, ribosomal protein phosphorylation and rate of protein synthesis are well-documented facets of the cellular response to hyperthermia in non-vertebrates. We have tried to reproduce this response pattern in 32P-labelled HeLa cells in order to investigate the hypothetical causal relationship between these effects. In HeLa cells shifted from 36 degrees C to 42 degrees C, dephosphorylation of S6 and inhibition of protein synthesis, owing to a decreased initiation rate, were observed, but could not have been mediated by changes in the cells' general energy charge since the ATP and GTP levels were not reduced. In addition, we found that the hyperthermic translation block developed faster than the overall dephosphorylation of S6, showing that S6 dephosphorylation cannot be responsible for the translation block unless site-specific effects play a critical role.

Phosphorylation of a 40S ribosomal subunit protein in Tetrahymena. Lack of correlation with cellular growth and ribosome stability

In Tetrahymena the small ribosomal subunit protein S7, which appears to be the equivalent of S6 of higher eukaryotes, undergoes reversible phosphorylation under a set of defined conditions. In an attempt to understand the physiological role of such reversible phosphorylation, we examined the status of ribosomal protein S7 in growing cells and growth-arrested cells, starving either non-specifically for nutrients or specifically for a single essential amino acid. These experiments allowed us to dissociate S7 phosphorylation from changes in the translational activity and the stability of ribosomes. The results revealed complete lack of correlation between phosphorylation of S7 and both the growth status of the cells and the in vivo stability of ribosomes. Taken together with the observation that phosphorylation of S7 occurs only when the cells are starved in buffers containing sodium chloride or high concentrations of Tris, non-essential ions for normal growth, our data suggest that this protein modification is required to maintain the functional integrity of the ribosomes in an altered electrostatic environment, induced by changes in the extracellular ionic conditions.

Effect of ouabain on the meiotic maturation of stage IV-V Xenopus laevis oocytes

Full-grown stage VI Xenopus laevis oocytes (1,200 to 1,300 micron) respond to progesterone stimulation by undergoing a series of physiological and morphological changes that are referred to as meiotic maturation. Oocytes in earlier stages of oogenesis (I through V) do not undergo these changes and remain in prophase arrest when exposed to this steroid. We have found that oocytes ranging from 850 micron (stage IV) to 1,000 micron (stage V) are capable of responding to progesterone under the appropriate conditions. Oocytes greater than or equal to 850 micron in diameter underwent germinal vesicle breakdown (GVBD) after 10-12 hr of exposure to progesterone when ouabain was added to the medium at a concentration greater than 2.5 X 10(-6) M. Under this culture condition, progesterone was now able to induce a 0.3- to 0.4-unit increase in the intracellular pH of stage IV-V oocytes, a 4- to 5-fold increase in 40s ribosomal protein S-6 phosphorylation, and a 2.3-fold increase in their rate of protein synthesis. All of these physiological changes are characteristic of full-grown stage VI oocytes undergoing meiotic maturation. In addition, we have found that oocytes greater than or equal to 750 micron are capable of amplifying maturation promoting factor (MPF) in their cytoplasm leading to GVBD. Therefore, stage IV-V Xenopus oocytes have the potential for undergoing meiotic maturation, but they are blocked at a point in prophase that appears to be alleviated by the combination of progesterone and ouabain.

Partial purification of the maturation-promoting factor MPF from unfertilized eggs of Xenopus laevis

A 200-fold purification of the maturation-promoting factor or MPF from unfertilized eggs of Xenopus laevis is reported for the first time. Purification was achieved by three successive column chromatographies on hydroxyapatite, trisacryl blue and L-arginine-agarose. The presence of MPF was assessed by the usual maturation criteria after injections of test material into immature stage VI unstimulated X. laevis oocytes: the precocious appearance of the maturation spot (within 45-120 min), the germinal vesicle breakdown, the presence of the first polar body and the second metaphase spindle. Purification was monitored by the decrease of the minimal amount of protein injected in a constant volume (50 nl) required to induce 50% frequency of germinal vesicle breakdown. This amount decreased from 500 ng in the crude extract to 2.5 ng in the 200-fold purified material. Analysis by SDS-PAGE of the crude extract showed about 40 Coomassie-blue-stained polypeptides with molecular masses ranging from 300 kDa to 20 kDa, whereas in the 200-fold purified MPF only 5 stained polypeptides were revealed, with molecular masses of 62, 53, 49, 39 and 37 kDa. In vitro phosphorylations for the detection of kinase activities for endogenous and exogenous substrates were monitored by analysis of autoradiograms of SDS-PAGE, after treatment of fractions with [gamma-32P]ATP. Only inactive fractions eluted from columns ahead of MPF, and fractions containing MPF activity were tested. Phosphorylation of numerous stained polypeptides was demonstrated in the crude MPF extract and exogenous substrates such as phosvitin, casein and histone type II-AS were also strongly phosphorylated. In the MPF fraction, purified on hydroxyapatite, a polypeptide of 53 kDa was more highly and specifically phosphorylated and the presence of kinase activities was observed for the above three exogenous substrates. In the 100-fold and 200-fold purified MPF, phosphorylation of endogenous substrates could not be shown and kinase activities for the above three substrates were drastically decreased as compared with the crude and purified MPF obtained after hydroxyapatite column chromatography. However, neither endogenous phosphorylations nor kinase activities with the above exogenous substrates could be shown in inactive fractions eluted ahead of MPF at the different purification steps. Some characteristics of the purified material are also described in this paper.

Phosphorylation and protein synthetic events in Xenopus laevis oocytes microinjected with pp60v-src

Microinjection of purified pp60v-src, the transforming protein of Rous sarcoma virus, into Xenopus laevis oocytes accelerated the rate of progesterone- or insulin-induced meiotic maturation. This acceleration was abolished by incubating the oocytes with cycloheximide or puromycin during a 2-h interval between pp60v-src microinjection and progesterone addition. In contrast, exposure to actinomycin D did not alter the acceleration of maturation by microinjected pp60v-src. Associated with progesterone treatment and pp60v-src microinjection were a number of qualitative changes in phosphoproteins; a few of these changes are common to both stimuli. These results indicate that the action of pp60v-src in oocytes involves both phosphorylation and protein synthetic events that affect oocyte maturation.

Increased phosphorylation of ribosomal protein S6 following microinjection of insulin receptor-kinase into Xenopus oocytes

The protein products of several transforming retroviruses as well as the receptors for several hormones and growth factors, including insulin, have been shown to possess a protein kinase activity in vitro specific for tyrosine residues in protein substrates, including themselves. In the case of pp60src and the insulin receptor, autophosphorylation activates the tyrosine kinase activity towards exogenous substrates. Experiments indicate that, in vivo, many of these viruses or growth factors induce an increase in cellular phosphotyrosine, as well as an increase in the phosphorylation of serine residues on proteins, including ribosomal protein S6. It seems likely that some of the effects of insulin might be mediated by phosphorylation of intracellular substrates by its receptor. As the beta subunit of the receptor is a transmembrane protein, such phosphorylation could occur either while the receptor is still in the membrane or after its internalization. In various cell systems, internalized receptors are degraded, reshuttled back to the plasmalemma or maintained in a separate compartment before reinsertion in the membrane; shuttling of the insulin receptor could provide the opportunity for it to phosphorylate various intracellular components as part of its mechanism of signal transduction. To approach directly the question of whether the receptor can elicit a signal while acting at an intracellular location, we have microinjected Xenopus oocytes with the insulin receptor kinase. The results indicate that an S6 protein-serine kinase is stimulated or an S6 protein-serine phosphatase inhibited by the activity of the insulin receptor, supporting the concept that the insulin receptor acting within the cell can elicit a biological response.

Stimulation of ribosomal protein S6 kinase activity by pp60v-src or by serum: dissociation from phorbol ester-stimulated activity

Ribosomal protein S6 kinase activity was measured in lysates prepared from serum-deprived chicken embryo fibroblasts (CEF) treated for various times with phorbol 12-myristate 13-acetate (PMA). Maximal activity was observed within 15 min, and it declined to the initial level by 4 hr. Incubation of these cells with PMA 4-60 hr after the initial treatment did not result in an additional increase in S6 protein kinase activity. These results are consistent with down-regulation of the PMA receptor, protein kinase C, and the dependence of PMA-stimulated S6 kinase activity on this enzyme. Long-term pretreatment of CEF with PMA only partially attenuated the stimulation of the S6 protein kinase activity by serum or by expression of the Rous sarcoma virus transforming gene product, pp60v-src. A similar protein kinase activity also was stimulated in cells treated with cycloheximide or sodium vanadate. Pretreatment with PMA had little effect on this response. These data indicate that it is likely that there are at least two mechanisms through which S6 kinase activity can be regulated, one of which apparently utilizes protein kinase C whereas the other(s) does not. Additional experiments show PMA-stimulated glucose transport was not attenuated by long-term incubation with phorbol ester, suggesting that another mechanism, which is not dependent on the presence of protein kinase C, maintains this response after the proposed down-regulation of the PMA receptor.

An altered ribosomal protein in an edeine-resistant mutant of Saccharomyces cerevisiae

The r-proteins of an edeine-resistant mutant of Saccharomyces cerevisiae were compared to those of the wild-type strain by using two different two-dimensional electrophoretic techniques: (1) the Kaltschmidt-Wittmann method and, (2) the Kaltschmidt-Wittmann system, in the first dimension and the Na Dodecyl-SO4 system in the second. With the first technique, the results indicate that the patterns of basic ribosomal proteins are similar in the two strains. However, the pattern of acidic ribosomal proteins of the mutant revealed an additional protein band with respect to the normal one. Using the other technique, the patterns of basic and acidic ribosomal proteins of the mutant demonstrated a similarity to the corresponding pattern of the wild-type strain. The data disclose that an acidic ribosomal protein of the mutant may have two forms with different electrophoretic mobilities and similar molecular weights.

Phosphorylation and protein synthetic events in Xenopus laevis oocytes microinjected with pp60v-src

Microinjection of purified pp60v-src, the transforming protein of Rous sarcoma virus, into Xenopus laevis oocytes accelerated the rate of progesterone- or insulin-induced meiotic maturation. This acceleration was abolished by incubating the oocytes with cycloheximide or puromycin during a 2-h interval between pp60v-src microinjection and progesterone addition. In contrast, exposure to actinomycin D did not alter the acceleration of maturation by microinjected pp60v-src. Associated with progesterone treatment and pp60v-src microinjection were a number of qualitative changes in phosphoproteins; a few of these changes are common to both stimuli. These results indicate that the action of pp60v-src in oocytes involves both phosphorylation and protein synthetic events that affect oocyte maturation.

Regulation of a ribosomal protein S6 kinase activity by the Rous sarcoma virus transforming protein, serum, or phorbol ester

Protein kinase capable of phosphorylating 40S ribosomal protein S6 on serine residues has been detected in chicken embryo fibroblasts. This activity appears to be regulated in direct response to expression of pp60v-src in chicken embryo fibroblasts infected with a temperature-sensitive transformation mutant of Rous sarcoma virus. Partially purified S6 kinase was highly specific for S6 in 40S ribosomal subunits. The S6 kinase was not inhibited by calcium or by the heat-stable inhibitor of cAMP-dependent protein kinase, nor was it activated by phosphatidylserine, diacylglycerol, and calcium. Thus, it is distinct from protein kinase C and cAMP-dependent protein kinase, which are capable of phosphorylating S6 in vitro. The tumor-promoter phorbol 12-myristate 13-acetate also stimulated ribosomal protein S6 kinase activity in serum-starved chicken embryo fibroblasts, whereas phorbol, the inactive analog of phorbol 12-myristate 13-acetate, had no effect. S6 kinase activity stimulated by expression of pp60v-src, by phorbol 12-myristate 13-acetate, or by serum growth factors exhibited similar chromatographic properties upon ion-exchange chromatography. These results suggest that a common protein kinase may be activated by three diverse stimuli all involved in regulating cell proliferation.

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

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.

Induction of selective phosphorylation of a fat body protein of Sarcophaga peregrina larvae by 20-hydroxyecdysone

20-Hydroxyecdysone was shown to induce selective phosphorylation of a fat body protein of Sarcophaga peregrina larvae with a molecular mass of 30 kDa. This phosphorylation was not associated with synthesis of new protein. Fractionation of 32P-labelled fat body by differential centrifugation showed that this protein was mainly present in the membrane-rich fraction, although we could not specify the membrane. Thus, 20-hydroxyecdysone may modify the function of the fat body by inducing phosphorylation of a specific membrane protein.

A protein kinase from Xenopus eggs specific for ribosomal protein S6

A protein kinase specific for ribosomal protein S6 has been purified from eggs of Xenopus laevis. As visualized on a silver-stained polyacrylamide gel, the major protein in the preparation migrated with a Mr of 90,000. Incubation of the enzyme preparation with [gamma-32P]ATP led to phosphorylation of this protein on serine residues. Upon glycerol gradient centrifugation, the S6 kinase activity and the Mr 90,000 protein both sedimented with a Mr of 50,000-55,000. Two-dimensional gel electrophoresis demonstrated that up to 4-5 phosphate groups per S6 molecule could be incorporated with this enzyme in vitro, and two-dimensional peptide mapping demonstrated that the phosphopeptides from S6 labeled in vitro with the enzyme comigrated with those from highly phosphorylated S6 labeled in vivo in response to progesterone treatment. The purified S6 protein kinase did not phosphorylate at a significant rate ribosomal protein S10, histone H1, histone H4, mixed histones, casein, or phosvitin, indicating a high degree of substrate specificity. These results indicate that activation of a single S6 protein kinase may be sufficient to account for increased S6 phosphorylation after a growth stimulus.

The biosynthesis of biologically active proteins in mRNA-microinjected Xenopus oocytes

The basic properties of mRNA-injected Xenopus oocytes as a heterologous system for the production of biologically active proteins will be reviewed. The advantages and limitations involved in the use of this in ovo system will be discussed, as compared with in vitro cell-free translation systems and with in vivo microinjected mammalian cells in culture. The different assay systems that have been utilized for the identification of the biological properties of oocyte-produced proteins will be described. This section will review the determination of properties such as binding of natural ligands, like heme or alpha-bungarotoxin; immunological recognition by antibodies; subcellular compartmentalization and/or secretion; various enzymatic catalytic activities; and induction in ovo of biological activities that affect other living cells in culture, such as those of interferon and of the T-cell receptor. The limitations involved in interpretation of results obtained using mRNA-injected oocytes will be critically reviewed. Special attention will be given to the effect of oocyte proteases and of changes in the endogenous translation rate on quantitative measurements of oocyte-produced proteins. In addition, the validity of the various measurement techniques will be evaluated. The various uses of bioassays of proteins produced in mRNA-injected Xenopus oocytes throughout the last decade will be reviewed. Nuclear and cytoplasmic injections, mRNA and protein turnover measurements and abundance calculations, and the use of in ovo bioassays for molecular cloning experiments will be discussed in this section. Finally, potential future uses of the oocyte system in various fields of research, such as immunology, neurobiology, and cell biology will be suggested.

OOcyte maturation in amphibians

This chapter presents a complex picture of the regulation of oocyte maturation. The events that have been identified can be expressed temporally on a relative time scale, as shown in Figure 8. A major question that lies ahead concerns which of these events is causal for subsequent events and for GVBD. Clearly, the first event at 0.1 GVBD50, a decrease in cAMP, is both necessary and sufficient to induce GVBD. The increase in protein synthesis at 0.3 GVBD50 is apparently necessary, and the activity of MPF appearing at 0.4 GVBD50 is apparently sufficient, as judged by the microinjection assay. It is important to identify further steps after MPF appearance in order to establish the mechanism of action of MPF. The fact that events highly correlated with cell growth (i.e., pHi increase, S6 phosphorylation, and MPF appearance) all occur during amphibian oocyte maturation provides strong support for continued use of the oocyte in studying the general control of cell proliferation, particularly with the technique of microinjection. Investigation of the process of amphibian oocyte maturation has also led to the discovery of a totally new type of steroid hormone mechanism in which a plasma membrane steroid receptor is linked to the adenylate cyclase system. The oocyte system is the first example of a steroid hormone effect mediated by a direct effect on a target enzyme without a requirement for protein or RNA synthesis. The generality of this type of steroid hormone mechanism remains to be established, but its similarity to the P site on adenylate cyclase is suggestive evidence of a general mechanism, since the P site is present in all eukaryotic plasma membranes. Most steroid hormone systems have been investigated with respect to the mechanism of increased protein synthesis after steroid addition, with little attention to any cell division occurring. In the oocyte, the cell division effect of steroids is the predominant one; it is possible that this type of effect is mediated by a different receptor system than the one associated with changes in gene expression in somatic cells. The information described in this chapter hints at the complexity underlying the activation of development during the process of oocyte maturation. The finding that so many of the specific events and general mechanisms operating in oocyte maturation are also used in differentiated tissues strongly indicates that further study of amphibian oocyte maturation will lead to findings of general significance in terms of how information stored in the oocyte is used during development to result in a new member of the species.

Phosphorylation of ribosomal protein S6 on serine after microinjection of the Abelson murine leukemia virus tyrosine-specific protein kinase into Xenopus oocytes

Phosphorylation of ribosomal protein S6 in NIH 3T3 fibroblasts is dependent on the presence of serum, but after transformation of these cells by Abelson murine leukemia virus (Ab-MuLV), S6 remained highly phosphorylated on serine residues either in the absence or the presence of serum. To investigate whether S6 phosphorylation in this system was a consequence of the action of the Ab-MuLV tyrosine-specific protein kinase, purified Ab-MuLV kinase made in Escherichia coli was microinjected into Xenopus oocytes and was observed to cause a 7- to 15-fold increase in the phosphorylation of S6 on serine residues. Two-dimensional phosphopeptide maps of S6 phosphorylated in Ab-MuLV-transformed NIH cells in the absence of serum were identical to those of S6 isolated from normal cells grown in the presence of serum. In addition, S6 from oocytes injected with Ab-MuLV kinase yielded an S6 phosphopeptide map indistinguishable from that of serum-stimulated NIH 3T3 cells, whereas S6 from control oocytes lacked several phosphopeptides. Ab-MuLV kinase did not phosphorylate S6 directly in vitro, and microinjection of a mutant Ab-MuLV protein lacking kinase activity had no effect. These results indicate that the Ab-MuLV kinase interacts with a cellular pathway to enhance S6 phosphorylation by directly or indirectly activating an S6 protein kinase and/or inactivating an S6 protein phosphatase.

Microinjection of pp60v-src into Xenopus oocytes increases phosphorylation of ribosomal protein S6 and accelerates the rate of progesterone-induced meiotic maturation

Microinjection of purified pp60v-src into Xenopus oocytes caused the phosphorylation of ribosomal protein S6 on serine residues and also increased total protein phosphorylation, with almost a two-fold increase in the percentage of phosphotyrosine present. In addition, pp60v-src accelerated the time course of progesterone-induced oocyte maturation, suggesting that the biochemical pathway influenced by pp60v-src is related to that induced by progesterone.

Phorbol ester, serum, and rous sarcoma virus transforming gene product induce similar phosphorylations of ribosomal protein S6

The addition of phorbol 12-myristate 13-acetate (PMA), a potent tumor promoter, to serum-starved quiescent chicken embryo fibroblasts (CEF) or C127 murine cells resulted in increased phosphorylation of 40S ribosomal protein S6. The effect of PMA on S6 phosphorylation in quiescent CEF was half-maximal at approximately equal to 100 nM and was readily observed at 16 nM. In addition, S6 phosphorylation was increased in serum-starved CEF incubated with the diacylglycerol derivative, 1-oleoyl-2-acetylglycerol. S6 phosphorylation in PMA-stimulated, serum-stimulated, and serum-starved Rous sarcoma virus-transformed CEF was analyzed by phospho amino acid analysis, two-dimensional polyacrylamide gel electrophoresis, limited proteolysis with V8 protease, and two-dimensional thin-layer electrophoresis of chymotryptic digests. Comparison of S6 phosphorylation by these methods suggests that phosphorylation of S6 stimulated by PMA, serum, or oncogenic transformation with Rous sarcoma virus occurs through common pathways. This is further supported by the observation that the simultaneous addition of PMA and serum to CEF or of either PMA or serum to Rous sarcoma virus-transformed CEF did not significantly further increase the incorporation of phosphate into S6.

Microinjection of pp60v-src into Xenopus oocytes increases phosphorylation of ribosomal protein S6 and accelerates the rate of progesterone-induced meiotic maturation

Microinjection of purified pp60v-src into Xenopus oocytes caused the phosphorylation of ribosomal protein S6 on serine residues and also increased total protein phosphorylation, with almost a two-fold increase in the percentage of phosphotyrosine present. In addition, pp60v-src accelerated the time course of progesterone-induced oocyte maturation, suggesting that the biochemical pathway influenced by pp60v-src is related to that induced by progesterone.

Phosphorylation of the ribosomal protein S6 is elevated in cells transformed by a variety of tumor viruses

We have analyzed the phosphorylation of the ribosomal protein S6 in several cultures of normal cells and cells transformed with RNA or DNA tumor viruses. Serum deprivation of confluent, normal cells results in low levels of S6 phosphorylation. In contrast, under identical conditions, this protein is highly phosphorylated in virally transformed cells. No differences in the phosphopeptides generated by one-dimensional limited proteolysis were detected in S6 prepared from serum-stimulated or transformed cells. These results suggest that a common event in oncogenic transformation is the phosphorylation of S6.

Changes in protein phosphorylation during the maturation of mammalian oocytes in vitro

Cumulus-enclosed sheep oocytes were cultured in gonadotrophin-containing medium for up to 9 hr and were then incubated for 3 hr in the presence of [32P]phosphate. The incorporation of 32P into TCA-insoluble material was measured, and oocyte proteins were separated by one- and two-dimensional gel electrophoresis. Incorporation of [32P]phosphate into protein increased after 3 hr culture and again after 9 hr, the time of germinal vesicle breakdown (GVBD). Qualitative and quantitative changes in the phosphorylation of proteins occurred over the 12-hr period studied. One of the most prominent changes was the appearance of a band of Mr 33,000, which was absent at 0-3 hr but appeared with increasing intensity with longer periods of culture. Two-dimensional electrophoresis revealed that the bulk of material in this band was a neutral polypeptide. No significant incorporation of [32P]phosphate was found in ribosomal extracts of oocytes.

Purified rat brain calcium- and phospholipid-dependent protein kinase phosphorylates ribosomal protein S6

The Ca2+-phospholipid-regulated protein kinase has been purified to homogeneity from a 100,000 X g supernatant fluid of rat brain homogenate by a procedure that includes DEAE-cellulose chromatography and successive filtrations on Ultrogel AcA 34 in EGTA and in phosphatidylserine and Ca2+. A more rapid purification consisting of DEAE-cellulose chromatography, Ultrogel AcA 34 gel filtration chromatography, and DEAE-trisacryl chromatography, all in the presence of EGTA, was also developed. Although the enzyme obtained by the latter procedure is not homogeneous, it exhibits properties similar to those of the pure enzyme and is more stable. In addition, the DEAE-trisacryl step permitted resolution of a contaminating Ca2+-inhibitable protein kinase that can interfere with studies of the Ca2+-phospholipid-stimulated enzyme. The homogeneous enzyme, purified about 300-fold, was estimated to have a Mr of 84,000. Its activity was 20- to 30-fold higher in the presence of phospholipid and Ca2+ than in the presence of phospholipid and EGTA, EGTA, or Ca2+ alone. The specific activity of the activated kinase was 852 nmol of P incorporated into histone per min/mg at 20 degrees C. The pure enzyme underwent autophosphorylation in a Ca2+- and phospholipid-dependent manner. This reaction was inhibited in the presence of histones without affecting the kinetic properties of the enzyme. Under optimal assay conditions, the homogeneous enzyme was activated 10-20% by either 10 microM diolein or 100 nM phorbol 12-myristate 13-acetate. Activation of the purified enzyme by diolein or the phorbol ester was far greater (3- to 4-fold) when aggregated instead of freshly sonicated phospholipids were used, suggesting that these compounds affect the interaction of the enzyme with phospholipids and Ca2+. The purified enzyme catalyzed the phosphorylation of the 40S ribosomal subunit protein S6. The Km for S6 was approximately equal to 1 microM and it was estimated that 2 mol of phosphate were incorporated per mol of S6. The observation that protein S6 can be phosphorylated by the purified Ca2+-phospholipid-dependent protein kinase may link recent reports that phorbol ester tumor promoters activate the Ca2+-phospholipid-dependent protein kinase in vitro and stimulate phosphorylation of the ribosomal protein S6 in vivo.

Ordered phosphorylation of 40S ribosomal protein S6 after serum stimulation of quiescent 3T3 cells

The amino acids and tryptic peptides that become phosphorylated in 40S ribosomal protein S6 after serum stimulation of quiescent 3T3 cells were examined by two-dimensional thin-layer electrophoresis. In the maximally phosphorylated form of the protein, most of the phosphate was incorporated into serine and a small amount, into threonine. Digestion of this form of the protein with trypsin revealed 10 major phosphopeptides. All 10 contained phosphoserine and 2 of the 10 also contained phosphothreonine. Next, the five forms of increasingly phosphorylated S6 were individually separated on two-dimensional polyacrylamide gels or total S6 was isolated from cells that were stimulated for only a short time and their phosphotryptic maps were analyzed. The results showed that, as larger amounts of phosphate were added to S6, the phosphopeptides appeared in a specific order.