Regulation of translation initiation and modulation of cellular physiology

Characterization of the 5' region of the Leishmania infantum LORIEN/MAT2 gene cluster and role of LORIEN flanking regions in post-transcriptional regulation

LORIEN (encoding a protein that contains a SP-RING/Miz zinc-finger motif present in a group of proteins involved in the Small Ubiquitin-related Modifier -SUMO- conjugation pathway) and MAT2 (encoding the methionine adenosyltransferase -MAT-) genes are arranged as two alternating copies in a head-to-tail configuration, with the LORIEN gene as the first copy of the cluster. The 5880bp preceding the first LORIEN gene copy were compared to the same region of L. major, showing a 93% identity between them. Bioinformatic analysis of this region predicted the presence of a 747-bp ORF encoding a hypothetical protein of 248 amino acids. Transcription of this ORF was confirmed by run-on assays and RT-PCR. Expression of the LORIEN gene was tested in both the promastigote and amastigote stages. Transcription arrest evidenced that LORIEN mRNA stability was very similar in both stages of the parasite life cycle. Protein synthesis inhibition by cycloheximide led to an increase in the steady-state levels of LORIEN transcripts only during the promastigote stage, pointing out to the existence of different stage-dependent mechanisms operating on the post-transcriptional regulation of this gene. The role of the LORIEN untranslated regions (5'UTR and 3'UTR) in post-transcriptional regulation was analysed using the luciferase (luc) reporter gene. Results evidenced that the 5'UTR was responsible for a low reporter gene expression, whereas the intergenic region (IR) between LORIEN and MAT2 genes provided high luc levels. However, the 3'UTR seemed to lack regulatory elements. Basing on these results, a model of regulation for the LORIEN gene is proposed.

Initiation factor eIF2 alpha phosphorylation in stress responses and apoptosis

The alpha subunit of polypeptide chain initiation factor eIF2 can be phosphorylated by a number of related protein kinases which are activated in response to cellular stresses. Physiological conditions which result in eIF2 alpha phosphorylation include virus infection, heat shock, iron deficiency, nutrient deprivation, changes in intracellular calcium, accumulation of unfolded or denatured proteins and the induction of apoptosis. Phosphorylated eIF2 acts as a dominant inhibitor of the guanine nucleotide exchange factor eIF2B and prevents the recycling of eIF2 between successive rounds of protein synthesis. Extensive phosphorylation of eIF2 alpha and strong inhibition of eIF2B activity can result in the downregulation of the overall rate of protein synthesis; less marked changes may lead to alterations in the selective translation of alternative open reading frames in polycistronic mRNAs, as demonstrated in yeast. These mechanisms can provide a signal transduction pathway linking eukaryotic cellular stress responses to alterations in the control of gene expression at the translational level.

Growth factor-stimulated protein synthesis is inhibited by sodium orthovanadate

The study of intracellular signaling pathways has been aided by the use of sodium orthovanadate, a cell-permeable inhibitor of tyrosine phosphatases. However, long-term addition of sodium orthovanadate is often cytotoxic. In this study we demonstrate that the growth factor-mediated increase in the rate of protein synthesis was inhibited by sodium orthovanadate. This effect of sodium orthovanadate was dose-dependent, with an IC50 of 40 microM and maximal inhibition obtained at 100 microM. As a consequence, the fetal bovine serum-mediated induction of the immediate-early genes, c-Fos and MKP-1, at the protein level was inhibited by orthovanadate. Orthovanadate's ability to attenuate protein synthesis was partially reversible, and was no longer evident when the agent was added 6 h after addition of growth factor to cells. Analysis of several elements of signaling pathways which are known to regulate protein synthesis in a positive manner (p42/p44 MAPK, AKT and p70 S6K stimulation, and hyperphosphorylation of PHAS-I) were not inhibited but rather were stimulated by orthovanadate. Thus, sodium orthovanadate is a potent inhibitor of growth factor-stimulated protein synthesis independent of p42/p44 MAPK or PI3K-p70 S6K activation.

Differential regulation of cyclooxygenase-2 (COX-2) mRNA stability by interleukin-1 beta (IL-1 beta) and tumor necrosis factor-alpha (TNF-alpha) in human in vitro differentiated macrophages

Cyclooxygenase-2 (COX-2) is a highly inducible gene in macrophages by pro-inflammatory cytokines. A major mechanism for cytokine-induced COX-2 expression is stabilization of COX-2 mRNA. In this study, we examined the induction of COX-2 expression by interleukin-1 beta (IL-1 beta) and tumor necrosis factor-alpha (TNF-alpha) in human primary in vitro differentiated macrophages. IL-1 beta (5 ng/mL) or TNF-alpha (1 ng/mL) induced up to an approximately 40-fold increase of COX-2 mRNA in macrophages during a 2 to 2.5-hr incubation. Run-off experiments demonstrated that cytokine stimulation had only a mild effect on the COX-2 transcription rate (approximately 10-40% increase). The translation blocker cycloheximide (CHM) (10 mg/mL) superinduced COX-2 mRNA during 2 hr of incubation and further stabilized the COX-2 mRNA (T1/2 > 4 hr). The CHM-superinduced COX-2 mRNA was subject to a rapid degradation after removal of CHM (T1/2 < 1 hr). Both IL-1 beta and TNF-alpha stabilized cytokine-induced COX-2 mRNA (T1/2 > or = 2 hr). Maximal stabilization of COX-2 mRNA after a short-term stimulation required the continued presence of IL-1 beta in the medium. Long-term treatment of TNF-alpha destabilized the induced COX-2 mRNA. Cells simultaneously treated with both IL-1 beta and TNF-alpha had a reduced induction of COX-2, IL-1 beta, and IL-6 mRNA. In transcription-arrested cells, the translation blocker puromycin affected the TNF-alpha-induced stabilization and destabilization of COX-2 mRNA, but not the IL-1 beta-induced stabilization. The studies suggest that positive and negative regulation of mRNA stability may play a major role in cytokine-mediated COX-2 induction in human macrophages. TNF-alpha may play both pro-inflammatory and protective roles during inflammation by regulation of pro-inflammatory gene transcripts.

Regulation and adaptation of glucose metabolism of the parasitic protist Leishmania donovani at the enzyme and mRNA levels

Adaptation of the glucose metabolism of Leishmania donovani promastigotes (insect stage) was investigated by simultaneously measuring metabolic rates, enzyme activities, message levels, and cellular parameters under various conditions. Chemostats were used to adapt cells to different growth rates with growth rate-limiting or excess glucose concentrations. L. donovani catabolized glucose to CO(2), succinate, acetate, and pyruvate in ratios that depended on growth rate and glucose availability. Rates of glucose consumption were a linear function of growth rate and were twice as high in excess glucose-grown cells as in glucose-limited organisms. The major end product was CO(2), but organic end products were also formed in ratios that varied strongly with growth conditions. The specific activities of the 14 metabolic enzymes measured varied by factors of 3 to 17. Two groups of enzymes adapted specific activities in parallel, but there was no correlation between the groups. The activities of only one group correlated with specific rates of glucose metabolism. Total RNA content per cellular protein varied by a factor of 6 and showed a linear relationship with the rate of glucose consumption. There was no correlation between steady-state message levels and activities of the corresponding enzymes, suggesting regulation at the posttranscriptional level. A comparison of the adaptation of energy metabolism in L. donovani and other species suggests that the energy metabolism of L. donovani is inefficient but is well suited to the environmental challenges that it encounters during residence in the sandfly, its insect vector.

Translational recruitment of Xenopus maternal mRNAs in response to poly(A) elongation requires initiation factor eIF4G-1

Xenopus oocytes accumulate maternal mRNAs which are then recruited to ribosomes during meiotic cell cycle progression in response to progesterone and coincident with poly(A) elongation. Prior to stimulation, most protein synthesis ( approximately 70%) does not require intact translation factor eIF4G (B. D. Keiper and R. E. Rhoads, 1997, Nucleic Acids Res. 25, 395-402). In the present study we have addressed the requirement of eIF4G in the recruitment of mRNAs during meiosis. Cleavage of eIF4G by coxsackievirus protease 2A inhibited progesterone-induced meiotic progression in 88% of the oocytes; prevented the recruitment of maternal mRNAs encoding cyclin B1, c-Mos, D7, and B9; and disrupted the association of eIF4G with poly(A)-binding protein. Poly(A) elongation, however, was not inhibited by eIF4G cleavage. Injection of MPF restored meiotic cell cycle progression to >60% of the oocytes but not the recruitment of cyclin B1 or B9 mRNA. Previously recruited maternal mRNAs were removed from polyribosomes following subsequent cleavage of eIF4G, indicating that eIF4G is required both to recruit and also to maintain maternal mRNAs on polyribosomes. The expression of a cleavage-resistant variant of human eIF4G-1 (G486E) significantly restored the ability to synthesize c-Mos in response to progesterone and to translate exogenous beta-globin mRNA, indicating that the inhibition by protease 2A is due to cleavage of eIF4G alone. These results indicate that intact eIF4G is required for the poly(A)-dependent recruitment of several maternal mRNAs (cyclin B1, c-Mos, D7, and B9) during meiotic cell cycle progression but not for the synthesis of most proteins.

Signalling pathways regulating protein turnover in skeletal muscle

The protein content of skeletal muscle is determined by the relative rates of synthesis and degradation which must be regulated coordinately to maintain equilibrium. However, in conditions such as fasting where amino acids are required for gluconeogenesis, or in cancer cachexia, this equilibrium is disrupted and a net loss of protein ensues. This review, utilising studies performed in several situations, summarizes the current state of knowledge on the possible signalling pathways regulating protein turnover in skeletal muscle and highlights areas for future work.

Disruption of the gene encoding the mitogen-regulated translational modulator PHAS-I in mice

PHAS-I is the prototype of a group of eIF4E-binding proteins that can regulate mRNA translation in response to hormones and growth factors. To investigate the importance of PHAS-I in the physiology of the intact animal, we disrupted the PHAS-I gene in mice. Tissues and cells derived from the knockout mice contained no detectable PHAS-I protein. A related protein, PHAS-II, and eIF4E were readily detectable in tissues from these animals, but neither appeared to be changed in a compensatory manner. Mice lacking PHAS-I appeared normal at birth. However, male knockout mice weighed approximately 10% less than controls at all ages, whereas female weights were similar to those of controls. Both males and females were fertile. Tissues from adult animals appeared to be normal by routine histological staining techniques, as were routine blood cell counts and chemistries. Fibroblasts derived from PHAS-I-deficient mouse embryos exhibited normal rates of growth and overall protein synthesis, responded normally to serum stimulation of ornithine decarboxylase activity and cell growth, and rapamycin inhibition of cell growth. Under these experimental conditions, PHAS-I is apparently not required for the normal development and reproductive behavior of female mice, but is required for normal body weight in male mice; the mechanisms responsible for this phenotype remain to be determined.

Molecular mechanism of heme biosynthesis

Two of the major organs producing heme are bone marrow and the liver. delta-Aminolevulinate synthase (ALAS) plays the key role to regulate heme biosynthesis in hepatocytes as well as in erythroid cells. In the liver, nonspecific (or housekeeping) isozyme of ALAS (ALAS-N) is expressed to be regulated by its end product, heme, in a negative feedback manner. The way to regulate ALAS-N in the liver is suitable to supply a constant level of heme for a family of drug metabolizing enzymes, cytochrome P-450 (CYP). In erythroid tissues, not only erythroid-specific isozyme of ALAS (ALAS-E) but also ALAS-N are expressed, and regulated by distinctive manners. Although heme regulates ALAS-N in a negative feedback manner even in erythroid cells, ALAS-E is upregulated by induced heme concentration. ALAS-N in undifferentiated erythroid cells, therefore, is suggested to produce heme for CYP, whereas heme for accumulating hemoglobin (Hb) in cells undergoing differentiation is synthesized via ALAS-E. In this article, we describe the molecular mechanisms to regulate heme biosynthesis in non-erythroid as well as in erythroid tissues, and discuss the pathological significance of the mechanisms in patients with inherited disorders, porphyrias.

KIS is a protein kinase with an RNA recognition motif

Protein phosphorylation is involved at multiple steps of RNA processing and in the regulation of protein expression. We present here the first identification of a serine/threonine kinase that possesses an RNP-type RNA recognition motif: KIS. We originally isolated KIS in a two-hybrid screen through its interaction with stathmin, a small phosphoprotein proposed to play a general role in the relay and integration of diverse intracellular signaling pathways. Determination of the primary sequence of KIS shows that it is formed by the juxtaposition of a kinase core with little homology to known kinases and a C-terminal domain that contains a characteristic RNA recognition motif with an intriguing homology to the C-terminal motif of the splicing factor U2AF. KIS produced in bacteria has an autophosphorylating activity and phosphorylates stathmin on serine residues. It also phosphorylates in vitro other classical substrates such as myelin basic protein and synapsin but not histones that inhibit its autophosphorylating activity. Immunofluorescence and biochemical analyses indicate that KIS overexpressed in HEK293 fibroblastic cells is partly targetted to the nucleus. Altogether, these results suggest the implication of KIS in the control of trafficking and/or splicing of RNAs probably through phosphorylation of associated factors.

Mutants of eukaryotic initiation factor eIF-4E with altered mRNA cap binding specificity reprogram mRNA selection by ribosomes in Saccharomyces cerevisiae

Recognition of the 5'-end of eukaryotic mRNA by the ribosomal 43 S preinitiation complex involves the eukaryotic translation initiation factor eIF-4E (eIF-4alpha). Deletion mutants of the eIF-4E gene of Saccharomyces cerevisiae (CDC33) encoded proteins with reduced affinity for the 5'-cap. One of these mutant proteins lacked any detectable binding to a cap analogue binding column, yet was still able to support cell growth. More than 17% of the total eIF-4E amino acid sequence could be removed without fully inactivating this factor. At least 30 of the N-terminal amino acids are not essential for function. The minimal functional eIF-4E protein segment therefore comprises at most 176 amino acids. The translation and growth defects of the deletion mutants could be at least partially compensated by increases in eIF-4E synthesis, possibly due to a mass-action effect on mRNA binding. Electroporation of yeast spheroplasts with in vitro synthesized mRNA allowed us to characterize the ability of eIF-4E mutant strains to distinguish between capped and uncapped mRNAs in vivo. Our data show that the cap specificity of eIF-4E determines to what extent the translational apparatus differentiates between capped and uncapped mRNAs and indicate the minimum relative mRNA (cap) binding activity of eIF-4E required for yeast cell viability.

Initiation of protein synthesis in eukaryotic cells

It is becoming increasingly apparent that translational control plays an important role in the regulation of gene expression in eukaryotic cells. Most of the known physiological effects on translation are exerted at the level of polypeptide chain initiation. Research on initiation of translation over the past five years has yielded much new information, which can be divided into three main areas: (a) structure and function of initiation factors (including identification by sequencing studies of consensus domains and motifs) and investigation of protein-protein and protein-RNA interactions during initiation; (b) physiological regulation of initiation factor activities and (c) identification of features in the 5' and 3' untranslated regions of messenger RNA molecules that regulate the selection of these mRNAs for translation. This review aims to assess recent progress in these three areas and to explore their interrelationships.

Regulation of both glycogen synthase and PHAS-I by insulin in rat skeletal muscle involves mitogen-activated protein kinase-independent and rapamycin-sensitive pathways

Incubating rat diaphragm muscles with insulin increased the glycogen synthase activity ratio (minus glucose 6-phosphate/plus glucose 6-phosphate) by approximately 2-fold. Insulin increased the activities of mitogen-activated protein (MAP) kinase and the Mr = 90,000 isoform of ribosomal protein S6 kinase (Rsk) by approximately 1.5-2.0-fold. Epidermal growth factor (EGF) was more effective than insulin in increasing MAP kinase and Rsk activity, but in contrast to insulin, EGF did not affect glycogen synthase activity. The activation of both MAP kinase and Rsk by insulin was abolished by incubating muscles with the MAP kinase kinase (MEK) inhibitor, PD 098059; however, the MEK inhibitor did not significantly reduce the effect of insulin on activating glycogen synthase. Incubating muscles with concentrations of rapamycin that inhibited activation of p70S6K abolished the activation of glycogen synthase. Insulin also increased the phosphorylation of PHAS-I (phosphorylated heat- and acid-stable protein) and promoted the dissociation of the PHAS-I*eIF-4E complex. Increasing MAP kinase activity with EGF did not mimic the effect of insulin on PHAS-I phosphorylation, and the effect of insulin on increasing MAP kinase could be abolished with the MEK inhibitor without decreasing the effect of insulin on PHAS-I. The effects of insulin on PHAS-I were attenuated by rapamycin. Thus, activation of the MAP kinase/Rsk signaling pathway appears to be neither necessary nor sufficient for insulin action on glycogen synthase and PHAS-I in rat skeletal muscle. The results indicate that the effects of insulin on increasing the synthesis of glycogen and protein in skeletal muscle, two of the most important actions of the hormone, involve a rapamycin-sensitive mechanism that may include elements of the p70S6K signaling pathway.

Insulin and phorbol ester stimulate initiation factor eIF-4E phosphorylation by distinct pathways in Chinese hamster ovary cells overexpressing the insulin receptor

We have developed a one-dimensional isoelectric focusing technique to measure changes in the steady-state phosphorylation of the cap-binding initiation factor, eIF-4E. We have used a Chinese hamster ovary cell line transfected with the human insulin receptor (CHO.T cells) to study the regulation of eIF-4E phosphorylation by insulin and other stimuli. Exposure of CHO.T cells to insulin, phorbol ester or serum resulted in a rapid increase (up to twofold) in eIF-4E phosphorylation. As a control, we have also performed experiments with the parental cell line, CHO.K1 cells, in which both serum and phorbol ester, but not nanomolar concentrations of insulin, produce similar changes in eIF-4E phosphorylation. We have used two complementary approaches to study the role of protein kinase C (PKC) in these responses: a highly specific inhibitor of PKC and down-regulation of PKC by prior treatment of the cells with phorbol ester. In CHO.T cells, both approaches indicate that PKC is required for the response to phorbol ester but that insulin and serum each increase eIF-4E phosphorylation by a mechanism(s) independent of this protein kinase. Similarly, PKC is necessary for the effects of phorbol ester, but not of serum, on eIF-4E phosphorylation in CHO.K1 cells. These data indicate that multiple signal transduction mechanisms are involved in the modulation of eIF-4E phosphorylation and the implications of these findings are discussed.

A novel dystrophin/utrophin-associated protein is an enzymatically inactive member of the phosphoglucomutase superfamily

A 60-kDa protein localised in adherens-type cellular junctions, and previously called aciculin, has been found to interact with the cytoskeletal proteins dystrophin and utrophin [Belkin, A. M. & Burridge, K. (1995) J. Biol. Chem. 270, 6328-6337]. In this study, we report the complete sequence of this protein, and show that it is a novel member of the phosphoglucomutase (PGM) family of proteins. The PGM-related protein (PGM-RP), which contains 506 amino acids (55.6 kDa), is smaller than PGM1 (566 amino acids, 61 kDa). The active site consensus sequences of prokaryotic and eukaryotic mutases are not conserved in PGM-RP, a finding consistent with the lack of enzymatic activity of PGM-RP in vitro, and the absence of a phosphorylated intermediate in vivo. The organisation of the PGM-RP gene is essentially identical to that of PGM1. We propose that the PGM-RP gene, which we have mapped to human chromosome 9qcen-q13, evolved from the PGM1 gene, and encodes a protein with a structural rather than an enzymatic role. PGM-RP is expressed predominantly in muscle with the highest levels in smooth muscle. The significance of the interaction between dystrophin/utrophin and an increasing number of cytoplasmic proteins including PGM-RP remains to be explored.

Characterization of the in vivo phosphorylation sites of the mRNA.cap-binding complex proteins eukaryotic initiation factor-4E and p20 in Saccharomyces cerevisiae

Eukaryotic translation is believed to be regulated via the phosphorylation of specific eukaryotic initiation factors (eIFs), including one of the cap-binding complex proteins, eIF-4E. We show that in the yeast Saccharomyces cerevisiae, both eIF-4E and another cap-binding complex protein, p20, are phosphoproteins. The major sites of phosphorylation of yeast eIF-4E are found to be located in the N-terminal region of its sequence (Ser2 and Ser15) and are thus in a different part of the protein from the main phosphorylation sites (Ser53 and Ser209) proposed previously for mammalian eIF-4E. The most likely sites of p20 phosphorylation are at Ser91 and/or Ser154. All of the major sites in the two yeast proteins are phosphorylated by casein kinase II in vitro. Casein kinase II phosphorylation of cap-complex proteins should therefore be considered as potentially involved in the control of yeast protein synthesis. Mutagenesis experiments revealed that yeast eIF-4E activity is not dependent on the presence of Ser2 or Ser15. On the other hand, we observed variations in the amount of (phosphorylated) p20 associated with the cap-binding complex as a function of cell growth conditions. Our results suggest that interactions of yeast eIF-4E with other phosphorylatable proteins, such as p20, could play a pivotal role in translational control.

Deregulation of translational control of the 65-kDa regulatory subunit (PR65 alpha) of protein phosphatase 2A leads to multinucleated cells

Efficient translation of the mRNA encoding the 65-kDa regulatory subunit (PR65 alpha) of protein phosphatase 2A (PP2A) is prevented by an out of frame upstream AUG and a stable stem-loop structure (delta G = -55.9 kcal/mol) in the 5'-untranslated region (5'-UTR). Deletion of the 5'-UTR allows efficient translation of the PR65 alpha message in vitro and overexpression in COS-1 cells. Insertion of the 5'-UTR into the beta-galactosidase leader sequence dramatically inhibits translation of the beta-galactosidase message in vitro and in vivo, confirming that this sequence functions as a potent translation regulatory sequence. Cells transfected or microinjected with a PR65 alpha expression vector lacking the 5'-UTR, express high levels of PR65 alpha, accumulating in both nucleus and cytoplasm. PR65 alpha overexpressing rat embryo fibroblasts (REF-52 cells) become multinucleated. These data and previous results (Mayer-Jaekel, R. E., Ohkura, H., Gomes, R., Sunkel, C. E., Baumgartner, S., Hemmings, B. A., and Glover, D. M. (1993) Cell 72, 621-633) suggest that PP2A participates in the regulation of both mitosis and cytokinesis.

Control of PHAS-I by insulin in 3T3-L1 adipocytes. Synthesis, degradation, and phosphorylation by a rapamycin-sensitive and mitogen-activated protein kinase-independent pathway

PHAS-I levels increased 8-fold as 3T3-L1 fibroblasts differentiated into adipocytes and acquired sensitivity to insulin. Insulin increased PHAS-I protein (3.3-fold after 2 days), the rate of PHAS-I synthesis (3-fold after 1 h), and the half-life of the protein (from 1.5 to 2.5 days). Insulin also increased the phosphorylation of PHAS-I and promoted dissociation of the PHAS-I eukaryotic initiation factor-4E (eIF-4E) complex, effects that were maximal within 10 min. With recombinant [H6]PHAS-I as substrate, mitogen-activated protein (MAP) kinase was the only insulin-stimulated PHAS-I kinase detected after fractionation of extracts by Mono Q chromatography; however, MAP kinase did not readily phosphorylate [H6]PHAS-I when the [H6]PHAS-I.eIF-4E complex was the substrate. Thus, while MAP kinase may phosphorylate free PHAS-I, it is not sufficient to dissociate the complex. Moreover, rapamycin attenuated the stimulation of PHAS-I phosphorylation by insulin and markedly inhibited dissociation of PHAS-I.eIF-4E, without decreasing MAP kinase activity. Rapamycin abolished the effects of insulin on increasing phosphorylation of ribosomal protein S6 and on activating p70S6K. The MAP kinase kinase inhibitor, PD 098059, markedly decreased MAP kinase activation by insulin, but it did not change PHAS-I phosphorylation or the association of PHAS-I with eIF-4E. In summary, insulin increases the expression of PHAS-I and promotes phosphorylation of multiple sites in the protein via multiple transduction pathways, one of which is rapamycin-sensitive and independent of MAP kinase. Rapamycin may inhibit translation initiation by increasing PHAS-I binding to eIF-4E.

cAMP- and rapamycin-sensitive regulation of the association of eukaryotic initiation factor 4E and the translational regulator PHAS-I in aortic smooth muscle cells

Incubating rat aortic smooth muscle cells with either platelet-derived growth factor BB (PDGF) or insulin-like growth factor I (IGF-I) increased the phosphorylation of PHAS-I, an inhibitor of the mRNA cap binding protein, eukaryotic initiation factor (eIF) 4E. Phosphorylation of PHAS-I promoted dissociation of the PHAS-I-eIF-4E complex, an effect that could partly explain the stimulation of protein synthesis by the two growth factors. Increasing cAMP with forskolin decreased PHAS-I phosphorylation and markedly increased the amount of eIF-4E bound to PHAS-I, effects consistent with an action of cAMP to inhibit protein synthesis. Both PDGF and IGF-I activated p70S6K, but only PDGF increased mitogen-activated protein kinase activity. Forskolin decreased by 50% the effect of PDGF on increasing p70S6K, and forskolin abolished the effect of IGF-I on the kinase. The effects of PDGF and IGF-I on increasing PHAS-I phosphorylation, on dissociating the PHAS-I-eIF-4E complex, and on increasing p70S6K were abolished by rapamycin. The results indicate that IGF-I and PDGF increase PHAS-I phosphorylation in smooth muscle cells by the same rapamycin-sensitive pathway that leads to activation of p70S6K.

Cytoplasmic mRNA-protein interactions in eukaryotic gene expression

Post-transcriptional mechanisms contribute in many important ways to the overall control and regulation of gene expression, and in doing so employ a veritable army of proteins that bind a wide range of targets in messenger RNA (mRNA). The full range of these RNA-protein interactions is only just beginning to emerge, and much remains to be learned about the mechanisms underlying the rapidly increasing number of regulatory systems now being described.

Post-transcriptional regulation of the content of spermidine/spermine N1-acetyltransferase by N1N12-bis(ethyl)spermine

Spermidine/spermine N1-acetyltransferase (SSAT) is the rate-limiting enzyme for the degradation and excretion of polyamines in mammalian cells, and its activity is known to be increased enormously on exposure to polyamines and polyamine analogues. The mechanism by which such an analogue, BESM [N1N12-bis(ethyl)spermine], increases the content of SSAT was investigated by transfecting COS-7 cells with plasmids containing SSAT cDNA in the pEUK expression vector. Despite a large increase in mRNA production, there was only a very small increase in SSAT activity in the transfected cells. When BESM was added at 36 h after transfection, there was a large and very rapid increase in SSAT protein amounting to 380-fold in 12 h without any increase in the mRNA. SSAT protein turned over very rapidly, with a half-life of about 20 min. In the presence of BESM, this turnover was greatly reduced, and the half-life increased to more than 13 h. However, this increase was not sufficient to account for all of the increase in SSAT protein, suggesting that there is also regulation of the translation of the mRNA by BESM. Further evidence for such translation regulation was obtained by studying the polysomal distribution of the SSAT mRNA. In the absence of BESM, most of the mRNA was present in fractions which sedimented more slowly than the monoribosome peak. In BESM-treated cells, a significant proportion of the SSAT mRNA was moved into the small-polysome region of the gradient. The expression of SSAT and the effects of BESM on the polysomal distribution of SSAT mRNA were not affected by the 5'- or 3'-untranslated regions of the mRNA, since constructs which lacked all of these regions gave similar results to constructs containing the entire mRNA sequence. These results show that the increased transcription of the SSAT gene that occurs in the presence of polyamine analogues such as BESM is not sufficient for SSAT expression and that post-transcriptional regulation is critical for the control of SSAT content.

Initiation codons within 5'-leaders of mRNAs as regulators of translation

Why should a preponderance of proto-oncogenes, growth factor genes and growth factor receptor genes contain translation initiation codons and associated open reading frames in their 5'-leaders? An increasing number of open reading frames are being shown to function as cis-acting regulatory signals able to moderate expression of the downstream reading frame. These regulatory elements could play a fundamental role in the regulation of proliferation of vertebrate cells.

The yeast Saccharomyces cerevisiae system: a powerful tool to study the mechanism of protein synthesis initiation in eukaryotes

This review summarizes recent progress in the study of initiation of protein synthesis in the yeast Saccharomyces cerevisiae. Biochemical and genetic approaches provide new insight into the recognition of the 5'-end of mRNA by initiation factors and 40S ribosomes, unwinding of mRNA secondary structures in the untranslated region and proper recognition of the AUG start codon. Experiments with initiation factor-dependent cell-free systems have facilitated studies of factor functions and factor requirements for translation of different mRNAs. The analysis of mutations which suppress the inhibitory effect on translation of RNA secondary structure in the 5'-untranslated region of yeast mRNAs has led to the identification of gene products which may be involved in both transcription and translation.