Eukaryotic protein synthesis: an in vitro analysis

Maternal immune activation in mice disrupts proteostasis in the fetal brain

Maternal infection and inflammation during pregnancy are associated with neurodevelopmental disorders in offspring, but little is understood about the molecular mechanisms underlying this epidemiologic phenomenon. Here, we leveraged single-cell RNA sequencing to profile transcriptional changes in the mouse fetal brain in response to maternal immune activation (MIA) and identified perturbations in cellular pathways associated with mRNA translation, ribosome biogenesis and stress signaling. We found that MIA activates the integrated stress response (ISR) in male, but not female, MIA offspring in an interleukin-17a-dependent manner, which reduced global mRNA translation and altered nascent proteome synthesis. Moreover, blockade of ISR activation prevented the behavioral abnormalities as well as increased cortical neural activity in MIA male offspring. Our data suggest that sex-specific activation of the ISR leads to maternal inflammation-associated neurodevelopmental disorders.

Measuring mRNA Translation by Polysome Profiling

Determination of mRNA translation rates is essential to understanding the regulatory pathways governing eukaryotic gene expression. In this chapter, we present a transcriptome-wide method to assess translation by association of mRNAs with polysomes on sucrose density gradients. After sedimentation, the fractions are spiked with a control RNA mixture and the RNA content is measured by high-throughput sequencing. Normalization to the spike-ins provides a global quantitative view on the translational status of cellular mRNAs, with the ability to measure changes and identify active and silent subpopulations of each.

Insights from a Paradigm Shift: How the Poly(A)-Binding Protein Brings Translating mRNAs Full Circle

In recent years, our thinking of how the initiation of protein synthesis occurs has changed dramatically. Initiation was thought to involve only events occurring at or near the 5′-cap structure, which serves as the binding site for the cap-binding complex, a group of translation initiation factors (eIFs) that facilitate the binding of the 40 S ribosomal subunit to an mRNA. Because the poly(A)-binding protein (PABP) binds the poly(A) tail present at the 3′-terminus of an mRNA, it was long thought to play no role in translation initiation. In this review, I present evidence from my laboratory that has contributed to the paradigm shift in how we think of mRNAs during translation. The depiction of mRNAs as straight molecules in which the poly(A) tail is far from events occurring at the 5′-end has now been replaced by the concept of a circular mRNA where the interaction between PABP and the cap-binding complex bridges the termini of an mRNA and promotes translation initiation. The research from my laboratory supports the new paradigm that translation of most mRNAs requires a functional and physical interaction between the termini of an mRNA.

Wheat eukaryotic initiation factor 4B organizes assembly of RNA and eIFiso4G, eIF4A, and poly(A)-binding protein

The eukaryotic translation initiation factor (eIF) 4B promotes the RNA-dependent ATP hydrolysis activity and ATP-dependent RNA helicase activity of eIF4A and eIF4F during translation initiation. Although this function is conserved among plants, animals, and yeast, eIF4B is one of the least conserved of initiation factors at the sequence level. To gain insight into its functional conservation, the organization of the functional domains of eIF4B from wheat has been investigated. Plant eIF4B contains three RNA binding domains, one more than reported for mammalian or yeast eIF4B, and each domain exhibits a preference for purine-rich RNA. In addition to a conserved RNA recognition motif and a C-terminal RNA binding domain, wheat eIF4B contains a novel N-terminal RNA binding domain that requires a short, lysine-rich containing sequence. Both the lysine-rich motif and an adjacent, C-proximal motif are conserved with an N-proximal sequence in human and yeast eIF4B. The C-proximal motif within the N-terminal RNA binding domain in wheat eIF4B is required for interaction with eIFiso4G, an interaction not reported for other eIF4B proteins. Moreover, each RNA binding domain requires dimerization for binding activity. Two binding sites for the poly(A)-binding protein were mapped to a region within each of two conserved 41-amino acid repeat domains on either side of the C-terminal RNA binding domain. eIF4A bound to an adjacent region within each repeat, supporting a central role for these conserved eIF4B domains in facilitating interaction with other components of the translational machinery. These results support the notion that eIF4B functions by organizing multiple components of the translation initiation machinery and RNA.

Initiation factor modifications in the preapoptotic phase

Recent studies have identified several mechanistic links between the regulation of translation and the process of apoptosis. Rates of protein synthesis are controlled by a wide range of agents that induce cell death, and in many instances, the changes that occur to the translational machinery precede overt apoptosis and loss of cell viability. The two principal ways in which factors required for translational activity are modified prior to and during apoptosis involve (i) changes in protein phosphorylation and (ii) specific proteolytic cleavages. In this review, we summarise the principal targets for such regulation, with particular emphasis on polypeptide chain initiation factors eIF2 and eIF4G and the eIF4E-binding proteins. We indicate how the functions of these factors and of other proteins with which they interact may be altered as a result of activation of apoptosis and we discuss the potential significance of such changes for translational control and cell growth regulation.

Tissue-specific regulation of protein synthesis by insulin and free fatty acids

The purpose of the study described herein was to investigate how the mammalian target of rapamycin (mTOR)-signaling pathway and eukaryotic initiation factor 2B (eIF2B) activity, both having key roles in the translational control of protein synthesis in skeletal muscle, are regulated in cardiac muscle of rats in response to two different models of altered free fatty acid (FFA) and insulin availability. Protein synthetic rates were reduced in both gastrocnemius and heart of 3-day diabetic rats. The reduction was associated with diminished mTOR-mediated signaling and eIF2B activity in the gastrocnemius but only with diminished mTOR signaling in the heart. In response to the combination of acute hypoinsulinemia and hypolipidemia induced by administration of niacin, protein synthetic rates were also diminished in both gastrocnemius and heart. The niacin-induced changes were associated with diminished mTOR signaling and eIF2B activity in the heart but only with decreased mTOR signaling in the gastrocnemius. In the heart, mTOR signaling and eIF2B activity correlated with cellular energy status and/or redox potential. Thus FFAs may contribute to the translational control of protein synthesis in the heart but not in the gastrocnemius. In contrast, insulin, but not FFAs, is required for the maintenance of protein synthesis in the gastrocnemius.

Translational initiation regulators are hypophosphorylated in rat liver during ethionine-mediated ATP depletion

Administration of ethionine to female rats is known to inhibit hepatic protein synthesis by reducing the level of hepatic ATP. Administration of methionine and/or adenine rapidly restores the ATP levels and protein synthesis. The ethionine administration causes a progressive disaggregation of hepatic polysomes, suggesting that the initiation step of protein synthesis is inhibited. Recent studies indicate that changes in initiation are associated with alterations in the phosphorylation states of translational initiation regulators such as eukaryotic initiation factor (eIF) 4E, eIF4E-binding protein 1 (4E-BP1), and the 70-kDa ribosomal protein S6 kinase (S6K1). We found that these initiation regulators are hypophosphorylated in rat liver during ethionine-mediated ATP depletion (60% of the control value). Furthermore, the restoration of the ATP levels by the administration of methionine and adenine brought about a complete recovery of the phosphorylation states of all these regulators. The present data suggest that hypophosphorylation of various initiation regulators represents the primary event in the ethionine-induced breakdown of polysomes and inhibition of protein synthesis in the liver. Possible involvement of mammalian target of rapamycin (mTOR), as a sensor of intracellular ATP level, was also discussed.

DUG is a novel homologue of translation initiation factor 4G that binds eIF4A

To elucidate the molecular mechanisms of cell death, we have cloned a new gene, designated death-upregulated gene (DUG), from rat insulinoma cells. DUG is constitutively expressed at very low levels in normal cells but is dramatically upregulated in apoptotic cells following serum/glucose starvation or death receptor ligation by Fas ligand. The DUG mRNA is present in two splicing forms: a long form that encodes a protein of 469 amino acids and a short form that gives rise to a polypeptide of 432 amino acids. The predicted DUG protein sequence contains two putative nuclear localization signals and multiple phosphorylation sites for protein kinases and two conserved MA3 domains. Importantly, DUG is homologous to eukaryotic translation initiation factor (eIF) 4G and binds to eIF4A presumably through MA3 domains. Upon transfection, DUG inhibits both intrinsic and extrinsic pathways of apoptosis. Thus, DUG is a novel homologue of eIF4G that regulates apoptosis.

Disruption of the interaction of mammalian protein synthesis eukaryotic initiation factor 4B with the poly(A)-binding protein by caspase- and viral protease-mediated cleavages

Eukaryotic initiation factor (eIF) 4B interacts with several components of the initiation pathway and is targeted for cleavage during apoptosis. In a cell-free system, cleavage of eIF4B by caspase-3 coincides with a general inhibition of protein synthetic activity. Affinity chromatography demonstrates that mammalian eIF4B interacts with the poly(A)-binding protein and that a region consisting of the N-terminal 80 amino acids of eIF4B is both necessary and sufficient for such binding. This interaction is lost when eIF4B is cleaved by caspase-3, which removes the N-terminal 45 amino acids. Similarly, the association of eIF4B with the poly(A)-binding protein in vivo is reduced when cells are induced to undergo apoptosis. Cleavage of the poly(A)-binding protein itself, using human rhinovirus 3C protease, also eliminates the interaction with eIF4B. Thus, disruption of the association between mammalian eIF4B and the poly(A)-binding protein can occur during both apoptosis and picornaviral infection and is likely to contribute to the inhibition of translation observed under these conditions.

Kinetic proofreading scanning models for eukaryotic translational initiation: the cap and poly(A) tail dependency of translation

Two simplified kinetic proofreading scanning (KPS) models were proposed to describe the 5' cap and 3' poly(A) tail dependency of eukaryotic translation initiation. In Model I, the initiation factor complex starts scanning and unwinding the secondary structure of the 5' untranslated region (UTR) from the 5' terminus of mRNA. In Model II, the initiation factor complex starts scanning from any binding site in the 5' UTR. In both models, following ATP hydrolysis, the initiation factor complex either dissociates from mRNA or continues to scan and unwind RNA secondary structure in the 5' UTR. This step repeats n times until the AUG codon is reached. These two models show very different cap and/or poly(A) tail dependency of translation initiation. The models predict that both cap and poly(A) tail dependencies of translation, and translatability of mRNAs are coupled with the structure of 5' UTR: the translation of mRNA with structured 5' UTR is strongly cap- and poly(A) tail-dependent; while translation of mRNA with unstructured 5' UTR is less cap- and poly(A) tail-dependent. We use these two models to explain: (1) the cap and poly(A) tail dependence of translation; (2) the effect of exogenous poly(A) on translation; (3) repression of host mRNA and translation of late adenovirus mRNA in the late phase of adenovirus infection; (4) repression of host mRNA and translation of Vaccinia virus mRNA in virus-infected cell; (5) heat shock repression of translation of normal mRNA and stimulation of translation of hsp mRNA; and (6) the synergistic effect of cap and poly(A) tail on stimulating translation. The kinetic proofreading scanning models provide a coherent interpretation of those phenomena.

Human La antigen is required for the hepatitis C virus internal ribosome entry site-mediated translation

The 5'-noncoding region (5'-NCR) of the hepatitis C virus (HCV) RNA genome serves as an internal ribosome entry site (IRES) and mediates translation initiation in a cap-independent manner. Previously, we reported the interaction between La antigen and the HCV IRES, which appeared to occur in the context of initiator AUG. It was further shown that HCV IRES-mediated translation was stimulated in the presence of human La antigen. In this study, we have defined the cis- and trans-acting elements responsible for La-5'-NCR interactions and established the dependence of the HCV IRES efficiency on cellular La antigen. During the La-IRES interaction, initiator AUG but not the neighboring codons was found to be the direct target of La binding. The C terminus effector domain-dependent modulation of La binding to the HCV IRES is demonstrated by deletion and substitution mutagenesis of the protein. An RNA systematic evolution of ligands by exponential enrichment (SELEX), generated against La protein that selectively binds La in HeLa lysates and competes for the protein binding to the 5'-NCR, was used to demonstrate the requirement of La for the HCV IRES function in the context of mono- and dicistronic mRNAs. Sequestration of La antigen by the RNA SELEX in HeLa translation lysates blocked the HCV and poliovirus IRES-mediated translation in vitro. The functional requirement of La protein for the HCV IRES activity was further established in a liver-derived cell line and in an add-back experiment in which the inhibited IRES was rescued by recombinant human La. These results strongly argue for the novel role of La protein during selection of the initiator AUG and its participation during internal initiation of translation of the HCV RNA genome.

Translation initiation factor modifications and the regulation of protein synthesis in apoptotic cells

The rate of protein synthesis is rapidly down-regulated in mammalian cells following the induction of apoptosis. Inhibition occurs at the level of polypeptide chain initiation and is accompanied by the phosphorylation of the alpha subunit of initiation factor eIF2 and the caspase-dependent cleavage of initiation factors eIF4G, eIF4B, eIF2alpha and the p35 subunit of eIF3. Proteolytic cleavage of these proteins yields characteristic products which may exert regulatory effects on the translational machinery. Inhibition of caspase activity protects protein synthesis from long-term inhibition in cells treated with some, but not all, inducers of apoptosis. This review describes the initiation factor modifications and the possible signalling pathways by which translation may be regulated during apoptosis. We discuss the significance of the initiation factor cleavages and other changes for protein synthesis, and the implications of these events for our understanding of the cellular changes associated with apoptosis.

Modulation of molecular mechanisms involved in protein synthesis machinery as a new tool for the control of cell proliferation

In the past years, the attention of scientists has focused mainly on the study of the genetic information and alterations that regulate eukaryotic cell proliferation and that lead to neoplastic transformation. All therapeutic strategies against cancer are, to date, directed at DNA either with cytotoxic drugs or gene therapy. Little or no interest has been aroused by protein synthesis mechanisms. However, an increasing body of data is emerging about the involvement of translational processes and factors in control of cell proliferation, indicating that protein synthesis can be an additional target for anticancer strategies. In this paper we review the novel insights on the biochemical and molecular events leading to protein biosynthesis and we describe their involvement in cell proliferation and tumorigenesis. A possible mechanistic explanation is given by the interactions that occur between protein synthesis machinery and the proliferative signal transduction pathways and that are therefore suitable targets for indirect modulation of protein synthesis. We briefly describe the molecular tools used to block protein synthesis and the attempts made at increasing their efficacy. Finally, we propose a new multimodal strategy against cancer based on the simultaneous intervention on protein synthesis and signal transduction.

Cleavage of polypeptide chain initiation factor eIF4GI during apoptosis in lymphoma cells: characterisation of an internal fragment generated by caspase-3-mediated cleavage

Polypeptide chain initiation factor eIF4GI undergoes caspase-mediated degradation during apoptosis to give characteristic fragments. The most prominent of these has an estimated mass of approximately 76 kDa (Middle-Fragment of Apoptotic cleavage of eIF4G; M-FAG). Subcellular fractionation of the BJAB lymphoma cell line after induction of apoptosis indicates that M-FAG occurs in both ribosome-bound and soluble forms. Affinity chromatography on m7GTP-Sepharose shows that M-FAG retains the ability of eIF4GI to associate with both the mRNA cap-binding protein eIF4E and initiation factor eIF4A and that the ribosome-bound form of M-FAG is also present as a complex with eIF4E and eIF4A. These data suggest that the binding sites for eIF4E, eIF4A and eIF3 on eIF4GI are retained in the caspase-generated fragment. M-FAG is also a substrate for cleavage by the Foot-and-Mouth-Disease Virus-encoded L protease. These properties, together with the pattern of recognition by a panel of antibodies, define the origin of the apoptotic cleavage fragment. N-terminal sequencing of the products of caspase-3-mediated eIF4GI cleavage has identified the major cleavage sites. The pattern of eIF4GI degradation and the possible roles of the individual cleavage products in cells undergoing apoptosis are discussed.

Wheat germ poly(A)-binding protein increases the ATPase and the RNA helicase activity of translation initiation factors eIF4A, eIF4B, and eIF-iso4F

Recent studies demonstrated that wheat germ poly(A)-binding protein (PABP) interacted with translation eukaryotic initiation factor (eIF)-iso4G and eIF4B, and these interactions increased the poly(A) binding activity of PABP (Le, H., Tanguay, R. L., Balasta, M. L., Wei, C. C., Browning, K. S., Metz, A. M., Goss, D. J., and Gallie, D. R. (1997) J. Biol. Chem. 272, 16247-16255) and the cap binding activity of eIF-iso4F (Wei, C. C., Balasta, M. L., Ren, J., and Goss, D. J. (1998) Biochemistry 37, 1910-1916). We report here that the interaction between PABP and eIF-iso4G has a substantial effect on the ATPase activity and RNA helicase activity of (eIF4A + eIF4B + eIF-iso4F) complex. ATPase kinetic assays show, in the presence of poly(U), PABP can increase the parameter (k(cat)/K(m)) by 3.5-fold with a 2-fold decrease of K(m) for the (eIF4A + eIF-iso4F) complex. In the presence of globin messenger RNA, the ATPase activity of the complex (eIF4A + eIF-iso4F) was increased 2-fold by the presence of PABP. RNA helicase assays demonstrated that the presence of PABP enhanced the RNA duplex unwinding activity of the initiation factor complex. These results suggest that, in terms of the scanning model of translation initiation, PABP may enhance the mRNA scanning rate of the complex formed by eIF4A, eIF4B, and eIF4F or eIF-(iso)4F and increase the rate of translation.

Caspase-3 is necessary and sufficient for cleavage of protein synthesis eukaryotic initiation factor 4G during apoptosis

Induction of apoptosis BJAB cells is accompanied by the rapid cleavage of protein synthesis eukaryotic initiation factor 4G and the appearance of a fragment of approximately 76 kDa. Inhibition of apoptotic proteases (caspases) has previously been shown to prevent the cleavage of eukaryotic initiation factor 4G. In MCF-7 breast carcinoma cells, which are deficient in caspase-3, eukaryotic initiation factor 4G is not cleaved but in vivo expression of caspase-3 restores eukaryotic initiation factor 4G cleavage following induction of apoptosis. Recombinant caspase-3 can also cleave eukaryotic initiation factor 4G to yield the 76 kDa fragment both in cell extracts and when the eukaryotic initiation factor 4G is presented in a purified eukaryotic initiation factor 4F complex. These results indicate that caspase-3 activity is necessary and sufficient for eukaryotic initiation factor 4G degradation.

Formation of alpha-Pal/Max heterodimers synergistically activates the eIF2-alpha promoter

The transcription factor alpha-Pal recognizes two tandem palindromic repeats within the promoter of eukaryotic translation initiation factor 2-alpha (eIF2-alpha). Whereas both binding sites have the same "core domain" sequence (CGCATGCG), they differ with respect to their flanking sequences. Of the two sites, the 5'-cap proximal site has a higher binding affinity for alpha-Pal than does the 5'-cap distal site (Jacob, W. F., Silverman, T. A., Cohen, R. B., and Safer, B. (1989) J. Biol. Chem. 264, 20372-20384). The well characterized transcription factor Max binds to sequences that are remarkably similar to the core domain that alpha-Pal recognizes. To date, all of the Max heterodimer partners lack DNA binding domains and are thus dependent on Max interacting with DNA. Here we report that the two alpha-Pal sites have very different binding activities with respect to the E-box-binding protein Max. The 5'-cap distal or low alpha-Pal affinity site binds both alpha-Pal and Max. Furthermore, both heterodimers and homodimers of each of these proteins bind to this site. In contrast to the low affinity site, the high affinity site does not bind Max as a homodimer. This is the first documented case where Max heterodimerizes with a transcription factor that has affinity for DNA independent of Max.

The phosphorylation state of the wheat translation initiation factors eIF4B, eIF4A, and eIF2 is differentially regulated during seed development and germination

The translation initiation factors (eIF) 4B and eIF2 are phosphoproteins whose phosphorylation state differs between mature seed and leaves. We examined the isoforms of eIF4B and the alpha and beta subunits of eIF2 during the development and germination of wheat seed to determine whether the differences in their phosphorylation state are because of tissue-specific regulation or occur concomitant with changes in protein synthetic activity during development. eIF2alpha underwent phosphorylation through several intermediate isoforms that correlated with the increase and subsequent reduction in protein synthetic activity characteristic of seed development. eIF2beta and eIF4B, present as highly phosphorylated isoforms during early seed development, underwent dephosphorylation during late development. eIF4B was rapidly phosphorylated within 20 h of germination, whereas eIF2alpha did not undergo dephosphorylation until 48-60 h of growth. A third factor, eIF4A, was predominantly nonphosphorylated throughout most of seed development and germination. These observations suggest that the phosphorylation state of eIF2alpha, eIF2beta, and eIF4B is developmentally regulated in a way that correlates with the changes in protein synthetic activity but that some differences were also observed.

Modulation of translation initiation in rat skeletal muscle and liver in response to food intake

Protein synthesis is altered in both skeletal muscle and liver in response to nutritional status with food deprivation being associated with an inhibition of mRNA translation. In the present study, the effect of food-intake on the initiation of mRNA translation was examined in rats fasted for 18-h and then refed a complete diet. Fasting and refeeding caused alterations in translation initiation in both skeletal muscle and liver that were not associated with any detectable changes in the activity of eIF2B or in the phosphorylation state of eIF2 alpha. Instead, alterations in initiation were associated with changes in the phosphorylation state of eIF4E and/or the association of eIF4E with eIF4G as well as the eIF4E binding protein, 4E-BP1. In muscle from fasted rats, the amount of eIF4E present in an inactive complex with 4E-BP1 was increased 5-fold compared to freely fed control animals. One hour after refeeding a complete diet, the amount of 4E-BP1 bound to eIF4E was reduced to freely fed control values. Reduced association of the two proteins was the result of increased phosphorylation of 4E-BP1. Refeeding a complete diet also stimulated the binding of eIF4E to eIF4G to form the active eIF4F complex. In liver, the amount of eIF4E associated with eIF4G, but not the amount of eIF4E associated with 4E-BP1, was altered by fasting and refeeding. Furthermore, in liver, but not in skeletal muscle, fasting and refeeding resulted in modulation of the phosphorylation state of eIF4E. Overall, the results suggest that protein synthesis may be differentially regulated in muscle and liver in response to fasting and refeeding. In muscle, protein synthesis is regulated through modulation of the binding of eIF4E to eIF4G and in liver through modulation of both phosphorylation of eIF4E as well as binding of eIF4E to eIF4G.

A viral sequence in the 3'-untranslated region mimics a 5' cap in facilitating translation of uncapped mRNA

For recognition by the translational machinery, most eukaryotic cellular mRNAs have a 5' cap structure [e.g. m7G(5')ppp(5')N]. We describe a translation enhancer sequence (3'TE) located in the 3'-untranslated region (UTR) of the genome of the PAV barley yellow dwarf virus (BYDV-PAV) which stimulates translation from uncapped mRNA by 30- to 100-fold in vitro and in vivo to a level equal to that of efficient capped mRNAs. A four base duplication within the 3'TE destroyed the stimulatory activity. Efficient translation was recovered by addition of a 5' cap to this mRNA. Translation of both uncapped mRNA containing the 3'TE in cis and capped mRNA lacking any BYDV-PAV sequence was inhibited specifically by added 3'TE RNA in trans. This inhibition was reversed by adding initiation factor 4F (eIF4F), suggesting that the 3'TE, like the 5' cap, mediates eIF4F-dependent translation initiation. The BYDV-PAV 5'UTR was necessary for the 3'TE to function, except when the 3'TE itself was moved to the 5'UTR. Thus, the 3'TE is sufficient for recruiting the translation factors and ribosomes, while the viral 5'UTR may serve only for the long distance 3'-5' communication. Models are proposed to explain this novel mechanism of cap-independent translation initiation facilitated by the 3'UTR.

Macrophage Inflammatory Protein-1α and Interferon-Inducible Protein 10 Inhibit Synergistically Induced Growth Factor Stimulation of MAP Kinase Activity and Suppress Phosphorylation of Eukaryotic Initiation Factor 4E and 4E Binding Protein 1

Granulocyte-macrophage colony-stimulating factor (GM-CSF ) and Steel factor (SLF ) synergistically stimulate Raf-1 kinase activity, protein synthesis, and proliferation in hematopoietic MO7e cells; synergistic action of these factors is blocked by the suppressive chemokines macrophage inflammatory protein-1α (MIP-1α) and interferon-inducible protein 10 (IP-10; Aronica et al, J Biol Chem 270:21998, 1995). We assessed the potential for both stimulatory and inhibitory factors to act through the MAP kinase signaling pathway by studying the effects of growth factors and chemokines on MAP kinase activation. Also, because activation of kinase signaling pathways and stimulation of protein synthesis by peptide growth factors are associated with increased phosphorylation of eukaryotic initiation factor 4E (eIF-4E) and the translational repressor 4E-binding protein 1 (4E-BP1) in some target cells, we investigated whether growth factor treatment could alter eIF-4E or 4E-BP1 phosphorylation state in MO7e cells. We report that treatment of MO7e cells with GM-CSF and SLF stimulated significant, greater-than-additive increases in MAP kinase activity and the phosphorylation of both eIF-4E and 4E-BP1. Increased 4E-BP1 phosphorylation correlated with a decrease in the association of 4E-BP1 with eIF-4E. Growth factor-induced phosphorylation of 4E-BP1 and dissociation of 4E-BP1 from eIF-4E was blocked in cells treated with rapamycin, wortmannin, or PD098059. Treatment of cells with IP-10 or MIP-1α blocked the stimulatory effects of GM-CSF and SLF, resulting in suppression of MAP kinase activity, eIF-4E and 4E-BP1 phosphorylation, and eIF-4E/4E-BP1 dissociation. Our results suggest that GM-CSF and SLF exert part of their combined growth-promoting effects on MO7e cells through activation of MAP kinase and enhancement of eIF-4E and 4E-BP1 phosphorylation and dissociation and that suppression of growth factor-induced protein synthesis by MIP-1α and IP-10 involves translational repression at the level of eIF-4E.

Macrophage Inflammatory Protein-1α and Interferon-Inducible Protein 10 Inhibit Synergistically Induced Growth Factor Stimulation of MAP Kinase Activity and Suppress Phosphorylation of Eukaryotic Initiation Factor 4E and 4E Binding Protein 1

Granulocyte-macrophage colony-stimulating factor (GM-CSF ) and Steel factor (SLF ) synergistically stimulate Raf-1 kinase activity, protein synthesis, and proliferation in hematopoietic MO7e cells; synergistic action of these factors is blocked by the suppressive chemokines macrophage inflammatory protein-1α (MIP-1α) and interferon-inducible protein 10 (IP-10; Aronica et al, J Biol Chem 270:21998, 1995). We assessed the potential for both stimulatory and inhibitory factors to act through the MAP kinase signaling pathway by studying the effects of growth factors and chemokines on MAP kinase activation. Also, because activation of kinase signaling pathways and stimulation of protein synthesis by peptide growth factors are associated with increased phosphorylation of eukaryotic initiation factor 4E (eIF-4E) and the translational repressor 4E-binding protein 1 (4E-BP1) in some target cells, we investigated whether growth factor treatment could alter eIF-4E or 4E-BP1 phosphorylation state in MO7e cells. We report that treatment of MO7e cells with GM-CSF and SLF stimulated significant, greater-than-additive increases in MAP kinase activity and the phosphorylation of both eIF-4E and 4E-BP1. Increased 4E-BP1 phosphorylation correlated with a decrease in the association of 4E-BP1 with eIF-4E. Growth factor-induced phosphorylation of 4E-BP1 and dissociation of 4E-BP1 from eIF-4E was blocked in cells treated with rapamycin, wortmannin, or PD098059. Treatment of cells with IP-10 or MIP-1α blocked the stimulatory effects of GM-CSF and SLF, resulting in suppression of MAP kinase activity, eIF-4E and 4E-BP1 phosphorylation, and eIF-4E/4E-BP1 dissociation. Our results suggest that GM-CSF and SLF exert part of their combined growth-promoting effects on MO7e cells through activation of MAP kinase and enhancement of eIF-4E and 4E-BP1 phosphorylation and dissociation and that suppression of growth factor-induced protein synthesis by MIP-1α and IP-10 involves translational repression at the level of eIF-4E.

The phosphorylation state of translation initiation factors is regulated developmentally and following heat shock in wheat

Several translation initiation factors in mammals and yeast are regulated by phosphorylation. The phosphorylation state of these factors is subject to alteration during development, environmental stress (heat shock, starvation, or heme deprivation), or viral infection. The phosphorylation state and the effect of changes in phosphorylation of the translation initiation factors of higher plants have not been previously investigated. We have determined the isoelectric states for the wheat translation initiation factors eIF-4A, eIF-4B, eIF-4F, eIF-iso4F, and eIF-2 and the poly(A)-binding protein in the seed, during germination, and following heat shock of wheat seedlings using two-dimensional gel electrophoresis and Western analysis. We found that the developmentally induced changes in isoelectric state observed during germination or the stress-induced changes were consistent with changes in phosphorylation. Treatment of the phosphorylated forms of the factors with phosphatases confirmed that the nature of the modification was due to phosphorylation. The isoelectric states of eIF-4B, eIF-4F (eIF-4E, p26), eIF-iso4F (eIF-iso4E, p28), and eIF-2alpha (p42) were altered during germination, suggesting that phosphorylation of these factors is developmentally regulated and correlates with the resumption of protein synthesis that occurs during germination. The phosphorylation of eIF-2beta (p38) or poly(A)-binding protein did not change either during germination or following a thermal stress. Only the phosphorylation state of two factors, eIF-4A and eIF-4B, changed following a heat shock, suggesting that plants may differ significantly from animals in the way in which their translational machinery is modified in response to a thermal stress.

Mutational analysis of the functional domains of the large subunit of the isozyme form of wheat initiation factor eIF4F

The isozyme form of plant eukaryotic initiation factor 4F (eIF(iso)4F) contains two subunits: p28, a cap-binding protein, and p86. To identify the functional domains of p86, truncations of the p86 cDNA were made, and the protein was expressed in Escherichia coli and purified. The deletion mutants were tested for the ability to bind the p28 subunit by two methods. In addition, these deletion mutants were evaluated in vitro by the ability to catalyze eIF4A and RNA-dependent ATP hydrolysis and to support polypeptide synthesis. The loss of the ability to bind p28 occurs within the first 90 amino acids of the N terminus and abrogates the ability of p86 to participate in translation initiation and bind to eIF4A, but does not affect ATP hydrolysis. Up to 299 amino acid residues from the C terminus of p86 must be deleted before an effect is observed on the ATP hydrolysis activity. Thus, the p28 binding and ATP hydrolysis activities appear to lie on two separate domains and are functionally uncoupled. In addition, at least a portion of the eIF4A binding domain appears to be in close proximity to the p28 binding domain and is also uncoupled from the ATP hydrolysis activity.

Sequences of two expressed sequence tags (EST) from rice encoding different cap-binding proteins

Wheat has been shown to have two forms of the cap-binding protein that participate in the initiation of translation. To identify cap-binding proteins from other higher plant species, the expressed sequence tag (EST) database was searched. Several rice ESTs were identified with similarity to both forms of the wheat cap-binding proteins. Two of the rice ESTs were obtained and the cDNA sequences completed. The deduced amino acid sequences of the rice cap-binding proteins are compared to the wheat cap-binding proteins and cap-binding proteins from Saccharomyces cerevisiae, Drosophila melanogaster, Xenopus laevis and human.

Translation in plants--rules and exceptions

Translation processes in plants are very similar to those in other eukaryotic organisms and can in general be explained with the scanning model. Particularly among plant viruses, unconventional mRNAs are frequent, which use modulated translation processes for their expression: leaky scanning, translational stop codon readthrough or frameshifting, and transactivation by virus-encoded proteins are used to translate polycistronic mRNAs; leader and trailer sequences confer (cap-independent) efficient ribosome binding, usually in an end-dependent mechanism, but true internal ribosome entry may occur as well; in a ribosome shunt, sequences within an RNA can be bypassed by scanning ribosomes. Translation in plant cells is regulated under conditions of stress and during development, but the underlying molecular mechanisms have not yet been determined. Only a small number of plant mRNAs, whose structure suggests that they might require some unusual translation mechanisms, have been described.

The plant translational apparatus

Protein synthesis in both eukaryotic and prokaryotic cells is a complex process requiring a large number of macromolecules: initiation factors, elongation factors, termination factors, ribosomes, mRNA, amino-acylsynthetases and tRNAs. This review focuses on our current knowledge of protein synthesis in higher plants.

Evidence for involvement of trans-acting factors in selection of the AUG start codon during eukaryotic translational initiation

The molecular mechanism with which an appropriate AUG codon is selected as the start site for translational initiation by eukaryotic ribosomes is not known. By using a cell-free translation system, small RNA molecules containing single AUG codons, surrounded by various nucleotide sequences, were tested for their abilities to interfere with the translation of a reporter mRNA. RNAs containing the AUG in an ACCAUGG context (Kozak consensus sequence) were able to inhibit translation of the reporter mRNA. In contrast, RNAs containing the AUG in a less favorable context for start site selection (for example, CAGAUGG) had no effect on the translation of the reporter mRNA. The effect mediated by the ACCAUGC-containing RNAs was not due to sequestration of ribosomal subunits or to particular structural features in these RNAs. To identify potential trans-acting factors that might be preferentially bound by ACCAUGG-containing RNAs, ACCAUGG- and CAGAUGC-containing RNAs with a single 4-thiouridine residue at the AUG were incubated with partially fractionated extracts, and AUG-binding proteins were identified after irradiation of the complexes with UV light and subsequent analysis by gel electrophoresis. The analysis (of such complexes in competition experiments revealed that proteins, approximately 50 and 100 kDa in size, were found to bind directly at the AUG codon embedded in the ACCAUGG motif. One of these proteins has been identified as the La autoantigen. These findings indicate that trans-acting factors may play a role in AUG start site selection during translational initiation.

A reevaluation of the cap-binding protein, eIF4E, as a rate-limiting factor for initiation of translation in reticulocyte lysate

The cap-binding eukaryotic initiation factor, eIF4E, is a key target for the regulation of translation in mammalian cells and is widely thought to be present at very low molar concentrations. Here we present observations with the reticulocyte lysate that challenge this view. When reticulocyte ribosomes are harvested by centrifugation, most (approximately 75%) of the eIF4E remains in the postribosomal supernatant (PRS). In a reconstituted translation system we find that the ribosome-associated eIF4E alone can sustain much of the overall activity, suggesting that much of the factor in the PRS is functionally redundant. Consistent with this, our estimates of eIF4E in the reticulocyte lysate reveal much higher concentrations than previously reported. The association of a small proportion of eIF4E with the ribosome fraction appears to be functional and dependent on interaction with the factor eIF4G. This fraction of eIF4E is, as expected, more highly phosphorylated than that in the PRS; however, at least half the total phosphorylated eIF4E in reticulocyte lysate translation systems resides in the PRS fraction, suggesting that, while phosphorylation may enhance activity, it is not in itself sufficient to promote utilization of the factor. We also show that the eIF4E-binding factor, eIF4E-BP1 or PHAS-I, which regulates eIF4E activity in insulin-responsive cells, is present in the reticulocyte PRS at an approximately 1:1 molar ratio relative to eIF4E and demonstrate by co-immunoprecipitation studies that the binding of PHAS-I and eIF4G to eIF4E is mutually exclusive. These data are consistent with a potential regulatory role for PHAS-I in the reticulocyte lysate.

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.

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.

Insulin and diabetes cause reciprocal changes in the association of eIF-4E and PHAS-I in rat skeletal muscle

We have investigated the roles of eukaryotic initiation factor 4E (eIF-4E), the cap-binding protein, and the translational regulator, PHAS-I, in the effects of insulin and alloxan-induced diabetes on protein synthesis in rat skeletal muscle. Diabetes increased the amount of eIF-4E found in the inactive PHAS-I.eIF-4E complex by threefold, explaining in part the inhibitory effect of insulin deficiency on translation initiation. Insulin treatment of diabetic rats caused dissociation of the complex, consistent with the action of the hormone on reversing the inhibitory effect of diabetes on translation initiation. The effects of both insulin and diabetes on PHAS-I binding to eIF-4E appeared to be due to changes in PHAS-I phosphorylation. Neither insulin nor diabetes changed the phosphorylation state of eIF-4E. The results indicate that the effects of both insulin and diabetes on protein synthesis in skeletal muscle involve modulation of the interaction of PHAS-I and eIF-4E.

Participation of initiation factors in the recruitment of mRNA to ribosomes

The step of protein synthesis which is normally rate limiting, formation of the 48S initiation complex, is catalyzed by the group 4 initiation factors. Collectively they recognize the 7-methylguanosine-containing cap of mRNA, unwind mRNA secondary structure, and allow scanning for the initiation codon by the small ribosomal subunit. The activities of the eIF-4 polypeptides are modulated by phosphorylation. Recent studies shed new light on the mechanism of assembly of the 48S initiation complex and the effect of phosphorylation of one of the eIF-4 polypeptides, the cap-binding protein eIF-4E.

Regulation of translation and cell growth by eIF-4E

This review discusses the regulation of a key controlling step in the initiation of protein synthesis, the binding of mRNA to ribosomes. Particular focus is given to the phosphorylation of the cap-binding factor, eIF-4E, and the role of this factor in the regulation of cell growth.

Translational control during heat shock

Regulation of translation during heat shock of Drosophila and mammalian cells is reviewed. Protein synthesis is severely inhibited by elevated temperatures but synthesis of heat shock proteins (HSPs) is resistant to this inhibition. The primary site of regulation is polypeptide chain initiation. The activities of two initiation factors, eIF-2 and eIF-4F, are modulated during heat shock. A protein kinase which modulates eIF-2 activity appears to be associated with heat shock proteins (HSPs). Evidence is emerging that HSP70 acts as a heat sensor by detecting the presence of accumulating denatured proteins. In the rabbit reticulocyte lysate denatured proteins bind HSP70 releasing an eIF-2 kinase to shut down protein synthesis. It appears highly likely that a similar mechanism is acting in heat shocked cells. Cell-free protein synthesizing systems prepared from heat shocked cells are deficient in eIF-4F. Modulation of eIF-4F can explain in part the apparent preferential translation of HSP mRNAs.