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

Yeast Tor complex 1 phosphorylates eIF4E-binding protein, Caf20

Tor complex 1 (TORC1), a master regulator of cell growth, is an evolutionarily conserved protein kinase within eukaryotic organisms. To control cell growth, TORC1 governs translational processes by phosphorylating its substrate proteins in response to cellular nutritional cues. Mammalian TORC1 (mTORC1) assumes the responsibility of phosphorylating the eukaryotic translation initiation factor 4E (eIF4E)-binding protein 1 (4E-BP1) to regulate its interaction with eIF4E. The budding yeast Saccharomyces cerevisiae possesses a pair of 4E-BP genes, CAF20 and EAP1. However, the extent to which the TORC1-4E-BP axis regulates translational initiation in yeast remains uncertain. In this study, we demonstrated the influence of TORC1 on the phosphorylation status of Caf20 in vivo, as well as the direct phosphorylation of Caf20 by TORC1 in vitro. Furthermore, we found the TORC1-dependent recruitment of Caf20 to the 80S ribosome. Consequently, our study proposes a plausible involvement of yeast's 4E-BP in the efficacy of translation initiation, an aspect under the control of TORC1.

eIF4E and Interactors from Unicellular Eukaryotes

eIF4E, the mRNA cap-binding protein, is well known as a general initiation factor allowing for mRNA-ribosome interaction and cap-dependent translation in eukaryotic cells. In this review we focus on eIF4E and its interactors in unicellular organisms such as yeasts and protozoan eukaryotes. In a first part, we describe eIF4Es from yeast species such as Saccharomyces cerevisiae, Candida albicans, and Schizosaccharomyces pombe. In the second part, we will address eIF4E and interactors from parasite unicellular species—trypanosomatids and marine microorganisms—dinoflagellates. We propose that different strategies have evolved during evolution to accommodate cap-dependent translation to differing requirements. These evolutive “adjustments” involve various forms of eIF4E that are not encountered in all microorganismic species. In yeasts, eIF4E interactors, particularly p20 and Eap1 are found exclusively in Saccharomycotina species such as S. cerevisiae and C. albicans. For protozoan parasites of the Trypanosomatidae family beside a unique cap4-structure located at the 5′UTR of all mRNAs, different eIF4Es and eIF4Gs are active depending on the life cycle stage of the parasite. Additionally, an eIF4E-interacting protein has been identified in Leishmania major which is important for switching from promastigote to amastigote stages. For dinoflagellates, little is known about the structure and function of the multiple and diverse eIF4Es that have been identified thanks to widespread sequencing in recent years.

Mechanisms by which PE21, an extract from the white willow Salix alba, delays chronological aging in budding yeast

We have recently found that PE21, an extract from the white willow Salix alba, slows chronological aging and prolongs longevity of the yeast Saccharomyces cerevisiae more efficiently than any of the previously known pharmacological interventions. Here, we investigated mechanisms through which PE21 delays yeast chronological aging and extends yeast longevity. We show that PE21 causes a remodeling of lipid metabolism in chronologically aging yeast, thereby instigating changes in the concentrations of several lipid classes. We demonstrate that such changes in the cellular lipidome initiate three mechanisms of aging delay and longevity extension. The first mechanism through which PE21 slows aging and prolongs longevity consists in its ability to decrease the intracellular concentration of free fatty acids. This postpones an age-related onset of liponecrotic cell death promoted by excessive concentrations of free fatty acids. The second mechanism of aging delay and longevity extension by PE21 consists in its ability to decrease the concentrations of triacylglycerols and to increase the concentrations of glycerophospholipids within the endoplasmic reticulum membrane. This activates the unfolded protein response system in the endoplasmic reticulum, which then decelerates an age-related decline in protein and lipid homeostasis and slows down an aging-associated deterioration of cell resistance to stress. The third mechanisms underlying aging delay and longevity extension by PE21 consists in its ability to change lipid concentrations in the mitochondrial membranes. This alters certain catabolic and anabolic processes in mitochondria, thus amending the pattern of aging-associated changes in several key aspects of mitochondrial functionality.

Properties of the ternary complex formed by yeast eIF4E, p20 and mRNA

Yeast p20 is a small, acidic protein that binds eIF4E, the cap-binding protein. It has been proposed to affect mRNA translation and degradation, however p20′s function as an eIF4E-binding protein (4E-BP) and its physiological significance has not been clearly established. In this paper we present data demonstrating that p20 is capable of binding directly to mRNA due to electrostatic interaction of a stretch of arginine and histidine residues in the protein with negatively charged phosphates in the mRNA backbone. This interaction contributes to formation of a ternary eIF4E/p20/capped mRNA complex that is more stable than complexes composed of capped mRNA bound to eIF4E in the absence of p20. eIF4E/p20 complex was found to have a more pronounced stimulatory effect on capped mRNA translation than purified eIF4E alone. Addition of peptides containing the eIF4E-binding domains present in p20 (motif  YTIDELF), in eIF4G (motif  YGPTFLL) or Eap1 (motif  YSMNELY) completely inhibited eIF4E-dependent capped mRNA translation (in vitro), but had a greatly reduced inhibitory effect when eIF4E/p20 complex was present. We propose that the eIF4E/p20/mRNA complex serves as a stable depository of mRNAs existing in a dynamic equilibrium with other complexes such as eIF4E/eIF4G (required for translation) and eIF4E/Eap1 (required for mRNA degradation).

The unique Leishmania EIF4E4 N-terminus is a target for multiple phosphorylation events and participates in critical interactions required for translation initiation

The eukaryotic initiation factor 4E (eIF4E) recognizes the mRNA cap structure and, together with eIF4G and eIF4A, form the eIF4F complex that regulates translation initiation in eukaryotes. In trypanosomatids, 2 eIF4E homologues (EIF4E3 and EIF4E4) have been shown to be part of eIF4F-like complexes with presumed roles in translation initiation. Both proteins possess unique N-terminal extensions, which can be targeted for phosphorylation. Here, we provide novel insights on the Leishmania infantum EIF4E4 function and regulation. We show that EIF4E4 is constitutively expressed throughout the parasite development but is preferentially phosphorylated in exponentially grown promastigote and amastigote life stages, hence correlating with high levels of translation. Phosphorylation targets multiple serine-proline or threonine-proline residues within the N-terminal extension of EIF4E4 but does not require binding to the EIF4E4's partner, EIF4G3, or to the cap structure. We also report that EIF4E4 interacts with PABP1 through 3 conserved boxes at the EIF4E4 N-terminus and that this interaction is a prerequisite for efficient EIF4E4 phosphorylation. EIF4E4 is essential for Leishmania growth and an EIF4E4 null mutant was only obtained in the presence of an ectopically provided wild type gene. Complementation for the loss of EIF4E4 with several EIF4E4 mutant proteins affecting either phosphorylation or binding to mRNA or to EIF4E4 protein partners revealed that, in contrast to other eukaryotes, only the EIF4E4-PABP1 interaction but neither the binding to EIF4G3 nor phosphorylation is essential for translation. These studies also demonstrated that the lack of both EIF4E4 phosphorylation and EIF4G3 binding leads to a non-functional protein. Altogether, these findings further highlight the unique features of the translation initiation process in trypanosomatid protozoa.

eIF4E-binding proteins: new factors, new locations, new roles

The cap-binding translation initiation factor eIF4E (eukaryotic initiation factor 4E) is central to protein synthesis in eukaryotes. As an integral component of eIF4F, a complex also containing the large bridging factor eIF4G and eIF4A RNA helicase, eIF4E enables the recruitment of the small ribosomal subunit to the 5' end of mRNAs. The interaction between eIF4E and eIF4G via a YXXXXLϕ motif is regulated by small eIF4E-binding proteins, 4E-BPs, which use the same sequence to competitively bind eIF4E thereby inhibiting cap-dependent translation. Additional eIF4E-binding proteins have been identified in the last 10-15 years, characterized by the YXXXXLϕ motif, and by interactions (many of which remain to be detailed) with RNA-binding proteins, or other factors in complexes that recognize the specific mRNAs. In the present article, we focus on the metazoan 4E-T (4E-transporter)/Cup family of eIF4E-binding proteins, and also discuss very recent examples in yeast, fruitflies and humans, some of which predictably inhibit translation, while others may result in mRNA decay or even enhance translation; altogether considerably expanding our understanding of the roles of eIF4E-binding proteins in gene expression regulation.

An in vivo control map for the eukaryotic mRNA translation machinery

A new quantitative strategy has generated a comprehensive rate control map for protein synthesis in exponentially growing yeast cells. This analysis reveals the modularity of the system as well as highly non-stoichiometric relationships between components.

A ‘genetic titration' method has generated a map of the in vivo rate control properties of components of the protein synthesis machinery in Saccharomyces cerevisiae and has been used to parameterize a new comprehensive model of the translation pathway.

The translation machinery is found to be a highly modular system in functional terms yet the intracellular concentrations of its components range from a few thousand to one million molecules per cell.

This approach identifies non-intuitive features of the system such as the strongest rate control being exercised by high abundance elongation factors.

The rate control analysis allows us to identify a surprising fine-control function for duplicated translation factor genes.

Rate control analysis defines the in vivo control map governing yeast protein synthesis and generates an extensively parameterized digital model of the translation pathway. Among other non-intuitive outcomes, translation demonstrates a high degree of functional modularity and comprises a non-stoichiometric combination of proteins manifesting functional convergence on a shared maximal translation rate. In exponentially growing cells, polypeptide elongation (eEF1A, eEF2, and eEF3) exerts the strongest control. The two other strong control points are recruitment of mRNA and tRNA_i to the 40S ribosomal subunit (eIF4F and eIF2) and termination (eRF1; Dbp5). In contrast, factors that are found to promote mRNA scanning efficiency on a longer than-average 5′untranslated region (eIF1, eIF1A, Ded1, eIF2B, eIF3, and eIF5) exceed the levels required for maximal control. This is expected to allow the cell to minimize scanning transition times, particularly for longer 5′UTRs. The analysis reveals these and other collective adaptations of control shared across the factors, as well as features that reflect functional modularity and system robustness. Remarkably, gene duplication is implicated in the fine control of cellular protein synthesis.

Eukaryotic initiation factor 4E-3 is essential for meiotic chromosome segregation, cytokinesis and male fertility in Drosophila

Gene expression is translationally regulated during many cellular and developmental processes. Translation can be modulated by affecting the recruitment of mRNAs to the ribosome, which involves recognition of the 5' cap structure by the cap-binding protein eIF4E. Drosophila has several genes encoding eIF4E-related proteins, but the biological role of most of them remains unknown. Here, we report that Drosophila eIF4E-3 is required specifically during spermatogenesis. Males lacking eIF4E-3 are sterile, showing defects in meiotic chromosome segregation, cytokinesis, nuclear shaping and individualization. We show that eIF4E-3 physically interacts with both eIF4G and eIF4G-2, the latter being a factor crucial for spermatocyte meiosis. In eIF4E-3 mutant testes, many proteins are present at different levels than in wild type, suggesting widespread effects on translation. Our results imply that eIF4E-3 forms specific eIF4F complexes that are essential for spermatogenesis.

On the Diversification of the Translation Apparatus across Eukaryotes

Diversity is one of the most remarkable features of living organisms. Current assessments of eukaryote biodiversity reaches 1.5 million species, but the true figure could be several times that number. Diversity is ingrained in all stages and echelons of life, namely, the occupancy of ecological niches, behavioral patterns, body plans and organismal complexity, as well as metabolic needs and genetics. In this review, we will discuss that diversity also exists in a key biochemical process, translation, across eukaryotes. Translation is a fundamental process for all forms of life, and the basic components and mechanisms of translation in eukaryotes have been largely established upon the study of traditional, so-called model organisms. By using modern genome-wide, high-throughput technologies, recent studies of many nonmodel eukaryotes have unveiled a surprising diversity in the configuration of the translation apparatus across eukaryotes, showing that this apparatus is far from being evolutionarily static. For some of the components of this machinery, functional differences between different species have also been found. The recent research reviewed in this article highlights the molecular and functional diversification the translational machinery has undergone during eukaryotic evolution. A better understanding of all aspects of organismal diversity is key to a more profound knowledge of life.

Assessing the components of the eIF3 complex and their phosphorylation status

The eukaryotic initiation factor 3 (eIF3) is an essential, highly conserved multiprotein complex that is a key component in the recruitment and assembly of the translation initiation machinery. To better understand the molecular function of eIF3, we examined its composition and phosphorylation status in Saccharomyces cerevisiae. The yeast eIF3 complex contains five core components: Rpg1, Nip1, Prt1, Tif34, and Tif35. 2-D LC-MS/MS analysis of affinity purified eIF3 complexes showed that several other initiation factors (Fun12, Tif5, Sui3, Pab1, Hcr1, and Sui1) and the casein kinase 2 complex (CK2) copurify. In Vivo metabolic labeling of proteins with (32)P revealed that Nip1 is phosphorylated. Using 2-D LC-MS/MS analysis of eIF3 complexes, we identified Prt1 phosphopeptides indicating phosphorylation at S22 and T707 and a Tif5 phosphopeptide with phosphorylation at T191. Additionally, we used immobilized metal affinity chromatography (IMAC) to enrich for eIF3 phosphopeptides and tandem mass spectrometry to identify phosphorylated residues. We found that three CK2 consensus sequences in Nip1 are phosphorylated: S98, S99, and S103. Using in vitro kinase assays, we showed that CK2 phophorylates Nip1 and that a synthetic Nip1 peptide containing S98, S99, and S103 competitively inhibits the reaction. Replacement of these three Nip1 serines with alanines causes a slow growth phenotype.

The role of PAS kinase in PASsing the glucose signal

PAS kinase is an evolutionarily conserved nutrient responsive protein kinase that regulates glucose homeostasis. Mammalian PAS kinase is activated by glucose in pancreatic beta cells, and knockout mice are protected from obesity, liver triglyceride accumulation, and insulin resistance when fed a high-fat diet. Yeast PAS kinase is regulated by both carbon source and cell integrity stress and stimulates the partitioning of glucose toward structural carbohydrate biosynthesis. In our current model for PAS kinase regulation, a small molecule metabolite binds the sensory PAS domain and activates the enzyme. Although bona fide PAS kinase substrates are scarce, in vitro substrate searches provide putative targets for exploration.

Analyzing Proteomes and Protein Function Using Graphical Comparative Analysis of Tandem Mass Spectrometry Results

Although generating large amounts of proteomic data using tandem mass spectrometry has become routine, there is currently no single set of comprehensive tools for the rigorous analysis of tandem mass spectrometry results given the large variety of possible experimental aims. Currently available applications are typically designed for displaying proteins and posttranslational modifications from the point of view of the mass spectrometrist and are not versatile enough to allow investigators to develop biological models of protein function, protein structure, or cell state. In addition, storage and dissemination of mass spectrometry-based proteomic data are problems facing the scientific community. To address these issues, we have developed a relational database model that efficiently stores and manages large amounts of tandem mass spectrometry results. We have developed an integrated suite of multifunctional analysis software for interpreting, comparing, and displaying these results. Our system, Bioinformatic Graphical Comparative Analysis Tools (BIGCAT), allows sophisticated analysis of tandem mass spectrometry results in a biologically intuitive format and provides a solution to many data storage and dissemination issues.

Expression of heterologous proteins in Pichia pastoris: a useful experimental tool in protein engineering and production

The use of the methylotrophic yeast, Pichia pastoris, as a cellular host for the expression of recombinant proteins has become increasing popular in recent times. P. pastoris is easier to genetically manipulate and culture than mammalian cells and can be grown to high cell densities. Equally important, P. pastoris is also a eukaryote, and thereby provides the potential for producing soluble, correctly folded recombinant proteins that have undergone all the post-translational modifications required for functionality. Additionally, linearized foreign DNA can be inserted in high efficiency via homologous recombination procedures to generate stable cell lines whilst expression vectors can be readily prepared that allow multiple copies of the target protein, multimeric proteins with different subunit structures, or alternatively the target protein and its cognate binding partners, to be expressed. A further benefit of the P. pastoris system is that strong promoters are available to drive the expression of a foreign gene(s) of interest, thus enabling production of large amounts of the target protein(s) with relative technical ease and at a lower cost than most other eukaryotic systems. The purpose of this review is to summarize important developments and features of this expression system and, in particular, to examine from an experimental perspective the genetic engineering, protein chemical and molecular design considerations that have to be taken into account for the successful expression of the target recombinant protein. Included in these considerations are the influences of P. pastoris strain selection; the choice of expression vectors and promoters; procedures for the transformation and integration of the vectors into the P. pastoris genome; the consequences of rare codon usage and truncated transcripts; and techniques employed to achieve multi-copy integration numbers. The impact of the alcohol oxidase (AOX) pathways in terms of the mut+ and mut(s) phenotypes, intracellular expression and folding pathways is examined. The roles of pre-pro signal sequences such as the alpha mating factor (alpha-MF) and the Glu-Ala repeats at the kex2p cleavage site on the processing of the protein translate(s) have also been considered. Protocols for the generation of protein variants and mutants for screening for orphan cognate binding partners and the use of experimental platforms addressing the molecular recognition behaviour of recombinant proteins such as the extracellular domains of transmembrane receptors with their physiological ligands are also described. Finally, the palindromic patterns of glycosylation that can occur with these expression systems, in terms of the role and location of the sequon in the primary structure, the number of mannose units and the types of oligosaccharides incorporated as Asn- or O-linkages and their impact on the thermostability and immunogenicity of the recombinant protein are considered. Procedures to prevent glycosylation through manipulation of cell culture conditions or via enzymatic and site-directed mutagenesis methods are also discussed.

Overproduction of a conserved domain of fission yeast and mammalian translation initiation factor eIF4G causes aberrant cell morphology and results in disruption of the localization of F-actin and the organization of microtubules

BACKGROUND

The recruitment of mRNA for translation involves the assembly at the 5'cap of a complex of three initiation factors: the cap binding protein eIF4E, the ATP-dependent RNA helicase eIF4A and the scaffold protein eIF4G. eIF4G mediates the binding of this mRNA-protein complex to the 43S ribosomal preinitiation complex. There is growing recognition that the components of the translational apparatus interact functionally with cytoskeletal components. Here we report specific effects of the over-expression of human and fission yeast eIF4G domains on cell morphology in Schizosaccharomyces pombe.

RESULTS

A single gene encoding fission yeast eIF4G was identified and demonstrated to be essential. We have over-expressed fragments corresponding to the conserved functional domains of eIF4G. At expression levels that did not disrupt rates of overall translation or protein accumulation, a fragment of S. pombe eIF4G, 4G-NOB, corresponding to the minimal region of human eIF4G required to support cap-independent mRNA recruitment, was found to impair cell proliferation in fission yeast. This resulted from defects in cytokinesis, and was associated with the disruption of both microtubules and actin microfilaments. The over-expressed fragment was itself localized to the cell ends, the nuclear periphery and the septum.

CONCLUSIONS

This is the first demonstration of a link between a translation initiation factor and mechanisms controlling cell morphology. The data suggest a direct or indirect interaction between the functional domains of eIF4G and cellular structures involved in cytokinesis.

Coordinate regulation of sugar flux and translation by PAS kinase

PAS kinase is a serine/threonine kinase regulated in cis by a PAS domain. A genetic study of the two PAS kinase genes in budding yeast gave evidence of the involvement of these enzymes in the control of sugar metabolism and translation. Using a biochemical screen for PAS kinase substrates, three translation factors were identified as direct phosphorylation targets. PAS kinase was also found to phosphorylate UDP-glucose pyrophosphorylase and glycogen synthase, the enzymes catalyzing the two final steps in the glycogen biosynthetic pathway. Genetic, biochemical, and physiological data provide evidence that both of these enzymes are inhibited by PAS kinase-dependent phosphorylation, thereby downregulating carbohydrate storage. These studies provide evidence of a cell-autonomous signaling system that both controls and connects the balance of fuel consumption/storage to protein synthesis.

Intracellular translation initiation factor levels in Saccharomyces cerevisiae and their role in cap-complex function

Knowledge of the balance of activities of eukaryotic initiation factors (eIFs) is critical to our understanding of the mechanisms underlying translational control. We have therefore estimated the intracellular levels of 11 eIFs in logarithmically growing cells of Saccharomyces cerevisiae using polyclonal antibodies raised in rabbits against recombinant proteins. Those factors involved in 43S complex formation occur at levels comparable (i.e. within a 0.5- to 2.0-fold range) to those published for ribosomes. In contrast, the subunits of the cap-binding complex eIF4F showed considerable variation in their abundance. The helicase eIF4A was the most abundant eIF of the yeast cell, followed by eIF4E at multiple copies per ribosome, and eIF4B at approximately one copy per ribosome. The adaptor protein eIF4G was the least abundant of the eIF4 factors, with a copy number per cell that is substoichiometric to the ribosome and similar to the abundance of mRNA. The observed excess of eIF4E over its functional partner eIF4G is not strictly required during exponential growth: at eIF4E levels artificially reduced to 30% of those in wild-type yeast, growth rates and the capacity for general protein synthesis are only minimally affected. This demonstrates that eIF4E does not exercise a higher level of rate control over translation than other eIFs. However, other features of the yeast life cycle, such as the control of cell size, are more sensitive to changes in eIF4E abundance. Overall, these data constitute an important basis for developing a quantitative model of the workings of the eukaryotic translation apparatus.

Modulation of eukaryotic mRNA stability via the cap-binding translation complex eIF4F

Decapping by Dcp1 in Saccharomyces cerevisiae is a key step in mRNA degradation. However, the cap also binds the eukaryotic initiation factor (eIF) complex 4F and its associated proteins. Characterisation of the relationship between decapping and interactions involving eIF4F is an essential step towards understanding polysome disassembly and mRNA decay. Three types of observation suggest how changes in the functional status of eIF4F modulate mRNA stability in vivo. First, partial disruption of the interaction between eIF4E and eIF4G, caused by mutations in eIF4E or the presence of the yeast 4E-binding protein p20, stabilised mRNAs. The interactions of eIF4G and p20 with eIF4E may therefore act to modulate the decapping process. Since we also show that the in vitro decapping rate is not directly affected by the nature of the body of the mRNA, this suggests that changes in eIF4F structure could play a role in triggering decapping during mRNA decay. Second, these effects were seen in the absence of extreme changes in global translation rates in the cell, and are therefore relevant to normal mRNA turnover. Third, a truncated form of eIF4E (Delta196) had a reduced capacity to inhibit Dcp1-mediated decapping in vitro, yet did not change cellular mRNA half-lives. Thus, the accessibility of the cap to Dcp1 in vivo is not simply controlled by competition with eIF4E, but is subject to switching between molecular states with different levels of access.

Characterization of human lactoferricin as a potent protein kinase CK2 activator regulated by A-kinase in vitro

Lactoferricin (LFcin) hydrolyzed from lactoferrin (LF), a major 80 kDa iron-binding protein in milk and other exocrine secretions, was characterized as a potent activator of protein kinase CK2 (CK2) in vitro. Human LFcin (hLFcin) at 0.5 microg stimulated approx. 5-fold CK2 activity [phosphorylation of 60S acidic ribosomal proteins (P0, P1, P2) and Hsp90 (p98)] in a manner similar to other functional proteins with oligo-Arg clusters, such as salmine A1, sperm histone H2B and HIV-1 Rev. Interestingly, this stimulatory effect of hLFcin was significantly reduced when it was phosphorylated by A-kinase in vitro. These results suggest that (i) hLFcin acts as a potent CK2 activator in vitro; and (ii) the stimulatory effect of hLFcin on CK2 activity is regulated by its phosphorylation by A-kinase in vitro.

Glycosylated and phosphorylated proteins--expression in yeast and oocytes of Xenopus: prospects and challenges--relevance to expression of thermostable proteins

Phosphorylation and glycosylation are important posttranslational events in the biosynthesis of proteins. The different degrees of phosphorylation and glycosylation of proteins have been an intriguing phenomenon. Advances in genetic engineering have made it possible to control the degree of glycosylation and phosphorylation of proteins. Structural biology of phosphorylated and glycosylated proteins has been advancing at a much slower pace due to difficulties in using high-resolution NMR studies in solution phase. Major difficulties have arisen from the inherent mobilities of phosphorylated and glycosylated side chains. This paper reviews molecular and structural biology of phosphorylated and glycosylated proteins expressed in eukaryotic expression systems which are especially suited for large-scale production of these proteins. In our laboratory, we have observed that eukaryotic expression systems are particularly suited for the expression of thermostable light-activated proteins, e.g., bacteriorhodopsins and plastocyanins.

A quantitative molecular model for modulation of mammalian translation by the eIF4E-binding protein 1

Translation initiation is a key point of regulation in eukaryotic gene expression. 4E-binding proteins (4E-BPs) inhibit initiation by blocking the association of eIF4E with eIF4G, two integral components of the mRNA cap-binding complex. Phosphorylation of 4E-BP1 reduces its ability to bind to eIF4E and thereby to compete with eIF4G. A novel combination of biophysical and biochemical tools was used to measure the impact of phosphorylation and acidic side chain substitution at each potentially modulatory site in 4E-BP1. For each individual site, we have analyzed the effects of modification on eIF4E binding using affinity chromatography and surface plasmon resonance analysis, and on the regulatory function of the 4E-BP1 protein using a yeast in vivo model system and a mammalian in vitro translation assay. We find that modifications at the two sites immediately flanking the eIF4E-binding domain, Thr(46) and Ser(65), consistently have the most significant effects, and that phosphorylation of Ser(65) causes the greatest reduction in binding affinity. These results establish a quantitative framework that should contribute to understanding of the molecular interactions underlying 4E-BP1-mediated translational regulation.

The eukaryotic mRNA decapping protein Dcp1 interacts physically and functionally with the eIF4F translation initiation complex

Dcp1 plays a key role in the mRNA decay process in Saccharomyces cerevisiae, cleaving off the 5' cap to leave an end susceptible to exonucleolytic degradation. The eukaryotic initiation factor complex eIF4F, which in yeast contains the core components eIF4E and eIF4G, uses the cap as a binding site, serving as an initial point of assembly for the translation apparatus, and also binds the poly(A) binding protein Pab1. We show that Dcp1 binds to eIF4G and Pab1 as free proteins, as well as to the complex eIF4E-eIF4G-Pab1. Dcp1 interacts with the N-terminal region of eIF4G but does not compete significantly with eIF4E or Pab1 for binding to eIF4G. Most importantly, eIF4G acts as a function-enhancing recruitment factor for Dcp1. However, eIF4E blocks this effect as a component of the high affinity cap-binding complex eIF4E-eIF4G. Indeed, cooperative enhancement of the eIF4E-cap interaction stabilizes yeast mRNAs in vivo. These data on interactions at the interface between translation and mRNA decay suggest how events at the 5' cap and 3' poly(A) tail might be coupled.

eIF4 initiation factors: effectors of mRNA recruitment to ribosomes and regulators of translation

Eukaryotic translation initiation factor 4F (eIF4F) is a protein complex that mediates recruitment of ribosomes to mRNA. This event is the rate-limiting step for translation under most circumstances and a primary target for translational control. Functions of the constituent proteins of eIF4F include recognition of the mRNA 5' cap structure (eIF4E), delivery of an RNA helicase to the 5' region (eIF4A), bridging of the mRNA and the ribosome (eIF4G), and circularization of the mRNA via interaction with poly(A)-binding protein (eIF4G). eIF4 activity is regulated by transcription, phosphorylation, inhibitory proteins, and proteolytic cleavage. Extracellular stimuli evoke changes in phosphorylation that influence eIF4F activity, especially through the phosphoinositide 3-kinase (PI3K) and Ras signaling pathways. Viral infection and cellular stresses also affect eIF4F function. The recent determination of the structure of eIF4E at atomic resolution has provided insight about how translation is initiated and regulated. Evidence suggests that eIF4F is also implicated in malignancy and apoptosis.

Eap1p, a novel eukaryotic translation initiation factor 4E-associated protein in Saccharomyces cerevisiae

Ribosome binding to eukaryotic mRNA is a multistep process which is mediated by the cap structure [m(7)G(5')ppp(5')N, where N is any nucleotide] present at the 5' termini of all cellular (with the exception of organellar) mRNAs. The heterotrimeric complex, eukaryotic initiation factor 4F (eIF4F), interacts directly with the cap structure via the eIF4E subunit and functions to assemble a ribosomal initiation complex on the mRNA. In mammalian cells, eIF4E activity is regulated in part by three related translational repressors (4E-BPs), which bind to eIF4E directly and preclude the assembly of eIF4F. No structural counterpart to 4E-BPs exists in the budding yeast, Saccharomyces cerevisiae. However, a functional homolog (named p20) has been described which blocks cap-dependent translation by a mechanism analogous to that of 4E-BPs. We report here on the characterization of a novel yeast eIF4E-associated protein (Eap1p) which can also regulate translation through binding to eIF4E. Eap1p shares limited homology to p20 in a region which contains the canonical eIF4E-binding motif. Deletion of this domain or point mutation abolishes the interaction of Eap1p with eIF4E. Eap1p competes with eIF4G (the large subunit of the cap-binding complex, eIF4F) and p20 for binding to eIF4E in vivo and inhibits cap-dependent translation in vitro. Targeted disruption of the EAP1 gene results in a temperature-sensitive phenotype and also confers partial resistance to growth inhibition by rapamycin. These data indicate that Eap1p plays a role in cell growth and implicates this protein in the TOR signaling cascade of S. cerevisiae.

The phosphorylation state of poly(A)-binding protein specifies its binding to poly(A) RNA and its interaction with eukaryotic initiation factor (eIF) 4F, eIFiso4F, and eIF4B

The poly(A)-binding protein (PABP) interacts with the eukaryotic initiation factor (eIF) 4G (or eIFiso4G), the large subunit of eIF4F (or eIFiso4F) to promote translation initiation. In plants, PABP also interacts with eIF4B, a factor that assists eIF4F function. PABP is a phosphoprotein, although the function of its phosphorylation has not been previously investigated. In this study, we have purified the phosphorylated and hypophosphorylated isoforms of PABP from wheat to examine whether its phosphorylation state affects its binding to poly(A) RNA and its interaction with eIF4G, eIFiso4G, or eIF4B. Phosphorylated PABP exhibited cooperative binding to poly(A) RNA even under non-stoichiometric binding conditions, whereas multiple molecules of hypophosphorylated PABP bound to poly(A) RNA only after free poly(A) RNA was no longer available. Together, phosphorylated and hypophosphorylated PABP exhibited synergistic binding. eIF4B interacted with PABP in a phosphorylation state-specific manner; native eIF4B increased the RNA binding activity specifically of phosphorylated PABP and was greater than 14-fold more effective than was recombinant eIF4B, whereas eIF4F promoted the cooperative binding of hypophosphorylated PABP. These data suggest that the phosphorylation state of PABP specifies the type of binding to poly(A) RNA and its interaction with its partner proteins.

Glucose-regulated turnover of mRNA and the influence of poly(A) tail length on half-life

Glucose repression in Saccharomyces cerevisiae can now be seen to operate at two levels: regulation of transcription of certain genes and control of the half-life of the corresponding mRNAs (Scheffler, I. E., de la Cruz, B. J., and Prieto, S. (1998) Int. J. Biochem Cell Biol. 30, 1175-1193). For example, the steady state levels of SDH2 mRNA and SUC2 mRNA are significantly determined by their differential rates of turnover. A current model for the mechanism of mRNA turnover includes three distinct steps: a rate-limiting deadenylation, removal of the 5'-7-methyl-G (decapping), and 5'-3' exonuclease digestion. We have investigated the same three reactions during glucose-induced degradation of these transcripts. Our results indicate that while decapping (by Dcp1p) and 5'-3' exonuclease digestion (by Xrn1p) are obligatory steps for the rapid degradation of these mRNAs, the dependence on deadenylation is more complicated. At steady state in glycerol these transcripts have very short poly(A) tails but are nevertheless very stable; the addition of glucose causes immediate decapping and degradation without further deadenylation; in contrast, newly made SUC2 mRNA (after a shift from glucose to glycerol) has significantly longer poly(A) tails, and such transcripts are not rapidly degraded upon addition of glucose. A constitutive deadenylation reaction that is independent of the carbon source eventually makes the stability of these transcripts very sensitive to glucose. These results are interpreted in terms of a working hypothesis proposing a competition between translational initiation and decapping influenced by the carbon source. The presence of a long poly(A) tail may also affect this competition in favor of translational initiation and mRNA stabilization.

Inhibition of mRNA turnover in yeast by an xrn1 mutation enhances the requirement for eIF4E binding to eIF4G and for proper capping of transcripts by Ceg1p

Null mutants of XRN1, encoding the major cytoplasmic exoribonuclease in yeast, are viable but accumulate decapped, deadenylated transcripts. A screen for mutations synthetic lethal with xrn1Delta identified a mutation in CDC33, encoding eIF4E. This mutation (glutamate to glycine at position 72) affected a highly conserved residue involved in interaction with eIF4G. Synthetic lethality between xrn1 and cdc33 was not relieved by high-copy expression of eIF4G or by disruption of the yeast eIF4E binding protein Caf20p. High-copy expression of a mutant eIF4G defective for eIF4E binding resulted in a dominant negative phenotype in an xrn1 mutant, indicating the importance of this interaction in an xrn1 mutant. Another allele of CDC33, cdc33-1, along with mutations in CEG1, encoding the nuclear guanylyltransferase, were also synthetic lethal with xrn1Delta, whereas mutations in PRT1, encoding a subunit of eIF3, were not. Mutations in CDC33, CEG1, PRT1, PAB1, and TIF4631, encoding eIF4G1, have been shown to lead to destabilization of mRNAs. Although such destabilization in cdc33, ceg1, and pab1 mutants can be partially suppressed by an xrn1 mutation, we observed synthetic lethality between xrn1 and either cdc33 or ceg1 and no suppression of the inviability of a pab1 null mutation by xrn1Delta. Thus, the inhibition of mRNA turnover by blocking Xrn1p function does not suppress the lethality of defects upstream in the turnover pathway but it does enhance the requirement for (7)mG caps and for proper formation of the eIF4E/eIF4G cap recognition complex.

The Cap-binding protein eIF4E promotes folding of a functional domain of yeast translation initiation factor eIF4G1

The association of eucaryotic translation initiation factor eIF4G with the cap-binding protein eIF4E establishes a critical link between the mRNA and the ribosome during translation initiation. This association requires a conserved seven amino acid peptide within eIF4G that binds to eIF4E. Here we report that a 98-amino acid fragment of S. cerevisiae eIF4G1 that contains this eIF4E binding peptide undergoes an unfolded to folded transition upon binding to eIF4E. The folding of the eIF4G1 domain was evidenced by the eIF4E-dependent changes in its protease sensitivity and (1)H-(15)N HSQC NMR spectrum. Analysis of a series of charge-to-alanine mutations throughout the essential 55.4-kDa core of yeast eIF4G1 also revealed substitutions within this 98-amino acid region that led to reduced eIF4E binding in vivo and in vitro. These data suggest that the association of yeast eIF4E with eIF4G1 leads to the formation of a structured domain within eIF4G1 that could serve as a specific site for interactions with other components of the translational apparatus. They also suggest that the stability of the native eIF4E-eIF4G complex is determined by amino acid residues outside of the conserved seven-residue consensus sequence.

A systematic nomenclature for new translation initiation factor genes from S. pombe and other fungi

Eukaryotic translation initiation factors and their corresponding genes have been characterized using biochemical and genetic methods from a variety of different organisms. The designations of the factors relate to their apparent roles in the biochemical process. Many gene names indicate genetic interactions with other genes or the functional attributes used to identify them. On the other hand, progress in systematic sequencing of the genomes of organisms like Saccharomyces cerevisiae and Schizosaccharomyces pombe has revealed many genes homologous to known translation initiation factor genes. The genes defined by the systematic sequencing approach are assigned numerical designations completely unrelated to their biological function. So far there have been publications on only three genes encoding translation initiation factors from Schizosaccharomyces pombe. We therefore see this an an ideal opportunity to propose a systematic and logical nomenclature for genes encoding translation initiation factor genes that can be applied to all further genes of this type that are characterized in this fission yeast.

Translational repression by human 4E-BP1 in yeast specifically requires human eIF4E as target

4E-binding proteins (4E-BPs) are believed to have important regulatory functions in controlling the rate of translation initiation in mammalian cells. They do so by binding to the mRNA cap-binding protein, eIF4E, thereby inhibiting formation of the cap-binding complex, a process essential for cap-dependent translation initiation. We have reproduced the translation-repressive function of human 4E-BP1 in yeast and find its activity to be dependent on substitution of human eIF4E for its yeast counterpart. Translation initiation and growth are inhibited when human 4E-BP1 is expressed in a strain with the human eIF4E substitution, but not in an unmodified strain. We have compared the relative affinities of human 4E-BP1 for human and yeast eIF4E, both in vitro using an m7GTP cap-binding assay and in vivo using a yeast two-hybrid assay, and find that the affinity of human 4E-BP1 for human eIF4E is markedly greater than for yeast eIF4E. Thus yeast eIF4E lacks structural features required for binding to human 4E-BP1. These results therefore demonstrate that the features of eIF4E required for binding to 4E-BP1 are distinct from those required for cap-complex assembly.

Posttranscriptional control of gene expression in yeast

Studies of the budding yeast Saccharomyces cerevisiae have greatly advanced our understanding of the posttranscriptional steps of eukaryotic gene expression. Given the wide range of experimental tools applicable to S. cerevisiae and the recent determination of its complete genomic sequence, many of the key challenges of the posttranscriptional control field can be tackled particularly effectively by using this organism. This article reviews the current knowledge of the cellular components and mechanisms related to translation and mRNA decay, with the emphasis on the molecular basis for rate control and gene regulation. Recent progress in characterizing translation factors and their protein-protein and RNA-protein interactions has been rapid. Against the background of a growing body of structural information, the review discusses the thermodynamic and kinetic principles that govern the translation process. As in prokaryotic systems, translational initiation is a key point of control. Modulation of the activities of translational initiation factors imposes global regulation in the cell, while structural features of particular 5' untranslated regions, such as upstream open reading frames and effector binding sites, allow for gene-specific regulation. Recent data have revealed many new details of the molecular mechanisms involved while providing insight into the functional overlaps and molecular networking that are apparently a key feature of evolving cellular systems. An overall picture of the mechanisms governing mRNA decay has only very recently begun to develop. The latest work has revealed new information about the mRNA decay pathways, the components of the mRNA degradation machinery, and the way in which these might relate to the translation apparatus. Overall, major challenges still to be addressed include the task of relating principles of posttranscriptional control to cellular compartmentalization and polysome structure and the role of molecular channelling in these highly complex expression systems.

Control of mRNA turnover as a mechanism of glucose repression in Saccharomyces cerevisiae

The phenomenon of glucose repression in yeast is concerned with the repression of a large number of genes when glucose is an abundant carbon source and almost all of the energy requirements of the cell can be satisfied from glycolysis. Prominent among the repressed genes are those encoding mitochondrial proteins required for respiration and oxidative phosphorylation. Past studies have characterized a pathway by which a signal generated from extracellular glucose is transmitted to the nucleus. The ultimate outcome is the repression of transcription of numerous genes, but also the induction of a limited number of others. The emphasis has been almost exclusively on transcriptional control mechanisms. A discovery made originally with the transcript of the SDH2 gene prompted an investigation of post-transcriptional mechanisms, and more specifically a study of the turnover rate of this mRNA in the absence and presence of glucose. SDH2 mRNA has a very short half-life in medium with glucose (YPD) and a significantly longer half-life in medium with glycerol (YPG). Experimental evidence and recent progress in understanding of (1) mRNA turnover in yeast and (2) initiation of translation on the 5' untranslated region of mRNAs, lead to a working hypothesis with the following major features: the carbon source, via a signaling pathway involving kinase/phosphatase activities, controls the rate of initiation, and thus influences a competition between eukaryotic initiation factors (prominently eIF4E, eIF4G, eIF3) binding to the capped mRNA and a decapping activity (DCP1) which is one of the rate limiting activities in the turnover of such mRNAs.

Cooperative modulation by eIF4G of eIF4E-binding to the mRNA 5' cap in yeast involves a site partially shared by p20

Interaction between the mRNA 5'-cap-binding protein eIF4E and the multiadaptor protein eIF4G has been demonstrated in all eukaryotic translation assemblies examined so far. This study uses immunological, genetic and biochemical methods to map the surface amino acids on eIF4E that contribute to eIF4G binding. Cap-analogue chromatography and surface plasmon resonance (SPR) analyses demonstrate that one class of mutations in these surface regions disrupts eIF4E-eIF4G association, and thereby polysome formation and growth. The residues at these positions in wild-type eIF4E mediate positive cooperativity between the binding of eIF4G to eIF4E and the latter's cap-affinity. Moreover, two of the mutations confer temperature sensitivity in eIF4G binding to eIF4E which correlates with the formation of large numbers of inactive ribosome 80S couples in vivo and the loss of cellular protein synthesis activity. The yeast 4E-binding protein p20 is estimated by SPR to have a ten times lower binding affinity than eIF4G for eIF4E. Investigation of a second class of eIF4E mutations reveals that p20 shares only part of eIF4G's binding site on the cap-binding protein. The results presented provide a basis for understanding how cycling of eIF4E and eIF4G occurs in yeast translation and explains how p20 can act as a fine, but not as a coarse, regulator of protein synthesis.

Protein kinase CK2 ("casein kinase-2") and its implication in cell division and proliferation

Protein kinase CK2 (also termed casein kinase-2 or -II) is a ubiquitous Ser/Thr-specific protein kinase required for viability and for cell cycle progression. CK2 is especially elevated in proliferating tissues, either normal or transformed, and the expression of its catalytic subunit in transgenic mice is causative of lymphomas. CK2 is highly pleiotropic: more than 160 proteins phosphorylated by it at sites specified by multiple acidic residues are known. Despite its heterotetrameric structure generally composed by two catalytic (alpha and/or alpha') and two non catalytic beta-subunits, the regulation of CK2 is still enigmatic. A number of functional features of the beta-subunit which could cooperate to the modulation of CK2 targeting/activity will be discussed.

Binding of eukaryotic translation initiation factor 4E (eIF4E) to eIF4G represses translation of uncapped mRNA

mRNA translation in crude extracts from the yeast Saccharomyces cerevisiae is stimulated by the cap structure and the poly(A) tail through the binding of the cap-binding protein eukaryotic translation initiation factor 4E (eIF4E) and the poly(A) tail-binding protein Pab1p. These proteins also bind to the translation initiation factor eIF4G and thereby link the mRNA to the general translational apparatus. In contrast, uncapped, poly(A)-deficient mRNA is translated poorly in yeast extracts, in part because of the absence of eIF4E and Pab1p binding sites on the mRNA. Here, we report that uncapped-mRNA translation is also repressed in yeast extracts due to the binding of eIF4E to eIF4G. Specifically, we find that mutations which weaken the eIF4E binding site on the yeast eIF4G proteins Tif4631p and Tif4632p lead to temperature-sensitive growth in vivo and the stimulation of uncapped-mRNA translation in vitro. A mutation in eIF4E which disturbs its ability to interact with eIF4G also leads to a stimulation of uncapped-mRNA translation in vitro. Finally, overexpression of eIF4E in vivo or the addition of excess eIF4E in vitro reverses these effects of the mutations. These data support the hypothesis that the eIF4G protein can efficiently stimulate translation of exogenous uncapped mRNA in extracts but is prevented from doing so as a result of its association with eIF4E. They also suggest that some mRNAs may be translationally regulated in vivo in response to the amount of free eIF4G in the cell.

The Su(Ste) repeat in the Y chromosome and betaCK2tes gene encode predicted isoforms of regulatory beta-subunit of protein kinase CK2 in Drosophila melanogaster

We report an exon-intron structure of the Su(Ste) repeat capable of encoding an isoform of the beta-subunit of protein kinase CK2. The predicted Su(Ste) gene product contains a drastically changed amino acid sequence of the N-terminal fragment as compared to the earlier described bCK2tes gene considered to be an ancestor of the Su(Ste) repeats. The following peculiarities of molecular divergence of the Su(Ste) and betaCK2tes genes are revealed: damages of the autophosphorylation site; usage of an alternative splicing site instead of a damaged one; conservation of the zinc finger domain in spite of local ORF alterations.

The p20 and Ded1 proteins have antagonistic roles in eIF4E-dependent translation in Saccharomyces cerevisiae

The translation initiation factor eIF4E mediates the binding of the small ribosomal subunit to the cap structure at the 5' end of the mRNA. In Saccharomyces cerevisiae, the cap-binding protein eIF4E is mainly associated with eIF4G, forming the cap-binding complex eIF4F. Other proteins are detected upon purification of the complex on cap-affinity columns. Among them is p20, a protein of unknown function encoded by the CAF20 gene. Here, we show a negative regulatory role for the p20 protein in translation initiation. Deletion of CAF20 partially suppresses mutations in translation initiation factors. Overexpression of the p20 protein results in a synthetic enhancement of translation mutation phenotypes. Similar effects are observed for mutations in the DED1 gene, which we have isolated as a multicopy suppressor of a temperature-sensitive eIF4E mutation. The DED1 gene encodes a putative RNA helicase of the DEAD-box family. The analyses of its suppressor activity, of polysome profiles of ded1 mutant strains, and of synthetic lethal interactions with different translation mutants indicate that the Ded1 protein has a role in translation initiation in S. cerevisiae.

A novel inhibitor of cap-dependent translation initiation in yeast: p20 competes with eIF4G for binding to eIF4E

In the yeast Saccharomyces cerevisiae a small protein named p20 is found associated with translation initiation factor eIF4E, the mRNA cap-binding protein. We demonstrate here that p20 is a repressor of cap-dependent translation initiation. p20 shows amino acid sequence homology to a region of eIF4G, the large subunit of the cap-binding protein complex eIF4F, which carries the binding site for eIF4E. Both, eIF4G and p20 bind to eIF4E and compete with each other for binding to eIF4E. The eIF4E-p20 complex can bind to the cap structure and inhibit cap-dependent but not cap-independent translation initiation: the translation of a mRNA with the 67 nucleotide omega sequence of tobacco mosaic virus in its 5' untranslated region (which was previously shown to render translation cap-independent) is not inhibited by p20. Whereas the translation of the same mRNA lacking the omega sequence is strongly inhibited by p20. Disruption of CAF20, the gene encoding p20, stimulates the growth of yeast cells, overexpression of p20 causes slower growth of yeast cells. These results show that p20 is a regulator of eIF4E activity which represses cap-dependent initiation of translation by interfering with the interaction of eIF4E with eIF4G, e.g. the formation of the eIF4F-complex.

Schizosaccharomyces pombe has a novel eukaryotic initiation factor 4F complex containing a cap-binding protein with the human eIF4E C-terminal motif KSGST

Genetic and biochemical analyses were performed on the cytoplasmic cap-binding complex (eukaryotic initiation factor (eIF) 4F) of Schizosaccharomyces pombe. Genomic and cDNA sequencing of the S. pombe gene (tif1) encoding the cap-binding component eIF4E revealed the presence of two introns in a reading frame of 219 codons. The encoded sequence of 218 amino acids shows a greater degree of identity to the mammalian eIF4E sequence than does its counterpart from Saccharomyces cerevisiae. In particular, unlike its S. cerevisiae counterpart, S.pombe eIF4E has a C-terminal Ser209 within the motif KSGST that is a site of phosphorylation in hamster and rabbit eIF4E. Of relevance to its potential regulatory role, eIF4E was found to be encoded by an mRNA with a six-nucleotide leader and to be of low abundance in vivo. Cross-linking experiments identified S. pombe eIF4E as the major cap-binding protein while a further protein, p36, also showed cap-dependent binding. eIF4A was not associated with the cap-binding complex. While S. pombe eIF4E was shown capable of binding S. cerevisiae p20, an equivalent protein was absent from the eIF4F complex isolated from S. pombe cells. S. pombe 4F therefore shows a remarkable combination of structural and functional properties, some of which it shares with its higher and its lower eukaryotic counterparts.

Intracellular targeting and mRNA interactions of the eukaryotic translation initiation factor eIF4E in the yeast Saccharomyces cerevisiae

The 5' cap structure of eukaryotic mRNAs is believed to play a role in a number of cellular processes, including pre-mRNA splicing, nuclear export and translation. An essential cap-binding protein that is likely to mediate the participation of the cap in at least one of these processes is the eukaryotic translation initiation factor eIF4E. This protein is thought to facilitate the initial ribosomal interaction with the 5' end of the mRNA, involving the binding of eIF4E to the cap in the cytoplasm. Yet the subcellular distribution and mechanism of targeting of eIF4E has been an unresolved issue. We have therefore examined whether eIF4E in the yeast Saccharomyces cerevisiae is directed to the nucleus by virtue of a nuclear localization sequence (NLS) in its amino acid sequence. eIF4E was fused with the "marker proteins' yeast invertase and jellyfish green fluorescent protein. The distribution of these fusions could be followed using immunofluorescence and confocal microscopy of protoplasts and whole cells. These and other fusions were used to show that while yeast eIF4E does not possess an efficiently functioning NLS, it can be transported into the nucleus if provided with a known active NLS. However, an NLS-eIF4E fusion of this type cannot be stably supported by the cell, most likely because of its inhibitory effects when present in large quantities in the nucleus, whereas an NLS fusion with a mutant form of eIF4E that has reduced cap-affinity is tolerated.

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.