Identifying eIF4E-binding protein translationally-controlled transcripts reveals links to mRNAs bound by specific PUF proteins

eIF4E-binding proteins (4E-BPs) regulate translation of mRNAs in eukaryotes. However the extent to which specific mRNA targets are regulated by 4E-BPs remains unknown. We performed translational profiling by microarray analysis of polysome and monosome associated mRNAs in wild-type and mutant cells to identify mRNAs in yeast regulated by the 4E-BPs Caf20p and Eap1p; the first-global comparison of 4E-BP target mRNAs. We find that yeast 4E-BPs modulate the translation of >1000 genes. Most target mRNAs differ between the 4E-BPs revealing mRNA specificity for translational control by each 4E-BP. This is supported by observations that eap1Δ and caf20Δ cells have different nitrogen source utilization defects, implying different mRNA targets. To account for the mRNA specificity shown by each 4E-BP, we found correlations between our data sets and previously determined targets of yeast mRNA-binding proteins. We used affinity chromatography experiments to uncover specific RNA-stabilized complexes formed between Caf20p and Puf4p/Puf5p and between Eap1p and Puf1p/Puf2p. Thus the combined action of each 4E-BP with specific 3'-UTR-binding proteins mediates mRNA-specific translational control in yeast, showing that this form of translational control is more widely employed than previously thought.

Fusel alcohols regulate translation initiation by inhibiting eIF2B to reduce ternary complex in a mechanism that may involve altering the integrity and dynamics of the eIF2B body

This study highlights a connection between the eIF2B body and the regulation of translation initiation as a response to stress in Saccharomyces cerevisiae. Fusel alcohols are involved in signaling nitrogen scarcity to the cell and they inhibit protein synthesis by preventing the movement of the eIF2B body throughout the cell.

Recycling of eIF2-GDP to the GTP-bound form constitutes a core essential, regulated step in eukaryotic translation. This reaction is mediated by eIF2B, a heteropentameric factor with important links to human disease. eIF2 in the GTP-bound form binds to methionyl initiator tRNA to form a ternary complex, and the levels of this ternary complex can be a critical determinant of the rate of protein synthesis. Here we show that eIF2B serves as the target for translation inhibition by various fusel alcohols in yeast. Fusel alcohols are endpoint metabolites from amino acid catabolism, which signal nitrogen scarcity. We show that the inhibition of eIF2B leads to reduced ternary complex levels and that different eIF2B subunit mutants alter fusel alcohol sensitivity. A DNA tiling array strategy was developed that overcame difficulties in the identification of these mutants where the phenotypic distinctions were too subtle for classical complementation cloning. Fusel alcohols also lead to eIF2α dephosphorylation in a Sit4p-dependent manner. In yeast, eIF2B occupies a large cytoplasmic body where guanine nucleotide exchange on eIF2 can occur and be regulated. Fusel alcohols impact on both the movement and dynamics of this 2B body. Overall, these results confirm that the guanine nucleotide exchange factor, eIF2B, is targeted by fusel alcohols. Moreover, they highlight a potential connection between the movement or integrity of the 2B body and eIF2B regulation.

Discovery of chemical modulators of a conserved translational control pathway by parallel screening in yeast

Eukaryotic initiation factor 2 (eIF2) B is a guanine nucleotide exchange factor that plays a central role in translation initiation and its control, especially in response to diverse cellular stresses. In addition, inherited mutations in human eIF2B subunits cause a fatal brain disorder commonly called childhood ataxia with central nervous system hypomyelination or leukoencephalopathy with vanishing white matter. In yeast, inhibiting activity of eIF2B up-regulates expression of the transcriptional activator general control nondepressible (GCN) 4. We report here evaluation of high-throughput screening (HTS) using a yeast-based reporter gene assay, in which strains containing either wild-type or a mutant eIF2B were screened in parallel to identify compounds modifying eIF2B-dependent responses. The goals of the HTS were twofold: first, to discover compounds that restore normal function to mutant eIF2B, which may have therapeutic utility for the fatal human disease; and second, to identify compounds that activate a GCN4 response, which might be useful experimental tools. The HTS assay measured cell growth by absorbance, and activation of gene expression via a beta-galactosidase reporter gene fusion. Because mutant eIF2B activates GCN4 in the absence of stress inducers, the mutant strain was screened for reduction in GCN4 activation. HTS revealed apparent mutant-selective inhibitors but did not reliably predict selectivity as these hits affected both wild-type and mutant strains equally on dose-response confirmation. Wild-type strain results from the HTS identified two GCN4 response activators, both of which were confirmed to be wild-type selective in dose-response testing, suggesting that these compounds may activate GCN4 by a mechanism that down-regulates eIF2B activity.

Upstream sequence elements direct post-transcriptional regulation of gene expression under stress conditions in yeast

Background

The control of gene expression in eukaryotic cells occurs both transcriptionally and post-transcriptionally. Although many genes are now known to be regulated at the translational level, in general, the mechanisms are poorly understood. We have previously presented polysomal gradient and array-based evidence that translational control is widespread in a significant number of genes when yeast cells are exposed to a range of stresses. Here we have re-examined these gene sets, considering the role of UTR sequences in the translational responses of these genes using recent large-scale datasets which define 5' and 3' transcriptional ends for many yeast genes. In particular, we highlight the potential role of 5' UTRs and upstream open reading frames (uORFs).

Results

We show a highly significant enrichment in specific GO functional classes for genes that are translationally up- and down-regulated under given stresses (e.g. carbohydrate metabolism is up-regulated under amino acid starvation). Cross-referencing these data with the stress response data we show that translationally upregulated genes have longer 5' UTRs, consistent with their role in translational regulation. In the first genome-wide study of uORFs in a set of mapped 5' UTRs, we show that uORFs are rare, being statistically under-represented in UTR sequences. However, they have distinct compositional biases consistent with their putative role in translational control and are more common in genes which are apparently translationally up-regulated.

Conclusion

These results demonstrate a central regulatory role for UTR sequences, and 5' UTRs in particular, highlighting the significant role of uORFs in post-transcriptional control in yeast. Yeast uORFs are more highly conserved than has been suggested, lending further weight to their significance as functional elements involved in gene regulation. It also suggests a more complex and novel mechanism of control, whereby uORFs permit genes to escape from a more general attenuation of translation under conditions of stress. However, since uORFs are relatively rare (only ~13% of yeast genes have them) there remain many unanswered questions as to how UTR elements can direct translational control of many hundreds of genes under stress.

Translation controlled

A report of the meeting 'Translational Control', Cold Spring Harbor, USA, 3-7 September 2008.

A report of the meeting 'Translational Control', Cold Spring Harbor, USA, 3-7 September 2008.

A Saccharomyces cerevisiae cell-based quantitative beta-galactosidase assay compatible with robotic handling and high-throughput screening

Reporter-gene assays that employ the Escherichia coli lacZ gene are ubiquitously employed in biological research. However, we were not able to readily identify a quantitative method that worked reliably with yeast (Saccharomyces cerevisiae) cells and that was compatible with high-throughput screening and robotic liquid handling tools. We have therefore adapted a commercially available assay employing a 6-O-beta-galactopyranosyl-luciferin substrate to provide the required sensitivity with minimal sample handling times. Our assay uses only one-tenth of the reagents suggested by the reagent manufacturer (Promega) for equivalent assays with mammalian cell cultures and produces rapid, sensitive and reproducible analysis with as little as 1 microl yeast cell culture and with < 100 cells. We demonstrate that the assay is compatible with yeast strains generated by the systematic yeast deletion project and functions equally well with genomically integrated or plasmid-encoded lacZ reporters and with cells grown in complex or defined media. The high-sensitivity, miniaturized format reduced sample handling required will make this assay useful for a wide range of applications.

Critical contacts between the eukaryotic initiation factor 2B (eIF2B) catalytic domain and both eIF2beta and -2gamma mediate guanine nucleotide exchange

Diverse guanine nucleotide exchange factors (GEFs) regulate the activity of GTP binding proteins. One of the most complicated pairs is eukaryotic initiation factor 2B (eIF2B) and eIF2, which function during protein synthesis initiation in eukaryotes. We have mutated conserved surface residues within the eIF2B GEF domain, located at the eIF2Bepsilon C terminus. Extensive genetic and biochemical characterization established how these residues contribute to GEF activity. We find that the universally conserved residue E569 is critical for activity and that even a conservative E569D substitution is lethal in vivo. Several mutations within residues close to E569 have no discernible effect on growth or GCN4 expression, but an alanine substitution at the adjacent L568 is cold sensitive and deregulates GCN4 activity at 15 degrees C. The mutation of W699, found on a separate surface approximately 40 A from E569, is also lethal. Binding studies show that W699 is critical for interaction with eIF2beta, while L568 and E569 are not. In contrast, all three residues are critical for interaction with eIF2gamma. These data show that multiple contacts between eIF2gamma and eIF2Bepsilon mediate nucleotide exchange.

Change in nutritional status modulates the abundance of critical pre-initiation intermediate complexes during translation initiation in vivo

In eukaryotic translation initiation, eIF2GTP-Met-tRNA(i)(Met) ternary complex (TC) interacts with eIF3-eIF1-eIF5 complex to form the multifactor complex (MFC), while eIF2GDP associates with eIF2B for guanine nucleotide exchange. Gcn2p phosphorylates eIF2 to inhibit eIF2B. Here we evaluate the abundance of eIFs and their pre-initiation intermediate complexes in gcn2 deletion mutant grown under different conditions. We show that ribosomes are three times as abundant as eIF1, eIF2 and eIF5, while eIF3 is half as abundant as the latter three and hence, the limiting component in MFC formation. By quantitative immunoprecipitation, we estimate that approximately 15% of the cellular eIF2 is found in TC during rapid growth in a complex rich medium. Most of the TC is found in MFC, and important, approximately 40% of the total eIF2 is associated with eIF5 but lacks tRNA(i)(Met). When the gcn2Delta mutant grows less rapidly in a defined complete medium, TC abundance increases threefold without altering the abundance of each individual factor. Interestingly, the TC increase is suppressed by eIF5 overexpression and Gcn2p expression. Thus, eIF2B-catalyzed TC formation appears to be fine-tuned by eIF2 phosphorylation and the novel eIF2/eIF5 complex lacking tRNA(i)(Met).

Purification of FLAG-tagged eukaryotic initiation factor 2B complexes, subcomplexes, and fragments from Saccharomyces cerevisiae

The eukaryotic initiation factor 2B (eIF2B) is a five-subunit guanine nucleotide exchange factor, that functions during translation initiation to catalyze the otherwise slow exchange of GDP for GTP on its substrate eIF2. Assays to measure substrate interaction and guanine nucleotide release ability of eIF2B require the complex to be purified free of interacting proteins. We have also found that a subcomplex of two subunits, gamma and epsilon or the largest one, epsilon alone, promotes this activity. Within eIF2Bepsilon, the catalytic center requires the C-terminal 200 residues only. Here, we describe our protocols for purifying the Saccharomyces cerevisiae eIF2B complexes and the catalytic subunit using FLAG-tagged proteins overexpressed in yeast cells. Using commercially available FLAG-affinity resin and high salt buffer, we are able to purify active eIF2B virtually free of contaminants.

An eIF5/eIF2 complex antagonizes guanine nucleotide exchange by eIF2B during translation initiation

In eukaryotic translation initiation, the eIF2.GTP/Met-tRNA(i)(Met) ternary complex (TC) binds the eIF3/eIF1/eIF5 complex to form the multifactor complex (MFC), whereas eIF2.GDP binds the pentameric factor eIF2B for guanine nucleotide exchange. eIF5 and the eIF2Bvarepsilon catalytic subunit possess a conserved eIF2-binding site. Nearly half of cellular eIF2 forms a complex with eIF5 lacking Met-tRNA(i)(Met), and here we investigate its physiological significance. eIF5 overexpression increases the abundance of both eIF2/eIF5 and TC/eIF5 complexes, thereby impeding eIF2B reaction and MFC formation, respectively. eIF2Bvarepsilon mutations, but not other eIF2B mutations, enhance the ability of overexpressed eIF5 to compete for eIF2, indicating that interaction of eIF2Bvarepsilon with eIF2 normally disrupts eIF2/eIF5 interaction. Overexpression of the catalytic eIF2Bvarepsilon segment similarly exacerbates eIF5 mutant phenotypes, supporting the ability of eIF2Bvarepsilon to compete with MFC. Moreover, we show that eIF5 overexpression does not generate aberrant MFC lacking tRNA(i)(Met), suggesting that tRNA(i)(Met) is a vital component promoting MFC assembly. We propose that the eIF2/eIF5 complex represents a cytoplasmic reservoir for eIF2 that antagonizes eIF2B-promoted guanine nucleotide exchange, enabling coordinated regulation of translation initiation.

Global translational responses to oxidative stress impact upon multiple levels of protein synthesis

Global inhibition of protein synthesis is a common response to stress conditions. We have analyzed the regulation of protein synthesis in response to oxidative stress induced by exposure to H(2)O(2) in the yeast Saccharomyces cerevisiae. Our data show that H(2)O(2) causes an inhibition of translation initiation dependent on the Gcn2 protein kinase, which phosphorylates the alpha-subunit of eukaryotic initiation factor-2. Additionally, our data indicate that translation is regulated in a Gcn2-independent manner because protein synthesis was still inhibited in response to H(2)O(2) in a gcn2 mutant. Polysome analysis indicated that H(2)O(2) causes a slower rate of ribosomal runoff, consistent with an inhibitory effect on translation elongation or termination. Furthermore, analysis of ribosomal transit times indicated that oxidative stress increases the average mRNA transit time, confirming a post-initiation inhibition of translation. Using microarray analysis of polysome- and monosome-associated mRNA pools, we demonstrate that certain mRNAs, including mRNAs encoding stress protective molecules, increase in association with ribosomes following H(2)O(2) stress. For some candidate mRNAs, we show that a low concentration of H(2)O(2) results in increased protein production. In contrast, a high concentration of H(2)O(2) promotes polyribosome association but does not necessarily lead to increased protein production. We suggest that these mRNAs may represent an mRNA store that could become rapidly activated following relief of the stress condition. In summary, oxidative stress elicits complex translational reprogramming that is fundamental for adaptation to the stress.

Global gene expression profiling reveals widespread yet distinctive translational responses to different eukaryotic translation initiation factor 2B-targeting stress pathways

Global inhibition of protein synthesis is a hallmark of many cellular stress conditions. Even though specific mRNAs defy this (e.g., yeast GCN4 and mammalian ATF4), the extent and variation of such resistance remain uncertain. In this study, we have identified yeast mRNAs that are translationally maintained following either amino acid depletion or fusel alcohol addition. Both stresses inhibit eukaryotic translation initiation factor 2B, but via different mechanisms. Using microarray analysis of polysome and monosome mRNA pools, we demonstrate that these stress conditions elicit widespread yet distinct translational reprogramming, identifying a fundamental role for translational control in the adaptation to environmental stress. These studies also highlight the complex interplay that exists between different stages in the gene expression pathway to allow specific preordained programs of proteome remodeling. For example, many ribosome biogenesis genes are coregulated at the transcriptional and translational levels following amino acid starvation. The transcriptional regulation of these genes has recently been connected to the regulation of cellular proliferation, and on the basis of our results, the translational control of these mRNAs should be factored into this equation.

eIF2B, a mediator of general and gene-specific translational control

eIF2B (eukaryotic initiation factor 2B) is a multisubunit protein that is required for protein synthesis initiation and its regulation in all eukaryotic cells. Mutations in eIF2B have also recently been found to cause a fatal human disease called CACH (childhood ataxia with central nervous system hypomyelination) or VWM (vanishing white matter disease). This review provides a general background to translation initiation and mechanisms known to control eIF2B function, before describing molecular genetic and biochemical analysis of eIF2B structure and function, integrating work from studies of the yeast and mammalian eIF2B proteins.

Mutations causing childhood ataxia with central nervous system hypomyelination reduce eukaryotic initiation factor 2B complex formation and activity

Childhood ataxia with central nervous system hypomyelination (CACH), or vanishing white matter leukoencephalopathy (VWM), is a fatal brain disorder caused by mutations in eukaryotic initiation factor 2B (eIF2B). eIF2B is essential for protein synthesis and regulates translation in response to cellular stresses. We performed mutagenesis to introduce changes equivalent to 12 human CACH/VWM mutations in three subunits of the equivalent factor from yeast (Saccharomyces cerevisiae) and analyzed effects on cell growth, translation, and gene expression in response to stresses. None of the mutations is lethal or temperature sensitive, but almost all confer some defect in eIF2B function significant enough to alter growth or gene expression under normal or stress conditions. Biochemical analyses indicate that mutations analyzed in eIF2Balpha and -epsilon reduce the steady-state level of the affected subunit, while the most severe mutant tested, eIF2Bbeta(V341D) (human eIF2B(betaV316D)), forms complexes with reduced stability and lower eIF2B activity. eIF2Bdelta is excluded from eIF2Bbeta(V341D) complexes. eIF2B(betav341D) function can be rescued by overexpression of eIF2Bdelta alone. Our findings imply CACH/VWM mutations do not specifically impair responses to eIF2 phosphorylation, but instead cause protein structure defects that impair eIF2B activity. Altered protein folding is characteristic of other diseases, including cystic fibrosis and neurodegenerative disorders such as Huntington, Alzheimer's, and prion diseases.

Structure of the Catalytic Fragment of Translation Initiation Factor 2B and Identification of a Critically Important Catalytic Residue

Eukaryotic initiation factor (eIF) 2B catalyzes the nucleotide activation of eIF2 to its active GTP-bound state. The exchange activity has been mapped to the C terminus of the eIF2Bepsilon subunit. We have determined the crystal structure of residues 544-704 from yeast eIF2Bepsilon at 2.3-A resolution, and this fragment is an all-helical protein built around the conserved aromatic acidic (AA) boxes also found in eIF4G and eIF5. The eight helices are organized in a manner similar to HEAT repeats. The molecule is highly asymmetric with respect to surface charge and conservation. One area in the N terminus is proposed to be directly involved in catalysis. In agreement with this hypothesis, mutation of glutamate 569 is shown to be lethal. An acidic belt and a second area in the C terminus containing residues from the AA boxes are important for binding to eIF2. Two mutations causing the fatal human genetic disease leukoencephalopathy with vanishing white matter are buried and appear to disrupt the structural integrity of the catalytic domain rather than interfering directly with catalysis or binding of eIF2.

Characterization of the minimal catalytic domain within eIF2B: the guanine-nucleotide exchange factor for translation initiation

For protein synthesis initiation in eukaryotes, eIF2B is the guanine-nucleotide exchange factor for eIF2. eIF2B is an essential multi-subunit factor and a major target for translational control in both yeast and mammalian cells. It was shown previously that the largest eIF2B subunit, eIF2Bepsilon, is the only single subunit with catalytic function. Here we report the results of a molecular dissection of the yeast epsilon subunit encoded by GCD6 in which we have identified the catalytic domain. By analysis of a series of N-terminal deletions in vitro we find that the smallest catalytically active fragment contains residues 518-712 (termed Gcd6p(518-712)). Further deletion to position 581 (Gcd6p(581-712)) results in loss of nucleotide exchange function, but eIF2-binding activity is retained. C- terminal deletion of only 61 residues (Gcd6p(1-651)) results in loss of both functions. Thus Gcd6p(518-712) contains two regions that together constitute the catalytic domain of eIF2B. Finally, we show that the catalytic domain can provide eIF2B biological function in vivo when elevated levels eIF2 and tRNA(i)(Met) are also present.

Tight binding of the phosphorylated alpha subunit of initiation factor 2 (eIF2alpha) to the regulatory subunits of guanine nucleotide exchange factor eIF2B is required for inhibition of translation initiation

Translation initiation factor 2 (eIF2) is a heterotrimeric protein that transfers methionyl-initiator tRNA(Met) to the small ribosomal subunit in a ternary complex with GTP. The eIF2 phosphorylated on serine 51 of its alpha subunit [eIF2(alphaP)] acts as competitive inhibitor of its guanine nucleotide exchange factor, eIF2B, impairing formation of the ternary complex and thereby inhibiting translation initiation. eIF2B is comprised of catalytic and regulatory subcomplexes harboring independent eIF2 binding sites; however, it was unknown whether the alpha subunit of eIF2 directly contacts any eIF2B subunits or whether this interaction is modulated by phosphorylation. We found that recombinant eIF2alpha (glutathione S-transferase [GST]-SUI2) bound to the eIF2B regulatory subcomplex in vitro, in a manner stimulated by Ser-51 phosphorylation. Genetic data suggest that this direct interaction also occurred in vivo, allowing overexpressed SUI2 to compete with eIF2(alphaP) holoprotein for binding to the eIF2B regulatory subcomplex. Mutations in SUI2 and in the eIF2B regulatory subunit GCD7 that eliminated inhibition of eIF2B by eIF2(alphaP) also impaired binding of phosphorylated GST-SUI2 to the eIF2B regulatory subunits. These findings provide strong evidence that tight binding of phosphorylated SUI2 to the eIF2B regulatory subcomplex is crucial for the inhibition of eIF2B and attendant downregulation of protein synthesis exerted by eIF2(alphaP). We propose that this regulatory interaction prevents association of the eIF2B catalytic subcomplex with the beta and gamma subunits of eIF2 in the manner required for GDP-GTP exchange.

Characterization of the initiation factor eIF2B and its regulation in Drosophila melanogaster

Eukaryotic initiation factor (eIF) 2B catalyzes a key regulatory step in the initiation of mRNA translation. eIF2B is well characterized in mammals and in yeast, although little is known about it in other eukaryotes. eIF2B is a hetropentamer which mediates the exchange of GDP for GTP on eIF2. In mammals and yeast, its activity is regulated by phosphorylation of eIF2alpha. Here we have cloned Drosophila melanogaster cDNAs encoding polypeptides showing substantial similarity to eIF2B subunits from yeast and mammals. They also exhibit the other conserved features of these proteins. D. melanogaster eIF2Balpha confers regulation of eIF2B function in yeast, while eIF2Bepsilon shows guanine nucleotide exchange activity. In common with mammalian eIF2Bepsilon, D. melanogaster eIF2Bepsilon is phosphorylated by glycogen synthase kinase-3 and casein kinase II. Phosphorylation of partially purified D. melanogaster eIF2B by glycogen synthase kinase-3 inhibits its activity. Extracts of D. melanogaster S2 Schneider cells display eIF2B activity, which is inhibited by phosphorylation of eIF2alpha, showing the insect factor is regulated similarly to eIF2B from other species. In S2 cells, serum starvation increases eIF2alpha phosphorylation, which correlates with inhibition of eIF2B, and both effects are reversed by serum treatment. This shows that eIF2alpha phosphorylation and eIF2B activity are under dynamic regulation by serum. eIF2alpha phosphorylation is also increased by endoplasmic reticulum stress in S2 cells. These are the first data concerning the structure, function or control of eIF2B from D. melanogaster.

Purification and kinetic analysis of eIF2B from Saccharomyces cerevisiae

Eukaryotic translation initiation factor 2B (eIF2B) is the heteropentameric guanine nucleotide exchange factor for translation initiation factor 2 (eIF2). Recent studies in the yeast Saccharomyces cerevisiae have served to characterize genetically the exchange factor. However, enzyme kinetic studies of the yeast enzyme have been hindered by the lack of sufficient quantities of protein suitable for biochemical analysis. We have purified yeast eIF2B and characterized its catalytic properties in vitro. Values for K(m) and V(max) were determined to be 12.2 nm and 250.7 fmol/min, respectively, at 0 degrees C. The calculated turnover number (K(cat)) of 43.2 pmol of GDP released per min/pmol of eIF2B at 30 degrees C is approximately 1 order of magnitude lower than values previously reported for the mammalian factor. Reciprocal plots at varying fixed concentrations of the second substrate were linear and intersected to the left of the y axis. This is consistent with a sequential catalytic mechanism and argues against a ping-pong mechanism similar to that proposed for EF-Tu/EF-Ts. In support of this model, our yeast eIF2B preparations bind guanine nucleotides, with an apparent dissociation constant for GTP in the low micromolar range.

Identification of domains and residues within the epsilon subunit of eukaryotic translation initiation factor 2B (eIF2Bepsilon) required for guanine nucleotide exchange reveals a novel activation function promoted by eIF2B complex formation

Eukaryotic translation initiation factor 2B (eIF2B) is the guanine nucleotide exchange factor for protein synthesis initiation factor 2 (eIF2). Composed of five subunits, it converts eIF2 from a GDP-bound form to the active eIF2-GTP complex. This is a regulatory step of translation initiation. In vitro, eIF2B catalytic function can be provided by the largest (epsilon) subunit alone (eIF2Bepsilon). This activity is stimulated by complex formation with the other eIF2B subunits. We have analyzed the roles of different regions of eIF2Bepsilon in catalysis, in eIF2B complex formation, and in binding to eIF2 by characterizing mutations in the Saccharomyces cerevisiae gene encoding eIF2Bepsilon (GCD6) that impair the essential function of eIF2B. Our analysis of nonsense mutations indicates that the C terminus of eIF2Bepsilon (residues 518 to 712) is required for both catalytic activity and interaction with eIF2. In addition, missense mutations within this region impair the catalytic activity of eIF2Bepsilon without affecting its ability to bind eIF2. Internal, in-frame deletions within the N-terminal half of eIF2Bepsilon disrupt eIF2B complex formation without affecting the nucleotide exchange activity of eIF2Bepsilon alone. Finally, missense mutations identified within this region do not affect the catalytic activity of eIF2Bepsilon alone or its interactions with the other eIF2B subunits or with eIF2. Instead, these missense mutations act indirectly by impairing the enhancement of the rate of nucleotide exchange that results from complex formation between eIF2Bepsilon and the other eIF2B subunits. This suggests that the N-terminal region of eIF2Bepsilon is an activation domain that responds to eIF2B complex formation.

Conserved bipartite motifs in yeast eIF5 and eIF2Bepsilon, GTPase-activating and GDP-GTP exchange factors in translation initiation, mediate binding to their common substrate eIF2

In the initiation phase of eukaryotic translation, eIF5 stimulates the hydrolysis of GTP bound to eIF2 in the 40S ribosomal pre-initiation complex, and the resultant GDP on eIF2 is replaced with GTP by the complex nucleotide exchange factor, eIF2B. Bipartite motifs rich in aromatic and acidic residues are conserved at the C-termini of eIF5 and the catalytic (epsilon) subunit of eIF2B. Here we show that these bipartite motifs are important for the binding of these factors, both in vitro and in vivo, to the beta subunit of their common substrate eIF2. We also find that three lysine-rich boxes in the N-terminal segment of eIF2beta mediate the binding of eIF2 to both eIF5 and eIF2B. Thus, eIF5 and eIF2Bepsilon employ the same sequence motif to facilitate interaction with the same segment of their common substrate. In agreement with this, archaea appear to lack eIF5, eIF2B and the lysine-rich binding domain for these factors in their eIF2beta homolog. The eIF5 bipartite motif is also important for its interaction with the eIF3 complex through the NIP1-encoded subunit of eIF3. Thus, the bipartite motif in eIF5 appears to be multifunctional, stimulating its recruitment to the 40S pre-initiation complex through interaction with eIF3 in addition to binding of its substrate eIF2.

Regulation of guanine nucleotide exchange through phosphorylation of eukaryotic initiation factor eIF2alpha. Role of the alpha- and delta-subunits of eiF2b

The guanine nucleotide exchange activity of eIF2B plays a key regulatory role in the translation initiation phase of protein synthesis. The activity is markedly inhibited when the substrate, i. e. eIF2, is phosphorylated on Ser51 of its alpha-subunit. Genetic studies in yeast implicate the alpha-, beta-, and delta-subunits of eIF2B in mediating the inhibition by substrate phosphorylation. However, the mechanism involved in the inhibition has not been defined biochemically. In the present study, we have coexpressed the five subunits of rat eIF2B in Sf9 cells using the baculovirus system and have purified the recombinant holoprotein to >90% homogeneity. We have also expressed and purified a four-subunit eIF2B complex lacking the alpha-subunit. Both the five- and four-subunit forms of eIF2B exhibit similar rates of guanine nucleotide exchange activity using unphosphorylated eIF2 as substrate. The five-subunit form is inhibited by preincubation with phosphorylated eIF2 (eIF2(alphaP)) and exhibits little exchange activity when eIF2(alphaP) is used as substrate. In contrast, eIF2B lacking the alpha-subunit is insensitive to inhibition by eIF2(alphaP) and is able to exchange guanine nucleotide using eIF2(alphaP) as substrate at a faster rate compared with five-subunit eIF2B. Finally, a double point mutation in the delta-subunit of eIF2B has been identified that results in insensitivity to inhibition by eIF2(alphaP) and exhibits little exchange activity when eIF2(alphaP) is used as substrate. The results provide the first direct biochemical evidence that the alpha- and delta-subunits of eIF2B are involved in mediating the effect of substrate phosphorylation.

eIF2 independently binds two distinct eIF2B subcomplexes that catalyze and regulate guanine-nucleotide exchange

eIF2B is a heteropentameric guanine-nucleotide exchange factor essential for protein synthesis initiation in eukaryotes. Its activity is inhibited in response to starvation or stress by phosphorylation of the alpha subunit of its substrate, translation initiation factor eIF2, resulting in reduced rates of translation and cell growth. We have used an in vitro nucleotide-exchange assay to show that wild-type yeast eIF2B is inhibited by phosphorylated eIF2 [eIF2(alphaP)] and to characterize eIF2B regulatory mutations that render translation initiation insensitive to eIF2 phosphorylation in vivo. Unlike wild-type eIF2B, eIF2B complexes with mutated GCN3 or GCD7 subunits efficiently catalyzed GDP exchange using eIF2(alphaP) as a substrate. Using an affinity-binding assay, we show that an eIF2B subcomplex of the GCN3, GCD7, and GCD2 subunits binds to eIF2 and has a higher affinity for eIF2(alphaP), but it lacks nucleotide-exchange activity. In contrast, the GCD1 and GCD6 subunits form an eIF2B subcomplex that binds equally to eIF2 and eIF2(alphaP). Remarkably, this second subcomplex has higher nucleotide-exchange activity than wild-type eIF2B that is not inhibited by eIF2(alphaP). The identification of regulatory and catalytic eIF2B subcomplexes leads us to propose that binding of eIF2(alphaP) to the regulatory subcomplex prevents a productive interaction with the catalytic subcomplex, thereby inhibiting nucleotide exchange.