A conformational change in the eukaryotic translation preinitiation complex and release of eIF1 signal recognition of the start codon

Critical cis-parameters influence STructure assisted RNA translation (START) initiation on non-AUG codons in eukaryotes

In eukaryotes, translation initiation is a highly regulated process, which combines cis-regulatory sequences located on the messenger RNA along with trans-acting factors like eukaryotic initiation factors (eIF). One critical step of translation initiation is the start codon recognition by the scanning 43S particle, which leads to ribosome assembly and protein synthesis. In this study, we investigated the involvement of secondary structures downstream the initiation codon in the so-called START (STructure-Assisted RNA translation) mechanism on AUG and non-AUG translation initiation. The results demonstrate that downstream secondary structures can efficiently promote non-AUG translation initiation if they are sufficiently stable to stall a scanning 43S particle and if they are located at an optimal distance from non-AUG codons to stabilize the codon-anticodon base pairing in the P site. The required stability of the downstream structure for efficient translation initiation varies in distinct cell types. We extended this study to genome-wide analysis of functionally characterized alternative translation initiation sites in Homo sapiens. This analysis revealed that about 25% of these sites have an optimally located downstream secondary structure of adequate stability which could elicit START, regardless of the start codon. We validated the impact of these structures on translation initiation for several selected uORFs.

Translation initiation factor eIF1.2 promotes Toxoplasma stage conversion by regulating levels of key differentiation factors

The parasite Toxoplasma gondii persists in its hosts by converting from replicating tachyzoites to latent bradyzoites housed in tissue cysts. The molecular mechanisms that mediate T. gondii differentiation remain poorly understood. Through a mutagenesis screen, we identified translation initiation factor eIF1.2 as a critical factor for T. gondii differentiation. A F97L mutation in eIF1.2 or the genetic ablation of eIF1.2 (∆eif1.2) markedly impeded bradyzoite cyst formation in vitro and in vivo. We demonstrated, at single-molecule level, that the eIF1.2 F97L mutation impacts the scanning process of the ribosome preinitiation complex on a model mRNA. RNA sequencing and ribosome profiling experiments unveiled that ∆eif1.2 parasites are defective in upregulating bradyzoite induction factors BFD1 and BFD2 during stress-induced differentiation. Forced expression of BFD1 or BFD2 significantly restored differentiation in ∆eif1.2 parasites. Together, our findings suggest that eIF1.2 functions by regulating the translation of key differentiation factors necessary to establish chronic toxoplasmosis.

Translation initiation factor eIF1.2 promotes Toxoplasma stage conversion by regulating levels of key differentiation factors

The parasite Toxoplasma gondii persists in its hosts by converting from replicating tachyzoites to latent bradyzoites housed in tissue cysts. The molecular mechanisms that mediate T. gondii differentiation remain poorly understood. Through a mutagenesis screen, we identified translation initiation factor eIF1.2 as a critical factor for T. gondii differentiation. A F97L mutation in eIF1.2 or the genetic ablation of eIF1.2 (Δ eif1.2 ) markedly impeded bradyzoite cyst formation in vitro and in vivo . We demonstrated, at single-molecule level, that the eIF1.2 F97L mutation impacts the scanning process of the ribosome preinitiation complex on a model mRNA. RNA sequencing and ribosome profiling experiments unveiled that Δ eif1.2 parasites are defective in upregulating bradyzoite induction factors BFD1 and BFD2 during stress-induced differentiation. Forced expression of BFD1 or BFD2 significantly restored differentiation in Δ eif1.2 parasites. Together, our findings suggest that eIF1.2 functions by regulating the translation of key differentiation factors necessary to establish chronic toxoplasmosis.

Translation initiation factor eIF1A rescues hygromycin B sensitivity caused by deleting the carboxy-terminal tail in the GPN-loop GTPase Npa3

The essential yeast protein GPN-loop GTPase 1 (Npa3) plays a critical role in RNA polymerase II (RNAPII) assembly and subsequent nuclear import. We previously identified a synthetic lethal interaction between a mutant lacking the carboxy-terminal 106-amino acid tail of Npa3 (npa3ΔC) and a bud27Δ mutant. As the prefoldin-like Bud27 protein participates in ribosome biogenesis and translation, we hypothesized that Npa3 may also regulate these biological processes. We investigated this proposal by using Saccharomyces cerevisiae strains episomally expressing either wild-type Npa3 or hypomorphic mutants (Npa3ΔC, Npa3K16R, and Npa3G70A). The Npa3ΔC mutant fully supports RNAPII nuclear localization and activity. However, the Npa3K16R and Npa3G70A mutants only partially mediate RNAPII nuclear targeting and exhibit a higher reduction in Npa3 function. Cell proliferation in these strains displayed an increased sensitivity to protein synthesis inhibitors hygromycin B and geneticin/G418 (npa3G70A > npa3K16R > npa3ΔC > NPA3 cells) but not to transcriptional elongation inhibitors 6-azauracil, mycophenolic acid or 1,10-phenanthroline. In all three mutant strains, the increase in sensitivity to both aminoglycoside antibiotics was totally rescued by expressing NPA3. Protein synthesis, visualized by quantifying puromycin incorporation into nascent-polypeptide chains, was markedly more sensitive to hygromycin B inhibition in npa3ΔC, npa3K16R, and npa3G70A than NPA3 cells. Notably, high-copy expression of the TIF11 gene, that encodes the eukaryotic translation initiation factor 1A (eIF1A) protein, completely suppressed both phenotypes (of reduced basal cell growth and increased sensitivity to hygromycin B) in npa3ΔC cells but not npa3K16R or npa3G70A cells. We conclude that Npa3 plays a critical RNAPII-independent and previously unrecognized role in translation initiation.

The molecular basis of translation initiation and its regulation in eukaryotes

The regulation of gene expression is fundamental for life. Whereas the role of transcriptional regulation of gene expression has been studied for several decades, it has been clear over the past two decades that post-transcriptional regulation of gene expression, of which translation regulation is a major part, can be equally important. Translation can be divided into four main stages: initiation, elongation, termination and ribosome recycling. Translation is controlled mainly during its initiation, a process which culminates in a ribosome positioned with an initiator tRNA over the start codon and, thus, ready to begin elongation of the protein chain. mRNA translation has emerged as a powerful tool for the development of innovative therapies, yet the detailed mechanisms underlying the complex process of initiation remain unclear. Recent studies in yeast and mammals have started to shed light on some previously unclear aspects of this process. In this Review, we discuss the current state of knowledge on eukaryotic translation initiation and its regulation in health and disease. Specifically, we focus on recent advances in understanding the processes involved in assembling the 43S pre-initiation complex and its recruitment by the cap-binding complex eukaryotic translation initiation factor 4F (eIF4F) at the 5' end of mRNA. In addition, we discuss recent insights into ribosome scanning along the 5' untranslated region of mRNA and selection of the start codon, which culminates in joining of the 60S large subunit and formation of the 80S initiation complex.

Molecular architecture of 40S translation initiation complexes on the hepatitis C virus IRES

Hepatitis C virus mRNA contains an internal ribosome entry site (IRES) that mediates end-independent translation initiation, requiring a subset of eukaryotic initiation factors (eIFs). Biochemical studies revealed that direct binding of the IRES to the 40S ribosomal subunit places the initiation codon into the P site, where it base pairs with eIF2-bound Met-tRNAiMet forming a 48S initiation complex. Subsequently, eIF5 and eIF5B mediate subunit joining, yielding an elongation-competent 80S ribosome. Initiation can also proceed without eIF2, in which case Met-tRNAiMet is recruited directly by eIF5B. However, the structures of initiation complexes assembled on the HCV IRES, the transitions between different states, and the accompanying conformational changes have remained unknown. To fill these gaps, we now obtained cryo-EM structures of IRES initiation complexes, at resolutions up to 3.5 Å, that cover all major stages from the initial ribosomal association, through eIF2-containing 48S initiation complexes, to eIF5B-containing complexes immediately prior to subunit joining. These structures provide insights into the dynamic network of 40S/IRES contacts, highlight the role of IRES domain II, and reveal conformational changes that occur during the transition from eIF2- to eIF5B-containing 48S complexes and prepare them for subunit joining.

Selection of start codon during mRNA scanning in eukaryotic translation initiation

Accurate and high-speed scanning and subsequent selection of the correct start codon are important events in protein synthesis. Eukaryotic mRNAs have long 5′ UTRs that are inspected for the presence of a start codon by the ribosomal 48S pre-initiation complex (PIC). However, the conformational state of the 48S PIC required for inspecting every codon is not clearly understood. Here, atomistic molecular dynamics (MD) simulations and energy calculations suggest that the scanning conformation of 48S PIC may reject all but 4 (GUG, CUG, UUG and ACG) of the 63 non-AUG codons, and initiation factor eIF1 is crucial for this discrimination. We provide insights into the possible role of initiation factors eIF1, eIF1A, eIF2α and eIF2β in scanning. Overall, the study highlights how the scanning conformation of ribosomal 48S PIC acts as a coarse selectivity checkpoint for start codon selection and scans long 5′ UTRs in eukaryotic mRNAs with accuracy and high speed.

Molecular simulations of start codon selection by the eukaryotic ribosome during mRNA scanning provide further insight into high speed of scanning and how initiation factors contribute toward codon-anticodon-ribosome network stability.

Conformational rearrangements upon start codon recognition in human 48S translation initiation complex

Selection of the translation start codon is a key step during protein synthesis in human cells. We obtained cryo-EM structures of human 48S initiation complexes and characterized the intermediates of codon recognition by kinetic methods using eIF1A as a reporter. Both approaches capture two distinct ribosome populations formed on an mRNA with a cognate AUG codon in the presence of eIF1, eIF1A, eIF2–GTP–Met-tRNA_i^Met and eIF3. The ‘open’ 40S subunit conformation differs from the human 48S scanning complex and represents an intermediate preceding the codon recognition step. The ‘closed’ form is similar to reported structures of complexes from yeast and mammals formed upon codon recognition, except for the orientation of eIF1A, which is unique in our structure. Kinetic experiments show how various initiation factors mediate the population distribution of open and closed conformations until 60S subunit docking. Our results provide insights into the timing and structure of human translation initiation intermediates and suggest the differences in the mechanisms of start codon selection between mammals and yeast.

Eukaryotic initiation factor 4F promotes a reorientation of eukaryotic initiation factor 3 binding on the 5' and the 3' UTRs of barley yellow dwarf virus mRNA

Viral mRNAs that lack a 5′ m⁷GTP cap and a 3′ poly-A tail rely on structural elements in their untranslated regions (UTRs) to form unique RNA-protein complexes that regulate viral translation. Recent studies of the barley yellow dwarf virus (BYDV) have revealed eukaryotic initiation factor 3 (eIF3) plays a significant role in facilitating communication between its 5′ and 3′ UTRs by binding both UTRs simultaneously. This report uses in vitro translation assays, fluorescence anisotropy binding assays, and selective 2′-hydroxyl acylation analyzed by primer extension (SHAPE) footprinting to identify secondary structures that are selectively interacting with eIF3. SHAPE data also show that eIF3 alters its interaction with BYDV structures when another factor crucial for BYDV translation, eIF4F, is introduced by the 3′ BYDV translational enhancer (BTE). The observed BTE and eIF4F-induced shift of eIF3 position on the 5’ UTR and the translational effects of altering eIF3-binding structures (SLC and SLII) support a new model for BYDV translation initiation that requires the reorientation of eIF3 on BYDV UTRs. This eIF3 function in BYDV translation initiation is both reminiscent of and distinct from eIF3–RNA interactions found in other non-canonically translating mRNAs (e.g. HCV). This characterization of a new role in translation initiation expands the known functionality of eIF3 and may be broadly applicable to other non-canonically translating mRNAs.

Altered proteome in translation initiation fidelity defective eIF5G31R mutant causes oxidative stress and DNA damage

The recognition of the AUG start codon and selection of an open reading frame (ORF) is fundamental to protein biosynthesis. Defect in the fidelity of start codon selection adversely affect proteome and have a pleiotropic effect on cellular function. Using proteomic techniques, we identified differential protein abundance in the translation initiation fidelity defective eIF5^G31R mutant that initiates translation using UUG codon in addition to the AUG start codon. Consistently, the eIF5^G31R mutant altered proteome involved in protein catabolism, nucleotide biosynthesis, lipid biosynthesis, carbohydrate metabolism, oxidation–reduction pathway, autophagy and re-programs the cellular pathways. The utilization of the upstream UUG codons by the eIF5^G31R mutation caused downregulation of uridylate kinase expression, sensitivity to hydroxyurea, and DNA damage. The eIF5^G31R mutant cells showed lower glutathione levels, high ROS activity, and sensitivity to H₂O₂.

Ribosomal RNA 2'-O-methylations regulate translation by impacting ribosome dynamics

SignificanceThe presence of RNA chemical modifications has long been known, but their precise molecular consequences remain unknown. 2'-O-methylation is an abundant modification that exists in RNA in all domains of life. Ribosomal RNA (rRNA) represents a functionally important RNA that is heavily modified by 2'-O-methylations. Although abundant at functionally important regions of the rRNA, the contribution of 2'-O-methylations to ribosome activities is unknown. By establishing a method to disturb rRNA 2'-O-methylation patterns, we show that rRNA 2'-O-methylations affect the function and fidelity of the ribosome and change the balance between different ribosome conformational states. Our work links 2'-O-methylation to ribosome dynamics and defines a set of critical rRNA 2'-O-methylations required for ribosome biogenesis and others that are dispensable.

Rebirth of the translational machinery: The importance of recycling ribosomes

Translation of the genetic code occurs in a cycle where ribosomes engage mRNAs, synthesize protein, and then disengage in order to repeat the process again. The final part of this process-ribosome recycling, where ribosomes dissociate from mRNAs-involves a complex molecular choreography of specific protein factors to remove the large and small subunits of the ribosome in a coordinated fashion. Errors in this process can lead to the accumulation of ribosomes at stop codons or translation of downstream open reading frames (ORFs). Ribosome recycling is also critical when a ribosome stalls during the elongation phase of translation and must be rescued to allow continued translation of the mRNA. Here we discuss the molecular interactions that drive ribosome recycling, and their regulation in the cell. We also examine the consequences of inefficient recycling with regards to disease, and its functional roles in synthesis of novel peptides, regulation of gene expression, and control of mRNA-associated proteins. Alterations in ribosome recycling efficiency have the potential to impact many cellular functions but additional work is needed to understand how this regulatory power is utilized.

Mechanistic convergence across initiation sites for RAN translation in fragile X associated tremor ataxia syndrome

Repeat associated non-AUG (RAN) translation of CGG repeats in the 5'UTR of FMR1 produces toxic proteins that contribute to fragile X-associated tremor/ataxia syndrome (FXTAS) pathogenesis. The most abundant RAN product, FMRpolyG, initiates predominantly at an ACG upstream of the repeat. Accurate FMRpolyG measurements in FXTAS patients are lacking. We used data-dependent acquisition and parallel reaction monitoring (PRM) mass spectrometry coupled with stable isotope labeled standard peptides to identify signature FMRpolyG fragments in patient samples. Following immunoprecipitation, PRM detected FMRpolyG signature peptides in transfected cells, and FXTAS tissues and cells, but not in controls. We identified two amino-terminal peptides: an ACG-initiated Ac-MEAPLPGGVR and a GUG-initiated Ac-TEAPLPGGVR, as well as evidence for RAN translation initiation within the CGG repeat itself in two reading frames. Initiation at all sites increased following cellular stress, decreased following eIF1 overexpression and was eIF4A and M7G cap-dependent. These data demonstrate that FMRpolyG is quantifiable in human samples and FMR1 RAN translation initiates via similar mechanisms for near-cognate codons and within the repeat through processes dependent on available initiation factors and cellular environment.

Dynamic competition between SARS-CoV-2 NSP1 and mRNA on the human ribosome inhibits translation initiation

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a beta-CoV that recently emerged as a human pathogen and is the causative agent of the COVID-19 pandemic. A molecular framework of how the virus manipulates host cellular machinery to facilitate infection remains unclear. Here, we focus on SARS-CoV-2 NSP1, which is proposed to be a virulence factor that inhibits protein synthesis by directly binding the human ribosome. We demonstrate biochemically that NSP1 inhibits translation of model human and SARS-CoV-2 messenger RNAs (mRNAs). NSP1 specifically binds to the small (40S) ribosomal subunit, which is required for translation inhibition. Using single-molecule fluorescence assays to monitor NSP1-40S subunit binding in real time, we determine that eukaryotic translation initiation factors (eIFs) allosterically modulate the interaction of NSP1 with ribosomal preinitiation complexes in the absence of mRNA. We further elucidate that NSP1 competes with RNA segments downstream of the start codon to bind the 40S subunit and that the protein is unable to associate rapidly with 80S ribosomes assembled on an mRNA. Collectively, our findings support a model where NSP1 proteins from viruses in at least two subgenera of beta-CoVs associate with the open head conformation of the 40S subunit to inhibit an early step of translation, by preventing accommodation of mRNA within the entry channel.

Dynamic competition between SARS-CoV-2 NSP1 and mRNA on the human ribosome inhibits translation initiation

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a beta-CoV that recently emerged as a human pathogen and is the causative agent of the COVID-19 pandemic. A molecular framework of how the virus manipulates host cellular machinery to facilitate infection remains unclear. Here, we focus on SARS-CoV-2 NSP1, which is proposed to be a virulence factor that inhibits protein synthesis by directly binding the human ribosome. We demonstrate biochemically that NSP1 inhibits translation of model human and SARS-CoV-2 messenger RNAs (mRNAs). NSP1 specifically binds to the small (40S) ribosomal subunit, which is required for translation inhibition. Using single-molecule fluorescence assays to monitor NSP1-40S subunit binding in real time, we determine that eukaryotic translation initiation factors (eIFs) allosterically modulate the interaction of NSP1 with ribosomal preinitiation complexes in the absence of mRNA. We further elucidate that NSP1 competes with RNA segments downstream of the start codon to bind the 40S subunit and that the protein is unable to associate rapidly with 80S ribosomes assembled on an mRNA. Collectively, our findings support a model where NSP1 proteins from viruses in at least two subgenera of beta-CoVs associate with the open head conformation of the 40S subunit to inhibit an early step of translation, by preventing accommodation of mRNA within the entry channel.

eIF1 discriminates against suboptimal initiation sites to prevent excessive uORF translation genome-wide

The translation preinitiation complex (PIC) scans the mRNA for an AUG codon in a favorable context. Previous findings suggest that the factor eIF1 discriminates against non-AUG start codons by impeding full accommodation of Met-tRNA_i in the P site of the 40S ribosomal subunit, necessitating eIF1 dissociation for start codon selection. Consistent with this, yeast eIF1 substitutions that weaken its binding to the PIC increase initiation at UUG codons on a mutant his4 mRNA and particular synthetic mRNA reporters; and also at the AUG start codon of the mRNA for eIF1 itself owing to its poor Kozak context. It was not known however whether such eIF1 mutants increase initiation at suboptimal start codons genome-wide. By ribosome profiling, we show that the eIF1-L96P variant confers increased translation of numerous upstream open reading frames (uORFs) initiating with either near-cognate codons (NCCs) or AUGs in poor context. The increased uORF translation is frequently associated with the reduced translation of the downstream main coding sequences (CDS). Initiation is also elevated at certain NCCs initiating amino-terminal extensions, including those that direct mitochondrial localization of the GRS1 and ALA1 products, and at a small set of main CDS AUG codons with especially poor context, including that of eIF1 itself. Thus, eIF1 acts throughout the yeast translatome to discriminate against NCC start codons and AUGs in poor context; and impairing this function enhances the repressive effects of uORFs on CDS translation and alters the ratios of protein isoforms translated from near-cognate versus AUG start codons.

Eukaryotic initiation factor (eIF) 3 mediates Barley Yellow Dwarf Viral mRNA 3'-5' UTR interactions and 40S ribosomal subunit binding to facilitate cap-independent translation

Barley Yellow Dwarf Virus (BYDV) is a positive strand RNA virus that lacks the canonical 5′ 7-methylguanosine cap and a 3′ poly-A tail. Instead, BYDV utilizes a cruciform cap independent translation element (CITE) in its 3′UTR RNA (BYDV-like CITE or BTE) that binds eukaryotic translation initiation factor (eIF) 4F and recruits 40S ribosomal subunits in the presence of active helicase factors (eIF4A, eIF4B, eIF4F and ATP). A long-range, 5-nucleotide, base-pairing kissing loop interaction between the 3′BTE and a 5′UTR stem-loop is necessary for translation to initiate. The 40S–eIF complex does not bind to the BYDV 5′UTR, suggesting the involvement of additional factors. We identified eIF3 as a component of the 3′BTE recruited complex using affinity-tagged 3′BTE RNA pull-down assays. Fluorescence anisotropy binding and gel shift assays showed that the 3′BTE and 5′UTR RNAs can simultaneously and non-competitively bind eIF3 in the presence of active helicase factors forming a single, macromolecular complex. Further, quantitative studies showed eIF3 increased recruitment of the 40S subunit by more than 25-fold. We propose a new role for eIF3, where eIF3 bridges BYDV’s UTRs, stabilizes the long-range 5′-3′ interaction, and facilitates recruitment of the 40S–eIF complex to the 5′UTR, leading to translation initiation.

Approaches for measuring the dynamics of RNA-protein interactions

RNA-protein interactions are pivotal for the regulation of gene expression from bacteria to human. RNA-protein interactions are dynamic; they change over biologically relevant timescales. Understanding the regulation of gene expression at the RNA level therefore requires knowledge of the dynamics of RNA-protein interactions. Here, we discuss the main experimental approaches to measure dynamic aspects of RNA-protein interactions. We cover techniques that assess dynamics of cellular RNA-protein interactions that accompany biological processes over timescales of hours or longer and techniques measuring the kinetic dynamics of RNA-protein interactions in vitro. This article is categorized under: RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications RNA Interactions with Proteins and Other Molecules > Protein-RNA Recognition RNA Interactions with Proteins and Other Molecules > RNA-Protein Complexes RNA Evolution and Genomics > Ribonomics.

Temperature-dependent regulation of upstream open reading frame translation in S. cerevisiae

BACKGROUND

Translation of an mRNA in eukaryotes starts at an AUG codon in most cases, but near-cognate codons (NCCs) such as UUG, ACG, and AUU can also be used as start sites at low levels in Saccharomyces cerevisiae. Initiation from NCCs or AUGs in the 5'-untranslated regions (UTRs) of mRNAs can lead to translation of upstream open reading frames (uORFs) that might regulate expression of the main ORF (mORF). Although there is some circumstantial evidence that the translation of uORFs can be affected by environmental conditions, little is known about how it is affected by changes in growth temperature.

RESULTS

Using reporter assays, we found that changes in growth temperature can affect translation from NCC start sites in yeast cells, suggesting the possibility that gene expression could be regulated by temperature by altering use of different uORF start codons. Using ribosome profiling, we provide evidence that growth temperature regulates the efficiency of translation of nearly 200 uORFs in S. cerevisiae. Of these uORFs, most that start with an AUG codon have increased translational efficiency at 37 °C relative to 30 °C and decreased efficiency at 20 °C. For translationally regulated uORFs starting with NCCs, we did not observe a general trend for the direction of regulation as a function of temperature, suggesting mRNA-specific features can determine the mode of temperature-dependent regulation. Consistent with this conclusion, the position of the uORFs in the 5'-leader relative to the 5'-cap and the start codon of the main ORF correlates with the direction of temperature-dependent regulation of uORF translation. We have identified several novel cases in which changes in uORF translation are inversely correlated with changes in the translational efficiency of the downstream main ORF. Our data suggest that translation of these mRNAs is subject to temperature-dependent, uORF-mediated regulation.

CONCLUSIONS

Our data suggest that alterations in the translation of specific uORFs by temperature can regulate gene expression in S. cerevisiae.

MEHMO syndrome mutation EIF2S3-I259M impairs initiator Met-tRNAiMet binding to eukaryotic translation initiation factor eIF2

The heterotrimeric eukaryotic translation initiation factor (eIF) 2 plays critical roles in delivering initiator Met-tRNA_i^Met to the 40S ribosomal subunit and in selecting the translation initiation site. Genetic analyses of patients with MEHMO syndrome, an X-linked intellectual disability syndrome, have identified several unique mutations in the EIF2S3 gene that encodes the γ subunit of eIF2. To gain insights into the molecular consequences of MEHMO syndrome mutations on eIF2 function, we generated a yeast model of the human eIF2γ-I259M mutant, previously identified in a patient with MEHMO syndrome. The corresponding eIF2γ-I318M mutation impaired yeast cell growth and derepressed GCN4 expression, an indicator of defective eIF2–GTP–Met-tRNA_i^Met complex formation, and, likewise, overexpression of human eIF2γ-I259M derepressed ATF4 messenger RNA translation in human cells. The yeast eIF2γ-I318M mutation also increased initiation from near-cognate start codons. Biochemical analyses revealed a defect in Met-tRNA_i^Met binding to the mutant yeast eIF2 complexes in vivo and in vitro. Overexpression of tRNA_i^Met restored Met-tRNA_i^Met binding to eIF2 in vivo and rescued the growth defect in the eIF2γ-I318M strain. Based on these findings and the structure of eIF2, we propose that the I259M mutation impairs Met-tRNA_i^Met binding, causing altered control of protein synthesis that underlies MEHMO syndrome.

Role of aIF1 in Pyrococcus abyssi translation initiation

In archaeal translation initiation, a preinitiation complex (PIC) made up of aIF1, aIF1A, the ternary complex (TC, e/aIF2-GTP-Met-tRNA_i^Met) and mRNA bound to the small ribosomal subunit is responsible for start codon selection. Many archaeal mRNAs contain a Shine-Dalgarno (SD) sequence allowing the PIC to be prepositioned in the vicinity of the start codon. Nevertheless, cryo-EM studies have suggested local scanning to definitely establish base pairing of the start codon with the tRNA anticodon. Here, using fluorescence anisotropy, we show that aIF1 and mRNA have synergistic binding to the Pyrococcus abyssi 30S. Stability of 30S:mRNA:aIF1 strongly depends on the SD sequence. Further, toeprinting experiments show that aIF1-containing PICs display a dynamic conformation with the tRNA not firmly accommodated in the P site. AIF1-induced destabilization of the PIC is favorable for proofreading erroneous initiation complexes. After aIF1 departure, the stability of the PIC increases reflecting initiator tRNA fully base-paired to the start codon. Altogether, our data support the idea that some of the main events governing start codon selection in eukaryotes and archaea occur within a common structural and functional core. However, idiosyncratic features in loop 1 sequence involved in 30S:mRNA binding suggest adjustments of e/aIF1 functioning in the two domains.

Ribosomal mutation in helix 32 of 18S rRNA alters fidelity of eukaryotic translation start site selection

The 40S ribosome plays a critical role in start codon selection. To gain insights into the role of its 18S rRNA in start codon selection, a suppressor screen was performed that suppressed the preferential UUG start codon recognition (Suppressor of initiation codon: Sui^- phenotype) associated with the eIF5^G31R mutant. The C1209U mutation in helix h32 of 18S rRNA was found to suppress the Sui^- and Gcn^- (failure to derepress GCN4 expression) phenotype of the eIF5^G31R mutant. The C1209U mutation suppressed Sui^- and Gcd^- (constitutive derepression of GCN4 expression) phenotype of eIF2β^S264Y , eIF1^K60E , and eIF1A-ΔC mutation. We propose that the C1209U mutation in 40S ribosomal may perturb the premature head rotation in 'Closed/P_IN ' state and enhance the stringency of translation start site selection.

Toward a Kinetic Understanding of Eukaryotic Translation

The eukaryotic translation pathway has been studied for more than four decades, but the molecular mechanisms that regulate each stage of the pathway are not completely defined. This is in part because we have very little understanding of the kinetic framework for the assembly and disassembly of pathway intermediates. Steps of the pathway are thought to occur in the subsecond to second time frame, but most assays to monitor these events require minutes to hours to complete. Understanding translational control in sufficient detail will therefore require the development of assays that can precisely monitor the kinetics of the translation pathway in real time. Here, we describe the translation pathway from the perspective of its kinetic parameters, discuss advances that are helping us move toward the goal of a rigorous kinetic understanding, and highlight some of the challenges that remain.

Protein Synthesis Initiation in Eukaryotic Cells

This review summarizes our current understanding of the major pathway for the initiation phase of protein synthesis in eukaryotic cells, with a focus on recent advances. We describe the major scanning or messenger RNA (mRNA) m⁷G cap-dependent mechanism, which is a highly coordinated and stepwise regulated process that requires the combined action of at least 12 distinct translation factors with initiator transfer RNA (tRNA), ribosomes, and mRNAs. We limit our review to studies involving either mammalian or budding yeast cells and factors, as these represent the two best-studied experimental systems, and only include a reference to other organisms where particular insight has been gained. We close with a brief description of what we feel are some of the major unknowns in eukaryotic initiation.

Translational initiation factor eIF5 replaces eIF1 on the 40S ribosomal subunit to promote start-codon recognition

In eukaryotic translation initiation, AUG recognition of the mRNA requires accommodation of Met-tRNAi in a 'PIN' state, which is antagonized by the factor eIF1. eIF5 is a GTPase activating protein (GAP) of eIF2 that additionally promotes stringent AUG selection, but the molecular basis of its dual function was unknown. We present a cryo-electron microscopy (cryo-EM) reconstruction of a yeast 48S pre-initiation complex (PIC), at an overall resolution of 3.0 Å, featuring the N-terminal domain (NTD) of eIF5 bound to the 40S subunit at the location vacated by eIF1. eIF5 interacts with and allows a more accommodated orientation of Met-tRNAi. Substitutions of eIF5 residues involved in the eIF5-NTD/tRNAi interaction influenced initiation at near-cognate UUG codonsin vivo, and the closed/open PIC conformation in vitro, consistent with direct stabilization of the codon:anticodon duplex by the wild-type eIF5-NTD. The present structure reveals the basis for a key role of eIF5 in start-codon selection.

eIF1 Loop 2 interactions with Met-tRNAi control the accuracy of start codon selection by the scanning preinitiation complex

The eukaryotic 43S preinitiation complex (PIC), bearing initiator methionyl transfer RNA (Met-tRNA_i) in a ternary complex (TC) with eukaryotic initiation factor 2 (eIF2)-GTP, scans the mRNA leader for an AUG codon in favorable context. AUG recognition evokes rearrangement from an open PIC conformation with TC in a "P_OUT" state to a closed conformation with TC more tightly bound in a "P_IN" state. eIF1 binds to the 40S subunit and exerts a dual role of enhancing TC binding to the open PIC conformation while antagonizing the P_IN state, necessitating eIF1 dissociation for start codon selection. Structures of reconstituted PICs reveal juxtaposition of eIF1 Loop 2 with the Met-tRNA_i D loop in the P_IN state and predict a distortion of Loop 2 from its conformation in the open complex to avoid a clash with Met-tRNA_i We show that Ala substitutions in Loop 2 increase initiation at both near-cognate UUG codons and AUG codons in poor context. Consistently, the D71A-M74A double substitution stabilizes TC binding to 48S PICs reconstituted with mRNA harboring a UUG start codon, without affecting eIF1 affinity for 40S subunits. Relatively stronger effects were conferred by arginine substitutions; and no Loop 2 substitutions perturbed the rate of TC loading on scanning 40S subunits in vivo. Thus, Loop 2-D loop interactions specifically impede Met-tRNA_i accommodation in the P_IN state without influencing the P_OUT mode of TC binding; and Arg substitutions convert the Loop 2-tRNA_i clash to an electrostatic attraction that stabilizes P_IN and enhances selection of poor start codons in vivo.

Translation initiation by cap-dependent ribosome recruitment: Recent insights and open questions

Gene expression universally relies on protein synthesis, where ribosomes recognize and decode the messenger RNA template by cycling through translation initiation, elongation, and termination phases. All aspects of translation have been studied for decades using the tools of biochemistry and molecular biology available at the time. Here, we focus on the mechanism of translation initiation in eukaryotes, which is remarkably more complex than prokaryotic initiation and is the target of multiple types of regulatory intervention. The "consensus" model, featuring cap-dependent ribosome entry and scanning of mRNA leader sequences, represents the predominantly utilized initiation pathway across eukaryotes, although several variations of the model and alternative initiation mechanisms are also known. Recent advances in structural biology techniques have enabled remarkable molecular-level insights into the functional states of eukaryotic ribosomes, including a range of ribosomal complexes with different combinations of translation initiation factors that are thought to represent bona fide intermediates of the initiation process. Similarly, high-throughput sequencing-based ribosome profiling or "footprinting" approaches have allowed much progress in understanding the elongation phase of translation, and variants of them are beginning to reveal the remaining mysteries of initiation, as well as aspects of translation termination and ribosomal recycling. A current view on the eukaryotic initiation mechanism is presented here with an emphasis on how recent structural and footprinting results underpin axioms of the consensus model. Along the way, we further outline some contested mechanistic issues and major open questions still to be addressed. This article is categorized under: Translation > Translation Mechanisms Translation > Translation Regulation RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications.

Inteins: Localized Distribution, Gene Regulation, and Protein Engineering for Biological Applications

Inteins are self-splicing polypeptides with an ability to excise themselves from flanking host protein regions with remarkable precision; in the process, they ligate flanked host protein fragments. Inteins are distributed sporadically across all three domains of life (bacteria, archaea, and unicellular eukaryotes). However, their apparent localized distribution in DNA replication, repair, and recombination proteins (the 3Rs), particularly in bacteria and archaea, is enigmatic. Our understanding of the localized distribution of inteins in the 3Rs, and their possible regulatory role in such distribution, is still only partial. Nevertheless, understanding the chemistry of post-translational self-splicing of inteins has opened up opportunities for protein chemists to modify, manipulate, and bioengineer proteins. Protein-splicing technology is adapted to a wide range of applications, starting with untagged protein purification, site-specific protein labeling, protein biotinylation, isotope incorporation, peptide cyclization, as an antimicrobial target, and so on. This review is focused on the chemistry of splicing; the localized distribution of inteins, particularly in the 3Rs and their possible role in regulating host protein function; and finally, the use of protein-splicing technology in various protein engineering applications.

Yeast eIF4A enhances recruitment of mRNAs regardless of their structural complexity

eIF4A is a DEAD-box RNA-dependent ATPase thought to unwind RNA secondary structure in the 5'-untranslated regions (UTRs) of mRNAs to promote their recruitment to the eukaryotic translation pre-initiation complex (PIC). We show that eIF4A's ATPase activity is markedly stimulated in the presence of the PIC, independently of eIF4E•eIF4G, but dependent on subunits i and g of the heteromeric eIF3 complex. Surprisingly, eIF4A accelerated the rate of recruitment of all mRNAs tested, regardless of their degree of structural complexity. Structures in the 5'-UTR and 3' of the start codon synergistically inhibit mRNA recruitment in a manner relieved by eIF4A, indicating that the factor does not act solely to melt hairpins in 5'-UTRs. Our findings that eIF4A functionally interacts with the PIC and plays important roles beyond unwinding 5'-UTR structure is consistent with a recent proposal that eIF4A modulates the conformation of the 40S ribosomal subunit to promote mRNA recruitment.

Defect in the GTPase activating protein (GAP) function of eIF5 causes repression of GCN4 translation

In eukaryotes, the eIF5 protein plays an important role in translation start site selection by providing the GAP (GTPase activating protein) function. However, in yeast translation initiation fidelity defective eIF5G31R mutant causes preferential utilization of UUG as initiation codon and is termed as Suppressor of initiation codon (Sui-) phenotype due to its hyper GTPase activity. The eIF5G31R mutant dominantly represses GCN4 expression and confers sensitivity to 3-Amino-1,2,4-Trizole (3AT) induced starvation. The down-regulation of the GCN4 expression (Gcn- phenotype) in the eIF5G31R mutant was not because of leaky scanning defects; rather was due to the utilization of upUUG initiation codons at the 5' regulatory region present between uORF1 and the main GCN4 ORF.

Toward the mechanism of eIF4F-mediated ribosomal attachment to mammalian capped mRNAs

Ribosomal attachment to mammalian capped mRNAs is achieved through the cap-eukaryotic initiation factor 4E (eIF4E)-eIF4G-eIF3-40S chain of interactions, but the mechanism by which mRNA enters the mRNA-binding channel of the 40S subunit remains unknown. To investigate this process, we recapitulated initiation on capped mRNAs in vitro using a reconstituted translation system. Formation of initiation complexes at 5'-terminal AUGs was stimulated by the eIF4E-cap interaction and followed "the first AUG" rule, indicating that it did not occur by backward scanning. Initiation complexes formed even at the very 5' end of mRNA, implying that Met-tRNAi (Met) inspects mRNA from the first nucleotide and that initiation does not have a "blind spot." In assembled initiation complexes, the cap was no longer associated with eIF4E. Omission of eIF4A or disruption of eIF4E-eIF4G-eIF3 interactions converted eIF4E into a specific inhibitor of initiation on capped mRNAs. Taken together, these results are consistent with the model in which eIF4E-eIF4G-eIF3-40S interactions place eIF4E at the leading edge of the 40S subunit, and mRNA is threaded into the mRNA-binding channel such that Met-tRNAi (Met) can inspect it from the first nucleotide. Before entering, eIF4E likely dissociates from the cap to overcome steric hindrance. We also found that the m(7)G cap specifically interacts with eIF3l.

Molecular Landscape of the Ribosome Pre-initiation Complex during mRNA Scanning: Structural Role for eIF3c and Its Control by eIF5

During eukaryotic translation initiation, eIF3 binds the solvent-accessible side of the 40S ribosome and recruits the gate-keeper protein eIF1 and eIF5 to the decoding center. This is largely mediated by the N-terminal domain (NTD) of eIF3c, which can be divided into three parts: 3c0, 3c1, and 3c2. The N-terminal part, 3c0, binds eIF5 strongly but only weakly to the ribosome-binding surface of eIF1, whereas 3c1 and 3c2 form a stoichiometric complex with eIF1. 3c1 contacts eIF1 through Arg-53 and Leu-96, while 3c2 faces 40S protein uS15/S13, to anchor eIF1 to the scanning pre-initiation complex (PIC). We propose that the 3c0:eIF1 interaction diminishes eIF1 binding to the 40S, whereas 3c0:eIF5 interaction stabilizes the scanning PIC by precluding this inhibitory interaction. Upon start codon recognition, interactions involving eIF5, and ultimately 3c0:eIF1 association, facilitate eIF1 release. Our results reveal intricate molecular interactions within the PIC, programmed for rapid scanning-arrest at the start codon.

Mechanism and Regulation of Protein Synthesis in Saccharomyces cerevisiae

In this review, we provide an overview of protein synthesis in the yeast Saccharomyces cerevisiae The mechanism of protein synthesis is well conserved between yeast and other eukaryotes, and molecular genetic studies in budding yeast have provided critical insights into the fundamental process of translation as well as its regulation. The review focuses on the initiation and elongation phases of protein synthesis with descriptions of the roles of translation initiation and elongation factors that assist the ribosome in binding the messenger RNA (mRNA), selecting the start codon, and synthesizing the polypeptide. We also examine mechanisms of translational control highlighting the mRNA cap-binding proteins and the regulation of GCN4 and CPA1 mRNAs.

Eukaryotic translation initiation factor 3 plays distinct roles at the mRNA entry and exit channels of the ribosomal preinitiation complex

Eukaryotic translation initiation factor 3 (eIF3) is a central player in recruitment of the pre-initiation complex (PIC) to mRNA. We probed the effects on mRNA recruitment of a library of S. cerevisiae eIF3 functional variants spanning its 5 essential subunits using an in vitro-reconstituted system. Mutations throughout eIF3 disrupt its interaction with the PIC and diminish its ability to accelerate recruitment to a native yeast mRNA. Alterations to the eIF3a CTD and eIF3b/i/g significantly slow mRNA recruitment, and mutations within eIF3b/i/g destabilize eIF2•GTP•Met-tRNA_i binding to the PIC. Using model mRNAs lacking contacts with the 40S entry or exit channels, we uncovered a critical role for eIF3 requiring the eIF3a NTD, in stabilizing mRNA interactions at the exit channel, and an ancillary role at the entry channel requiring residues of the eIF3a CTD. These functions are redundant: defects at each channel can be rescued by filling the other channel with mRNA.

DOI:http://dx.doi.org/10.7554/eLife.20934.001

Cells use the genetic information stored within genes to build proteins, which are largely responsible for performing the molecular tasks essential for life. The ribosome is the molecular machine that translates the information within genes to assemble proteins in all cells, from bacteria to humans. To make a protein, the corresponding gene is first copied to make molecules of messenger ribonucleic acid (or mRNA for short). Then the ribosome binds to the mRNA in a process called translation initiation.

Cells tightly regulate translation initiation so that they can decide which proteins to make, according to their needs and in response to changes in the environment. In fact, regulation of translation initiation is often disrupted during viral infections, cancer and other human diseases. A set of proteins called translation initiation factors drive translation initiation; the largest and least understood of these is called eIF3.

Cells are unable to load the mRNA onto the ribosome without eIF3, which has two “arms” that sit near where the mRNA enters and exits the ribosome. Aitken et al. used mutant forms of eIF3 from genetically modified yeast to investigate how the arms of the protein work, and if they help the ribosome hold onto the mRNA.

These experiments show that the two arms of eIF3 have unique roles. One arm sits near where mRNA exits the ribosome and is important for holding onto the mRNA. The other arm – which is near where mRNA enters the ribosome – helps hold the ribosome and other components of the translation machinery together. This arm may also help to open and close the channel through which messenger RNA enters the ribosome. The next challenges are to find out the precise role this arm plays in translation – in particular, how it helps to open and close the channel in the ribosome, and whether this helps the ribosome load the messenger RNA or even move along it.

DOI:http://dx.doi.org/10.7554/eLife.20934.002

What Is the Impact of mRNA 5' TL Heterogeneity on Translational Start Site Selection and the Mammalian Cellular Phenotype?

A major determinant in the efficiency of ribosome loading onto mRNAs is the 5′ TL (transcript leader or 5′ UTR). In addition, elements within this region also impact on start site selection demonstrating that it can modulate the protein readout at both quantitative and qualitative levels. With the increasing wealth of data generated by the mining of the mammalian transcriptome, it has become evident that a genes 5′ TL is not homogeneous but actually exhibits significant heterogeneity. This arises due to the utilization of alternative promoters, and is further compounded by significant variability with regards to the precise transcriptional start sites of each (not to mention alternative splicing). Consequently, the transcript for a protein coding gene is not a unique mRNA, but in-fact a complexed quasi-species of variants whose composition may respond to the changing physiological environment of the cell. Here we examine the potential impact of these events with regards to the protein readout.

Linking Α to Ω: diverse and dynamic RNA-based mechanisms to regulate gene expression by 5'-to-3' communication

Communication between the 5′ and 3′ ends of a eukaryotic messenger RNA (mRNA) or viral genomic RNA is a ubiquitous and important strategy used to regulate gene expression. Although the canonical interaction between initiation factor proteins at the 5′ end of an mRNA and proteins bound to the polyadenylate tail at the 3′ end is well known, in fact there are many other strategies used in diverse ways. These strategies can involve “non-canonical” proteins, RNA structures, and direct RNA-RNA base-pairing between distal elements to achieve 5′-to-3′ communication. Likewise, the communication induced by these interactions influences a variety of processes linked to the use and fate of the RNA that contains them. Recent studies are revealing how dynamic these interactions are, possibly changing in response to cellular conditions or to link various phases of the mRNA’s life, from translation to decay. Thus, 5′-to-3′ communication is about more than just making a closed circle; the RNA elements and associated proteins are key players in controlling gene expression at the post-transcriptional level.

A bacterial homolog YciH of eukaryotic translation initiation factor eIF1 regulates stress-related gene expression and is unlikely to be involved in translation initiation fidelity

YciH is a bacterial protein, homologous to eukaryotic translation initiation factor eIF1. Preceding evidence obtained with the aid of in vitro translation initiation system suggested that it may play a role of a translation initiation factor, ensuring selection against suboptimal initiation complexes. Here we studied the effect of Escherichia coli yciH gene inactivation on translation of model mRNAs. Neither the translation efficiency of leaderless mRNAs, nor mRNAs with non AUG start codons, was found to be affected by YciH in vivo. Comparative proteome analysis revealed that yciH gene knockout leads to a more than fold2- increase in expression of 66 genes and a more than fold2- decrease in the expression of 20 genes. Analysis of these gene sets allowed us to suggest a role of YciH as an inhibitor of translation in a stress response rather than the role of a translation initiation factor.

The molecular choreography of protein synthesis: translational control, regulation, and pathways

Translation of proteins by the ribosome regulates gene expression, with recent results underscoring the importance of translational control. Misregulation of translation underlies many diseases, including cancer and many genetic diseases. Decades of biochemical and structural studies have delineated many of the mechanistic details in prokaryotic translation, and sketched the outlines of eukaryotic translation. However, translation may not proceed linearly through a single mechanistic pathway, but likely involves multiple pathways and branchpoints. The stochastic nature of biological processes would allow different pathways to occur during translation that are biased by the interaction of the ribosome with other translation factors, with many of the steps kinetically controlled. These multiple pathways and branchpoints are potential regulatory nexus, allowing gene expression to be tuned at the translational level. As research focus shifts toward eukaryotic translation, certain themes will be echoed from studies on prokaryotic translation. This review provides a general overview of the dynamic data related to prokaryotic and eukaryotic translation, in particular recent findings with single-molecule methods, complemented by biochemical, kinetic, and structural findings. We will underscore the importance of viewing the process through the viewpoints of regulation, translational control, and heterogeneous pathways.

Principles of start codon recognition in eukaryotic translation initiation

Selection of the correct start codon during initiation of translation on the ribosome is a key event in protein synthesis. In eukaryotic initiation, several factors have to function in concert to ensure that the initiator tRNA finds the cognate AUG start codon during mRNA scanning. The two initiation factors eIF1 and eIF1A are known to provide important functions for the initiation process and codon selection. Here, we have used molecular dynamics free energy calculations to evaluate the energetics of initiator tRNA binding to different near-cognate codons on the yeast 40S ribosomal subunit, in the presence and absence of these two initiation factors. The results show that eIF1 and eIF1A together cause a relatively uniform and high discrimination against near-cognate codons. This works such that eIF1 boosts the discrimination against a first position near-cognate G-U mismatch, and also against a second position A-A base pair, while eIF1A mainly acts on third codon position. The computer simulations further reveal the structural basis of the increased discriminatory effect caused by binding of eIF1 and eIF1A to the 40S ribosomal subunit.

DHX29 reduces leaky scanning through an upstream AUG codon regardless of its nucleotide context

During eukaryotic translation initiation, the 43S preinitiation complex (43S PIC), consisting of the 40S ribosomal subunit, eukaryotic initiation factors (eIFs) and initiator tRNA scans mRNA to find an appropriate start codon. Key roles in the accuracy of initiation codon selection belong to eIF1 and eIF1A, whereas the mammalian-specific DHX29 helicase substantially contributes to ribosomal scanning of structured mRNAs. Here, we show that DHX29 stimulates the recognition of the AUG codon but not the near-cognate CUG codon regardless of its nucleotide context during ribosomal scanning. The stimulatory effect depends on the contact between DHX29 and eIF1A. The unique DHX29 N-terminal domain binds to the ribosomal site near the mRNA entrance, where it contacts the eIF1A OB domain. UV crosslinking assays revealed that DHX29 may rearrange eIF1A and eIF2α in key nucleotide context positions of ribosomal complexes. Interestingly, DHX29 impedes the 48S initiation complex formation in the absence of eIF1A perhaps due to forming a physical barrier that prevents the 43S PIC from loading onto mRNA. Mutational analysis allowed us to split the mRNA unwinding and codon selection activities of DHX29. Thus, DHX29 is another example of an initiation factor contributing to start codon selection.

RAN translation-What makes it run?

Nucleotide-repeat expansions underlie a heterogeneous group of neurodegenerative and neuromuscular disorders for which there are currently no effective therapies. Recently, it was discovered that such repetitive RNA motifs can support translation initiation in the absence of an AUG start codon across a wide variety of sequence contexts, and that the products of these atypical translation initiation events contribute to neuronal toxicity. This review examines what we currently know and do not know about repeat associated non-AUG (RAN) translation in the context of established canonical and non-canonical mechanisms of translation initiation. We highlight recent findings related to RAN translation in three repeat expansion disorders: CGG repeats in fragile X-associated tremor ataxia syndrome (FXTAS), GGGGCC repeats in C9orf72 associated amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) and CAG repeats in Huntington disease. These studies suggest that mechanistic differences may exist for RAN translation dependent on repeat type, repeat reading frame, and the surrounding sequence context, but that for at least some repeats, RAN translation retains a dependence on some of the canonical translational initiation machinery. This article is part of a Special Issue entitled SI:RNA Metabolism in Disease.

eIF2 interactions with initiator tRNA and eIF2B are regulated by post-translational modifications and conformational dynamics

Translation of messenger RNA (mRNA) into proteins is key to eukaryotic gene expression and begins when initiation factor-2 (eIF2) delivers methionyl initiator tRNA (Met-tRNA_i^Met) to ribosomes. This first step is controlled by eIF2B mediating guanine nucleotide exchange on eIF2. We isolated eIF2 from yeast and used mass spectrometry to study the intact complex, and found that eIF2β is the most labile of the three subunits (eIF2α/β/γ). We then compared conformational dynamics of the ternary complex eIF2:GTP:Met-tRNA_i^Met with apo eIF2 using comparative chemical cross-linking. Results revealed high conformational dynamics for eIF2α in apo eIF2 while in the ternary complex all three subunits are constrained. Novel post-translational modifications identified here in both eIF2 and eIF2B were combined with established sites, and located within protein sequences and homology models. We found clustering at subunit interfaces and highly phosphorylated unstructured regions, at the N-terminus of eIF2β, and also between the eIF2Bε core and catalytic domains. We propose that modifications of these unstructured regions have a key role in regulating interactions between eIF2 and eIF2B, as well as other eIFs.

The β-hairpin of 40S exit channel protein Rps5/uS7 promotes efficient and accurate translation initiation in vivo

The eukaryotic 43S pre-initiation complex bearing tRNAi(Met) scans the mRNA leader for an AUG start codon in favorable context. Structural analyses revealed that the β-hairpin of 40S protein Rps5/uS7 protrudes into the 40S mRNA exit-channel, contacting the eIF2∙GTP∙Met-tRNAi ternary complex (TC) and mRNA context nucleotides; but its importance in AUG selection was unknown. We identified substitutions in β-strand-1 and C-terminal residues of yeast Rps5 that reduced bulk initiation, conferred 'leaky-scanning' of AUGs; and lowered initiation fidelity by exacerbating the effect of poor context of the eIF1 AUG codon to reduce eIF1 abundance. Consistently, the β-strand-1 substitution greatly destabilized the 'PIN' conformation of TC binding to reconstituted 43S·mRNA complexes in vitro. Other substitutions in β-hairpin loop residues increased initiation fidelity and destabilized PIN at UUG, but not AUG start codons. We conclude that the Rps5 β-hairpin is as crucial as soluble initiation factors for efficient and accurate start codon recognition.

ABC50 mutants modify translation start codon selection

ATP-binding cassette 50 (ABC50; also known as ABCF1) binds to eukaryotic initiation factor 2 (eIF2) and is required for efficient translation initiation. An essential step of this process is accurate recognition and selection of the initiation codon. It is widely accepted that the presence and movement of eIF1, eIF1A and eIF5 are key factors in modulating the stringency of start-site selection, which normally requires an AUG codon in an appropriate sequence context. In the present study, we show that expression of ABC50 mutants, which cannot hydrolyse ATP, decreases general translation and relaxes the discrimination against the use of non-AUG codons at translation start sites. These mutants do not appear to alter the association of key initiation factors to 40S subunits. The stringency of start-site selection can be restored through overexpression of eIF1, consistent with the role of that factor in enhancing stringency. The present study indicates that interfering with the function of ABC50 influences the accuracy of initiation codon selection.

Recruitment of the 40S ribosome subunit to the 3'-untranslated region (UTR) of a viral mRNA, via the eIF4 complex, facilitates cap-independent translation

Barley yellow dwarf virus mRNA, which lacks both cap and poly(A) tail, has a translation element (3'-BTE) in its 3'-UTR essential for efficient translation initiation at the 5'-proximal AUG. This mechanism requires eukaryotic initiation factor 4G (eIF4G), subunit of heterodimer eIF4F (plant eIF4F lacks eIF4A), and 3'-BTE-5'-UTR interaction. Using fluorescence anisotropy, SHAPE (selective 2'-hydroxyl acylation analyzed by primer extension) analysis, and toeprinting, we found that (i) 40S subunits bind to BTE (Kd = 350 ± 30 nm), (ii) the helicase complex eIF4F-eIF4A-eIF4B-ATP increases 40S subunit binding (Kd = 120 ± 10 nm) to the conserved stem-loop I of the 3'-BTE by exposing more unpaired bases, and (iii) long distance base pairing transfers this complex to the 5'-end of the mRNA, where translation initiates. Although 3'-5' interactions have been recognized as important in mRNA translation, barley yellow dwarf virus employs a novel mechanism utilizing the 3'-UTR as the primary site of ribosome recruitment.

Translational tolerance of mitochondrial genes to metabolic energy stress involves TISU and eIF1-eIF4GI cooperation in start codon selection

Protein synthesis is a major energy-consuming process, which is rapidly repressed upon energy stress by AMPK. How energy deficiency affects translation of mRNAs that cope with the stress response is poorly understood. We found that mitochondrial genes remain translationally active upon energy deprivation. Surprisingly, inhibition of translation is partially retained in AMPKα1/AMPKα2 knockout cells. Mitochondrial mRNAs are enriched with TISU, a translation initiator of short 5' UTR, which confers resistance specifically to energy stress. Purified 48S preinitiation complex is sufficient for initiation via TISU AUG, when preceded by a short 5' UTR. eIF1 stimulates TISU but inhibits non-TISU-directed initiation. Remarkably, eIF4GI shares this activity and also interacts with eIF1. Furthermore, eIF4F is released upon 48S formation on TISU. These findings describe a specialized translation tolerance mechanism enabling continuous translation of TISU genes under energy stress and reveal that a key step in start codon selection of short 5' UTR is eIF4F release.

Quantitative studies of mRNA recruitment to the eukaryotic ribosome

The process of peptide bond synthesis by ribosomes is conserved between species, but the initiation step differs greatly between the three kingdoms of life. This is illustrated by the evolution of roughly an order of magnitude more initiation factor mass found in humans compared with bacteria. Eukaryotic initiation of translation is comprised of a number of sub-steps: (i) recruitment of an mRNA and initiator methionyl-tRNA to the 40S ribosomal subunit; (ii) migration of the 40S subunit along the 5' UTR to locate the initiation codon; and (iii) recruitment of the 60S subunit to form the 80S initiation complex. Although the mechanism and regulation of initiation has been studied for decades, many aspects of the pathway remain unclear. In this review, I will focus discussion on what is known about the mechanism of mRNA selection and its recruitment to the 40S subunit. I will summarize how the 43S preinitiation complex (PIC) is formed and stabilized by interactions between its components. I will discuss what is known about the mechanism of mRNA selection by the eukaryotic initiation factor 4F (eIF4F) complex and how the selected mRNA is recruited to the 43S PIC. The regulation of this process by secondary structure located in the 5' UTR of an mRNA will also be discussed. Finally, I present a possible kinetic model with which to explain the process of mRNA selection and recruitment to the eukaryotic ribosome.

Identification of a second GTP-bound magnesium ion in archaeal initiation factor 2

Eukaryotic and archaeal translation initiation processes involve a heterotrimeric GTPase e/aIF2 crucial for accuracy of start codon selection. In eukaryotes, the GTPase activity of eIF2 is assisted by a GTPase-activating protein (GAP), eIF5. In archaea, orthologs of eIF5 are not found and aIF2 GTPase activity is thought to be non-assisted. However, no in vitro GTPase activity of the archaeal factor has been reported to date. Here, we show that aIF2 significantly hydrolyses GTP in vitro. Within aIF2γ, H97, corresponding to the catalytic histidine found in other translational GTPases, and D19, from the GKT loop, both participate in this activity. Several high-resolution crystal structures were determined to get insight into GTP hydrolysis by aIF2γ. In particular, a crystal structure of the H97A mutant was obtained in the presence of non-hydrolyzed GTP. This structure reveals the presence of a second magnesium ion bound to GTP and D19. Quantum chemical/molecular mechanical simulations support the idea that the second magnesium ion may assist GTP hydrolysis by helping to neutralize the developing negative charge in the transition state. These results are discussed in light of the absence of an identified GAP in archaea to assist GTP hydrolysis on aIF2.