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

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.

The structure of a human translation initiation complex reveals two independent roles for the helicase eIF4A

Eukaryotic translation initiation involves recruitment of the 43S pre-initiation complex to the 5' end of mRNA by the cap-binding complex eIF4F, forming the 48S translation initiation complex (48S), which then scans along the mRNA until the start codon is recognized. We have previously shown that eIF4F binds near the mRNA exit channel of the 43S, leaving open the question of how mRNA secondary structure is removed as it enters the mRNA channel on the other side of the 40S subunit. Here we report the structure of a human 48S that shows that, in addition to the eIF4A that is part of eIF4F, there is a second eIF4A helicase bound at the mRNA entry site, which could unwind RNA secondary structures as they enter the 48S. The structure also reveals conserved interactions between eIF4F and the 43S, probaby explaining how eIF4F can promote mRNA recruitment in all eukaryotes.

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.

Functional analysis of the AUG initiator codon context reveals novel conserved sequences that disfavor mRNA translation in eukaryotes

mRNA translation is a fundamental process for life. Selection of the translation initiation site (TIS) is crucial, as it establishes the correct open reading frame for mRNA decoding. Studies in vertebrate mRNAs discovered that a purine at -3 and a G at +4 (where A of the AUG initiator codon is numbered + 1), promote TIS recognition. However, the TIS context in other eukaryotes has been poorly experimentally analyzed. We analyzed in vitro the influence of the -3, -2, -1 and + 4 positions of the TIS context in rabbit, Drosophila, wheat, and yeast. We observed that -3A conferred the best translational efficiency across these species. However, we found variability at the + 4 position for optimal translation. In addition, the Kozak motif that was defined from mammalian cells was only weakly predictive for wheat and essentially non-predictive for yeast. We discovered eight conserved sequences that significantly disfavored translation. Due to the big differences in translational efficiency observed among weak TIS context sequences, we define a novel category that we termed 'barren AUG context sequences (BACS)', which represent sequences disfavoring translation. Analysis of mRNA-ribosomal complexes structures provided insights into the function of BACS. The gene ontology of the BACS-containing mRNAs is presented.

The interplay between cis- and trans-acting factors drives selective mRNA translation initiation in eukaryotes

Translation initiation consists in the assembly of the small and large ribosomal subunits on the start codon. This important step directly modulates the general proteome in living cells. Recently, genome wide studies revealed unexpected translation initiation events from unsuspected novel open reading frames resulting in the synthesis of a so-called 'dark proteome'. Indeed, the identification of the start codon by the translation machinery is a critical step that defines the translational landscape of the cell. Therefore, translation initiation is a highly regulated process in all organisms. In this review, we focus on the various cis- and trans-acting factors that rule the regulation of translation initiation in eukaryotes. Recent discoveries have shown that the guidance of the translation machinery for the choice of the start codon require sophisticated molecular mechanisms. In particular, the 5'UTR and the coding sequences contain cis-acting elements that trigger the use of AUG codons but also non-AUG codons to initiate protein synthesis. The use of these alternative start codons is also largely influenced by numerous trans-acting elements that drive selective mRNA translation in response to environmental changes.

Initiation factor 3 bound to the 30S ribosomal subunit in an initial step of translation

Bacterial ribosomes require three initiation factors IF1, IF2, and IF3 during the initial steps of translation. These IFs ensure correct base pairing of the initiator tRNA anticodon with the start codon in the mRNA located at the P-site of the 30S ribosomal subunit. IF3 is one of the first IFs to bind to the 30S and plays a crucial role in the selection of the correct start codon and codon: anticodon base pairing. IF3 also prevents the premature association of the 50S subunit of ribosomes and aids in ribosome recycling. IF3 is reported to change binding sites and conformation to ensure translation initiation fidelity. A recent study suggested an initial binding of IF3 CTD away from the P-site and that IF1 and IF2 promote the movement of CTD to the P-site and concomitant movement of NTD. Hence, to visualize the position of IF3 in the absence of any other IFs, we determined cryo-EM structure of the 30S-IF3 complex. The map shows that IF3 is present in an extended conformation with CTD present at the P-site and NTD near the platform even in the absence of IF1 and IF2. Hence, IF3 CTD binds at the P-site and moves away during the accommodation of the initiator tRNA at the P-site in the later steps of translation initiation. Overall, we report the structure of 30S-IF3 which demystifies the starting binding site and conformation of IF3 on the 30S ribosomal subunit.

LARP1 senses free ribosomes to coordinate supply and demand of ribosomal proteins

Terminal oligopyrimidine motif-containing mRNAs (TOPs) encode all ribosomal proteins in mammals and are regulated to tune ribosome synthesis to cell state. Previous studies implicate LARP1 in 40S- or 80S-ribosome complexes that repress and stabilize TOPs. However, a mechanistic understanding of how LARP1 and TOPs interact with these complexes to coordinate TOP outcomes is lacking. Here, we show that LARP1 senses the cellular supply of ribosomes by directly binding non-translating ribosomal subunits. Cryo-EM structures reveal a previously uncharacterized domain of LARP1 bound to and occluding the 40S mRNA channel. Free cytosolic ribosomes induce sequestration of TOPs in repressed 80S-LARP1-TOP complexes independent of alterations in mTOR signaling. Together, this work demonstrates a general ribosome-sensing function of LARP1 that allows it to tune ribosome protein synthesis to cellular demand.

Human TRMT1 catalyzes m2G or m22G formation on tRNAs in a substrate-dependent manner

TRMT1 is an N2-methylguanosine (m2G) and N2,N2-methylguanosine (m22G) methyltransferase that targets G26 of both cytoplasmic and mitochondrial tRNAs. In higher eukaryotes, most cytoplasmic tRNAs with G26 carry m22G26, although the majority of mitochondrial G26-containing tRNAs carry m2G26 or G26, suggesting differences in the mechanisms by which TRMT1 catalyzes modification of these tRNAs. Loss-of-function mutations of human TRMT1 result in neurological disorders and completely abrogate tRNA:m22G26 formation. However, the mechanism underlying the independent catalytic activity of human TRMT1 and identity of its specific substrate remain elusive, hindering a comprehensive understanding of the pathogenesis of neurological disorders caused by TRMT1 mutations. Here, we showed that human TRMT1 independently catalyzes formation of the tRNA:m2G26 or m22G26 modification in a substrate-dependent manner, which explains the distinct distribution of m2G26 and m22G26 on cytoplasmic and mitochondrial tRNAs. For human TRMT1-mediated tRNA:m22G26 formation, the semi-conserved C11:G24 serves as the determinant, and the U10:A25 or G10:C25 base pair is also required, while the size of the variable loop has no effect. We defined the requirements of this recognition mechanism as the "m22G26 criteria". We found that the m22G26 modification occurred in almost all the higher eukaryotic tRNAs conforming to these criteria, suggesting the "m22G26 criteria" are applicable to other higher eukaryotic tRNAs.

Translation machinery captured in motion

Translation accuracy is one of the most critical factors for protein synthesis. It is regulated by the ribosome and its dynamic behavior, along with translation factors that direct ribosome rearrangements to make translation a uniform process. Earlier structural studies of the ribosome complex with arrested translation factors laid the foundation for an understanding of ribosome dynamics and the translation process as such. Recent technological advances in time-resolved and ensemble cryo-EM have made it possible to study translation in real time at high resolution. These methods provided a detailed view of translation in bacteria for all three phases: initiation, elongation, and termination. In this review, we focus on translation factors (in some cases GTP activation) and their ability to monitor and respond to ribosome organization to enable efficient and accurate translation. This article is categorized under: Translation > Ribosome Structure/Function Translation > Mechanisms.

Molecular mechanisms of eukaryotic translation fidelity and their associations with diseases

Converting genetic information into functional proteins is a complex, multi-step process, with each step being tightly regulated to ensure the accuracy of translation, which is critical to cellular health. In recent years, advances in modern biotechnology, especially the development of cryo-electron microscopy and single-molecule techniques, have enabled a clearer understanding of the mechanisms of protein translation fidelity. Although there are many studies on the regulation of protein translation in prokaryotes, and the basic elements of translation are highly conserved in prokaryotes and eukaryotes, there are still great differences in the specific regulatory mechanisms. This review describes how eukaryotic ribosomes and translation factors regulate protein translation and ensure translation accuracy. However, a certain frequency of translation errors does occur in translation, so we describe diseases that arise when the rate of translation errors reaches or exceeds a threshold of cellular tolerance.

eIF3d: A driver of noncanonical cap-dependent translation of specific mRNAs and a trigger of biological/pathological processes

Eukaryotic initiation factor 3d (eIF3d), a known RNA-binding subunit of the eIF3 complex, is a 66 to 68-kDa protein with an RNA-binding motif and a cap-binding domain. Compared with other eIF3 subunits, eIF3d is relatively understudied. However, recent progress in studying eIF3d has revealed a number of intriguing findings on its role in maintaining eIF3 complex integrity, global protein synthesis, and in biological and pathological processes. It has also been reported that eIF3d has noncanonical functions in regulating translation of a subset of mRNAs by binding to 5'-UTRs or interacting with other proteins independent of the eIF3 complex and additional functions in regulating protein stability. The noncanonical regulation of mRNA translation or protein stability may contribute to the role of eIF3d in biological processes such as metabolic stress adaptation and in disease onset and progression including severe acute respiratory syndrome coronavirus 2 infection, tumorigenesis, and acquired immune deficiency syndrome. In this review, we critically evaluate the recent studies on these aspects of eIF3d and assess prospects in understanding the function of eIF3d in regulating protein synthesis and in biological and pathological processes.

Role of aIF5B in archaeal translation initiation

In eukaryotes and in archaea late steps of translation initiation involve the two initiation factors e/aIF5B and e/aIF1A. In eukaryotes, the role of eIF5B in ribosomal subunit joining is established and structural data showing eIF5B bound to the full ribosome were obtained. To achieve its function, eIF5B collaborates with eIF1A. However, structural data illustrating how these two factors interact on the small ribosomal subunit have long been awaited. The role of the archaeal counterparts, aIF5B and aIF1A, remains to be extensively addressed. Here, we study the late steps of Pyrococcus abyssi translation initiation. Using in vitro reconstituted initiation complexes and light scattering, we show that aIF5B bound to GTP accelerates subunit joining without the need for GTP hydrolysis. We report the crystallographic structures of aIF5B bound to GDP and GTP and analyze domain movements associated to these two nucleotide states. Finally, we present the cryo-EM structure of an initiation complex containing 30S bound to mRNA, Met-tRNA_i^Met, aIF5B and aIF1A at 2.7 Å resolution. Structural data shows how archaeal 5B and 1A factors cooperate to induce a conformation of the initiator tRNA favorable to subunit joining. Archaeal and eukaryotic features of late steps of translation initiation are discussed.

Regulation of the interactions between human eIF5 and eIF1A by the CK2 kinase

Translation initiation in eukaryotes relies on a complex network of interactions that are continuously reorganized throughout the process. As more information becomes available about the structure of the ribosomal preinitiation complex (PIC) at various points in translation initiation, new questions arise about which interactions occur when, their roles, and regulation. The eukaryotic translation factor (eIF) 5 is the GTPase-activating protein (GAP) for the GTPase eIF2, which brings the initiator Met-tRNA_i to the PIC. eIF5 also plays a central role in PIC assembly and remodeling through interactions with other proteins, including eIFs 1, 1A, and 3c. Phosphorylation by casein kinase 2 (CK2) significantly increases the eIF5 affinity for eIF2. The interaction between eIF5 and eIF1A was reported to be mediated by the eIF5 C-terminal domain (CTD) and the eIF1A N-terminal tail. Here, we report a new contact interface, between eIF5-CTD and the oligonucleotide/oligosaccharide-binding fold (OB) domain of eIF1A, which contributes to the overall affinity between the two proteins. We also show that the interaction is modulated by dynamic intramolecular interactions within both eIF5 and eIF1A. CK2 phosphorylation of eIF5 increases its affinity for eIF1A, offering new insights into the mechanisms by which CK2 stimulates protein synthesis and cell proliferation.

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eIF5-CTD interacts with both the N-terminal tail and the OB domain of eIF1A.

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The OB domain contacts stabilize the overall interaction.

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The eIF1A C-terminal tail and the eIF5 DWEAR motif interfere with OB domain binding.

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CK2 phosphorylation of eIF5 increases its affinity for eIF1A.