Ribosome-mediated translational pause and protein domain organization

MALAT1: A key regulator in lung cancer pathogenesis and therapeutic targeting

Lung cancer remains a formidable global health burden, necessitating a comprehensive understanding of the underlying molecular mechanisms driving its progression. Recently, lncRNAs have become necessary controllers of various biological functions, including cancer development. MALAT1 has garnered significant attention due to its multifaceted role in lung cancer progression. Lung cancer, among other malignancies, upregulates MALAT1. Its overexpression has been associated with aggressive tumor behavior and poor patient prognosis. MALAT1 promotes cellular proliferation, epithelial-mesenchymal transition (EMT), and angiogenesis in lung cancer, collectively facilitating tumor growth and metastasis. Additionally, MALAT1 enhances cancer cell invasion by interacting with numerous signaling pathways. Furthermore, MALAT1 has been implicated in mediating drug resistance in lung cancer, contributing to the limited efficacy of conventional therapies. Recent advancements in molecular biology and high-throughput sequencing technologies have offered fresh perspectives into the regulatory networks of MALAT1 in lung cancer. It exerts its oncogenic effects by acting as a ceRNA to sponge microRNAs, thereby relieving their inhibitory effects on target genes. Moreover, MALAT1 also influences chromatin remodeling and post-translational modifications to modulate gene expression, further expanding its regulatory capabilities. This review sheds light on the multifaceted roles of MALAT1 in lung cancer progression, underscoring its potential as an innovative therapeutic target and diagnostic biomarker. Targeting MALAT1 alone or combined with existing therapies holds promise to mitigate lung cancer progression and improve patient outcomes.

Bioinformatic Analysis Reveals the Role of Translation Elongation Efficiency Optimisation in the Evolution of Ralstonia Genus

Translation efficiency modulates gene expression in prokaryotes. The comparative analysis of translation elongation efficiency characteristics of Ralstonia genus bacteria genomes revealed that these characteristics diverge in accordance with the phylogeny of Ralstonia. The first branch of this genus is a group of bacteria commonly found in moist environments such as soil and water that includes the species R. mannitolilytica, R. insidiosa, and R. pickettii, which are also described as nosocomial infection pathogens. In contrast, the second branch is plant pathogenic bacteria consisting of R. solanacearum, R. pseudosolanacearum, and R. syzygii. We found that the soil Ralstonia have a significantly lower number and energy of potential secondary structures in mRNA and an increased role of codon usage bias in the optimization of highly expressed genes' translation elongation efficiency, not only compared to phytopathogenic Ralstonia but also to Cupriavidus necator, which is closely related to the Ralstonia genus. The observed alterations in translation elongation efficiency of orthologous genes are also reflected in the difference of potentially highly expressed gene' sets' content among Ralstonia branches with different lifestyles. Analysis of translation elongation efficiency characteristics can be considered a promising approach for studying complex mechanisms that determine the evolution and adaptation of bacteria in various environments.

Principles of gene regulation quantitatively connect DNA to RNA and proteins in bacteria

Protein concentrations are set by a complex interplay between gene-specific regulatory processes and systemic factors, including cell volume and shared gene expression machineries. Elucidating this interplay is crucial for discerning and designing gene regulatory systems. We quantitatively characterized gene-specific and systemic factors that affect transcription and translation genome-wide for Escherichia coli across many conditions. The results revealed two design principles that make regulation of gene expression insulated from concentrations of shared machineries: RNA polymerase activity is fine-tuned to match translational output, and translational characteristics are similar across most messenger RNAs (mRNAs). Consequently, in bacteria, protein concentration is set primarily at the promoter level. A simple mathematical formula relates promoter activities and protein concentrations across growth conditions, enabling quantitative inference of gene regulation from omics data.

Mechanisms governing codon usage bias and the implications for protein expression in the chloroplast of Chlamydomonas reinhardtii

Chloroplasts possess a considerably reduced genome that is decoded via an almost minimal set of tRNAs. These features make an excellent platform for gaining insights into fundamental mechanisms that govern protein expression. Here, we present a comprehensive and revised perspective of the mechanisms that drive codon selection in the chloroplast of Chlamydomonas reinhardtii and the functional consequences for protein expression. In order to extract this information, we applied several codon usage descriptors to genes with different expression levels. We show that highly expressed genes strongly favor translationally optimal codons, while genes with lower functional importance are rather affected by directional mutational bias. We demonstrate that codon optimality can be deduced from codon-anticodon pairing affinity and, for a small number of amino acids (leucine, arginine, serine, and isoleucine), tRNA concentrations. Finally, we review, analyze, and expand on the impact of codon usage on protein yield, secondary structures of mRNA, translation initiation and termination, and amino acid composition of proteins, as well as cotranslational protein folding. The comprehensive analysis of codon choice provides crucial insights into heterologous gene expression in the chloroplast of C. reinhardtii, which may also be applicable to other chloroplast-containing organisms and bacteria.

Bioinformatic Assessment of Factors Affecting the Correlation between Protein Abundance and Elongation Efficiency in Prokaryotes

Protein abundance is crucial for the majority of genetically regulated cell functions to act properly in prokaryotic organisms. Therefore, developing bioinformatic methods for assessing the efficiency of different stages of gene expression is of great importance for predicting the actual protein abundance. One of these steps is the evaluation of translation elongation efficiency based on mRNA sequence features, such as codon usage bias and mRNA secondary structure properties. In this study, we have evaluated correlation coefficients between experimentally measured protein abundance and predicted elongation efficiency characteristics for 26 prokaryotes, including non-model organisms, belonging to diverse taxonomic groups The algorithm for assessing elongation efficiency takes into account not only codon bias, but also number and energy of secondary structures in mRNA if those demonstrate an impact on predicted elongation efficiency of the ribosomal protein genes. The results show that, for a number of organisms, secondary structures are a better predictor of protein abundance than codon usage bias. The bioinformatic analysis has revealed several factors associated with the value of the correlation coefficient. The first factor is the elongation efficiency optimization type-the organisms whose genomes are optimized for codon usage only have significantly higher correlation coefficients. The second factor is taxonomical identity-bacteria that belong to the class Bacilli tend to have higher correlation coefficients among the analyzed set. The third is growth rate, which is shown to be higher for the organisms with higher correlation coefficients between protein abundance and predicted translation elongation efficiency. The obtained results can be useful for further improvement of methods for protein abundance prediction.

Mitochondrial mRNA localization is governed by translation kinetics and spatial transport

For many nuclear-encoded mitochondrial genes, mRNA localizes to the mitochondrial surface co-translationally, aided by the association of a mitochondrial targeting sequence (MTS) on the nascent peptide with the mitochondrial import complex. For a subset of these co-translationally localized mRNAs, their localization is dependent on the metabolic state of the cell, while others are constitutively localized. To explore the differences between these two mRNA types we developed a stochastic, quantitative model for MTS-mediated mRNA localization to mitochondria in yeast cells. This model includes translation, applying gene-specific kinetics derived from experimental data; and diffusion in the cytosol. Even though both mRNA types are co-translationally localized we found that the steady state number, or density, of ribosomes along an mRNA was insufficient to differentiate the two mRNA types. Instead, conditionally-localized mRNAs have faster translation kinetics which modulate localization in combination with changes to diffusive search kinetics across metabolic states. Our model also suggests that the MTS requires a maturation time to become competent to bind mitochondria. Our work indicates that yeast cells can regulate mRNA localization to mitochondria by controlling mitochondrial volume fraction (influencing diffusive search times) and gene translation kinetics (adjusting mRNA binding competence) without the need for mRNA-specific binding proteins. These results shed light on both global and gene-specific mechanisms that enable cells to alter mRNA localization in response to changing metabolic conditions.

Biosynthetic Protein Folding and Molecular Chaperons

The problem of linear polypeptide chain folding into a unique tertiary structure is one of the fundamental scientific challenges. The process of folding cannot be fully understood without its biological context, especially for big multidomain and multisubunit proteins. The principal features of biosynthetic folding are co-translational folding of growing nascent polypeptide chains and involvement of molecular chaperones in the process. The review summarizes available data on the early events of nascent chain folding, as well as on later advanced steps, including formation of elements of native structure. The relationship between the non-uniformity of translation rate and folding of the growing polypeptide is discussed. The results of studies on the effect of biosynthetic folding features on the parameters of folding as a physical process, its kinetics and mechanisms, are presented. Current understanding and hypotheses on the relationship of biosynthetic folding with the fundamental physical parameters and current views on polypeptide folding in the context of energy landscapes are discussed.

Synonymous Variants: Necessary Nuance in our Understanding of Cancer Drivers and Treatment Outcomes

Once called "silent mutations" and assumed to have no effect on protein structure and function, synonymous variants are now recognized to be drivers for some cancers. There have been significant advances in our understanding of the numerous mechanisms by which synonymous single nucleotide variants (sSNVs) can affect protein structure and function by affecting pre-mRNA splicing, mRNA expression, stability, folding, miRNA binding, translation kinetics, and co-translational folding. This review highlights the need for considering sSNVs in cancer biology to gain a better understanding of the genetic determinants of human cancers and to improve their diagnosis and treatment. We surveyed the literature for reports of sSNVs in cancer and found numerous studies on the consequences of sSNVs on gene function with supporting in vitro evidence. We also found reports of sSNVs that have statistically significant associations with specific cancer types but for which in vitro studies are lacking to support the reported associations. Additionally, we found reports of germline and somatic sSNVs that were observed in numerous clinical studies and for which in silico analysis predicts possible effects on gene function. We provide a review of these investigations and discuss necessary future studies to elucidate the mechanisms by which sSNVs disrupt protein function and are play a role in tumorigeneses, cancer progression, and treatment efficacy. As splicing dysregulation is one of the most well recognized mechanisms by which sSNVs impact protein function, we also include our own in silico analysis for predicting which sSNVs may disrupt pre-mRNA splicing.

Peptides before and during the nucleotide world: an origins story emphasizing cooperation between proteins and nucleic acids

Recent developments in Origins of Life research have focused on substantiating the narrative of an abiotic emergence of nucleic acids from organic molecules of low molecular weight, a paradigm that typically sidelines the roles of peptides. Nevertheless, the simple synthesis of amino acids, the facile nature of their activation and condensation, their ability to recognize metals and cofactors and their remarkable capacity to self-assemble make peptides (and their analogues) favourable candidates for one of the earliest functional polymers. In this mini-review, we explore the ramifications of this hypothesis. Diverse lines of research in molecular biology, bioinformatics, geochemistry, biophysics and astrobiology provide clues about the progression and early evolution of proteins, and lend credence to the idea that early peptides served many central prebiotic roles before they were encodable by a polynucleotide template, in a putative 'peptide-polynucleotide stage'. For example, early peptides and mini-proteins could have served as catalysts, compartments and structural hubs. In sum, we shed light on the role of early peptides and small proteins before and during the nucleotide world, in which nascent life fully grasped the potential of primordial proteins, and which has left an imprint on the idiosyncratic properties of extant proteins.

A Code Within a Code: How Codons Fine-Tune Protein Folding in the Cell

The genetic code sets the correspondence between the sequence of a given nucleotide triplet in an mRNA molecule, called a codon, and the amino acid that is added to the growing polypeptide chain during protein synthesis. With four bases (A, G, U, and C), there are 64 possible triplet codons: 61 sense codons (encoding amino acids) and 3 nonsense codons (so-called, stop codons that define termination of translation). In most organisms, there are 20 common/standard amino acids used in protein synthesis; thus, the genetic code is redundant with most amino acids (with the exception of Met and Trp) are being encoded by more than one (synonymous) codon. Synonymous codons were initially presumed to have entirely equivalent functions, however, the finding that synonymous codons are not present at equal frequencies in mRNA suggested that the specific codon choice might have functional implications beyond coding for amino acid. Observation of nonequivalent use of codons in mRNAs implied a possibility of the existence of auxiliary information in the genetic code. Indeed, it has been found that genetic code contains several layers of such additional information and that synonymous codons are strategically placed within mRNAs to ensure a particular translation kinetics facilitating and fine-tuning co-translational protein folding in the cell via step-wise/sequential structuring of distinct regions of the polypeptide chain emerging from the ribosome at different points in time. This review summarizes key findings in the field that have identified the role of synonymous codons and their usage in protein folding in the cell.

From the notebook to recombinant protein production in Escherichia coli: Design of expression vectors and gene cloning

Research in recombinant protein expression in microorganism hosts spans half a century. The field has evolved from mostly trial-and-error approaches to more rational strategies, including careful design of the expression vectors and the coding sequence for the protein of interest. It is important to reflect on many aspects about vector construction, such as codon usage, integration site, coding sequence mutagenesis and many others. In this chapter, we overview methods and considerations to generate a suitable construct and anticipate possible experimental roadblocks.

Nonrefoldability is Pervasive Across the E. coli Proteome

Decades of research on protein folding have primarily focused on a subset of small proteins that can reversibly refold from a denatured state. However, these studies have generally not been representative of the complexity of natural proteomes, which consist of many proteins with complex architectures and domain organizations. Here, we introduce an experimental approach to probe protein refolding kinetics for whole proteomes using mass spectrometry-based proteomics. Our study covers the majority of the soluble E. coli proteome expressed during log-phase growth, and among this group, we find that one-third of the E. coli proteome is not intrinsically refoldable on physiological time scales, a cohort that is enriched with certain fold-types, domain organizations, and other biophysical features. We also identify several properties and fold-types that are correlated with slow refolding on the minute time scale. Hence, these results illuminate when exogenous factors and processes, such as chaperones or cotranslational folding, might be required for efficient protein folding.

Deep conservation of ribosome stall sites across RNA processing genes

The rate of translation can vary depending on the mRNA template. During the elongation phase the ribosome can transiently pause or permanently stall. A pause can provide the nascent protein with the time to fold or be transported, while stalling can serve as quality control and trigger degradation of aberrant mRNA and peptide. Ribosome profiling has allowed for the genome-wide detection of such pauses and stalls, but due to library-specific biases, these predictions are often unreliable. Here, we take advantage of the deep conservation of protein synthesis machinery, hypothesizing that similar conservation could exist for functionally important locations of ribosome slowdown, here collectively called stall sites. We analyze multiple ribosome profiling datasets from phylogenetically diverse eukaryotes: yeast, fruit fly, zebrafish, mouse and human to identify conserved stall sites. We find thousands of stall sites across multiple species, with the enrichment of proline, glycine and negatively charged amino acids around conserved stalling. Many of the sites are found in RNA processing genes, suggesting that stalling might have a conserved role in RNA metabolism. In summary, our results provide a rich resource for the study of conserved stalling and indicate possible roles of stalling in gene regulation.

Quantitative Proteomics Reveals that Hsp90 Inhibition Dynamically Regulates Global Protein Synthesis in Leishmania mexicana

Heat shock protein 90 (Hsp90) is a conserved molecular chaperone responsible for the folding and maturation of nascent proteins. Hsp90 is regarded as a master regulator of protein homeostasis in the cell, and its inhibition affects the functions of a large array of client proteins. The classical Hsp90 inhibitor tanespimycin has shown potent antileishmanial activity. Despite the increasing importance of Hsp90 inhibition in the development of antileishmanial agents, the global effects of these inhibitors on the parasite proteome remain unknown. By combining tanespimycin treatment with bioorthogonal noncanonical amino acid tagging (BONCAT) metabolic labeling and isobaric tags for relative and absolute quantitation (iTRAQ)-based quantitative proteomic mass spectrometry, for the first time, we robustly profiled the relative changes in the synthesis of hundreds of parasite proteins as functions of dose and duration of the inhibitor treatment. We showed that Hsp90 inhibition dynamically regulates nascent protein synthesis in Leishmania mexicana, with many chaperones and virulence factors showing inhibitor concentration- and treatment duration-dependent changes in relative expression. Many ribosomal proteins showed a downregulation upon severe Hsp90 inhibition, providing the first protein-level evidence that Hsp90 inhibition affects the protein synthesis capacity of the ribosome in this organism. We also provide an unbiased target validation of tanespimycin in L. mexicana using live parasite photoaffinity labeling with a novel chemical probe and quantitative proteomic mass spectrometry. We showed that the classical Hsp90 inhibitor not only engages with its presumed target, Hsp83-1, in L. mexicana promastigotes but also affects multiple proteins involved in protein synthesis and quality control in the parasite. This study defines the Leishmania parasites' response to Hsp90 inhibition at the level of nascent global protein synthesis and provides a rich resource for future studies on Leishmania spp. biology and antileishmanial drug development.IMPORTANCE Leishmania spp. are the causative agents of leishmaniasis, a poverty-related disease, which is endemic in >90 countries worldwide, affecting approximately 12 million people, with an estimated 700,000 to 1 million new cases and around 70,000 deaths annually. Inhibitors of the chaperone protein Hsp90 have shown promising antileishmanial activity. However, further development of the Hsp90 inhibitors as antileishmanials is hampered by a lack of direct information of their downstream effects on the parasite proteome. Using a combination of mass spectrometry-based quantitative proteomics and chemical and metabolic labeling, we provide the first protein-level evidence that Hsp90 inhibition affects global protein synthesis in Leishmania We also provide the precise relative quantitative changes in the expressions of hundreds of affected proteins as functions of both the concentration and duration of the inhibitor treatment. We find that Leishmania regulates its ribosomal proteins under Hsp90 inhibition while a set of virulence factors and chaperones are preferentially synthesized.

The effects of codon bias and optimality on mRNA and protein regulation

The central dogma of molecular biology entails that genetic information is transferred from nucleic acid to proteins. Notwithstanding retro-transcribing genetic elements, DNA is transcribed to RNA which in turn is translated into proteins. Recent advancements have shown that each stage is regulated to control protein abundances for a variety of essential physiological processes. In this regard, mRNA regulation is essential in fine-tuning or calibrating protein abundances. In this review, we would like to discuss one of several mRNA-intrinsic features of mRNA regulation that has been gaining traction of recent-codon bias and optimality. Specifically, we address the effects of codon bias with regard to codon optimality in several biological processes centred on translation, such as mRNA stability and protein folding among others. Finally, we examine how different organisms or cell types, through this system, are able to coordinate physiological pathways to respond to a variety of stress or growth conditions.

Synonymous variants in holoprosencephaly alter codon usage and impact the Sonic Hedgehog protein

Synonymous single nucleotide variants (sSNVs) have been implicated in various genetic disorders through alterations of pre-mRNA splicing, mRNA structure and miRNA regulation. However, their impact on synonymous codon usage and protein translation remains to be elucidated in clinical context. Here, we explore the functional impact of sSNVs in the Sonic Hedgehog (SHH) gene, identified in patients affected by holoprosencephaly, a congenital brain defect resulting from incomplete forebrain cleavage. We identified eight sSNVs in SHH, selectively enriched in holoprosencephaly patients as compared to healthy individuals, and systematically assessed their effect at both transcriptional and translational levels using a series of in silico and in vitro approaches. Although no evidence of impact of these sSNVs on splicing, mRNA structure or miRNA regulation was found, five sSNVs introduced significant changes in codon usage and were predicted to impact protein translation. Cell assays demonstrated that these five sSNVs are associated with a significantly reduced amount of the resulting protein, ranging from 5% to 23%. Inhibition of the proteasome rescued the protein levels for four out of five sSNVs, confirming their impact on protein stability and folding. Remarkably, we found a significant correlation between experimental values of protein reduction and computational measures of codon usage, indicating the relevance of in silico models in predicting the impact of sSNVs on translation. Considering the critical role of SHH in brain development, our findings highlight the clinical relevance of sSNVs in holoprosencephaly and underline the importance of investigating their impact on translation in human pathologies.

Construction of anti-codon table of the plant kingdom and evolution of tRNA selenocysteine (tRNASec)

Background

The tRNAs act as a bridge between the coding mRNA and incoming amino acids during protein translation. The anti-codon of tRNA recognizes the codon of the mRNA and deliver the amino acid into the protein translation chain. However, we did not know about the exact abundance of anti-codons in the genome and whether the frequency of abundance remains same across the plant lineage or not.

Results

Therefore, we analysed the tRNAnome of 128 plant species and reported an anti-codon table of the plant kingdom. We found that CAU anti-codon of tRNA^Met has highest (5.039%) whereas GCG anti-codon of tRNA^Arg has lowest (0.004%) abundance. However, when we compared the anti-codon frequencies according to the tRNA isotypes, we found tRNA^Leu (7.808%) has highest abundance followed by tRNA^Ser (7.668%) and tRNA^Gly (7.523%). Similarly, suppressor tRNA (0.036%) has lowest abundance followed by tRNA^Sec (0.066%) and tRNA^His (2.109). The genome of Ipomoea nil, Papaver somniferum, and Zea mays encoded the highest number of anti-codons (isoacceptor) at 59 each whereas the genome of Ostreococcus tauri was found to encode only 18 isoacceptors. The tRNA^Sec genes undergone losses more frequently than duplication and we found that tRNA^Sec showed anti-codon switch during the course of evolution.

Conclusion

The anti-codon table of the plant tRNA will enable us to understand the synonymous codon usage of the plant kingdom and can be very helpful to understand which codon is preferred over other during the translation.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-020-07216-3.

Does proteostasis get lost in translation? Implications for protein aggregation across the lifespan

Protein aggregation is a phenomenon of major relevance in neurodegenerative and neuromuscular disorders, cataracts, diabetes and many other diseases. Research has unveiled that proteins also aggregate in multiple tissues during healthy aging yet, the biological and biomedical relevance of this apparently asymptomatic phenomenon remains to be understood. It is known that proteome homeostasis (proteostasis) is maintained by a balanced protein synthesis rate, high protein synthesis accuracy, efficient protein folding and continual tagging of damaged proteins for degradation, suggesting that protein aggregation during healthy aging may be associated with alterations in both protein synthesis and the proteostasis network (PN) pathways. In particular, dysregulation of protein synthesis and alterations in translation fidelity are hypothesized to lead to the production of misfolded proteins which could explain the occurrence of age-related protein aggregation. Nevertheless, some data on this topic is controversial and the biological mechanisms that lead to widespread protein aggregation remain to be elucidated. We review the recent literature about the age-related decline of proteostasis, highlighting the need to build an integrated view of protein synthesis rate, fidelity and quality control pathways in order to better understand the proteome alterations that occur during aging and in age-related diseases.

Presenting a codon-optimized palette of fluorescent proteins for use in Candida albicans

Fluorescent proteins with varying colors are indispensable tools for the life sciences research community. These fluorophores are often developed for use in mammalian systems, with incremental enhancements or new versions published frequently. However, the successful application of these labels in other organisms in the tree of life, such as the fungus Candida albicans, can be difficult to achieve due to the difficulty in engineering constructs for good expression in these organisms. In this contribution, we present a palette of Candida-optimized fluorescent proteins ranging from cyan to red and assess their application potential. We also compare a range of reported expression optimization techniques, and find that none of these strategies is generally applicable, and that even very closely related proteins require the application of different strategies to achieve good expression. In addition to reporting new fluorescent protein variants for applications in Candida albicans, our work highlights the ongoing challenges in optimizing protein expression in heterologous systems.

[Synonymous Codon Usage-a Guide for Co-Translational Protein Folding in the Cell]

In the cell, protein folding begins during protein synthesis/translation and thus is a co-translational process. Co-translational protein folding is tightly linked to translation elongation, which is not a uniform process. While there are many reasons for translation non-uniformity, it is generally believed that non-uniform synonymous codon usage is one of the key factors modulating translation elongation rates. Frequent/optimal codons as a rule are translated more rapidly than infrequently used ones and vice versa. Over 30 years ago, it was hypothesized that changes in synonymous codon usage affecting translation elongation rates could impinge on co-translation protein folding and that many synonymous codons are strategically placed within mRNA to ensure a particular translation kinetics facilitating productive step-by-step co-translational folding of proteins. It was suggested that this particular translation kinetics (and, specifically, translation pause sites) may define the window of opportunity for the protein parts to fold locally, particularly at the critical points where folding is far from equilibrium. It was thus hypothesized that synonymous codons may provide a secondary code for protein folding in the cell. Although, mostly accepted now, this hypothesis appeared to be difficult to prove and many convincing results were obtained only relatively recently. Here, I review the progress in the field and explain, why this simple idea appeared to be so challenging to prove.

A pair of non-optimal codons are necessary for the correct biosynthesis of the Aspergillus nidulans urea transporter, UreA

In both prokaryotic and eukaryotic genomes, synonymous codons are unevenly used. Such differential usage of optimal or non-optimal codons has been suggested to play a role in the control of translation initiation and elongation, as well as at the level of transcription and mRNA stability. In the case of membrane proteins, codon usage has been proposed to assist in the establishment of a pause necessary for the correct targeting of the nascent chains to the translocon. By using as a model UreA, the Aspergillus nidulans urea transporter, we revealed that a pair of non-optimal codons encoding amino acids situated at the boundary between the N-terminus and the first transmembrane segment are necessary for proper biogenesis of the protein at 37°C. These codons presumably regulate the translation rate in a previously undescribed fashion, possibly contributing to the correct interaction of ureA-translating ribosome-nascent chain complexes with the signal recognition particle and/or other factors, while the polypeptide has not yet emerged from the ribosomal tunnel. Our results suggest that the presence of the pair of non-optimal codons would not be functionally important in all cellular conditions. Whether this mechanism would affect other proteins remains to be determined.

Effects of codon optimization on coagulation factor IX translation and structure: Implications for protein and gene therapies

Synonymous codons occur with different frequencies in different organisms, a phenomenon termed codon usage bias. Codon optimization, a common term for a variety of approaches used widely by the biopharmaceutical industry, involves synonymous substitutions to increase protein expression. It had long been presumed that synonymous variants, which, by definition, do not alter the primary amino acid sequence, have no effect on protein structure and function. However, a critical mass of reports suggests that synonymous codon variations may impact protein conformation. To investigate the impact of synonymous codons usage on protein expression and function, we designed an optimized coagulation factor IX (FIX) variant and used multiple methods to compare its properties to the wild-type FIX upon expression in HEK293T cells. We found that the two variants differ in their conformation, even when controlling for the difference in expression levels. Using ribosome profiling, we identified robust changes in the translational kinetics of the two variants and were able to identify a region in the gene that may have a role in altering the conformation of the protein. Our data have direct implications for codon optimization strategies, for production of recombinant proteins and gene therapies.

Protein synthesis rates and ribosome occupancies reveal determinants of translation elongation rates

Although protein synthesis dynamics has been studied both with theoretical models and by profiling ribosome footprints, the determinants of ribosome flux along open reading frames (ORFs) are not fully understood. Combining measurements of protein synthesis rate with ribosome footprinting data, we here inferred translation initiation and elongation rates for over a 1,000 ORFs in exponentially growing wild-type yeast cells. We found that the amino acid composition of synthesized proteins is as important a determinant of translation elongation rate as parameters related to codon and transfer RNA (tRNA) adaptation. We did not find evidence of ribosome collisions curbing the protein output of yeast transcripts, either in high translation conditions associated with exponential growth, or in strains in which deletion of individual ribosomal protein (RP) genes leads to globally increased or decreased translation. Slow translation elongation is characteristic of RP-encoding transcripts, which have markedly lower protein output compared with other transcripts with equally high ribosome densities.

Evolutionary Changes after Translational Challenges Imposed by Horizontal Gene Transfer

Genes acquired by horizontal gene transfer (HGT) may provide the recipient organism with potentially new functions, but proper expression level and integration of the transferred genes in the novel environment are not granted. Notably, transferred genes can differ from the receiving genome in codon usage preferences, leading to impaired translation and reduced functionality. Here, we characterize the genomic and proteomic changes undergone during experimental evolution of Escherichia coli after HGT of three synonymous versions, presenting very different codon usage preference, of an antibiotic resistance gene. The experimental evolution was conducted with and without the corresponding antibiotic and the mutational patterns and proteomic profiles after 1,000 generations largely depend on the experimental growth conditions (e.g., mutations in antibiotic off-target genes), and on the synonymous gene version transferred (e.g., mutations in genes responsive to translational stress). The transfer of an exogenous gene extensively modifies the whole proteome, and these proteomic changes are different for the different version of the transferred gene. Additionally, we identified conspicuous changes in global regulators and in intermediate metabolism, confirmed the evolutionary ratchet generated by mutations in DNA repair genes and highlighted the plasticity of bacterial genomes accumulating large and occasionally transient duplications. Our results support a central role of HGT in fuelling evolution as a powerful mechanism promoting rapid, often dramatic genotypic and phenotypic changes. The profound reshaping of the pre-existing geno/phenotype allows the recipient bacteria to explore new ways of functioning, far beyond the mere acquisition of a novel function.

Enhancing co-translational folding of heterologous protein by deleting non-essential ribosomal proteins in Pichia pastoris

BACKGROUND

Translational regulation played an important role in the correct folding of heterologous proteins to form bioactive conformations during biogenesis. Translational pausing coordinates protein translation and co-translational folding. Decelerating translation elongation speed has been shown to improve the soluble protein yield when expressing heterologous proteins in industrial expression hosts. However, rational redesign of translational pausing via synonymous mutations may not be feasible in many cases. Our goal was to develop a general and convenient strategy to improve heterologous protein synthesis in Pichia pastoris without mutating the expressed genes.

RESULTS

Here, a large-scale deletion library of ribosomal protein (RP) genes was constructed for heterologous protein expression in Pichia pastoris, and 59% (16/27) RP deletants have significantly increased heterologous protein yield. This is due to the delay of 60S subunit assembly by deleting non-essential ribosomal protein genes or 60S subunit processing factors, thus globally decreased the translation elongation speed and improved the co-translational folding, without perturbing the relative transcription level and translation initiation.

CONCLUSION

Global decrease in the translation elongation speed by RP deletion enhanced co-translational folding efficiency of nascent chains and decreased protein aggregates to improve heterologous protein yield. A potential expression platform for efficient pharmaceutical proteins and industrial enzymes production was provided without synonymous mutation.

Differential interaction strategies of hepatitis c virus genotypes during entry - An in silico investigation of envelope glycoprotein E2 - CD81 interaction

Hepatitis C Virus is a blood borne pathogen responsible for chronic hepatitis in more than 71 million people. Wide variations across strains and genotypes are one of the major hurdles in therapeutic development. While genotype 1 remains the most extensively studied and abundant strain, genotype 3 is more virulent and second most prevalent. This study aimed to compare differences in the glycoprotein E2 across HCV genotypes at nucleotide, protein and structural levels. Nucleotide sequences of E2 from 29 strains across genotypes 1a, 1b, 3a and 3b revealed a stark preference for C-richness which was attributed to a distinct bias for C-rich codons in genotype 1. Genotype 3 exhibited a similar preference to a lesser extent. Amino acid level comparison revealed majority of the changes at the C-terminal half of the proteins leaving the N-terminal region conspicuously conserved apart from the two hyper variable regions. Amino acid changes across genotypes were mostly polar-nonpolar alterations. In silico models of E2 glycoproteins and docking analysis with the energy minimized PDB-CD81 model revealed unique interacting residues in both E2 and CD81. While several CD81 binding residues were common for all four genotypes, number and composition of interacting residues varied. The interacting residues of E2 were however unique for each genotype. E2 of genotype 3a and CD81 had the strongest interaction. In conclusion this is the first comprehensive study comparing E2 sequences across genotypes 1a, 1b, 3a and 3b revealing stark genotype-specific differences which requires more extensive investigation.

Plasmodium apicoplast tyrosyl-tRNA synthetase recognizes an unusual, simplified identity set in cognate tRNATyr

The life cycle of Plasmodium falciparum, the agent responsible for malaria, depends on both cytosolic and apicoplast translation fidelity. Apicoplast aminoacyl-tRNA synthetases (aaRS) are bacterial-like enzymes devoted to organellar tRNA aminoacylation. They are all encoded by the nuclear genome and are translocated into the apicoplast only after cytosolic biosynthesis. Apicoplast aaRSs contain numerous idiosyncratic sequence insertions: An understanding of the roles of these insertions has remained elusive and they hinder efforts to heterologously overexpress these proteins. Moreover, the A/T rich content of the Plasmodium genome leads to A/U rich apicoplast tRNA substrates that display structural plasticity. Here, we focus on the P. falciparum apicoplast tyrosyl-tRNA synthetase (Pf-apiTyrRS) and its cognate tRNATyr substrate (Pf-apitRNATyr). Cloning and expression strategies used to obtain an active and functional recombinant Pf-apiTyrRS are reported. Functional analyses established that only three weak identity elements in the apitRNATyr promote specific recognition by the cognate Pf-apiTyrRS and that positive identity elements usually found in the tRNATyr acceptor stem are excluded from this set. This finding brings to light an unusual behavior for a tRNATyr aminoacylation system and suggests that Pf-apiTyrRS uses primarily negative recognition elements to direct tyrosylation specificity.

Repertoires of tRNAs: The Couplers of Genomics and Proteomics

The pool of transfer RNA (tRNA) molecules in cells allows the ribosome to decode genetic information. This repertoire of molecular decoders is positioned in the crossroad of the genome, the transcriptome, and the proteome. Omics and systems biology now allow scientists to explore the entire repertoire of tRNAs of many organisms, revealing basic exciting biology. The tRNA gene set of hundreds of species is now characterized, in addition to the tRNA genes of organelles and viruses. Genes encoding tRNAs for certain anticodon types appear in dozens of copies in a genome, while others are universally absent from any genome. Transcriptome measurement of tRNAs is challenging, but in recent years new technologies have allowed researchers to determine the dynamic expression patterns of tRNAs. These advances reveal that availability of ready-to-translate tRNA molecules is highly controlled by several transcriptional and posttranscriptional regulatory processes. This regulation shapes the proteome according to the cellular state. The tRNA pool profoundly impacts many aspects of cellular and organismal life, including protein expression level, translation accuracy, adequacy of folding, and even mRNA stability. As a result, the shape of the tRNA pool affects organismal health and may participate in causing conditions such as cancer and neurological conditions.

Genome scale analysis of Escherichia coli with a comprehensive prokaryotic sequence-based biophysical model of translation initiation and elongation

Translation initiation in prokaryotes is affected by the mRNA folding and interaction of the ribosome binding site with the ribosomal RNA. The elongation rate is affected, among other factors, by the local biophysical properties of the coding regions, the decoding rates of different codons, and the interactions among ribosomes. Currently, there is no comprehensive biophysical model of translation that enables the prediction of mRNA translation dynamics based only on the transcript sequence and while considering all of these fundamental aspects of translation. In this study, we provide, for the first time, a computational simulative biophysical model of both translation initiation and elongation with all aspects mentioned above. We demonstrate our model performance and advantages focusing on Escherichia coli genes. We further show that the model enables prediction of translation rate, protein levels, and ribosome densities. In addition, our model enables quantifying the effect of silent mutations on translation rate in different parts of the transcript, the relative effect of mutations on translation initiation and elongation, and the effect of mutations on ribosome traffic jams. Thus, unlike previous models, the proposed one provides comprehensive information, facilitating future research in disciplines such as molecular evolution, synthetic biology, and functional genomics. A toolkit to estimate translation dynamics of transcripts is available at: https://www.cs.tau.ac.il/∼tamirtul/transim.

Pausing of Chloroplast Ribosomes Is Induced by Multiple Features and Is Linked to the Assembly of Photosynthetic Complexes

Many mRNAs contain pause sites that briefly interrupt the progress of translation. Specific features that induce ribosome pausing have been described; however, their individual contributions to pause-site formation, and the overall biological significance of ribosome pausing, remain largely unclear. We have taken advantage of the compact genome of chloroplasts to carry out a plastid genome-wide survey of pause sites, as a basis for studying the impact of pausing on posttranslational processes. Based on ribosomal profiling of Arabidopsis (Arabidopsis thaliana) chloroplast mRNAs, we demonstrate that a combination of factors-mRNA secondary structure, internal Shine-Dalgarno sequences, and positively charged amino acids in the nascent peptide chain-explains 95% of the major pause sites on plastid mRNAs, whereas codon usage has little impact. The distribution of the pause sites is nonrandom and conforms to distinct patterns in the vicinity of sequences coding for transmembrane domains, which depend on their orientation within the membrane as well as being next to sequences coding for cofactor binding sites. We found strong indications that the mechanisms causing ribosomal pausing and at least some of the ribosomes pause sites are conserved between distantly related plant species. In addition, the positions of features that cause pausing are well conserved in photoautotrophic plants, but less so in their nonphotosynthetic, parasitic relatives, implying that the synthesis and assembly of photosynthetic multiprotein complexes requires localized ribosome pausing.

Scikit-ribo Enables Accurate Estimation and Robust Modeling of Translation Dynamics at Codon Resolution

Ribosome profiling (Ribo-seq) is a powerful technique for measuring protein translation; however, sampling errors and biological biases are prevalent and poorly understood. Addressing these issues, we present Scikit-ribo (https://github.com/schatzlab/scikit-ribo), an open-source analysis package for accurate genome-wide A-site prediction and translation efficiency (TE) estimation from Ribo-seq and RNA sequencing data. Scikit-ribo accurately identifies A-site locations and reproduces codon elongation rates using several digestion protocols (r = 0.99). Next, we show that the commonly used reads per kilobase of transcript per million mapped reads-derived TE estimation is prone to biases, especially for low-abundance genes. Scikit-ribo introduces a codon-level generalized linear model with ridge penalty that correctly estimates TE, while accommodating variable codon elongation rates and mRNA secondary structure. This corrects the TE errors for over 2,000 genes in S. cerevisiae, which we validate using mass spectrometry of protein abundances (r = 0.81), and allows us to determine the Kozak-like sequence directly from Ribo-seq. We conclude with an analysis of coverage requirements needed for robust codon-level analysis and quantify the artifacts that can occur from cycloheximide treatment.

Evolutionary analysis of polyproline motifs in Escherichia coli reveals their regulatory role in translation

Translation of consecutive prolines causes ribosome stalling, which is alleviated but cannot be fully compensated by the elongation factor P. However, the presence of polyproline motifs in about one third of the E. coli proteins underlines their potential functional importance, which remains largely unexplored. We conducted an evolutionary analysis of polyproline motifs in the proteomes of 43 E. coli strains and found evidence of evolutionary selection against translational stalling, which is especially pronounced in proteins with high translational efficiency. Against the overall trend of polyproline motif loss in evolution, we observed their enrichment in the vicinity of translational start sites, in the inter-domain regions of multi-domain proteins, and downstream of transmembrane helices. Our analysis demonstrates that the time gain caused by ribosome pausing at polyproline motifs might be advantageous in protein regions bracketing domains and transmembrane helices. Polyproline motifs might therefore be crucial for co-translational folding and membrane insertion.

Polyproline motifs induce ribosome stalling during translation, but the functional significance of this effect remains unclear. Our evolutionary analysis of polyproline motifs reveals that they are disfavored in E. coli proteomes as a consequence of the reduced translation efficiency, supporting the conjecture that translation efficiency-based evolutionary pressure shapes protein sequences. Enrichment of polyproline motifs in the protein regions bracketing structural domains and transmembrane helices indicates their regulatory role in co-translational protein folding and transmembrane helix insertion. Polyproline motifs could thus serve as protein-level cis-acting elements, which directly regulate the rate of translation elongation.

Evidence of evolutionary selection for cotranslational folding

Recent experiments and simulations have demonstrated that proteins can fold on the ribosome. However, the extent and generality of fitness effects resulting from cotranslational folding remain open questions. Here we report a genome-wide analysis that uncovers evidence of evolutionary selection for cotranslational folding. We describe a robust statistical approach to identify loci within genes that are both significantly enriched in slowly translated codons and evolutionarily conserved. Surprisingly, we find that domain boundaries can explain only a small fraction of these conserved loci. Instead, we propose that regions enriched in slowly translated codons are associated with cotranslational folding intermediates, which may be smaller than a single domain. We show that the intermediates predicted by a native-centric model of cotranslational folding account for the majority of these loci across more than 500 Escherichia coli proteins. By making a direct connection to protein folding, this analysis provides strong evidence that many synonymous substitutions have been selected to optimize translation rates at specific locations within genes. More generally, our results indicate that kinetics, and not just thermodynamics, can significantly alter the efficiency of self-assembly in a biological context.

Comprehensive structural analysis of designed incomplete polypeptide chains of the replicase nonstructural protein 1 from the severe acute respiratory syndrome coronavirus

The cotranslational folding is recognized as a very cooperative process that occurs after the nearly completion of the polypeptide sequence of a domain. Here we investigated the challenges faced by polypeptide segments of a non-vectorial β-barrel fold. Besides the biological interest behind the SARS coronavirus non-structural protein 1 (nsp1, 117 amino acids), this study model has two structural features that motivated its use in this work: 1- its recombinant production is dependent on the temperature, with greater solubility when expressed at low temperatures. This is an indication of the cotranslational guidance to the native protein conformation. 2- Conversely, nsp1 has a six-stranded, mixed parallel/antiparallel β-barrel with intricate long-range interactions, indicating it will need the full-length protein to fold properly. We used non-denaturing purification conditions that allowed the characterization of polypeptide chains of different lengths, mimicking the landscape of the cotranslational fold of a β-barrel, and avoiding the major technical hindrances of working with the nascent polypeptide bound to the ribosome. Our results showed partially folded states formed as soon as the amino acids of the second β-strand were present (55 amino acids). These partially folded states are different based on the length of polypeptide chain. The native α-helix (amino acids 24-37) was identified as a transient structure (~20-30% propensity). We identified the presence of regular secondary structure after the fourth native β-strand is present (89 amino acids), in parallel to the collapse to a non-native 3D structure. Interestingly the polypeptide sequences of the native strands β2, β3 and β4 have characteristics of α-helices. Our comprehensive analyses support the idea that incomplete polypeptide chains, such as the ones of nascent proteins much earlier than the end of the translation, adopt an abundance of specific transient folds, instead of disordered conformations.

Computational analysis of nascent peptides that induce ribosome stalling and their proteomic distribution in Saccharomyces cerevisiae

Interactions between the ribosomal exit tunnel and the nascent peptide can affect translation elongation rates. While previous studies have already demonstrated the feasibility of such interactions, little is known about the nature of the stalling peptide sequences and their distribution in the proteome. Here we ask which peptide sequences tend to occupy the tunnel of stalled ribosomes and how they are distributed in the proteome. Using computational analysis of ribosome profiling data from S. cerevisiae, we identified for the first time dozens of short stalling peptide sequences and studied their statistical properties. We found that short peptide sequences associated with ribosome stalling tend significantly to be either over- or underrepresented in the proteome. We then showed that the stalling interactions may occur at different positions along the length of the tunnel, prominently close to the P-site. Our findings throw light on the determinants of nascent peptide-mediated ribosome stalling during translation elongation and support the novel conjecture that mRNA translation affects the proteomic distribution of short peptide sequences.

Protein charge distribution in proteomes and its impact on translation

As proteins are synthesized, the nascent polypeptide must pass through a negatively charged exit tunnel. During this stage, positively charged stretches can interact with the ribosome walls and slow the translation. Therefore, charged polypeptides may be important factors that affect protein expression. To determine the frequency and distribution of positively and negatively charged stretches in different proteomes, the net charge was calculated for every 30 consecutive amino acid residues, which corresponds to the length of the ribosome exit tunnel. The following annotated and reviewed proteins in the UniProt database (Swiss-Prot) were analyzed: 551,705 proteins from different organisms and a total of 180 million protein segments. We observed that there were more negative than positive stretches and that super-charged positive sequences (i.e., net charges ≥ 14) were underrepresented in the proteomes. Overall, the proteins were more positively charged at their N-termini and C-termini, and this feature was present in most organisms and subcellular localizations. To investigate whether the N-terminal charges affect the elongation rates, previously published ribosomal profiling data obtained from S. cerevisiae, without translation-interfering drugs, were analyzed. We observed a nonlinear effect of the charge on the ribosome occupancy in which values ≥ +5 and ≤ -6 showed increased and reduced ribosome densities, respectively. These groups also showed different distributions across 80S monosomes and polysomes. Basic polypeptides are more common within short proteins that are translated by monosomes, whereas negative stretches are more abundant in polysome-translated proteins. These findings suggest that the nascent peptide charge impacts translation and can be one of the factors that regulate translation efficiency and protein expression.

Which factors shape the sequence of amino acids that will form a protein? The biochemical features of amino acids, such as their charge and hydrophobicity, are important drivers of protein tridimensional folding, which creates interaction sites for binding other molecules and directs proteins to specific cellular compartments. These features all impact the activity of the proteins after they are produced. Another less obvious factor that influences the protein’s primary structure may be how efficiently a given amino acid sequence is produced by the ribosome. It is known that a repetitive stretch of positively charged amino acids may interact with the negative charges in the ribosome exit tunnel, slowing, or even halting, translation. By analyzing the charge of protein stretches in different organisms, we observed that proteins tend to present positively charged stretches at their extremities, and high charge values can slow (for positive charges) or speed (for negative charges) translation. An interesting consequence of this trend is that proteins that are translated in high quantities by several ribosomes at the same RNA (polysomes) tend to have more negatively charged stretches than proteins that are translated by a single ribosome per RNA (monosomes).

Widespread position-specific conservation of synonymous rare codons within coding sequences

Synonymous rare codons are considered to be sub-optimal for gene expression because they are translated more slowly than common codons. Yet surprisingly, many protein coding sequences include large clusters of synonymous rare codons. Rare codons at the 5’ terminus of coding sequences have been shown to increase translational efficiency. Although a general functional role for synonymous rare codons farther within coding sequences has not yet been established, several recent reports have identified rare-to-common synonymous codon substitutions that impair folding of the encoded protein. Here we test the hypothesis that although the usage frequencies of synonymous codons change from organism to organism, codon rarity will be conserved at specific positions in a set of homologous coding sequences, for example to tune translation rate without altering a protein sequence. Such conservation of rarity–rather than specific codon identity–could coordinate co-translational folding of the encoded protein. We demonstrate that many rare codon cluster positions are indeed conserved within homologous coding sequences across diverse eukaryotic, bacterial, and archaeal species, suggesting they result from positive selection and have a functional role. Most conserved rare codon clusters occur within rather than between conserved protein domains, challenging the view that their primary function is to facilitate co-translational folding after synthesis of an autonomous structural unit. Instead, many conserved rare codon clusters separate smaller protein structural motifs within structural domains. These smaller motifs typically fold faster than an entire domain, on a time scale more consistent with translation rate modulation by synonymous codon usage. While proteins with conserved rare codon clusters are structurally and functionally diverse, they are enriched in functions associated with organism growth and development, suggesting an important role for synonymous codon usage in organism physiology. The identification of conserved rare codon clusters advances our understanding of distinct, functional roles for otherwise synonymous codons and enables experimental testing of the impact of synonymous codon usage on the production of functional proteins.

Proteins are long linear polymers that must fold into complex three-dimensional shapes in order to carry out their cellular functions. Every protein is synthesized by the ribosome, which decodes each trinucleotide codon in an mRNA coding sequence in order to select the amino acid residue that will occupy each position in the protein sequence. Most amino acids can be encoded by more than one codon, but these synonymous codons are not used with equal frequency. Rare codons are associated with generally slower rates for protein synthesis, and for this reason have traditionally been considered mildly deleterious for efficient protein production. However, because synonymous codon substitutions do not change the sequence of the encoded protein, the majority view is that they merely reflect genomic ‘background noise’. To the contrary, here we show that the positions of many synonymous rare codons are conserved in mRNA sequences that encode structurally similar proteins from a diverse range of organisms. These results suggest that rare codons have a functional role related to the production of functional proteins, potentially to regulate the rate of protein synthesis and the earliest steps of protein folding, while synthesis is still underway.

Synonymous Codons: Choose Wisely for Expression

The genetic code, which defines the amino acid sequence of a protein, also contains information that influences the rate and efficiency of translation. Neither the mechanisms nor functions of codon-mediated regulation were well understood. The prevailing model was that the slow translation of codons decoded by rare tRNAs reduces efficiency. Recent genome-wide analyses have clarified several issues. Specific codons and codon combinations modulate ribosome speed and facilitate protein folding. However, tRNA availability is not the sole determinant of rate; rather, interactions between adjacent codons and wobble base pairing are key. One mechanism linking translation efficiency and codon use is that slower decoding is coupled to reduced mRNA stability. Changes in tRNA supply mediate biological regulationfor instance,, changes in tRNA amounts facilitate cancer metastasis.

The Impact of Selection at the Amino Acid Level on the Usage of Synonymous Codons

There are two main forces that affect usage of synonymous codons: directional mutational pressure and selection. The effectiveness of protein translation is usually considered as the main selectional factor. However, biased codon usage can also be a byproduct of a general selection at the amino acid level interacting with nucleotide replacements. To evaluate the validity and strength of such an effect, we superimposed >3.5 billion unrestricted mutational processes on the selection of nonsynonymous substitutions based on the differences in physicochemical properties of the coded amino acids. Using a modified evolutionary optimization algorithm, we determined the conditions in which the effect on the relative codon usage is maximized. We found that the effect is enhanced by mutational processes generating more adenine and thymine than guanine and cytosine, as well as more purines than pyrimidines. Interestingly, this effect is observed only under an unrestricted model of nucleotide substitution, and disappears when the mutational process is time-reversible. Comparison of the simulation results with data for real protein coding sequences indicates that the impact of selection at the amino acid level on synonymous codon usage cannot be neglected. Furthermore, it can considerably interfere, especially in AT-rich genomes, with other selections on codon usage, e.g., translational efficiency. It may also lead to difficulties in the recognition of other effects influencing codon bias, and an overestimation of protein coding sequences whose codon usage is subjected to adaptational selection.

Architecture of the yeast Elongator complex

The highly conserved eukaryotic Elongator complex performs specific chemical modifications on wobble base uridines of tRNAs, which are essential for proteome stability and homeostasis. The complex is formed by six individual subunits (Elp1-6) that are all equally important for its tRNA modification activity. However, its overall architecture and the detailed reaction mechanism remain elusive. Here, we report the structures of the fully assembled yeast Elongator and the Elp123 sub-complex solved by an integrative structure determination approach showing that two copies of the Elp1, Elp2, and Elp3 subunits form a two-lobed scaffold, which binds Elp456 asymmetrically. Our topological models are consistent with previous studies on individual subunits and further validated by complementary biochemical analyses. Our study provides a structural framework on how the tRNA modification activity is carried out by Elongator.

Role of mRNA structure in the control of protein folding

Specific structures in mRNA modulate translation rate and thus can affect protein folding. Using the protein structures from two eukaryotes and three prokaryotes, we explore the connections between the protein compactness, inferred from solvent accessibility, and mRNA structure, inferred from mRNA folding energy (ΔG). In both prokaryotes and eukaryotes, the ΔG value of the most stable 30 nucleotide segment of the mRNA (ΔGmin) strongly, positively correlates with protein solvent accessibility. Thus, mRNAs containing exceptionally stable secondary structure elements typically encode compact proteins. The correlations between ΔG and protein compactness are much more pronounced in predicted ordered parts of proteins compared to the predicted disordered parts, indicative of an important role of mRNA secondary structure elements in the control of protein folding. Additionally, ΔG correlates with the mRNA length and the evolutionary rate of synonymous positions. The correlations are partially independent and were used to construct multiple regression models which explain about half of the variance of protein solvent accessibility. These findings suggest a model in which the mRNA structure, particularly exceptionally stable RNA structural elements, act as gauges of protein co-translational folding by reducing ribosome speed when the nascent peptide needs time to form and optimize the core structure.

Structural basis for tRNA modification by Elp3 from Dehalococcoides mccartyi

During translation elongation decoding is based on the recognition of codons by corresponding tRNA anticodon triplets. Molecular mechanisms that regulate global protein synthesis via specific base modifications in tRNA anticodons have recently received increasing attention. The conserved eukaryotic Elongator complex specifically modifies uridines located in the wobble base position of tRNAs. Here, we present the crystal structure of Dehalococcoides mccartyi Elp3 (DmcElp3) at 2.15 Å resolution. Our results reveal the unexpected arrangement of Elp3 lysine acetyl transferase (KAT) and radical S-adenosyl-methionine (SAM) domains that share a large interface to form a composite active site and tRNA binding pocket with an iron sulfur cluster located in the dimerization interface of two DmcElp3 molecules. Structure-guided mutagenesis studies of yeast Elp3 confirm the relevance of our findings for eukaryotic Elp3s and for understanding Elongator’s role in the onset of various neurodegenerative diseases and cancer in humans.

Adjacent Codons Act in Concert to Modulate Translation Efficiency in Yeast

Translation elongation efficiency is largely thought of as the sum of decoding efficiencies for individual codons. Here, we find that adjacent codon pairs modulate translation efficiency. Deploying an approach in Saccharomyces cerevisiae that scored the expression of over 35,000 GFP variants in which three adjacent codons were randomized, we have identified 17 pairs of adjacent codons associated with reduced expression. For many pairs, codon order is obligatory for inhibition, implying a more complex interaction than a simple additive effect. Inhibition mediated by adjacent codons occurs during translation itself as GFP expression is restored by increased tRNA levels or by non-native tRNAs with exact-matching anticodons. Inhibition operates in endogenous genes, based on analysis of ribosome profiling data. Our findings suggest translation efficiency is modulated by an interplay between tRNAs at adjacent sites in the ribosome and that this concerted effect needs to be considered in predicting the functional consequences of codon choice.

The Yin and Yang of codon usage

The genetic code is degenerate. With the exception of two amino acids (Met and Trp), all other amino acid residues are each encoded by multiple, so-called synonymous codons. Synonymous codons were initially presumed to have entirely equivalent functions, however, the finding that synonymous codons are not present at equal frequencies in genes/genomes suggested that codon choice might have functional implications beyond amino acid coding. The pattern of non-uniform codon use (known as codon usage bias) varies between organisms and represents a unique feature of an organism. Organism-specific codon choice is related to organism-specific differences in populations of cognate tRNAs. This implies that, in a given organism, frequently used codons will be translated more rapidly than infrequently used ones and vice versa A theory of codon-tRNA co-evolution (necessary to balance accurate and efficient protein production) was put forward to explain the existence of codon usage bias. This model suggests that selection favours preferred (frequent) over un-preferred (rare) codons in order to sustain efficient protein production in cells and that a given un-preferred codon will have the same effect on an organism's fitness regardless of its position within an mRNA's open reading frame. However, many recent studies refute this prediction. Un-preferred codons have been found to have important functional roles and their effects appeared to be position-dependent. Synonymous codon usage affects the efficiency/stringency of mRNA decoding, mRNA biogenesis/stability, and protein secretion and folding. This review summarizes recent developments in the field that have identified novel functions of synonymous codons and their usage.

Modeling the effect of codon translation rates on co-translational protein folding mechanisms of arbitrary complexity

In a cell, the folding of a protein molecule into tertiary structure can begin while it is synthesized by the ribosome. The rate at which individual amino acids are incorporated into the elongating nascent chain has been shown to affect the likelihood that proteins will populate their folded state, indicating that co-translational protein folding is a far from equilibrium process. Developing a theoretical framework to accurately describe this process is, therefore, crucial for advancing our understanding of how proteins acquire their functional conformation in living cells. Current state-of-the-art computational approaches, such as molecular dynamics simulations, are very demanding in terms of the required computer resources, making the simulation of co-translational protein folding difficult. Here, we overcome this limitation by introducing an efficient approach that predicts the effects that variable codon translation rates have on co-translational folding pathways. Our approach is based on Markov chains. By using as an input a relatively small number of molecular dynamics simulations, it allows for the computation of the probability that a nascent protein is in any state as a function of the translation rate of individual codons along a mRNA's open reading frame. Due to its computational efficiency and favorable scalability with the complexity of the folding mechanism, this approach could enable proteome-wide computational studies of the influence of translation dynamics on co-translational folding.

Silent Polymorphisms: Can the tRNA Population Explain Changes in Protein Properties?

Silent mutations are being intensively studied. We previously showed that the estrogen receptor alpha Ala87’s synonymous polymorphism affects its functional properties. Whereas a link has been clearly established between the effect of silent mutations, tRNA abundance and protein folding in prokaryotes, this connection remains controversial in eukaryotic systems. Although a synonymous polymorphism can affect mRNA structure or the interaction with specific ligands, it seems that the relative frequencies of isoacceptor tRNAs could play a key role in the protein-folding process, possibly through modulation of translation kinetics. Conformational changes could be subtle but enough to cause alterations in solubility, proteolysis profiles, functional parameters or intracellular targeting. Interestingly, recent advances describe dramatic changes in the tRNA population associated with proliferation, differentiation or response to chemical, physical or biological stress. In addition, several reports reveal changes in tRNAs’ posttranscriptional modifications in different physiological or pathological conditions. In consequence, since changes in the cell state imply quantitative and/or qualitative changes in the tRNA pool, they could increase the likelihood of protein conformational variants, related to a particular codon usage during translation, with consequences of diverse significance. These observations emphasize the importance of genetic code flexibility in the co-translational protein-folding process.

Improved Ribosome-Footprint and mRNA Measurements Provide Insights into Dynamics and Regulation of Yeast Translation

Ribosome-footprint profiling provides genome-wide snapshots of translation, but technical challenges can confound its analysis. Here, we use improved methods to obtain ribosome-footprint profiles and mRNA abundances that more faithfully reflect gene expression in Saccharomyces cerevisiae. Our results support proposals that both the beginning of coding regions and codons matching rare tRNAs are more slowly translated. They also indicate that emergent polypeptides with as few as three basic residues within a ten-residue window tend to slow translation. With the improved mRNA measurements, the variation attributable to translational control in exponentially growing yeast was less than previously reported, and most of this variation could be predicted with a simple model that considered mRNA abundance, upstream open reading frames, cap-proximal structure and nucleotide composition, and lengths of the coding and 5' UTRs. Collectively, our results provide a framework for executing and interpreting ribosome-profiling studies and reveal key features of translational control in yeast.