The Ribosome as a Platform for mRNA and Nascent Polypeptide Quality Control

The MADS-box transcription factor CmFYF promotes the production of male flowers and inhibits the fruit development in melon (Cucumis melo L.)

The FOREVER YOUNG FLOWER (FYF/AGL42) gene plays an important role in regulating the flower development especially the flowering time, and senescence and abscission of floral organs. Melon is an important horticultural crop, and the flower development has an important influence on pollination, fertilization and final fruit yield. However, the function of FYF homolog is still unknown in melon. In this study, the characteristic of melon CmFYF was analyzed combining with molecular biology, developmental biology and biochemical tools. CmFYF was present in all detected tissues of melon, but its expression level was significantly higher in shoot apex of lateral branches and male flowers than that in other tissues. Subcellular localization revealed that CmFYF was located in the nucleus. CmFYF was able to respond to multiple hormone and environmental signals including GA3, SA, MeJA, cold and drought. Ectopic expression of CmFYF in Arabidopsis resulted in the early flowering phenotype and increased plant height, but had no effect on the flower organs or fruits development. In melon, overexpression of CmFYF increased the number of male flowers, but inhibited the size of fruit. These results suggested that CmFYF of melon was partially equivalent to AtFYF of Arabidopsis. Further biochemical analyses indicated that CmFYF directly interacted with CmAGb (a homolog of Arabidopsis AGAMOUS), CmKNAT7 (KNOTTED-LIKE HOMEOBOX OF ARABIDOPSIS THALIANA 7) and itself at the protein level. Therefore, this study enriched the function of FYF homologs and revealed a preliminary molecular mechanism underlying the male flower production and fruit development in melon.

Human DDX6 regulates translation and decay of inefficiently translated mRNAs

Recent findings indicate that the translation elongation rate influences mRNA stability. One of the factors that has been implicated in this link between mRNA decay and translation speed is the yeast DEAD-box helicase Dhh1p. Here, we demonstrated that the human ortholog of Dhh1p, DDX6, triggers the deadenylation-dependent decay of inefficiently translated mRNAs in human cells. DDX6 interacts with the ribosome through the Phe-Asp-Phe (FDF) motif in its RecA2 domain. Furthermore, RecA2-mediated interactions and ATPase activity are both required for DDX6 to destabilize inefficiently translated mRNAs. Using ribosome profiling and RNA sequencing, we identified two classes of endogenous mRNAs that are regulated in a DDX6-dependent manner. The identified targets are either translationally regulated or regulated at the steady-state-level and either exhibit signatures of poor overall translation or of locally reduced ribosome translocation rates. Transferring the identified sequence stretches into a reporter mRNA caused translation- and DDX6-dependent degradation of the reporter mRNA. In summary, these results identify DDX6 as a crucial regulator of mRNA translation and decay triggered by slow ribosome movement and provide insights into the mechanism by which DDX6 destabilizes inefficiently translated mRNAs.

YTHDF2 facilitates aggresome formation via UPF1 in an m6A-independent manner

YTHDF2 has been extensively studied and typified as an RNA-binding protein that specifically recognizes and destabilizes RNAs harboring N6-methyladenosine (m6A), the most prevalent internal modification found in eukaryotic RNAs. In this study, we unravel the m6A-independent role of YTHDF2 in the formation of an aggresome, where cytoplasmic protein aggregates are selectively sequestered upon failure of protein homeostasis mediated by the ubiquitin-proteasome system. Downregulation of YTHDF2 in HeLa cells reduces the circularity of aggresomes and the rate of movement of misfolded polypeptides, inhibits aggresome formation, and thereby promotes cellular apoptosis. Mechanistically, YTHDF2 is recruited to a misfolded polypeptide-associated complex composed of UPF1, CTIF, eEF1A1, and DCTN1 through its interaction with UPF1. Subsequently, YTHDF2 increases the interaction between the dynein motor protein and the misfolded polypeptide-associated complex, facilitating the diffusion dynamics of the movement of misfolded polypeptides toward aggresomes. Therefore, our data reveal that YTHDF2 is a cellular factor involved in protein quality control.

The DEAD-box RNA helicase Dbp5 is a key protein that couples multiple steps in gene expression

Cell viability largely depends on the surveillance of mRNA export and translation. Upon pre-mRNA processing and nuclear quality control, mature mRNAs are exported into the cytoplasm via Mex67-Mtr2 attachment. At the cytoplasmic site of the nuclear pore complex, the export receptor is displaced by the action of the DEAD-box RNA helicase Dbp5. Subsequent quality control of the open reading frame requires translation. Our studies suggest an involvement of Dbp5 in cytoplasmic no-go-and non-stop decay. Most importantly, we have also identified a key function for Dbp5 in translation termination, which identifies this helicase as a master regulator of mRNA expression.

Improving the soluble expression of difficult-to-express proteins in prokaryotic expression system via protein engineering and synthetic biology strategies

Solubility and folding stability are key concerns for difficult-to-express proteins (DEPs) restricted by amino acid sequences and superarchitecture, resolved by the precise distribution of amino acids and molecular interactions as well as the assistance of the expression system. Therefore, an increasing number of tools are available to achieve efficient expression of DEPs, including directed evolution, solubilization partners, chaperones, and affluent expression hosts, among others. Furthermore, genome editing tools, such as transposons and CRISPR Cas9/dCas9, have been developed and expanded to construct engineered expression hosts capable of efficient expression ability of soluble proteins. Accounting for the accumulated knowledge of the pivotal factors in the solubility and folding stability of proteins, this review focuses on advanced technologies and tools of protein engineering, protein quality control systems, and the redesign of expression platforms in prokaryotic expression systems, as well as advances of the cell-free expression technologies for membrane proteins production.

Re-evaluating the impact of alternative RNA splicing on proteomic diversity

Alternative splicing (AS) constitutes a mechanism by which protein-coding genes and long non-coding RNA (lncRNA) genes produce more than a single mature transcript. From plants to humans, AS is a powerful process that increases transcriptome complexity. Importantly, splice variants produced from AS can potentially encode for distinct protein isoforms which can lose or gain specific domains and, hence, differ in their functional properties. Advances in proteomics have shown that the proteome is indeed diverse due to the presence of numerous protein isoforms. For the past decades, with the help of advanced high-throughput technologies, numerous alternatively spliced transcripts have been identified. However, the low detection rate of protein isoforms in proteomic studies raised debatable questions on whether AS contributes to proteomic diversity and on how many AS events are really functional. We propose here to assess and discuss the impact of AS on proteomic complexity in the light of the technological progress, updated genome annotation, and current scientific knowledge.

Co-Translational Quality Control Induced by Translational Arrest

Genetic mutations, mRNA processing errors, and lack of availability of charged tRNAs sometimes slow down or completely stall translating ribosomes. Since an incomplete nascent chain derived from stalled ribosomes may function anomalously, such as by forming toxic aggregates, surveillance systems monitor every step of translation and dispose of such products to prevent their accumulation. Over the past decade, yeast models with powerful genetics and biochemical techniques have contributed to uncovering the mechanism of the co-translational quality control system, which eliminates the harmful products generated from aberrant translation. We here summarize the current knowledge of the molecular mechanism of the co-translational quality control systems in yeast, which eliminate the incomplete nascent chain, improper mRNAs, and faulty ribosomes to maintain cellular protein homeostasis.

Nascent peptide-induced translation discontinuation in eukaryotes impacts biased amino acid usage in proteomes

Robust translation elongation of any given amino acid sequence is required to shape proteomes. Nevertheless, nascent peptides occasionally destabilize ribosomes, since consecutive negatively charged residues in bacterial nascent chains can stochastically induce discontinuation of translation, in a phenomenon termed intrinsic ribosome destabilization (IRD). Here, using budding yeast and a human factor-based reconstituted translation system, we show that IRD also occurs in eukaryotic translation. Nascent chains enriched in aspartic acid (D) or glutamic acid (E) in their N-terminal regions alter canonical ribosome dynamics, stochastically aborting translation. Although eukaryotic ribosomes are more robust to ensure uninterrupted translation, we find many endogenous D/E-rich peptidyl-tRNAs in the N-terminal regions in cells lacking a peptidyl-tRNA hydrolase, indicating that the translation of the N-terminal D/E-rich sequences poses an inherent risk of failure. Indeed, a bioinformatics analysis reveals that the N-terminal regions of ORFs lack D/E enrichment, implying that the translation defect partly restricts the overall amino acid usage in proteomes.

Distinct elongation stalls during translation are linked with distinct pathways for mRNA degradation

Key protein adapters couple translation to mRNA decay on specific classes of problematic mRNAs in eukaryotes. Slow decoding on non-optimal codons leads to codon-optimality-mediated decay (COMD) and prolonged arrest at stall sites leads to no-go decay (NGD). The identities of the decay factors underlying these processes and the mechanisms by which they respond to translational distress remain open areas of investigation. We use carefully designed reporter mRNAs to perform genetic screens and functional assays in Saccharomyces cerevisiae. We characterize the roles of Hel2, Syh1, and Smy2 in coordinating translational repression and mRNA decay on NGD reporter mRNAs, finding that Syh1 and, to a lesser extent its paralog Smy2, act in a distinct pathway from Hel2. This Syh1/Smy2-mediated pathway acts as a redundant, compensatory pathway to elicit NGD when Hel2-dependent NGD is impaired. Importantly, we observe that these NGD factors are not involved in the degradation of mRNAs enriched in non-optimal codons. Further, we establish that a key factor previously implicated in COMD, Not5, contributes modestly to the degradation of an NGD-targeted mRNA. Finally, we use ribosome profiling to reveal distinct ribosomal states associated with each reporter mRNA that readily rationalize the contributions of NGD and COMD factors to degradation of these reporters. Taken together, these results provide new insight into the role of Syh1 and Smy2 in NGD and into the ribosomal states that correlate with the activation of distinct pathways targeting mRNAs for degradation in yeast.

Complex Inclusion Bodies and Defective Proteome Hubs in Neurodegenerative Disease: New Clues, New Challenges

A healthy physiological environment of cells represents the dynamic homeostasis of crowded molecules. A subset of cellular proteome forms protein quality control (PQC) machinery to maintain an uninterrupted synthesis of new polypeptides and targeted elimination of old or defective proteins. The process of PQC may get overwhelmed under specific genetic mutations, environmental stress conditions, and aging-associated perturbances. Many of these conditions may lead to the generation of various types of aberrant protein species that may or may not accumulate as large cellular inclusions. These proteinaceous formations, referred to as inclusion bodies (IBs), could be membrane-bound or membrane-less, cytoplasmic, or nuclear. Most importantly, they could either be toxic or protective. Under acute stress conditions, the formation of aggregates may cause proteostasis failure, leading to large-scale changes in the cellular proteome compositions. However, the large insoluble IBs may act as reservoirs for many soluble proteins with high aggregation propensities, which can overwhelm the cellular chaperoning capacity and protein degradation machinery. The kinetic equilibrium between folding and unfolding, misfolding, and refolding; aggregation and degradation is perturbed in one or many neurodegenerative disorders (NDDs) associated with dementia, cognitive impairments, movement, and behavioural losses. However, a detailed interplay of IBs into the manifestation of the NDDs is unknown, and a very primitive knowledge of structural compositions of amyloid inclusions is present. The present article presents a brief evolutionary background of IBs; their functional relevance for prokaryotes, plants, and animals; and associated involvement in neuronal proteostasis.

Canary in a coal mine: collided ribosomes as sensors of cellular conditions

The recent discovery that collision of ribosomes triggers quality control and stress responses in eukaryotes has shifted the perspective of the field. Collided eukaryotic ribosomes adopt a unique structure, acting as a ubiquitin signaling platform for various response factors. While several of the signals that determine which downstream pathways are activated have been uncovered, we are only beginning to learn how the specificity for the activation of each process is achieved during collisions. This review will summarize those findings and how ribosome-associated factors act as molecular sentinels, linking aberrations in translation to the overall cellular state. Insights into how cells respond to ribosome collision events will provide greater understanding of the role of the ribosome in the maintenance of cellular homeostasis.

Cytosolic Quality Control of Mitochondrial Protein Precursors-The Early Stages of the Organelle Biogenesis

With few exceptions, proteins that constitute the proteome of mitochondria originate outside of this organelle in precursor forms. Such protein precursors follow dedicated transportation paths to reach specific parts of mitochondria, where they complete their maturation and perform their functions. Mitochondrial precursor targeting and import pathways are essential to maintain proper mitochondrial function and cell survival, thus are tightly controlled at each stage. Mechanisms that sustain protein homeostasis of the cytosol play a vital role in the quality control of proteins targeted to the organelle. Starting from their synthesis, precursors are constantly chaperoned and guided to reduce the risk of premature folding, erroneous interactions, or protein damage. The ubiquitin-proteasome system provides proteolytic control that is not restricted to defective proteins but also regulates the supply of precursors to the organelle. Recent discoveries provide evidence that stress caused by the mislocalization of mitochondrial proteins may contribute to disease development. Precursors are not only subject to regulation but also modulate cytosolic machinery. Here we provide an overview of the cellular pathways that are involved in precursor maintenance and guidance at the early cytosolic stages of mitochondrial biogenesis. Moreover, we follow the circumstances in which mitochondrial protein import deregulation disturbs the cellular balance, carefully looking for rescue paths that can restore proteostasis.

Proteostasis regulated by testis-specific ribosomal protein RPL39L maintains mouse spermatogenesis

Maintaining proteostasis is important for animal development. How proteostasis influences spermatogenesis that generates male gametes, spermatozoa, is not clear. We show that testis-specific paralog of ribosomal large subunit protein RPL39, RPL39L, is required for mouse spermatogenesis. Deletion of Rpl39l in mouse caused reduced proliferation of spermatogonial stem cells, malformed sperm mitochondria and flagella, leading to sub-fertility in males. Biochemical analyses revealed that lack of RPL39L deteriorated protein synthesis and protein quality control in spermatogenic cells, partly due to reduced biogenesis of ribosomal subunits and ribosome homeostasis. RPL39/RPL39L is likely assembled into ribosomes via H/ACA domain containing NOP10 complex early in ribosome biogenesis pathway. Furthermore, Rpl39l null mice exhibited compromised regenerative spermatogenesis after chemical insult and early degenerative spermatogenesis in aging mice. These data demonstrate that maintaining proteostasis is important for spermatogenesis, of which ribosome homeostasis maintained by ribosomal proteins coordinates translation machinery to the regulation of cellular growth.

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Rpl39l deletion causes reduced spermatogenesis and subfertility in male mice

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SSC proliferation, mitochondria and sperm flagella compromised in Rpl39l^–/– mice

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Rpl39l deletion affects ribosomal LSU formation and protein quality control

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Aberrant proteostasis affects spermatogenesis and regeneration

Nascent polypeptide within the exit tunnel stabilizes the ribosome to counteract risky translation

Continuous translation elongation, irrespective of amino acid sequences, is a prerequisite for living organisms to produce their proteomes. However, nascent polypeptide products bear an inherent risk of elongation abortion. For example, negatively charged sequences with occasional intermittent prolines, termed intrinsic ribosome destabilization (IRD) sequences, weaken the translating ribosomal complex, causing certain nascent chain sequences to prematurely terminate translation. Here, we show that most potential IRD sequences in the middle of open reading frames remain cryptic and do not interrupt translation, due to two features of the nascent polypeptide. Firstly, the nascent polypeptide itself spans the exit tunnel, and secondly, its bulky amino acid residues occupy the tunnel entrance region, thereby serving as a bridge and protecting the large and small ribosomal subunits from dissociation. Thus, nascent polypeptide products have an inbuilt ability to ensure elongation continuity.

Assessment of Sacsin Turnover in Patients With ARSACS: Implications for Molecular Diagnosis and Pathogenesis

Background and Objectives

Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) is caused by variations in SACS gene encoding sacsin, a huge multimodular protein of unknown function. More than 200 SACS variations have been described worldwide to date. Because ARSACS presents phenotypic variability, previous empirical studies attempted to correlate the nature and position of SACS variations with the age at onset or with disease severity, although not considering the effect of the various variations on protein stability. In this work, we studied genotype-phenotype correlation in ARSACS at a functional level.

Methods

We analyzed a large set of skin fibroblasts derived from patients with ARSACS, including both new and already published cases, carrying variations of different types affecting diverse domains of the protein.

Results

We found that sacsin is almost absent in patients with ARSACS, regardless of the nature of the variation. As expected, we did not detect sacsin in patients with truncating variations. We found it strikingly reduced or absent also in compound heterozygotes carrying diverse missense variations. In this case, we excluded SACS mRNA decay, defective translation, or faster posttranslational degradation as possible causes of protein reduction. Conversely, our results demonstrate that nascent mutant sacsin protein undergoes cotranslational ubiquitination and degradation.

Discussion

Our results provide a mechanistic explanation for the lack of genotype-phenotype correlation in ARSACS. We also propose a new and unambiguous criterion for ARSACS diagnosis that is based on the evaluation of sacsin level. Last, we identified preemptive degradation of a mutant protein as a novel cause of a human disease.

UPF1: From mRNA Surveillance to Protein Quality Control

Selective recognition and removal of faulty transcripts and misfolded polypeptides are crucial for cell viability. In eukaryotic cells, nonsense-mediated mRNA decay (NMD) constitutes an mRNA surveillance pathway for sensing and degrading aberrant transcripts harboring premature termination codons (PTCs). NMD functions also as a post-transcriptional gene regulatory mechanism by downregulating naturally occurring mRNAs. As NMD is activated only after a ribosome reaches a PTC, PTC-containing mRNAs inevitably produce truncated and potentially misfolded polypeptides as byproducts. To cope with the emergence of misfolded polypeptides, eukaryotic cells have evolved sophisticated mechanisms such as chaperone-mediated protein refolding, rapid degradation of misfolded polypeptides through the ubiquitin–proteasome system, and sequestration of misfolded polypeptides to the aggresome for autophagy-mediated degradation. In this review, we discuss how UPF1, a key NMD factor, contributes to the selective removal of faulty transcripts via NMD at the molecular level. We then highlight recent advances on UPF1-mediated communication between mRNA surveillance and protein quality control.

Genome-wide Survey of Ribosome Collision

Ribosome movement is not always smooth and is rather often impeded. For ribosome pauses, fundamental issues remain to be addressed, including where ribosomes pause on mRNAs, what kind of RNA/amino acid sequence causes this pause, and the physiological significance of this attenuation of protein synthesis. Here, we survey the positions of ribosome collisions caused by ribosome pauses in humans and zebrafish using modified ribosome profiling. Collided ribosomes, i.e., disomes, emerge at various sites: Pro-Pro/Gly/Asp motifs; Arg-X-Lys motifs; stop codons; and 3' untranslated regions. The electrostatic interaction between the charged nascent chain and the ribosome exit tunnel determines the eIF5A-mediated disome rescue at the Pro-Pro sites. In particular, XBP1u, a precursor of endoplasmic reticulum (ER)-stress-responsive transcription factor, shows striking queues of collided ribosomes and thus acts as a degradation substrate by ribosome-associated quality control. Our results provide insight into the causes and consequences of ribosome pause by dissecting collided ribosomes.

Ribosome stalling caused by the Argonaute-microRNA-SGS3 complex regulates the production of secondary siRNAs in plants

The path of ribosomes on mRNAs can be impeded by various obstacles. One such example is halting of ribosome movement by microRNAs, but the exact mechanism and physiological role remain unclear. Here, we find that ribosome stalling caused by the Argonaute-microRNA-SGS3 complex regulates production of secondary small interfering RNAs (siRNAs) in plants. We show that the double-stranded RNA-binding protein SGS3 interacts directly with the 3' end of the microRNA in an Argonaute protein, resulting in ribosome stalling. Importantly, microRNA-mediated ribosome stalling correlates positively with efficient production of secondary siRNAs from target mRNAs. Our results illustrate a role of paused ribosomes in regulation of small RNA function that may have broad biological implications across the plant kingdom.

Assessing the functional relevance of splice isoforms

Alternative splicing of messenger RNA can generate an array of mature transcripts, but it is not clear how many go on to produce functionally relevant protein isoforms. There is only limited evidence for alternative proteins in proteomics analyses and data from population genetic variation studies indicate that most alternative exons are evolving neutrally. Determining which transcripts produce biologically important isoforms is key to understanding isoform function and to interpreting the real impact of somatic mutations and germline variations. Here we have developed a method, TRIFID, to classify the functional importance of splice isoforms. TRIFID was trained on isoforms detected in large-scale proteomics analyses and distinguishes these biologically important splice isoforms with high confidence. Isoforms predicted as functionally important by the algorithm had measurable cross species conservation and significantly fewer broken functional domains. Additionally, exons that code for these functionally important protein isoforms are under purifying selection, while exons from low scoring transcripts largely appear to be evolving neutrally. TRIFID has been developed for the human genome, but it could in principle be applied to other well-annotated species. We believe that this method will generate valuable insights into the cellular importance of alternative splicing.

Influence of nascent polypeptide positive charges on translation dynamics

Protein segments with a high concentration of positively charged amino acid residues are often used in reporter constructs designed to activate ribosomal mRNA/protein decay pathways, such as those involving nonstop mRNA decay (NSD), no-go mRNA decay (NGD) and the ribosome quality control (RQC) complex. It has been proposed that the electrostatic interaction of the positively charged nascent peptide with the negatively charged ribosomal exit tunnel leads to translation arrest. When stalled long enough, the translation process is terminated with the degradation of the transcript and an incomplete protein. Although early experiments made a strong argument for this mechanism, other features associated with positively charged reporters, such as codon bias and mRNA and protein structure, have emerged as potent inducers of ribosome stalling. We carefully reviewed the published data on the protein and mRNA expression of artificial constructs with diverse compositions as assessed in different organisms. We concluded that, although polybasic sequences generally lead to lower translation efficiency, it appears that an aggravating factor, such as a nonoptimal codon composition, is necessary to cause translation termination events.

Nuclear SR-protein mediated mRNA quality control is continued in cytoplasmic nonsense-mediated decay

One important task of eukaryotic cells is to translate only mRNAs that were correctly processed to prevent the production of truncated proteins, found in neurodegenerative diseases and cancer. Nuclear quality control of splicing requires the SR-like proteins Gbp2 and Hrb1 in S. cerevisiae, where they promote the degradation of faulty pre-mRNAs. Here we show that Gbp2 and Hrb1 also function in nonsense mediated decay (NMD) of spliced premature termination codon (PTC)-containing mRNAs. Our data support a model in which they are in a complex with the Upf-proteins and help to transmit the Upf1-mediated PTC recognition to the transcripts ends. Most importantly they appear to promote translation repression of spliced transcripts that contain a PTC and to finally facilitate degradation of the RNA, presumably by supporting the recruitment of the degradation factors. Therefore, they seem to control mRNA quality beyond the nuclear border and may thus be global surveillance factors. Identification of SR-proteins as general cellular surveillance factors in yeast will help to understand the complex human system in which many diseases with defects in SR-proteins or NMD are known, but the proteins were not yet recognized as general RNA surveillance factors.

A ribosome-associated chaperone enables substrate triage in a cotranslational protein targeting complex

Protein biogenesis is essential in all cells and initiates when a nascent polypeptide emerges from the ribosome exit tunnel, where multiple ribosome-associated protein biogenesis factors (RPBs) direct nascent proteins to distinct fates. How distinct RPBs spatiotemporally coordinate with one another to affect accurate protein biogenesis is an emerging question. Here, we address this question by studying the role of a cotranslational chaperone, nascent polypeptide-associated complex (NAC), in regulating substrate selection by signal recognition particle (SRP), a universally conserved protein targeting machine. We show that mammalian SRP and SRP receptors (SR) are insufficient to generate the biologically required specificity for protein targeting to the endoplasmic reticulum. NAC co-binds with and remodels the conformational landscape of SRP on the ribosome to regulate its interaction kinetics with SR, thereby reducing the nonspecific targeting of signalless ribosomes and pre-emptive targeting of ribosomes with short nascent chains. Mathematical modeling demonstrates that the NAC-induced regulations of SRP activity are essential for the fidelity of cotranslational protein targeting. Our work establishes a molecular model for how NAC acts as a triage factor to prevent protein mislocalization, and demonstrates how the macromolecular crowding of RPBs at the ribosome exit site enhances the fidelity of substrate selection into individual protein biogenesis pathways.

An analysis of tissue-specific alternative splicing at the protein level

The role of alternative splicing is one of the great unanswered questions in cellular biology. There is strong evidence for alternative splicing at the transcript level, and transcriptomics experiments show that many splice events are tissue specific. It has been suggested that alternative splicing evolved in order to remodel tissue-specific protein-protein networks. Here we investigated the evidence for tissue-specific splicing among splice isoforms detected in a large-scale proteomics analysis. Although the data supporting alternative splicing is limited at the protein level, clear patterns emerged among the small numbers of alternative splice events that we could detect in the proteomics data. More than a third of these splice events were tissue-specific and most were ancient: over 95% of splice events that were tissue-specific in both proteomics and RNAseq analyses evolved prior to the ancestors of lobe-finned fish, at least 400 million years ago. By way of contrast, three in four alternative exons in the human gene set arose in the primate lineage, so our results cannot be extrapolated to the whole genome. Tissue-specific alternative protein forms in the proteomics analysis were particularly abundant in nervous and muscle tissues and their genes had roles related to the cytoskeleton and either the structure of muscle fibres or cell-cell connections. Our results suggest that this conserved tissue-specific alternative splicing may have played a role in the development of the vertebrate brain and heart.

We manually curated a set of 255 splice events detected in a large-scale tissue-based proteomics experiment and found that more than a third had evidence of significant tissue-specific differences. Events that were significantly tissue-specific at the protein level were highly conserved; almost 75% evolved over 400 million years ago. The tissues in which we found most evidence for tissue-specific splicing were nervous tissues and cardiac tissues. Genes with tissue-specific events in these two tissues had functions related to important cellular structures in brain and heart tissues. These splice events may have been essential for the development of vertebrate heart and muscle. However, our data set may not be representative of alternative exons as a whole. We found that most tissue specific splicing was strongly conserved, but just 5% of annotated alternative exons in the human gene set are ancient. More than three quarters of alternative exons are primate-derived. Although the analysis does not provide a definitive answer to the question of the functional role of alternative splicing, our results do indicate that alternative splice variants may have played a significant part in the evolution of brain and heart tissues in vertebrates.

Translational read-through promotes aggregation and shapes stop codon identity

Faithful translation of genetic information depends on the ability of the translational machinery to decode stop codons as termination signals. Although termination of protein synthesis is highly efficient, errors in decoding of stop codons may lead to the synthesis of C-terminally extended proteins. It was found that in eukaryotes such elongated proteins do not accumulate in cells. However, the mechanism for sequestration of C-terminally extended proteins is still unknown. Here we show that 3′-UTR-encoded polypeptides promote aggregation of the C-terminally extended proteins, and targeting to lysosomes. We demonstrate that 3′-UTR-encoded polypeptides can promote different levels of protein aggregation, similar to random sequences. We also show that aggregation of endogenous proteins can be induced by aminoglycoside antibiotics that promote stop codon read-through, by UAG suppressor tRNA, or by knokcdown of release factor 1. Furthermore, we find correlation between the fidelity of termination signals, and the predicted propensity of downstream 3′-UTR-encoded polypeptides to form intrinsically disordered regions. Our data highlight a new quality control mechanism for elimination of C-terminally elongated proteins.

Nonsense-mediated mRNA decay factor UPF1 promotes aggresome formation

Nonsense-mediated mRNA decay (NMD) typifies an mRNA surveillance pathway. Because NMD necessitates a translation event to recognize a premature termination codon on mRNAs, truncated misfolded polypeptides (NMD-polypeptides) could potentially be generated from NMD substrates as byproducts. Here, we show that when the ubiquitin–proteasome system is overwhelmed, various misfolded polypeptides including NMD-polypeptides accumulate in the aggresome: a perinuclear nonmembranous compartment eventually cleared by autophagy. Hyperphosphorylation of the key NMD factor UPF1 is required for selective targeting of the misfolded polypeptide aggregates toward the aggresome via the CTIF–eEF1A1–DCTN1 complex: the aggresome-targeting cellular machinery. Visualization at a single-particle level reveals that UPF1 increases the frequency and fidelity of movement of CTIF aggregates toward the aggresome. Furthermore, the apoptosis induced by proteotoxic stresses is suppressed by UPF1 hyperphosphorylation. Altogether, our data provide evidence that UPF1 functions in the regulation of a protein surveillance as well as an mRNA quality control.

Nonsense-mediated mRNA decay (NMD) is a translation-coupled process that eliminates mRNAs containing premature translation-termination codons. Here the authors identify a role for the NMD factor UPF1 in protein quality control, whereby truncated misfolded polypeptides are cleared through autophagy.

Translational Control of Secretory Proteins in Health and Disease

Secretory proteins are synthesized in a form of precursors with additional sequences at their N-terminal ends called signal peptides. The signal peptides are recognized co-translationally by signal recognition particle (SRP). This interaction leads to targeting to the endoplasmic reticulum (ER) membrane and translocation of the nascent chains into the ER lumen. It was demonstrated recently that in addition to a targeting function, SRP has a novel role in protection of secretory protein mRNAs from degradation. It was also found that the quality of secretory proteins is controlled by the recently discovered Regulation of Aberrant Protein Production (RAPP) pathway. RAPP monitors interactions of polypeptide nascent chains during their synthesis on the ribosomes and specifically degrades their mRNAs if these interactions are abolished due to mutations in the nascent chains or defects in the targeting factor. It was demonstrated that pathological RAPP activation is one of the molecular mechanisms of human diseases associated with defects in the secretory proteins. In this review, we discuss recent progress in understanding of translational control of secretory protein biogenesis on the ribosome and pathological consequences of its dysregulation in human diseases.

RQT complex dissociates ribosomes collided on endogenous RQC substrate SDD1

Ribosome-associated quality control (RQC) represents a rescue pathway in eukaryotic cells that is triggered upon translational stalling. Collided ribosomes are recognized for subsequent dissociation followed by degradation of nascent peptides. However, endogenous RQC-inducing sequences and the mechanism underlying the ubiquitin-dependent ribosome dissociation remain poorly understood. Here, we identified SDD1 messenger RNA from Saccharomyces cerevisiae as an endogenous RQC substrate and reveal the mechanism of its mRNA-dependent and nascent peptide-dependent translational stalling. In vitro translation of SDD1 mRNA enabled the reconstitution of Hel2-dependent polyubiquitination of collided disomes and, preferentially, trisomes. The distinct trisome architecture, visualized using cryo-EM, provides the structural basis for the more-efficient recognition by Hel2 compared with that of disomes. Subsequently, the Slh1 helicase subunit of the RQC trigger (RQT) complex preferentially dissociates the first stalled polyubiquitinated ribosome in an ATP-dependent manner. Together, these findings provide fundamental mechanistic insights into RQC and its physiological role in maintaining cellular protein homeostasis.

Identification of a novel trigger complex that facilitates ribosome-associated quality control in mammalian cells

Ribosome stalling triggers the ribosome-associated quality control (RQC) pathway, which targets collided ribosomes and leads to subunit dissociation, followed by proteasomal degradation of the nascent peptide. In yeast, RQC is triggered by Hel2-dependent ubiquitination of uS10, followed by subunit dissociation mediated by the RQC-trigger (RQT) complex. In mammals, ZNF598-dependent ubiquitination of collided ribosomes is required for RQC, and activating signal cointegrator 3 (ASCC3), a component of the ASCC complex, facilitates RQC. However, the roles of other components and associated factors of the ASCC complex remain unknown. Here, we demonstrate that the human RQC-trigger (hRQT) complex, an ortholog of the yeast RQT complex, plays crucial roles in RQC. The hRQT complex is composed of ASCC3, ASCC2, and TRIP4, which are orthologs of the RNA helicase Slh1(Rqt2), ubiquitin-binding protein Cue3(Rqt3), and zinc-finger type protein yKR023W(Rqt4), respectively. The ATPase activity of ASCC3 and the ubiquitin-binding activity of ASCC2 are crucial for triggering RQC. Given the proposed function of the RQT complex in yeast, we propose that the hRQT complex recognizes the ubiquitinated stalled ribosome and induces subunit dissociation to facilitate RQC.

Quality controls induced by aberrant translation

During protein synthesis, translating ribosomes encounter many challenges imposed by various types of defective mRNAs that can lead to reduced cellular fitness and, in some cases, even threaten cell viability. Aberrant translation leads to activation of one of several quality control pathways depending on the nature of the problem. These pathways promote the degradation of the problematic mRNA as well as the incomplete translation product, the nascent polypeptide chain. Many of these quality control systems feature critical roles for specialized regulatory factors that work in concert with conventional factors. This review focuses on the mechanisms used by these quality control pathways to recognize aberrant ribosome stalling and discusses the conservation of these systems.

A dual role of the ribosome-bound chaperones RAC/Ssb in maintaining the fidelity of translation termination

The yeast ribosome-associated complex RAC and the Hsp70 homolog Ssb are anchored to the ribosome and together act as chaperones for the folding and co-translational assembly of nascent polypeptides. In addition, the RAC/Ssb system plays a crucial role in maintaining the fidelity of translation termination; however, the latter function is poorly understood. Here we show that the RAC/Ssb system promotes the fidelity of translation termination via two distinct mechanisms. First, via direct contacts with the ribosome and the nascent chain, RAC/Ssb facilitates the translation of stalling-prone poly-AAG/A sequences encoding for polylysine segments. Impairment of this function leads to enhanced ribosome stalling and to premature nascent polypeptide release at AAG/A codons. Second, RAC/Ssb is required for the assembly of fully functional ribosomes. When RAC/Ssb is absent, ribosome biogenesis is hampered such that core ribosomal particles are structurally altered at the decoding and peptidyl transferase centers. As a result, ribosomes assembled in the absence of RAC/Ssb bind to the aminoglycoside paromomycin with high affinity (K_D = 76.6 nM) and display impaired discrimination between stop codons and sense codons. The combined data shed light on the multiple mechanisms by which the RAC/Ssb system promotes unimpeded biogenesis of newly synthesized polypeptides.

Release factor-dependent ribosome rescue by BrfA in the Gram-positive bacterium Bacillus subtilis

Rescue of the ribosomes from dead-end translation complexes, such as those on truncated (non-stop) mRNA, is essential for the cell. Whereas bacteria use trans-translation for ribosome rescue, some Gram-negative species possess alternative and release factor (RF)-dependent rescue factors, which enable an RF to catalyze stop-codon-independent polypeptide release. We now discover that the Gram-positive Bacillus subtilis has an evolutionarily distinct ribosome rescue factor named BrfA. Genetic analysis shows that B. subtilis requires the function of either trans-translation or BrfA for growth, even in the absence of proteotoxic stresses. Biochemical and cryo-electron microscopy (cryo-EM) characterization demonstrates that BrfA binds to non-stop stalled ribosomes, recruits homologous RF2, but not RF1, and induces its transition into an open active conformation. Although BrfA is distinct from E. coli ArfA, they use convergent strategies in terms of mode of action and expression regulation, indicating that many bacteria may have evolved as yet unidentified ribosome rescue systems.

In bacteria, the conserved trans-translation system serves as the primary pathway of ribosome rescue, but many species can also use alternative rescue pathways. Here the authors report that in B. subtilis, the rescue factor BrfA binds to non-stop stalled ribosomes, recruits RF2 but not RF1, and induces transition of the ribosome into an open active conformation.

Perspective on Alternative Splicing and Proteome Complexity in Plants

Alternative splicing (AS) generates multiple transcripts from the same gene, however, AS contribution to proteome complexity remains elusive in plants. AS is prevalent under stress conditions in plants, but it is counterintuitive why plants would invest in protein synthesis under declining energy supply. We propose that plants employ AS not only to potentially increasing proteomic complexity, but also to buffer against the stress-responsive transcriptome to reduce the metabolic cost of translating all AS transcripts. To maximise efficiency under stress, plants may make fewer proteins with disordered domains via AS to diversify substrate specificity and maintain sufficient regulatory capacity. Furthermore, we suggest that chromatin state-dependent AS engenders short/long-term stress memory to mediate reproducible transcriptional response in the future.

Translation arrest as a protein quality control system for aberrant translation of the 3'-UTR in mammalian cells

Read-through or mutations of a stop codon resulting in translation of the 3'-UTR produce potentially toxic C-terminally extended proteins. However, quality control mechanisms for such proteins are poorly understood in mammalian cells. Here, a comprehensive analysis of the 3'-UTRs of genes associated with hereditary diseases identified novel arrest-inducing sequences in the 3'-UTRs of 23 genes that can repress the levels of their protein products. In silico analysis revealed that the hydrophobicity of the polypeptides encoded in the 3'-UTRs is correlated with arrest efficiency. These results provide new insight into quality control mechanisms mediated by 3'-UTRs to prevent the production of C-terminally extended cytotoxic proteins.

RNA Regulation in Plant Cold Stress Response

In addition to plants, all organisms react to environmental stimuli via the perception of signals and subsequently respond through alterations of gene expression. However, genes/mRNAs are usually not the functional unit themselves, and instead, resultant protein products with individual functions result in various acquired phenotypes. In order to fully characterize the adaptive responses of plants to environmental stimuli, it is essential to determine the level of proteins, in addition to the regulation of mRNA expression. This regulatory step, which is referred to as "mRNA posttranscriptional regulation," occurs subsequent to mRNA transcription and prior to translation. Although these RNA regulatory mechanisms have been well-studied in many organisms, including plants, it is not fully understood how plants respond to environmental stimuli, such as cold stress, via these RNA regulations.A recent study described several RNA regulatory factors in relation to environmental stress responses, including plant cold stress tolerance. In this chapter, the functions of RNA regulatory factors and comprehensive analyses related to the RNA regulations involved in cold stress response are summarized, such as mRNA maturation, including capping, splicing, polyadenylation of mRNA, and the quality control system of mRNA; mRNA degradation, including the decapping step; and mRNA stabilization. In addition, the putative roles of messenger ribonucleoprotein (mRNP) granules, such as processing bodies (PBs) and stress granules (SGs), which are cytoplasmic particles, are described in relation to RNA regulations under stress conditions. These RNA regulatory systems are important for adjusting or fine-tuning and determining the final levels of mRNAs and proteins in order to adapt or respond to environmental stresses. Collectively, these new areas of study revealed that plants possess precise novel regulatory mechanisms which specifically function in the response to cold stress.

Insights into RNA-processing pathways and associated RNA-degrading enzymes in Archaea

RNA-processing pathways are at the centre of regulation of gene expression. All RNA transcripts undergo multiple maturation steps in addition to covalent chemical modifications to become functional in the cell. This includes destroying unnecessary or defective cellular RNAs. In Archaea, information on mechanisms by which RNA species reach their mature forms and associated RNA-modifying enzymes are still fragmentary. To date, most archaeal actors and pathways have been proposed in light of information gathered from Bacteria and Eukarya. In this context, this review provides a state of the art overview of archaeal endoribonucleases and exoribonucleases that cleave and trim RNA species and also of the key small archaeal proteins that bind RNAs. Furthermore, synthetic up-to-date views of processing and biogenesis pathways of archaeal transfer and ribosomal RNAs as well as of maturation of stable small non-coding RNAs such as CRISPR RNAs, small C/D and H/ACA box guide RNAs, and other emerging classes of small RNAs are described. Finally, prospective post-transcriptional mechanisms to control archaeal messenger RNA quality and quantity are discussed.

Release of Ubiquitinated and Non-ubiquitinated Nascent Chains from Stalled Mammalian Ribosomal Complexes by ANKZF1 and Ptrh1

The ribosome-associated quality control (RQC) pathway degrades nascent chains (NCs) arising from interrupted translation. First, recycling factors split stalled ribosomes, yielding NC-tRNA/60S ribosome-nascent chain complexes (60S RNCs). 60S RNCs associate with NEMF, which recruits the E3 ubiquitin ligase Listerin that ubiquitinates NCs. The mechanism of subsequent ribosomal release of Ub-NCs remains obscure. We found that, in non-ubiquitinated 60S RNCs and 80S RNCs formed on non-stop mRNAs, tRNA is not firmly fixed in the P site, which allows peptidyl-tRNA hydrolase Ptrh1 to cleave NC-tRNA, suggesting the existence of a pathway involving release of non-ubiquitinated NCs. Association with NEMF and Listerin and ubiquitination of NCs results in accommodation of NC-tRNA, rendering 60S RNCs resistant to Ptrh1 but susceptible to ANKZF1, which induces specific cleavage in the tRNA acceptor arm, releasing proteasome-degradable Ub-NCs linked to four 3'-terminal tRNA nucleotides. We also found that TCF25, a poorly characterized RQC component, ensures preferential formation of the K48-ubiquitin linkage.

Molecular mechanism of ER stress-induced pre-emptive quality control involving association of the translocon, Derlin-1, and HRD1

The maintenance of endoplasmic reticulum (ER) homeostasis is essential for cell function. ER stress-induced pre-emptive quality control (ERpQC) helps alleviate the burden to a stressed ER by limiting further protein loading. We have previously reported the mechanisms of ERpQC, which includes a rerouting step and a degradation step. Under ER stress conditions, Derlin family proteins (Derlins), which are components of ER-associated degradation, reroute specific ER-targeting proteins to the cytosol. Newly synthesized rerouted polypeptides are degraded via the cytosolic chaperone Bag6 and the AAA-ATPase p97 in the ubiquitin-proteasome system. However, the mechanisms by which ER-targeting proteins are rerouted from the ER translocation pathway to the cytosolic degradation pathway and how the E3 ligase ubiquitinates ERpQC substrates remain unclear. Here, we show that ERpQC substrates are captured by the carboxyl-terminus region of Derlin-1 and ubiquitinated by the HRD1 E3 ubiquitin ligase prior to degradation. Moreover, HRD1 forms a large ERpQC-related complex composed of Sec61α and Derlin-1 during ER stress. These findings indicate that the association of the degradation factor HRD1 with the translocon and the rerouting factor Derlin-1 may be necessary for the smooth and effective clearance of ERpQC substrates.

Polylysine is a Proteostasis Network-Engaging Structural Determinant

C-terminal polylysine (PL) can be synthesized from the polyadenine tail of prematurely cleaved mRNAs or when a read-though of a stop codon happens. Due to the highly positive charge, PL stalls in the electrostatically negative ribosomal exit channel. The stalled polypeptide recruits the Ribosome-associated quality control (RQC) complex which processes and extracts the nascent chain. Dysfunction of the RQC leads to the accumulation of PL-tagged proteins, induction of a stress response, and cellular toxicity. Not much is known about the PL-specific aspect of protein quality control. Using quantitative mass spectrometry, we uncovered the post-ribosomal PL-processing machinery in human cytosol. It encompasses key cytosolic complexes of the proteostasis network, such as chaperonin TCP-1 ring complexes (TRiC) and half-capped 19S-20S proteasomes. Furthermore, we found that the nuclear transport machinery associates with PL, which suggests a novel mechanism by which faulty proteins can be compartmentalized in the cell. The enhanced nuclear import of a PL-tagged polypeptide confirmed this implication, which leads to questions regarding the biological rationale behind it.

Ribothrypsis, a novel process of canonical mRNA decay, mediates ribosome-phased mRNA endonucleolysis

mRNAs transmit the genetic information that dictates protein production and are a nexus for numerous pathways that regulate gene expression. The prevailing view of canonical mRNA decay is that it is mediated by deadenylation and decapping followed by exonucleolysis from the 3' and 5' ends. By developing Akron-seq, a novel approach that captures the native 3' and 5' ends of capped and polyadenylated RNAs, respectively, we show that canonical human mRNAs are subject to repeated cotranslational and ribosome-phased endonucleolytic cuts at the exit site of the mRNA ribosome channel, in a process that we term ribothrypsis. We uncovered RNA G quadruplexes among likely ribothrypsis triggers and show that ribothrypsis is a conserved process. Strikingly, we found that mRNA fragments are abundant in living cells and thus have important implications for the interpretation of experiments, such as RNA-seq, that rely on the assumption that mRNAs exist largely as full-length molecules in vivo.

Control of mitochondrial biogenesis and function by the ubiquitin-proteasome system

Mitochondria are pivotal organelles in eukaryotic cells. The complex proteome of mitochondria comprises proteins that are encoded by nuclear and mitochondrial genomes. The biogenesis of mitochondrial proteins requires their transport in an unfolded state with a high risk of misfolding. The mislocalization of mitochondrial proteins is deleterious to the cell. The electron transport chain in mitochondria is a source of reactive oxygen species that damage proteins. Mitochondrial dysfunction is linked to many pathological conditions and, together with the loss of cellular protein homeostasis (proteostasis), are hallmarks of ageing and ageing-related degeneration diseases. The pathogenesis of neurodegenerative disorders, such as Alzheimer's disease and Parkinson's disease, has been associated with mitochondrial and proteostasis failure. Thus, mitochondrial proteins require sophisticated surveillance mechanisms. Although mitochondria form a proteasome-exclusive compartment, multiple lines of evidence indicate a crucial role for the cytosolic ubiquitin-proteasome system (UPS) in the quality control of mitochondrial proteins. The proteasome affects mitochondrial proteins at stages of their biogenesis and maturity. The effects of the UPS go beyond the removal of damaged proteins and include the adjustment of mitochondrial proteome composition, the regulation of organelle dynamics and the protection of cellular homeostasis against mitochondrial failure. In turn, mitochondrial activity and mitochondrial dysfunction adjust the activity of the UPS, with implications at the cellular level.

Intrinsic Ribosome Destabilization Underlies Translation and Provides an Organism with a Strategy of Environmental Sensing

Nascent polypeptides can modulate the polypeptide elongation speed on the ribosome. Here, we show that nascent chains can even destabilize the translating Escherichia coli ribosome from within. This phenomenon, termed intrinsic ribosome destabilization (IRD), occurs in response to a special amino acid sequence of the nascent chain, without involving the release or the recycling factors. Typically, a consecutive array of acidic residues and those intermitted by alternating prolines induce IRD. The ribosomal protein bL31, which bridges the two subunits, counteracts IRD, such that only strong destabilizing sequences abort translation in living cells. We found that MgtL, the leader peptide of a Mg²⁺ transporter (MgtA), contains a translation-aborting sequence, which sensitizes the ribosome to a decline in Mg²⁺ concentration and thereby triggers the MgtA-upregulating genetic scheme. Translation proceeds at an inherent risk of ribosomal destabilization, and nascent chain-ribosome complexes can function as a Mg²⁺ sensor by harnessing IRD.

Preribosomes escaping from the nucleus are caught during translation by cytoplasmic quality control

Assembly of fully functional ribosomes is a prerequisite for failsafe translation. This explains why maturing preribosomal subunits have to pass through an array of quality-control checkpoints, including nuclear export, to ensure that only properly assembled ribosomes engage in translation. Despite these safeguards, we found that nuclear pre-60S particles unable to remove a transient structure composed of ITS2 pre-rRNA and associated assembly factors, termed the 'foot', escape to the cytoplasm, where they can join with mature 40S subunits to catalyze protein synthesis. However, cells harboring these abnormal ribosomes show translation defects indicated by the formation of 80S ribosomes poised with pre-60S subunits carrying tRNAs in trapped hybrid states. To overcome this translational stress, the cytoplasmic surveillance machineries RQC and Ski-exosome target these malfunctioning ribosomes. Thus, pre-60S subunits that escape nuclear quality control can enter translation, but are caught by cytoplasmic surveillance mechanisms.

Cryo-EM structures of the 80S ribosomes from human parasites Trichomonas vaginalis and Toxoplasma gondii

As an indispensable molecular machine universal in all living organisms, the ribosome has been selected by evolution to be the natural target of many antibiotics and small-molecule inhibitors. High-resolution structures of pathogen ribosomes are crucial for understanding the general and unique aspects of translation control in disease-causing microbes. With cryo-electron microscopy technique, we have determined structures of the cytosolic ribosomes from two human parasites, Trichomonas vaginalis and Toxoplasma gondii, at resolution of 3.2-3.4 Å. Although the ribosomal proteins from both pathogens are typical members of eukaryotic families, with a co-evolution pattern between certain species-specific insertions/extensions and neighboring ribosomal RNA (rRNA) expansion segments, the sizes of their rRNAs are sharply different. Very interestingly, rRNAs of T. vaginalis are in size comparable to prokaryotic counterparts, with nearly all the eukaryote-specific rRNA expansion segments missing. These structures facilitate the dissection of evolution path for ribosomal proteins and RNAs, and may aid in design of novel translation inhibitors.

In vitro analysis of RQC activities provides insights into the mechanism and function of CAT tailing

Ribosomes can stall during translation due to defects in the mRNA template or translation machinery, leading to the production of incomplete proteins. The Ribosome-associated Quality control Complex (RQC) engages stalled ribosomes and targets nascent polypeptides for proteasomal degradation. However, how each RQC component contributes to this process remains unclear. Here we demonstrate that key RQC activities-Ltn1p-dependent ubiquitination and Rqc2p-mediated Carboxy-terminal Alanine and Threonine (CAT) tail elongation-can be recapitulated in vitro with a yeast cell-free system. Using this approach, we determined that CAT tailing is mechanistically distinct from canonical translation, that Ltn1p-mediated ubiquitination depends on the poorly characterized RQC component Rqc1p, and that the process of CAT tailing enables robust ubiquitination of the nascent polypeptide. These findings establish a novel system to study the RQC and provide a framework for understanding how RQC factors coordinate their activities to facilitate clearance of incompletely synthesized proteins.

Ribosomal Stalling During Translation: Providing Substrates for Ribosome-Associated Protein Quality Control

Cells of all organisms survey problems during translation elongation, which may happen as a consequence of mRNA aberrations, inefficient decoding, or other sources. In eukaryotes, ribosome-associated quality control (RQC) senses elongation-stalled ribosomes and promotes dissociation of ribosomal subunits. This so-called ribosomal rescue releases the mRNA for degradation and allows 40S subunits to be recycled for new rounds of translation. However, the nascent polypeptide chains remain linked to tRNA and associated with the rescued 60S subunits. As a final critical step in this pathway, the Ltn1/Listerin E3 ligase subunit of the RQC complex (RQCc) ubiquitylates the nascent chain, which promotes clearance of the 60S subunit while simultaneously marking the nascent chain for elimination. Here we review the ribosomal stalling and rescue steps upstream of the RQCc, where one witnesses intersection with cellular machineries implicated in translation elongation, translation termination, ribosomal subunit recycling, and mRNA quality control. We emphasize both recent progress and future directions in this area, as well as examples linking ribosomal rescue with the production of Ltn1-RQCc substrates.

Pervasive yet nonuniform contributions of Dcp2 and Cnot7 to maternal mRNA clearance in zebrafish

mRNA degradation is a fundamental biological process that erases transcribed genetic information from cells. During maternal-to-zygotic transition of animal development, thousands of maternal mRNAs are degraded by multiple mechanisms including microRNAs and codon-mediated decay. Enzymatic requirements for maternal mRNA clearance, however, are not fully understood. Here, we analyzed a contribution of the decapping enzyme Dcp2 to maternal mRNA clearance in zebrafish by over-expressing catalytically inactive Dcp2 and performing RNA-seq analysis. As expected, Dcp2 had a widespread role in maternal mRNA clearance. Interestingly, each mRNA showed differential dependency on Dcp2-mediated decapping and Cnot7-mediated deadenylation for degradation. Correlation analysis identified several mRNA features that were associated with the observed differential dependency. Our results show pervasive yet nonuniform contributions of the decapping enzyme Dcp2 and the deadenylase Cnot7 to maternal mRNA clearance.

The Growing Toolbox for Protein Synthesis Studies

Protein synthesis stands at the last stage of the central dogma of molecular biology, providing a final regulatory layer for gene expression. Reacting to environmental cues and internal signals, the translation machinery can quickly tune the translatome from a pre-existing pool of RNAs, before the transcriptome changes. Although the translation reaction itself has been known since the 1950s, the quantitative or even qualitative measurement of its efficacy in cells has posed experimental and analytic hurdles. In this review, we outline the array of state-of-the-art methods that have emerged to tackle the hidden aspects of translational control.

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).