Structural basis of co-translational quality control by ArfA and RF2 bound to ribosome

Allosteric activation of VCP, an AAA unfoldase, by small molecule mimicry

The loss of function of AAA (ATPases associated with diverse cellular activities) mechanoenzymes has been linked to diseases, and small molecules that activate these proteins can be powerful tools to probe mechanisms and test therapeutic hypotheses. Unlike chemical inhibitors that can bind a single conformational state to block enzyme function, activator binding must be permissive to different conformational states needed for mechanochemistry. However, we do not know how AAA proteins can be activated by small molecules. Here, we focus on valosin-containing protein (VCP)/p97, an AAA unfoldase whose loss of function has been linked to protein aggregation-based disorders, to identify druggable sites for chemical activators. We identified VCP ATPase Activator 1 (VAA1), a compound that dose-dependently stimulates VCP ATPase activity up to ~threefold. Our cryo-EM studies resulted in structures (ranging from ~2.9 to 3.7 Å-resolution) of VCP in apo and ADP-bound states and revealed that VAA1 binds an allosteric pocket near the C-terminus in both states. Engineered mutations in the VAA1-binding site confer resistance to VAA1, and furthermore, modulate VCP activity. Mutation of a phenylalanine residue in the VCP C-terminal tail that can occupy the VAA1 binding site also stimulates ATPase activity, suggesting that VAA1 acts by mimicking this interaction. Together, our findings uncover a druggable allosteric site and a mechanism of enzyme regulation that can be tuned through small molecule mimicry.

Mechanistic insights into the alternative ribosome recycling by HflXr

During stress conditions such as heat shock and antibiotic exposure, ribosomes stall on messenger RNAs, leading to inhibition of protein synthesis. To remobilize ribosomes, bacteria use rescue factors such as HflXr, a homolog of the conserved housekeeping GTPase HflX that catalyzes the dissociation of translationally inactive ribosomes into individual subunits. Here we use time-resolved cryo-electron microscopy to elucidate the mechanism of ribosome recycling by Listeria monocytogenes HflXr. Within the 70S ribosome, HflXr displaces helix H69 of the 50S subunit and induces long-range movements of the platform domain of the 30S subunit, disrupting inter-subunit bridges B2b, B2c, B4, B7a and B7b. Our findings unveil a unique ribosome recycling strategy by HflXr which is distinct from that mediated by RRF and EF-G. The resemblance between HflXr and housekeeping HflX suggests that the alternative ribosome recycling mechanism reported here is universal in the prokaryotic kingdom.

Allosteric activation of VCP, a AAA unfoldase, by small molecule mimicry

The loss of function of AAA (ATPases associated with diverse cellular activities) mechanoenzymes has been linked to diseases, and small molecules that activate these proteins can be powerful tools to probe mechanisms and test therapeutic hypotheses. Unlike chemical inhibitors that can bind a single conformational state to block enzyme activity, activator binding must be permissive to different conformational states needed for enzyme function. However, we do not know how AAA proteins can be activated by small molecules. Here, we focus on valosin-containing protein (VCP)/p97, a AAA unfoldase whose loss of function has been linked to protein aggregation-based disorders, to identify druggable sites for chemical activators. We identified VCP Activator 1 (VA1), a compound that dose-dependently stimulates VCP ATPase activity up to ∼3-fold. Our cryo-EM studies resulted in structures (∼2.9-3.5 Å-resolution) of VCP in apo and ADP-bound states, and revealed VA1 binding an allosteric pocket near the C-terminus in both states. Engineered mutations in the VA1 binding site confer resistance to VA1, and furthermore, modulate VCP activity to a similar level as VA1-mediated activation. The VA1 binding site can alternatively be occupied by a phenylalanine residue in the VCP C-terminal tail, a motif that is post-translationally modified and interacts with cofactors. Together, our findings uncover a druggable allosteric site and a mechanism of enzyme regulation that can be tuned through small molecule mimicry.

Significance

The loss of function of valosin-containing protein (VCP/p97), a mechanoenzyme from the AAA superfamily that hydrolyzes ATP and uses the released energy to extract or unfold substrate proteins, is linked to protein aggregation-based disorders. However, druggable allosteric sites to activate VCP, or any AAA mechanoenzyme, have not been identified. Here, we report cryo-EM structures of VCP in two states in complex with VA1, a compound we identified that dose-dependently stimulates VCP's ATP hydrolysis activity. The VA1 binding site can also be occupied by a phenylalanine residue in the VCP C-terminal tail, suggesting that VA1 acts through mimicry of this interaction. Our study reveals a druggable allosteric site and a mechanism of enzyme regulation.

Channel capacity of the ribosome

Translation is one of the most fundamental processes in the biological cell. Because of the central role that translation plays across all domains of life, the enzyme that carries out this process, the ribosome, is required to process information with high accuracy. This accuracy often approaches values near unity experimentally. In this paper, we model the ribosome as an information channel and demonstrate mathematically that this biological machine has information-processing capabilities that have not been recognized previously. In particular, we calculate bounds on the ribosome's theoretical Shannon capacity and numerically approximate this capacity. Finally, by incorporating estimates on the ribosome's operation time, we show that the ribosome operates at speeds safely below its capacity, allowing the ribosome to process information with an arbitrary degree of error. Our results show that the ribosome achieves a high accuracy in line with purely information-theoretic means.

Prophage excision switches the primary ribosome rescue pathway and rescue-associated gene regulations in Escherichia coli

Escherichia coli has multiple pathways to release nonproductive ribosome complexes stalled at the 3' end of nonstop mRNA: tmRNA (SsrA RNA)-mediated trans-translation and stop codon-independent termination by ArfA/RF2 or ArfB (YaeJ). The arfA mRNA lacks a stop codon and its expression is repressed by trans-translation. Therefore, ArfA is considered to complement the ribosome rescue activity of trans-translation, but the physiological situations in which ArfA is expressed have not been elucidated. Here, we found that the excision of CP4-57 prophage adjacent to E. coli ssrA leads to the inactivation of tmRNA and switches the primary rescue pathway from trans-translation to ArfA/RF2. This "rescue-switching" rearranges not only the proteome landscape in E. coli but also the phenotype such as motility. Furthermore, among the proteins with significantly increased abundance in the ArfA⁺ cells, we found ZntR, whose mRNA is transcribed together as the upstream part of nonstop arfA mRNA. Repression of ZntR and reconstituted model genes depends on the translation of the downstream nonstop ORFs that trigger the trans-translation-coupled exonucleolytic degradation by polynucleotide phosphorylase (PNPase). Namely, our studies provide a novel example of trans-translation-dependent regulation and re-define the physiological roles of prophage excision.

The ribosome rescue pathways SsrA-SmpB, ArfA, and ArfB mediate tolerance to heat and antibiotic stresses in Azotobacter vinelandii

Bacteria have a mechanism to rescue stalled ribosomes known as trans-translation consisting of SsrA, a transfer-messenger RNA (tmRNA), and the small protein SmpB. Other alternative rescue mechanisms mediated by ArfA and ArfB proteins are present only in some species. Ribosome rescue mechanisms also play a role in tolerance to antibiotics and various stresses such as heat. This study shows that the genome of the soil bacterium A. vinelandii harbours genes encoding for tmRNA, SmpB, two paralogs of ArfA (arfA1 and arfA2), and ArfB. A number of mutant strains carrying mutations in the ssrA, arfA1, arfA2, and arfB genes were constructed and tested for their growth and susceptibility to heat and the antibiotic tetracycline. We found that the inactivation of both ssrA and one or the two arfA genes was detrimental to growth and caused a higher susceptibility to heat and to the antibiotic tetracycline. Interestingly, the arfB mutant strain was unable to grow after 2 h of incubation at 45°C. Inactivation of arfB in the ssrA-arfA1-arfA2 strain caused a lethal phenotype since the quadruple mutant could not be isolated. Taken together, our data suggest that both arfA1 and arfA2, as well as arfB, are functional as back up mechanisms, and that the ArfB pathway has an essential role that confers A. vinelandii resistance to high temperatures.

Sequential rescue and repair of stalled and damaged ribosome by bacterial PrfH and RtcB

RtcB is involved in transfer RNA (tRNA) splicing in archaeal and eukaryotic organisms. However, most RtcBs are found in bacteria, whose tRNAs have no introns. Because tRNAs are the substrates of archaeal and eukaryotic RtcB, it is assumed that bacterial RtcBs are for repair of damaged tRNAs. Here, we show that a subset of bacterial RtcB, denoted RtcB2 herein, specifically repair ribosomal damage in the decoding center. To access the damage site for repair, however, the damaged 70S ribosome needs to be dismantled first, and this is accomplished by bacterial PrfH. Peptide-release assays revealed that PrfH is only active with the damaged 70S ribosome but not with the intact one. A 2.55-Å cryo-electron microscopy structure of PrfH in complex with the damaged 70S ribosome provides molecular insight into PrfH discriminating between the damaged and the intact ribosomes via specific recognition of the cleaved 3'-terminal nucleotide. RNA repair assays demonstrated that RtcB2 efficiently repairs the damaged 30S ribosomal subunit but not the damaged tRNAs. Cell-based assays showed that the RtcB2-PrfH pair reverse the damage inflicted by ribosome-specific ribotoxins in vivo. Thus, our combined biochemical, structural, and cell-based studies have uncovered a bacterial defense system specifically evolved to reverse the lethal ribosomal damage in the decoding center for cell survival.

Maintaining mitochondrial ribosome function: The role of ribosome rescue and recycling factors

The universally conserved process of protein biosynthesis is crucial for maintaining cellular homoeostasis and in eukaryotes, mitochondrial translation is essential for aerobic energy production. Mitochondrial ribosomes (mitoribosomes) are highly specialized to synthesize 13 core subunits of the oxidative phosphorylation (OXPHOS) complexes. Although the mitochondrial translation machinery traces its origin from a bacterial ancestor, it has acquired substantial differences within this endosymbiotic environment. The cycle of mitoribosome function proceeds through the conserved canonical steps of initiation, elongation, termination and mitoribosome recycling. However, when mitoribosomes operate in the context of limited translation factors or on aberrant mRNAs, they can become stalled and activation of rescue mechanisms is required. This review summarizes recent advances in the understanding of protein biosynthesis in mitochondria, focusing especially on the mechanistic and physiological details of translation termination, and mitoribosome recycling and rescue.

Conserved heterodimeric GTPase Rbg1/Tma46 promotes efficient translation in eukaryotic cells

Conserved developmentally regulated guanosine triphosphate (GTP)-binding proteins (Drgs) and their binding partner Drg family regulatory proteins (Dfrps) are important for embryonic development, cellular growth control, differentiation, and proliferation. Here, we report that the yeast Drg1/Dfrp1 ortholog Rbg1/Tma46 facilitates translational initiation, elongation, and termination by suppressing prolonged ribosome pausing. Consistent with the genome-wide observations, deletion of Rbg1 exacerbates the growth defect resulting from translation stalling, and Rbg1 stabilizes mRNAs against no-go decay. Furthermore, we provide a cryoelectron microscopy (cryo-EM) structure of the 80S ribosome bound with Rbg1/Tma46 that reveals the molecular interactions responsible for Rbg1/Tma46 function. The Rbg1 subunit binds to the GTPase association center of the ribosome and the A-tRNA, and the N-terminal zinc finger domain of the Tma46 subunit binds to the 40S, establishing an interaction critical for the ribosomal association. Our results answer the fundamental question of how a paused ribosome resumes translation and show that Drg1/Dfrp1 play a critical role in ensuring orderly translation.

RR3DD: an RNA global structure-based RNA three-dimensional structural classification database

The three-dimensional (3D) structure of RNA usually plays an important role in the recognition with RNA-binding protein. Along with the discovering of RNAs, several RNA databases are developed to study the functions of RNA based on sequence, secondary structure, local 3D structural motif and global structure. Based on RNA function and structure, different RNAs are classified and stored in SCOR and DARTS, respectively. The classification of RNA structures is useful in RNA structure prediction and function annotation. However, the SCOR and DARTS are not updated any more. In this study, we present an RNA classification database RR3DD based on RNA fold with the global 3D structural similarity. The RR3DD includes 13,601 RNA chains from PDB and mmCIF format structures which are classified into 780 RNA folds. The RNA chains from PDB and mmCIF format structures are aligned and clustered into 675 and 220 RNA folds, respectively. By analysing the RNA structure in RR3DD, we find that there are 11 clusters with more than 50 members. These clusters include rRNAs, riboswitches, tRNAs and so on. By mapping RR3DD into Rfam, we found that some RNAs without annotation by Rfam can be annotated through structural alignment. For example, we analysed tRNAs and found that tRNA were successfully grouped in RR3DD for which Rfam did not classify them into one family. Finally, we provide a web interface of RR3DD offering functions of browsing RR3DD, annotating RNA 3D structure and finding templates for RNA homology modelling.

Diversity and Similarity of Termination and Ribosome Rescue in Bacterial, Mitochondrial, and Cytoplasmic Translation

When a ribosome encounters the stop codon of an mRNA, it terminates translation, releases the newly made protein, and is recycled to initiate translation on a new mRNA. Termination is a highly dynamic process in which release factors (RF1 and RF2 in bacteria; eRF1•eRF3•GTP in eukaryotes) coordinate peptide release with large-scale molecular rearrangements of the ribosome. Ribosomes stalled on aberrant mRNAs are rescued and recycled by diverse bacterial, mitochondrial, or cytoplasmic quality control mechanisms. These are catalyzed by rescue factors with peptidyl-tRNA hydrolase activity (bacterial ArfA•RF2 and ArfB, mitochondrial ICT1 and mtRF-R, and cytoplasmic Vms1), that are distinct from each other and from release factors. Nevertheless, recent structural studies demonstrate a remarkable similarity between translation termination and ribosome rescue mechanisms. This review describes how these pathways rely on inherent ribosome dynamics, emphasizing the active role of the ribosome in all translation steps.

Structural basis of l-tryptophan-dependent inhibition of release factor 2 by the TnaC arrest peptide

In Escherichia coli, elevated levels of free l-tryptophan (l-Trp) promote translational arrest of the TnaC peptide by inhibiting its termination. However, the mechanism by which translation-termination by the UGA-specific decoding release factor 2 (RF2) is inhibited at the UGA stop codon of stalled TnaC-ribosome-nascent chain complexes has so far been ambiguous. This study presents cryo-EM structures for ribosomes stalled by TnaC in the absence and presence of RF2 at average resolutions of 2.9 and 3.5 Å, respectively. Stalled TnaC assumes a distinct conformation composed of two small α-helices that act together with residues in the peptide exit tunnel (PET) to coordinate a single L-Trp molecule. In addition, while the peptidyl-transferase center (PTC) is locked in a conformation that allows RF2 to adopt its canonical position in the ribosome, it prevents the conserved and catalytically essential GGQ motif of RF2 from adopting its active conformation in the PTC. This explains how translation of the TnaC peptide effectively allows the ribosome to function as a L-Trp-specific small-molecule sensor that regulates the tnaCAB operon.

Schrödinger's T Cells: Molecular Insights Into Stemness and Exhaustion

T cell stemness and exhaustion coexist as two key contrasting phenomena during chronic antigen stimulation, such as infection, transplant, cancer, and autoimmunity. T cell exhaustion refers to the progressive loss of effector function caused by chronic antigen exposure. Exhausted T (T_EX) cells highly express multiple inhibitory receptors and exhibit severe defects in cell proliferation and cytokine production. The term T cell stemness describes the stem cell-like behaviors of T cells, including self-renewal, multipotency, and functional persistence. It is well accepted that naïve and some memory T cell subsets have stem cell-like properties. When investigating the exhaustive differentiation of T cells in chronic infection and cancer, recent studies highlighted the stemness of "precursors of exhausted" T (T_PEX) cells prior to their terminal differentiation to T_EX cells. Clinically successful checkpoint blockades for cancer treatment appear to invigorate antitumor T_PEX cells but not T_EX cells. Here we discuss the transcriptional and epigenetic regulations of T cell stemness and exhaustion, with a focus on how systems immunology was and will be utilized to define the molecular basis underlying the transition of T_PEX to T_EX cells. We suggest a "stepwise model" of T cell stemness and exhaustion, in which loss of stemness and exhaustion progression are gradual multi-step processes. We provide perspectives on the research needed to define T cell stemness and exhaustion in the transplantation setting, in which allogenic T cells are also chronically exposed to alloantigens. A better understanding of T cell stemness and exhaustion will shed light on developing novel strategies for immunotherapies.

Ribosome Rescue Pathways in Bacteria

Ribosomes that become stalled on truncated or damaged mRNAs during protein synthesis must be rescued for the cell to survive. Bacteria have evolved a diverse array of rescue pathways to remove the stalled ribosomes from the aberrant mRNA and return them to the free pool of actively translating ribosomes. In addition, some of these pathways target the damaged mRNA and the incomplete nascent polypeptide chain for degradation. This review highlights the recent developments in our mechanistic understanding of bacterial ribosomal rescue systems, including drop-off, trans-translation mediated by transfer-messenger RNA and small protein B, ribosome rescue by the alternative rescue factors ArfA and ArfB, as well as Bacillus ribosome rescue factor A, an additional rescue system found in some Gram-positive bacteria, such as Bacillus subtilis. Finally, we discuss the recent findings of ribosome-associated quality control in particular bacterial lineages mediated by RqcH and RqcP. The importance of rescue pathways for bacterial survival suggests they may represent novel targets for the development of new antimicrobial agents against multi-drug resistant pathogenic bacteria.

ArfB can displace mRNA to rescue stalled ribosomes

Ribosomes stalled during translation must be rescued to replenish the pool of translation-competent ribosomal subunits. Bacterial alternative rescue factor B (ArfB) releases nascent peptides from ribosomes stalled on mRNAs truncated at the A site, allowing ribosome recycling. Prior structural work revealed that ArfB recognizes such ribosomes by inserting its C-terminal α-helix into the vacant mRNA tunnel. In this work, we report that ArfB can efficiently recognize a wider range of mRNA substrates, including longer mRNAs that extend beyond the A-site codon. Single-particle cryo-EM unveils that ArfB employs two modes of function depending on the mRNA length. ArfB acts as a monomer to accommodate a shorter mRNA in the ribosomal A site. By contrast, longer mRNAs are displaced from the mRNA tunnel by more than 20 Å and are stabilized in the intersubunit space by dimeric ArfB. Uncovering distinct modes of ArfB function resolves conflicting biochemical and structural studies, and may lead to re-examination of other ribosome rescue pathways, whose functions depend on mRNA lengths.

Mechanism of ribosome rescue by alternative ribosome-rescue factor B

Alternative ribosome-rescue factor B (ArfB) rescues ribosomes stalled on non-stop mRNAs by releasing the nascent polypeptide from the peptidyl-tRNA. By rapid kinetics we show that ArfB selects ribosomes stalled on short truncated mRNAs, rather than on longer mRNAs mimicking pausing on rare codon clusters. In combination with cryo-electron microscopy we dissect the multistep rescue pathway of ArfB, which first binds to ribosomes very rapidly regardless of the mRNA length. The selectivity for shorter mRNAs arises from the subsequent slow engagement step, as it requires longer mRNA to shift to enable ArfB binding. Engagement results in specific interactions of the ArfB C-terminal domain with the mRNA entry channel, which activates peptidyl-tRNA hydrolysis by the N-terminal domain. These data reveal how protein dynamics translate into specificity of substrate recognition and provide insights into the action of a putative rescue factor in mitochondria.

Rescue of ribosomes stalled on non-stop mRNA is essential for cell viability, and several rescue systems to resolve stalling exist in bacteria. Here, the authors use rapid kinetics and cryo-EM to reveal the pathway and selectivity mechanism of ArfB-mediated ribosome rescue.

Molecular determinants of release factor 2 for ArfA-mediated ribosome rescue

Translation termination in bacteria requires that the stop codon be recognized by release factor RF1 or RF2, leading to hydrolysis of the ester bond between the peptide and tRNA on the ribosome. As a consequence, normal termination cannot proceed if the translated mRNA lacks a stop codon. In Escherichia coli, the ribosome rescue factor ArfA releases the nascent polypeptide from the stalled ribosome with the help of RF2 in a stop codon-independent manner. Interestingly, the reaction does not proceed if RF1 is instead provided, even though the structures of RF1 and RF2 are very similar. Here, we identified the regions of RF2 required for the ArfA-dependent ribosome rescue system. Introduction of hydrophobic residues from RF2 found at the interface between RF2 and ArfA into RF1 allowed RF1 to associate with the ArfA-ribosome complex to a certain extent but failed to promote peptidyl-tRNA hydrolysis, whereas WT RF1 did not associate with the complex. We also identified the key residues required for the process after ribosome binding. Our findings provide a basis for understanding how the ArfA-ribosome complex is specifically recognized by RF2 and how RF2 undergoes a conformational change upon binding to the ArfA-ribosome complex.

Rescuing stalled mammalian mitoribosomes - what can we learn from bacteria?

In the canonical process of translation, newly completed proteins escape from the ribosome following cleavage of the ester bond that anchors the polypeptide to the P-site tRNA, after which the ribosome can be recycled to initiate a new round of translation. Not all protein synthesis runs to completion as various factors can impede the progression of ribosomes. Rescuing of stalled ribosomes in mammalian mitochondria, however, does not share the same mechanisms that many bacteria use. The classic method for rescuing bacterial ribosomes is trans-translation. The key components of this system are absent from mammalian mitochondria; however, four members of a translation termination factor family are present, with some evidence of homology to members of a bacterial back-up rescue system. To date, there is no definitive demonstration of any other member of this family functioning in mitoribosome rescue. Here, we provide an overview of the processes and key players of canonical translation termination in both bacteria and mammalian mitochondria, followed by a perspective of the bacterial systems used to rescue stalled ribosomes. We highlight any similarities or differences with the mitochondrial translation release factors, and suggest potential roles for these proteins in ribosome rescue in mammalian mitochondria.

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.

The structural basis for release-factor activation during translation termination revealed by time-resolved cryogenic electron microscopy

When the ribosome encounters a stop codon, it recruits a release factor (RF) to hydrolyze the ester bond between the peptide chain and tRNA. RFs have structural motifs that recognize stop codons in the decoding center and a GGQ motif for induction of hydrolysis in the peptidyl transfer center 70 Å away. Surprisingly, free RF2 is compact, with only 20 Å between its codon-reading and GGQ motifs. Cryo-EM showed that ribosome-bound RFs have extended structures, suggesting that RFs are compact when entering the ribosome and then extend their structures upon stop codon recognition. Here we use time-resolved cryo-EM to visualize transient compact forms of RF1 and RF2 at 3.5 and 4 Å resolution, respectively, in the codon-recognizing ribosome complex on the native pathway. About 25% of complexes have RFs in the compact state at 24 ms reaction time, and within 60 ms virtually all ribosome-bound RFs are transformed to their extended forms.

Translation termination is under strong selection pressure for high speed and accuracy. Here the authors provide a 3D view of the dynamics of a translating bacterial ribosome as it recruits a class-1 release factor (RF1 or RF2) upon encountering a stop codon, and propose a structure-based kinetic model for the early steps in bacterial translation termination.

Mechanism of Inhibition of Translation Termination by Blasticidin S

Understanding the mechanisms of inhibitors of translation termination may inform development of new antibacterials and therapeutics for premature termination diseases. We report the crystal structure of the potent termination inhibitor blasticidin S bound to the ribosomal 70S•release factor 1 (RF1) termination complex. Blasticidin S shifts the catalytic domain 3 of RF1 and restructures the peptidyl transferase center. Universally conserved uridine 2585 in the peptidyl transferase center occludes the catalytic backbone of the GGQ motif of RF1, explaining the structural mechanism of inhibition. Rearrangement of domain 3 relative to the codon-recognition domain 2 provides insight into the dynamics of RF1 implicated in termination accuracy.

A New Mechanism for Ribosome Rescue Can Recruit RF1 or RF2 to Nonstop Ribosomes

Francisella tularensis is a highly infectious intracellular pathogen that kills more than half of infected humans if left untreated. F. tularensis has also been classified as a potential bioterrorism agent with a great risk for deliberate misuse. Recently, compounds that inhibit ribosome rescue have been shown to have antibiotic activity against F. tularensis and other important pathogens. Like all bacteria that have been studied, F. tularensis uses trans-translation as the main pathway to rescue stalled ribosomes. However, unlike most bacteria, F. tularensis can survive without any of the known factors for ribosome rescue. Our work identified a F. tularensis protein, ArfT, that rescues stalled ribosomes in the absence of trans-translation using a new mechanism. These results indicate that ribosome rescue activity is essential in F. tularensis and suggest that ribosome rescue activity might be essential in all bacteria.

Bacterial ribosomes frequently translate to the 3′ end of an mRNA without terminating at an in-frame stop codon. In all bacteria studied to date, these “nonstop” ribosomes are rescued using trans-translation. Genes required for trans-translation are essential in some species, but other species can survive without trans-translation because they express an alternative ribosome rescue factor, ArfA or ArfB. Francisella tularensis cells lacking trans-translation are viable, but F. tularensis does not encode ArfA or ArfB. Transposon mutagenesis followed by deep sequencing (Tn-seq) identified a new alternative ribosome rescue factor, now named ArfT. arfT can be deleted in wild-type (wt) cells but not in cells that lack trans-translation activity. Overexpression of ArfT suppresses the slow-growth phenotype in cells lacking trans-translation and counteracts growth arrest caused by trans-translation inhibitors, indicating that ArfT rescues nonstop ribosomes in vivo. Ribosome rescue assays in vitro show that ArfT promotes hydrolysis of peptidyl-tRNA on nonstop ribosomes in conjunction with F. tularensis release factors. Unlike ArfA, which requires RF2 for activity, ArfT can function with either RF1 or RF2. Overall, these results indicate that ArfT is a new alternative ribosome rescue factor with a distinct mechanism from ArfA and ArfB.

Mechanism of premature translation termination on a sense codon

Accurate translation termination by release factors (RFs) is critical for the integrity of cellular proteomes. Premature termination on sense codons, for example, results in truncated proteins, whose accumulation could be detrimental to the cell. Nevertheless, some sense codons are prone to triggering premature termination, but the structural basis for this is unclear. To investigate premature termination, we determined a cryo-EM structure of the Escherichia coli 70S ribosome bound with RF1 in response to a UAU (Tyr) sense codon. The structure reveals that RF1 recognizes a UAU codon similarly to a UAG stop codon, suggesting that sense codons induce premature termination because they structurally mimic a stop codon. Hydrophobic interaction between the nucleobase of U3 (the third position of the UAU codon) and conserved Ile-196 in RF1 is important for misreading the UAU codon. Analyses of RNA binding in ribonucleoprotein complexes or by amino acids reveal that Ile-U packing is a frequent protein-RNA-binding motif with key functional implications. We discuss parallels with eukaryotic translation termination by the release factor eRF1.

Interaction between rpsL and gyrA mutations affects the fitness and dual resistance of Mycobacterium tuberculosis clinical isolates against streptomycin and fluoroquinolones

Background

The interaction between different drug-resistant mutations is important to the development of drug resistance and its evolution. In this study, we aimed to reveal the potential relationships between mutations conferring resistance to two important antituberculosis drugs streptomycin (STR) and fluoroquinolones (FLQ).

Materials and methods

We used an in vitro competitive fitness assay to reveal the interactions between different mutations of rpsL and gyrA in drug-resistant Mycobacterium smegmatis, followed by the analysis of the frequency of rpsL and gyrA mutation combinations in 213 STR-FLQ dual-resistant clinical Mycobacterium tuberculosis isolates from Sichuan region, which was also investigated by the whole genome data from 3,056 global clinical M. tuberculosis isolates.

Results

The strains with K43R and K88R mutation in rpsL showed no difference in relative fitness compared with their susceptible ancestor, while K43N, K43M, K43T, and K88E exhibited a significantly lower relative fitness (P<0.05). For the FLQ-resistant mutants, all mutation types showed no difference in their relative fitness. Among STR-FLQ dual-resistant M. smegmatis strains, a lower fitness was detected in those with K43N/M/T and K88E instead of K43R and K88R mutations in rpsL. Among M. tuberculosis isolates harboring rpsL and gyrA dual mutations, the most two frequent combinatorial mutation types were K43R/D94G (n=37) and K43R/A90V (n=24), with the former being the most frequent one by both in vitro tests and clinical survey.

Conclusion

Our results suggest that the interaction between rpsL and gyrA mutations affects the fitness cost in STR-FLQ dual-resistant M. smegmatis and also the predilection of mutation combinations in clinical M. tuberculosis isolates.

Operative Binding of Class I Release Factors and YaeJ Stabilizes the Ribosome in the Nonrotated State

During translation, the small subunit of the ribosome rotates with respect to the large subunit primarily between two states as mRNA is being translated into a protein. At the termination of bacterial translation, class I release factors (RFs) bind to a stop codon in the A-site and catalyze the release of the peptide chain from the ribosome. Periodically, mRNA is truncated prematurely, and the translating ribosome stalls at the end of the mRNA forming a nonstop complex requiring one of several ribosome rescue factors to intervene. One factor, YaeJ, is structurally homologous with the catalytic region of RFs but differs by binding to the ribosome directly through its C-terminal tail. Structures of the ribosome show that the ribosome adopts the nonrotated state conformation when these factors are bound. However, these studies do not elucidate the influence of binding to cognate or noncognate codons on the dynamics of intersubunit rotation. Here, we investigate the effects of wild-type and mutant forms of RF1, RF2, and YaeJ binding on ribosome intersubunit rotation using single-molecule Förster resonance energy transfer. We show that both RF1 binding and RF2 binding are sufficient to shift the population of posthydrolysis ribosome complexes from primarily the rotated to the nonrotated state only when a cognate stop codon is present in the A-site. Similarly, YaeJ binding stabilizes nonstop ribosomal complexes in the nonrotated state. Along with previous studies, these results are consistent with the idea that directed conformational changes and binding of subsequent factors to the ribosome are requisite for efficient termination and ribosome recycling.

Conformation of methylated GGQ in the Peptidyl Transferase Center during Translation Termination

The universally conserved Gly-Gly-Gln (GGQ) tripeptide in release factors or release factor-like surveillance proteins is required to catalyze the release of nascent peptide in the ribosome. The glutamine of the GGQ is methylated post-translationally at the N⁵ position in vivo; this covalent modification is essential for optimal cell growth and efficient translation termination. However, the precise conformation of the methylated-GGQ tripeptide in the ribosome remains unknown. Using cryoEM and X-ray crystallography, we report the conformation of methylated-GGQ in the peptidyl transferase center of the ribosome during canonical translational termination and co-translation quality control. It has been suggested that the GGQ motif arose independently through convergent evolution among otherwise unrelated proteins that catalyze peptide release. The requirement for this tripeptide in the highly conserved peptidyl transferase center suggests that the conformation reported here is likely shared during termination of protein synthesis in all domains of life.

Ribosomal proteins: insight into molecular roles and functions in hepatocellular carcinoma

Ribosomes, which are important sites for the synthesis of proteins related to expression and transmission of genetic information in humans, have a complex structure and diverse functions. They consist of a variety of ribosomal proteins (RPs), ribosomal RNAs (rRNAs) and small nucleolar RNAs. Owing to the involvement of ribosomes in many important biological processes of cells, their major components, rRNAs and RPs, have an important role in human diseases, including the initiation and evolvement of malignancies. However, the main mechanisms underlying the involvement of ribosomes in cancer remain unclear. This review describes the crucial role of ribosomes in various common malignant tumors; in particular, it examines the effects of RPs, including S6, the receptor for activated C-kinase and RPS15A, on the development and progression of hepatocellular carcinoma.

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.

Structural Basis for Ribosome Rescue in Bacteria

Ribosomes that translate mRNAs lacking stop codons become stalled at the 3' end of the mRNA. Recycling of these stalled ribosomes is essential for cell viability. In bacteria three ribosome rescue systems have been identified so far, with the most ubiquitous and best characterized being the trans-translation system mediated by transfer-messenger RNA (tmRNA) and small protein B (SmpB). The two additional rescue systems present in some bacteria employ alternative rescue factor (Arf) A and release factor (RF) 2 or ArfB. Recent structures have revealed how ArfA mediates ribosome rescue by recruiting the canonical termination factor RF2 to ribosomes stalled on truncated mRNAs. This now provides us with the opportunity to compare and contrast the available structures of all three bacterial ribosome rescue systems.

Mechanism of ribosome rescue by ArfA and RF2

ArfA rescues ribosomes stalled on truncated mRNAs by recruiting release factor RF2, which normally binds stop codons to catalyze peptide release. We report two 3.2 Å resolution cryo-EM structures – determined from a single sample – of the 70S ribosome with ArfA•RF2 in the A site. In both states, the ArfA C-terminus occupies the mRNA tunnel downstream of the A site. One state contains a compact inactive RF2 conformation. Ordering of the ArfA N-terminus in the second state rearranges RF2 into an extended conformation that docks the catalytic GGQ motif into the peptidyl-transferase center. Our work thus reveals the structural dynamics of ribosome rescue. The structures demonstrate how ArfA ‘senses’ the vacant mRNA tunnel and activates RF2 to mediate peptide release without a stop codon, allowing stalled ribosomes to be recycled.

DOI:http://dx.doi.org/10.7554/eLife.23687.001