Complementary charge-based interaction between the ribosomal-stalk protein L7/12 and IF2 is the key to rapid subunit association

Structure-function comparison of Arbekacin with other aminoglycosides elucidates its higher potency as bacterial translation inhibitor

Aminoglycoside antibiotics are well-known inhibitors of bacterial protein synthesis, which act mainly by inhibiting ribosomal translocation and inducing miscoding errors. Arbekacin (ABK) is a semisynthetic aminoglycoside that was developed by adding a 3-amino-2-hydroxybutyric (AHB) moiety to the 2-deoxystreptamine (2-DOS) ring of dibekacin for counteracting the problem of enzyme-mediated resistance of aminoglycosides. Here, we have systematically compared the inhibition efficacy of ABK with other aminoglycosides by in vivo MIC determination and in vitro fast-kinetics based translocation and termination assays complemented with a high-resolution cryo-EM structure. ABK presents significantly lower MIC50 value compared to its parent antibiotics kanamycin and dibekacin. Consistent with that, ABK inhibits translocation with lower inhibition constant and reside on the ribosome for significantly longer time than the classical aminoglycosides. Our 3.1 Å resolution cryo-EM structure of ABK-bound ribosome containing mRNA and initiator-tRNA, shows interactions of the unique AHB moiety of ABK with rRNA nucleobases, which likely provide additional stabilization of ABK at the canonical aminoglycoside binding pocket and contribute to its prolonged dwelling time. Our structural and functional analyses provide molecular basis for higher potency of ABK in bacterial translation inhibition and opens the possibility of rational design of new antibiotics.

Structural mechanism of FusB-mediated rescue from fusidic acid inhibition of protein synthesis

The antibiotic resistance protein FusB rescues protein synthesis from inhibition by fusidic acid (FA), which locks elongation factor G (EF-G) to the ribosome after GTP hydrolysis. Here, we present time-resolved single-particle cryo-EM structures explaining the mechanism of FusB-mediated rescue. FusB binds to the FA-trapped EF-G on the ribosome, causing large-scale conformational changes of EF-G that break interactions with the ribosome, tRNA, and mRNA. This leads to dissociation of EF-G from the ribosome, followed by FA release. We also observe two independent binding sites of FusB on the classical-state ribosome, overlapping with the binding site of EF-G to each of the ribosomal subunits, yet not inhibiting tRNA delivery. The affinity of FusB to the ribosome and the concentration of FusB in S. aureus during FusB-mediated resistance support that direct binding of FusB to ribosomes could occur in the cell. Our results reveal an intricate resistance mechanism involving specific interactions of FusB with both EF-G and the ribosome, and a non-canonical release pathway of EF-G.

Ribosome customization and functional diversification among P-stalk proteins regulate late poxvirus protein synthesis

Growing evidence suggests that ribosomes selectively regulate translation of specific mRNA subsets. Here, quantitative proteomics and cryoelectron microscopy demonstrate that poxvirus infection does not alter ribosomal subunit protein (RP) composition but skews 40S rotation states and displaces the 40S head domain. Genetic knockout screens employing metabolic assays and a dual-reporter virus further identified two RPs that selectively regulate non-canonical translation of late poxvirus mRNAs, which contain unusual 5' poly(A) leaders: receptor of activated C kinase 1 (RACK1) and RPLP2. RACK1 is a component of the altered 40S head domain, while RPLP2 is a subunit of the P-stalk, wherein RPLP0 anchors two heterodimers of RPLP1 and RPLP2 to the large 60S subunit. RPLP0 was required for global translation, yet RPLP1 was dispensable, while RPLP2 was specifically required for non-canonical poxvirus protein synthesis. From these combined results, we demonstrate that poxviruses structurally customize ribosomes and become reliant upon traditionally non-essential RPs from both ribosomal subunits for efficient initiation on their late mRNAs.

The ribosome termination complex remodels release factor RF3 and ejects GDP

Translation termination involves release factors RF1, RF2 and the GTPase RF3 that recycles RF1 and RF2 from the ribosome. RF3 dissociates from the ribosome in the GDP-bound form and must then exchange GDP for GTP. The 70S ribosome termination complex (70S-TC) accelerates GDP exchange in RF3, suggesting that the 70S-TC can function as the guanine nucleotide exchange factor for RF3. Here, we use cryogenic-electron microscopy to elucidate the mechanism of GDP dissociation from RF3 catalyzed by the Escherichia coli 70S-TC. The non-rotated ribosome bound to RF1 remodels RF3 and induces a peptide flip in the phosphate-binding loop, efficiently ejecting GDP. Binding of GTP allows RF3 to dock at the GTPase center, promoting the dissociation of RF1 from the ribosome. The structures recapitulate the functional cycle of RF3 on the ribosome and uncover the mechanism by which the 70S-TC allosterically dismantles the phosphate-binding groove in RF3, a previously overlooked function of the ribosome.

Extraribosomal Functions of Bacterial Ribosomal Proteins-An Update, 2023

Ribosomal proteins (r-proteins) are abundant, highly conserved, and multifaceted cellular proteins in all domains of life. Most r-proteins have RNA-binding properties and can form protein-protein contacts. Bacterial r-proteins govern the co-transcriptional rRNA folding during ribosome assembly and participate in the formation of the ribosome functional sites, such as the mRNA-binding site, tRNA-binding sites, the peptidyl transferase center, and the protein exit tunnel. In addition to their primary role in a cell as integral components of the protein synthesis machinery, many r-proteins can function beyond the ribosome (the phenomenon known as moonlighting), acting either as individual regulatory proteins or in complexes with various cellular components. The extraribosomal activities of r-proteins have been studied over the decades. In the past decade, our understanding of r-protein functions has advanced significantly due to intensive studies on ribosomes and gene expression mechanisms not only in model bacteria like Escherichia coli or Bacillus subtilis but also in little-explored bacterial species from various phyla. The aim of this review is to update information on the multiple functions of r-proteins in bacteria.

Second messenger c-di-AMP regulates multiple antibiotic sensitivity pathways in Mycobacterium smegmatis by discrete mechanisms

One of the debilitating causes of high mortality in the case of tuberculosis and other bacterial infections is the resistance development against standard drugs. There are limited studies so far to describe how a bacterial second messenger molecule can directly participate in distinctive antibiotic tolerance characteristics of a cell in a mechanism-dependent manner. Here we show that intracellular cyclic di-AMP (c-di-AMP) concentration can modulate drug sensitivity of Mycobacterium smegmatis by interacting with an effector protein or interfering with the 5'-UTR regions in mRNA of the genes and thus causing transcriptional downregulation of important genes in the pathways. We studied four antibiotics with different mechanisms of action: rifampicin, ciprofloxacin, erythromycin, and tobramycin and subsequently found that the level of drug sensitivity of the bacteria is directly proportional to the c-di-AMP concentration inside the cell. Further, we unraveled the underlying molecular mechanisms to delineate the specific genes and pathways regulated by c-di-AMP and hence result in differential drug sensitivity in M. smegmatis.

Ribosomal stalk-captured CARF-RelE ribonuclease inhibits translation following CRISPR signaling

Prokaryotic type III CRISPR-Cas antiviral systems employ cyclic oligoadenylate (cAn) signaling to activate a diverse range of auxiliary proteins that reinforce the CRISPR-Cas defense. Here we characterize a class of cAn-dependent effector proteins named CRISPR-Cas-associated messenger RNA (mRNA) interferase 1 (Cami1) consisting of a CRISPR-associated Rossmann fold sensor domain fused to winged helix-turn-helix and a RelE-family mRNA interferase domain. Upon activation by cyclic tetra-adenylate (cA4), Cami1 cleaves mRNA exposed at the ribosomal A-site thereby depleting mRNA and leading to cell growth arrest. The structures of apo-Cami1 and the ribosome-bound Cami1-cA4 complex delineate the conformational changes that lead to Cami1 activation and the mechanism of Cami1 binding to a bacterial ribosome, revealing unexpected parallels with eukaryotic ribosome-inactivating proteins.

Early divergence of translation initiation and elongation factors

Protein translation is a foundational attribute of all living cells. The translation function carried out by the ribosome critically depends on an assortment of protein interaction partners, collectively referred to as the translation machinery. Various studies suggest that the diversification of the translation machinery occurred prior to the last universal common ancestor, yet it is unclear whether the predecessors of the extant translation machinery factors were functionally distinct from their modern counterparts. Here we reconstructed the shared ancestral trajectory and subsequent evolution of essential translation factor GTPases, elongation factor EF-Tu (aEF-1A/eEF-1A), and initiation factor IF2 (aIF5B/eIF5B). Based upon their similar functions and structural homologies, it has been proposed that EF-Tu and IF2 emerged from an ancient common ancestor. We generated the phylogenetic tree of IF2 and EF-Tu proteins and reconstructed ancestral sequences corresponding to the deepest nodes in their shared evolutionary history, including the last common IF2 and EF-Tu ancestor. By identifying the residue and domain substitutions, as well as structural changes along the phylogenetic history, we developed an evolutionary scenario for the origins, divergence and functional refinement of EF-Tu and IF2 proteins. Our analyses suggest that the common ancestor of IF2 and EF-Tu was an IF2-like GTPase protein. Given the central importance of the translation machinery to all cellular life, its earliest evolutionary constraints and trajectories are key to characterizing the universal constraints and capabilities of cellular evolution.

Compact IF2 allows initiator tRNA accommodation into the P site and gates the ribosome to elongation

During translation initiation, initiation factor 2 (IF2) holds initiator transfer RNA (fMet-tRNAifMet) in a specific orientation in the peptidyl (P) site of the ribosome. Upon subunit joining IF2 hydrolyzes GTP and, concomitant with inorganic phosphate (Pi) release, changes conformation facilitating fMet-tRNAifMet accommodation into the P site and transition of the 70 S ribosome initiation complex (70S-IC) to an elongation-competent ribosome. The mechanism by which IF2 separates from initiator tRNA at the end of translation initiation remains elusive. Here, we report cryo-electron microscopy (cryo-EM) structures of the 70S-IC from Pseudomonas aeruginosa bound to compact IF2-GDP and initiator tRNA. Relative to GTP-bound IF2, rotation of the switch 2 α-helix in the G-domain bound to GDP unlocks a cascade of large-domain movements in IF2 that propagate to the distal tRNA-binding domain C2. The C2-domain relocates 35 angstroms away from tRNA, explaining how IF2 makes way for fMet-tRNAifMet accommodation into the P site. Our findings provide the basis by which IF2 gates the ribosome to the elongation phase.

Regulation of Translation by Lysine Acetylation in Escherichia coli

Numerous cellular processes are regulated in response to the metabolic state of the cell. One such regulatory mechanism involves lysine acetylation, a covalent modification involving the transfer of an acetyl group from central metabolite acetyl-coenzyme A or acetyl phosphate to a lysine residue in a protein.

Optimization of a fluorescent-mRNA based real-time assay for precise kinetic measurements of ribosomal translocation

Kinetic characterization of ribosomal translocation is important for understanding the mechanism of elongation in protein synthesis. Here we have optimized a popular fluorescent-mRNA based translocation assay conducted in stopped-flow, by calibrating it with the functional tripeptide formation assay in quench-flow. We found that a fluorescently labelled mRNA, ten bases long from position +1 (mRNA+10), is best suited for both assays as it forms tripeptide at a fast rate equivalent to the longer mRNAs, and yet produces a large fluorescence change upon mRNA movement. Next, we compared the commonly used peptidyl tRNA analog, N-acetyl-Phe-tRNA^Phe, with the natural dipeptidyl fMet-Phe-tRNA^Phe in the stopped-flow assay. This analog translocates about two times slower than the natural dipeptidyl tRNA and produces biphasic kinetics. The rates reduce further at lower temperatures and with higher Mg²⁺ concentration, but improve with higher elongation factor G (EF-G) concentration, which increase both rate and amplitude of the fast phase significantly. In summary, we present here an improved real time assay for monitoring mRNA-translocation with the natural- and an N-Ac-analog of dipeptidyl tRNA.

GGQ methylation enhances both speed and accuracy of stop codon recognition by bacterial class-I release factors

Accurate translation termination in bacteria requires correct recognition of the stop codons by the class-I release factors (RFs) RF1 and RF2, which release the nascent peptide from the peptidyl tRNA after undergoing a "compact to open" conformational transition. These RFs possess a conserved Gly-Gly-Gln (GGQ) peptide release motif, of which the Q residue is posttranslationally methylated. GGQ-methylated RFs have been shown to be faster in peptide release than the unmethylated ones, but it was unknown whether this modification had additional roles. Using a fluorescence-based real-time in vitro translation termination assay in a stopped-flow instrument, we demonstrate that methylated RF1 and RF2 are two- to four-fold more accurate in the cognate stop codon recognition than their unmethylated variants. Using pH titration, we show that the lack of GGQ methylation facilitates the "compact to open" transition, which results in compromised accuracy of the unmethylated RFs. Furthermore, thermal melting studies using circular dichroism and SYPRO-orange fluorescence demonstrate that GGQ methylation increases overall stability of the RF proteins. This increased stability, we suspect, is the basis for the more controlled conformational change of the methylated RFs upon codon recognition, which enhances both their speed and accuracy. This GGQ methylation-based modulation of the accuracy of RFs can be a tool for regulating translational termination in vivo.

Kinetic Analysis Suggests Evolution of Ribosome Specificity in Modern Elongation Factor-Tus from "Generalist" Ancestors

It has been hypothesized that early enzymes are more promiscuous than their extant orthologs. Whether or not this hypothesis applies to the translation machinery, the oldest molecular machine of life, is not known. Efficient protein synthesis relies on a cascade of specific interactions between the ribosome and the translation factors. Here, using elongation factor-Tu (EF-Tu) as a model system, we have explored the evolution of ribosome specificity in translation factors. Employing presteady state fast kinetics using quench flow, we have quantitatively characterized the specificity of two sequence-reconstructed 1.3- to 3.3-Gy-old ancestral EF-Tus toward two unrelated bacterial ribosomes, mesophilic Escherichia coli and thermophilic Thermus thermophilus. Although the modern EF-Tus show clear preference for their respective ribosomes, the ancestral EF-Tus show similar specificity for diverse ribosomes. In addition, despite increase in the catalytic activity with temperature, the ribosome specificity of the thermophilic EF-Tus remains virtually unchanged. Our kinetic analysis thus suggests that EF-Tu proteins likely evolved from the catalytically promiscuous, “generalist” ancestors. Furthermore, compatibility of diverse ribosomes with the modern and ancestral EF-Tus suggests that the ribosomal core probably evolved before the diversification of the EF-Tus. This study thus provides important insights regarding the evolution of modern translation machinery.

SHP2 Nuclear/Cytoplasmic Trafficking in Granulosa Cells Is Essential for Oocyte Meiotic Resumption and Maturation

Src-homology-2-containing phosphotyrosine phosphatase (SHP2), a classic cytoplasmic protein and a major regulator of receptor tyrosine kinases and G protein-coupled receptors, plays a significant role in preimplantation embryo development. In this study, we deciphered the role of SHP2 in the somatic compartment of oocytes during meiotic maturation. SHP2 showed nuclear/cytoplasmic localization in bovine cumulus and human granulosa (COV434) cells. Follicle-stimulating hormone (FSH) treatment significantly enhanced cytoplasmic SHP2 localization, in contrast to the E₂ treatment, which augmented nuclear localization. Enhanced cytoplasmic SHP2 was found to negatively regulate the expression of the ERα-transcribed NPPC and NPR2 mRNAs, which are vital for oocyte meiotic arrest. The co-immunoprecipitation results revealed the presence of the SHP2/ERα complex in the germinal vesicle-stage cumulus-oocyte complexes, and this complex significantly decreased with the progression of meiotic maturation. The complex formation between ERα and SHP2 was also confirmed by using a series of computational modeling methods. To verify the correlation between SHP2 and NPPC/NPR2, SHP2 was knocked down via RNA interference, and NPPC and NPR2 mRNAs were analyzed in the control, E₂, and FSH-stimulated COV434 cells. Furthermore, phenyl hydrazonopyrazolone sulfonate 1, a site-directed inhibitor of active SHP2, showed no significant effect on the ERα-transcribed NPPC and NPR2 mRNAs. Taken together, these findings support a novel nuclear/cytoplasmic role of SHP2 in oocyte meiotic resumption and maturation.

Tetrapeptide Ac-HAEE-NH2 Protects α4β2 nAChR from Inhibition by Aβ

The cholinergic deficit in Alzheimer’s disease (AD) may arise from selective loss of cholinergic neurons caused by the binding of Aβ peptide to nicotinic acetylcholine receptors (nAChRs). Thus, compounds preventing such an interaction are needed to address the cholinergic dysfunction. Recent findings suggest that the ¹¹EVHH¹⁴ site in Aβ peptide mediates its interaction with α4β2 nAChR. This site contains several charged amino acid residues, hence we hypothesized that the formation of Aβ-α4β2 nAChR complex is based on the interaction of ¹¹EVHH¹⁴ with its charge-complementary counterpart in α4β2 nAChR. Indeed, we discovered a ³⁵HAEE³⁸ site in α4β2 nAChR, which is charge-complementary to ¹¹EVHH¹⁴, and molecular modeling showed that a stable Aβ₄₂-α4β2 nAChR complex could be formed via the ¹¹EVHH¹⁴:³⁵HAEE³⁸ interface. Using surface plasmon resonance and bioinformatics approaches, we further showed that a corresponding tetrapeptide Ac-HAEE-NH₂ can bind to Aβ via ¹¹EVHH¹⁴ site. Finally, using two-electrode voltage clamp in Xenopus laevis oocytes, we showed that Ac-HAEE-NH₂ tetrapeptide completely abolishes the Aβ₄₂-induced inhibition of α4β2 nAChR. Thus, we suggest that ³⁵HAEE³⁸ is a potential binding site for Aβ on α4β2 nAChR and Ac-HAEE-NH₂ tetrapeptide corresponding to this site is a potential therapeutic for the treatment of α4β2 nAChR-dependent cholinergic dysfunction in AD.

Alterations in the ribosomal protein bL12 of E. coli affecting the initiation, elongation and termination of protein synthesis

In bacteria, ribosomal protein bL12 forms the prominent stalk structure on the ribosome and binds to multiple, distinct translational GTPase factors during the sequential steps of translation. Using a genetic selection in E. coli for altered readthrough of UGA stop codons, we have isolated seven different mutations affecting the C-terminal domain of the protein that forms the interaction surface with translation factors. Analysis of these altered proteins, along with four additional alterations previously shown to affect IF2-ribosome interactions, indicates that multiple steps of translation are affected, consistent with bL12's interaction with multiple factors. Surprisingly, deletion of the release factor GTPase, RF3, has relatively little effect on bL12-promoted stop codon readthrough, suggesting that other steps in termination are also influenced by bL12.

Loss of a single methylation in 23S rRNA delays 50S assembly at multiple late stages and impairs translation initiation and elongation

Ribosome biogenesis is a complex process, and dozens of factors are required to facilitate and regulate the subunit assembly in bacteria. The 2'-O-methylation of U2552 in 23S rRNA by methyltransferase RrmJ is a crucial step in late-stage assembly of the 50S subunit. Its absence results in severe growth defect and marked accumulation of pre50S assembly intermediates. In the present work, we employed cryoelectron microscopy to characterize a set of late-stage pre50S particles isolated from an Escherichia coli ΔrrmJ strain. These assembly intermediates (solved at 3.2 to 3.8 Å resolution) define a collection of late-stage particles on a progressive assembly pathway. Apart from the absence of L16, L35, and L36, major structural differences between these intermediates and the mature 50S subunit are clustered near the peptidyl transferase center, such as H38, H68-71, and H89-93. In addition, the ribosomal A-loop of the mature 50S subunit from ΔrrmJ strain displays large local flexibility on nucleotides next to unmethylated U2552. Fast kinetics-based biochemical assays demonstrate that the ΔrrmJ 50S subunit is only 50% active and two times slower than the WT 50S subunit in rapid subunit association. While the ΔrrmJ 70S ribosomes show no defect in peptide bond formation, peptide release, and ribosome recycling, they translocate with 20% slower rate than the WT ribosomes in each round of elongation. These defects amplify during synthesis of the full-length proteins and cause overall defect in protein synthesis. In conclusion, our data reveal the molecular roles of U2552 methylation in both ribosome biogenesis and protein translation.

Ribosome assembly defects subvert initiation Factor3 mediated scrutiny of bona fide start signal

In bacteria, the assembly factors tightly orchestrate the maturation of ribosomes whose competency for protein synthesis is validated by translation machinery at various stages of translation cycle. However, what transpires to the quality control measures when the ribosomes are produced with assembly defects remains enigmatic. In Escherichia coli, we show that 30S ribosomes that harbour assembly defects due to the lack of assembly factors such as RbfA and KsgA display suboptimal initiation codon recognition and bypass the critical codon–anticodon proofreading steps during translation initiation. These premature ribosomes on entering the translation cycle compromise the fidelity of decoding that gives rise to errors during initiation and elongation. We show that the assembly defects compromise the binding of initiation factor 3 (IF3), which in turn appears to license the rapid transition of 30S (pre) initiation complex to 70S initiation complex by tempering the validation of codon–anticodon interaction during translation initiation. This suggests that the premature ribosomes harbouring the assembly defects subvert the IF3 mediated proofreading of cognate initiation codon to enter the translation cycle.

Probing the dynamic stalk region of the ribosome using solution NMR

We describe an NMR approach based on the measurement of residual dipolar couplings (RDCs) to probe the structural and motional properties of the dynamic regions of the ribosome. Alignment of intact 70S ribosomes in filamentous bacteriophage enabled measurement of RDCs in the mobile C-terminal domain (CTD) of the stalk protein bL12. A structural refinement of this domain using the observed RDCs did not show large changes relative to the isolated protein in the absence of the ribosome, and we also found that alignment of the CTD was almost independent of the presence of the core ribosome particle, indicating that the inter-domain linker has significant flexibility. The nature of this linker was subsequently probed in more detail using a paramagnetic alignment strategy, which revealed partial propagation of alignment between neighbouring domains, providing direct experimental validation of a structural ensemble previously derived from SAXS and NMR relaxation measurements. Our results demonstrate the prospect of better characterising dynamical and functional regions of more challenging macromolecular machines and systems, for example ribosome–nascent chain complexes.

Computational Approaches for Elucidating Protein-Protein Interactions in Cation Channel Signaling

BACKGROUND

The lipid bilayer of the plasma membrane is impermeable to ions, yet changes in the flux of ions across the cell membrane are critical regulatory events in cells. Because of their regulatory roles in a range of physiological processes, such as electrical signaling in muscles and neurons, to name a few, these proteins are one of the most important drug targets.

OBJECTIVE

This review mainly focused on the computational approaches for elucidating proteinprotein interactions in cation channel signaling.

DISCUSSION

Due to continuously advanced facilities and technologies in computer sciences, the physical contacts of macromolecules of channel structures have been virtually visualized. Indeed, techniques like protein-protein docking, homology modeling, and molecular dynamics simulation are valuable tools for predicting the protein complex and refining channels with unreleased structures. Undoubtedly, these approaches will greatly expand the cation channel signaling research, thereby speeding up structure-based drug design and discovery.

CONCLUSION

We introduced a series of valuable computational tools for elucidating protein-protein interactions in cation channel signaling, including molecular graphics, protein-protein docking, homology modeling, and molecular dynamics simulation.

The enigmatic ribosomal stalk

The large ribosomal subunit has a distinct feature, the stalk, extending outside the ribosome. In bacteria it is called the L12 stalk. The base of the stalk is protein uL10 to which two or three dimers of proteins bL12 bind. In archea and eukarya P1 and P2 proteins constitute the stalk. All these extending proteins, that have a high degree of flexibility due to a hinge between their N- and C-terminal parts, are essential for proper functionalization of some of the translation factors. The role of the stalk proteins has remained enigmatic for decades but is gradually approaching an understanding. In this review we summarise the knowhow about the structure and function of the ribosomal stalk till date starting from the early phase of ribosome research.

Large-scale simulations of nucleoprotein complexes: ribosomes, nucleosomes, chromatin, chromosomes and CRISPR

Recent advances in biotechnology such as Hi-C, CRISPR/Cas9 and ribosome display have placed nucleoprotein complexes at center stage. Understanding the structural dynamics of these complexes aids in optimizing protocols and interpreting data for these new technologies. The integration of simulation and experiment has helped advance mechanistic understanding of these systems. Coarse-grained simulations, reduced-description models, and explicit solvent molecular dynamics simulations yield useful complementary perspectives on nucleoprotein complex structural dynamics. When combined with Hi-C, cryo-EM, and single molecule measurements, these simulations integrate disparate forms of experimental data into a coherent mechanism.

Visualization of translation termination intermediates trapped by the Apidaecin 137 peptide during RF3-mediated recycling of RF1

During translation termination in bacteria, the release factors RF1 and RF2 are recycled from the ribosome by RF3. While high-resolution structures of the individual termination factors on the ribosome exist, direct structural insight into how RF3 mediates dissociation of the decoding RFs has been lacking. Here we have used the Apidaecin 137 peptide to trap RF1 together with RF3 on the ribosome and visualize an ensemble of termination intermediates using cryo-electron microscopy. Binding of RF3 to the ribosome induces small subunit (SSU) rotation and swivelling of the head, yielding intermediate states with shifted P-site tRNAs and RF1 conformations. RF3 does not directly eject RF1 from the ribosome, but rather induces full rotation of the SSU that indirectly dislodges RF1 from its binding site. SSU rotation is coupled to the accommodation of the GTPase domain of RF3 on the large subunit (LSU), thereby promoting GTP hydrolysis and dissociation of RF3 from the ribosome.

In bacteria, the process of translation termination is performed by three termination release factors RF1, RF2 and RF3. Here the authors provide detailed structural insights into the mechanism by which RF1 is dissociated from the ribosome by RF3 during termination.