The ribosome as a molecular machine: the mechanism of tRNA-mRNA movement in translocation

Translocation of tRNA and mRNA through the ribosome is one of the most dynamic events during protein synthesis. In the cell, translocation is catalysed by EF-G (elongation factor G) and driven by GTP hydrolysis. Major unresolved questions are: how the movement is induced and what the moving parts of the ribosome are. Recent progress in time-resolved cryoelectron microscopy revealed trajectories of tRNA movement through the ribosome. Driven by thermal fluctuations, the ribosome spontaneously samples a large number of conformational states. The spontaneous movement of tRNAs through the ribosome is loosely coupled to the motions within the ribosome. EF-G stabilizes conformational states prone to translocation and promotes a conformational rearrangement of the ribosome (unlocking) that accelerates the rate-limiting step of translocation: the movement of the tRNA anticodons on the small ribosomal subunit. EF-G acts as a Brownian ratchet providing directional bias for movement at the cost of GTP hydrolysis.     

Molecular dynamics of EF-G during translocation

Elongation factor G (EF‐G) plays a crucial role in two stages of mRNA‐(tRNA)₂ translocation. First, EF‐G?GTP enters the pre‐translocational ribosome in its intersubunit‐rotated state, with tRNAs in their hybrid (P/E and A/P) positions. Second, a conformational change in EF‐G's Domain IV induced by GTP hydrolysis disengages the mRNA‐anticodon stem‐loops of the tRNAs from the decoding center to advance relative to the small subunit when the ribosome undergoes a backward inter‐subunit rotation. These events take place as EF‐G undergoes a series of large conformational changes as visualized by cryo‐EM and X‐ray studies. The number and variety of these structures leave open many questions on how these different configurations form during the dynamic translocation process. To understand the molecular mechanism of translocation, we examined the molecular motions of EF‐G in solution by means of molecular dynamics simulations. Our results show: (1) rotations of the super‐domain formed by Domains III–V with respect to the super‐domain formed by I–II, and rotations of Domain IV with respect to Domain III; (2) flexible conformations of both 503‐ and 575‐loops; (3) large conformational variability in the bound form caused by the interaction between Domain V and the GTPase‐associated center; (4) after GTP hydrolysis, the Switch I region seems to be instrumental for effecting the conformational change at the end of Domain IV implicated in the disengagement of the codon‐anticodon helix from the decoding center. Proteins 2011; © 2011 Wiley‐Liss, Inc.     

Mechanism of tRNA Translocation on the Ribosome

During the translocation step of the elongation cycle of peptide synthesis two tRNAs together with the mRNA move synchronously and rapidly on the ribosome. Translocation is catalyzed by the elongation factor G (EF-G) and requires GTP hydrolysis. The fundamental biochemical features of the process were worked out in the 1970–80s, to a large part by A.S. Spirin and his colleagues. Recent results from pre-steady-state kinetic analysis and cryoelectron microscopy suggest that translocation is a multistep dynamic process that entails large-scale structural rearrangements of both ribosome and EF-G. Kinetic and thermodynamic data, together with the structural information on the conformational changes in the ribosome and EF-G, provide a detailed mechanistic model of translocation and suggest a mechanism of translocation catalysis by EF-G.     

Structural dynamics of the ribosome

Protein synthesis is inherently a dynamic process, requiring both small-scale and large-scale movements of tRNA and mRNA. It has long been suspected that these movements might be coupled to conformational changes in the ribosome, and in its RNA moieties in particular. Recently, the nature of ribosome structural dynamics has begun to emerge from a combination of approaches, most notably cryo-EM, X-ray crystallography, and FRET. Ribosome movement occurs both on a grand scale, as in the intersubunit rotational movements that are coupled to tRNA-mRNA translocation, and in intricate localized rearrangements such as those that accompany codon-anticodon recognition and peptide bond formation. In spite of much progress, our understanding of the mechanics of translation is now beset with countless new questions, reflecting the vast molecular architecture of the ribosome itself.     

Single-molecule structural dynamics of EF-G--ribosome interaction during translocation

EF-G catalyzes translocation of mRNA and tRNAs within the ribosome during protein synthesis. Detection of structural states in the reaction sequence that are not highly populated can be facilitated by studying the process one molecule at a time. Here we present single-molecule studies of translocation showing that, for ribosomes engaged in poly(Phe) synthesis, fluorescence resonance energy transfer (FRET) between the G' domain of EF-G and the N-terminal domain of ribosomal protein L11 occurs within two rapidly interconverting states, having FRET efficiencies of 0.3 and 0.6. The antibiotic fusidic acid increases the population of the 0.6 state, indicating that it traps the ribosome.EF-G complex in a preexisting conformation formed during translation. Only the 0.3 state is observed when poly(Phe) synthesis is prevented by omission of EF-Tu, or in studies on vacant ribosomes. These results suggest that the 0.6 state results from the conformational lability of unlocked ribosomes formed during translocation. An idling state, possibly pertinent to regulation of protein synthesis, is detected in some ribosomes in the poly(Phe) system.     

The hybrid state of tRNA binding is an authentic translation elongation intermediate

The GTPase elongation factor (EF)-G is responsible for promoting the translocation of the messenger RNA-transfer RNA complex on the ribosome, thus opening up the A site for the next aminoacyl-tRNA. Chemical modification and cryo-EM studies have indicated that tRNAs can bind the ribosome in an alternative 'hybrid' state after peptidyl transfer and before translocation, though the relevance of this state during translation elongation has been a subject of debate. Here, using pre-steady-state kinetic approaches and mutant analysis, we show that translocation by EF-G is most efficient when tRNAs are bound in a hybrid state, supporting the argument that this state is an authentic intermediate during translation.     

Visualization of the hybrid state of tRNA binding promoted by spontaneous ratcheting of the ribosome

A crucial step in translation is the translocation of tRNAs through the ribosome. In the transition from one canonical site to the other, the tRNAs acquire intermediate configurations, so-called hybrid states. At this stage, the small subunit is rotated with respect to the large subunit, and the anticodon stem loops reside in the A and P sites of the small subunit, while the acceptor ends interact with the P and E sites of the large subunit. In this work, by means of cryo-EM and particle classification procedures, we visualize the hybrid state of both A/P and P/E tRNAs in an authentic factor-free ribosome complex during translocation. In addition, we show how the repositioning of the tRNAs goes hand in hand with the change in the interplay between S13, L1 stalk, L5, H68, H69, and H38 that is caused by the ratcheting of the small subunit.     

Observation of intersubunit movement of the ribosome in solution using FRET

Protein synthesis is believed to be a dynamic process, involving structural rearrangements of the ribosome. Cryo-EM reconstructions of certain elongation factor G (EF-G)-containing complexes have led to the proposal that translocation of tRNA and mRNA through the ribosome, from the A to P to E sites, is accompanied by a rotational movement between the two ribosomal subunits. Here, we have used F?rster resonance energy transfer (FRET) to monitor changes in the relative orientation of the ribosomal subunits in different complexes trapped at intermediate stages of translocation in solution. Binding of EF-G to the ribosome in the presence of the non-hydrolyzable GTP analogue GDPNP or GTP plus fusidic acid causes an increase in the efficiency of energy transfer between fluorophores introduced into proteins S11 in the 30 S subunit and L9 in the 50 S subunit, and a decrease in energy transfer between S6 and L9. Similar anti-correlated changes in energy transfer occur upon binding the GTP-requiring release factor RF3. These changes are consistent with the counter-clockwise rotation of the 30 S subunit relative to the 50 S subunit observed in cryo-EM studies. Reaction of ribosomal complexes containing the peptidyl-tRNA analogues N-Ac-Phe-tRNAPhe, N-Ac-Met-tRNAMet or f-Met-tRNAfMet with puromycin, conditions favoring movement of the resulting deacylated tRNAs into the P/E hybrid state, leads to similar changes in FRET. Conversely, treatment of a ribosomal complex containing deacylated and peptidyl-tRNAs bound in the A/P and P/E states, respectively, with EF-G.GTP causes reversal of the FRET changes. The use of FRET has enabled direct observation of intersubunit movement in solution, provides independent evidence that formation of the hybrid state is coupled to rotation of the 30 S subunit and shows that the intersubunit movement is reversed during the second step of translocation.     

Molecular basis for ATPase-powered substrate translocation by the Lon AAA+ protease

The Lon AAA+ (adenosine triphosphatases associated with diverse cellular activities) protease (LonA) converts ATP-fuelled conformational changes into sufficient mechanical force to drive translocation of a substrate into a hexameric proteolytic chamber. To understand the structural basis for the substrate translocation process, we determined the cryo-electron microscopy (cryo-EM) structure of Meiothermus taiwanensis LonA (MtaLonA) in a substrate-engaged state at 3.6 Å resolution. Our data indicate that substrate interactions are mediated by the dual pore loops of the ATPase domains, organized in spiral staircase arrangement from four consecutive protomers in different ATP-binding and hydrolysis states. However, a closed AAA+ ring is maintained by two disengaged ADP-bound protomers transiting between the lowest and highest position. This structure reveals a processive rotary translocation mechanism mediated by LonA-specific nucleotide-dependent allosteric coordination among the ATPase domains, which is induced by substrate binding.     

Domain movements of elongation factor eEF2 and the eukaryotic 80S ribosome facilitate tRNA translocation

An 11.7-A-resolution cryo-EM map of the yeast 80S.eEF2 complex in the presence of the antibiotic sordarin was interpreted in molecular terms, revealing large conformational changes within eEF2 and the 80S ribosome, including a rearrangement of the functionally important ribosomal intersubunit bridges. Sordarin positions domain III of eEF2 so that it can interact with the sarcin-ricin loop of 25S rRNA and protein rpS23 (S12p). This particular conformation explains the inhibitory action of sordarin and suggests that eEF2 is stalled on the 80S ribosome in a conformation that has similarities with the GTPase activation state. A ratchet-like subunit rearrangement (RSR) occurs in the 80S.eEF2.sordarin complex that, in contrast to Escherichia coli 70S ribosomes, is also present in vacant 80S ribosomes. A model is suggested, according to which the RSR is part of a mechanism for moving the tRNAs during the translocation reaction.     

Translocation of a tRNA with an extended anticodon through the ribosome

Coordinated translocation of the tRNA-mRNA complex by the ribosome occurs in a precise, stepwise movement corresponding to a distance of three nucleotides along the mRNA. Frameshift suppressor tRNAs generally contain an extra nucleotide in the anticodon loop and they subvert the normal mechanisms used by the ribosome for frame maintenance. The mechanism by which suppressor tRNAs traverse the ribosome during translocation is poorly understood. Here, we demonstrate translocation of a tRNA by four nucleotides from the A site to the P site, and from the P site to the E site. We show that translocation of a punctuated mRNA is possible with an extra, unpaired nucleotide between codons. Interestingly, the NMR structure of the four nucleotide anticodon stem-loop reveals a conformation different from the canonical tRNA structure. Flexibility within the loop may allow conformational adjustment upon A site binding and for interacting with the four nucleotide codon in order to shift the mRNA reading frame.     

Mechanism of tRNA translocation on the ribosome

During the translocation step of the elongation cycle of peptide synthesis two tRNAs together with the mRNA move synchronously and rapidly on the ribosome. Translocation is catalyzed by the elongation factor G (EF-G) and requires GTP hydrolysis. The fundamental biochemical features of the process were worked out in the 1970-80s, to a large part by A.S. Spirin and his colleagues. Recent results from pre-steady-state kinetic analysis and cryoelectron microscopy suggest that translocation is a multistep dynamic process that entails large-scale structural rearrangements of both ribosome and EF-G. Kinetic and thermodynamic data, together with the structural information on the conformational changes of the ribosome and of EF-G, provide a detailed mechanistic model of translocation and suggest a mechanism of translocation catalysis by EF-G.     

Recent advances in the structural biology of the 26S proteasome

There is growing appreciation for the fundamental role of structural dynamics in the function of macromolecules. In particular, the 26S proteasome, responsible for selective protein degradation in an ATP dependent manner, exhibits dynamic conformational changes that enable substrate processing. Recent cryo-electron microscopy (cryo-EM) work has revealed the conformational dynamics of the 26S proteasome and established the function of the different conformational states. Technological advances such as direct electron detectors and image processing algorithms allowed resolving the structure of the proteasome at atomic resolution. Here we will review those studies and discuss their contribution to our understanding of proteasome function.     

Cryo-EM of macromolecular assemblies at near-atomic resolution

With single-particle electron cryomicroscopy (cryo-EM), it is possible to visualize large, macromolecular assemblies in near-native states. Although subnanometer resolutions have been routinely achieved for many specimens, state of the art cryo-EM has pushed to near-atomic (3.3-4.6 ?) resolutions. At these resolutions, it is now possible to construct reliable atomic models directly from the cryo-EM density map. In this study, we describe our recently developed protocols for performing the three-dimensional reconstruction and modeling of Mm-cpn, a group II chaperonin, determined to 4.3 ? resolution. This protocol, utilizing the software tools EMAN, Gorgon and Coot, can be adapted for use with nearly all specimens imaged with cryo-EM that target beyond 5 ? resolution. Additionally, the feature recognition and computational modeling tools can be applied to any near-atomic resolution density maps, including those from X-ray crystallography.     

Hyper-swivel head domain motions are required for complete mRNA-tRNA translocation and ribosome resetting

Translocation of messenger RNA (mRNA) and transfer RNA (tRNA) substrates through the ribosome during protein synthesis, an exemplar of directional molecular movement in biology, entails a complex interplay of conformational, compositional, and chemical changes. The molecular determinants of early translocation steps have been investigated rigorously. However, the elements enabling the ribosome to complete translocation and reset for subsequent protein synthesis reactions remain poorly understood. Here, we have combined molecular simulations with single-molecule fluorescence resonance energy transfer imaging to gain insights into the rate-limiting events of the translocation mechanism. We find that diffusive motions of the ribosomal small subunit head domain to hyper-swivelled positions, governed by universally conserved rRNA, can maneuver the mRNA and tRNAs to their fully translocated positions. Subsequent engagement of peptidyl-tRNA and disengagement of deacyl-tRNA from mRNA, within their respective small subunit binding sites, facilitate the ribosome resetting mechanism after translocation has occurred to enable protein synthesis to resume.     

Modified nucleotides in tRNA(Lys) and tRNA(Val) are important for translocation

Ribosomes translate genetic information encoded by messenger RNAs (mRNAs) into proteins. Accurate decoding by the ribosome depends on the proper interaction between the mRNA codon and the anticodon of transfer RNA (tRNA). tRNAs from all kingdoms of life are enzymatically modified at distinct sites, particularly in and near the anticodon. Yet, the role of these naturally occurring tRNA modifications in translation is not fully understood. Here we show that modified nucleosides at the first, or wobble, position of the anticodon and 3'-adjacent to the anticodon are important for translocation of tRNA from the ribosome's aminoacyl site (A site) to the peptidyl site (P site). Thus, naturally occurring modifications in tRNA contribute functional groups and conformational dynamics that are critical for accurate decoding of mRNA and for translocation to the P site during protein synthesis.     

Consensus among flexible fitting approaches improves the interpretation of cryo-EM data

Cryo-elecron microscopy (cryo-EM) can provide important structural information of large macromolecular assemblies in different conformational states. Recent years have seen an increase in structures deposited in the Protein Data Bank (PDB) by fitting a high-resolution structure into its low-resolution cryo-EM map. A commonly used protocol for accommodating the conformational changes between the X-ray structure and the cryo-EM map is rigid body fitting of individual domains. With the emergence of different flexible fitting approaches, there is a need to compare and revise these different protocols for the fitting. We have applied three diverse automated flexible fitting approaches on a protein dataset for which rigid domain fitting (RDF) models have been deposited in the PDB. In general, a consensus is observed in the conformations, which indicates a convergence from these theoretically different approaches to the most probable solution corresponding to the cryo-EM map. However, the result shows that the convergence might not be observed for proteins with complex conformational changes or with missing densities in cryo-EM map. In contrast, RDF structures deposited in the PDB can represent conformations that not only differ from the consensus obtained by flexible fitting but also from X-ray crystallography. Thus, this study emphasizes that a "consensus" achieved by the use of several automated flexible fitting approaches can provide a higher level of confidence in the modeled configurations. Following this protocol not only increases the confidence level of fitting, but also highlights protein regions with uncertain fitting. Hence, this protocol can lead to better interpretation of cryo-EM data.     

Interactions of translational factor EF-G with the bacterial ribosome before and after mRNA translocation

A conserved translation factor, known as EF-G in bacteria, promotes the translocation of tRNA and mRNA in the ribosome during protein synthesis. Here, EF-G.ribosome complexes in two intermediate states, before and after mRNA translocation, have been probed with hydroxyl radicals generated from free Fe(II)-EDTA. Before mRNA translocation and GTP hydrolysis, EF-G protected a limited set of nucleotides in both subunits of the ribosome from cleavage by hydroxyl radicals. In this state, an extensive set of nucleotides, in the platform and head domains of the 30S subunit and in the L7/L12 stalk region of the 50S subunit, became more exposed to hydroxyl radical attack, suggestive of conformational changes in these domains. Following mRNA translocation, EF-G protected a larger set of nucleotides (23S rRNA helices H43, H44, H89, and H95; 16S rRNA helices h5 and h15). No nucleotide with enhanced reactivity to hydroxyl radicals was detected in this latter state. Both before and after mRNA translocation, EF-G protected identical nucleotides in h5 and h15 of the 30S subunit. These results suggest that h5 and h15 may remain associated with EF-G during the dynamic course of the translocation mechanism. Nucleotides in H43 and H44 of the 50S subunit were protected only after translocation and GTP hydrolysis, suggesting that these helices interact dynamically with EF-G. The effects in H95 suggest that EF-G interacts weakly with H95 before mRNA translocation and strongly and more extensively with this helix following mRNA translocation.     

Zinc binding alters the conformational dynamics and drives the transport cycle of the cation diffusion facilitator YiiP

YiiP is a secondary transporter that couples Zn²⁺ transport to the proton motive force. Structural studies of YiiP from prokaryotes and Znt8 from humans have revealed three different Zn²⁺ sites and a conserved homodimeric architecture. These structures define the inward-facing and outward-facing states that characterize the archetypal alternating access mechanism of transport. To study the effects of Zn²⁺ binding on the conformational transition, we use cryo-EM together with molecular dynamics simulation to compare structures of YiiP from Shewanella oneidensis in the presence and absence of Zn²⁺. To enable single-particle cryo-EM, we used a phage-display library to develop a Fab antibody fragment with high affinity for YiiP, thus producing a YiiP/Fab complex. To perform MD simulations, we developed a nonbonded dummy model for Zn²⁺ and validated its performance with known Zn²⁺-binding proteins. Using these tools, we find that, in the presence of Zn²⁺, YiiP adopts an inward-facing conformation consistent with that previously seen in tubular crystals. After removal of Zn²⁺ with high-affinity chelators, YiiP exhibits enhanced flexibility and adopts a novel conformation that appears to be intermediate between inward-facing and outward-facing states. This conformation involves closure of a hydrophobic gate that has been postulated to control access to the primary transport site. Comparison of several independent cryo-EM maps suggests that the transition from the inward-facing state is controlled by occupancy of a secondary Zn²⁺ site at the cytoplasmic membrane interface. This work enhances our understanding of individual Zn²⁺ binding sites and their role in the conformational dynamics that govern the transport cycle.