Capturing Transition States for tRNA Hybrid-State Formation in the Ribosome

Conformational changes of ribosomes during translation elongation resolved by molecular dynamics simulations

Molecular dynamics simulations have emerged as a powerful set of tools to unravel the intricate dynamics of ribosomes during protein synthesis. Recent advancements in this field have enabled simulations to delve deep into the conformational rearrangements of ribosomes and associated factors, providing invaluable insights into the intricacies of translation. Emphasis on simulations has recently been on translation elongation, such as tRNA selection, translocation, and ribosomal head-swivel motions. These studies have offered crucial structural interpretations of how genetic information is faithfully translated into proteins. This review outlines recent discoveries concerning ribosome conformational changes occurring during translation elongation, as elucidated through molecular dynamics simulations.

tRNA shape is an identity element for an archaeal pyrrolysyl-tRNA synthetase from the human gut

Protein translation is orchestrated through tRNA aminoacylation and ribosomal elongation. Among the highly conserved structure of tRNAs, they have distinguishing features which promote interaction with their cognate aminoacyl tRNA synthetase (aaRS). These key features are referred to as identity elements. In our study, we investigated the tRNA:aaRS pair that installs the 22nd amino acid, pyrrolysine (tRNAPyl:PylRS). Pyrrolysyl-tRNA synthetases (PylRSs) are naturally encoded in some archaeal and bacterial genomes to acylate tRNAPyl with pyrrolysine. Their large amino acid binding pocket and poor recognition of the tRNA anticodon have been instrumental in incorporating >200 noncanonical amino acids. PylRS enzymes can be divided into three classes based on their genomic structure. Two classes contain both an N-terminal and C-terminal domain, however the third class (ΔpylSn) lacks the N-terminal domain. In this study we explored the tRNA identity elements for a ΔpylSn tRNAPyl from Candidatus Methanomethylophilus alvus which drives the orthogonality seen with its cognate PylRS (MaPylRS). From aminoacylation and translation assays we identified five key elements in ΔpylSn tRNAPyl necessary for MaPylRS activity. The absence of a base (position 8) and a G-U wobble pair (G28:U42) were found to affect the high-resolution structure of the tRNA, while molecular dynamic simulations led us to acknowledge the rigidity imparted from the G-C base pairs (G3:C70 and G5:C68).

Uncovering translation roadblocks during the development of a synthetic tRNA

Ribosomes are remarkable in their malleability to accept diverse aminoacyl-tRNA substrates from both the same organism and other organisms or domains of life. This is a critical feature of the ribosome that allows the use of orthogonal translation systems for genetic code expansion. Optimization of these orthogonal translation systems generally involves focusing on the compatibility of the tRNA, aminoacyl-tRNA synthetase, and a non-canonical amino acid with each other. As we expand the diversity of tRNAs used to include non-canonical structures, the question arises as to the tRNA suitability on the ribosome. Specifically, we investigated the ribosomal translation of allo-tRNAUTu1, a uniquely shaped (9/3) tRNA exploited for site-specific selenocysteine insertion, using single-molecule fluorescence. With this technique we identified ribosomal disassembly occurring from translocation of allo-tRNAUTu1 from the A to the P site. Using cryo-EM to capture the tRNA on the ribosome, we pinpointed a distinct tertiary interaction preventing fluid translocation. Through a single nucleotide mutation, we disrupted this tertiary interaction and relieved the translation roadblock. With the continued diversification of genetic code expansion, our work highlights a targeted approach to optimize translation by distinct tRNAs as they move through the ribosome.

Elongation Factor Tu Switch I Element is a Gate for Aminoacyl-tRNA Selection

Selection of correct aminoacyl (aa)-tRNA at the ribosomal A site is fundamental to maintaining translational fidelity. Aa-tRNA selection is a multistep process facilitated by the guanosine triphosphatase elongation factor (EF)-Tu. EF-Tu delivers aa-tRNA to the ribosomal A site and participates in tRNA selection. The structural mechanism of how EF-Tu is involved in proofreading remains to be fully resolved. Here, we provide evidence that switch I of EF-Tu facilitates EF-Tu's involvement during aa-tRNA selection. Using structure-based and explicit solvent molecular dynamics simulations based on recent cryo-electron microscopy reconstructions, we studied the conformational change of EF-Tu from the guanosine triphosphate to guanine diphosphate conformation during aa-tRNA accommodation. Switch I of EF-Tu rapidly converts from an α-helix into a β-hairpin and moves to interact with the acceptor stem of the aa-tRNA. In doing so, switch I gates the movement of the aa-tRNA during accommodation through steric interactions with the acceptor stem. Pharmacological inhibition of the aa-tRNA accommodation pathway prevents the proper positioning of switch I with the aa-tRNA acceptor stem, suggesting that the observed interactions are specific for cognate aa-tRNA substrates, and thus capable of contributing to the fidelity mechanism.

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.

Dissecting the Energetics of Subunit Rotation in the Ribosome

The accurate expression of proteins requires the ribosome to efficiently undergo elaborate conformational rearrangements. The most dramatic of these motions is subunit rotation, which is necessary for tRNA molecules to transition between ribosomal binding sites. While rigid-body descriptions provide a qualitative picture of the process, obtaining quantitative mechanistic insights requires one to account for the relationship between molecular flexibility and collective dynamics. Using simulated rotation events, we assess the quality of experimentally accessible measures for describing the collective displacement of the ∼4000-residue small subunit. For this, we ask whether each coordinate is able to identify the underlying free-energy barrier and transition state ensemble (TSE). We find that intuitive structurally motivated coordinates (e.g., rotation angle, interprotein distances) can distinguish between the endpoints, though they are poor indicators of barrier-crossing events, and they underestimate the free-energy barrier. In contrast, coordinates based on intersubunit bridges can identify the TSE. We additionally verify that the committor probability for the putative TSE configurations is 0.5, a hallmark feature of any transition state. In terms of structural properties, these calculations implicate a transition state in which flexibility allows for asynchronous rearrangements of the bridges, as the ribosome adopts a partially rotated orientation. This provides a theoretical foundation, upon which experimental techniques may precisely quantify the energy landscape of the ribosome.

Ribosome Structure, Function, and Early Evolution

Ribosomes are among the largest and most dynamic molecular motors. The structure and dynamics of translation initiation and elongation are reviewed. Three ribosome motions have been identified for initiation and translocation. A swivel motion between the head/beak and the body of the 30S subunit was observed. A tilting dynamic of the head/beak versus the body of the 30S subunit was detected using simulations. A reversible ratcheting motion was seen between the 30S and the 50S subunits that slide relative to one another. The 30S⁻50S intersubunit contacts regulate translocation. IF2, EF-Tu, and EF-G are homologous G-protein GTPases that cycle on and off the same site on the ribosome. The ribosome, aminoacyl-tRNA synthetase (aaRS) enzymes, transfer ribonucleic acid (tRNA), and messenger ribonucleic acid (mRNA) form the core of information processing in cells and are coevolved. Surprisingly, class I and class II aaRS enzymes, with distinct and incompatible folds, are homologs. Divergence of class I and class II aaRS enzymes and coevolution of the genetic code are described by analysis of ancient archaeal species.

Dual DNA rulers reveal an 'mRNA looping' intermediate state during ribosome translocation

The precise 3-nucleotide movement of mRNA is critical for translation fidelity. One mRNA translocation error propagates to all of the following codons, which is detrimental to the cell. However, none of the current methods can reveal the mRNA dynamics near the ribosome entry site, which limits the understanding of this important issue. We have developed an assay of dual DNA rulers that provides such capability. By uniquely probing both the 3'- and 5'-ends of mRNA, we observed an antibiotic-trapped intermediate state that is consistent with a ribosomal conformation containing mRNA asymmetric partial displacements at its entry and exit sites. Based on the available ribosome structures and computational simulations, we proposed a 'looped' mRNA conformation, which suggested a stepwise 'inchworm' mechanism for ribosomal translocation. The same 'looped' intermediate state identified with the dual rulers persists with a '-1' frameshifting motif, indicating that the branching point of normal and frameshifting translocations occurs at a later stage of translocation.

How the Ribosomal A-Site Finger Can Lead to tRNA Species-Dependent Dynamics

Proteins are synthesized by the joint action of the ribosome and tRNA molecules, where the rate of synthesis can be affected by numerous factors, such as the concentration of tRNA, the binding affinity of tRNA for the ribosome, or post-transcriptional modifications. Here, we expand this range of contributors by demonstrating how differences in tRNA structure can give rise to tRNA species-specific dynamics in the ribosome. To show this, we perform simulations of A/P hybrid-state formation for two tRNA species (tRNA^Phe and tRNA^Leu), which differ in the size of their variable loops (VLs). These calculations reveal that steric interactions between the VL and the ribosomal A-site finger (ASF, i.e., H38 of 23S rRNA) can directly modulate the free-energy landscape for each tRNA species. We also find that tRNA and ASF motions are highly correlated, where fluctuations of the ASF are predictive of tRNA transition events. Finally, by introducing perturbations to the model, we demonstrate that ASF flexibility is a determinant of the rate of A/P hybrid-state formation.