Structural basis for the transition from translation initiation to elongation by an 80S-eIF5B complex.
2020; 11 (1): 5003
eIF5B gates the transition from translation initiation to elongation.
Recognition of a start codon by the initiator aminoacyl-tRNA determines the reading frame of messenger RNA (mRNA) translation by the ribosome. In eukaryotes, the GTPase eIF5B collaborates in the correct positioning of the initiator Met-tRNAiMet on the ribosome in the later stages of translation initiation, gating entrance into elongation. Leveraging the long residence time of eIF5B on the ribosome recently identified by single-molecule fluorescence measurements, we determine the cryoEM structure of the naturally long-lived ribosome complex with eIF5B and Met-tRNAiMet immediately before transition into elongation. The structure uncovers an unexpected, eukaryotic specific and dynamic fidelity checkpoint implemented by eIF5B in concert with components of the large ribosomal subunit.
View details for DOI 10.1038/s41467-020-18829-3
View details for PubMedID 33024099
Dynamics of the context-specific translation arrest by chloramphenicol and linezolid.
Nature chemical biology
Translation initiation determines both the quantity and identity of the protein that is encoded in an mRNA by establishing the reading frame for protein synthesis. In eukaryotic cells, numerous translation initiation factors prepare ribosomes for polypeptide synthesis; however, the underlying dynamics of this process remain unclear1,2. A central question is how eukaryotic ribosomes transition from translation initiation to elongation. Here we use in vitro single-molecule fluorescence microscopy approaches in a purified yeast Saccharomyces cerevisiae translation system to monitor directly, in real time, the pathways of late translation initiation and the transition to elongation. This transition was slower in our eukaryotic system than that reported for Escherichia coli3-5. The slow entry to elongation was defined by a long residence time of eukaryotic initiation factor 5B (eIF5B) on the 80S ribosome after the joining of individual ribosomal subunits-a process that is catalysed by this universally conserved initiation factor. Inhibition of the GTPase activity of eIF5B after the joining of ribosomal subunits prevented the dissociation of eIF5B from the 80S complex, thereby preventing elongation. Our findings illustrate how the dissociation of eIF5B serves as a kinetic checkpoint for the transition from initiation to elongation, and how its release may be governed by a change in the conformation of the ribosome complex that triggers GTP hydrolysis.
View details for DOI 10.1038/s41586-019-1561-0
View details for PubMedID 31534220
RACK1 on and off the ribosome
2019; 25 (7): 881–95
Kinetics of D-Amino Acid Incorporation in Translation
ACS CHEMICAL BIOLOGY
2019; 14 (2): 204–13
Chloramphenicol (CHL) and linezolid (LZD) are antibiotics that inhibit translation. Both were thought to block peptide-bond formation between all combinations of amino acids. Yet recently, a strong nascent peptide context-dependency of CHL- and LZD-induced translation arrest was discovered. Here we probed the mechanism of action of CHL and LZD by using single-molecule Forster resonance energy transfer spectroscopy to monitor translation arrest induced by antibiotics. The presence of CHL or LZD does not substantially alter dynamics of protein synthesis until the arrest-motif of the nascent peptide is generated. Inhibition of peptide-bond formation compels the fully accommodated A-site transfer RNA to undergo repeated rounds of dissociation and nonproductive rebinding. The glycyl amino-acid moiety on the A-site Gly-tRNA manages to overcome the arrest by CHL. Our results illuminate the mechanism of CHL and LZD action through their interactions with the ribosome, the nascent peptide and the incoming amino acid, perturbing elongation dynamics.
View details for DOI 10.1038/s41589-019-0423-2
View details for PubMedID 31844301
RACK1 on and off the ribosome.
RNA (New York, N.Y.)
Despite the stereospecificity of translation for l-amino acids (l-AAs) in vivo, synthetic biologists have enabled ribosomal incorporation of d-AAs in vitro toward encoding polypeptides with pharmacologically desirable properties. However, the steps in translation limiting d-AA incorporation need clarification. In this work, we compared d- and l-Phe incorporation in translation by quench-flow kinetics, measuring 250-fold slower incorporation into the dipeptide for the d isomer from a tRNAPhe-based adaptor (tRNAPheB). Incorporation was moderately hastened by tRNA body swaps and higher EF-Tu concentrations, indicating that binding by EF-Tu can be rate-limiting. However, from tRNAAlaB with a saturating concentration of EF-Tu, the slow d-Phe incorporation was unexpectedly very efficient in competition with incorporation of the l isomer, indicating fast binding to EF-Tu, fast binding of the resulting complex to the ribosome, and rate-limiting accommodation/peptide bond formation. Subsequent elongation with an l-AA was confirmed to be very slow and inefficient. This understanding helps rationalize incorporation efficiencies in vitro and stereospecific mechanisms in vivo and suggests approaches for improving incorporation.
View details for DOI 10.1021/acschembio.8b00952
View details for Web of Science ID 000459367200009
View details for PubMedID 30648860
How Messenger RNA and Nascent Chain Sequences Regulate Translation Elongation.
Annual review of biochemistry
2018; 87: 421–49
Receptor for activated C kinase 1 (RACK1) is a eukaryote-specific ribosomal protein implicated in diverse biological functions. To engineer ribosomes for specific fluorescent labeling, we selected RACK1 as a target given its location on the small ribosomal subunit and other properties. However, prior results suggested that RACK1 has roles both on and off the ribosome, and such an exchange might be related to its various cellular functions and hinder our ability to use RACK1 as a stable fluorescent tag for the ribosome. In addition, the kinetics of spontaneous exchange of RACK1 or any ribosomal protein from a mature ribosome in vitro remain unclear. To address these issues, we engineered fluorescently-labeled human ribosomes via RACK1, and applied bulk and single-molecule biochemical analyses to track RACK1 on and off the human ribosome. Our results demonstrate that, despite its cellular non-essentiality from yeast to humans, RACK1 readily re-associates with the ribosome, displays limited conformational dynamics, and remains stably bound to the ribosome for hours in vitro. This work sheds insight into the biochemical basis of ribosomal protein exchange on and off a mature ribosome and provides tools for single-molecule analysis of human translation.
View details for PubMedID 31023766
2'-O-methylation in mRNA disrupts tRNA decoding during translation elongation.
Nature structural & molecular biology
Translation elongation is a highly coordinated, multistep, multifactor process that ensures accurate and efficient addition of amino acids to a growing nascent-peptide chain encoded in the sequence of translated messenger RNA (mRNA). Although translation elongation is heavily regulated by external factors, there is clear evidence that mRNA and nascent-peptide sequences control elongation dynamics, determining both the sequence and structure of synthesized proteins. Advances in methods have driven experiments that revealed the basic mechanisms of elongation as well as the mechanisms of regulation by mRNA and nascent-peptide sequences. In this review, we highlight how mRNA and nascent-peptide elements manipulate the translation machinery to alter the dynamics and pathway of elongation.
View details for PubMedID 29925264
Ribosomal incorporation of unnatural amino acids: lessons and improvements from fast kinetics studies.
Current opinion in chemical biology
2018; 46: 180–87
Chemical modifications of mRNA may regulate many aspects of mRNA processing and protein synthesis. Recently, 2'-O-methylation of nucleotides was identified as a frequent modification in translated regions of human mRNA, showing enrichment in codons for certain amino acids. Here, using single-molecule, bulk kinetics and structural methods, we show that 2'-O-methylation within coding regions of mRNA disrupts key steps in codon reading during cognate tRNA selection. Our results suggest that 2'-O-methylation sterically perturbs interactions of ribosomal-monitoring bases (G530, A1492 and A1493) with cognate codon-anticodon helices, thereby inhibiting downstream GTP hydrolysis by elongation factor Tu (EF-Tu) and A-site tRNA accommodation, leading to excessive rejection of cognate aminoacylated tRNAs in initial selection and proofreading. Our current and prior findings highlight how chemical modifications of mRNA tune the dynamics of protein synthesis at different steps of translation elongation.
View details for PubMedID 29459784
Dynamic basis of fidelity and speed in translation: Coordinated multistep mechanisms of elongation and termination.
Technologies for genetically programming ribosomal incorporation of unnatural amino acids are expanding and have created many exciting applications. However, these applications are generally limited by low efficiencies of the unnatural incorporations. Here we review our current mechanistic understanding of these limitations delineated from in vitro fast kinetics. Rate limitation occurs by different mechanisms, depending on the classes of the unnatural amino acids and the tRNA adaptors. This new understanding has led to several ways of improving the incorporation efficiencies, as well as challenges of dogma on rate-limiting steps in protein synthesis in natural cells.
View details for PubMedID 30125734
Translational roles of the C75 2'OH in an in vitro tRNA transcript at the ribosomal A, P and E sites.
2017; 7 (1): 6709
As the universal machine that transfers genetic information from RNA to protein, the ribosome synthesizes proteins with remarkably high fidelity and speed. This is a result of the accurate and efficient decoding of mRNA codons via multistep mechanisms during elongation and termination stages of translation. These mechanisms control how the correct sense codon is recognized by a tRNA for peptide elongation, how the next codon is presented to the decoding center without change of frame during translocation, and how the stop codon is discriminated for timely release of the nascent peptide. These processes occur efficiently through coupling of chemical energy expenditure, ligand interactions, and conformational changes. Understanding this coupling in detail required integration of many techniques that were developed in the past two decades. This multidisciplinary approach has revealed the dynamic nature of translational control and uncovered how external cellular factors such as tRNA abundance and mRNA modifications affect the synthesis of the protein product. Insights from these studies will aid synthetic biology and therapeutic approaches to translation.
View details for DOI 10.1002/pro.3190
View details for PubMedID 28480640
De novo design and synthesis of a 30-cistron translation-factor module.
Nucleic acids research
Aminoacyl-tRNAs containing a deoxy substitution in the penultimate nucleotide (C75 2'OH → 2'H) have been widely used in translation for incorporation of unnatural amino acids (AAs). However, this supposedly innocuous modification surprisingly increased peptidyl-tRNA(Ala)ugc drop off in biochemical assays of successive incorporations. Here we predict the function of this tRNA 2'OH in the ribosomal A, P and E sites using recent co-crystal structures of ribosomes and tRNA substrates and test these structure-function models by systematic kinetics analyses. Unexpectedly, the C75 2'H did not affect A- to P-site translocation nor peptidyl donor activity of tRNA(Ala)ugc. Rather, the peptidyl acceptor activity of the A-site Ala-tRNA(Ala)ugc and the translocation of the P-site deacylated tRNA(Ala)ugc to the E site were impeded. Delivery by EF-Tu was not significantly affected. This broadens our view of the roles of 2'OH groups in tRNAs in translation.
View details for DOI 10.1038/s41598-017-06991-6
View details for PubMedID 28751745
View details for PubMedCentralID PMC5532260
Kinetics of tRNA(Pyl)-mediated amber suppression in Escherichia coli translation reveals unexpected limiting steps and competing reactions
BIOTECHNOLOGY AND BIOENGINEERING
2016; 113 (7): 1552-1559
Two of the many goals of synthetic biology are synthesizing large biochemical systems and simplifying their assembly. While several genes have been assembled together by modular idempotent cloning, it is unclear if such simplified strategies scale to very large constructs for expression and purification of whole pathways. Here we synthesize from oligodeoxyribonucleotides a completely de-novo-designed, 58-kb multigene DNA. This BioBrick plasmid insert encodes 30 of the 31 translation factors of the PURE translation system, each His-tagged and in separate transcription cistrons. Dividing the insert between three high-copy expression plasmids enables the bulk purification of the aminoacyl-tRNA synthetases and translation factors necessary for affordable, scalable reconstitution of an in vitro transcription and translation system, PURE 3.0.
View details for DOI 10.1093/nar/gkx753
View details for PubMedID 28977654
Ribosomal Peptide Syntheses from Activated Substrates Reveal Rate Limitation by an Unexpected Step at the Peptidyl Site.
Journal of the American Chemical Society
2016; 138 (48): 15587–95
The utility of ribosomal incorporation of unnatural amino acids (AAs) in vivo is generally restricted by low efficiencies, even with the most widely used suppressor tRNA(Pyl) . Because of the difficulties of studying incorporation in vivo, almost nothing is known about the limiting steps after tRNA charging. Here, we measured the kinetics of all subsequent steps using a purified Escherichia coli translation system. Dipeptide formation from initiator fMet-tRNA(fMet) and tRNA(Pyl) charged with allylglycine or methylserine displayed unexpectedly sluggish biphasic kinetics, ∼30-fold slower than for native substrates. The amplitude of the fast phases increased with increasing EF-Tu concentration, allowing measurement of Kd values of EF-Tu binding, both of which were ∼25-fold weaker than normal. However, binding could be increased ∼30-fold by lowering temperature. The fast phase rates were limited by the surprisingly ∼10-fold less efficient binding of EF-Tu:GTP:AA-tRNA(Pyl) ternary complex to the ribosomes, not GTP hydrolysis or peptide bond formation. Furthermore, processivity was unexpectedly impaired as ∼40% of the dipeptidyl-tRNA(Pyl) could not be elongated to tripeptide. Dipeptide formation was slow enough that termination due to misreading the UAG codon by non-cognate RF2 became very significant. This new understanding provides a framework for improving unnatural AA incorporation by amber suppression. Biotechnol. Bioeng. 2016;113: 1552-1559. © 2015 Wiley Periodicals, Inc.
View details for DOI 10.1002/bit.25917
View details for Web of Science ID 000377527900017
View details for PubMedID 26705134
Kinetics of Ribosome-Catalyzed Polymerization Using Artificial Aminoacyl-tRNA Substrates Clarifies Inefficiencies and Improvements
ACS CHEMICAL BIOLOGY
2015; 10 (10): 2187-2192
Protein synthesis (translation) is central to cellular function and antibiotic development. Interestingly, the key chemical step of translation, peptide bond formation, is among the slower enzymatic reactions. The reason for this remains controversial because of reliance on studies using highly modified, severely minimized, or unreactive substrate analogues. Here, we investigated this problem by fast kinetics using full-length aminoacyl-tRNA substrates with atomic substitutions that activated the ester electrophile. While trifluoro substitution of hydrogens in nonconserved positions of the peptidyl-site substrate dramatically increased the ester reactivity in solution assays, a large hastening of the combined rates of ribosomal accommodation and peptidyl transfer was observed only with a slowly reacting aminoacyl-site nucleophile, proline. With a fast-reacting A-site nucleophile, phenylalanine, effects did not correlate at all with electrophilicities. As effects were observed using the same, natural, aminoacyl-tRNA at the A site and all rates of accommodation/peptidyl transfer were pH dependent, we concluded that rate limitation was not by A-site accommodation but rather by peptidyl transfer and a hitherto unexpected step at the P site. This new slow step, which we term P-site accommodation, has implications for the activation or inhibition of ribosome function in vitro and in vivo.
View details for DOI 10.1021/jacs.6b06936
View details for PubMedID 27934010
Facile Synthesis of N-Acyl-aminoacyl-pCpA for Preparation of Mischarged Fully Ribo tRNA
2014; 25 (11): 2086-2091
Ribosomal synthesis of polymers of unnatural amino acids (AAs) is limited by low incorporation efficiencies using the artificial AA-tRNAs, but the kinetics have yet to be studied. Here, kinetics were performed on five consecutive incorporations using various artificial AA-tRNAs with all intermediate products being analyzed. Yields within a few seconds displayed similar trends to our prior yields after 30 min without preincubation, demonstrating the relevance of fast kinetics to traditional long-incubation translations. Interestingly, the two anticodon swaps were much less inhibitory in the present optimized system, which should allow more flexibility in the engineering of artificial AA-tRNAs. The biggest kinetic defect was caused by the penultimate dC introduced from the standard, chemoenzymatic, charging method. This prompted peptidyl-tRNA drop-off, decreasing processivities during five consecutive AA incorporations. Indeed, two tRNA charging methods that circumvented the dC dramatically improved efficiencies of ribosomal, consecutive, unnatural AA incorporations to give near wild-type kinetics.
View details for DOI 10.1021/acschembio.5b00335
View details for Web of Science ID 000363225100002
View details for PubMedID 26191973
Peptide Formation by N-Methyl Amino Acids in Translation Is Hastened by Higher pH and tRNA(Pro)
ACS CHEMICAL BIOLOGY
2014; 9 (6): 1303-1311
Chemical synthesis of N-acyl-aminoacyl-pdCpA and its ligation to tRNA(minus CA) is widely used for the preparation of unnatural aminoacyl-tRNA substrates for ribosomal translation. However, the presence of the unnatural deoxyribose can decrease incorporation yield in translation and there is no straightforward method for chemical synthesis of the natural ribo version. Here, we show that pCpA is surprisingly stable to treatment with strong organic bases provided that anhydrous conditions are used. This allowed development of a facile method for chemical aminoacylation of pCpA. Preparative synthesis of pCpA was also simplified by using t-butyl-dithiomethyl protecting group methodology, and a more reliable pCpA postpurification treatment method was developed. Such aminoacyl-pCpA analogues ligated to tRNA(minus CA) transcripts are highly active in a purified translation system, demonstrating utility of our synthetic method.
View details for DOI 10.1021/bc500441b
View details for Web of Science ID 000345309000020
View details for PubMedID 25338217
Applications of N-methyl amino acids (NMAAs) in drug discovery are limited by their low efficiencies of ribosomal incorporation, and little is known mechanistically about the steps leading to incorporation. Here, we demonstrate that a synthetic tRNA body based on a natural N-alkyl amino acid carrier, tRNA(Pro), increases translation incorporation rates of all three studied NMAAs compared with tRNA(Phe)- and tRNA(Ala)-based bodies. We also investigate the pH dependence of the incorporation rates and find that the rates increase dramatically in the range of pH 7 to 8.5 with the titration of a single proton. Results support a rate-limiting peptidyl transfer step dependent on deprotonation of the N-nucleophile of the NMAA. Competition experiments demonstrate that several futile cycles of delivery and rejection of A-site NMAA-tRNA are required per peptide bond formed and that increasing magnesium ion concentration increases incorporation yield. Data clarify the mechanism of ribosomal NMAA incorporation and provide three generalizable ways to improve incorporation of NMAAs in translation.
View details for DOI 10.1021/cb500036a
View details for Web of Science ID 000337870500012
View details for PubMedID 24673854