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Two Proofreading Steps Amplify the Accuracy of Genetic Code Translation

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Two Proofreading Steps Amplify the Accuracy of Genetic Code Translation Two proofreading steps amplify the accuracy of genetic code translation Ka-Weng Ieonga, Ülkü Uzuna,1, Maria Selmera, and Måns Ehrenberga,2 aDepartment of Cell and Molecular Biology, Uppsala University, Uppsala 75124, Sweden Edited by Ada Yonath, Weizmann Institute of Science, Rehovot, Israel, and approved October 12, 2016 (received for review July 4, 2016) Aminoacyl-tRNAs (aa-tRNAs) are selected by the messenger RNA kinetic efficiency to substrate-selective, enzyme-catalyzed reactions programmed ribosome in ternary complex with elongation factor than single-step proofreading (5, 14, 15), it has been taken for Tu (EF-Tu) and GTP and then, again, in a proofreading step after granted that there is but a single proofreading step in tRNA se- GTP hydrolysis on EF-Tu. We use tRNA mutants with different lection by the translating ribosome (16). Here, we present data affinities for EF-Tu to demonstrate that proofreading of aa- showing that the proofreading factor (F), by which the accuracy (A) tRNAs occurs in two consecutive steps. First, aa-tRNAs in ternary is amplified from its initial selection value (I) by aa-tRNA in ternary complex with EF-Tu·GDP are selected in a step where the accu- complex with EF-Tu and GTP, increases linearly with increasing racy increases linearly with increasing aa-tRNA affinity to EF-Tu. association equilibrium constant, KA, for aa-tRNA binding to EF- Then, following dissociation of EF-Tu·GDP from the ribosome, the Tu. We suggest the cause of this linear increase to be the activity of accuracy is further increased in a second and apparently EF- a first proofreading step, in which aa-tRNA is discarded in complex Tu−independent step. Our findings identify the molecular basis with EF-Tu and GDP whereas the forward reaction is release of of proofreading in bacteria, highlight the pivotal role of EF-Tu for EF-Tu·GDP. In the limit of zero affinity between aa-tRNA and EF- fast and accurate protein synthesis, and illustrate the importance Tu, where the first proofreading step is expected to be completely of multistep substrate selection in intracellular processing of turned off, we observe a remaining accuracy amplification by genetic information. proofreading. This amplification, we suggest, comes from the ac- tivity of a second proofreading step, involving aa-tRNA only (Fig. ribosome | error correction | fidelity | EF-Tu | ternary complex 1). We use the present results to discuss the molecular basis for proofreading of aminoacyl-tRNAs, which, until now, has remained e have found that the bacterial ribosome uses two proof- obscure (17). We suggest that multistep proofreading in genetic Wreading steps following initial selection of transfer RNAs code translation has evolved to neutralize potential error hot spots (tRNAs) to maintain high accuracy of translation of the genetic originating in error-prone initial selection of aa-tRNA in ternary code. This means that there are three selection steps for codon complex with EF-Tu and GTP (18, 19). Recent cryo-EM data of recognition by aa-tRNAs. First, there is initial codon selection by ribosomes from live human cells show two states of ribosome-bound aa-tRNA in ternary complex with elongation factor Tu (EF-Tu) preaccommodated aa-tRNA, one with aa-tRNA in complex with and GTP. Second, there is proofreading of aa-tRNA in ternary the EF-Tu homolog eEF1α in the GDP form, the other with only complex with EF-Tu and GDP. Third, there is proofreading of aa-tRNA after release of eEF1α·GDP (20). With support from aa-tRNA in an EF-Tu−independent manner, presumably after these cryo-EM data in conjunction with the present findings, we dissociation of EF-Tu·GDP from the ribosome (Fig. 1). Seven suggest that two-step proofreading mechanisms are at work not only decades ago, Linus Pauling suggested that the precision by which in bacteria but also in eukaryotes and, perhaps, in all three king- proteins can choose between cognate and near-cognate sub- doms of life. strates of similar structures is greatly limited, and he predicted high amino acid substitution frequency in intracellular protein Significance synthesis (1). However, experiments by Robert Loftfield with rabbit reticulocyte hemoglobin demonstrated an error frequency We have discovered that two proofreading steps amplify the much lower than Pauling’s prediction (2). One way to resolve this accuracy of genetic code reading, not one step, as hitherto apparent paradox was offered by the principle of proofreading, believed. We have characterized the molecular basis of each first formulated by Hopfield (3) and Ninio (4). By this principle, one of these steps, paving the way for structural analysis in the very same standard free energy difference, ΔΔG0, between conjunction with structure-based standard free energy com- enzyme-bound noncognate and cognate substrate can be used putations. Our work highlights the essential role of elongation in both initial selection (I) and subsequent proofreading se- factor Tu for accurate genetic code translation in both initial lection (F) to boost the total accuracy (A = I × F)ofselective codon selection and proofreading. Our results have implica- enzymes above their single-step selection limits. Accuracy am- tions for the evolution of efficient and accurate genetic code plification by proofreading requires substrate discarding to be reading through multistep proofreading, which attenuates the driven by a chemical potential decrease from the entering of a otherwise harmful effects of the obligatory tradeoff between substrate to its exit along the proofreading path. One way to im- efficiency and accuracy in substrate selection by enzymes. plement such a drop in chemical potential is to couple the discarding of substrates by proofreading to hydrolysis of GTP or ATP at high Author contributions: K.-W.I. and M.E. designed research; K.-W.I. and Ü.U. performed chemical potential to the low chemical potential of their hydrolytic research; M.S. performed structural analysis; K.-W.I. and M.E. analyzed data; and K.-W.I. products (5, 6). and M.E. wrote the paper. Proofreading was first discovered for the IleRS enzyme, which The authors declare no conflict of interest. Ile Ile aminoacylates tRNA to cognate Ile-tRNA and efficiently sup- This article is a PNAS Direct Submission. Ile presses near-cognate Val-tRNA formation by discarding valine Freely available online through the PNAS open access option. through ATP hydrolysis-driven proofreading (7). Soon thereafter, 1Present address: Department of Biochemistry, University of Oxford, Oxford OX1 3QU, GTP-driven proofreading of codon reading by aa-tRNAs on the United Kingdom. mRNA-translating ribosome was discovered (8, 9) and subsequently 2To whom correspondence should be addressed. Email: [email protected]. – subjected to extensive research (10 13). Although it was early rec- This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. ognized that multistep proofreading confers higher accuracy and 1073/pnas.1610917113/-/DCSupplemental. 13744–13749 | PNAS | November 29, 2016 | vol. 113 | no. 48 www.pnas.org/cgi/doi/10.1073/pnas.1610917113 Downloaded by guest on September 25, 2021 I I F II F Initial selection ( ) Proofreading ( 1 ) Proofreading ( 2 ) EF-Tu·GDP is discarded. Furthermore, because the accuracy of 2+ Binding of ternary AA-tRNA release from Accommodation / factor-free codon selection has comparatively low sensitivity to Mg GTP hydrolysis + complex EF-Tu•GDP Peptidyl transfer concentration (Fig. 2F), such a low Mg2 sensitivity could be typical of EPAT EPA c(nc) EPA E c(nc) EP kc(nc) k kc(nc) k a T c D Tu pep a second proofreading step after dissociation of EF-Tu·GDP from the E A E E E E P A c(nc) kd ribosome. We decided to subject this hypothesis to stringent testing in P D i qc(nc) qc(nc) 1 EF-Tu• GDP 2 a series of experiments that were inspired by the observation that the E PAT D E A T choice of base pairs in the T stem of any tRNA strongly affects its rejection of rejection of AA-tRNA EF-Tu • GDP • AA-tRNA affinity to EF-Tu · GTP (23). For these experiments, we first devel- E A E A oped a kinetic model for two-step proofreading in ribosomal D protein synthesis in bacteria (Fig. 1) as described in Mechanistic EF-Tu• GDP + AA-tRNA Model of a Two-Step Proofreading Mechanism. Total Proofreading FFF=⋅12 Mechanistic Model of a Two-Step Proofreading Mechanism in Fig. 1. Proposed schematic of tRNA selection in bacterial protein synthesis. A Bacterial Protein Synthesis. We designed a kinetic model for codon · ternary complex with aa-tRNA and EF-Tu GTP binds to A/T state of the pre- selection in an initial step, before GTP hydrolysis on EF-Tu, followed translocation ribosome with association rate constant ka. Then, the ternary complex dissociates with rate constant k or GTP is hydrolyzed with rate constant by accuracy amplification in two proofreading steps (Fig. 1). Initial d codon selection by aa-tRNA in ternary complex with EF-Tu·GTP is kc, leading to ribosome-bound ternary complex EF-Tu·GDP·aa-tRNA, which dis- sociates with rate constant q1,orEF-Tu·GDP dissociates with rate constant kTu, followed by a first proofreading step in which aa-tRNA in ternary leadingtoanaa-tRNA−bound preaccommodation state of the ribosome. From complex with EF-Tu·GDP is discarded or, alternatively, EF-Tu·GDP this state, aa-tRNA dissociates with rate constant q2 or accommodates into the A dissociates from the ribosome-bound aa-tRNA. In a second proof- site with rate constant kpep. There are three selection steps in this scheme: Near- reading step, aa-tRNA is discarded in an EF-Tu−independent manner cognate tRNA can be rejected during initial selection (I), first proofreading step or, alternatively, accommodated in the A site.
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