Year: 2011
Type: article
Abstract: Mistranslation broadly encompasses the introduction of errors during any step of protein synthesis, leading to the incorporation of an amino acid different from the one encoded in the gene.Recent research has enhanced our understanding of the mechanisms that control mistranslation at the molecular level and has led to the discovery that the rates of mistranslation in vivo are not fixed but variable.Protein synthesis has an overall error rate of approximately 10 -4 for each mRNA codon translated.The fidelity of translation is mainly determined by two events: synthesis of cognate amino acid:tRNA pairs by aminoacyl-tRNA synthetases (aaRSs) and accurate selection of aminoacyl-tRNAs (aa-tRNAs) by the ribosome.To ensure faithful aa-tRNA synthesis many aaRSs employ a proofreading ("editing") activity, such as phenylalanyl-tRNA synthetases (PheRS) that hydrolyze mischarged Tyr-tRNA Phe .Eukaryotes maintain two distinct PheRS enzymes, a cytoplasmic (ctPheRS) and an organellar form.CtPheRS is similar to bacterial enzymes in that it consists of a heterotetramer in which the α-subunits contain the active site and the β-subunits harbor the editing site.In contrast, mitochondrial PheRS (mtPheRS) is an α-subunit monomer that does not edit Tyr-tRNA Phe , and a comparable trans-acting activity does not exist in organelles.Although mtPheRS does not edit, the Saccharomyces cerevisiae mtPheRS is extremely specific as only one Tyr-tRNA Phe is synthesized for every ~7300 Phe-tRNA Phe , iii compatible with an error rate in translation of ~10 -4 .When the error rate of mtPheRS was increased 17-fold, the corresponding strain could not grow on respiratory media and the mitochondrial genome was rapidly lost.In contrast, error-prone mtPheRS, editingdeficient ctPheRS, and their wild type counterparts all supported cytoplasmic protein synthesis and cell growth.However, alterations of the tyrosine concentrations provided in the growth media reveal a titratable growth phenotype in editing-deficient ctPheRS cells.This suggest that the loss of editing by ctPheRS reduces the efficiency of translation and that the editing function of ctPheRS acts as a constitutive response to amino acid stress conditions.The differences seen in the requirements for quality control in different cellular compartments and with different extracellular amino acid conditions suggest that the limits of translational accuracy may be largely determined by cellular physiology.Defects in organellar translation are the underlying cause of a number of mitochondrial diseases including diabetes, deafness, encephalopathy and other mitochondrial myopathies.The most common causes of these diseases are mutations in mitochondrially encoded tRNAs.It has recently become apparent that mutations in nuclear-encoded components of the mitochondrial translation machinery, such as the aaRSs, can also lead to disease.In some cases, mutations can be directly linked to losses in enzymatic activity, but for many their effect is unknown.To investigate how aaRS mutations impact function without changing enzymatic activity, we chose nonsynonymous single nucleotide polymorphisms (nsSNPs) that encode residues distal from the active site of human mitochondrial phenylalanyl-tRNA synthetase (hmtPheRS).The hmtPheRS variants S57C and N280S both displayed wild type aminoacylation activity iv and stability with respect to their free energies of unfolding, but were less stable at low pH.Mitochondrial proteins undergo partial unfolding/refolding during import, and both the S57C and N280S variants retained less activity than wild type after refolding, consistent with their reduced stability at low pH.These data show nsSNPs can impact mitochondrial translation by changing a biophysical property of a protein, in this case refolding, without affecting the corresponding enzymatic activity.v Dedication Dedicated to my family
Author: Noah M. Reynolds
Institution: The Ohio State University
Language: English
Subfield: Molecular Biology
Domain: Life Sciences
Open Access status: green
OpenAlex ID: W428403808