Trends in Biochemical Sciences
Impact of disease-related mitochondrial mutations on tRNA structure and function
Section snippets
An overview of human mitochondrial tRNAs
tRNAs are essential components of protein synthesis because they function to transport amino acids to the ribosome, match them to the codons of mRNAs and facilitate their incorporation into polypeptides with high fidelity [6]. The human mitochondrial genome contains a minimal set of tRNAs. The 16.6 kb circular genome encodes 22 tRNAs, along with 13 proteins that correspond to respiratory chain subunits, and two rRNAs 1, 7. The tRNAs encoded in the mitochondria are the only ones used in protein
tRNALeu(UUR): loss of native structure hinders function
The gene encoding human mitochondrial tRNALeu(UUR) harbors a significant portion of the known pathogenic mutations within the human mitochondrial genome. Of the 150 documented mutations, 20 occur within the tRNALeu(UUR) gene (see http://www.mitomap.org; Figure 3). The mutations (or single-base deletions) affect 18 of the 76 nucleotides within this tRNA. Varied clinical consequences result from mutations in tRNALeu(UUR), including diabetes, myopathies and encephalopathies (see //www.mitomap.org
tRNAIle: weak structures amplify effects of mutations
The gene encoding tRNAIle contains ten pathogenic mutations associated with cardiomyopathy and ophthalmoplegia 4, 47. Interestingly, the four ophthalmoplegia mutations all introduce CA mispairs into stem regions of this tRNA, whereas the six cardiomyopathy mutations are in varied locations (two produce GU mispairs, three are substitutions of unpaired bases and one replaces a CA mispair in the wild-type structure with a Watson–Crick UA pair; Figure 3). Although a link between a certain type of
tRNALys: importance of post-transcriptional modifications
The human mitochondrial tRNALys is an example of a mitochondrial tRNA with a minimized structure (Figure 3). The D-loop of this molecule is proposed to consist of only three bases, which limits the number of contacts stabilizing the tertiary fold. Post-transcriptional modifications appear to play a significant role in stabilizing the structure of human mitochondrial tRNALys and might contribute to the cellular effects of the pathogenic mutations identified within this gene. A study of cybrid
tRNASer(UCN): decreased tRNA levels with pathogenic mutations
Lowered steady-state tRNA levels have been correlated with two deafness-related mutations within the human mitochondrial tRNASer(UCN) gene. The U7445C mutation affects a nucleotide proximal to the site of 3′ end processing (Figure 3), and in vitro studies indicate that cleavage by 3′-tRNase is significantly inhibited [21]. The defect in processing would affect the levels of tRNASer(UCN). In addition, analysis of cybrid cell lines containing the 7472C insertion that affects tRNASer(UCN) appears
Concluding remarks
Recent efforts to catalog the effects of disease-related mutations affecting mitochondrial tRNAs have highlighted the sensitivity of the function of these important biomolecules to subtle structural perturbations. Disruptions in elements of secondary structure or the loss of contacts stabilizing tertiary structure can significantly impair function, as revealed by in vitro and in vivo studies of protein synthesis efficiency, aminoacylation, post-transcriptional modification and processing. The
Acknowledgements
We acknowledge the NIH (GM063890–01A2) for financial support of the work on human mitochondrial tRNAs conducted in our laboratory.
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