In vitro mutagenesis of the mitochondrial leucyl-tRNA synthetase of S. cerevisiae reveals residues critical for its in vivo activities

The phenotypic expression of mitochondrial tRNA-mutations can be modulated by either mitochondrial leucyl-tRNA synthetase or the C-terminal domain thereof

Mutations in mitochondrial (mt) DNA determine important human diseases. The majority of the known pathogenic mutations are located in transfer RNA (tRNA) genes and are responsible for a wide range of currently untreatable disorders. Experimental evidence both in yeast and in human cells has shown that the detrimental effects of mt-tRNA point mutations can be attenuated by increasing the expression of the cognate mt-aminoacyl-tRNA synthetases (aaRSs). In addition, constitutive high levels of isoleucyl-tRNA syntethase have been shown to reduce the penetrance of a homoplasmic mutation in mt-tRNA^Ile in a small kindred. More recently, we showed that the isolated carboxy-terminal domain of human mt-leucyl tRNA synthetase (LeuRS-Cterm) localizes to mitochondria and ameliorates the energetic defect in transmitochondrial cybrids carrying mutations either in the cognate mt-tRNA^Leu(UUR) or in the non-cognate mt-tRNA^Ile gene. Since the mt-LeuRS-Cterm does not possess catalytic activity, its rescuing ability is most likely mediated by a chaperon-like effect, consisting in the stabilization of the tRNA structure altered by the mutation. All together, these observations open potential therapeutic options for mt-tRNA mutations-associated diseases.

The isolated carboxy-terminal domain of human mitochondrial leucyl-tRNA synthetase rescues the pathological phenotype of mitochondrial tRNA mutations in human cells

Mitochondrial (mt) diseases are multisystem disorders due to mutations in nuclear or mtDNA genes. Among the latter, more than 50% are located in transfer RNA (tRNA) genes and are responsible for a wide range of syndromes, for which no effective treatment is available at present. We show that three human mt aminoacyl-tRNA syntethases, namely leucyl-, valyl-, and isoleucyl-tRNA synthetase are able to improve both viability and bioenergetic proficiency of human transmitochondrial cybrid cells carrying pathogenic mutations in the mt-tRNA(Ile) gene. Importantly, we further demonstrate that the carboxy-terminal domain of human mt leucyl-tRNA synthetase is both necessary and sufficient to improve the pathologic phenotype associated either with these "mild" mutations or with the "severe" m.3243A>G mutation in the mt-tRNA(L)(eu(UUR)) gene. Furthermore, we provide evidence that this small, non-catalytic domain is able to directly and specifically interact in vitro with human mt-tRNA(Leu(UUR)) with high affinity and stability and, with lower affinity, with mt-tRNA(Ile). Taken together, our results sustain the hypothesis that the carboxy-terminal domain of human mt leucyl-tRNA synthetase can be used to correct mt dysfunctions caused by mt-tRNA mutations.

Genetic evidence for interaction between Cbp1 and specific nucleotides in the 5' untranslated region of mitochondrial cytochrome b mRNA in Saccharomyces cerevisiae

The cytochrome b (COB) gene is encoded by the mitochondrial genome; however, its expression requires the participation of several nuclearly encoded protein factors. The yeast Cbp1 protein, which is encoded by the nuclear CBP1 gene, is required for the stabilization of COB mRNA. A previous deletion analysis identified an 11-nucleotide-long sequence within the 5' untranslated region of COB mRNA that is important for Cbp1-dependent COB mRNA stability. In the present study, site-directed mutagenesis experiments were carried out to define further the features of this cis element. The CCG sequence within this region was shown to be necessary for stability. A change in residue 533 of Cbp1 from aspartate to tyrosine suppresses the effects of a single-base change in the CCG element. This is strong genetic evidence that the nuclearly encoded Cbp1 protein recognizes and binds directly to the sequence containing CCG and thus protects COB mRNA from degradation.

In vitro mutagenesis of the mitochondrial leucyl tRNA synthetase of Saccharomyces cerevisiae shows that the suppressor activity of the mutant proteins is related to the splicing function of the wild-type protein

The NAM2 gene of Saccharomyces cerevisiae encodes the mitochondrial leucyl tRNA synthetase (mLRS), which is necessary for the excision of the fourth intron of the mitochondrial cytb gene (bI4) and the fourth intron of the mitochondrial coxI gene (aI4), as well as for mitochondrial protein synthesis. Some dominant mutant alleles of the gene are able to suppress mutations that inactivate the bI4 maturase, which is essential for the excision of the introns aI4 and bI4. Here we report mutagenesis studies which focus on the splicing and suppressor functions of the protein. Small deletions in the C-terminal region of the protein preferentially reduce the splicing, but not the synthetase activity; and all the C-terminal deletions tested abolish the suppressor activity. Mutations which increase the volume of the residue at position 240 in the wild-type mLRS without introducing a charge, lead to a suppressor activity. The mutant 238C, which is located in the suppressor region, has a reduced synthetase activity and no detectable splicing activity. These data show that the splicing and suppressor functions are linked and that the suppressor activity of the mutant alleles results from a modification of the wild-type splicing activity.

Nucleo-mitochondrial interactions in mitochondrial gene expression

All proteins encoded by mitochondrial DNA (mtDNA) are dependent on proteins encoded by nuclear genes for their synthesis and function. Recent developments in the identification of these genes and the elucidation of the roles their products play at various stages of mitochondrial gene expression are covered in this review, which focuses mainly on work with the yeast Saccharomyces cerevisiae. The high degree of evolutionary conservation of many cellular processes between this yeast and higher eukaryotes, the ease with which mitochondrial biogenesis can be manipulated both genetically and physiologically, and the fact that it will be the first organism for which a complete genomic sequence will be available within the next 2 to 3 years makes it the organism of choice for drawing up an inventory of all nuclear genes involved in mitochondrial biogenesis and for the identification of their counterparts in other organisms.