Protein AMPylation by an Evolutionarily Conserved Pseudokinase.

Approximately 10% of human protein kinases are believed to be inactive and named pseudokinases because they lack residues required for catalysis. Here, we show that the highly conserved pseudokinase selenoprotein-O (SelO) transfers AMP from ATP to Ser, Thr, and Tyr residues on protein substrates (AMPylation), uncovering a previously unrecognized activity for a member of the protein kinase superfamily. The crystal structure of a SelO homolog reveals a protein kinase-like fold with ATP flipped in the active site, thus providing a structural basis for catalysis. SelO pseudokinases localize to the mitochondria and AMPylate proteins involved in redox homeostasis. Consequently, SelO activity is necessary for the proper cellular response to oxidative stress. Our results suggest that AMPylation may be a more widespread post-translational modification than previously appreciated and that pseudokinases should be analyzed for alternative transferase activities.

Molecular basis of diseases induced by the mitochondrial DNA mutation m.9032T>C.

The mitochondrial DNA mutation m.9032T>C was previously identified in patients presenting with NARP (Neuropathy Ataxia Retinitis Pigmentosa). Their clinical features had a maternal transmission and patient's cells showed a reduced oxidative phosphorylation capacity, elevated reactive oxygen species (ROS) production and hyperpolarization of the mitochondrial inner membrane, providing evidence that m.9032T>C is truly pathogenic. This mutation leads to replacement of a highly conserved leucine residue with proline at position 169 of ATP synthase subunit a (L169P). This protein and a ring of identical c-subunits (c-ring) move protons through the mitochondrial inner membrane coupled to ATP synthesis. We herein investigated the consequences of m.9032T>C on ATP synthase in a strain of Saccharomyces cerevisiae with an equivalent mutation (L186P). The mutant enzyme assembled correctly but was mostly inactive as evidenced by a > 95% drop in the rate of mitochondrial ATP synthesis and absence of significant ATP-driven proton pumping across the mitochondrial membrane. Intragenic suppressors selected from L186P yeast restoring ATP synthase function to varying degrees (30-70%) were identified at the original mutation site (L186S) or in another position of the subunit a (H114Q, I118T). In light of atomic structures of yeast ATP synthase recently described, we conclude from these results that m.9032T>C disrupts proton conduction between the external side of the membrane and the c-ring, and that H114Q and I118T enable protons to access the c-ring through a modified pathway.

The pathogenic m.8993 T > G mutation in mitochondrial ATP6 gene prevents proton release from the subunit c-ring rotor of ATP synthase.

The human ATP synthase is an assembly of 29 subunits of 18 different types, of which only two (a and 8) are encoded in the mitochondrial genome. Subunit a, together with an oligomeric ring of c-subunit (c-ring), forms the proton pathway responsible for the transport of protons through the mitochondrial inner membrane, coupled to rotation of the c-ring and ATP synthesis. Neuromuscular diseases have been associated to a number of mutations in the gene encoding subunit a, ATP6. The most common, m.8993 T > G, leads to replacement of a strictly conserved leucine residue with arginine (aL156R). We previously showed that the equivalent mutation (aL173R) dramatically compromises respiratory growth of Saccharomyces cerevisiae and causes a 90% drop in the rate of mitochondrial ATP synthesis. Here, we isolated revertants from the aL173R strain that show improved respiratory growth. Four first-site reversions at codon 173 (aL173M, aL173S, aL173K and aL173W) and five second-site reversions at another codon (aR169M, aR169S, aA170P, aA170G and aI216S) were identified. Based on the atomic structures of yeast ATP synthase and the biochemical properties of the revertant strains, we propose that the aL173R mutation is responsible for unfavorable electrostatic interactions that prevent the release of protons from the c-ring into a channel from which protons move from the c-ring to the mitochondrial matrix. The results provide further evidence that yeast aL173 (and thus human aL156) optimizes the exit of protons from ATP synthase, but is not essential despite its strict evolutionary conservation.

ATP Synthase Subunit a Supports Permeability Transition in Yeast Lacking Dimerization Subunits and Modulates yPTP Conductance.

BACKGROUND/AIMS: Mitochondrial ATP synthase, in addition to being involved in ATP synthesis, is involved in permeability transition pore (PTP) formation, which precedes apoptosis in mammalian cells and programmed cell death in yeast. Mutations in genes encoding ATP synthase subunits cause neuromuscular disorders and have been identified in cancer samples. PTP is also involved in pathology. We previously found that in Saccharomyces cerevisiae, two mutations in ATP synthase subunit a (atp6-P163S and atp6-K90E, equivalent to those detected in prostate and thyroid cancer samples, respectively) in the OM45-GFP background affected ROS and calcium homeostasis and delayed yeast PTP (yPTP) induction upon calcium treatment by modulating the dynamics of ATP synthase dimer/oligomer formation. The Om45 protein is a component of the porin complex, which is equivalent to mammalian VDAC. We aimed to investigate yPTP function in atp6-P163S and atp6-K90E mutants lacking the e and g dimerization subunits of ATP synthase. METHODS: Triple mutants with the atp6-P163S or atp6-K90E mutation, the OM45-GFP gene and deletion of the TIM11 gene encoding subunit e were constructed by crossing and tetrad dissection. In spores capable of growing, the original atp6 mutations reverted to wild type, and two compensatory mutations, namely, atp6-C33S-T215C, were selected. The effects of these mutations on cellular physiology, mitochondrial morphology, bioenergetics and permeability transition (PT) were analyzed by fluorescence and electron microscopy, mitochondrial respiration, ATP synthase activity, calcium retention capacity and swelling assays. RESULTS: The atp6-C33S-T215C mutations in the OM45-GFP background led to delayed growth at elevated temperature on both fermentative and respiratory media and increased sensitivity to high calcium ions concentration or hydrogen peroxide in the medium. The ATP synthase activity was reduced by approximately 50% and mitochondrial network was hyperfused in these cells grown at elevated temperature. The atp6-C33S-T215C stabilized ATP synthase dimers and restored the yPTP properties in Tim11∆ cells. In OM45-GFP cells, in which Tim11 is present, these mutations increased the fraction of swollen mitochondria by up to 85% vs 60% in the wild type, although the time required for calcium release doubled. CONCLUSION: ATP synthase subunit e is essential in the S. cerevisiae atp6-P163S and atp6-K90E mutants. In addition to subunits e and g, subunit a is critical for yPTP induction and conduction. The increased yPTP conduction decrease the S. cerevisiae cell fitness.

Molecular Basis of the Pathogenic Mechanism Induced by the m.9191T>C Mutation in Mitochondrial ATP6 Gene.

Probing the pathogenicity and functional consequences of mitochondrial DNA (mtDNA) mutations from patient's cells and tissues is difficult due to genetic heteroplasmy (co-existence of wild type and mutated mtDNA in cells), occurrence of numerous mtDNA polymorphisms, and absence of methods for genetically transforming human mitochondria. Owing to its good fermenting capacity that enables survival to loss-of-function mtDNA mutations, its amenability to mitochondrial genome manipulation, and lack of heteroplasmy, Saccharomyces cerevisiae is an excellent model for studying and resolving the molecular bases of human diseases linked to mtDNA in a controlled genetic background. Using this model, we previously showed that a pathogenic mutation in mitochondrial ATP6 gene (m.9191T>C), that converts a highly conserved leucine residue into proline in human ATP synthase subunit a (aL222P), severely compromises the assembly of yeast ATP synthase and reduces by 90% the rate of mitochondrial ATP synthesis. Herein, we report the isolation of intragenic suppressors of this mutation. In light of recently described high resolution structures of ATP synthase, the results indicate that the m.9191T>C mutation disrupts a four α-helix bundle in subunit a and that the leucine residue it targets indirectly optimizes proton conduction through the membrane domain of ATP synthase.

Two mutations in mitochondrial ATP6 gene of ATP synthase, related to human cancer, affect ROS, calcium homeostasis and mitochondrial permeability transition in yeast.

The relevance of mitochondrial DNA (mtDNA) mutations in cancer process is still unknown. Since the mutagenesis of mitochondrial genome in mammals is not possible yet, we have exploited budding yeast S. cerevisiae as a model to study the effects of tumor-associated mutations in the mitochondrial MTATP6 gene, encoding subunit 6 of ATP synthase, on the energy metabolism. We previously reported that four mutations in this gene have a limited impact on the production of cellular energy. Here we show that two mutations, Atp6-P163S and Atp6-K90E (human MTATP6-P136S and MTATP6-K64E, found in prostate and thyroid cancer samples, respectively), increase sensitivity of yeast cells both to compounds inducing oxidative stress and to high concentrations of calcium ions in the medium, when Om45p, the component of porin complex in outer mitochondrial membrane (OM), was fused to GFP. In OM45-GFP background, these mutations affect the activation of yeast permeability transition pore (yPTP, also called YMUC, yeast mitochondrial unspecific channel) upon calcium induction. Moreover, we show that calcium addition to isolated mitochondria heavily induced the formation of ATP synthase dimers and oligomers, recently proposed to form the core of PTP, which was slower in the mutants. We show the genetic evidence for involvement of mitochondrial ATP synthase in calcium homeostasis and permeability transition in yeast. This paper is a first to show, although in yeast model organism, that mitochondrial ATP synthase mutations, which accumulate during carcinogenesis process, may be significant for cancer cell escape from apoptosis.

Molecular basis of diseases caused by the mtDNA mutation m.8969G>A in the subunit a of ATP synthase.

The ATP synthase which provides aerobic eukaryotes with ATP, organizes into a membrane-extrinsic catalytic domain, where ATP is generated, and a membrane-embedded FO domain that shuttles protons across the membrane. We previously identified a mutation in the mitochondrial MT-ATP6 gene (m.8969G>A) in a 14-year-old Chinese female who developed an isolated nephropathy followed by brain and muscle problems. This mutation replaces a highly conserved serine residue into asparagine at amino acid position 148 of the membrane-embedded subunit a of ATP synthase. We showed that an equivalent of this mutation in yeast (aS175N) prevents FO-mediated proton translocation. Herein we identified four first-site intragenic suppressors (aN175D, aN175K, aN175I, and aN175T), which, in light of a recently published atomic structure of yeast FO indicates that the detrimental consequences of the original mutation result from the establishment of hydrogen bonds between aN175 and a nearby glutamate residue (aE172) that was proposed to be critical for the exit of protons from the ATP synthase towards the mitochondrial matrix. Interestingly also, we found that the aS175N mutation can be suppressed by second-site suppressors (aP12S, aI171F, aI171N, aI239F, and aI200M), of which some are very distantly located (by 20-30 Å) from the original mutation. The possibility to compensate through long-range effects the aS175N mutation is an interesting observation that holds promise for the development of therapeutic molecules.

High-Conductance Channel Formation in Yeast Mitochondria is Mediated by F-ATP Synthase e and g Subunits.

BACKGROUND/AIMS: The permeability transition pore (PTP) is an unselective, Ca2+-dependent high conductance channel of the inner mitochondrial membrane whose molecular identity has long remained a mystery. The most recent hypothesis is that pore formation involves the F-ATP synthase, which consistently generates Ca2+-activated channels. Available structures do not display obvious features that can accommodate a channel; thus, how the pore can form and whether its activity can be entirely assigned to F-ATP synthase is the matter of debate. In this study, we investigated the role of F-ATP synthase subunits e, g and b in PTP formation. METHODS: Yeast null mutants for e, g and the first transmembrane (TM) α-helix of subunit b were generated and evaluated for mitochondrial morphology (electron microscopy), membrane potential (Rhodamine123 fluorescence) and respiration (Clark electrode). Homoplasmic C23S mutant of subunit a was generated by in vitro mutagenesis followed by biolistic transformation. F-ATP synthase assembly was evaluated by BN-PAGE analysis. Cu2+ treatment was used to induce the formation of F-ATP synthase dimers in the absence of e and g subunits. The electrophysiological properties of F-ATP synthase were assessed in planar lipid bilayers. RESULTS: Null mutants for the subunits e and g display dimer formation upon Cu2+ treatment and show PTP-dependent mitochondrial Ca2+ release but not swelling. Cu2+ treatment causes formation of disulfide bridges between Cys23 of subunits a that stabilize dimers in absence of e and g subunits and favors the open state of wild-type F-ATP synthase channels. Absence of e and g subunits decreases conductance of the F-ATP synthase channel about tenfold. Ablation of the first TM of subunit b, which creates a distinct lateral domain with e and g, further affected channel activity. CONCLUSION: F-ATP synthase e, g and b subunits create a domain within the membrane that is critical for the generation of the high-conductance channel, thus is a prime candidate for PTP formation. Subunits e and g are only present in eukaryotes and may have evolved to confer this novel function to F-ATP synthase.

[Molecular bases of diseases caused by mutations in genes encoding subunits of ATP synthase]. / Molekularne podloze chorób spowodowanych mutacjami w genach kodujacych podjednostki syntazy ATP.

ATP synthase is the last enzyme of the OXPHOS system synthesizing ATP. Mutations in either mitochondrial or nuclear genes encoding subunits of this enzyme (17 polypeptides) cause neurodegenerative diseases. The ATP synthase subunits 8 (ATP8, alias A6L) and a (ATP6) are encoded by the MT-ATP8 and MT-ATP6 mitochondrial genes, respectively. 17 diseases associated mutations were identified in five nuclear genes coding for subunits of this enzyme. 58 mutations were described in the MT-ATP6 and MT-ATP8 genes, among them 36 were deposited in MITOMAP database. For most of them neither their pathogenic character nor the mechanisms are known. This review summarizes what is known about the molecular basis of the ATP synthase deficiencies. We review the mutations in the ATP synthase genes as well as biochemical data obtained from studies of patient's cells and cybrid or yeast models. We include yeast research about drugs selection and their mechanism of action. Moreover we position the mutations into a recently published structural model of the Fo complex and discuss their structural/functional consequences.

Experimental relocation of the mitochondrial ATP9 gene to the nucleus reveals forces underlying mitochondrial genome evolution.

Only a few genes remain in the mitochondrial genome retained by every eukaryotic organism that carry out essential functions and are implicated in severe diseases. Experimentally relocating these few genes to the nucleus therefore has both therapeutic and evolutionary implications. Numerous unproductive attempts have been made to do so, with a total of only 5 successes across all organisms. We have taken a novel approach to relocating mitochondrial genes that utilizes naturally nuclear versions from other organisms. We demonstrate this approach on subunit 9/c of ATP synthase, successfully relocating this gene for the first time in any organism by expressing the ATP9 genes from Podospora anserina in Saccharomyces cerevisiae. This study substantiates the role of protein structure in mitochondrial gene transfer: expression of chimeric constructs reveals that the P. anserina proteins can be correctly imported into mitochondria due to reduced hydrophobicity of the first transmembrane segment. Nuclear expression of ATP9, while permitting almost fully functional oxidative phosphorylation, perturbs many cellular properties, including cellular morphology, and activates the heat shock response. Altogether, our study establishes a novel strategy for allotopic expression of mitochondrial genes, demonstrates the complex adaptations required to relocate ATP9, and indicates a reason that this gene was only transferred to the nucleus during the evolution of multicellular organisms.

A yeast-based assay identifies drugs active against human mitochondrial disorders.

Due to the lack of relevant animal models, development of effective treatments for human mitochondrial diseases has been limited. Here we establish a rapid, yeast-based assay to screen for drugs active against human inherited mitochondrial diseases affecting ATP synthase, in particular NARP (neuropathy, ataxia, and retinitis pigmentosa) syndrome. This method is based on the conservation of mitochondrial function from yeast to human, on the unique ability of yeast to survive without production of ATP by oxidative phosphorylation, and on the amenability of the yeast mitochondrial genome to site-directed mutagenesis. Our method identifies chlorhexidine by screening a chemical library and oleate through a candidate approach. We show that these molecules rescue a number of phenotypes resulting from mutations affecting ATP synthase in yeast. These compounds are also active on human cybrid cells derived from NARP patients. These results validate our method as an effective high-throughput screening approach to identify drugs active in the treatment of human ATP synthase disorders and suggest that this type of method could be applied to other mitochondrial diseases.

Fmp40 ampylase regulates cell survival upon oxidative stress by controlling Prx1 and Trx3 oxidation.

Reactive oxygen species (ROS), play important roles in cellular signaling, nonetheless are toxic at higher concentrations. Cells have many interconnected, overlapped or backup systems to neutralize ROS, but their regulatory mechanisms remain poorly understood. Here, we reveal an essential role for mitochondrial AMPylase Fmp40 from budding yeast in regulating the redox states of the mitochondrial 1-Cys peroxiredoxin Prx1, which is the only protein shown to neutralize H2O2 with the oxidation of the mitochondrial glutathione and the thioredoxin Trx3, directly involved in the reduction of Prx1. Deletion of FMP40 impacts a cellular response to H2O2 treatment that leads to programmed cell death (PCD) induction and an adaptive response involving up or down regulation of genes encoding, among others the catalase Cta1, PCD inducing factor Aif1, and mitochondrial redoxins Trx3 and Grx2. This ultimately perturbs the reduced glutathione and NADPH cellular pools. We further demonstrated that Fmp40 AMPylates Prx1, Trx3, and Grx2 in vitro and interacts with Trx3 in vivo. AMPylation of the threonine residue 66 in Trx3 is essential for this protein's proper endogenous level and its precursor forms' maturation under oxidative stress conditions. Additionally, we showed the Grx2 involvement in the reduction of Trx3 in vivo. Taken together, Fmp40, through control of the reduction of mitochondrial redoxins, regulates the hydrogen peroxide, GSH and NADPH signaling influencing the yeast cell survival.

ATP synthase interactome analysis identifies a new subunit l as a modulator of permeability transition pore in yeast.

The mitochondrial ATP synthase, an enzyme that synthesizes ATP and is involved in the formation of the mitochondrial mega-channel and permeability transition, is a multi-subunit complex. In S. cerevisiae, the uncharacterized protein Mco10 was previously found to be associated with ATP synthase and referred as a new 'subunit l'. However, recent cryo-EM structures could not ascertain Mco10 with the enzyme making questionable its role as a structural subunit. The N-terminal part of Mco10 is very similar to k/Atp19 subunit, which along with subunits g/Atp20 and e/Atp21 plays a major role in stabilization of the ATP synthase dimers. In our effort to confidently define the small protein interactome of ATP synthase we found Mco10. We herein investigate the impact of Mco10 on ATP synthase functioning. Biochemical analysis reveal in spite of similarity in sequence and evolutionary lineage, that Mco10 and Atp19 differ significantly in function. The Mco10 is an auxiliary ATP synthase subunit that only functions in permeability transition.

Analysis of MT-ATP8 gene variants reported in patients by modeling in silico and in yeast model organism.

Defects in ATP synthase functioning due to the substitutions in its two mitochondrially encoded subunits a and 8 lead to untreatable mitochondrial diseases. Defining the character of variants in genes encoding these subunits is challenging due to their low frequency, heteroplasmy of mitochondrial DNA in patients' cells and polymorphisms of mitochondrial genome. We successfully used yeast S. cerevisiae as a model to study the effects of variants in MT-ATP6 gene and our research led to understand how eight amino acid residues substitutions impact the proton translocation through the channel formed by subunit a and c-ring of ATP synthase at the molecular level. Here we applied this approach to study the effects of the m.8403T>C variant in MT-ATP8 gene. The biochemical data from yeast mitochondria indicate that equivalent mutation is not detrimental for the yeast enzyme functioning. The structural analysis of substitutions in subunit 8 introduced by m.8403T>C and five other variants in MT-ATP8 provides indications about the role of subunit 8 in the membrane domain of ATP synthase and potential structural consequences of substitutions in this subunit.

Probing the pathogenicity of patient-derived variants of MT-ATP6 in yeast.

The list of mitochondrial DNA (mtDNA) variants detected in individuals with neurodegenerative diseases is constantly growing. Evaluating their functional consequences and pathogenicity is not easy, especially when they are found in only a limited number of patients together with wild-type mtDNA (heteroplasmy). Owing to its amenability to mitochondrial genetic transformation and incapacity to stably maintain heteroplasmy, and the strong evolutionary conservation of the proteins encoded in mitochondria, Saccharomyces cerevisiae provides a convenient model to investigate the functional consequences of human mtDNA variants. We herein report the construction and energy-transducing properties of yeast models of eight MT-ATP6 gene variants identified in patients with various disorders: m.8843T>C, m.8950G>A, m.9016A>G, m.9025G>A, m.9029A>G, m.9058A>G, m.9139G>A and m.9160T>C. Significant defect in growth dependent on respiration and deficits in ATP production were observed in yeast models of m.8950G>A, m.9025G>A and m.9029A>G, providing evidence of pathogenicity for these variants. Yeast models of the five other variants showed very mild, if any, effect on mitochondrial function, suggesting that the variants do not have, at least alone, the potential to compromise human health.

Arf1 coordinates fatty acid metabolism and mitochondrial homeostasis.

Lipid mobilization through fatty acid ß-oxidation is a central process essential for energy production during nutrient shortage. In yeast, this catabolic process starts in the peroxisome from where ß-oxidation products enter mitochondria and fuel the tricarboxylic acid cycle. Little is known about the physical and metabolic cooperation between these organelles. Here we found that expression of fatty acid transporters and of the rate-limiting enzyme involved in ß-oxidation is decreased in cells expressing a hyperactive mutant of the small GTPase Arf1, leading to an accumulation of fatty acids in lipid droplets. Consequently, mitochondria became fragmented and ATP synthesis decreased. Genetic and pharmacological depletion of fatty acids phenocopied the arf1 mutant mitochondrial phenotype. Although ß-oxidation occurs in both mitochondria and peroxisomes in mammals, Arf1's role in fatty acid metabolism is conserved. Together, our results indicate that Arf1 integrates metabolism into energy production by regulating fatty acid storage and utilization, and presumably organelle contact sites.

[Structure, biogenesis and mechanism of function of the mitochondrial ATP synthase complex]. / Budowa, biogeneza i mechanizm dzialania kompleksu mitochondrialnej syntazy ATP.

Mitochondria are organelles present in all eukaryotic organisms. Their primary function is production of energy in the form of ATP by oxidative phosphorylation. The final step of this process is catalyzed by an enzyme of internal mitochondrial membrane - ATP synthase. The ATP synthase consists of the seventeen subunits in yeast (in vertebrate sixteen is identified to date) organized in hydrophobic, membrane localized unit, referred to as F0 and hydrophilic domain F1 directed into mitochondria matrix. Genes encoding the ATP synthase subunits are mainly nuclear, but few of them, encoding hydrophobic subunits, are retained in mitochondrial genome in most Eukaryotes. Biogenesis of the ATP synthase is a sophisticated process, depending on the activity of proteins, which are not ATP synthase subunits, coordinating expression of the nuclear and mitochondrial genes and their assembly in active complex. This review summarizes the present knowledge about structure, biogenesis and mechanism of ATP synthase complex function.

Visualizing Mitochondrial Importability of a Protein Using the Yeast Bi-Genomic Mitochondrial-Split-GFP Strain and an Ordinary Fluorescence Microscope.

Proving with certainty that a GFP-tagged protein is imported inside mitochondria by visualizing its fluorescence emission with an epifluorescence microscope is currently impossible using regular GFP-tagging. This is particularly true for proteins dual localized in the cytosol and mitochondria, which have been estimated to represent up to one third of the established mitoproteomes. These proteins are usually composed of a surpassingly abundant pool of the cytosolic isoform compared to the mitochondrial isoform. As a consequence, when tagged with a regular GFP, the fluorescence emission of the cytosolic isoform will inevitably eclipse that of the mitochondrial one and prevent the detection of the mitochondrial echoform. To overcome this technical limit, we engineered a yeast strain expressing a new type of GFP called Bi-Genomic Mitochondrial-Split-GFP (BiG Mito-Split-GFP). In this strain, one moiety of the GFP is encoded by the mitochondrial DNA while the second moiety of the GFP can be tagged to any nuclear-encoded protein (suspected to be dual localized or bona fide mitochondrial). By doing so, only mitochondrial proteins or echoforms of dual localized proteins, regardless of their organismal origin, trigger GFP reconstitution that can be visualized by regular fluorescence microscopy. The strength of the BiG Mito-Split-GFP system is that proof of the mitochondrial localization of a given protein rests on a simple and effortless microscopy observation.

Creation of Yeast Models for Evaluating the Pathogenicity of Mutations in the Human Mitochondrial Gene MT-ATP6 and Discovering Therapeutic Molecules.

Numerous diseases in humans have been associated with mutations of the mitochondrial genome (mtDNA). This genome encodes 13 protein subunits of complexes involved in oxidative phosphorylation (OXPHOS), a process that provides aerobic eukaryotes with the energy-rich adenosine triphosphate molecule (ATP). Mutations of the mtDNA may therefore have dramatic consequences especially in tissues and organs with high energy demand. Evaluating the pathogenicity of these mutations may be difficult because they often affect only a fraction of the numerous copies of the mitochondrial genome (up to several thousands in a single cell), which is referred to as heteroplasmy. Furthermore, due to its exposure to reactive oxygen species (ROS) produced in mitochondria, the mtDNA is prone to mutations, and some may be simply neutral polymorphisms with no detrimental consequences on human health. Another difficulty is the absence of methods for genetically transforming human mitochondria. Face to these complexities, the yeast Saccharomyces cerevisiae provides a convenient model for investigating the consequences of human mtDNA mutations in a defined genetic background. Owing to its good fermentation capacity, it can survive the loss of OXPHOS, its mitochondrial genome can be manipulated, and genetic heterogeneity in its mitochondria is unstable. Taking advantage of these unique attributes, we herein describe a method we have developed for creating yeast models of mitochondrial ATP6 gene mutations detected in patients, to determine how they impact OXPHOS. Additionally, we describe how these models can be used to discover molecules with therapeutic potential.

Assembly-dependent translation of subunits 6 (Atp6) and 9 (Atp9) of ATP synthase in yeast mitochondria.

The yeast mitochondrial ATP synthase is an assembly of 28 subunits of 17 types of which 3 (subunits 6, 8, and 9) are encoded by mitochondrial genes, while the 14 others have a nuclear genetic origin. Within the membrane domain (FO) of this enzyme, the subunit 6 and a ring of 10 identical subunits 9 transport protons across the mitochondrial inner membrane coupled to ATP synthesis in the extra-membrane structure (F1) of ATP synthase. As a result of their dual genetic origin, the ATP synthase subunits are synthesized in the cytosol and inside the mitochondrion. How they are produced in the proper stoichiometry from two different cellular compartments is still poorly understood. The experiments herein reported show that the rate of translation of the subunits 9 and 6 is enhanced in strains with mutations leading to specific defects in the assembly of these proteins. These translation modifications involve assembly intermediates interacting with subunits 6 and 9 within the final enzyme and cis-regulatory sequences that control gene expression in the organelle. In addition to enabling a balanced output of the ATP synthase subunits, these assembly-dependent feedback loops are presumably important to limit the accumulation of harmful assembly intermediates that have the potential to dissipate the mitochondrial membrane electrical potential and the main source of chemical energy of the cell.