SLS1, a new Saccharomyces cerevisiae gene involved in mitochondrial metabolism, isolated as a syntheticlethal in association with an SSM4 deletion

Translational activators and mitoribosomal isoforms cooperate to mediate mRNA-specific translation in Schizosaccharomyces pombe mitochondria

Mitochondrial mRNAs encode key subunits of the oxidative phosphorylation complexes that produce energy for the cell. In Saccharomyces cerevisiae, mitochondrial translation is under the control of translational activators, specific to each mRNA. In Schizosaccharomyces pombe, which more closely resembles the human system by its mitochondrial DNA structure and physiology, most translational activators appear to be either lacking, or recruited for post-translational functions. By combining bioinformatics, genetic and biochemical approaches we identified two interacting factors, Cbp7 and Cbp8, controlling Cytb production in S. pombe. We show that their absence affects cytb mRNA stability and impairs the detection of the Cytb protein. We further identified two classes of Cbp7/Cbp8 partners and showed that they modulated Cytb or Cox1 synthesis. First, two isoforms of bS1m, a protein of the small mitoribosomal subunit, that appear mutually exclusive and confer translational specificity. Second, a complex of four proteins dedicated to Cox1 synthesis, which includes an RNA helicase that interacts with the mitochondrial ribosome. Our results suggest that S. pombe contains, in addition to complexes of translational activators, a heterogeneous population of mitochondrial ribosomes that could specifically modulate translation depending on the mRNA translated, in order to optimally balance the production of different respiratory complex subunits.

Simultaneous Ablation of the Catalytic AMPK α-Subunit SNF1 and Mitochondrial Matrix Protease CLPP Results in Pronounced Lifespan Extension

Organismic aging is known to be controlled by genetic and environmental traits. Pathways involved in the control of cellular metabolism play a crucial role. Previously, we identified a role of PaCLPP, a mitochondrial matrix protease, in the control of the mitochondrial energy metabolism, aging, and lifespan of the fungal aging model Podospora anserina. Most surprisingly, we made the counterintuitive observation that the ablation of this component of the mitochondrial quality control network leads to lifespan extension. In the current study, we investigated the role of energy metabolism of P. anserina. An age-dependent metabolome analysis of the wild type and a PaClpP deletion strain verified differences and changes of various metabolites in cultures of the PaClpP mutant and the wild type. Based on these data, we generated and analyzed a PaSnf1 deletion mutant and a ΔPaSnf1/ΔPaClpP double mutant. In both mutants PaSNF1, the catalytic α-subunit of AMP-activated protein kinase (AMPK) is ablated. PaSNF1 was found to be required for the development of fruiting bodies and ascospores and the progeny of sexual reproduction of this ascomycete and impact mitochondrial dynamics and autophagy. Most interestingly, while the single PaSnf1 deletion mutant is characterized by a slight lifespan increase, simultaneous deletion of PaSnf1 and PaClpP leads to a pronounced lifespan extension. This synergistic effect is strongly reinforced in the presence of the mating-type "minus"-linked allele of the rmp1 gene. Compared to the wild type, culture temperature of 35°C instead of the standard laboratory temperature of 27°C leads to a short-lived phenotype of the ΔPaSnf1/ΔPaClpP double mutant. Overall, our study provides novel evidence for complex interactions of different molecular pathways involved in mitochondrial quality control, gene expression, and energy metabolism in the control of organismic aging.

Mam33 promotes cytochrome c oxidase subunit I translation in Saccharomyces cerevisiae mitochondria

Expression of genes encoded by the mitochondrial genome is dependent on gene-specific translational activators. Mam33, the yeast homologue of p32/gC1qR/C1QBP/HABP1, promotes the translation of Cox1, a core catalytic subunit of respiratory chain complex IV.

Three mitochondrial DNA–encoded proteins, Cox1, Cox2, and Cox3, comprise the core of the cytochrome c oxidase complex. Gene-specific translational activators ensure that these respiratory chain subunits are synthesized at the correct location and in stoichiometric ratios to prevent unassembled protein products from generating free oxygen radicals. In the yeast Saccharomyces cerevisiae, the nuclear-encoded proteins Mss51 and Pet309 specifically activate mitochondrial translation of the largest subunit, Cox1. Here we report that Mam33 is a third COX1 translational activator in yeast mitochondria. Mam33 is required for cells to adapt efficiently from fermentation to respiration. In the absence of Mam33, Cox1 translation is impaired, and cells poorly adapt to respiratory conditions because they lack basal fermentative levels of Cox1.

Mitochondrial protein synthesis, import, and assembly

The mitochondrion is arguably the most complex organelle in the budding yeast cell cytoplasm. It is essential for viability as well as respiratory growth. Its innermost aqueous compartment, the matrix, is bounded by the highly structured inner membrane, which in turn is bounded by the intermembrane space and the outer membrane. Approximately 1000 proteins are present in these organelles, of which eight major constituents are coded and synthesized in the matrix. The import of mitochondrial proteins synthesized in the cytoplasm, and their direction to the correct soluble compartments, correct membranes, and correct membrane surfaces/topologies, involves multiple pathways and macromolecular machines. The targeting of some, but not all, cytoplasmically synthesized mitochondrial proteins begins with translation of messenger RNAs localized to the organelle. Most proteins then pass through the translocase of the outer membrane to the intermembrane space, where divergent pathways sort them to the outer membrane, inner membrane, and matrix or trap them in the intermembrane space. Roughly 25% of mitochondrial proteins participate in maintenance or expression of the organellar genome at the inner surface of the inner membrane, providing 7 membrane proteins whose synthesis nucleates the assembly of three respiratory complexes.

Roles of Oxa1-related inner-membrane translocases in assembly of respiratory chain complexes

Members of the family of the polytopic inner membrane proteins are related to Saccharomyces cerevisiae Oxa1 function in the assembly of energy transducing complexes of mitochondria and chloroplasts. Here we focus on the two mitochondrial members of this family, Oxa1 and Cox18, reviewing studies on their biogenesis as well as their functions, reflected in the phenotypic consequences of their absence in various organisms. In yeast, cytochrome c oxidase subunit II (Cox2) is a key substrate of these proteins. Oxa1 is required for co-translational translocation and insertion of Cox2, while Cox18 is necessary for the export of its C-terminal domain. Genetic and biochemical strategies have been used to investigate the functions of distinct domains of Oxa1 and to identify its partners in protein insertion/translocation. Recent work on the related bacterial protein YidC strongly indicates that it is capable of functioning alone as a translocase for hydrophilic domains and an insertase for TM domains. Thus, the Oxa1 and Cox18 probably catalyze these reactions directly in a co- and/or posttranslational way. In various species, Oxa1 appears to assist in the assembly of different substrate proteins, although it is still unclear how Oxa1 recognizes its substrates, and whether additional factors participate in this beyond its direct interaction with mitochondrial ribosomes, demonstrated in S. cerevisiae. Oxa1 is capable of assisting posttranslational insertion and translocation in isolated mitochondria, and Cox18 may posttranslationally translocate its only known substrate, the Cox2 C-terminal domain, in vivo. Detailed understanding of the mechanisms of action of these two proteins must await the resolution of their structure in the membrane and the development of a true in vitro mitochondrial translation system.

Rmd9p controls the processing/stability of mitochondrial mRNAs and its overexpression compensates for a partial deficiency of oxa1p in Saccharomyces cerevisiae

Oxa1p is a key component of the general membrane insertion machinery of eukaryotic respiratory complex subunits encoded by the mitochondrial genome. In this study, we have generated a respiratory-deficient mutant, oxa1-E65G-F229S, that contains two substitutions in the predicted intermembrane space domain of Oxa1p. The respiratory deficiency due to this mutation is compensated for by overexpressing RMD9. We show that Rmd9p is an extrinsic membrane protein facing the matrix side of the mitochondrial inner membrane. Its deletion leads to a pleiotropic effect on respiratory complex biogenesis. The steady-state level of all the mitochondrial mRNAs encoding respiratory complex subunits is strongly reduced in the Deltarmd9 mutant, and there is a slight decrease in the accumulation of two RNAs encoding components of the small subunit of the mitochondrial ribosome. Overexpressing RMD9 leads to an increase in the steady-state level of mitochondrial RNAs, and we discuss how this increase could suppress the oxa1 mutations and compensate for the membrane insertion defect of the subunits encoded by these mRNAs.

Interaction between the oxa1 and rmp1 genes modulates respiratory complex assembly and life span in Podospora anserina

A causal link between deficiency of the cytochrome respiratory pathway and life span was previously shown in the filamentous fungus Podospora anserina. To gain more insight into the relationship between mitochondrial function and life span, we have constructed a strain carrying a thermosensitive mutation of the gene oxa1. OXA1 is a membrane protein conserved from bacteria to human. The mitochondrial OXA1 protein is involved in the assembly/insertion of several respiratory complexes. We show here that oxa1 is an essential gene in P. anserina. The oxa1(ts) mutant exhibits severe defects in the respiratory complexes I and IV, which are correlated with an increased life span, a strong induction of the alternative oxidase, and a reduction in ROS production. However, there is no causal link between alternative oxidase level and life span. We also show that in the oxa1(ts) mutant, the extent of the defects in complexes I and IV and the life-span increase depends on the essential gene rmp1. The RMP1 protein, whose function is still unknown, can be localized in the mitochondria and/or the cytosolic compartment, depending on the developmental stage. We propose that the RMP1 protein could be involved in the process of OXA1-dependent protein insertion.

The mitochondrial message-specific mRNA protectors Cbp1 and Pet309 are associated in a high-molecular weight complex

In Saccharomyces cerevisiae, the nuclear-encoded protein Cbp1 promotes stability and translation of mitochondrial cytochrome b transcripts through interaction with the 5' untranslated region. Fusion of a biotin binding peptide tag to the C terminus of Cbp1 has now allowed detection in mitochondrial extracts by using peroxidase-coupled avidin. Cbp1 is associated with the mitochondrial membranes when high ionic strength extraction conditions are used. However, the protein is easily solubilized by omitting salt from the extraction buffer, which suggests Cbp1 is loosely associated with the membrane through weak hydrophobic interactions. Gel filtration analysis and blue native PAGE showed that Cbp1 is part of a single 900,000-Da complex. The complex was purified using the biotin tag and a sequence-specific protease cleavage site. In addition to Cbp1, the complex contains several polypeptides of molecular weights between 113 and 40 kDa. Among these, we identified another message-specific factor, Pet309, which promotes the stability and translation of mitochondrial cytochrome oxidase subunit I mRNA. A hypothesis is presented in which the Cbp1-Pet309 complex contains several message-specific RNA binding proteins and links transcription to translation of the mRNAs at the membrane.

Multiple interactions involving the amino-terminal domain of yeast mtRNA polymerase determine the efficiency of mitochondrial protein synthesis

The amino-terminal domain (ATD) of Saccharomyces cerevisiae mitochondrial RNA polymerase has been shown to provide a functional link between transcription and post-transcriptional events during mitochondrial gene expression. This connection is mediated in large part by its interactions with the matrix protein Nam1p and, based on genetic phenotypes, the mitochondrial membrane protein Sls1p. These observations led us to propose previously that mtRNA polymerase, Nam1p, and Sls1p work together to coordinate transcription and translation of mtDNA-encoded gene products. Here we demonstrate by specific labeling of mitochondrial gene products in vivo that Nam1p and Sls1p indeed work together in a pathway that is required globally for efficient mitochondrial translation. Likewise, mutations in the ATD result in similar global reductions in mitochondrial translation efficiency and sensitivity to the mitochondrial translation inhibitor erythromycin. These data, coupled with the observation that the ATD is required to co-purify Sls1p in association with mtDNA nucleoids, suggest that efficient expression of mtDNA-encoded genes in yeast involves a complex series of interactions that localize active transcription complexes to the inner membrane in order to coordinate translation with transcription.

Maintenance and integrity of the mitochondrial genome: a plethora of nuclear genes in the budding yeast

Instability of the mitochondrial genome (mtDNA) is a general problem from yeasts to humans. However, its genetic control is not well documented except in the yeast Saccharomyces cerevisiae. From the discovery, 50 years ago, of the petite mutants by Ephrussi and his coworkers, it has been shown that more than 100 nuclear genes directly or indirectly influence the fate of the rho(+) mtDNA. It is not surprising that mutations in genes involved in mtDNA metabolism (replication, repair, and recombination) can cause a complete loss of mtDNA (rho(0) petites) and/or lead to truncated forms (rho(-)) of this genome. However, most loss-of-function mutations which increase yeast mtDNA instability act indirectly: they lie in genes controlling functions as diverse as mitochondrial translation, ATP synthase, iron homeostasis, fatty acid metabolism, mitochondrial morphology, and so on. In a few cases it has been shown that gene overexpression increases the levels of petite mutants. Mutations in other genes are lethal in the absence of a functional mtDNA and thus convert this petite-positive yeast into a petite-negative form: petite cells cannot be recovered in these genetic contexts. Most of the data are explained if one assumes that the maintenance of the rho(+) genome depends on a centromere-like structure dispensable for the maintenance of rho(-) mtDNA and/or the function of mitochondrially encoded ATP synthase subunits, especially ATP6. In fact, the real challenge for the next 50 years will be to assemble the pieces of this puzzle by using yeast and to use complementary models, especially in strict aerobes.