Absence of cardiolipin in the crd1 null mutant results in decreased mitochondrial membrane potential and reduced mitochondrial function

Cardiolipin (CL) is a unique phospholipid which is present throughout the eukaryotic kingdom and is localized in mitochondrial membranes. Saccharomyces cerevisiae cells containing a disruption of CRD1, the structural gene encoding CL synthase, have no CL in mitochondrial membranes. To elucidate the physiological role of CL, we compared mitochondrial functions in the crd1Delta mutant and isogenic wild type. The crd1Delta mutant loses viability at elevated temperature, and prolonged culture at 37 degrees C leads to loss of the mitochondrial genome. Mutant membranes have increased phosphatidylglycerol (PG) when grown in a nonfermentable carbon source but have almost no detectable PG in medium containing glucose. In glucose-grown cells, maximum respiratory rate, ATPase and cytochrome oxidase activities, and protein import are deficient in the mutant. The ADP/ATP carrier is defective even during growth in a nonfermentable carbon source. The mitochondrial membrane potential is decreased in mutant cells. The decrease is more pronounced in glucose-grown cells, which lack PG, but is also apparent in membranes containing PG (i.e. in nonfermentable carbon sources). We propose that CL is required for maintaining the mitochondrial membrane potential and that reduced membrane potential in the absence of CL leads to defects in protein import and other mitochondrial functions.     

Impact of mitochondrial function on yeast susceptibility to antifungal compounds

Saccharomyces cerevisiae pell and crd1 mutants deficient in the biosynthesis of mitochondrial phosphatidylglycerol (PG) and cardiolipin (CL) as well as Kluyveromyces lactis mutants impaired in the respiratory chain function (RCF) containing dysfunctional mitochondria show altered sensitivity to metabolic inhibitors. The S. cerevisiae pell mutant displayed increased sensitivity to cycloheximide, chloramphenicol, oligomycin and the cell-wall perturbing agents caffeine, caspofungin and hygromycin. On the other hand, the pel1 mutant was less sensitive to fluconazole, similarly as the K. lactis mutants impaired in the function of mitochondrial cytochromes. Mitochondrial dysfunction resulting either from the absence of PG and CL or impairment of the RCF presumably renders the cells more resistant to fluconazole. The increased tolerance of K. lactis respiratory chain mutants to amphotericin B, caffeine and hygromycin is probably related to a modification of the cell wall.     

Growth of eukaryotic cells in relation to the structure of mitochondrial membranes and mitochondrial genome

Viability ofpetite-negative yeast, such asKluyveromyces lactis, is dependent on functional mitochondrial genome encoding essential components of both mitochondrial protein synthesizing system and oxidative phosphorylation. We have isolated several nuclear mutants impaired in mitochondrial functions that were unable to grow on non-fermentable carbon and energy sources. They were used for the isolation and molecular characterization of the three genes encoding apocytochromec, apocytochromec ₁ and the protein involved in the biogenesis of cytochrome oxidase. All cytochrome-deficient mutants were viable and did not survive the ethidium bromide mutagenesis.Petite-positiveSaccharomyces cerevisiae requires intact mitochondrial genome when its phosphatidylglycerolphosphate synthase was inactivated due to mutation in thePEL1 gene. UsingPEL-lacZ fusion genes it was demonstrated that Pel1p is a mitochondrial protein (expressed in response tomyo-inositol and choline). Thepel1 mutant was deficient in phosphatidylglycerol (PG) and cardiolipin (CL) and itsrho ⁻/rho ⁰ mutants grew extremely slowly on complex medium with glucose. Under the same conditions the growth rate of thecrd1 rho ⁻ double mutants was similar to that of its parentcrd1 mutant deficient in cardiolipin synthase and accumulating PG. The results demonstrate that thepetite negativity in yeast is not dependent on an intact respiratory chain or functional oxidative phosphorylation. The presence of the negatively charged PG or CL seems to be essential for the maintenance of specific mitochondrial functions required for the normal mitotic growth of yeast cells. Presented at theInternational Conference on Recent Problems in Microbiology and Immunology, Košice (Slovakia), 13–15 October 1999.     

Cardiolipin is not required to maintain mitochondrial DNA stability or cell viability for Saccharomyces cerevisiae grown at elevated temperatures

In eukaryotic cells, the phospholipid cardiolipin (CL) is primarily found in the inner mitochondrial membrane. Saccharomyces cerevisiae mutants, unable to synthesize CL because of a null allele of the CRD1 gene (encodes CL synthase), have been reported with different phenotypes. Some mutants, when grown on a nonfermentable carbon source at elevated temperatures, exhibit mitochondrial DNA instability, loss of viability, and significant defects in several functions that rely on the mitochondrial energy transducing system (ETS). These mutants also lack the immediate precursor to CL, phosphatidylglycerol (PG), when grown on glucose as a carbon source. Other mutants show reduced growth efficiency on a nonfermentable carbon source but much milder phenotypes associated with growth at elevated temperatures and increased levels of PG when grown on glucose. We present evidence that mitochondrial DNA instability, loss of viability, and defects in the ETS exhibited at elevated temperatures by some mutants are caused by the reduced expression of the PET56 gene in the presence of the his3 Delta 200 allele and not the lack of CL alone. We also found that PG is present and elevated in all crd1 Delta strains when grown on glucose. A supermolecular complex between complex III and complex IV of the mitochondrial ETS detected in wild type cells was missing in all of the above crd1 Delta cells. The level of components of the ETS was also reduced in crd1 Delta cells grown at elevated temperatures because of reduced gene expression and not reduced stability. These results suggest that all phenotypes reported for cells carrying the his3 Delta 200 allele and lacking CL should be re-evaluated.     

Synthetic lethal interaction of the mitochondrial phosphatidylethanolamine and cardiolipin biosynthetic pathways in Saccharomyces cerevisiae

Saccharomyces cerevisiae mitochondria contain enzymes required for synthesis of the phospholipids cardiolipin (CL) and phosphatidylethanolamine (PE), which are enriched in mitochondrial membranes. Previous studies indicated that PE may compensate for the lack of CL, and vice versa. These data suggest that PE and CL have overlapping functions and that the absence of both lipids may be lethal. To address this hypothesis, we determined whether the crd1delta mutant, which lacks CL, was viable in genetic backgrounds in which PE synthesis was genetically blocked. Deletion of the mitochondrial PE pathway gene PSD1 was synthetically lethal with the crd1delta mutant, whereas deletion of the Golgi and endoplasmic reticulum pathway genes PSD2 and DPL1 did not result in synthetic lethality. A 20-fold reduction in phosphatidylcholine did not affect the growth of crd1delta cells. Supplementation with ethanolamine, which led to increased PE synthesis, or with propanolamine, which led to synthesis of the novel phospholipid phosphatidylpropanolamine, failed to rescue the synthetic lethality of the crd1delta psd1delta cells. These results suggest that mitochondrial biosynthesis of PE is essential for the viability of yeast mutants lacking CL.     

Loss of tafazzin in yeast leads to increased oxidative stress during respiratory growth

The tafazzin (TAZ) gene is highly conserved from yeast to humans, and the yeast taz1 null mutant shows alterations in cardiolipin (CL) metabolism, mitochondrial dysfunction and stabilization of supercomplexes similar to those found in Barth syndrome, a human disorder resulting from loss of tafazzin. We have previously shown that the yeast tafazzin mutant taz1Delta, which cannot remodel CL, is ethanol-sensitive at elevated temperature. In the current report, we show that in response to ethanol, CL mutants taz1Delta as well as crd1Delta, which cannot synthesize CL, exhibited increased protein carbonylation, an indicator of reactive oxygen species (ROS). The increase in ROS is most likely not due to defective oxidant defence systems, as the CL mutants do not display sensitivity to paraquat, menadione or hydrogen peroxide (H2O2). Ethanol sensitivity and increased protein carbonylation in the taz1Delta mutant but not in crd1Delta can be rescued by supplementation with oleic acid, suggesting that oleoyl-CL and/or oleoyl-monolyso-CL enables growth of taz1Delta in ethanol by decreasing oxidative stress. Our findings of increased oxidative stress in the taz1Delta mutant during respiratory growth may have important implications for understanding the pathogenesis of Barth syndrome.     

Cardiolipin mediates cross-talk between mitochondria and the vacuole

Cardiolipin (CL) is an anionic phospholipid with a dimeric structure predominantly localized in the mitochondrial inner membrane, where it is closely associated with mitochondrial function, biogenesis, and genome stability (Daum, 1985; Janitor and Subik, 1993; Jiang et al., 2000; Schlame et al., 2000; Zhong et al., 2004). Previous studies have shown that yeast mutant cells lacking CL due to a disruption in CRD1, the structural gene encoding CL synthase, exhibit defective colony formation at elevated temperature even on glucose medium (Jiang et al., 1999; Zhong et al., 2004), suggesting a role for CL in cellular processes apart from mitochondrial bioenergetics. In the current study, we present evidence that the crd1Delta mutant exhibits severe vacuolar defects, including swollen vacuole morphology and loss of vacuolar acidification, at 37 degrees C. Moreover, vacuoles from crd1Delta show decreased vacuolar H(+)-ATPase activity and proton pumping, which may contribute to loss of vacuolar acidification. Deletion mutants in RTG2 and NHX1, which mediate vacuolar pH and ion homeostasis, rescue the defective colony formation phenotype of crd1Delta, strongly suggesting that the temperature sensitivity of crd1Delta is a consequence of the vacuolar defects. Our results demonstrate the existence of a novel mitochondria-vacuole signaling pathway mediated by CL synthesis.     

Dispensable nature of phosphatidylglycerol in Escherichia coli: dual roles of anionic phospholipids

The major anionic phospholipids of Escherichia coli, phosphatidylglycerol (PG) and cardiolipin (CL), have been considered to be indispensable for essential cellular functions, such as the initiation of DNA replication and translocation of proteins across the cytoplasmic membrane. However, we successfully constructed a null pgsA mutant of E. coli that had undetectable levels of PG and CL if the major outer membrane lipoprotein was deficient, clearly indicating that these anionic phospholipids are not indispensable. In the null mutant, we observed the accumulation of phosphatidic acid, an acidic biosynthetic precursor. This suggests a functionally substitutable nature of these anionic phospholipids and allows us to formulate a dual role model for the physiological roles of the anionic phospholipids in E. coli. The anionic phospholipids may play dual roles in E. coli as (i) substrates for head group-specific enzyme reactions, albeit the viability of null PG mutants indicates that the products of head group-specific reactions are not essential; and (ii) those that are replaceable, partly or entirely, by other phospholipids bearing net negative charges, because of their rather loose head group specificity. These two aspects of the physiological roles of anionic phospholipids are discussed with special reference to the phospholipids of other bacteria and eukaryotic organelles.     

Loss of function of KRE5 suppresses temperature sensitivity of mutants lacking mitochondrial anionic lipids

Disruption of PGS1, which encodes the enzyme that catalyzes the committed step of cardiolipin (CL) synthesis, results in loss of the mitochondrial anionic phospholipids phosphatidylglycerol (PG) and CL. The pgs1Delta mutant exhibits severe growth defects at 37 degrees C. To understand the essential functions of mitochondrial anionic lipids at elevated temperatures, we isolated suppressors of pgs1Delta that grew at 37 degrees C. One of the suppressors has a loss of function mutation in KRE5, which is involved in cell wall biogenesis. The cell wall of pgs1Delta contained markedly reduced beta-1,3-glucan, which was restored in the suppressor. Stabilization of the cell wall with osmotic support alleviated the cell wall defects of pgs1Delta and suppressed the temperature sensitivity of all CL-deficient mutants. Evidence is presented suggesting that the previously reported inability of pgs1Delta to grow in the presence of ethidium bromide was due to defective cell wall integrity, not from "petite lethality." These findings demonstrated that mitochondrial anionic lipids are required for cellular functions that are essential in cell wall biogenesis, the maintenance of cell integrity, and survival at elevated temperature.     

Cardiolipin-deficient cells depend on anaplerotic pathways to ameliorate defective TCA cycle function

Previous studies have shown that the cardiolipin (CL)-deficient yeast mutant, crd1Δ, has decreased levels of acetyl-CoA and decreased activities of the TCA cycle enzymes aconitase and succinate dehydrogenase. These biochemical phenotypes are expected to lead to defective TCA cycle function. In this study, we report that signaling and anaplerotic metabolic pathways that supplement defects in the TCA cycle are essential in crd1Δ mutant cells. The crd1Δ mutant is synthetically lethal with mutants in the TCA cycle, retrograde (RTG) pathway, glyoxylate cycle, and pyruvate carboxylase 1. Glutamate levels were decreased, and the mutant exhibited glutamate auxotrophy. Glyoxylate cycle genes were up-regulated, and the levels of glyoxylate metabolites succinate and citrate were increased in crd1Δ. Import of acetyl-CoA from the cytosol into mitochondria is essential in crd1Δ, as deletion of the carnitine-acetylcarnitine translocase led to lethality in the CL mutant. β-oxidation was functional in the mutant, and oleate supplementation rescued growth defects. These findings suggest that TCA cycle deficiency caused by the absence of CL necessitates activation of anaplerotic pathways to replenish acetyl-CoA and TCA cycle intermediates. Implications for Barth syndrome, a genetic disorder of CL metabolism, are discussed.     

Interplay of Packing and Flip-flop in Local Bilayer Deformation. How Phosphatidylglycerol Could Rescue Mitochondrial Function in a Cardiolipin-deficient Yeast Mutant

In a previous work, we have shown that a spatially localized transmembrane pH gradient, produced by acid micro-injection near the external side of cardiolipin-containing giant unilamellar vesicles, leads to the formation of tubules that retract after the dissipation of this gradient. These tubules have morphologies similar to mitochondrial cristae. The tubulation effect is attributable to direct phospholipid packing modification in the outer leaflet, that is promoted by protonation of cardiolipin headgroups. In this study, we compare the case of cardiolipin-containing giant unilamellar vesicles with that of giant unilamellar vesicles that contain phosphatidylglycerol (PG). Local acidification also promotes formation of tubules in the latter. However, compared with cardiolipin-containing giant unilamellar vesicles the tubules are longer, exhibit a visible pearling, and have a much longer lifetime after acid micro-injection is stopped. We attribute these differences to an additional mechanism that increases monolayer surface imbalance, namely inward PG flip-flop promoted by the local transmembrane pH gradient. Simulations using a fully nonlinear membrane model as well as geometrical calculations are in agreement with this hypothesis. Interestingly, among yeast mutants deficient in cardiolipin biosynthesis, only the crd1-null mutant, which accumulates phosphatidylglycerol, displays significant mitochondrial activity. Our work provides a possible explanation of such a property and further emphasizes the salient role of specific lipids in mitochondrial function.     

Cardiolipin and mitochondrial phosphatidylethanolamine have overlapping functions in mitochondrial fusion in Saccharomyces cerevisiae

The two non-bilayer forming mitochondrial phospholipids cardiolipin (CL) and phosphatidylethanolamine (PE) play crucial roles in maintaining mitochondrial morphology. We have shown previously that CL and PE have overlapping functions, and the loss of both is synthetically lethal. Because the lack of CL does not lead to defects in the mitochondrial network in Saccharomyces cerevisiae, we hypothesized that PE may compensate for CL in the maintenance of mitochondrial tubular morphology and fusion. To test this hypothesis, we constructed a conditional mutant crd1Δpsd1Δ containing null alleles of CRD1 (CL synthase) and PSD1 (mitochondrial phosphatidylserine decarboxylase), in which the wild type CRD1 gene is expressed on a plasmid under control of the TET(OFF) promoter. In the presence of tetracycline, the mutant exhibited highly fragmented mitochondria, loss of mitochondrial DNA, and reduced membrane potential, characteristic of fusion mutants. Deletion of DNM1, required for mitochondrial fission, restored the tubular mitochondrial morphology. Loss of CL and mitochondrial PE led to reduced levels of small and large isoforms of the fusion protein Mgm1p, possibly accounting for the fusion defect. Taken together, these data demonstrate for the first time in vivo that CL and mitochondrial PE are required to maintain tubular mitochondrial morphology and have overlapping functions in mitochondrial fusion.     

Absence of cardiolipin results in temperature sensitivity, respiratory defects, and mitochondrial DNA instability independent of pet56

Cardiolipin (CL) is a dimeric phospholipid localized primarily in the mitochondrial membrane. Previous studies have shown that yeast cells containing a disruption of CRD1, the structural gene encoding CL synthase, exhibit temperature-sensitive colony formation and multiple mitochondrial defects. A recent report (Zhang, M., Su, X., Mileykovskaya, E., Amoscato, A. A., and Dowhan, W. (2003) J. Biol. Chem. 278, 35204-35210) suggested that defects associated with CL deficiency may result from the reduced expression of PET56 in crd1 Delta mutant backgrounds and should be reevaluated. In the current study, we present evidence that CL deficiency leads to mitochondrial DNA instability, loss of viability, and defects in oxidative phosphorylation at elevated temperatures. The observed mutant phenotypes are characteristic of crd1 Delta mutant cells of both PET56 and pet56 backgrounds and are complemented by an episomal copy of CRD1 but not by expression of the PET56 gene. Phosphatidylglycerol is elevated in crd1 Delta mutant cells when grown in the presence of fermentable and non-fermentable carbon sources, although the extent of the increase is higher in nonfermentable medium. An increase in the ratio of phosphatidylethanolamine to phosphatidylcholine was also apparent in the mutant. These findings demonstrate that CRD1, independent of PET56, is required for optimal mitochondrial function and for an essential cellular function at elevated temperatures.     

Interplay of Packing and Flip-flop in Local Bilayer Deformation. How Phosphatidylglycerol Could Rescue Mitochondrial Function in a Cardiolipin-deficient Yeast Mutant

In a previous work, we have shown that a spatially localized transmembrane pH gradient, produced by acid micro-injection near the external side of cardiolipin-containing giant unilamellar vesicles, leads to the formation of tubules that retract after the dissipation of this gradient. These tubules have morphologies similar to mitochondrial cristae. The tubulation effect is attributable to direct phospholipid packing modification in the outer leaflet, that is promoted by protonation of cardiolipin headgroups. In this study, we compare the case of cardiolipin-containing giant unilamellar vesicles with that of giant unilamellar vesicles that contain phosphatidylglycerol (PG). Local acidification also promotes formation of tubules in the latter. However, compared with cardiolipin-containing giant unilamellar vesicles the tubules are longer, exhibit a visible pearling, and have a much longer lifetime after acid micro-injection is stopped. We attribute these differences to an additional mechanism that increases monolayer surface imbalance, namely inward PG flip-flop promoted by the local transmembrane pH gradient. Simulations using a fully nonlinear membrane model as well as geometrical calculations are in agreement with this hypothesis. Interestingly, among yeast mutants deficient in cardiolipin biosynthesis, only the crd1-null mutant, which accumulates phosphatidylglycerol, displays significant mitochondrial activity. Our work provides a possible explanation of such a property and further emphasizes the salient role of specific lipids in mitochondrial function.     

CARDIOLIPIN CONTENT OF WILD TYPE AND MUTANT YEASTS IN RELATION TO MITOCHONDRIAL FUNCTION AND DEVELOPMENT

The phospholipid composition of various strains of the yeast, Saccharomyces cerevisiae, and several of their derived mitochondrial mutants grown under conditions designed to induce variations in the complement of mitochondrial membranes has been examined. Wild type and petite (cytoplasmic respiratory deficient) yeasts were fractionated into various subcellular fractions, which were monitored by electron microscopy and analyzed for cytochrome oxidase (in wild type) and phospholipid composition. 90% or more of the phospholipid, cardiolipin was found in the mitochondrial membranes of wild type and petite yeast. Cardiolipin content differed markedly under various growth conditions. Stationary yeast grown in glucose had better developed mitochondria and more cardiolipin than repressed log phase yeast. Aerobic yeast contained more cardiolipin than anaerobic yeast. Respiration-deficient cytoplasmic mitochondrial mutants, both suppressive and neutral, contained less cardiolipin than corresponding wild types. A chromosomal mutant lacking respiratory function had normal cardiolipin content. Log phase cells grown in galactose and lactate, which do not readily repress the development of mitochondrial membranes, contained as much cardiolipin as stationary phase cells grown in glucose. Cytoplasmic mitochondrial mutants respond to changes in the glucose concentration of the growth medium by variations in their cardiolipin content in the same way as wild type yeast does under similar growth conditions. It is concluded that cardiolipin content of yeast is correlated with, and is a good indicator of, the state of development of mitochondrial membrane.     

Growth defects in intramitochondrial energy depleted cells: role of mitochondrial biogenesis

Simultaneous inhibition of oxidative phosphorylation by rho- mutation and adenine nucleotide exchange by op1 mutation or bongkrekic acid results in intramitochondrial energy depletion and cessation of growth in yeast. Effect of energy depletion of mitochondria on mitochondrial biogenesis was studied in intact yeast cells. Immunoblot analysis revealed an overall decrease in cellular content of two mitochondrial proteins - ADP/ATP translocase and beta subunit of mitochondrial ATPase - together with their lower ability to reach the proper intramitochondrial compartment. Both effects indicate disturbed biogenesis of energy depleted mitochondria. Quantitative differences in growth abilities and mitochondrial damage observed in two studied systems - op1 rho- double mutants and rho- cells treated with bongkrekic acid - can be explained by different degree of intramitochondrial energy depletion due to leakiness of op1 mutation in op1 rho- cells.     

Role of anionic lipid in bacterial membranes

The major phospholipids of Bacillus stearothermophilus are phosphatidylethanolamine (PE), phosphatidylglycerol (PG), and cardiolipin (CL). Under the growth conditions used in this study the concentration of anionic lipid (PG + CL) was determined by the pH of the culture medium. Cells grown in a complex medium at pH 5.8, 7.0, and 8.0 contained 17, 29 and 36 nmol of anionic (PG + CL) lipid/mg cell (dry weight). The concentration of the zwitterionic lipid phosphatidylethanolamine (PE) was 17-20 nmol/mg cell (dry weight) under all conditions. Analysis of isolated membrane preparations suggested that the amount of anionic lipid per unit area of membrane increased as the pH of the growth medium was increased. Membranes from cells grown at pH 5.8 and 8.0 contained 130 and 320 nmol anionic lipid/mg membrane protein, respectively. Phosphatidylethanolamine appeared to be localized on the inner membrane surface in cells grown under all conditions. Increasing the ionic strength of the culture medium by the addition of NaCl or KCl had little effect on growth at pH 5.8 but inhibited growth at pH 7 and 8. It was concluded that anionic phospholipid plays an important physiological role in maintaining an acidic pH at the outer membrane surface.     

Identification and characterization of human cardiolipin synthase

The mitochondrial phospholipid cardiolipin is synthesized from cytidinediphosphate-diacylglycerol and phosphatidylglycerol, a process catalyzed by the enzyme cardiolipin synthase. In this study, we identified a human candidate gene/cDNA for cardiolipin synthase, C20orf155. Expression of this candidate cDNA in the (cardiolipin synthase-deficient) crd1Delta yeast confirmed that it indeed encodes human cardiolipin synthase. Purified mitochondria of the crd1Delta expressing human cardiolipin synthase were used to characterize the enzyme. It has an alkaline pH optimum, requires divalent cations for activity and appears to have a different substrate preference for cytidinediphosphate-diacylglycerol species when compared to phosphatidylglycerol species. The possible implications for CL synthesis and remodeling are discussed.     

Loss of Cardiolipin Leads to Perturbation of Mitochondrial and Cellular Iron Homeostasis

Cardiolipin (CL) is the signature phospholipid of mitochondrial membranes, where it is synthesized locally and plays a critical role in mitochondrial bioenergetic functions. The importance of CL in human health is underscored by the observation that perturbation of CL biosynthesis causes the severe genetic disorder Barth syndrome. To fully understand the cellular response to the loss of CL, we carried out genome-wide expression profiling of the yeast CL mutant crd1Δ. Our results show that the loss of CL in this mutant leads to increased expression of iron uptake genes accompanied by elevated levels of mitochondrial iron and increased sensitivity to iron and hydrogen peroxide. Previous studies have shown that increased mitochondrial iron levels result from perturbations in iron-sulfur (Fe-S) cluster biogenesis. Consistent with an Fe-S defect, deletion of ISU1, one of two ISU genes that encode the mitochondrial Fe-S scaffolding protein essential for the synthesis of Fe-S clusters, led to synthetic growth defects with the crd1Δ mutant. We further show that crd1Δ cells have reduced activities of mitochondrial Fe-S enzymes (aconitase, succinate dehydrogenase, and ubiquinol-cytochrome c oxidoreductase), as well as cytosolic Fe-S enzymes (sulfite reductase and isopropylmalate isomerase). Increased expression of ATM1 or YAP1 did not rescue the Fe-S defects in crd1Δ. These findings show for the first time that CL is required for Fe-S biogenesis to maintain mitochondrial and cellular iron homeostasis.     

FMP30 is required for the maintenance of a normal cardiolipin level and mitochondrial morphology in the absence of mitochondrial phosphatidylethanolamine synthesis

Mitochondria of the yeast Saccharomyces cerevisiae contain enzymes Crd1p and Psd1p, which synthesize cardiolipin (CL) and phosphatidylethanolamine respectively. A previous study indicated that crd1Δ is synthetically lethal with psd1Δ. In this study, to identify novel genes involved in CL metabolism, we searched for genes that genetically interact with Psd1p, and found that deletion of FMP30 encoding a mitochondrial inner membrane protein results in a synthetic growth defect with psd1Δ. Although fmp30Δ cells grew normally and exhibited a slightly decreased CL level, fmp30Δpsd1Δ cells exhibited a severe growth defect and an about 20-fold reduction in the CL level, as compared with the wild-type control. We found also that deletion of FMP30 caused a defect in mitochondrial morphology. Furthermore, FMP30 genetically interacted with seven mitochondrial morphology genes. These results indicated that Fmp30p is involved in the maintenance of mitochondrial morphology and required for the accumulation of a normal level of CL in the absence of mitochondrial phosphatidylethanolamine synthesis.