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

Phosphatidylethanolamine homeostasis under conditions of impaired CDP-ethanolamine pathway or phosphatidylserine decarboxylation

Phosphatidylethanolamine is the major inner-membrane lipid in the plasma and mitochondrial membranes. It is synthesized in the endoplasmic reticulum from ethanolamine and diacylglycerol (DAG) by the CDP-ethanolamine pathway and from phosphatidylserine by decarboxylation in the mitochondria. Recently, multiple genetic disorders that impact these pathways have been identified, including hereditary spastic paraplegia 81 and 82, Liberfarb syndrome, and a new type of childhood-onset neurodegeneration-CONATOC. Individuals with these diseases suffer from multisystem disorders mainly affecting neuronal function. This indicates the importance of maintaining proper phospholipid homeostasis when major biosynthetic pathways are impaired. This study summarizes the current knowledge of phosphatidylethanolamine metabolism in order to identify areas of future research that might lead to the development of treatment options.

Phosphatidylglycerol Supplementation Alters Mitochondrial Morphology and Cardiolipin Composition

The pathogenic variant of the TAZ gene is directly associated with Barth syndrome. Because tafazzin in the mitochondria is responsible for cardiolipin (CL) remodeling, all molecules related to the metabolism of CL can affect or be affected by TAZ mutation. In this study, we intend to recover the distortion of the mitochondrial lipid composition, especially CL, for Barth syndrome treatment. The genetically edited TAZ knockout HAP1 cells were demonstrated to be a suitable cellular model, where CL desaturation occurred and monolyso-CL (MLCL) was accumulated. From the species analysis by mass spectrometry, phosphatidylethanolamine showed changed species content after TAZ knockout. TAZ knockout also caused genetic down-regulation of PGS gene and up-regulation of PNPLA8 gene, which may decrease the biosynthesis of CLs and increase the hydrolysis product MLCL. Supplemented phosphatidylglycerol(18:1)₂ (PG(18:1)₂) was successfully biosynthesized to mature symmetrical CL and drastically decrease the concentration of MLCL to recover the morphology of mitochondria and the cristae shape of inner mitochondria. Newly synthesized mature CL may induce the down-regulation of PLA2G6 and PNPLA8 genes to potentially decrease MLCL production. The excess supplemented PG was further metabolized into phosphatidylcholine and phosphatidylethanolamine.

ER-localized phosphatidylethanolamine synthase plays a conserved role in lipid droplet formation

The asymmetric distribution of phospholipids in membranes is a fundamental principle of cellular compartmentalization and organization. Phosphatidylethanolamine (PE), a nonbilayer phospholipid that contributes to organelle shape and function, is synthesized at several subcellular localizations via semiredundant pathways. Previously, we demonstrated in budding yeast that the PE synthase Psd1, which primarily operates on the mitochondrial inner membrane, is additionally targeted to the ER. While ER-localized Psd1 is required to support cellular growth in the absence of redundant pathways, its physiological function is unclear. We now demonstrate that ER-localized Psd1 sublocalizes on the ER to lipid droplet (LD) attachment sites and show it is specifically required for normal LD formation. We also find that the role of phosphatidylserine decarboxylase (PSD) enzymes in LD formation is conserved in other organisms. Thus we have identified PSD enzymes as novel regulators of LDs and demonstrate that both mitochondria and LDs in yeast are organized and shaped by the spatial positioning of a single PE synthesis enzyme.

Shaping the mitochondrial inner membrane in health and disease

Mitochondria play central roles in cellular energetics, metabolism and signalling. Efficient respiration, mitochondrial quality control, apoptosis and inheritance of mitochondrial DNA depend on the proper architecture of the mitochondrial membranes and a dynamic remodelling of inner membrane cristae. Defects in mitochondrial architecture can result in severe human diseases affecting predominantly the nervous system and the heart. Inner membrane morphology is generated and maintained in particular by the mitochondrial contact site and cristae organizing system (MICOS), the F₁ F_o -ATP synthase, the fusion protein OPA1/Mgm1 and the nonbilayer-forming phospholipids cardiolipin and phosphatidylethanolamine. These protein complexes and phospholipids are embedded in a network of functional interactions. They communicate with each other and additional factors, enabling them to balance different aspects of cristae biogenesis and to dynamically remodel the inner mitochondrial membrane. Genetic alterations disturbing these membrane-shaping factors can lead to human pathologies including fatal encephalopathy, dominant optic atrophy, Leigh syndrome, Parkinson's disease and Barth syndrome.

SUNny Ways: The Role of the SUN-Domain Protein Mps3 Bridging Yeast Nuclear Organization and Lipid Homeostasis

Mps3 is a SUN (Sad1-UNC-84) domain-containing protein that is located in the inner nuclear membrane (INM). Genetic screens with multiple Mps3 mutants have suggested that distinct regions of Mps3 function in relative isolation and underscore the broad involvement of Mps3 in multiple pathways including mitotic spindle formation, telomere maintenance, and lipid metabolism. These pathways have largely been characterized in isolation, without a holistic consideration for how key regulatory events within one pathway might impinge on other aspects of biology at the nuclear membrane. Mps3 is uniquely positioned to function in these multiple pathways as its N- terminus is in the nucleoplasm, where it is important for telomere anchoring at the nuclear periphery, and its C-terminus is in the lumen, where it has links with lipid metabolic processes. Emerging work suggests that the role of Mps3 in nuclear organization and lipid homeostasis are not independent, but more connected. For example, a failure in regulating Mps3 levels through the cell cycle leads to nuclear morphological abnormalities and loss of viability, suggesting a link between the N-terminal domain of Mps3 and nuclear envelope homeostasis. We will highlight work suggesting that Mps3 is pivotal factor in communicating events between the nucleus and the lipid bilayer.

Multifaceted roles of porin in mitochondrial protein and lipid transport

Mitochondria are essential eukaryotic organelles responsible for primary cellular energy production. Biogenesis, maintenance, and functions of mitochondria require correct assembly of resident proteins and lipids, which require their transport into and within mitochondria. Mitochondrial normal functions also require an exchange of small metabolites between the cytosol and mitochondria, which is primarily mediated by a metabolite channel of the outer membrane (OM) called porin or voltage-dependent anion channel. Here, we describe recently revealed novel roles of porin in the mitochondrial protein and lipid transport. First, porin regulates the formation of the mitochondrial protein import gate in the OM, the translocase of the outer membrane (TOM) complex, and its dynamic exchange between the major form of a trimer and the minor form of a dimer. The TOM complex dimer lacks a core subunit Tom22 and mediates the import of a subset of mitochondrial proteins while the TOM complex trimer facilitates the import of most other mitochondrial proteins. Second, porin interacts with both a translocating inner membrane (IM) protein like a carrier protein accumulated at the small TIM chaperones in the intermembrane space and the TIM22 complex, a downstream translocator in the IM for the carrier protein import. Porin thereby facilitates the efficient transfer of carrier proteins to the IM during their import. Third, porin facilitates the transfer of lipids between the OM and IM and promotes a back-up pathway for the cardiolipin synthesis in mitochondria. Thus, porin has roles more than the metabolite transport in the protein and lipid transport into and within mitochondria, which is likely conserved from yeast to human.

Mitochondrial reactive oxygen species-mediated cytotoxicity of intracellularly accumulated dihydrosphingosine in the yeast Saccharomyces cerevisiae

In eukaryotic cells, the content of sphingoid long-chain bases (LCBs) is generally much lower than that of complex sphingolipids and ceramides, and the quantitative balance of these metabolites in cells is tightly regulated. In the budding yeast Saccharomyces cerevisiae, it has been demonstrated that exogenously added phytosphingosine (PHS) causes a strong growth defect in tryptophan auxotrophic cells, due to delayed uptake of tryptophan from the culture medium; however, the growth inhibitory effect of dihydrosphingosine (DHS) is less than that of PHS in tryptophan auxotrophic cells. Here, we found that, in tryptophan-prototrophic yeast cells, exogenously added DHS is much more toxic than PHS. Exogenously added DHS is converted to PHS, Cers, or LCB 1-phosphates through the action of sphingolipid C4-hydroxylase, Cer synthases, or LCB kinases, respectively; however, suppression of further metabolism of DHS in cells resulted in an increase in the growth inhibitory activity of exogenously added DHS, indicating that DHS itself is causative of the cytotoxicity. The cytotoxicity of DHS was not mediated by Pkh1/2, Sch9, and Ypk1/2 kinases, intracellular targets of LCBs. DHS treatment caused an increase in mitochondria-derived reactive oxygen species, and the cytotoxic effect of DHS was suppressed by depletion of mitochondrial DNA or antioxidant N-acetylcysteine, but enhanced by deletion of SOD1 and SOD2 encoding superoxide dismutases. Thus, collectively, these results indicated that intracellularly accumulated DHS has mitochondrial reactive oxygen species-mediated cytotoxic activity, which is much more potent than that of PHS.

Porin proteins have critical functions in mitochondrial phospholipid metabolism in yeast

Mitochondrial synthesis of cardiolipin (CL) and phosphatidylethanolamine requires the transport of their precursors, phosphatidic acid and phosphatidylserine, respectively, to the mitochondrial inner membrane. In yeast, the Ups1-Mdm35 and Ups2-Mdm35 complexes transfer phosphatidic acid and phosphatidylserine, respectively, between the mitochondrial outer and inner membranes. Moreover, a Ups1-independent CL accumulation pathway requires several mitochondrial proteins with unknown functions including Mdm31. Here, we identified a mitochondrial porin, Por1, as a protein that interacts with both Mdm31 and Mdm35 in budding yeast (Saccharomyces cerevisiae). Depletion of the porins Por1 and Por2 destabilized Ups1 and Ups2, decreased CL levels by ∼90%, and caused loss of Ups2-dependent phosphatidylethanolamine synthesis, but did not affect Ups2-independent phosphatidylethanolamine synthesis in mitochondria. Por1 mutations that affected its interactions with Mdm31 and Mdm35, but not respiratory growth, also decreased CL levels. Using HeLa cells, we show that mammalian porins also function in mitochondrial CL metabolism. We conclude that yeast porins have specific and critical functions in mitochondrial phospholipid metabolism and that porin-mediated regulation of CL metabolism appears to be evolutionarily conserved.

Lipid Homeostasis Is Maintained by Dual Targeting of the Mitochondrial PE Biosynthesis Enzyme to the ER

Spatial organization of phospholipid synthesis in eukaryotes is critical for cellular homeostasis. The synthesis of phosphatidylcholine (PC), the most abundant cellular phospholipid, occurs redundantly via the ER-localized Kennedy pathway and a pathway that traverses the ER and mitochondria via membrane contact sites. The basis of the ER-mitochondrial PC synthesis pathway is the exclusive mitochondrial localization of a key pathway enzyme, phosphatidylserine decarboxylase Psd1, which generates phosphatidylethanolamine (PE). We find that Psd1 is localized to both mitochondria and the ER. Our data indicate that Psd1-dependent PE made at mitochondria and the ER has separable cellular functions. In addition, the relative organellar localization of Psd1 is dynamically modulated based on metabolic needs. These data reveal a critical role for ER-localized Psd1 in cellular phospholipid homeostasis, question the significance of an ER-mitochondrial PC synthesis pathway to cellular phospholipid homeostasis, and establish the importance of fine spatial regulation of lipid biosynthesis for cellular functions.

Cooperative function of Fmp30, Mdm31, and Mdm32 in Ups1-independent cardiolipin accumulation in the yeast Saccharomyces cerevisiae

Cardiolipin (CL) is synthesized from phosphatidic acid (PA) through a series of enzymatic reactions occurring at the mitochondrial inner membrane (MIM). Ups1-Mdm35 mediates PA transfer from the mitochondrial outer membrane (MOM) to the MIM in the yeast Saccharomyces cerevisiae. Deletion of UPS1 leads to a ~80% decrease in the cellular CL level. However, the CL accumulation in ups1∆ cells is enhanced by the depletion of Ups2, which forms a protein complex with Mdm35 and mediates phosphatidylserine (PS) transfer from the MOM to the MIM for phosphatidylethanolamine (PE) synthesis by a PS decarboxylase, Psd1. In this study, we found that the accumulation of CL in ups1∆ cells was enhanced by deletion of not only UPS2, but also PSD1 and CHO1 encoding a PS synthase, suggesting that low PE levels in mitochondria were relevant to the enhancement of CL accumulation in ups1∆ cells. Furthermore, the Ups1-independent and low-level PE-enhanced CL accumulation was shown to depend on the functions of FMP30, MDM31, and MDM32. In addition, the physical interactions of Fmp30 with Mdm31 and Mdm32 were revealed. Thus, when the mitochondrial PE level is reduced, Fmp30, Mdm31, and Mdm32 seem to function cooperatively for the accumulation of CL in a UPS1-independent manner.

Identification and characterization of the mitochondrial membrane sorting signals in phosphatidylserine decarboxylase 1 from Saccharomyces cerevisiae

Phosphatidylserine decarboxylase 1 (Psd1p) catalyzes the formation of the majority of phosphatidylethanolamine (PE) in the yeast Saccharomyces cerevisiae. Psd1p is localized to mitochondria, anchored to the inner mitochondrial membrane (IMM) through membrane spanning domains and oriented towards the mitochondrial intermembrane space. We found that Psd1p harbors at least two inner membrane-associated domains, which we named IM1 and IM2. IM1 is important for proper orientation of Psd1p within the IMM (Horvath et al., J. Biol. Chem. 287 (2012) 36744-55), whereas it remained unclear whether IM2 is important for membrane-association of Psd1p. To discover the role of IM2 in Psd1p import, processing and assembly into the mitochondria, we constructed Psd1p variants with deletions in IM2. Removal of the complete IM2 led to an altered topology of the protein with the soluble domain exposed to the matrix and to decreased enzyme activity. Psd1p variants lacking portions of the N-terminal moiety of IM2 were inserted into IMM with an altered topology. Psd1p variants with deletions of C-terminal portions of IM2 accumulated at the outer mitochondrial membrane and lost their enzyme activity. In conclusion we showed that IM2 is essential for full enzymatic activity, maturation and correct integration of yeast Psd1p into the inner mitochondrial membrane.

Cardiolipin deficiency causes triacylglycerol accumulation in Saccharomyces cerevisiae

In yeast, the synthesis of cardiolipin (CL) and phosphatidylethanolamine (PE) occurs mainly in mitochondria. CL and PE have overlapping functions, and they are required for mitochondrial function. PE is physiologically linked with triacylglycerol (TAG) metabolism in Saccharomyces cerevisiae, involving an acyl-CoA-independent pathway through the phospholipid:diacylglycerol acyltransferase activity of the Lro1 protein. There is no report on the physiological link between CL and TAG metabolism. Here we report a metabolic link between CL and TAG accumulation in the S. cerevisiae. Our data indicated that CL deficiency causes TAG accumulation, involving an acyl-CoA-dependent pathway through the diacylglycerol acyltransferase activity of the Dga1 protein with no changes in the TAG molecular species. The DGA1 gene deletion from the CL-deficient strains reduced the TAG levels. Data from in vitro and in vivo analyses showed that CL did not affect the enzymatic activity of Dga1. Our data also showed that CL deficiency leads to the up-regulation of acetyl-CoA synthetase genes (ACS1 and ACS2) of the cytosolic pyruvate dehydrogenase bypass pathway. This study establishes a physiological link between CL and TAG metabolism in S. cerevisiae.

Cell biology, physiology and enzymology of phosphatidylserine decarboxylase

Phosphatidylethanolamine is one of the most abundant phospholipids whose major amounts are formed by phosphatidylserine decarboxylases (PSD). Here we provide a comprehensive description of different types of PSDs in the different kingdoms of life. In eukaryotes, type I PSDs are mitochondrial enzymes, whereas other PSDs are localized to other cellular compartments. We describe the role of mitochondrial Psd1 proteins, their function, enzymology, biogenesis, assembly into mitochondria and their contribution to phospholipid homeostasis in much detail. We also discuss briefly the cellular physiology and the enzymology of Psd2. This article is part of a Special Issue entitled: Lipids of Mitochondria edited by Guenther Daum.

Phosphatidylcholine Affects Inner Membrane Protein Translocases of Mitochondria

Two protein translocases transport precursor proteins into or across the inner mitochondrial membrane. The presequence translocase (TIM23 complex) sorts precursor proteins with a cleavable presequence either into the matrix or into the inner membrane. The carrier translocase (TIM22 complex) inserts multispanning proteins into the inner membrane. Both protein import pathways depend on the presence of a membrane potential, which is generated by the activity of the respiratory chain. The non-bilayer-forming phospholipids cardiolipin and phosphatidylethanolamine are required for the activity of the respiratory chain and therefore to maintain the membrane potential for protein import. Depletion of cardiolipin further affects the stability of the TIM23 complex. The role of bilayer-forming phospholipids like phosphatidylcholine (PC) in protein transport into the inner membrane and the matrix is unknown. Here, we report that import of presequence-containing precursors and carrier proteins is impaired in PC-deficient mitochondria. Surprisingly, depletion of PC does not affect stability and activity of respiratory supercomplexes, and the membrane potential is maintained. Instead, the dynamic TIM23 complex is destabilized when the PC levels are reduced, whereas the TIM22 complex remains intact. Our analysis further revealed that initial precursor binding to the TIM23 complex is impaired in PC-deficient mitochondria. We conclude that reduced PC levels differentially affect the TIM22 and TIM23 complexes in mitochondrial protein transport.

Effects of lipids on mitochondrial functions

Mitochondria contain two membranes: the outer and inner membrane. Whereas the outer membrane is particularly enriched in phospholipids, the inner membrane has an unusual high protein content and forms large invaginations termed cristae. The proper phospholipid composition of the membranes is crucial for mitochondrial functions. Phospholipids affect activity, biogenesis and stability of protein complexes including protein translocases and respiratory chain supercomplexes. Negatively charged phospholipids such as cardiolipin are important for the architecture of the membranes and recruit soluble factors to the membranes to support mitochondrial dynamics. Thus, phospholipids not only form the hydrophobic core of biological membranes that surround mitochondria, but also create a specific environment to promote functions of various protein machineries. This article is part of a Special Issue entitled: Lipids of Mitochondria edited by Guenther Daum.

Get1p and Get2p are required for maintenance of mitochondrial morphology and normal cardiolipin levels

Cardiolipin (CL) is the signature phospholipid of mitochondrial membranes. CL deficiency leads to defects in mitochondrial function. Using a targeted synthetic lethality screen to identify defects that exacerbate CL deficiency, we determined that deletion of mitochondrial morphology genes in cells lacking CL leads to severe growth defects. We show that ER membrane proteins Get1p and Get2p are required for maintaining normal levels of CL. We propose that these proteins regulate the level of CL by maintaining wild type-like tubular mitochondrial morphology. The genetic interactions observed in this study identify novel physiological modifiers that are required for maintenance of CL levels and mitochondrial morphology.

Interaction of MDM33 with mitochondrial inner membrane homeostasis pathways in yeast

Membrane homeostasis affects mitochondrial dynamics, morphology, and function. Here we report genetic and proteomic data that reveal multiple interactions of Mdm33, a protein essential for normal mitochondrial structure, with components of phospholipid metabolism and mitochondrial inner membrane homeostasis. We screened for suppressors of MDM33 overexpression-induced growth arrest and isolated binding partners by immunoprecipitation of cross-linked cell extracts. These approaches revealed genetic and proteomic interactions of Mdm33 with prohibitins, Phb1 and Phb2, which are key components of mitochondrial inner membrane homeostasis. Lipid profiling by mass spectrometry of mitochondria isolated from Mdm33-overexpressing cells revealed that high levels of Mdm33 affect the levels of phosphatidylethanolamine and cardiolipin, the two key inner membrane phospholipids. Furthermore, we show that cells lacking Mdm33 show strongly decreased mitochondrial fission activity indicating that Mdm33 is critical for mitochondrial membrane dynamics. Our data suggest that MDM33 functionally interacts with components important for inner membrane homeostasis and thereby supports mitochondrial division.

VID22 is required for transcriptional activation of the PSD2 gene in the yeast Saccharomyces cerevisiae

Regulation of expression of the PS decarboxylase 2 (PSD2) gene in Saccharomyces cerevisiae is poorly understood. We found that deletion of VID22 resulted in a decrease in the activity of the Psd2p enzyme through down-regulation of PSD2 gene expression.

Phospholipid methylation controls Atg32-mediated mitophagy and Atg8 recycling

Degradation of mitochondria via selective autophagy, termed mitophagy, contributes to mitochondrial quality and quantity control whose defects have been implicated in oxidative phosphorylation deficiency, aberrant cell differentiation, and neurodegeneration. How mitophagy is regulated in response to cellular physiology remains obscure. Here, we show that mitophagy in yeast is linked to the phospholipid biosynthesis pathway for conversion of phosphatidylethanolamine to phosphatidylcholine by the two methyltransferases Cho2 and Opi3. Under mitophagy-inducing conditions, cells lacking Opi3 exhibit retardation of Cho2 repression that causes an anomalous increase in glutathione levels, leading to suppression of Atg32, a mitochondria-anchored protein essential for mitophagy. In addition, loss of Opi3 results in accumulation of phosphatidylmonomethylethanolamine (PMME) and, surprisingly, generation of Atg8-PMME, a mitophagy-incompetent lipid conjugate of the autophagy-related ubiquitin-like modifier. Amelioration of Atg32 expression and attenuation of Atg8-PMME conjugation markedly rescue mitophagy in opi3-null cells. We propose that proper regulation of phospholipid methylation is crucial for Atg32-mediated mitophagy.

Phosphatidylcholine affects the role of the sorting and assembly machinery in the biogenesis of mitochondrial β-barrel proteins

Two protein translocases drive the import of β-barrel precursor proteins into the mitochondrial outer membrane: The translocase of the outer membrane (TOM complex) promotes transport of the precursor to the intermembrane space, whereas the sorting and assembly machinery (SAM complex) mediates subsequent folding of the β-barrel and its integration into the target membrane. The non-bilayer-forming phospholipids phosphatidylethanolamine (PE) and cardiolipin (CL) are required for the biogenesis of β-barrel proteins. Whether bilayer-forming phospholipids such as phosphatidylcholine (PC), the most abundant phospholipid of the mitochondrial outer membrane, play a role in the import of β-barrel precursors is unclear. In this study, we show that PC is required for stability and function of the SAM complex during the biogenesis of β-barrel proteins. PC further promotes the SAM-dependent assembly of the TOM complex, indicating a general role of PC for the function of the SAM complex. In contrast to PE-deficient mitochondria precursor accumulation at the TOM complex is not affected by depletion of PC. We conclude that PC and PE affect the function of distinct protein translocases in mitochondrial β-barrel biogenesis.

Phospholipid transport via mitochondria

In eukaryotic cells, complex membrane structures called organelles are highly developed to exert specialized functions. Mitochondria are one of such organelles consisting of the outer and inner membranes (OM and IM) with characteristic protein and phospholipid compositions. Maintaining proper phospholipid compositions of the membranes is crucial for mitochondrial integrity, thereby contributing to normal cell activities. As cellular locations for phospholipid synthesis are restricted to specific compartments such as the endoplasmic reticulum (ER) membrane and the mitochondrial inner membrane, newly synthesized phospholipids have to be transported and distributed properly from the ER or mitochondria to other cellular membranes. Although understanding of molecular mechanisms of phospholipid transport are much behind those of protein transport, recent studies using yeast as a model system began to provide intriguing insights into phospholipid exchange between the ER and mitochondria as well as between the mitochondrial OM and IM. In this review, we summarize the latest findings of phospholipid transport via mitochondria and discuss the implicated molecular mechanisms.

Does the study of genetic interactions help predict the function of mitochondrial proteins in Saccharomyces cerevisiae?

Mitochondria are complex organelles of eukaryotic cells that contain their own genome, encoding key subunits of the respiratory complexes. The successive steps of mitochondrial gene expression are intimately linked, and are under the control of a large number of nuclear genes encoding factors that are imported into mitochondria. Investigating the relationships between these genes and their interaction networks, and whether they reveal direct or indirect partners, can shed light on their role in mitochondrial biogenesis, as well as identify new actors in this process. These studies, mainly developed in yeasts, are significant because mammalian equivalents of such yeast genes are candidate genes in mitochondrial pathologies. In practice, studies of physical, chemical and genetic interactions can be undertaken. The search for genetic interactions, either aggravating or alleviating the phenotype of the starting mutants, has proved to be particularly powerful in yeast since even subtle changes in respiratory phenotypes can be screened in a very efficient way. In addition, several high throughput genetic approaches have recently been developed. In this review we analyze the genetic network of three genes involved in different steps of mitochondrial gene expression, from the transcription and translation of mitochondrial RNAs to the insertion of newly synthesized proteins into the inner mitochondrial membrane, and we examine their relevance to our understanding of mitochondrial biogenesis. We find that these genetic interactions are seldom redundant with physical interactions, and thus bring a considerable amount of original and significant information as well as open new areas of research.

Role of phosphatidylethanolamine in the biogenesis of mitochondrial outer membrane proteins

The mitochondrial outer membrane contains proteinaceous machineries for the import and assembly of proteins, including TOM (translocase of the outer membrane) and SAM (sorting and assembly machinery). It has been shown that the dimeric phospholipid cardiolipin is required for the stability of TOM and SAM complexes and thus for the efficient import and assembly of β-barrel proteins and some α-helical proteins of the outer membrane. Here, we report that mitochondria deficient in phosphatidylethanolamine (PE), the second non-bilayer-forming phospholipid, are impaired in the biogenesis of β-barrel proteins, but not of α-helical outer membrane proteins. The stability of TOM and SAM complexes is not disturbed by the lack of PE. By dissecting the import steps of β-barrel proteins, we show that an early import stage involving translocation through the TOM complex is affected. In PE-depleted mitochondria, the TOM complex binds precursor proteins with reduced efficiency. We conclude that PE is required for the proper function of the TOM complex.

Processing and topology of the yeast mitochondrial phosphatidylserine decarboxylase 1

The inner mitochondrial membrane plays a crucial role in cellular lipid homeostasis through biosynthesis of the non-bilayer-forming lipids phosphatidylethanolamine and cardiolipin. In the yeast Saccharomyces cerevisiae, the majority of cellular phosphatidylethanolamine is synthesized by the mitochondrial phosphatidylserine decarboxylase 1 (Psd1). The biogenesis of Psd1 involves several processing steps. It was speculated that the Psd1 precursor is sorted into the inner membrane and is subsequently released into the intermembrane space by proteolytic removal of a hydrophobic sorting signal. However, components involved in the maturation of the Psd1 precursor have not been identified. We show that processing of Psd1 involves the action of the mitochondrial processing peptidase and Oct1 and an autocatalytic cleavage at a highly conserved LGST motif yielding the α- and β-subunit of the enzyme. The Psd1 β-subunit (Psd1β) forms the membrane anchor, which binds the intermembrane space-localized α-subunit (Psd1α). Deletion of a transmembrane segment in the β-subunit results in mislocalization of Psd1 and reduced enzymatic activity. Surprisingly, autocatalytic cleavage does not depend on proper localization to the inner mitochondrial membrane. In summary, membrane integration of Psd1 is crucial for its functionality and for maintenance of mitochondrial lipid homeostasis.

Phosphatidylserine decarboxylase 1 (Psd1) promotes mitochondrial fusion by regulating the biophysical properties of the mitochondrial membrane and alternative topogenesis of mitochondrial genome maintenance protein 1 (Mgm1)

BACKGROUND

Phosphatidylethanolamine is proposed to regulate mitochondrial fusion, but its mechanism of action is unknown.

RESULTS

Decreasing phosphatidylethanolamine reduces the rate of lipid mixing and the biogenesis of Mgm1, a mitochondrial fusion protein.

CONCLUSION

Psd1 regulates the lipid and protein machineries of mitochondrial fusion.

SIGNIFICANCE

Understanding how lipid metabolism regulates mitochondrial dynamics will reveal its role in cellular functions such as apoptosis and autophagy. Non-bilayer-forming lipids such as cardiolipin, phosphatidic acid, and phosphatidylethanolamine (PE) are proposed to generate negative membrane curvature, promoting membrane fusion. However, the mechanism by which lipids regulate mitochondrial fusion remains poorly understood. Here, we show that mitochondrial-localized Psd1, the key yeast enzyme that synthesizes PE, is required for proper mitochondrial morphology and fusion. Yeast cells lacking Psd1 exhibit fragmented and aggregated mitochondria with impaired mitochondrial fusion during mating. More importantly, we demonstrate that a reduction in PE reduces the rate of lipid mixing during fusion of liposomes with lipid compositions reflecting the mitochondrial membrane. This suggests that the mitochondrial fusion defect in the Δpsd1 strain could be due to the altered biophysical properties of the mitochondrial membrane, resulting in reduced fusion kinetics. The Δpsd1 strain also has impaired mitochondrial activity such as oxidative phosphorylation and reduced mitochondrial ATP levels which are due to a reduction in mitochondrial PE. The loss of Psd1 also impairs the biogenesis of s-Mgm1, a protein essential for mitochondrial fusion, further exacerbating the mitochondrial fusion defect of the Δpsd1 strain. Increasing s-Mgm1 levels in Δpsd1 cells markedly reduced mitochondrial aggregation. Our results demonstrate that mitochondrial PE regulates mitochondrial fusion by regulating the biophysical properties of the mitochondrial membrane and by enhancing the biogenesis of s-Mgm1. While several proteins are required to orchestrate the intricate process of membrane fusion, we propose that specific phospholipids of the mitochondrial membrane promote fusion by enhancing lipid mixing kinetics and by regulating the action of profusion proteins.

Phosphatidylethanolamine and cardiolipin differentially affect the stability of mitochondrial respiratory chain supercomplexes

The mitochondrial inner membrane contains two non-bilayer‐forming phospholipids, phosphatidylethanolamine (PE) and cardiolipin (CL). Lack of CL leads to destabilization of respiratory chain supercomplexes, a reduced activity of cytochrome c oxidase, and a reduced inner membrane potential Δψ. Although PE is more abundant than CL in the mitochondrial inner membrane, its role in biogenesis and assembly of inner membrane complexes is unknown. We report that similar to the lack of CL, PE depletion resulted in a decrease of Δψ and thus in an impaired import of preproteins into and across the inner membrane. The respiratory capacity and in particular the activity of cytochrome c oxidase were impaired in PE-depleted mitochondria, leading to the decrease of Δψ. In contrast to depletion of CL, depletion of PE did not destabilize respiratory chain supercomplexes but favored the formation of larger supercomplexes (megacomplexes) between the cytochrome bc₁ complex and the cytochrome c oxidase. We conclude that both PE and CL are required for a full activity of the mitochondrial respiratory chain and the efficient generation of the inner membrane potential. The mechanisms, however, are different since these non-bilayer‐forming phospholipids exert opposite effects on the stability of respiratory chain supercomplexes.

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.

The complexity of cardiolipin in health and disease

Cardiolipin, the signature phospholipid of mitochondria, is a lipid dimer that is important for a diverse range of mitochondrial activities beyond the process of ATP production. Thus not surprisingly, derangements in cardiolipin metabolism are now appreciated to contribute to an assortment of pathological conditions. A comprehensive inventory of enzymes involved in cardiolipin biosynthesis and remodeling was just recently obtained. Post-biosynthesis, the acyl chain composition of cardiolipin is modified by up to three distinct remodeling enzymes that produce either a homogeneous tissue-specific mature form of cardiolipin or alternatively 'bad' cardiolipin that has been linked to mitochondrial dysfunction. In this review, we initially focus on the newly identified players in cardiolipin metabolism and then shift our attention to how changes in cardiolipin metabolism contribute to human disease.

The complexity of cardiolipin in health and disease

Cardiolipin, the signature phospholipid of mitochondria, is a lipid dimer that is important for a diverse range of mitochondrial activities beyond the process of ATP production. Thus not surprisingly, derangements in cardiolipin metabolism are now appreciated to contribute to an assortment of pathological conditions. A comprehensive inventory of enzymes involved in cardiolipin biosynthesis and remodeling was just recently obtained. Post-biosynthesis, the acyl chain composition of cardiolipin is modified by up to three distinct remodeling enzymes that produce either a homogeneous tissue-specific mature form of cardiolipin or alternatively 'bad' cardiolipin that has been linked to mitochondrial dysfunction. In this review, we initially focus on the newly identified players in cardiolipin metabolism and then shift our attention to how changes in cardiolipin metabolism contribute to human disease.

The conserved GTPase Gem1 regulates endoplasmic reticulum-mitochondria connections

Mitochondria are connected to the endoplasmic reticulum (ER) through specialized protein complexes. We recently identified the ER-mitochondria encounter structure (ERMES) tethering complex, which plays a role in phospholipid exchange between the two organelles. ERMES also has been implicated in the coordination of mitochondrial protein import, mitochondrial DNA replication, and mitochondrial dynamics, suggesting that these interorganelle contact sites play central regulatory roles in coordinating various aspects of the physiology of the two organelles. Here we purified ERMES complexes and identified the Ca(2+)-binding Miro GTPase Gem1 as an integral component of ERMES. Gem1 regulates the number and size of the ERMES complexes. In vivo, association of Gem1 to ERMES required the first of Gem1's two GTPase domains and the first of its two functional Ca(2+)-binding domains. In contrast, Gem1's second GTPase domain was required for proper ERMES function in phospholipid exchange. Our results suggest that ERMES is not a passive conduit for interorganellar lipid exchange, but that it can be regulated in response to physiological needs. Furthermore, we provide evidence that the metazoan Gem1 ortholog Miro-1 localizes to sites of ER-mitochondrial contact, suggesting that some of the features ascribed to Gem1 may be evolutionarily conserved.