Phosphatidylethanolamine synthesized by three different pathways is supplied to peroxisomes of the yeast Saccharomyces cerevisiae

Rewiring phospholipid biosynthesis reveals resilience to membrane perturbations and uncovers regulators of lipid homeostasis

The organelles of eukaryotic cells differ in their membrane lipid composition. This heterogeneity is achieved by the localization of lipid synthesizing and modifying enzymes to specific compartments, as well as by intracellular lipid transport that utilizes vesicular and non‐vesicular routes to ferry lipids from their place of synthesis to their destination. For instance, the major and essential phospholipids, phosphatidylethanolamine (PE) and phosphatidylcholine (PC), can be produced by multiple pathways and, in the case of PE, also at multiple locations. However, the molecular components that underlie lipid homeostasis as well as the routes allowing their distribution remain unclear. Here, we present an approach in which we simplify and rewire yeast phospholipid synthesis by redirecting PE and PC synthesis reactions to distinct subcellular locations using chimeric enzymes fused to specific organelle targeting motifs. In rewired conditions, viability is expected to depend on homeostatic adaptation to the ensuing lipostatic perturbations and on efficient interorganelle lipid transport. We therefore performed genetic screens to identify factors involved in both of these processes. Among the candidates identified, we find genes linked to transcriptional regulation of lipid homeostasis, lipid metabolism, and transport. In particular, we identify a requirement for Csf1—an uncharacterized protein harboring a Chorein‐N lipid transport motif—for survival under certain rewired conditions as well as lipidomic adaptation to cold, implicating Csf1 in interorganelle lipid transport and homeostatic adaptation.

Yeast Vps13 is Crucial for Peroxisome Expansion in Cells With Reduced Peroxisome-ER Contact Sites

In the yeast Hansenula polymorpha the peroxisomal membrane protein Pex11 and three endoplasmic reticulum localized proteins of the Pex23 family (Pex23, Pex24 and Pex32) are involved in the formation of peroxisome-ER contact sites. Previous studies suggested that these contacts are involved in non-vesicular lipid transfer and important for expansion of the peroxisomal membrane. The absence of Pex32 results in a severe peroxisomal phenotype, while cells lacking Pex11, Pex23 or Pex24 show milder defects and still are capable to form peroxisomes and grow on methanol. We performed transposon mutagenesis on H. polymorpha pex11 cells and selected mutants that lost the capacity to grow on methanol and are severely blocked in peroxisome formation. This strategy resulted in the identification of Vps13, a highly conserved contact site protein involved in bulk lipid transfer. Our data show that peroxisome formation and function is normal in cells of a vps13 single deletion strain. However, Vps13 is essential for peroxisome biogenesis in pex11. Notably, Vps13 is also required for peroxisome formation in pex23 and pex24 cells. These data suggest that Vps13 is crucial for peroxisome formation in cells with reduced peroxisome-endoplasmic reticulum contact sites and plays a redundant function in lipid transfer from the ER to peroxisomes.

Fluidity and Lipid Composition of Membranes of Peroxisomes, Mitochondria and the ER From Oleic Acid-Induced Saccharomyces cerevisiae

The maintenance of a fluid lipid bilayer is key for organelle function and cell viability. Given the critical role of lipid compositions in determining membrane properties and organelle identity, it is clear that cells must have elaborate mechanism for membrane maintenance during adaptive responses to environmental conditions. Emphasis of the presented study is on peroxisomes, oleic acid-inducible organelles that are essential for the growth of yeast under conditions of oleic acid as single carbon source. Here, we isolated peroxisomes, mitochondria and ER from oleic acid-induced Saccharomyces cerevisiae and determined the lipid composition of their membranes using shotgun lipidomics and compared it to lipid ordering using fluorescence microscopy. In comparison to mitochondrial and ER membranes, the peroxisomal membranes were slightly more disordered and characterized by a distinct enrichment of phosphaditylinositol, indicating an important role of this phospholipid in peroxisomal membrane associated processes.

FSH1 encodes lysophospholipase activity in Saccharomyces cerevisiae

OBJECTIVES

To elucidate the role of FSH1 (family of serine hydrolase) in lipid homeostasis.

RESULTS

Proteins in various species containing alpha/beta hydrolase domain are known to be involved in lipid metabolism. In silico analysis of the FSH1 gene in Saccharomyces cerevisiae revealed the presence of alpha/beta hydrolase domain (ABHD) and a lipase motif (GXSXG), however its function in lipid metabolism remained elusive. The overexpression of FSH1 in WT and fsh1Δ cells showed a significant reduction in the cellular phospholipid levels and an increase in the triacylglycerol levels and lipid droplet (LD) number. Furthermore, the purified recombinant protein Fsh1p was identified as a lysophospholipase that specifically acts on lysophosphatidylserine (LPS) and impacts the lipid homeostasis in S. cerevisiae.

CONCLUSIONS

These results depicted that Fsh1p has a role on lipid homeostasis and is a lysophospholipase that hydrolyzes lysophosphatidylserine (LPS).

Pex24 and Pex32 are required to tether peroxisomes to the ER for organelle biogenesis, positioning and segregation in yeast

The yeast Hansenula polymorpha contains four members of the Pex23 family of peroxins, which characteristically contain a DysF domain. Here we show that all four H. polymorpha Pex23 family proteins localize to the endoplasmic reticulum (ER). Pex24 and Pex32, but not Pex23 and Pex29, predominantly accumulate at peroxisome-ER contacts. Upon deletion of PEX24 or PEX32 - and to a much lesser extent, of PEX23 or PEX29 - peroxisome-ER contacts are lost, concomitant with defects in peroxisomal matrix protein import, membrane growth, and organelle proliferation, positioning and segregation. These defects are suppressed by the introduction of an artificial peroxisome-ER tether, indicating that Pex24 and Pex32 contribute to tethering of peroxisomes to the ER. Accumulation of Pex32 at these contact sites is lost in cells lacking the peroxisomal membrane protein Pex11, in conjunction with disruption of the contacts. This indicates that Pex11 contributes to Pex32-dependent peroxisome-ER contact formation. The absence of Pex32 has no major effect on pre-peroxisomal vesicles that occur in pex3 atg1 deletion cells.

Targeting lipid droplet lysophosphatidylcholine for cisplatin chemotherapy

This study aims to explore lipidic mechanism towards low‐density lipoprotein receptor (LDLR)‐mediated platinum chemotherapy resistance. By using the lipid profiling technology, LDLR knockdown was found to increase lysosomal lipids and decrease membranous lipid levels in EOC cells. LDLR knockdown also down‐regulated ether‐linked phosphatidylethanolamine (PE‐O, lysosomes or peroxisomes) and up‐regulated lysophosphatidylcholine [LPC, lipid droplet (LD)]. This implies that the manner of using Lands cycle (conversion of lysophospholipids) for LDs might affect cisplatin sensitivity. The bioinformatics analyses illustrated that LDLR‐related lipid entry into LD, rather than an endogenous lipid resource (eg Kennedy pathway), controls the EOC prognosis of platinum chemotherapy patients. Moreover, LDLR knockdown increased the number of platinum‐DNA adducts and reduced the LD platinum amount. By using a manufactured LPC‐liposome‐cisplatin (LLC) drug, the number of platinum‐DNA adducts increased significantly in LLC‐treated insensitive cells. Moreover, the cisplatin content in LDs increased upon LLC treatment. Furthermore, lipid profiles of 22 carcinoma cells with differential cisplatin sensitivity (9 sensitive vs 13 insensitive) were acquired. These profiles revealed low storage lipid levels in insensitive cells. This result recommends that LD lipidome might be a common pathway in multiple cancers for platinum sensitivity in EOC. Finally, LLC suppressed both cisplatin‐insensitive human carcinoma cell training and testing sets. Thus, LDLR‐platinum insensitivity can be due to a defective Lands cycle that hinders LPC production in LDs. Using lipidome assessment with the newly formulated LLC can be a promising cancer chemotherapy method.

Peroxisome development in yeast is associated with the formation of Pex3-dependent peroxisome-vacuole contact sites

Using electron and fluorescence microscopy techniques, we identified various physical contacts between peroxisomes and other cell organelles in the yeast Hansenula polymorpha. In exponential glucose-grown cells, which typically contain a single small peroxisome, contacts were only observed with the endoplasmic reticulum and the plasma membrane. Here we focus on a novel peroxisome-vacuole contact site that is formed when glucose-grown cells are shifted to methanol containing media, conditions that induce strong peroxisome development. At these conditions, the small peroxisomes rapidly increase in size, a phenomenon that is paralleled by the formation of distinct intimate contacts with the vacuole. Localization studies showed that the peroxin Pex3 accumulated in patches at the peroxisome-vacuole contact sites. In wild-type cells growing exponentially on medium containing glucose, peroxisome-vacuole contact sites were never observed. However, upon overproduction of Pex3 peroxisomes also associated to vacuoles at these growth conditions. Our observations strongly suggest a role for Pex3 in the formation of a novel peroxisome-vacuole contact site. This contact likely plays a role in membrane growth as it is formed solely at conditions of strong peroxisome expansion.

Turnover of muscle lipids and response to exercise differ between neutral and polar fractions in a model songbird, the zebra finch

The turnover rates of tissues and their constituent molecules give us insights into animals' physiological demands and their functional flexibility over time. Thus far, most studies of this kind have focused on protein turnover, and few have considered lipid turnover despite an increasing appreciation of the functional diversity of this class of molecules. We measured the turnover rates of neutral and polar lipids from the pectoralis muscles of a model songbird, the zebra finch (Taeniopygia guttata, N=65), in a 256 day C₃/C₄ diet shift experiment, with tissue samples taken at 10 time points. We also manipulated the physiological state of a subset of these birds with a 10 week flight training regimen to test the effect of exercise on lipid turnover. We measured lipid δ¹³C values via isotope ratio mass spectrometry (IRMS) and estimated turnover in different fractions and treatment groups with non-linear mixed-effect regression. We found a significant difference between the mean retention times (τ) of neutral and polar lipids (t₁₁₉=-2.22, P=0.028), with polar lipids (τ=11.80±1.28 days) having shorter retention times than neutral lipids (τ=19.47±3.22 days). When all birds were considered, we also found a significant decrease in the mean retention time of polar lipids in exercised birds relative to control birds (difference=-2.2±1.83 days, t₅₆=-2.37, P=0.021), but not neutral lipids (difference=4.2± 7.41 days, t₅₆=0.57, P=0.57). A larger, more variable neutral lipid pool and the exposure of polar lipids in mitochondrial membranes to oxidative damage and increased turnover provide mechanisms consistent with our results.

Complex Relations Between Phospholipids, Autophagy, and Neutral Lipids

Research in the past decade has established the importance of autophagy to a large number of physiological processes and pathophysiological conditions. Originally characterized as a pathway responsible for protein turnover and recycling of amino acids in times of starvation, it has been recently recognized as a major regulator of lipid metabolism. Different lipid species play various roles in the regulation of autophagosomal biogenesis, both as membrane constituents and as signaling platforms. Distinct types of autophagy, in turn, facilitate specific steps in metabolic pathways of different lipid classes, best exemplified in recent studies on neutral lipid dynamics. We review the emerging notion of intricate links between phospholipids, autophagy, and neutral lipids.

The birth of yeast peroxisomes

This contribution describes the phenotypic differences of yeast peroxisome-deficient mutants (pex mutants). In some cases different phenotypes were reported for yeast mutants deleted in the same PEX gene. These differences are most likely related to the marker proteins and methods used to detect peroxisomal remnants. This is especially evident for pex3 and pex19 mutants, where the localization of receptor docking proteins (Pex13, Pex14) resulted in the identification of peroxisomal membrane remnants, which do not contain other peroxisomal membrane proteins, such as the ring proteins Pex2, Pex10 and Pex12. These structures in pex3 and pex19 cells are the template for peroxisome formation upon introduction of the missing gene. Taken together, these data suggest that in all yeast pex mutants analyzed so far peroxisomes are not formed de novo but use membrane remnant structures as a template for peroxisome formation upon reintroduction of the missing gene. The relevance of this model for peroxisomal membrane protein and lipid sorting to peroxisomes is discussed.

Phosphatidylcholine Supply to Peroxisomes of the Yeast Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, phosphatidylcholine (PC), the major phospholipid (PL) of all organelle membranes, is synthesized via two different pathways. Methylation of phosphatidylethanolamine (PE) catalyzed by the methyl transferases Cho2p/Pem1p and Opi3p/Pem2p as well as incorporation of choline through the CDP (cytidine diphosphate)-choline branch of the Kennedy pathway lead to PC formation. To determine the contribution of these two pathways to the supply of PC to peroxisomes (PX), yeast mutants bearing defects in the two pathways were cultivated under peroxisome inducing conditions, i.e. in the presence of oleic acid, and subjected to biochemical and cell biological analyses. Phenotype studies revealed compromised growth of both the cho20Δopi3Δ (mutations in the methylation pathway) and the cki1Δdpl1Δeki1Δ (mutations in the CDP-choline pathway) mutant when grown on oleic acid. Analysis of peroxisomes from the two mutant strains showed that both pathways produce PC for the supply to peroxisomes, although the CDP-choline pathway seemed to contribute with higher efficiency than the methylation pathway. Changes in the peroxisomal lipid pattern of mutants caused by defects in the PC biosynthetic pathways resulted in changes of membrane properties as shown by anisotropy measurements with fluorescent probes. In summary, our data define the origin of peroxisomal PC and demonstrate the importance of PC for peroxisome membrane formation and integrity.

Identification of a phospholipase B encoded by the LPL1 gene in Saccharomyces cerevisiae

Phospholipids also play a major role in maintaining the lipid droplet (LD) morphology. In our current study, deletion of LPL1 resulted in altered morphology of LDs and was confirmed by microscopic analysis. LPL1/YOR059c contains lipase specific motif GXSXG and acetate labeling in the LPL1 overexpressed strains depicted a decrease in glycerophospholipids and an increase in free fatty acids. The purified Lpl1p showed phospholipase activity with broader substrate specificity, acting on all glycerophospholipids primarily at sn-2 position and later at sn-1 position. Localization studies precisely revealed that Lpl1 is exclusively localized in the LD at the stationary phase. Site directed mutagenesis experiments clearly demonstrated that the lipase motif is vital for the phospholipase activity. In summary, our results demonstrate that yeast Lpl1 exerts phospholipase activity, plays a vital role in LD morphology, and its absence results in altered LD size. Based on the localization and enzyme activity we renamed YOR059c as LPL1 (LD phospholipase 1).

Genome-wide analysis of Saccharomyces cerevisiae identifies cellular processes affecting intracellular aggregation of Alzheimer's amyloid-β42: importance of lipid homeostasis

Amyloid-β (Aβ)-containing plaques are a major neuropathological feature of Alzheimer's disease (AD). The two major isoforms of Aβ peptide associated with AD are Aβ40 and Aβ42, of which the latter is highly prone to aggregation. Increased presence and aggregation of intracellular Aβ42 peptides is an early event in AD progression. Improved understanding of cellular processes affecting Aβ42 aggregation may have implications for development of therapeutic strategies. Aβ42 fused to green fluorescent protein (Aβ42-GFP) was expressed in ∼4600 mutants of a Saccharomyces cerevisiae genome-wide deletion library to identify proteins and cellular processes affecting intracellular Aβ42 aggregation by assessing the fluorescence of Aβ42-GFP. This screening identified 110 mutants exhibiting intense Aβ42-GFP-associated fluorescence. Four major cellular processes were overrepresented in the data set, including phospholipid homeostasis. Disruption of phosphatidylcholine, phosphatidylserine, and/or phosphatidylethanolamine metabolism had a major effect on intracellular Aβ42 aggregation and localization. Confocal microscopy indicated that Aβ42-GFP localization in the phospholipid mutants was juxtaposed to the nucleus, most likely associated with the endoplasmic reticulum (ER)/ER membrane. These data provide a genome-wide indication of cellular processes that affect intracellular Aβ42-GFP aggregation and may have important implications for understanding cellular mechanisms affecting intracellular Aβ42 aggregation and AD disease progression.

Making connections: interorganelle contacts orchestrate mitochondrial behavior

Mitochondria are highly dynamic organelles. During their life cycle they frequently fuse and divide, and damaged mitochondria are removed by autophagic degradation. These processes serve to maintain mitochondrial function and ensure optimal energy supply for the cell. It has recently become clear that this complex mitochondrial behavior is governed to a large extent by interactions with other organelles. In this review, we describe mitochondrial contacts with the endoplasmic reticulum (ER), plasma membrane, and peroxisomes. In particular, we highlight how mitochondrial fission, distribution, inheritance, and turnover are orchestrated by interorganellar contacts in yeast and metazoa. These interactions are pivotal for the integration of the dynamic mitochondrial network into the architecture of eukaryotic cells.

Mitochondria are targets for peroxisome-derived oxidative stress in cultured mammalian cells

Many cellular processes are driven by spatially and temporally regulated redox-dependent signaling events. Although mounting evidence indicates that organelles such as the endoplasmic reticulum and mitochondria can function as signaling platforms for oxidative stress-regulated pathways, little is known about the role of peroxisomes in these processes. In this study, we employ targeted variants of the genetically encoded photosensitizer KillerRed to gain a better insight into the interplay between peroxisomes and cellular oxidative stress. We show that the phototoxic effects of peroxisomal KillerRed induce mitochondria-mediated cell death and that this process can be counteracted by targeted overexpression of a select set of antioxidant enzymes, including peroxisomal glutathione S-transferase kappa 1, superoxide dismutase 1, and mitochondrial catalase. We also present evidence that peroxisomal disease cell lines deficient in plasmalogen biosynthesis or peroxisome assembly are more sensitive to KillerRed-induced oxidative stress than control cells. Collectively, these findings confirm and extend previous observations suggesting that disturbances in peroxisomal redox control and metabolism can sensitize cells to oxidative stress. In addition, they lend strong support to the ideas that peroxisomes and mitochondria share a redox-sensitive relationship and that the redox communication between these organelles is not only mediated by diffusion of reactive oxygen species from one compartment to the other. Finally, these findings indicate that mitochondria may act as dynamic receivers, integrators, and transmitters of peroxisome-derived mediators of oxidative stress, and this may have profound implications for our views on cellular aging and age-related diseases.

Screening for hydrolytic enzymes reveals Ayr1p as a novel triacylglycerol lipase in Saccharomyces cerevisiae

Saccharomyces cerevisiae, as well as other eukaryotes, preserves fatty acids and sterols in a biologically inert form, as triacylglycerols and steryl esters. The major triacylglycerol lipases of the yeast S. cerevisiae identified so far are Tgl3p, Tgl4p, and Tgl5p (Athenstaedt, K., and Daum, G. (2003) YMR313c/TGL3 encodes a novel triacylglycerol lipase located in lipid particles of Saccharomyces cerevisiae. J. Biol. Chem. 278, 23317-23323; Athenstaedt, K., and Daum, G. (2005) Tgl4p and Tgl5p, two triacylglycerol lipases of the yeast Saccharomyces cerevisiae, are localized to lipid particles. J. Biol. Chem. 280, 37301-37309). We observed that upon cultivation on oleic acid, triacylglycerol mobilization did not come to a halt in a yeast strain deficient in all currently known triacylglycerol lipases, indicating the presence of additional not yet characterized lipases/esterases. Functional proteome analysis using lipase and esterase inhibitors revealed a subset of candidate genes for yet unknown hydrolytic enzymes on peroxisomes and lipid droplets. Based on the conserved GXSXG lipase motif, putative functions, and subcellular localizations, a selected number of candidates were characterized by enzyme assays in vitro, gene expression analysis, non-polar lipid analysis, and in vivo triacylglycerol mobilization assays. These investigations led to the identification of Ayr1p as a novel triacylglycerol lipase of yeast lipid droplets and confirmed the hydrolytic potential of the peroxisomal Lpx1p in vivo. Based on these results, we discuss a possible link between lipid storage, lipid mobilization, and peroxisomal utilization of fatty acids as a carbon source.

The role of phospholipids in the biological activity and structure of the endoplasmic reticulum

The endoplasmic reticulum (ER) is an interconnected network of tubular and planar membranes that supports the synthesis and export of proteins, carbohydrates and lipids. Phospholipids, in particular phosphatidylcholine (PC), are synthesized in the ER where they have essential functions including provision of membranes required for protein synthesis and export, cholesterol homeostasis, and triacylglycerol storage and secretion. Coordination of these biological processes is essential, as highlighted by findings that link phospholipid metabolism in the ER with perturbations in lipid storage/secretion and stress responses, ultimately contributing to obesity/diabetes, atherosclerosis and neurological disorders. Phospholipid synthesis is not uniformly distributed in the ER but is localized at membrane interfaces or contact zones with other organelles, and in dynamic, proliferating ER membranes. The topology of phospholipid synthesis is an important consideration when establishing the etiology of diseases that arise from ER dysfunction. This review will highlight our current understanding of the contribution of phospholipid synthesis to proper ER function, and how alterations contribute to aberrant stress responses and disease. This article is part of a Special Issue entitled: Functional and structural diversity of endoplasmic reticulum.

Analysis of yeast lipid droplet proteome and lipidome

Lipid droplets (LD) are in the spotlight of lipid research because of the link of lipid storage to health and disease and the just incipient understanding of their involvement in cellular processes apart from nonpolar lipid metabolism. Yeast is an excellent model organism to study the lipidome and proteome of LD under different environmental conditions and to address new aspects of LD biology and chemistry. In this chapter, we describe a versatile protocol for the isolation of LD at high purity and address specific demands for handling different yeast species. Moreover, we discuss the analysis of the LD proteome and lipidome based on standard methods such as thin layer chromatography (TLC), gas liquid chromatography (GLC), mass spectrometry (MS) as well as GLC/MS. Finally, we point out similarities and disparities of LD proteome and lipidome from the three different yeasts Saccharomyces cerevisiae, Yarrowia lipolytica, and Pichia pastoris.

Endoplasmic reticulum: ER stress regulates mitochondrial bioenergetics

Endoplasmic reticulum (ER) stress activates an adaptive unfolded protein response (UPR) that facilitates cellular repair, however, under prolonged ER stress, the UPR can ultimately trigger apoptosis thereby terminating damaged cells. The molecular mechanisms responsible for execution of the cell death program are relatively well characterized, but the metabolic events taking place during the adaptive phase of ER stress remain largely undefined. Here we discuss emerging evidence regarding the metabolic changes that occur during the onset of ER stress and how ER influences mitochondrial function through mechanisms involving calcium transfer, thereby facilitating cellular adaptation. Finally, we highlight how dysregulation of ER-mitochondrial calcium homeostasis during prolonged ER stress is emerging as a novel mechanism implicated in the onset of metabolic disorders.

YPR139c/LOA1 encodes a novel lysophosphatidic acid acyltransferase associated with lipid droplets and involved in TAG homeostasis

LOA1, a yeast member of the glycerolipid acyltransferase family, encodes a novel lysophosphatidic acid acyltransferase associated with lipid droplets (LDs) and involved in triacylglycerol (TAG) accumulation. Loa1p, recruited during LD formation, preferentially directs oleic acid–containing phosphatidic acid species into the TAG biosynthetic pathway.

For many years, lipid droplets (LDs) were considered to be an inert store of lipids. However, recent data showed that LDs are dynamic organelles playing an important role in storage and mobilization of neutral lipids. In this paper, we report the characterization of LOA1 (alias VPS66, alias YPR139c), a yeast member of the glycerolipid acyltransferase family. LOA1 mutants show abnormalities in LD morphology. As previously reported, cells lacking LOA1 contain more LDs. Conversely, we showed that overexpression results in fewer LDs. We then compared the lipidome of loa1Δ mutant and wild-type strains. Steady-state metabolic labeling of loa1Δ revealed a significant reduction in triacylglycerol content, while phospholipid (PL) composition remained unchanged. Interestingly, lipidomic analysis indicates that both PLs and glycerolipids are qualitatively affected by the mutation, suggesting that Loa1p is a lysophosphatidic acid acyltransferase (LPA AT) with a preference for oleoyl-CoA. This hypothesis was tested by in vitro assays using both membranes of Escherichia coli cells expressing LOA1 and purified proteins as enzyme sources. Our results from purification of subcellular compartments and proteomic studies show that Loa1p is associated with LD and active in this compartment. Loa1p is therefore a novel LPA AT and plays a role in LD formation.

Staying in touch: the molecular era of organelle contact sites

Membrane contact sites (MCS) are close appositions between two organelles that facilitate both signaling and the passage of ions and lipids from one cellular compartment to another. Despite the fact that MCS have been observed for over 50 years now, we still know very little about the molecular machinery required to create them or their structure, function and regulation. In this review, we focus on the three best-characterized contact sites to date: the nucleus-vacuole junction and mitochondria-ER and plasma membrane-ER contact sites. In addition, we discuss principles arising from recent research and highlight several unanswered questions in the field.

Metabolic link between phosphatidylethanolamine and triacylglycerol metabolism in the yeast Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae triacylglycerols (TAG) are synthesized by the acyl-CoA dependent acyltransferases Dga1p, Are1p, Are2p and the acyl-CoA independent phospholipid:diacylglycerol acyltransferase (PDAT) Lro1p which uses phosphatidylethanolamine (PE) as a preferred acyl donor. In the present study we investigated a possible link between TAG and PE metabolism by analyzing the contribution of the four different PE biosynthetic pathways to TAG formation, namely de novo PE synthesis via Psd1p and Psd2p, the CDP-ethanolamine (CDP-Etn) pathway and lyso-PE acylation by Ale1p. In cells grown on the non-fermentable carbon source lactate supplemented with 5 mM ethanolamine (Etn) the CDP-Etn pathway contributed most to the cellular TAG level, whereas mutations in the other pathways displayed only minor effects. In cki1∆dpl1∆eki1∆ mutants bearing defects in the CDP-Etn pathway both the cellular and the microsomal levels of PE were markedly decreased, whereas in other mutants of PE biosynthetic routes depletion of this aminoglycerophospholipid was less pronounced in microsomes. This observation is important because Lro1p similar to the enzymes of the CDP-Etn pathway is a component of the ER. We conclude from these results that in cki1∆dpl1∆eki1∆ insufficient supply of PE to the PDAT Lro1p was a major reason for the strongly reduced TAG level. Moreover, we found that Lro1p activity was markedly decreased in cki1∆dpl1∆eki1∆, although transcription of LRO1 was not affected. Our findings imply that (i) TAG and PE syntheses in the yeast are tightly linked; and (ii) TAG formation by the PDAT Lro1p strongly depends on PE synthesis through the CDP-Etn pathway. Moreover, it is very likely that local availability of PE in microsomes is crucial for TAG synthesis through the Lro1p reaction.

Lipid particles/droplets of the yeast Saccharomyces cerevisiae revisited: lipidome meets proteome

In the yeast Saccharomyces cerevisiae as in other eukaryotes non-polar lipids are a reservoir of energy and building blocks for membrane lipid synthesis. The yeast non-polar lipids, triacylglycerols (TG) and steryl esters (SE) are stored in so-called lipid particles/droplets (LP) as biologically inert form of fatty acids and sterols. To understand LP structure and function in more detail we investigated the molecular equipment of this compartment making use of mass spectrometric analysis of lipids (TG, SE, phospholipids) and proteins. We addressed the question whether or not lipid and protein composition of LP influence each other and performed analyses of LP from cells grown on two different carbon sources, glucose and oleate. Growth of cells on oleate caused dramatic cellular changes including accumulation of TG at the expense of SE, enhanced the amount of glycerophospholipids and strongly increased the degree of unsaturation in all lipid classes. Most interestingly, oleate as a carbon source led to adaptation of the LP proteome resulting in the appearance of several novel LP proteins. Localization of these new LP proteins was confirmed by cell fractionation. Proteomes of LP variants from cells grown on glucose or oleate, respectively, were compared and are discussed with emphasis on the different groups of proteins detected through this analysis. In summary, we demonstrate flexibility of the yeast LP lipidome and proteome and the ability of LP to adapt to environmental changes.

The putative Saccharomyces cerevisiae hydrolase Ldh1p is localized to lipid droplets

Here, we report the identification of a novel hydrolase in Saccharomyces cerevisiae. Ldh1p (systematic name, Ybr204cp) comprises the typical GXSXG-type lipase motif of members of the α/β-hydrolase family and shares some features with the peroxisomal lipase Lpx1p. Both proteins carry a putative peroxisomal targeting signal type1 (PTS1) and can be aligned with two regions of homology. While Lpx1p is known as a peroxisomal enzyme, subcellular localization studies revealed that Ldh1p is predominantly localized to lipid droplets, the storage compartment of nonpolar lipids. Ldh1p is not required for the function and biogenesis of peroxisomes, and targeting of Ldh1p to lipid droplets occurs independently of the PTS1 receptor Pex5p.

Oleate inhibits steryl ester synthesis and causes liposensitivity in yeast

In the yeast Saccharomyces cerevisiae, neutral lipids can be synthesized by four acyltransferases, namely Dga1p and Lro1p producing triacylglycerols (TAG) and Are1p and Are2p forming steryl esters (SE). TAG and SE are stored in an organelle called lipid particles/droplet. Growth of yeast cells on oleate-supplemented media strongly induced proliferation of lipid particles and specifically the synthesis of TAG, which serve as the major pool for the excess of fatty acids. Surprisingly, SE synthesis was strongly inhibited under these conditions. Here, we show that this effect was not due to decreased expression of ARE2 encoding the major yeast SE synthase at the transcriptional level but to competitive enzymatic inhibition of Are2p by free oleate. Consequently, a triple mutant dga1Deltalro1Deltaare1DeltaARE2(+) grown on oleate did not form substantial amounts of SE and exhibited a growth phenotype similar to the dga1Deltalro1Deltaare1Deltaare2Delta quadruple mutant, including lack of lipid particles. Growth of these mutants on oleate was strongly delayed, and cell viability was decreased but rescued by adaptation. In these strains, oleate stress caused morphological changes of intracellular membranes, altered phospholipid composition and formation of an additional lipid class, ethyl esters of fatty acids. In summary, our data showed that exposure to oleate led to disturbed lipid and membrane homeostasis along with liposensitivity of the yeast.

Janus-faced enzymes yeast Tgl3p and Tgl5p catalyze lipase and acyltransferase reactions

The yeast triacylglycerol (TAG) lipases Tgl3p and Tgl5p not only contain a TAG lipase, but also an acyltransferase motif. We show that purified Tgl3p and Tgl5p indeed exhibit lysophospholipid acyltransferase activity. Both Tgl3p and Tgl5p affect the level of glycerophospholipids, and Tgl3p is also important for efficient sporulation of yeast.

In the yeast, mobilization of triacylglycerols (TAGs) is facilitated by the three TAG lipases Tgl3p, Tgl4p, and Tgl5p. Motif search analysis, however, indicated that Tgl3p and Tgl5p do not only contain the TAG lipase motif GXSXG but also an H-(X)₄-D acyltransferase motif. Interestingly, lipid analysis revealed that deletion of TGL3 resulted in a decrease and overexpression of TGL3 in an increase of glycerophospholipids. Similar results were obtained with TGL5. Therefore, we tested purified Tgl3p and Tgl5p for acyltransferase activity. Indeed, both enzymes not only exhibited lipase activity but also catalyzed acylation of lysophosphatidylethanolamine and lysophosphatidic acid, respectively. Experiments using variants of Tgl3p created by site-directed mutagenesis clearly demonstrated that the two enzymatic activities act independently of each other. We also showed that Tgl3p is important for efficient sporulation of yeast cells, but rather through its acyltransferase than lipase activity. In summary, our results demonstrate that yeast Tgl3p and Tgl5p play a dual role in lipid metabolism contributing to both anabolic and catabolic processes.