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

Lipid droplets degradation mechanisms from microalgae to mammals, a comparative overview

Lipid droplets (LDs) are organelles composed of a hydrophobic core (mostly triacylglycerols and steryl esters) delineated by a lipid monolayer and found throughout the tree of life. LDs were seen for a long time as simple energy storage organelles but recent works highlighted their versatile roles in several fundamental cellular processes, particularly during stress response. LDs biogenesis occurs in the ER and their number and size can be dynamically regulated depending on their function, e.g. during development or stress. Understanding their biogenesis and degradation mechanisms is thus essential to better apprehend their roles. LDs degradation can occur in the cytosol by lipolysis or after their internalization into lytic compartments (e.g. vacuoles or lysosomes) using diverse mechanisms that depend on the considered organism, tissue, developmental stage or environmental condition. In this review, we summarize our current knowledge on the different LDs degradation pathways in several main phyla of model organisms, unicellular or pluricellular, photosynthetic or not (budding yeast, mammals, land plants and microalgae). We highlight the conservation of the main degradation pathways throughout evolution, but also the differences between organisms, or inside an organism between different organs. Finally, we discuss how this comparison can help to shed light on relationships between LDs degradation pathways and LDs functions.

Nitrogen signaling factor triggers a respiration-like gene expression program in fission yeast

Microbes have evolved intricate communication systems that enable individual cells of a population to send and receive signals in response to changes in their immediate environment. In the fission yeast Schizosaccharomyces pombe, the oxylipin nitrogen signaling factor (NSF) is part of such communication system, which functions to regulate the usage of different nitrogen sources. Yet, the pathways and mechanisms by which NSF acts are poorly understood. Here, we show that NSF physically interacts with the mitochondrial sulfide:quinone oxidoreductase Hmt2 and that it prompts a change from a fermentation- to a respiration-like gene expression program without any change in the carbon source. Our results suggest that NSF activity is not restricted to nitrogen metabolism alone and that it could function as a rheostat to prepare a population of S. pombe cells for an imminent shortage of their preferred nutrients.

Systematic multi-level analysis of an organelle proteome reveals new peroxisomal functions

Seventy years following the discovery of peroxisomes, their complete proteome, the peroxi-ome, remains undefined. Uncovering the peroxi-ome is crucial for understanding peroxisomal activities and cellular metabolism. We used high-content microscopy to uncover peroxisomal proteins in the model eukaryote - Saccharomyces cerevisiae. This strategy enabled us to expand the known peroxi-ome by ~40% and paved the way for performing systematic, whole-organellar proteome assays. By characterizing the sub-organellar localization and protein targeting dependencies into the organelle, we unveiled non-canonical targeting routes. Metabolomic analysis of the peroxi-ome revealed the role of several newly identified resident enzymes. Importantly, we found a regulatory role of peroxisomes during gluconeogenesis, which is fundamental for understanding cellular metabolism. With the current recognition that peroxisomes play a crucial part in organismal physiology, our approach lays the foundation for deep characterization of peroxisome function in health and disease.

Crystal Structures of the Plant Phospholipase A1 Proteins Reveal a Unique Dimerization Domain

Phospholipase is an enzyme that hydrolyzes various phospholipid substrates at specific ester bonds and plays important roles such as membrane remodeling, as digestive enzymes, and the regulation of cellular mechanism. Phospholipase proteins are divided into following the four major groups according to the ester bonds they cleave off: phospholipase A1 (PLA1), phospholipase A2 (PLA2), phospholipase C (PLC), and phospholipase D (PLD). Among the four phospholipase groups, PLA1 has been less studied than the other phospholipases. Here, we report the first molecular structures of plant PLA1s: AtDSEL and CaPLA1 derived from Arabidopsis thaliana and Capsicum annuum, respectively. AtDSEL and CaPLA1 are novel PLA1s in that they form homodimers since PLAs are generally in the form of a monomer. The dimerization domain at the C-terminal of the AtDSEL and CaPLA1 makes hydrophobic interactions between each monomer, respectively. The C-terminal domain is also present in PLA1s of other plants, but not in PLAs of mammals and fungi. An activity assay of AtDSEL toward various lipid substrates demonstrates that AtDSEL is specialized for the cleavage of sn-1 acyl chains. This report reveals a new domain that exists only in plant PLA1s and suggests that the domain is essential for homodimerization.

The Diversity of the Mitochondrial Outer Membrane Protein Import Channels: Emerging Targets for Modulation

The functioning of mitochondria and their biogenesis are largely based on the proper function of the mitochondrial outer membrane channels, which selectively recognise and import proteins but also transport a wide range of other molecules, including metabolites, inorganic ions and nucleic acids. To date, nine channels have been identified in the mitochondrial outer membrane of which at least half represent the mitochondrial protein import apparatus. When compared to the mitochondrial inner membrane, the presented channels are mostly constitutively open and consequently may participate in transport of different molecules and contribute to relevant changes in the outer membrane permeability based on the channel conductance. In this review, we focus on the channel structure, properties and transported molecules as well as aspects important to their modulation. This information could be used for future studies of the cellular processes mediated by these channels, mitochondrial functioning and therapies for mitochondria-linked diseases.

Phosphatidic acid biosynthesis in the model organism yeast Saccharomyces cerevisiae - a survey

Phosphatidic acid biosynthesis represents the initial part of de novo formation of all glycerophospholipids (membrane lipids) as well as triacylglycerols (storage lipids), and is thus the centerpiece of glycerolipid metabolism. The universal route of phosphatidic acid biosynthesis starts from the precursor glycerol-3-phosphate and comprises two consecutive acylation reactions which are catalyzed by a glycerol-3-phosphate acyltransferase and a 1-acyl glycerol-3-phosphate acyltransferase. In addition, yeast and mammals harbor a set of enzymes which can synthesize phosphatidic acid from the precursor dihydroxyacetone phosphate. In the present review our current knowledge about enzymes contributing to phosphatidic acid biosynthesis in the invaluable model organism yeast, Saccharomyces cerevisiae, is summarized. A special focus is laid upon the regulation and the localization of these enzymes. Furthermore, research needs for a deeper insight into the high complexity of phosphatidic acid biosynthesis and consequently the entire lipid metabolic network is presented.

Lipid droplets and autophagy-links and regulations from yeast to humans

Recent advances in the yeast Saccharomyces cerevisiae and higher eukaryotes have been increasingly connecting lipid droplet (LD) dynamics to the regulation of autophagy. In this review we will discuss implications that connect LD de novo synthesis and LD mobilization to autophagy and how autophagy is regulated by these mechanisms. Elucidating these connections might pose a chance to further understand autophagy induction and membrane biogenesis for the growing autophagosome under different conditions. Increasing our understanding of these mechanisms might provide a chance to understand several conditions that might be related to LD dysregulation and, possibly, as a consequence of this, dysregulation of autophagy.

Metabolism of Storage Lipids and the Role of Lipid Droplets in the Yeast Schizosaccharomyces pombe

Storage lipids, triacylglycerols (TAG), and steryl esters (SE), are predominant constituents of lipid droplets (LD) in fungi. In several yeast species, metabolism of TAG and SE is linked to various cellular processes, including cell division, sporulation, apoptosis, response to stress, and lipotoxicity. In addition, TAG are an important source for the generation of value-added lipids for industrial and biomedical applications. The fission yeast Schizosaccharomyces pombe is a widely used unicellular eukaryotic model organism. It is a powerful tractable system used to study various aspects of eukaryotic cellular and molecular biology. However, the knowledge of S. pombe neutral lipids metabolism is quite limited. In this review, we summarize and discuss the current knowledge of the homeostasis of storage lipids and of the role of LD in the fission yeast S. pombe with the aim to stimulate research of lipid metabolism and its connection with other essential cellular processes. We also discuss the advantages and disadvantages of fission yeast in lipid biotechnology and recent achievements in the use of S. pombe in the biotechnological production of valuable lipid compounds.

Analysis of autophagy activated during changes in carbon source availability in yeast cells

Autophagy is a conserved intracellular degradation system in eukaryotes. Recent studies have revealed that autophagy can be induced not only by nitrogen starvation but also by many other stimuli. However, questions persist regarding the types of conditions that induce autophagy, as well as the particular kinds of autophagy that are induced under these specific conditions. In experimental studies, abrupt nutrient changes are often used to induce autophagy. In this study, we investigated autophagy induction in batch culture on low-glucose medium, in which growth of yeast (Saccharomyces cerevisiae) cells is clearly reflected exclusively by carbon source state. In this medium, cells pass sequentially through three stages: glucose-utilizing, ethanol-utilizing, and ethanol-depleted phases. Using GFP cleavage assay by immunoblotting methods, fluorescence microscopy, and transmission electron microscopy ultrastructural analysis, we found that bulk autophagy and endoplasmic reticulum-phagy are induced starting at the ethanol-utilizing phase, and bulk autophagy is activated to a greater extent in the ethanol-depleted phase. Furthermore, we found that mitophagy is induced by ethanol depletion. Microautophagy occurred after glucose depletion and involved incorporation of cytosolic components and lipid droplets into the vacuolar lumen. Moreover, we observed that autophagy-deficient cells grow more slowly in the ethanol-utilizing phase and exhibit a delay in growth resumption when they are shifted to fresh medium from the ethanol-depleted phase. Our findings suggest that distinct types of autophagy are induced in yeast cells undergoing gradual changes in carbon source availability.

Fatty Acids Effect on Lipase and Biosurfactant Induction in Rhodotorula Glutinis CMGB-RG5

Fatty acids represent important substrates for obtaining microbial lipases and biosurfactants. The yeast strain CMGBRG5 was identified as Rhodotorula glutinis using the BIOLOG MicroLog System. The opacity screening tests showed that R. glutinis CMGB-RG5 was able to produce lipases in presence of 1% Tween 80 after three days of incubation. Lipase induction was estimated as cell growth in presence of Tween 80, Tween 20, olive oil and tributyrin. After 48 hours, best results were obtained in presence of butyric acid, respectively, oleic acid: 2.2 × 107 cells/ml on tributyrin and 1.0 × 107 cells/ml on olive oil. Biosurfactant production was evaluated as emulsification index (E24%). After one week, high E24 values were obtained on fried sunflower oil (53%) and olive oil (35%). Crude and concentrated biosurfactants were tested against nine Candida strains. Best antimicrobial activity was observed for [20X] biosurfactants against C. tropicalis, C. guilliermondii and C. krusei. In conclusion, R. glutinis CGB-RG5 shows high potential for using fatty acids from various sources as unique carbon substrates for synthesis of biocompounds with high economic and biotechnological value.

Novel Channels of the Outer Membrane of Mitochondria: Recent Discoveries Change Our View

Ion channels mediate ion flux across biological membranes and regulate important organellar and cellular tasks. A recent study revealed the presence of four new proteins, the MIM complex (composed by Mim1 and Mim2), Ayr1, OMC7, and OMC8, that are able to form ion-conducting channels in the outer mitochondria membrane (OMM). These findings strongly indicate that the OMM is endowed with many solute-specific channels, in addition to porins and known channels mediating protein import into mitochondria. These solute-specific channels provide essential pathways for the controlled transport of ions and metabolites and may thus add a further layer of specificity to the regulation of mitochondrial function at the organelle-cytosol and/or inter-organellar interface. Future studies will be required to fully understand the way(s) of regulation of these new channels and to integrate them into signaling pathways within the cells.

Saccharomyces cerevisiae lipid droplet associated enzyme Ypr147cp shows both TAG lipase and ester hydrolase activities

Saccharomyces cerevisiae Ypr147cp was found localized to lipid droplets but the physiological role of Ypr147cp remains unknown. Sequence analysis of Ypr147cp revealed an α/β hydrolase domain along with the conserved GXSXG lipase motif. Recombinant Ypr147cp showed both triacylglycerol lipase and ester hydrolase activities. Knock out of YPR147C led to accumulation of TAG in ypr147cΔ when compared to wild type (WT). In addition, transmission electron microscopic analysis of ypr147cΔ cells revealed a greater number of lipid bodies, justifying the increase in TAG content, and the phenotype was rescued upon overexpression of YPR147C in ypr147cΔ. Moreover, the lipid profiling confirmed the accumulation of fatty acids derived from neutral and phospholipids in ypr147cΔ cells. Based on these results, Ypr147cp is identified as a lipid droplet associated triacylglycerol lipase along with an ester hydrolyzing capacity.

Lipid droplet-mediated lipid and protein homeostasis in budding yeast

Lipid droplets are conserved specialized organelles that store neutral lipids. Our view on this unique organelle has evolved from a simple fat deposit to a highly dynamic and functionally diverse hub—one that mediates the buffering of fatty acid stress and the adaptive reshaping of lipid metabolism to promote membrane and organelle homeostasis and the integrity of central proteostasis pathways, including autophagy, which ensure stress resistance and cell survival. This Review will summarize the recent developments in the budding yeast Saccharomyces cerevisiae, as this model organism has been instrumental in deciphering the fundamental mechanisms and principles of lipid droplet biology and interconnecting lipid droplets with many unanticipated cellular functions applicable to many other cell systems.

Metabolic engineering of Saccharomyces cerevisiae for overproduction of triacylglycerols

Triacylglycerols (TAGs) are valuable versatile compounds that can be used as metabolites for nutrition and health, as well as feedstocks for biofuel production. Although Saccharomyces cerevisiae is the favored microbial cell factory for industrial production of biochemicals, it does not produce large amounts of lipids and TAGs comprise only ~1% of its cell dry weight. Here, we engineered S. cerevisiae to reorient its metabolism for overproduction of TAGs, by regulating lipid droplet associated-proteins involved in TAG synthesis and hydrolysis. We implemented a push-and-pull strategy by overexpressing genes encoding a deregulated acetyl-CoA carboxylase, ACC1^S659A/S1157A(ACC1**), as well as the last two steps of TAG formation: phosphatidic phosphatase (PAH1) and diacylglycerol acyltransferase (DGA1), ultimately leading to 129 mg∙gCDW⁻¹ of TAGs. Disruption of TAG lipase genes TGL3, TGL4, TGL5 and sterol acyltransferase gene ARE1 increased the TAG content to 218 mg∙gCDW⁻¹. Further disruption of the beta-oxidation by deletion of POX1, as well as glycerol-3-phosphate utilization through deletion of GUT2, did not affect TAGs levels. Finally, disruption of the peroxisomal fatty acyl-CoA transporter PXA1 led to accumulation of 254 mg∙gCDW⁻¹. The TAG levels achieved here are the highest titer reported in S. cerevisiae, reaching 27.4% of the maximum theoretical yield in minimal medium with 2% glucose. This work shows the potential of using an industrially established and robust yeast species for high level lipid production.

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Saccharomyces cerevisiae was engineered towards triacylglycerol (TAG) accumulation.

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Overexpression of TAG biosynthesis genes, and deletion of degradation pathways were targeted.

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Final strain reached 254 mg/gCDW TAG on 2% glucose media, achieving 27.4% of the maximum theoretical yield.

Ymr210wp leads to the accumulation of phospholipids and steryl esters in yeast

Ymr210w was identified as a MAG (Monoacylglycerol) lipase. The accumulation of the phospholipids in the ymr210wΔ was not clearly understood. It was expressed in S. cerevisiae using pYES2/CT vector and His-tag purified recombinant protein confirmed TAG lipase activity. To further evaluate the role of YMR210w, ester hydrolase activity was also confirmed with pNP-acetate, pNP-butyrate and pNP - palmitate. GC-MS lipid profiling of ymr210wΔ showed an increase in the 15:0 Pentadecanoic acid by 76% among the total lipids. Phospholipid, Erucic acid 22:1 (Δ13) showed 43% increase while steryl esters showed significant changes with 16:0 hexadecanoic acid augmentations by 80% and 18:0 Octadecanoic acid by 165% when compared to wild type (WT). Increase in the steryl ester and TAG content supports the accumulation of lipid bodies in ymr210wΔ strain when compared with WT cells.

Identification of new channels by systematic analysis of the mitochondrial outer membrane

Channels in the mitochondrial outer membrane exchange metabolites, ions, and proteins with the rest of the cell. Kruger et al. identify several new types of channel and suggest that the outer mitochondrial membrane is a more selective molecular sieve with a greater variety of channel-forming proteins than previously appreciated.

The mitochondrial outer membrane is essential for communication between mitochondria and the rest of the cell and facilitates the transport of metabolites, ions, and proteins. All mitochondrial outer membrane channels known to date are β-barrel membrane proteins, including the abundant voltage-dependent anion channel and the cation-preferring protein-conducting channels Tom40, Sam50, and Mdm10. We analyzed outer membrane fractions of yeast mitochondria and identified four new channel activities: two anion-preferring channels and two cation-preferring channels. We characterized the cation-preferring channels at the molecular level. The mitochondrial import component Mim1 forms a channel that is predicted to have an α-helical structure for protein import. The short-chain dehydrogenase-related protein Ayr1 forms an NADPH-regulated channel. We conclude that the mitochondrial outer membrane contains a considerably larger variety of channel-forming proteins than assumed thus far. These findings challenge the traditional view of the outer membrane as an unspecific molecular sieve and indicate a higher degree of selectivity and regulation of metabolite fluxes at the mitochondrial boundary.

Analysis of the lipid body proteome of the oleaginous alga Lobosphaera incisa

BACKGROUND

Lobosphaera incisa (L. incisa) is an oleaginous microalga that stores triacylglycerol (TAG) rich in arachidonic acid in lipid bodies (LBs). This organelle is gaining attention in algal research, since evidence is accumulating that proteins attached to its surface fulfill important functions in TAG storage and metabolism.

RESULTS

Here, the composition of the LB proteome in L incisa was investigated by comparing different cell fractions in a semiquantitative proteomics approach. After applying stringent filters to the proteomics data in order to remove contaminating proteins from the list of possible LB proteins (LBPs), heterologous expression of candidate proteins in tobacco pollen tubes, allowed us to confirm 3 true LBPs: A member of the algal Major Lipid Droplet Protein family, a small protein of unknown function and a putative lipase. In addition, a TAG lipase that belongs to the SUGAR DEPENDENT 1 family of TAG lipases known from oilseed plants was identified. Its activity was verified by functional complementation of an Arabidopsis thaliana mutant lacking the major seed TAG lipases.

CONCLUSIONS

Here we describe 3 LBPs as well as a TAG lipase from the oleaginous microalga L. incisa and discuss their possible involvement in LB metabolism. This study highlights the importance of filtering LB proteome datasets and verifying the subcellular localization one by one, so that contaminating proteins can be recognized as such. Our dataset can serve as a valuable resource in the identification of additional LBPs, shedding more light on the intriguing roles of LBs in microalgae.

The protein and neutral lipid composition of lipid droplets isolated from the fission yeast, Schizosaccharomyces pombe

Lipid droplets consist of a core of neutral lipids surrounded by a phospholipid monolayer with bound proteins. Much of the information on lipid droplet function comes from proteomic and lipodomic studies that identify the components of droplets isolated from organisms throughout the phylogenetic tree. Here, we add to that important inventory by reporting lipid droplet factors from the fission yeast, Schizosaccharomyces pombe. Unique to this study was the fact that cells were cultured in three different environments: 1) late log growth phase in glucose-based media, 2) stationary phase in glucosebased media, and 3) late log growth phase in media containing oleic acid. We confirmed colocalization of major factors with lipid droplets using live-cell fluorescent microscopy. We also analyzed droplets from each of the three conditions for sterol ester (SE) and triacylglycerol (TAG) content, along with their respective fatty acid compositions. We identified a previously undiscovered lipid droplet protein, Vip1p, which affects droplet size distribution. The results provide further insight into the workings of these ubiquitous organelles.

Molecular characterization of human ABHD2 as TAG lipase and ester hydrolase

Alterations in lipid metabolism have been progressively documented as a characteristic property of cancer cells. Though, human ABHD2 gene was found to be highly expressed in breast and lung cancers, its biochemical functionality is yet uncharacterized. In the present study we report, human ABHD2 as triacylglycerol (TAG) lipase along with ester hydrolysing capacity. Sequence analysis of ABHD2 revealed the presence of conserved motifs G²⁰⁵XS²⁰⁷XG²⁰⁹ and H¹²⁰XXXXD¹²⁵. Phylogenetic analysis showed homology to known lipases, Drosophila melanogaster CG3488. To evaluate the biochemical role, recombinant ABHD2 was expressed in Saccharomyces cerevisiae using pYES2/CT vector and His-tag purified protein showed TAG lipase activity. Ester hydrolase activity was confirmed with pNP acetate, butyrate and palmitate substrates respectively. Further, the ABHD2 homology model was built and the modelled protein was analysed based on the RMSD and root mean square fluctuation (RMSF) of the 100 ns simulation trajectory. Docking the acetate, butyrate and palmitate ligands with the model confirmed covalent binding of ligands with the Ser²⁰⁷ of the GXSXG motif. The model was validated with a mutant ABHD2 developed with alanine in place of Ser²⁰⁷ and the docking studies revealed loss of interaction between selected ligands and the mutant protein active site. Based on the above results, human ABHD2 was identified as a novel TAG lipase and ester hydrolase.

MGL2/YMR210w encodes a monoacylglycerol lipase in Saccharomyces cerevisiae

In silico analysis of the uncharacterized open reading frame YMR210w in Saccharomyces cerevisiae revealed that it possesses both an α/β hydrolase domain (ABHD) and a typical lipase (GXSXG) motif. The purified protein displayed monoacylglycerol (MAG) lipase activity and preferred palmitoyl-MAG. Overexpression of YMR210w in the known MAG lipase mutant yju3Δ clearly revealed that the protein had MAG lipase activity, hence we named the ORF MGL2. Overexpression of YMR210w decreased the cellular triacylglycerol levels. Analysis of the overexpressed strains showed reduction in the lipid droplets number and size. Phenotype studies revealed that the double deletion yju3Δmgl2Δ displayed a growth defect that was partially restored by MGL2 overexpression.

An Energy-Independent Pro-longevity Function of Triacylglycerol in Yeast

Intracellular triacylglycerol (TAG) is a ubiquitous energy storage lipid also involved in lipid homeostasis and signaling. Comparatively, little is known about TAG’s role in other cellular functions. Here we show a pro-longevity function of TAG in the budding yeast Saccharomyces cerevisiae. In yeast strains derived from natural and laboratory environments a correlation between high levels of TAG and longer chronological lifespan was observed. Increased TAG abundance through the deletion of TAG lipases prolonged chronological lifespan of laboratory strains, while diminishing TAG biosynthesis shortened lifespan without apparently affecting vegetative growth. TAG-mediated lifespan extension was independent of several other known stress response factors involved in chronological aging. Because both lifespan regulation and TAG metabolism are conserved, this cellular pro-longevity function of TAG may extend to other organisms.

Triacylglycerol (TAG) is a ubiquitous lipid species well-known for its roles in storing surplus energy, providing insulation, and maintaining cellular lipid homeostasis. Here we present evidence for a novel pro-longevity function of TAG in the budding yeast, a model organism for aging research. Yeast cells that are genetically engineered to store more TAG live significantly longer without suffering obvious growth defects, whereas those lean cells that are depleted of TAG die early. Yeast strains isolated from the wild in general contain more fat and also display longer lifespan. One of the approaches taken here to force the increase of intracellular TAG is to delete lipases responsible for lipid hydrolysis. Energy extraction from TAG thus is unlikely an underlying cause of the observed lifespan extension. Our results are reminiscent of certain animal studies linking higher body fat to longer lifespan. Potential mechanisms for the connection of TAG and yeast lifespan regulation are discussed.

A Yeast Mutant Deleted of GPH1 Bears Defects in Lipid Metabolism

In a previous study we demonstrated up-regulation of the yeast GPH1 gene under conditions of phosphatidylethanolamine (PE) depletion caused by deletion of the mitochondrial (M) phosphatidylserine decarboxylase 1 (PSD1) (Gsell et al., 2013, PLoS One. 8(10):e77380. doi: 10.1371/journal.pone.0077380). Gph1p has originally been identified as a glycogen phosphorylase catalyzing degradation of glycogen to glucose in the stationary growth phase of the yeast. Here we show that deletion of this gene also causes decreased levels of phosphatidylcholine (PC), triacylglycerols and steryl esters. Depletion of the two non-polar lipids in a Δgph1 strain leads to lack of lipid droplets, and decrease of the PC level results in instability of the plasma membrane. In vivo labeling experiments revealed that formation of PC via both pathways of biosynthesis, the cytidine diphosphate (CDP)-choline and the methylation route, is negatively affected by a Δgph1 mutation, although expression of genes involved is not down regulated. Altogether, Gph1p besides its function as a glycogen mobilizing enzyme appears to play a regulatory role in yeast lipid metabolism.

Lipid droplets and their component triglycerides and steryl esters regulate autophagosome biogenesis

Autophagy is a major catabolic process responsible for the delivery of proteins and organelles to the lysosome/vacuole for degradation. Malfunction of this pathway has been implicated in numerous pathological conditions. Different organelles have been found to contribute to the formation of autophagosomes, but the exact mechanism mediating this process remains obscure. Here, we show that lipid droplets (LDs) are important for the regulation of starvation-induced autophagy. Deletion of Dga1 and Lro1 enzymes responsible for triacylglycerol (TAG) synthesis, or of Are1 and Are2 enzymes responsible for the synthesis of steryl esters (STE), results in the inhibition of autophagy. Moreover, we identified the STE hydrolase Yeh1 and the TAG lipase Ayr1 as well as the lipase/hydrolase Ldh1 as essential for autophagy. Finally, we provide evidence that the ER-LD contact-site proteins Ice2 and Ldb16 regulate autophagy. Our study thus highlights the importance of lipid droplet dynamics for the autophagic process under nitrogen starvation.

Regulatory link between steryl ester formation and hydrolysis in the yeast Saccharomyces cerevisiae

Steryl esters and triacylglycerols are the major storage lipids of the yeast Saccharomyces cerevisiae. Steryl esters are formed in the endoplasmic reticulum by the two acyl-CoA:sterol acyltransferases Are1p and Are2p, whereas steryl ester hydrolysis is catalyzed by the three steryl ester hydrolases Yeh1p, Yeh2p and Tgl1p. To shed light on the regulatory link between steryl ester formation and hydrolysis in the maintenance of cellular sterol and free fatty acid levels we employed yeast mutants which lacked the enzymes catalyzing the degradation of steryl esters. These studies revealed feedback regulation of steryl ester formation by steryl ester hydrolysis although in a Δtgl1Δyeh1Δyeh2 triple mutant the gene expression levels of ARE1 and ARE2 as well as protein levels and stability of Are1p and Are2p were not altered. Nevertheless, the capacity of the triple mutant to synthesize steryl esters was significantly reduced as shown by in vitro and in vivo labeling of lipids with [(14)C]oleic acid and [(14)C]acetate. Enzymatic analysis revealed that inhibition of steryl ester formation occurred at the enzyme level. As the amounts and the formation of sterols and fatty acids were also decreased in the triple mutant we concluded that defects in steryl ester hydrolysis also caused feedback inhibition on the formation of sterols and fatty acids which serve as precursors for steryl ester formation. In summary, this study demonstrates a regulatory link within the steryl ester metabolic network which contributes to non-polar lipid homeostasis in yeast cells.

Lipid droplet dynamics in budding yeast

Eukaryotic cells store excess fatty acids as neutral lipids, predominantly triacylglycerols and sterol esters, in organelles termed lipid droplets (LDs) that bulge out from the endoplasmic reticulum. LDs are highly dynamic and contribute to diverse cellular functions. The catabolism of the storage lipids within LDs is channeled to multiple metabolic pathways, providing molecules for energy production, membrane building blocks, and lipid signaling. LDs have been implicated in a number of protein degradation and pathogen infection processes. LDs may be linked to prevalent human metabolic diseases and have marked potential for biofuel production. The knowledge accumulated on LDs in recent years provides a foundation for diverse, and even unexpected, future research. This review focuses on recent advances in LD research, emphasizing the diverse physiological roles of LDs in the model system of budding yeast.

ROG1 encodes a monoacylglycerol lipase in Saccharomyces cerevisiae

Lipid metabolism is extensively studied in Saccharomyces cerevisiae. Here, we report that revertant of glycogen synthase kinase mutation-1 (Rog1p) possesses monoacylglycerol (MAG) lipase activity in S. cerevisiae. The lipase activity of Rog1p was confirmed in two ways: through analysis of a strain with a double deletion of ROG1 and monoglyceride lipase YJU3 (yju3Δrog1Δ) and by site-directed mutagenesis of the ROG1 lipase motif (GXSXG). Rog1p is localized in both the cytosol and the nucleus. Overexpression of ROG1 in a ROG1-deficient strain resulted in an accumulation of reactive oxygen species. These results suggest that Rog1p is a MAG lipase that regulates lipid homeostasis.

Modifications of the C terminus affect functionality and stability of yeast triacylglycerol lipase Tgl3p

Lipid droplets are specific organelles for the storage of triacylglycerols and steryl esters. They are surrounded by a phospholipid monolayer with a small but specific set of proteins embedded. Assembly and insertion of proteins into this surface membrane is an intriguing question of lipid droplet biology. To address this question we studied the topology of Tgl3p, the major triacylglycerol lipase of the yeast Saccharomyces cerevisiae, on lipid droplets. Employing the method of limited proteolysis of lipid droplet surface proteins, we found that the C terminus of Tgl3p faces the inside of the organelle, whereas the N terminus is exposed at the cytosolic side of lipid droplets. Detailed analysis of the C terminus revealed a stretch of seven amino acids that are critical for protein stability and functionality. The negative charge of two aspartate residues within this stretch is crucial for lipase activity of Tgl3p. A portion of Tgl3p, which is located to the endoplasmic reticulum, exhibits a different topology. In the phospholipid bilayer of the endoplasmic reticulum the C terminus faces the cytosol, which results in instability of the protein. Thus, the topology of Tgl3p is important for its function and strongly dependent on the membrane environment.

Storage lipids of yeasts: a survey of nonpolar lipid metabolism in Saccharomyces cerevisiae, Pichia pastoris, and Yarrowia lipolytica

Biosynthesis and storage of nonpolar lipids, such as triacylglycerols (TG) and steryl esters (SE), have gained much interest during the last decades because defects in these processes are related to severe human diseases. The baker's yeast Saccharomyces cerevisiae has become a valuable tool to study eukaryotic lipid metabolism because this single-cell microorganism harbors many enzymes and pathways with counterparts in mammalian cells. In this article, we will review aspects of TG and SE metabolism and turnover in the yeast that have been known for a long time and combine them with new perceptions of nonpolar lipid research. We will provide a detailed insight into the mechanisms of nonpolar lipid synthesis, storage, mobilization, and degradation in the yeast S. cerevisiae. The central role of lipid droplets (LD) in these processes will be addressed with emphasis on the prevailing view that this compartment is more than only a depot for TG and SE. Dynamic and interactive aspects of LD with other organelles will be discussed. Results obtained with S. cerevisiae will be complemented by recent investigations of nonpolar lipid research with Yarrowia lipolytica and Pichia pastoris. Altogether, this review article provides a comprehensive view of nonpolar lipid research in yeast.