Oleate inhibits steryl ester synthesis and causes liposensitivity in yeast

Lipid droplets control the negative effect of non-yeast sterols in membranes of Saccharomyces cerevisiae under hypoxic stress

The effectivity of utilization of exogenous sterols in the yeast Saccharomyces cerevisiae exposed to hypoxic stress is dependent on the sterol structure. The highly imported sterols include animal cholesterol or plant sitosterol, while ergosterol, typical of yeasts, is imported to a lesser extent. An elevated utilization of non-yeast sterols is associated with their high esterification and relocalization to lipid droplets (LDs). Here we present data showing that LDs and sterol esterification play a critical role in the regulation of the accumulation of non-yeast sterols in membranes. Failure to form LDs during anaerobic growth in media supplemented with cholesterol or sitosterol resulted in an extremely long lag phase, in contrast to normal growth in media with ergosterol or plant stigmasterol. Moreover, in hem1∆, which mimics anaerobiosis, neither cholesterol nor sitosterol supported the growth in an LD-less background. The incorporation of non-ergosterol sterols into the membranes affected fundamental membrane characteristics such as relative membrane potential, permeability, tolerance to osmotic stress and the formation of membrane domains. Our findings reveal that LDs assume an important role in scenarios wherein cells are dependent on the utilization of exogenous lipids, particularly under anoxia. Given the diverse lipid structures present in yeast niches, LDs fulfil a protective role, mitigating the risk of excessive accumulation of potentially toxic steroids and fatty acids in the membranes. Finally, we present a novel function for sterols in a model eukaryotic cell - alleviation of the lipotoxicity of unsaturated fatty acids.

Functions and substrate selectivity of diacylglycerol acyltransferases from Mortierella alpina

Mortierella alpina produces various polyunsaturated fatty acids in the form of triacylglycerols (TAG). Diacylglycerol acyltransferase (DGAT) catalyzes the binding of acyl-CoA to diacylglycerol to form TAG and is the key enzyme involved in TAG synthesis. A variety of DGATs are present in M. alpina; however, comparative analysis of the functional properties and substrate selectivity of these DGATs is insufficient. In this study, DGAT1 (MaDGAT1A/1B/1C) and DGAT2 (MaDGAT2A/2B) isoforms from M. alpina were analyzed and heterologously expressed in S. cerevisiae H1246. The results showed that MaDGAT1A/1B/2A/2B were able to restore TAG synthesis, and the corresponding TAG content in recombinant yeasts was 2.92 ± 0.42%, 3.62 ± 0.22%, 0.86 ± 0.34%, and 0.18 ± 0.09%, respectively. In S. cerevisiae H1246, MaDGAT1A preferred C16:1 among monounsaturated fatty acids, MaDGAT1B preferred C16:0 among saturated fatty acids (SFAs), and MaDGAT2A/2B preferred C18:0 among SFAs. Under exogenous addition of polyunsaturated fatty acids (PUFAs), MaDGAT1A and 2A preferentially assembled linoleic acid into TAG, and MaDGAT2B had substrate selectivity for eicosapentaenoic and linoleic acids in ω-6 PUFAs. In vitro, MaDGAT1A showed no obvious acyl-CoA selectivity and MaDGAT1B preferred C20:5-CoA. MaDGAT1A/1B preferred C18:1/C18:1-DAG compared with C20:4/C20:4-DAG. This study indicates that MaDGATs have the potential to be used in the production of LA/EPA-rich TAG and provide a reference for improving the production of TAGs in oleaginous fungi. KEY POINTS: • MaDGAT1A preferred C16:1 among MUFAs, MaDGAT1B and MaDGAT2A/2B preferred C16:0 and C18:0 among SFAs, respectively • MaDGAT1A/2A preferentially assembled linoleic acid into TAG, and MaDGAT2B has substrate selectivity for eicosapentaenoic acid and linoleic acid in ω-6 PUFAs • MaDGAT1A showed no obvious acyl-CoA selectivity, and MaDGAT1B preferred C20:5-CoA. MaDGAT1A/1B preferred to select C18:1/C18:1-DAG compared with C20:4/C20:4-DAG.

Genetic manipulation of the interconversion between diacylglycerols and triacylglycerols in Rhodosporidium toruloides

The basidiomycetous yeast Rhodosporidium toruloides (R. toruloides) is an excellent producer for neutral lipids, including triacylglycerols (TAG). Partially because genetic tools for this yeast were less developed, limited efforts were shown to explore its capacity for the production of higher-value lipids such as diacylglycerols (DAG). Here, four genes linked to the interconversion between DAG and TAG were manipulated to promote the production of DAG and free fatty acids (FFA). Among them, three TAG synthesis-related genes, DGA1, LRO1, and ARE1, were down-regulated successively via the RNA interference technology, and an endogenous TAG lipase encoded by TGL5 was fused with LDP1 and over-expressed to convert TAG into DAG and FFA. Results showed that those engineered R. toruloides strains grew normally under nutrient-rich conditions but notably slower than the parental strain NP11 in the lipid production stage. When cultivated in nitrogen-limited media, engineered strains were able to produce total lipids with improved contents of DAG and FFA by up to two-fold and three-fold, respectively. Further correlation analysis between lipid composition and cell density indicated that the formation of TAG correlated positively with cell growth; however, other lipids including DAG did negatively. This study offered valuable information and strains to engineer R. toruloides for advanced production of fatty acid derivatives.

Seipin concentrates distinct neutral lipids via interactions with their acyl chain carboxyl esters

Lipid droplets (LDs) are essential for cellular lipid homeostasis by storing diverse neutral lipids (NLs), such as triacylglycerol (TAG), steryl esters (SE), and retinyl esters (RE). A proper assembly of TAG-containing LDs at the ER requires Seipin, a conserved protein often mutated in lipodystrophies. Here, we show that the yeast Seipin Sei1 and its partner Ldb16 also promote the storage of other NL in LDs. Importantly, this role of Sei1/Ldb16 is evolutionarily conserved as expression of human-Seipin restored normal SE-containing LDs in yeast Seipin mutants. As in the case of TAG, the formation of SE-containing LDs requires interactions between hydroxyl-residues in human Seipin or yeast Ldb16 with NL carboxyl esters. These findings provide a universal mechanism for Seipin-mediated LD formation and suggest a model for how Seipin distinguishes NLs from aliphatic phospholipid acyl chains in the center of the membrane bilayer.

Acyl-CoA:diacylglycerol acyltransferase: Properties, physiological roles, metabolic engineering and intentional control

Acyl-CoA:diacylglycerol acyltransferase (DGAT, EC 2.3.1.20) catalyzes the last reaction in the acyl-CoA-dependent biosynthesis of triacylglycerol (TAG). DGAT activity resides mainly in membrane-bound DGAT1 and DGAT2 in eukaryotes and bifunctional wax ester synthase-diacylglycerol acyltransferase (WSD) in bacteria, which are all membrane-bound proteins but exhibit no sequence homology to each other. Recent studies also identified other DGAT enzymes such as the soluble DGAT3 and diacylglycerol acetyltransferase (EaDAcT), as well as enzymes with DGAT activities including defective in cuticular ridges (DCR) and steryl and phytyl ester synthases (PESs). This review comprehensively discusses research advances on DGATs in prokaryotes and eukaryotes with a focus on their biochemical properties, physiological roles, and biotechnological and therapeutic applications. The review begins with a discussion of DGAT assay methods, followed by a systematic discussion of TAG biosynthesis and the properties and physiological role of DGATs. Thereafter, the review discusses the three-dimensional structure and insights into mechanism of action of human DGAT1, and the modeled DGAT1 from Brassica napus. The review then examines metabolic engineering strategies involving manipulation of DGAT, followed by a discussion of its therapeutic applications. DGAT in relation to improvement of livestock traits is also discussed along with DGATs in various other eukaryotic organisms.

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 Malassezia Lipidome Disclosed Differences Among the Species and Reveals Presence of Unusual Yeast Lipids

Malassezia yeasts are lipid dependent and part of the human and animal skin microbiome. However, they are also associated with a variety of dermatological conditions and even cause systemic infections. How these yeasts can live as commensals on the skin and switch to a pathogenic stage has long been a matter of debate. Lipids are important cellular molecules, and understanding the lipid metabolism and composition of Malassezia species is crucial to comprehending their biology and host-microbe interaction. Here, we investigated the lipid composition of Malassezia strains grown to the stationary phase in a complex Dixon medium broth. In this study, we perform a lipidomic analysis of a subset of species; in addition, we conducted a gene prediction analysis for the detection of lipid metabolic proteins. We identified 18 lipid classes and 428 lipidic compounds. The most commonly found lipids were triglycerides (TAG), sterol (CH), diglycerides (DG), fatty acids (FAs), phosphatidylcholine (PC), phosphatidylethanolamine (PE), ceramides, cholesteryl ester (CE), sphingomyelin (SM), acylcarnitine, and lysophospholipids. Particularly, we found a low content of CEs in Malassezia furfur, atypical M. furfur, and Malassezia pachydermatis and undetectable traces of these components in Malassezia globosa, Malassezia restricta, and Malassezia sympodialis. Remarkably, uncommon lipids in yeast, like diacylglyceryltrimethylhomoserine and FA esters of hydroxyl FAs, were found in a variable concentration in these Malassezia species. The latter are bioactive lipids recently reported to have antidiabetic and anti-inflammatory properties. The results obtained can be used to discriminate different Malassezia species and offer a new overview of the lipid composition of these yeasts. We could confirm the presence and the absence of certain lipid-biosynthesis genes in specific species. Further analyses are necessary to continue disclosing the complex lipidome of Malassezia species and the impact of the lipid metabolism in connection with the host interaction.

Triglyceride deficiency and diacylglycerol kinase1 activity lead to the upregulation of mevalonate pathway in yeast: A study for the development of potential yeast platform for improved production of triterpenoid

Besides energy storage and membrane biogenesis, lipids are known for their numerous biological functions. The two essential lipids, diacylglycerol (DG) and phosphatidic acid (PA), are shown to be associated with cell signalling processes. In this study, we examined whether triglyceride-deficient yeast mutants (tgΔ), dga1Δ and dga1Δlro1Δ, may play an important role in mevalonate (MEV) pathway regulation. Our metabolite analyses revealed that tgΔ cells showed high levels of squalene (SQ) and ergosterol (ERG), which are key indicators of MEV pathway activity. In addition, gene expression studies indicated that the MEV pathway genes in tgΔ cells were significantly upregulated. Interestingly, tgΔ cells exhibited high diacylglycerol kinase1 (DGK1) expression. Furthermore, DGK1 overexpression in WT and tgΔ phenotypes causes a substantial elevation in SQ and ERG levels, and we also found a significant increase in transcript levels of MEV pathway genes, confirming the new role of DGK1 in MEV pathway regulation. This suggests that high DG phosphorylation activity increases the PA pool that may induce the upregulation of MEV pathway in tgΔ cells. The induced MEV pathway is one of the key strategies in the field of synthetic biology for improved production of terpenoids in yeast. Thus, to examine whether increased endogenous MEV pathway flux can be redirected to triterpenoid, β-Amyrin synthase gene was heterologously expressed in DGK1 overexpressing tgΔ cells that led to significant production of β-Amyrin, a natural triterpenoid. In conclusion, our findings provide a novel strategy to increase MEV pathway precursors by modulating endogenous signal lipids for improved production of terpenoids.

Phosphatidate-mediated regulation of lipid synthesis at the nuclear/endoplasmic reticulum membrane

In yeast and higher eukaryotes, phospholipids and triacylglycerol are derived from phosphatidate at the nuclear/endoplasmic reticulum membrane. In de novo biosynthetic pathways, phosphatidate is channeled into membrane phospholipids via its conversion to CDP-diacylglycerol. Its dephosphorylation to diacylglycerol is required for the synthesis of triacylglycerol as well as for the synthesis of phosphatidylcholine and phosphatidylethanolamine via the Kennedy pathway. In addition to the role of phosphatidate as a precursor, it is a regulatory molecule in the transcriptional control of phospholipid synthesis genes via the Henry regulatory circuit. Pah1 phosphatidate phosphatase and Dgk1 diacylglycerol kinase are key players that function counteractively in the control of the phosphatidate level at the nuclear/endoplasmic reticulum membrane. Loss of Pah1 phosphatidate phosphatase activity not only affects triacylglycerol synthesis but also disturbs the balance of the phosphatidate level, resulting in the alteration of lipid synthesis and related cellular defects. The pah1Δ phenotypes requiring Dgk1 diacylglycerol kinase exemplify the importance of the phosphatidate level in the misregulation of cellular processes. The catalytic function of Pah1 requires its translocation from the cytoplasm to the nuclear/endoplasmic reticulum membrane, which is regulated through its phosphorylation in the cytoplasm by multiple protein kinases as well as through its dephosphorylation by the membrane-associated Nem1-Spo7 protein phosphatase complex. This article is part of a Special Issue entitled Endoplasmic reticulum platforms for lipid dynamics edited by Shamshad Cockcroft and Christopher Stefan.

Impact of Selected Cosmetic Ingredients on Common Microorganisms of Healthy Human Skin

Human skin is a complex ecosystem and is host to a large number of microorganisms. When the bacterial ecosystem is balanced and differentiated, skin remains healthy. However, the use of cosmetics can change this balance and promote the appearance of skin diseases. The skin’s microorganisms can utilize some cosmetic components, which either promote their growth, or produce metabolites that influence the skin environment. In this study, we tested the ability of the Malassezia species and some bacterial strains to assimilate substances frequently used in dermal formulations. The growth capability of microorganisms was determined and their lipase activity was analyzed. The growth of all Malassezia spp. in the presence of free acids, free acid esters, and fatty alcohols with a fatty chain length above 12 carbon atoms was observed. No growth was observed in the presence of fatty alcohol ethers, secondary fatty alcohols, paraffin- and silicon-based substances, polymers, polyethylene glycols, quaternary ammonium salts, hydroxy fatty acid esters, or fatty acids and fatty acid esters with a fatty chain length shorter than 12 carbon atoms. The hydrolysis of esters by Malassezia lipases was detected using High Performance Thin Layer Chromatography (HPTLC). The production of free fatty acids as well as fatty alcohols was observed. The growth promotion or inhibition of bacterial strains was only found in the presence of a few ingredients. Based on these results, formulations containing microbiome inert ingredients were developed.

Identification of gene products that control lipid droplet size in yeast using a high-throughput quantitative image analysis

Lipid droplets (LDs) are important organelles involved in energy storage and expenditure. LD dynamics has been investigated using genome-wide image screening methods in yeast and other model organisms. For most studies, genes were identified using two-dimensional images with LD enlargement as readout. Due to imaging limitation, reduction of LD size is seldom explored. Here, we aim to set up a screen that specifically utilizes reduced LD size as the readout. To achieve this, a novel yeast screen is set up to quantitatively and systematically identify genes that regulate LD size through a three-dimensional imaging-based approach. Cidea which promotes LD fusion and growth in mammalian cells was overexpressed in a yeast knockout library to induce large LD formation. Next, an automated, high-throughput image analysis method that monitors LD size was utilized. With this screen, we identified twelve genes that reduced LD size when deleted. The effects of eight of these genes on LD size were further validated in fld1 null strain background. In addition, six genes were previously identified as LD-regulating genes. To conclude, this methodology represents a promising strategy to screen for players in LD size control in both yeast and mammalian cells to aid in the investigation of LD-associated metabolic diseases.

On the current role of hydratases in biocatalysis

Water addition to carbon-carbon double bonds provides access to value-added products from inexpensive organic feedstock. This interesting but relatively little-studied reaction is catalysed by hydratases in a highly regio- and enantiospecific fashion with excellent atom economy. Considering that asymmetric hydration of (non-activated) carbon-carbon double bonds is virtually impossible with current organic chemistry, enzymatic hydration reactions are highly attractive for industrial applications. Hydratases have been known for several decades but their biocatalytic potential has only been explored over the past 15 years. As a result, a considerable amount of information on this enzyme group has become available, enabling their development for practical applications. This review focuses on hydratases catalysing water addition to non-activated carbon-carbon double bonds, and examines hydratases from a biochemical, structural and mechanistic angle. Current challenges and opportunities in hydration biocatalysis are discussed, and, ultimately, their potential for organic synthesis is highlighted.

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.

Effects of glucose concentration on 1,18-cis-octadec-9-enedioic acid biotransformation efficiency and lipid body formation in Candida tropicalis

The unsaturated long-chain α,ω-dicarboxylic acid 1,18-cis-octadec-9-enedioic acid (cis-ODA) is a versatile precursor of various valuable compounds, such as polymers, and can be obtained from renewable resources. This makes cis-ODA highly attractive for the chemical industry where there is a growing interest in sustainable processes. However, chemical synthesis of the cis isomers is currently not feasible. In contrast, biotechnological production allows for highly specific and selective reactions. Therefore, we developed an efficient production strategy for cis-ODA using Candida tropicalis as a whole-cell biocatalyst for the biotransformation of oleic acid, which naturally occurs in various fats and oils. Applying a bench-top system comprising eight parallel bioreactors, the production process was characterised and optimised for high productivity. Glucose feed rate was identified as the most crucial process parameter influencing product yield, with high rates inducing oleic acid incorporation into triacylglycerols and storage in lipid bodies. Conversely, application of medium-chain length fatty acid as a substrate did not show any occurrence of lipid bodies. Applying the lowest possible molar ratio of glucose to oleic acid (1.5) resulted in marginal lipid body formation, but led to a peak volumetric productivity of 0.56 g/L/h and a final titre of approximately 45 g/L with a corresponding yield of 70%.

The impact of nonpolar lipids on the regulation of the steryl ester hydrolases Tgl1p and Yeh1p in the yeast Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae degradation of steryl esters is catalyzed by the steryl ester hydrolases Tgl1p, Yeh1p and Yeh2p. The two steryl ester hydrolases Tgl1p and Yeh1p localize to lipid droplets, a cell compartment storing steryl esters and triacylglycerols. In the present study we investigated regulatory aspects of these two hydrolytic enzymes, namely the gene expression level, protein amount, stability and enzyme activity of Tgl1p and Yeh1p in strains lacking both or only one of the two major nonpolar lipids, steryl esters and triacylglycerols. In a strain lacking both nonpolar lipids and consequently lipid droplets, Tgl1p as well as Yeh1p were present at low amount, became highly unstable compared to wild-type cells, and lost their enzymatic activity. Under these conditions both steryl ester hydrolases were retained in the endoplasmic reticulum. The lack of steryl esters alone was not sufficient to cause an altered intracellular localization of Tgl1p and Yeh1p. Surprisingly, the stability of Tgl1p and Yeh1p was markedly reduced in a strain lacking triacylglycerols, but their capacity to mobilize steryl esters remained unaffected. We also tested a possible cross-regulation of Tgl1p and Yeh1p by analyzing the behavior of each hydrolase in the absence of its counterpart steryl ester hydrolases. In summary, this study demonstrates a strong regulation of the two lipid droplet associated steryl ester hydrolases Tgl1p and Yeh1p due to the presence/absence of their host organelle.

Yeast ENV9 encodes a conserved lipid droplet (LD) short-chain dehydrogenase involved in LD morphology

Lipid droplets (LDs) have emerged as dynamic and interactive organelles with important roles in lipid metabolism and membrane biogenesis. Here, we report that Saccharomyces cerevisiae Env9 is a novel conserved oxidoreductase involved in LD morphology. Microscopic and biochemical studies confirm localization of tagged Env9 to LDs and implicate its C-terminal hydrophobic domain (aa241-265) in its membrane association and stability. Confocal studies reveal a role for Env9 in LD morphology. Env9 positively affects both formation of large LDs upon overexpression and LD proliferation under poor carbon source. In silico bioinformatic and modeling approaches establish that ENV9 is a widely conserved member of the short-chain dehydrogenase (SDR) superfamily. Bayesian phylogenetic studies strongly support ENV9 as an ortholog of human SDR retinol dehydrogenase 12 (RDH12). Dehydrogenase activity of Env9 was confirmed by in vitro oxidoreductase assays. RDH12 mutations have been linked to Leber Congenital Amaurosis. Similar site-directed point mutations in the predicted Env9 oxidoreductase active site (N146L) or cofactor-binding site (G23-24A) abolished its reductase activity in vitro, consistent with those reported in other retinol dehydrogenases. The same residues were essential for affecting LD size and number in vivo. Taken together, our results implicate oxidoreductase activity of Env9 in its cellular role in LD morphology.

The pathogen-related yeast protein Pry1, a member of the CAP protein superfamily, is a fatty acid-binding protein

Members of the CAP superfamily (cysteine-rich secretory proteins, antigen 5, and pathogenesis-related 1 proteins), also known as SCP superfamily (sperm-coating proteins), have been implicated in many physiological processes, including immune defenses, venom toxicity, and sperm maturation. Their mode of action, however, remains poorly understood. Three proteins of the CAP superfamily, Pry1, -2, and -3 (pathogen related in yeast), are encoded in the Saccharomyces cerevisiae genome. We have shown previously that Pry1 binds cholesterol in vitro and that Pry function is required for sterol secretion in yeast cells, indicating that members of this superfamily may generally bind sterols or related small hydrophobic compounds. On the other hand, tablysin-15, a CAP protein from the horsefly Tabanus yao, has been shown to bind leukotrienes and free fatty acids in vitro Therefore, here we assessed whether the yeast Pry1 protein binds fatty acids. Computational modeling and site-directed mutagenesis indicated that the mode of fatty acid binding is conserved between tablysin-15 and Pry1. Pry1 bound fatty acids with micromolar affinity in vitro, and its function was essential for fatty acid export in cells lacking the acyl-CoA synthetases Faa1 and Faa4. Fatty acid binding of Pry1 is independent of its capacity to bind sterols, and the two sterol- and fatty acid-binding sites are nonoverlapping. These results indicate that some CAP family members, such as Pry1, can bind different lipids, particularly sterols and fatty acids, at distinct binding sites, suggesting that the CAP domain may serve as a stable, secreted protein domain that can accommodate multiple ligand-binding sites.

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.

Trans 18-carbon monoenoic fatty acid has distinct effects from its isomeric cis fatty acid on lipotoxicity and gene expression in Saccharomyces cerevisiae

Epidemiological studies have suggested that an excess intake of trans-unsaturated fatty acids increases the risk of coronary heart disease. However, the mechanisms of action of trans-unsaturated fatty acids in eukaryotic cells remain unclear. Since the budding yeast Saccharomyces cerevisiae can grow using fatty acids as the sole carbon source, it is a simple and suitable model organism for understanding the effects of trans-unsaturated fatty acids at the molecular and cellular levels. In this study, we compared the physiological effects of Δ9 cis and trans 18-carbon monoenoic fatty acids (oleic acid and elaidic acid) in yeast cells. The results obtained revealed that the two types have distinct effects on the expression of OLE1, which encodes Δ9 desaturase, and lipotoxicity in are1Δare2Δdga1Δlro1Δ and gat1Δ cells. Our results suggest that cis and trans 18-carbon monoenoic fatty acids exert different physiological effects in the regulation of gene expression and processing of excess fatty acids in yeast.

The Phospholipid:Diacylglycerol Acyltransferase Lro1 Is Responsible for Hepatitis C Virus Core-Induced Lipid Droplet Formation in a Yeast Model System

Chronic infection with the hepatitis C virus frequently induces steatosis, which is a significant risk factor for liver pathogenesis. Steatosis is characterized by the accumulation of lipid droplets in hepatocytes. The structural protein core of the virus induces lipid droplet formation and localizes on the surface of the lipid droplets. However, the precise molecular mechanisms for the core-induced formation of lipid droplets remain elusive. Recently, we showed that the expression of the core protein in yeast as a model system could induce lipid droplet formation. In this study, we probed the cellular factors responsible for the formation of core-induced lipid-droplets in yeast cells. We demonstrated that one of the enzymes responsible for triglyceride synthesis, a phospholipid:diacylglycerol acyltransferase (Lro1), is required for the core-induced lipid droplet formation. While core proteins inhibit Lro1 degradation and alter Lro1 localization, the characteristic localization of Lro1 adjacent to the lipid droplets appeared to be responsible for the core-induced lipid droplet formation. RNA virus genomes have evolved using high mutation rates to maintain their ability to replicate. Our observations suggest a functional relationship between the core protein with hepatocytes and yeast cells. The possible interactions between core proteins and the endoplasmic reticulum membrane affect the mobilization of specific proteins.

Regulation of the yeast triacylglycerol lipases Tgl4p and Tgl5p by the presence/absence of nonpolar lipids

Tgl4p and Tgl5p are two yeast lipid droplet proteins exhibiting triacylglycerol lipase and lysophospholipid acyltransferase activity. In the absence of lipid droplets, Tgl4p and Tgl5p are retained in the ER, where their properties change. Thus triacylglycerol substrate availability is an important regulator of Tgl4p and Tgl5p functionality.

Tgl3p, Tgl4p, and Tgl5p are the major triacylglycerol lipases of the yeast Saccharomyces cerevisiae. Recently we demonstrated that properties of Tgl3p are regulated by the formation of nonpolar lipids. The present study extends these investigations to the two other yeast triacylglycerol lipases, Tgl4p and Tgl5p. We show that Tgl4p and Tgl5p, which are localized to lipid droplets in wild type, are partially retained in the endoplasmic reticulum in cells lacking triacylglycerols and localize exclusively to the endoplasmic reticulum in a mutant devoid of lipid droplets. In cells lacking steryl esters, the subcellular distribution of Tgl4p and Tgl5p is unaffected, but Tgl5p becomes unstable, whereas the stability of Tgl4p increases. In cells lacking nonpolar lipids, Tgl4p and Tgl5p lose their lipolytic activity but retain their side activity as lysophospholipid acyltransferases. To investigate the regulatory network of yeast triacylglycerol lipases in more detail, we also examined properties of Tgl3p, Tgl4p, and Tgl5p, respectively, in the absence of the other lipases. Surprisingly, lack of two lipases did not affect expression, localization, and stability of the remaining Tgl protein. These results suggest that Tgl3p, Tgl4p, and Tgl5p, although they exhibit similar functions, act as independent entities.

Lipid Droplets Form from Distinct Regions of the Cell in the Fission Yeast Schizosaccharomyces pombe

Eukaryotic cells store cholesterol/sterol esters (SEs) and triacylglycerols (TAGs) in lipid droplets, which form from the contiguous endoplasmic reticulum (ER) network. However, it is not known if droplets preferentially form from certain regions of the ER over others. Here, we used fission yeast Schizosaccharomyces pombe cells where the nuclear and cortical/peripheral ER domains are distinguishable by light microscopy to show that SE-enriched lipid droplets form away from the nucleus at the cell tips, whereas TAG-enriched lipid droplets form around the nucleus. Sterols localize to the regions of the cells where droplets enriched in SEs are observed. TAG droplet formation around the nucleus appears to be a strong function of diacylglycerol (DAG) homeostasis with Cpt1p, which coverts DAG into phosphatidylcholine and phosphatidylethanolamine localized exclusively to the nuclear ER. Also, Dgk1p, which converts DAG into phosphatidic acid localized strongly to the nuclear ER over the cortical/peripheral ER. We also show that TAG more readily translocates from the ER to lipid droplets than do SEs. The results augment the standard lipid droplet formation model, which has SEs and TAGs flowing into the same nascent lipid droplet regardless of its biogenesis point in the cell.

Yeast MRX deletions have short chronological life span and more triacylglycerols

Saccharomyces cerevisiae is an excellent model organism for lipid research. Here, we have used yeast haploid RAdiation Damage (RAD) deletion strains to study life span and lipid storage patterns. RAD genes are mainly involved in DNA repair mechanism and hence, their deletions have resulted in shorter life span. Viable RAD mutants were screened for non-polar lipid content, and some of the mutants showed significantly high amounts of triacylglycerol (TAG) and steryl ester, besides short chronological life span. Among these, RAD50, MRE11 and XRS2 form a complex, MRX that is involved in homologous recombination that showed an increase in the amount of TAG. Microarray data of single MRX deletions revealed that besides DNA damage signature genes, lipid metabolism genes are also differentially expressed. Lipid biosynthetic genes (LPP1, SLC1) were upregulated and lipid hydrolytic gene (TGL3) was downregulated. We observed that rad50Δ, mre11Δ, xrs2Δ and mrxΔ strains have high number of lipid droplets (LDs) with fragmented mitochondria. These mutants have a short chronological life span compared to wild type. Aged wild-type cells also accumulated TAG with LDs of ∼2.0 μm in diameter. These results suggest that TAG accumulation and big size LDs could be possible markers for premature or normal aging.

Squalene is lipotoxic to yeast cells defective in lipid droplet biogenesis

The toxic effect of overloaded lipids on cell physiology and viability was described in various organisms. In this study we focused on the potential lipotoxicity of squalene, a linear triterpene synthesized in eukaryotic cells as an intermediate in sterol biosynthesis. Squalene toxicity was studied in the yeast Saccharomyces cerevisiae, a model unicellular eukaryote established in lipotoxicity studies. Squalene levels in yeast are typically low but its accumulation can be induced under specific conditions, e.g. by inhibition of squalene monooxygenase with the antimycotic terbinafine. At higher levels squalene is stored in lipid droplets. We demonstrated that low doses of terbinafine caused severe impairment of growth and loss of viability of the yeast mutant dga1Δ lro1Δ are1Δ are2Δ unable to form lipid droplets and that these defects were linked to squalene accumulation. The hypersensitivity of the lipid droplet-less mutant to terbinafine was alleviated by decreasing squalene accumulation with low doses of squalene synthase inhibitor zaragozic acid. Our results proved that accumulated squalene is lipotoxic to yeast cells if it cannot be efficiently sequestered in lipid droplets. This supports the hypothesis about the role of squalene in the fungicidal activity of terbinafine. Squalene toxicity may represent also a limiting factor for production of this high-value lipid in yeast.

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.

Functional role of histone variant Htz1 in the stress response to oleate in Saccharomyces cerevisiae

In response to oleate stress in Saccharomyces cerevisiaes, Histone Two A Z1 (Htz1) undergoes a global redistribution during the glucose-oleate shift. The number of Htz1-bound genes increases, but the number of Htz1-bound ribosome genes decreases with stress. Citrate cycle-associated genes are enhanced and ribosome genes are repressed. Nucleosome dynamics are coupled with Htz1-binding changes upon stress. Multicopy suppressor of SNF1 protein 2 (Msn2) acts an important role in response to the oleate stress. We highlight the dynamics of Htz1 in the oleate stress.

Chromatin structure is implicated in regulating gene transcription in stress response. Transcription factors, transferases and deacetylases, such as multicopy suppressor of SNF1 protein 2 (Msn2), SET domain-containing protein 1 (Set1) and sucrose NonFermenting protein 1 (Snf1), have been identified as key regulators in stress response. In the present study, we reported the dynamics of nucleosome occupancy, Histone Two A Z1 (Htz1) deposition and histone H3 lysine 4 dimethylation (H3K4me2) and histone H3 lysine 79 trimethylation (H3K79me3) in Saccharomyces cerevisiae under oleate stress. Our results indicated that citrate cycle-associated genes are enhanced and ribosome genes are repressed during the glucose-oleate shift. Importantly, Htz1 acts as a sensor for oleate stress. High-throughput ChIP-chip analysis showed that Htz1 has redistributed across the genome during oleate stress. The number of Htz1-bound genes increases with stress and the number of Htz1-bound ribosome genes decreases with stress. The dynamics of Htz1 and H3K79me3 around transcription factor-binding sites correlate with transcriptional changes. Moreover, we found that nucleosome dynamics are coupled with Htz1 binding changes upon stress. In unstressed conditions (2% glucose), nucleosome occupancy is comparable between Htz1-bound genes and Htz1-depleted genes; in stressed conditions (0.2% oleate for 8 h), the nucleosome occupancy of Htz1-depleted genes is significantly lower than that of Htz1-bound genes. We also found that Msn2 acts an important role in response to the oleate stress and Htz1 is dynamic in Msn2-target genes. Htz1 senses the oleate stress and undergoes a global redistribution and this change couples dynamics of nucleosome occupancy. Our analysis suggests that Htz1 and nucleosome dynamics change in response to oleate stress.

Cytosolic lipid droplets: from mechanisms of fat storage to disease

The lipid droplet (LD) is a phylogenetically conserved organelle. In eukaryotes, it is born from the endoplasmic reticulum, but unlike its parent organelle, LDs are the only known cytosolic organelles that are micellar in structure. LDs are implicated in numerous physiological and pathophysiological functions. Many aspects of the LD has captured the attention of diverse scientists alike and has recently led to an explosion in information on the LD biogenesis, expansion and fusion, identification of LD proteomes and diseases associated with LD biology. This review will provide a brief history of this fascinating organelle and provide some contemporary views of unanswered questions in LD biogenesis.

Regulation of serum-responsive transmembrane kinase EhTMKB1-9 by an unsaturated lipid, oleic acid in protistan parasite Entamoeba histolytica

Transmembrane kinases of Entamoeba histolytica are known to play a wide range of roles from virulence, phagocytosis, and proliferation to stress response. Transmembrane kinase EhTMKB1-9 is thought to be involved in early proliferative response and it was originally identified as a serum inducible gene. Ability to stimulate EhTMKB1 expression of serum starved cells resides in unsaturated fatty acids associated with albumin fraction of serum and the mechanism of stimulation follows activation of EhTMKB1-9 promoter. Gel shift assay showed the presence of proteins that bind to the specific site of EhTMKB1-9 upstream region and the concentration of these protein(s) go down on serum starvation, but level of binding protein(s) go up on serum or fatty acid replenishment. This increase in concentration of binding molecule(s) is due to new synthesis rather than activation of existing molecule(s) as a protein synthesis inhibitor blocked enhanced level of gel shifted material on replenishment. The stimulating activity resides in the fatty acyl chain, but not in the head group. Moreover, the fatty acid initiates signaling through class I PI3 kinases that result in activation of EhTMKB1-9 expression. These results suggest a novel mechanism of gene regulation in E. histolytica, and unsaturated fatty acids as potential new signaling molecules.

A sterol-enriched vacuolar microdomain mediates stationary phase lipophagy in budding yeast

During stationary phase growth in yeast, a feed-forward loop exists in which lipophagy stimulates vacuolar microdomain formation, which, in turn, further promotes lipophagy.

Stationary phase (stat-phase) is a poorly understood physiological state under which cells arrest proliferation and acquire resistance to multiple stresses. Lipid droplets (LDs), organelles specialized for cellular lipid homeostasis, increase in size and number at the onset of stat-phase. However, little is known about the dynamics of LDs under this condition. In this paper, we reveal the passage of LDs from perinuclear endoplasmic reticulum association to entry into vacuoles during the transition to stat-phase. We show that the process requires the core autophagy machinery and a subset of autophagy-related (Atg) proteins involved in selective autophagy. Notably, the process that we term stat-phase lipophagy is mediated through a sterol-enriched vacuolar microdomain whose formation and integrity directly affect LD translocation. Intriguingly, cells defective in stat-phase lipophagy showed disrupted vacuolar microdomains, implying that LD contents, likely sterol esters, contribute to the maintenance of vacuolar microdomains. Together, we propose a feed-forward loop in which lipophagy stimulates vacuolar microdomain formation, which in turn promotes lipophagy during stat-phase.

Defects in triacylglycerol lipolysis affect synthesis of triacylglycerols and steryl esters in the yeast

Tgl3p, Tgl4p and Tgl5p are the major triacylglycerol lipases of the yeast Saccharomyces cerevisiae catalyzing degradation of triacylglycerols stored in lipid droplets. Previous results from our laboratory (Athenstaedt and Daum, 2005, J. Biol. Chem. 280, 37301-37309) demonstrated that a yeast strain lacking all three triacylglycerol lipases accumulates not only triacylglycerols at high amount, but also steryl esters. Here we show a metabolic link between synthesis and mobilization of non-polar lipids. In particular, we demonstrate that a block in tri-acylglycerol degradation in a tgl3∆tgl4∆tgl5∆ triple mutant lacking all major triacylglycerol lipases causes marked changes in non-polar lipid synthesis. Under these conditions formation of triacylglycerols is reduced, whereas steryl ester synthesis is enhanced as shown by quantification of non-polar lipids, in vivo labeling of lipids using [(14)C]oleic acid and [(14)C]acetic acid as precursors, and enzyme analyses in vitro. In summary, this study demonstrates that triacylglycerol metabolism and steryl ester metabolism are linked processes. The importance of balanced storage and degradation of these components for lipid homeostasis in the yeast is highlighted.

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.

Dictyostelium discoideum Dgat2 can substitute for the essential function of Dgat1 in triglyceride production but not in ether lipid synthesis

Triacylglycerol (TAG), the common energy storage molecule, is formed from diacylglycerol and a coenzyme A-activated fatty acid by the action of an acyl coenzyme A:diacylglycerol acyltransferase (DGAT). In order to conduct this step, most organisms rely on more than one enzyme. The two main candidates in Dictyostelium discoideum are Dgat1 and Dgat2. We show, by creating single and double knockout mutants, that the endoplasmic reticulum (ER)-localized Dgat1 enzyme provides the predominant activity, whereas the lipid droplet constituent Dgat2 contributes less activity. This situation may be opposite from what is seen in mammalian cells. Dictyostelium Dgat2 is specialized for the synthesis of TAG, as is the mammalian enzyme. In contrast, mammalian DGAT1 is more promiscuous regarding its substrates, producing diacylglycerol, retinyl esters, and waxes in addition to TAG. The Dictyostelium Dgat1, however, produces TAG, wax esters, and, most interestingly, also neutral ether lipids, which represent a significant constituent of lipid droplets. Ether lipids had also been found in mammalian lipid droplets, but the role of DGAT1 in their synthesis was unknown. The ability to form TAG through either Dgat1 or Dgat2 activity is essential for Dictyostelium to grow on bacteria, its natural food substrate.

Phospholipid:diacylglycerol acyltransferase-mediated triacylglycerol biosynthesis is crucial for protection against fatty acid-induced cell death in growing tissues of Arabidopsis

Phospholipid:diacylglycerol acyltransferase (PDAT) and diacylglycerol:acyl CoA acyltransferase play overlapping roles in triacylglycerol (TAG) assembly in Arabidopsis, and are essential for seed and pollen development, but the functional importance of PDAT in vegetative tissues remains largely unknown. Taking advantage of the Arabidopsis tgd1-1 mutant that accumulates oil in vegetative tissues, we demonstrate here that PDAT1 is crucial for TAG biosynthesis in growing tissues. We show that disruption of PDAT1 in the tgd1-1 mutant background causes serious growth retardation, gametophytic defects and premature cell death in developing leaves. Lipid analysis data indicated that knockout of PDAT1 results in increases in the levels of free fatty acids (FFAs) and diacylglycerol. In vivo ¹⁴C-acetate labeling experiments showed that, compared with wild-type, tgd1-1 exhibits a 3.8-fold higher rate of fatty acid synthesis (FAS), which is unaffected by disruption or over-expression of PDAT1, indicating a lack of feedback regulation of FAS in tgd1-1. We also show that detached leaves of both pdat1-2 and tgd1-1 pdat1-2 display increased sensitivity to FFA but not to diacylglycerol. Taken together, our results reveal a critical role for PDAT1 in mediating TAG synthesis and thereby protecting against FFA-induced cell death in fast-growing tissues of plants.

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 emergence of lipid droplets in yeast: current status and experimental approaches

The ‘discovery’ of lipid droplets as a metabolically highly active subcellular organelle has sparked great scientific interest in its research in recent years. The previous view of a rather inert storage pool of neutral lipids—triacylglycerol and sterols or steryl esters—has markedly changed. Driven by the endemic dimensions of lipid-associated disorders on the one hand, and the promising biotechnological application to generate oils (‘biodiesel’) from single-celled organisms on the other, multiple model organisms are exploited in basic and applied research to develop a better understanding of biogenesis and metabolism of this organelle. This article summarizes the current status of LD research in yeast and experimental approaches to obtain insight into the regulatory and structural components driving lipid droplet formation and their physiological and pathophysiological roles in lipid homeostasis.

Lipid droplets and peroxisomes: key players in cellular lipid homeostasis or a matter of fat--store 'em up or burn 'em down

Lipid droplets (LDs) and peroxisomes are central players in cellular lipid homeostasis: some of their main functions are to control the metabolic flux and availability of fatty acids (LDs and peroxisomes) as well as of sterols (LDs). Both fatty acids and sterols serve multiple functions in the cell—as membrane stabilizers affecting membrane fluidity, as crucial structural elements of membrane-forming phospholipids and sphingolipids, as protein modifiers and signaling molecules, and last but not least, as a rich carbon and energy source. In addition, peroxisomes harbor enzymes of the malic acid shunt, which is indispensable to regenerate oxaloacetate for gluconeogenesis, thus allowing yeast cells to generate sugars from fatty acids or nonfermentable carbon sources. Therefore, failure of LD and peroxisome biogenesis and function are likely to lead to deregulated lipid fluxes and disrupted energy homeostasis with detrimental consequences for the cell. These pathological consequences of LD and peroxisome failure have indeed sparked great biomedical interest in understanding the biogenesis of these organelles, their functional roles in lipid homeostasis, interaction with cellular metabolism and other organelles, as well as their regulation, turnover, and inheritance. These questions are particularly burning in view of the pandemic development of lipid-associated disorders worldwide.

Identification of triacylglycerol and steryl ester synthases of the methylotrophic yeast Pichia pastoris

In yeast like in many other eukaryotes, fatty acids are stored in the biologically inert form of triacylglycerols (TG) and steryl esters (SE) as energy reserve and/or as membrane building blocks. In the present study, we identified gene products catalyzing formation of TG and SE in the methylotrophic yeast Pichia pastoris. Based on sequence homologies to Saccharomyces cerevisiae, the two diacylglycerol acyltransferases Dga1p and Lro1p and one acyl CoA:sterol acyltransferase Are2p from P. pastoris were identified. Mutants bearing single and multiple deletions of the respective genes were analyzed for their growth phenotype, lipid composition and the ability to form lipid droplets. Our results indicate that the above mentioned gene products are most likely responsible for the entire TG and SE synthesis in P. pastoris. Lro1p which has low fatty acid substrate specificity in vivo is the major TG synthase in this yeast, whereas Dga1p contributes less to TG synthesis although with some preference to utilize polyunsaturated fatty acids as substrates. In contrast to S. cerevisiae, Are2p is the only SE synthase in P. pastoris. Also this enzyme exhibits some preference for certain fatty acids as judged from the fatty acid profile of SE compared to bulk lipids. Most interestingly, TG formation in P. pastoris is indispensable for lipid droplet biogenesis. The small amount of SE synthesized by Are2p in a dga1∆lro1∆ double deletion mutant is insufficient to initiate the formation of the storage organelle. In summary, our data provide a first insight into the molecular machinery of non-polar lipid synthesis and storage in P. pastoris and demonstrate specific features of this machinery in comparison to other eukaryotic cells, especially S. cerevisiae.

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.

A soluble diacylglycerol acyltransferase is involved in triacylglycerol biosynthesis in the oleaginous yeast Rhodotorula glutinis

The biosynthesis of triacylglycerol (TAG) occurs in the microsomal membranes of eukaryotes. Here, we report the identification and functional characterization of diacylglycerol acyltransferase (DGAT), a member of the 10 S cytosolic TAG biosynthetic complex (TBC) in Rhodotorula glutinis. Both a full-length and an N-terminally truncated cDNA clone of a single gene were isolated from R. glutinis. The DGAT activity of the protein encoded by RgDGAT was confirmed in vivo by the heterologous expression of cDNA in a Saccharomyces cerevisiae quadruple mutant (H1246) that is defective in TAG synthesis. RgDGAT overexpression in yeast was found to be capable of acylating diacylglycerol (DAG) in an acyl-CoA-dependent manner. Quadruple mutant yeast cells exhibit growth defects in the presence of oleic acid, but wild-type yeast cells do not. In an in vivo fatty acid supplementation experiment, RgDGAT expression rescued quadruple mutant growth in an oleate-containing medium. We describe a soluble acyl-CoA-dependent DAG acyltransferase from R. glutinis that belongs to the DGAT3 class of enzymes. The study highlights the importance of an alternative TAG biosynthetic pathway in oleaginous yeasts.

The dynamic roles of intracellular lipid droplets: from archaea to mammals

During the past decade, there has been a paradigm shift in our understanding of the roles of intracellular lipid droplets (LDs). New genetic, biochemical and imaging technologies have underpinned these advances, which are revealing much new information about these dynamic organelles. This review takes a comparative approach by examining recent work on LDs across the whole range of biological organisms from archaea and bacteria, through yeast and Drosophila to mammals, including humans. LDs probably evolved originally in microorganisms as temporary stores of excess dietary lipid that was surplus to the immediate requirements of membrane formation/turnover. LDs then acquired roles as long-term carbon stores that enabled organisms to survive episodic lack of nutrients. In multicellular organisms, LDs went on to acquire numerous additional roles including cell- and organism-level lipid homeostasis, protein sequestration, membrane trafficking and signalling. Many pathogens of plants and animals subvert their host LD metabolism as part of their infection process. Finally, malfunctions in LDs and associated proteins are implicated in several degenerative diseases of modern humans, among the most serious of which is the increasingly prevalent constellation of pathologies, such as obesity and insulin resistance, which is associated with metabolic syndrome.

Controlling lipid fluxes at glycerol-3-phosphate acyltransferase step in yeast: unique contribution of Gat1p to oleic acid-induced lipid particle formation

The ability to channel excess fatty acids into neutral lipids like triacylglycerol (TAG) is a critical strategy used by cells to maintain lipid homeostasis. Upon activation to acyl-CoA, fatty acids become readily available as substrates for acyltransferases involved in neutral lipid synthesis. Neutral lipids are then packed into organelles derived from the endoplasmic reticulum called lipid particles (LPs). The first acylation step in the de novo pathway for TAG synthesis is catalyzed by glycerol-3-phosphate acyltransferases (GPATs). Two isoforms, Gat1p/Gpt2p and Gat2p/Sct1p, are present in the yeast Saccharomyces cerevisiae. Previous evidence indicated that these enzymes contribute differentially to the synthesis of TAG in actively growing cells. In this work we studied the role of the yeast GPATs in the formation of LPs induced by a surplus of oleic acid. Yeast lacking Gat1p (but not Gat2p) were sensitive to oleate and failed to accumulate LPs induced by this unsaturated fatty acid. It is shown that oleate induces dephosphorylation of Gat1p as well as an increment in its levels. Most importantly, we identified novel Gat1p crescent structures that are formed in the presence of oleate. These structures are connected with the endoplasmic reticulum and are intimately associated with LPs. No such structures were observed for Gat2p. A crucial point of control of lipid fluxes at the GPAT step is proposed.

TAG, you're it! Chlamydomonas as a reference organism for understanding algal triacylglycerol accumulation

Photosynthetic organisms are responsible for converting sunlight into organic matter, and they are therefore seen as a resource for the renewable fuel industry. Ethanol and esterified fatty acids (biodiesel) are the most common fuel products derived from these photosynthetic organisms. The potential of algae as producers of biodiesel precursor (or triacylglycerols (TAGs)) has yet to be realized because of the limited knowledge of the underlying biochemistry, cell biology and genetics. Well-characterized pathways from fungi and land plants have been used to identify algal homologs of key enzymes in TAG synthesis, including diacylglcyerol acyltransferases, phospholipid diacylglycerol acyltransferase and phosphatidate phosphatases. Many laboratories have adopted Chlamydomonas reinhardtii as a reference organism for discovery of algal-specific adaptations of TAG metabolism. Stressed Chlamydomonas cells, grown either photoautotrophically or photoheterotrophically, accumulate TAG in plastid and cytoplasmic lipid bodies, reaching 46-65% of dry weight in starch accumulation (sta) mutants. State of the art genomic technologies including expression profiling and proteomics have identified new proteins, including key components of lipid droplets, candidate regulators and lipid/TAG degrading activities. By analogy with crop plants, it is expected that advances in algal breeding and genome engineering may facilitate realizing the potential in algae.