Neutral lipid metabolism influences phospholipid synthesis and deacylation in Saccharomyces cerevisiae

Membranes that make fat: roles of membrane lipids as acyl donors for triglyceride synthesis and organelle function

Triglycerides constitute an inert storage form for fatty acids deposited in lipid droplets and are mobilized to provide metabolic energy or membrane building blocks. The biosynthesis of triglycerides is highly conserved within eukaryotes and normally involves the sequential esterification of activated fatty acids with a glycerol backbone. Some eukaryotes, however, can also use cellular membrane lipids as direct fatty acid donors for triglyceride synthesis. The biological significance of a pathway that generates triglycerides at the expense of organelle membranes has remained elusive. Here we review current knowledge on how cells use membrane lipids as fatty acid donors for triglyceride synthesis and discuss the hypothesis that a primary function of this pathway is to regulate membrane lipid remodeling and organelle function.

Integrated pathway engineering and transcriptome analysis for improved astaxanthin biosynthesis in Yarrowia lipolytica

Astaxanthin is a high value carotenoid with a broad range of commercial applications due to its superior antioxidant properties. In this study, β-carotene-producing Yarrowia lipolytica XK17 constructed in the lab was employed for astaxanthin biosynthesis. The catalytic effects of β-carotene ketolase CrtW and β-carotene hydroxylase CrtZ from various species were investigated. The PspCrtW from Paracoccus sp. and HpCrtZ# from Haematococcus pluvialis were confirmed to be the best combination in converting β-carotene. Several key bottlenecks in biomass and astaxanthin biosynthesis were effectively eliminated by optimizing the expression of the above enzymes and restoring uracil/leucine biosynthesis. In addition, the effects of astaxanthin biosynthesis on cell metabolism were investigated by integrated analysis of pathway modification and transcriptome information. After further optimization, strain DN30 was able to synthesize up to 730.3 mg/L astaxanthin in laboratory 5-L fermenter. This study provides a good metabolic strategy and a sustainable development platform for high-value carotenoid production.

Oleaginous yeasts: Time to rethink the definition?

Oleaginous yeasts are typically defined as those able to accumulate more than 20% of their cell dry weight as lipids or triacylglycerides. Research on these yeasts has increased lately fuelled by an interest to use biotechnology to produce lipids and oleochemicals that can substitute those coming from fossil fuels or offer sustainable alternatives to traditional extractions (e.g., palm oil). Some oleaginous yeasts are attracting attention both in research and industry, with Yarrowia lipolytica one of the best-known and studied ones. Oleaginous yeasts can be found across several clades and different metabolic adaptations have been found, affecting not only fatty acid and neutral lipid synthesis, but also lipid particle stability and degradation. Recently, many novel oleaginous yeasts are being discovered, including oleaginous strains of the traditionally considered non-oleaginous Saccharomyces cerevisiae. In the face of this boom, a closer analysis of the definition of "oleaginous yeast" reveals that this term has instrumental value for biotechnology, while it does not give information about distinct types of yeasts. Having this perspective in mind, we propose to expand the term "oleaginous yeast" to those able to produce either intracellular or extracellular lipids, not limited to triacylglycerides, in at least one growth condition (including ex novo lipid synthesis). Finally, a critical look at Y. lipolytica as a model for oleaginous yeasts shows that the term "oleaginous" should be reserved only for strains and not species and that in the case of Y. lipolytica, it is necessary to distinguish clearly between the lipophilic and oleaginous phenotype.

Production of β-carotene in Saccharomyces cerevisiae through altering yeast lipid metabolism

Saccharomyces cerevisiae is a widely used cell factory for the production of fuels and chemicals. However, as a non-oleaginous yeast, S. cerevisiae has a limited production capacity for lipophilic compounds, such as β-carotene. To increase its accumulation of β-carotene, we engineered different lipid metabolic pathways in a β-carotene producing strain and investigated the relationship between lipid components and the accumulation of β-carotene. We found that overexpression of sterol ester synthesis genes ARE1 and ARE2 increased β-carotene yield by 1.5-fold. Deletion of phosphatidate phosphatase (PAP) genes (PAH1, DPP1, and LPP1) also increased β-carotene yield by twofold. Combining these two strategies resulted in a 2.4-fold improvement in β-carotene production compared with the starting strain. These results demonstrated that regulating lipid metabolism pathways is important for β-carotene accumulation in S. cerevisiae, and may also shed insights to the accumulation of other lipophilic compounds in yeast.

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.

FSH1 encodes lysophospholipase activity in Saccharomyces cerevisiae

OBJECTIVES

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

RESULTS

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

CONCLUSIONS

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

Combining proteomics and lipid analysis to unravel Confidor stress response in Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae is a useful model for studying the influence of different stress factors on eukaryotic cells. In this work we used the pesticide imidacloprid, in the Confidor formulation, as the stress factor and analyzed its influence on the metabolic activity, proteome and lipid content and composition of Saccharomyces cerevisiae yeast. During the cultivation of yeast, the lowest recommended application dose of Confidor (0.025%, v/v) was added to the growth media and its influence on the mitochondria, cytosol with microsomes, and the whole yeast cells was monitored. The results show that under the stress provoked by the toxic effects of Confidor, yeast cells density significantly decreased and the percentage of metabolically disturbed cells significantly increased comparing with untreated control. Also, there was a downregulation of majority of glycolytic, gluconeogenesis, and TCA cycle enzymes (Fba1, Adh1, Hxk2, Tal1, Tdh1,Tdh3, Eno1) thus providing enough acetyl-CoA for the lipid restructuring and accumulation mechanism since we have found the changes in the cell and mitochondrial lipid content and FA composition. This data suggest that lipids could be the molecules that orchestrate the answer of the cells in the stress response to the Confidor treatment.

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.

Genetic Engineering Approaches Used to Increase Lipid Production and Alter Lipid Profile in Microbes

Microbial production of lipids provides important alternative sources for a variety of fine chemicals and fuels. With the development of biotechnology, genetic engineering approaches are widely used to increase lipid production in microbes, as well as to alter the lipid profile with unique physicochemical properties. In this chapter, based on the well-known information of de novo lipid accumulation mechanisms in microbes, genetic engineering strategies at the direction of increased supply of substrates, regulation of lipid synthesis pathway, regulation of lipid catabolic pathway, and regulation of lipid profiles are described. These methods provide promising insights to promote the optimization of lipid accumulation and properties.

Redirection of lipid flux toward phospholipids in yeast increases fatty acid turnover and secretion

Bio-based production of fatty acids and fatty acid-derived products can enable sustainable substitution of petroleum-derived fuels and chemicals. However, developing new microbial cell factories for producing high levels of fatty acids requires extensive engineering of lipid metabolism, a complex and tightly regulated metabolic network. Here we generated a Saccharomyces cerevisiae platform strain with a simplified lipid metabolism network with high-level production of free fatty acids (FFAs) due to redirected fatty acid metabolism and reduced feedback regulation. Deletion of the main fatty acid activation genes (the first step in β-oxidation), main storage lipid formation genes, and phosphatidate phosphatase genes resulted in a constrained lipid metabolic network in which fatty acid flux was directed to a large extent toward phospholipids. This resulted in simultaneous increases of phospholipids by up to 2.8-fold and of FFAs by up to 40-fold compared with wild-type levels. Further deletion of phospholipase genes PLB1 and PLB2 resulted in a 46% decrease in FFA levels and 105% increase in phospholipid levels, suggesting that phospholipid hydrolysis plays an important role in FFA production when phospholipid levels are increased. The multiple deletion mutant generated allowed for a study of fatty acid dynamics in lipid metabolism and represents a platform strain with interesting properties that provide insight into the future development of lipid-related cell factories.

The role of phospholipid molecular species in determining the physical properties of yeast membranes

In most eukaryotes, including Saccharomyces cerevisiae, glycerophospholipids are the main membrane lipid constituents. Besides serving as general membrane ‘building blocks’, glycerophospholipids play an important role in determining the physical properties of the membrane, which are crucial for proper membrane function. To ensure optimal physical properties, membrane glycerophospholipid composition and synthesis are tightly regulated. This review will summarize our current knowledge of factors and processes determining the membrane glycerophospholipid composition of the reference eukaryote S. cerevisiae at the level of molecular species. Extrapolating from relevant model membrane data, we also discuss how modulation of the molecular species composition can regulate membrane physical properties.

Primary Metabolism and Medium-Chain Fatty Acid Alterations Precede Long-Chain Fatty Acid Changes Impacting Neutral Lipid Metabolism in Response to an Anticancer Lysophosphatidylcholine Analogue in Yeast

The nonmetabolizable lysophosphatidylcholine (LysoPC) analogue edelfosine is the prototype of a class of compounds being investigated for their potential as selective chemotherapeutic agents. Edelfosine targets membranes, disturbing cellular homeostasis. Is not clear at this point how membrane alterations are communicated between intracellular compartments leading to growth inhibition and eventual cell death. In the present study, a combined metabolomics/lipidomics approach for the unbiased identification of metabolic pathways altered in yeast treated with sublethal concentrations of the LysoPC analogue was employed. Mass spectrometry of polar metabolites, fatty acids, and lipidomic profiling was used to study the effects of edelfosine on yeast metabolism. Amino acid and sugar metabolism, the Krebs cycle, and fatty acid profiles were most disrupted, with polar metabolites and short-medium chain fatty acid changes preceding long and very long-chain fatty acid variations. Initial increases in metabolites such as trehalose, proline, and γ-amino butyric acid with a concomitant decrease in metabolites of the Krebs cycle, citrate and fumarate, are interpreted as a cellular attempt to offset oxidative stress in response to mitochondrial dysfunction induced by the treatment. Notably, alanine, inositol, and myristoleic acid showed a steady increase during the period analyzed (2, 4, and 6 h after treatment). Of importance was the finding that edelfosine induced significant alterations in neutral glycerolipid metabolism resulting in a significant increase in the signaling lipid diacylglycerol.

NuA4 Lysine Acetyltransferase Complex Contributes to Phospholipid Homeostasis in Saccharomyces cerevisiae

Actively proliferating cells constantly monitor and readjust their metabolic pathways to ensure the replenishment of phospholipids necessary for membrane biogenesis and intracellular trafficking. In Saccharomyces cerevisiae, multiple studies have suggested that the lysine acetyltransferase complex NuA4 plays a role in phospholipid homeostasis. For one, NuA4 mutants induce the expression of the inositol-3-phosphate synthase gene, INO1, which leads to excessive accumulation of inositol, a key metabolite used for phospholipid biosynthesis. Additionally, NuA4 mutants also display negative genetic interactions with sec14-1^ts, a mutant of a lipid-binding gene responsible for phospholipid remodeling of the Golgi. Here, using a combination of genetics and transcriptional profiling, we explore the connections between NuA4, inositol, and Sec14. Surprisingly, we found that NuA4 mutants did not suppress but rather exacerbated the growth defects of sec14-1^ts under inositol-depleted conditions. Transcriptome studies reveal that while loss of the NuA4 subunit EAF1 in sec14-1^ts does derepress INO1 expression, it does not derepress all inositol/choline-responsive phospholipid genes, suggesting that the impact of Eaf1 on phospholipid homeostasis extends beyond inositol biosynthesis. In fact, we find that NuA4 mutants have impaired lipid droplet levels and through genetic and chemical approaches, we determine that the genetic interaction between sec14-1^ts and NuA4 mutants potentially reflects a role for NuA4 in fatty acid biosynthesis. Altogether, our work identifies a new role for NuA4 in phospholipid homeostasis.

Novel Interconnections in Lipid Metabolism Revealed by Overexpression of Sphingomyelin Synthase-1

This study investigates the consequences of elevating sphingomyelin synthase 1 (SMS1) activity, which generates the main mammalian sphingolipid, sphingomyelin. HepG2 cells stably transfected with SMS1 (HepG2-SMS1) exhibit elevated enzyme activity in vitro and increased sphingomyelin content (mainly C22:0- and C24:0-sphingomyelin) but lower hexosylceramide (Hex-Cer) levels. HepG2-SMS1 cells have fewer triacylglycerols than controls but similar diacylglycerol acyltransferase activity, triacylglycerol secretion, and mitochondrial function. Treatment with 1 mm palmitate increases de novo ceramide synthesis in both cell lines to a similar degree, causing accumulation of C16:0-ceramide (and some C18:0-, C20:0-, and C22:0-ceramides) as well as C16:0- and C18:0-Hex-Cers. In these experiments, the palmitic acid is delivered as a complex with delipidated BSA (2:1, mol/mol) and does not induce significant lipotoxicity. Based on precursor labeling, the flux through SM synthase also increases, which is exacerbated in HepG2-SMS1 cells. In contrast, palmitate-induced lipid droplet formation is significantly reduced in HepG2-SMS1 cells. [¹⁴C]Choline and [³H]palmitate tracking shows that SMS1 overexpression apparently affects the partitioning of palmitate-enriched diacylglycerol between the phosphatidylcholine and triacylglycerol pathways, to the benefit of the former. Furthermore, triacylglycerols from HepG2-SMS1 cells are enriched in polyunsaturated fatty acids, which is indicative of active remodeling. Together, these results delineate novel metabolic interactions between glycerolipids and sphingolipids.

Lipid Acyl Chain Remodeling in Yeast

Membrane lipid homeostasis is maintained by de novo synthesis, intracellular transport, remodeling, and degradation of lipid molecules. Glycerophospholipids, the most abundant structural component of eukaryotic membranes, are subject to acyl chain remodeling, which is defined as the post-synthetic process in which one or both acyl chains are exchanged. Here, we review studies addressing acyl chain remodeling of membrane glycerophospholipids in Saccharomyces cerevisiae, a model organism that has been successfully used to investigate lipid synthesis and its regulation. Experimental evidence for the occurrence of phospholipid acyl chain exchange in cardiolipin, phosphatidylcholine, phosphatidylinositol, and phosphatidylethanolamine is summarized, including methods and tools that have been used for detecting remodeling. Progress in the identification of the enzymes involved is reported, and putative functions of acyl chain remodeling in yeast are discussed.

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.

Engineering the fatty acid metabolic pathway in Saccharomyces cerevisiae for advanced biofuel production

Fatty acid-derived fuels and chemicals have attracted a great deal of attention in recent decades, due to their following properties of high compatibility to gasoline-based fuels and existing infrastructure for their direct utilization, storage and distribution. The yeast Saccharomyces cerevisiae is the ideal biofuel producing candidate, based on the wealth of available genetic information and versatile tools designed to manipulate its metabolic pathways. Engineering the fatty acid metabolic pathways in S. cerevisiae is an effective strategy to increase its fatty acid biosynthesis and provide more pathway precursors for production of targeted products. This review summarizes the recent progress in metabolic engineering of yeast cells for fatty acids and fatty acid derivatives production, including the regulation of acetyl-CoA biosynthesis, NADPH production, fatty acid elongation, and the accumulation of activated precursors of fatty acids for converting enzymes. By introducing specific enzymes in the engineered strains, a powerful platform with a scalable, controllable and economic route for advanced biofuel production has been established.

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Recent progress in metabolic engineering for enhanced fatty acid production.

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Regulation of acetyl-CoA, NADPH pathway for fatty acid synthesis.

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Regulation of elongation and catabolic pathway to strength fatty acid synthesis.

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Enhanced production of activated precursors for fatty acid derivatives production.

MadR1, a Mycobacterium tuberculosis cell cycle stress response protein that is a member of a widely conserved protein class of prokaryotic, eukaryotic and archeal origin

Stress-induced molecular programs designed to stall division progression are nearly ubiquitous in bacteria, with one well-known example being the participation of the SulA septum inhibiting protein in the SOS DNA damage repair response. Mycobacteria similarly demonstrate stress-altered growth kinetics, however no such regulators have been found in these organisms. We therefore set out to identify SulA-like regulatory proteins in Mycobacterium tuberculosis. A bioinformatics modeling-based approach led to the identification of rv2216 as encoding for a protein with weak similarity to SulA, further analysis distinguished this protein as belonging to a group of uncharacterized growth promoting proteins. We have named the mycobacterial protein encoded by rv2216 morphology altering division regulator protein 1, MadR1. Overexpression of madR1 modulated cell length while maintaining growth kinetics similar to wild-type, and increased the proportion of bent or V-form cells in the population. The presence of MadR1-GFP at regions of cellular elongation (poles) and morphological differentiation (V-form) suggests MadR1 involvement in phenotypic heterogeneity and longitudinal cellular growth. Global transcriptional analysis indicated that MadR1 functionality is linked to lipid editing programs required for growth and persistence. This is the first report to differentiate the larger class of these conserved proteins from SulA proteins and characterizes MadR1 effects on the mycobacterial cell.

Increase in cellular triacylglycerol content and emergence of large ER-associated lipid droplets in the absence of CDP-DG synthase function

Cds1 is an evolutionarily conserved CDP-DG synthase. Fission yeast are used to demonstrate that cells deficient in its function exhibit markedly increased triacylglycerol content and assemble unusual ER-associated lipid droplets that recruit the triacylglycerol synthesis machinery and grow by expansion.

Excess fatty acids and sterols are stored as triacylglycerols and sterol esters in specialized cellular organelles, called lipid droplets. Understanding what determines the cellular amount of neutral lipids and their packaging into lipid droplets is of fundamental and applied interest. Using two species of fission yeast, we show that cycling cells deficient in the function of the ER-resident CDP-DG synthase Cds1 exhibit markedly increased triacylglycerol content and assemble large lipid droplets closely associated with the ER membranes. We demonstrate that these unusual structures recruit the triacylglycerol synthesis machinery and grow by expansion rather than by fusion. Our results suggest that interfering with the CDP-DG route of phosphatidic acid utilization rewires cellular metabolism to adopt a triacylglycerol-rich lifestyle reliant on the Kennedy pathway.

Enhanced production of fatty alcohols by engineering the TAGs synthesis pathway in Saccharomyces cerevisiae

The production of fatty acid-derived chemicals has received a great deal of attention in recent years. In yeast cells, the main storage forms of fatty acids are TAGs. However, the conversion of TAGs into fatty acid derivatives suffers from a practical standpoint. Herein, a more direct strategy was applied to accumulate cellular fatty acyl-CoAs in Saccharomyces cerevisiae, which are the activated forms of fatty acids and used as important precursors for various converting enzymes. The dga1 gene was deleted to block the fatty acyl-CoAs dependent pathway of TAGs synthesis and a significant decrease in lipid content was observed. The FAR gene was cloned and overexpressed in the wild type strain and gene disrupted strain, to convert the fatty acyl-CoAs to the corresponding fatty acid derivatives. The metabolic engineered pathway resulted in enhanced production of fatty alcohols. Compared with the wild type strain with overexpressed FAR gene, the yield of fatty alcohols in the Δdga1 strain with FAR was dramatically increased: the intracellular fatty alcohols increased from 26 mg/L to 45 mg/L, while the extracellular fatty alcohols increased from 2.2 mg/L to 4.3 mg/L. By optimizing the culture medium with increased carbon concentration and limited nitrogen concentration, the fatty alcohols yield in the Δdga1 strain with FAR was further increased to 84 mg/L in cells and 14 mg/L secreted in broth. The results in this study demonstrated the feasibility of using the designed strategy to solve the bottleneck in utilizing TAGs for fatty acid derivatives production.

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.

Dual role for phospholipid:diacylglycerol acyltransferase: enhancing fatty acid synthesis and diverting fatty acids from membrane lipids to triacylglycerol in Arabidopsis leaves

There is growing interest in engineering green biomass to expand the production of plant oils as feed and biofuels. Here, we show that phospholipid:diacylglycerol acyltransferase1 (PDAT1) is a critical enzyme involved in triacylglycerol (TAG) synthesis in leaves. Overexpression of PDAT1 increases leaf TAG accumulation, leading to oil droplet overexpansion through fusion. Ectopic expression of oleosin promotes the clustering of small oil droplets. Coexpression of PDAT1 with oleosin boosts leaf TAG content by up to 6.4% of the dry weight without affecting membrane lipid composition and plant growth. PDAT1 overexpression stimulates fatty acid synthesis (FAS) and increases fatty acid flux toward the prokaryotic glycerolipid pathway. In the trigalactosyldiacylglycerol1-1 mutant, which is defective in eukaryotic thylakoid lipid synthesis, the combined overexpression of PDAT1 with oleosin increases leaf TAG content to 8.6% of the dry weight and total leaf lipid by fourfold. In the plastidic glycerol-3-phosphate acyltransferase1 mutant, which is defective in the prokaryotic glycerolipid pathway, PDAT1 overexpression enhances TAG content at the expense of thylakoid membrane lipids, leading to defects in chloroplast division and thylakoid biogenesis. Collectively, these results reveal a dual role for PDAT1 in enhancing fatty acid and TAG synthesis in leaves and suggest that increasing FAS is the key to engineering high levels of TAG accumulation in green biomass.

Biochemical pathways in seed oil synthesis

Oil produced in plant seeds is utilized as a major source of calories for human nutrition, as feedstocks for non-food uses such as soaps and polymers, and can serve as a high-energy biofuel. The biochemical pathways leading to oil (triacylglycerol) synthesis in seeds involve multiple subcellular organelles, requiring extensive lipid trafficking. Phosphatidylcholine plays a central role in these pathways as a substrate for acyl modifications and likely as a carrier for the trafficking of acyl groups between organelles and membrane subdomains. Although much has been clarified regarding the enzymes and pathways responsible for acyl-group flux, there are still major gaps in our understanding. These include the identity of several key enzymes, how flux between alternative pathways is controlled and the specialized cell biology leading to biogenesis of oil bodies that store up to 80% of carbon in seeds.