Defining the extreme substrate specificity of Euonymus alatus diacylglycerol acetyltransferase, an unusual membrane-bound O-acyltransferase

Evaluation of 1,2-diacyl-3-acetyl triacylglycerol production in Yarrowia lipolytica

Plants produce many high-value oleochemical molecules. While oil-crop agriculture is performed at industrial scales, suitable land is not available to meet global oleochemical demand. Worse, establishing new oil-crop farms often comes with the environmental cost of tropical deforestation. The field of metabolic engineering offers tools to transplant oleochemical metabolism into tractable hosts while simultaneously providing access to molecules produced by non-agricultural plants. Here, we evaluate strategies for rewiring metabolism in the oleaginous yeast Yarrowia lipolytica to synthesize a foreign lipid, 3-acetyl-1,2-diacyl-sn-glycerol (acTAG). Oils made up of acTAG have a reduced viscosity and melting point relative to traditional triacylglycerol oils making them attractive as low-grade diesels, lubricants, and emulsifiers. This manuscript describes a metabolic engineering study that established acTAG production at g/L scale, exploration of the impact of lipid bodies on acTAG titer, and a techno-economic analysis that establishes the performance benchmarks required for microbial acTAG production to be economically feasible.

Functional analysis of alternative castor bean DGAT enzymes

The diversity of diacylglycerol acyltransferases (DGATs) indicates alternative roles for these enzymes in plant metabolism besides triacylglycerol (TAG) biosynthesis. In this work, we functionally characterized castor bean (Ricinus communis L.) DGATs assessing their subcellular localization, expression in seeds, capacity to restore triacylglycerol (TAG) biosynthesis in mutant yeast and evaluating whether they provide tolerance over free fatty acids (FFA) in sensitive yeast. RcDGAT3 displayed a distinct subcellular localization, located in vesicles outside the endoplasmic reticulum (ER) in most leaf epidermal cells. This enzyme was unable to restore TAG biosynthesis in mutant yeast; however, it was able to outperform other DGATs providing higher tolerance over FFA. RcDAcTA subcellular localization was associated with the ER membranes, resembling RcDGAT1 and RcDGAT2, but it failed to rescue the long-chain TAG biosynthesis in mutant yeast, even with fatty acid supplementation. Besides TAG biosynthesis, our results suggest that RcDGAT3 might have alternative functions and roles in lipid metabolism.

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.

Expression of a high-activity diacylglycerol acetyltransferase results in enhanced synthesis of acetyl-TAG in camelina seed oil

Acetyl-triacylglycerols (acetyl-TAG) contain an acetate group in the sn-3 position instead of the long-chain fatty acid present in regular triacylglycerol (TAG). The acetate group confers unique physical properties such as reduced viscosity and a lower freezing point to acetyl-TAG, providing advantages for use as emulsifiers, lubricants, and 'drop-in' biofuels. Previously, the synthesis of acetyl-TAG in the seeds of the oilseed crop camelina (Camelina sativa) was achieved through the heterologous expression of the diacylglycerol acetyltransferase gene EaDAcT, isolated from Euonymus alatus seeds that naturally accumulate high levels of acetyl-TAG. Subsequent work identified a similar acetyltransferase, EfDAcT, in the seeds of Euonymus fortunei, that possesses higher in vitro activity compared to EaDAcT. In this study, the seed-specific expression of EfDAcT in camelina led to a 20 mol% increase in acetyl-TAG levels over that of EaDAcT. Coupling EfDAcT expression with suppression of the endogenous competing enzyme DGAT1 further enhanced acetyl-TAG accumulation, up to 90 mol% in the best transgenic lines. Accumulation of high levels of acetyl-TAG was stable over multiple generations, with minimal effect on seed size, weight, and fatty acid content. Slight delays in germination were noted in transgenic seeds compared to the wild type. EfDAcT transcript and protein levels were correlated during seed development with a limited window of EfDAcT protein accumulation. In high acetyl-TAG producing lines, EfDAcT protein expression in developing seeds did not reflect the eventual acetyl-TAG levels in mature seeds, suggesting that other factors limit acetyl-TAG accumulation.

CRISPR/Cas9-Induced fad2 and rod1 Mutations Stacked With fae1 Confer High Oleic Acid Seed Oil in Pennycress (Thlaspi arvense L.)

Pennycress (Thlaspi arvense L.) is being domesticated as an oilseed cash cover crop to be grown in the off-season throughout temperate regions of the world. With its diploid genome and ease of directed mutagenesis using molecular approaches, pennycress seed oil composition can be rapidly tailored for a plethora of food, feed, oleochemical and fuel uses. Here, we utilized Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)/Cas9 technology to produce knockout mutations in the FATTY ACID DESATURASE2 (FAD2) and REDUCED OLEATE DESATURATION1 (ROD1) genes to increase oleic acid content. High oleic acid (18:1) oil is valued for its oxidative stability that is superior to the polyunsaturated fatty acids (PUFAs) linoleic (18:2) and linolenic (18:3), and better cold flow properties than the very long chain fatty acid (VLCFA) erucic (22:1). When combined with a FATTY ACID ELONGATION1 (fae1) knockout mutation, fad2 fae1 and rod1 fae1 double mutants produced ∼90% and ∼60% oleic acid in seed oil, respectively, with PUFAs in fad2 fae1 as well as fad2 single mutants reduced to less than 5%. MALDI-MS spatial imaging analyses of phosphatidylcholine (PC) and triacylglycerol (TAG) molecular species in wild-type pennycress embryo sections from mature seeds revealed that erucic acid is highly enriched in cotyledons which serve as storage organs, suggestive of a role in providing energy for the germinating seedling. In contrast, PUFA-containing TAGs are enriched in the embryonic axis, which may be utilized for cellular membrane expansion during seed germination and seedling emergence. Under standard growth chamber conditions, rod1 fae1 plants grew like wild type whereas fad2 single and fad2 fae1 double mutant plants exhibited delayed growth and overall reduced heights and seed yields, suggesting that reducing PUFAs below a threshold in pennycress had negative physiological effects. Taken together, our results suggest that combinatorial knockout of ROD1 and FAE1 may be a viable route to commercially increase oleic acid content in pennycress seed oil whereas mutations in FAD2 will likely require at least partial function to avoid fitness trade-offs.

Alterations in allocation and composition of lipid classes in Euonymus fruits and seeds

The spindle tree (Euonymus europaeus L.) is a much-branched deciduous shrub or small tree. Its fruit capsules contain seeds with remarkably high content of oil interesting for industry, especially the 3-acetyl-1,2-diacyl-sn-glycerols (AcDAG) synthesized by a specific acetyl-CoA diacylglycerol acetyltransferase. The distribution and amount of individual triacylglycerols (TAG) and especially acetyl-triacylglycerols (AcDAG) in Euonymus fruit have previously been assigned to specific tissues. Using anatomical and microscopical observations, we studied the fruit morphology, and for the first time, we identified a more detailed allocation of oil bodies in individual tissue structures. Thin layer chromatography separation of extracts from immature and mature fruits confirmed TAG and AcDAG as the most abundant lipid classes in both endosperm and embryo, followed by fatty acids and polar lipids. The abundance of fatty acids was further studied in the TAG and AcDAG fractions using gas chromatography. Data revealed particular FAs in both fractions allocated in tissue-specific manner and/or as indicators of maturation of E. europaeus seeds. While the abundance of cis-vaccenic-, linoleic as well as α-linolenic acids in the AcDAG structures generally drop with maturation in both embryo and endosperm, content of oleic acid increases. Abundance of cis-vaccenic acid in TAG was recorded in immature endosperm. For embryo, the abundance of stearic acid in AcDAG and oleic acid in TAG fraction was distinctive. Deeper understanding of underlying metabolic processes will be essential for targeted metabolic engineering and/or application for oilseed crops.

Towards the synthetic design of camelina oil enriched in tailored acetyl-triacylglycerols with medium-chain fatty acids

The ability to manipulate expression of key biosynthetic enzymes has allowed the development of genetically modified plants that synthesise unusual lipids that are useful for biofuel and industrial applications. By taking advantage of the unique activities of enzymes from different species, tailored lipids with a targeted structure can be conceived. In this study we demonstrate the successful implementation of such an approach by metabolically engineering the oilseed crop Camelina sativa to produce 3-acetyl-1,2-diacyl-sn-glycerols (acetyl-TAGs) with medium-chain fatty acids (MCFAs). Different transgenic camelina lines that had been genetically modified to produce MCFAs through the expression of MCFA-specific thioesterases and acyltransferases were retransformed with the Euonymus alatus gene for diacylglycerol acetyltransferase (EaDAcT) that synthesises acetyl-TAGs. Concomitant RNAi suppression of acyl-CoA:diacylglycerol acyltransferase increased the levels of acetyl-TAG, with up to 77 mole percent in the best lines. However, the total oil content was reduced. Analysis of the composition of the acetyl-TAG molecular species using electrospray ionisation mass spectrometry demonstrated the successful synthesis of acetyl-TAG containing MCFAs. Field growth of high-yielding plants generated enough oil for quantification of viscosity. As part of an ongoing design-test-learn cycle, these results, which include not only the synthesis of 'designer' lipids but also their functional analysis, will lead to the future production of such molecules tailored for specific applications.

Membrane topology and identification of key residues of EaDAcT, a plant MBOAT with unusual substrate specificity

Euonymus alatus diacylglycerol acetyltransferase (EaDAcT) catalyzes the transfer of an acetyl group from acetyl-CoA to the sn-3 position of diacylglycerol to form 3-acetyl-1,2-diacyl-sn-glycerol (acetyl-TAG). EaDAcT belongs to a small, plant-specific subfamily of the membrane bound O-acyltransferases (MBOAT) that acylate different lipid substrates. Sucrose gradient density centrifugation revealed that EaDAcT colocalizes to the same fractions as an endoplasmic reticulum (ER)-specific marker. By mapping the membrane topology of EaDAcT, we obtained an experimentally determined topology model for a plant MBOAT. The EaDAcT model contains four transmembrane domains (TMDs), with both the N- and C-termini orientated toward the lumen of the ER. In addition, there is a large cytoplasmic loop between the first and second TMDs, with the MBOAT signature region of the protein embedded in the third TMD close to the interface between the membrane and the cytoplasm. During topology mapping, we discovered two cysteine residues (C187 and C293) located on opposite sides of the membrane that are important for enzyme activity. In order to identify additional amino acid residues important for acetyltransferase activity, we isolated and characterized acetyltransferases from other acetyl-TAG-producing plants. Among them, the acetyltransferase from Euonymus fortunei possessed the highest activity in vivo and in vitro. Mutagenesis of conserved amino acids revealed that S253, H257, D258 and V263 are essential for EaDAcT activity. Alteration of residues unique to the acetyltransferases did not alter the unique acyl donor specificity of EaDAcT, suggesting that multiple amino acids are important for substrate recognition.