Kinetic improvement of an algal diacylglycerol acyltransferase 1 via fusion with an acyl-CoA binding protein

Protein interactomes for plant lipid biosynthesis and their biotechnological applications

Plant lipids have essential biological roles in plant development and stress responses through their functions in cell membrane formation, energy storage and signalling. Vegetable oil, which is composed mainly of the storage lipid triacylglycerol, also has important applications in food, biofuel and oleochemical industries. Lipid biosynthesis occurs in multiple subcellular compartments and involves the coordinated action of various pathways. Although biochemical and molecular biology research over the last few decades has identified many proteins associated with lipid metabolism, our current understanding of the dynamic protein interactomes involved in lipid biosynthesis, modification and channelling is limited. This review examines advances in the identification and characterization of protein interactomes involved in plant lipid biosynthesis, with a focus on protein complexes consisting of different subunits for sequential reactions such as those in fatty acid biosynthesis and modification, as well as transient or dynamic interactomes formed from enzymes in cooperative pathways such as assemblies of membrane-bound enzymes for triacylglycerol biosynthesis. We also showcase a selection of representative protein interactome structures predicted using AlphaFold2, and discuss current and prospective strategies involving the use of interactome knowledge in plant lipid biotechnology. Finally, unresolved questions in this research area and possible approaches to address them are also discussed.

High wax ester and triacylglycerol biosynthesis potential in coastal sediments of Antarctic and Subantarctic environments

The wax ester (WE) and triacylglycerol (TAG) biosynthetic potential of marine microorganisms is poorly understood at the microbial community level. The goal of this work was to uncover the prevalence and diversity of bacteria with the potential to synthesize these neutral lipids in coastal sediments of two high latitude environments, and to characterize the gene clusters related to this process. Homolog sequences of the key enzyme, the wax ester synthase/acyl-CoA:diacylglycerol acyltransferase (WS/DGAT) were retrieved from 13 metagenomes, including subtidal and intertidal sediments of a Subantarctic environment (Ushuaia Bay, Argentina), and subtidal sediments of an Antarctic environment (Potter Cove, Antarctica). The abundance of WS/DGAT homolog sequences in the sediment metagenomes was 1.23 ± 0.42 times the abundance of 12 single-copy genes encoding ribosomal proteins, higher than in seawater (0.13 ± 0.31 times in 338 metagenomes). Homolog sequences were highly diverse, and were assigned to the Pseudomonadota, Actinomycetota, Bacteroidota and Acidobacteriota phyla. The genomic context of WS/DGAT homologs included sequences related to WE and TAG biosynthesis pathways, as well as to other related pathways such as fatty-acid metabolism, suggesting carbon recycling might drive the flux to neutral lipid synthesis. These results indicate the presence of abundant and taxonomically diverse bacterial populations with the potential to synthesize lipid storage compounds in marine sediments, relating this metabolic process to bacterial survival.

Characterization of Auxenochlorella protothecoides acyltransferases and potential of their protein interactions to promote the enrichment of oleic acid

BACKGROUND

After centuries of heavy reliance on fossil fuel energy, the world suffers from an energy crisis and global warming, calling for carbon emission reduction and a transition to clean energy. Microalgae have attracted much attention as a potential feedstock for biofuel production due to their high triacylglycerol content and CO₂ sequestration ability. Many diacylglycerol acyltransferases (DGAT) species have been characterized, which catalyze the final committed step in triacylglycerol biosynthesis. However, the detailed structure-function features of DGATs and the role of the interactions among DGAT proteins in lipid metabolism remained largely unknown.

RESULTS

In this study, the three characterized DGATs of Auxenochlorella protothecoides 2341 showed distinct structural and functional conservation. Functional complementation analyses showed that ApDGAT1 had higher activity than ApDGAT2b in yeast and model microalgae, and ApDGAT2a had no activity in yeast. The N-terminus was not essential to the catalysis function of ApDGAT1 but was crucial to ApDGAT2b as its enzyme activity was sensitive to any N-terminus modifications. Similarly, when acyl-CoA binding proteins (ACBPs) were fused to the N-terminus of ApDGAT1 and ApDGAT2b, zero and significant activity changes were observed, respectively. Interestingly, the ApACBP3 + ApDGAT1 variant contributed to higher oil accumulation than the original DGAT1, and ApACBP1 + ApDGAT1 fusion boosted oleic acid content in yeast. Overexpression of the three DGATs and the variation of ApACBP3 + ApDGAT1 increased the content of C18:1 of Chlamydomonas reinhardtii CC-5235. Significantly, ApDGAT1 interacted with itself, ApDGAT2b, and ApACBP1, which indicated that these three lipid metabolic proteins might have been a part of a dynamic protein interactome that facilitated the enrichment of oleic acid.

CONCLUSIONS

This study provided new insights into the functional and structural characteristics of DGATs and elucidated the importance of these physical interactions in potential lipid channeling.

Genetic architecture of seed glycerolipids in Asian cultivated rice

Glycerolipids are essential for rice development and grain quality but its genetic regulation remains unknown. Here we report its genetic base using metabolite-based genome-wide association study and metabolite-based quantitative traits locus (QTL) analyses based on lipidomic profiles of seeds from 587 Asian cultivated rice accessions and 103 chromosomal segment substitution lines, respectively. We found that two genes encoding phosphatidylcholine (PC):diacylglycerol cholinephosphotransferase (OsLP1) and granule-bound starch synthase I (Waxy) contribute to variations in saturated triacylglycerol (TAG) and lyso-PC contents, respectively. We demonstrated that allelic variation in OsLP1 sequence between indica and japonica results in different enzymatic preference for substrate PC-16:0/16:0 and different saturated TAG levels. Further evidence demonstrated that OsLP1 also affects heading date, and that co-selection of OsLP1 and a flooding-tolerant QTL in Aus results in the abundance of saturated TAGs associated with flooding tolerance. Moreover, we revealed that the sequence polymorphisms in Waxy has pleiotropic effects on lyso-PC and amylose content. We proposed that rice seed glycerolipids have been unintentionally shaped during natural and artificial selection for adaptive or import seed quality traits. Collectively, our findings provide valuable genetic resources for rice improvement and evolutionary insights into seed glycerolipid variations in rice.

Biochemistry and Biotechnology of Lipid Accumulation in the Microalga Nannochloropsis oceanica

Oils are among the most important agricultural commodities and have wide applications in food/nutrition, biofuels, and oleochemicals. The oleaginous microalga Nannochloropsis oceanica can produce large amounts of oils and the high-value ω-3 eicosapentaenoic acid, which represents a promising resource for oil production targeting biodiesel, nutraceutical, and aquaculture industries. In recent years, with the availability of omics databases and the development of genetic tools, N. oceanica has been extensively investigated as a model photosynthetic organism for studying lipid metabolism and as a green cellular factory to produce lipids for industrial applications. This review summarizes the current knowledge on the lipid composition and biosynthetic pathways of N. oceanica and reviews the recent advances in metabolic engineering of lipid production in this microalga.

Polyacetylenes in herbal medicine: A comprehensive review of its occurrence, pharmacology, toxicology, and pharmacokinetics (2014-2021)

Polyacetylenes are a kind of small active compounds with carbon-carbon triple bond with vast occurrence in plants. Polyacetylenes have attracted considerable attention owing to their diverse biofunctions like tumor suppression, immunity regulation, depression resistance and neural protection. The present review intends to reconstruct data concerning the occurrence, pharmacology, toxicology and pharmacokinetics of polyacetylenes from herbal medicine in a systematic and integrated way, with a view to backing up their curative potential and healthcare properties (2014-2021). The natural polyacetylene-related data were all acquired from the scientific search engines and databases that are globally recognized, such as PubMed, Web of Science, Elsevier, Google Scholar, ResearchGate, SciFinderⁿ and CNKI. A total of 183 polyacetylenes were summarized in this paper. Modern pharmacological studies indicated that polyacetylenes possess multiple biological activities including antitumor, immunomodulatory, neuroprotective, anti-depression, anti-obesity, hypoglycemic, antiviral, antibacterial, antifungal, hepatoprotective and renoprotective activities. As important bioactive components of herbal medicine, the pharmacological curative potential of polyacetylenes has been described against carcinomas, inflammatory responses, central nervous system, endocrine disorders and microbial infection in this review. While, further in-depth studies on the aspects of polyacetylenes for toxicity, pharmacokinetics, and molecular mechanisms are still limited, thereby intensive research and assessments should be performed.

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.

Evolutionary and biochemical characterization of a Chromochloris zofingiensis MBOAT with wax synthase and diacylglycerol acyltransferase activity

Wax synthase (WS) catalyzes the last step in wax ester biosynthesis in green plants. Two unrelated sub-families of WS, including the bifunctional acyltransferase and plant-like WS have been reported, but the latter is largely uncharacterized in microalgae. Here, we functionally characterized a putative plant-like WS (CzWS1) from the emerging model green microalga Chromochloris zofingiensis. Our results showed that plant-like WS evolved under different selection constraints in plants and microalgae, with positive selection likely contributing to functional divergence. Unlike jojoba with high amounts of wax ester in seeds and a highly active WS enzyme, C. zofingiensis has no detectable wax ester but a high abundance of WS transcripts. Co-expression analysis showed that C. zofingiensis WS has different expression correlation with lipid biosynthetic genes from jojoba, and may have a divergent function. In vitro characterization indicated that CzWS1 had diacylglycerol acyltransferase activity along with WS activity, and overexpression of CzWS1 in yeast and Chlamydomonas reinhardtii affected triacylglycerol accumulation. Moreover, biochemical and bioinformatic analyses revealed the relevance of the C-terminal region of CzWS1 in enzyme function. Taken together, our results indicated a functional divergence of plant-like WS in plants and microalgae, and the importance of its C-terminal region in specialization of enzyme function.

The oleaginous astaxanthin-producing alga Chromochloris zofingiensis: potential from production to an emerging model for studying lipid metabolism and carotenogenesis

The algal lipids-based biodiesel, albeit having advantages over plant oils, still remains high in the production cost. Co-production of value-added products with lipids has the potential to add benefits and is thus believed to be a promising strategy to improve the production economics of algal biodiesel. Chromochloris zofingiensis, a unicellular green alga, has been considered as a promising feedstock for biodiesel production because of its robust growth and ability of accumulating high levels of triacylglycerol under multiple trophic conditions. This alga is also able to synthesize high-value keto-carotenoids and has been cited as a candidate producer of astaxanthin, the strongest antioxidant found in nature. The concurrent accumulation of triacylglycerol and astaxanthin enables C. zofingiensis an ideal cell factory for integrated production of the two compounds and has potential to improve algae-based production economics. Furthermore, with the advent of chromosome-level whole genome sequence and genetic tools, C. zofingiensis becomes an emerging model for studying lipid metabolism and carotenogenesis. In this review, we summarize recent progress on the production of triacylglycerol and astaxanthin by C. zofingiensis. We also update our understanding in the distinctive molecular mechanisms underlying lipid metabolism and carotenogenesis, with an emphasis on triacylglycerol and astaxanthin biosynthesis and crosstalk between the two pathways. Furthermore, strategies for trait improvements are discussed regarding triacylglycerol and astaxanthin synthesis in C. zofingiensis.

Lipid Droplets in Unicellular Photosynthetic Stramenopiles

The Heterokonta or Stramenopile phylum comprises clades of unicellular photosynthetic species, which are promising for a broad range of biotechnological applications, based on their capacity to capture atmospheric CO₂ via photosynthesis and produce biomolecules of interest. These molecules include triacylglycerol (TAG) loaded inside specific cytosolic bodies, called the lipid droplets (LDs). Understanding TAG production and LD biogenesis and function in photosynthetic stramenopiles is therefore essential, and is mostly based on the study of a few emerging models, such as the pennate diatom Phaeodactylum tricornutum and eustigmatophytes, such as Nannochloropsis and Microchloropsis species. The biogenesis of cytosolic LD usually occurs at the level of the endoplasmic reticulum. However, stramenopile cells contain a complex plastid deriving from a secondary endosymbiosis, limited by four membranes, the outermost one being connected to the endomembrane system. Recent cell imaging and proteomic studies suggest that at least some cytosolic LDs might be associated to the surface of the complex plastid, via still uncharacterized contact sites. The carbon length and number of double bonds of the acyl groups contained in the TAG molecules depend on their origin. De novo synthesis produces long-chain saturated or monounsaturated fatty acids (SFA, MUFA), whereas subsequent maturation processes lead to very long-chain polyunsaturated FA (VLC-PUFA). TAG composition in SFA, MUFA, and VLC-PUFA reflects therefore the metabolic context that gave rise to the formation of the LD, either via an early partitioning of carbon following FA de novo synthesis and/or a recycling of FA from membrane lipids, e.g., plastid galactolipids or endomembrane phosphor- or betaine lipids. In this review, we address the relationship between cytosolic LDs and the complex membrane compartmentalization within stramenopile cells, the metabolic routes leading to TAG accumulation, and the physiological conditions that trigger LD production, in response to various environmental factors.

Characterization of a Haematococcus pluvialis Diacylglycerol Acyltransferase 1 and Its Potential in Unsaturated Fatty Acid-Rich Triacylglycerol Production

The unicellular green alga Haematococcus pluvialis has been recognized as an industry strain to produce simultaneously esterified astaxanthin (EAST) and triacylglycerol (TAG) under stress induction. It is necessary to identify the key enzymes involving in synergistic accumulation of EAST and TAG in H. pluvialis. In this study, a novel diacylglycerol acyltransferase 1 was systematically characterized by in vivo and in silico assays. The upregulated expression of HpDGAT1 gene was positively associated with the significant increase of TAG and EAST contents under stress conditions. Functional complementation by overexpressing HpDGAT1 in a TAG-deficient yeast strain H1246 revealed that HpDGAT1 could restore TAG biosynthesis and exhibited a high substrate preference for monounsaturated fatty acyl-CoAs (MUFAs) and polyunsaturated fatty acyl-CoAs (PUFAs). Notably, heterogeneous expression of HpDGAT1 in Chlamydomonas reinhardtii and Arabidopsis thaliana resulted in a significant enhancement of total oils and concurrently a high accumulation of MUFAs- and PUFAs-rich TAGs. Furthermore, molecular docking analysis indicated that HpDGAT1 contained AST-binding sites. These findings evidence a possible dual-function role for HpDGAT1 involving in TAG and EAST synthesis, demonstrating that it is a potential target gene to enrich AST accumulation in this alga and to design oil production in both commercial algae and oil crops.