sn-Glycerol-3-phosphate acyltransferases in plants

Mitochondrial acyltransferases and glycerophospholipid metabolism

Our understanding of the synthesis and remodeling of mitochondrial phospholipids remains incomplete. Two isoforms of glycerol-3-phosphate acyltransferase (GPAT1 and 2) and two isoforms of acylglycerol-3-phosphate acyltransferase (AGPAT4 and 5) are located on the outer mitochondrial membrane, suggesting that both lysophosphatidic acid and phosphatidic acid are synthesized in situ for de novo glycerolipid biosynthesis. However, it is believed that the phosphatidic acid substrate for cardiolipin and phosphatidylethanolamine biosynthesis is produced at the endoplasmic reticulum whereas the phosphatidic acid synthesized in the mitochondria must be transferred to the endoplasmic reticulum before it undergoes additional steps to form the mature phospholipids that are trafficked back to the mitochondria. It is unclear whether mitochondrial phospholipids are remodeled by mitochondrial acyltransferases or whether lysophospholipids must return to the endoplasmic reticulum or to the mitochondrial associated membrane for reesterification. In this review we will focus on the few glycerolipid acyltransferases that are known to be mitochondrial. This article is part of a Special Issue entitled: Lipids of Mitochondria edited by Guenther Daum.

Regulation of FA and TAG biosynthesis pathway genes in endosperms and embryos of high and low oil content genotypes of Jatropha curcas L

The rising demand for biofuels has raised concerns about selecting alternate and promising renewable energy crops which do not compete with food supply. Jatropha (Jatropha curcas L.), a non-edible energy crop of the family euphorbiaceae, has the potential of providing biodiesel feedstock due to the presence of high proportion of unsaturated fatty acids (75%) in seed oil which is mainly accumulated in endosperm and embryo. The molecular basis of seed oil biosynthesis machinery has been studied in J. curcas, however, what genetic differences contribute to differential oil biosynthesis and accumulation in genotypes varying for oil content is poorly understood. We investigated expression profile of 18 FA and TAG biosynthetic pathway genes in different developmental stages of embryo and endosperm from high (42%) and low (30%) oil content genotypes grown at two geographical locations. Most of the genes showed relatively higher expression in endosperms of high oil content genotype, whereas no significant difference was observed in endosperms versus embryos of low oil content genotype. The promoter regions of key genes from FA and TAG biosynthetic pathways as well as other genes implicated in oil accumulation were analyzed for regulatory elements and transcription factors specific to oil or lipid accumulation in plants such as Dof, CBF (LEC1), SORLIP, GATA and Skn-1_motif etc. Identification of key genes from oil biosynthesis and regulatory elements specific to oil deposition will be useful not only in dissecting the molecular basis of high oil content but also improving seed oil content through transgenic or molecular breeding approaches.

Flowers and mycorrhizal roots--closer than we think?

Roots and flowers are formed at the extreme ends of plants and they differ in almost every aspect of their development and function; even so, they exhibit surprising molecular commonalities. For example, the calcium and calmodulin-dependent protein kinase (CCaMK) plays a central role in root symbioses with fungi and bacteria, but is also highly expressed in developing anthers. Moreover, independent evidence from transcriptomics, phylogenomics, and genetics reveals common developmental elements in root symbioses and reproductive development. We discuss the significance of these overlaps, and we argue that an integrated comparative view of the two phenomena will stimulate research and provide new insight, not only into shared components, but also into the specific aspects of anther development and root symbioses.

Cloning and functional analysis of three diacylglycerol acyltransferase genes from peanut (Arachis hypogaea L.)

Diacylglycerol acyltransferase (DGAT) catalyzes the final and only committed acylation step in the synthesis of triacylglycerols. In this study, three novel AhDGATs genes were identified and isolated from peanut. Quantitative real-time RT-PCR analysis indicated that the AhDGAT1-2 transcript was more abundant in roots, seeds, and cotyledons, whereas the transcript abundances of AhDGAT1-1 and AhDGAT3-3 were higher in flowers than in the other tissues examined. During seed development, transcript levels of AhDGAT1-1 remained relatively low during the initial developmental stage but increased gradually during later stages, peaking at 50 days after pegging (DAP). Levels of AhDGAT1-2 transcripts were higher at 10 and 60 DAPs and much lower during other stages, whereas AhDGAT3-3 showed higher expression levels at 20 and 50 DAPs. In addition, AhDGAT transcripts were differentially expressed following exposure to abiotic stresses or abscisic acid. The activity of the three AhDGAT genes was confirmed by heterologous expression in a Saccharomyces cerevisiae TAG-deficient quadruple mutant. The recombinant yeasts restored lipid body formation and TAG biosynthesis, and preferentially incorporated unsaturated C18 fatty acids into lipids. The present study provides significant information useful in modifying the oil deposition of peanut through molecular breeding.

Glycerol-3-phosphate acyltransferase 4 is essential for the normal development of reproductive organs and the embryo in Brassica napus

The enzyme sn-glycerol-3-phosphate acyltransferase 4 (GPAT4) is involved in the biosynthesis of plant lipid poly-esters. The present study further characterizes the enzymatic activities of three endoplasmic reticulum-bound GPAT4 isoforms of Brassica napus and examines their roles in the development of reproductive organs and the embryo. All three BnGPAT4 isoforms exhibited sn-2 acyltransferase and phosphatase activities with dicarboxylic acid-CoA as acyl donor. When non-substituted acyl-CoA was used as acyl donor, the rate of acylation was considerably lower and phosphatase activity was not manifested. RNA interference (RNAi)-mediated down-regulation of all GPAT4 homologues in B. napus under the control of the napin promoter caused abnormal development of several reproductive organs and reduced seed set. Microscopic examination and reciprocal crosses revealed that both pollen grains and developing embryo sacs of the B. napus gpat4 lines were affected. The gpat4 mature embryos showed decreased cutin content and altered monomer composition. The defective embryo development further affected the oil body morphology, oil content, and fatty acid composition in gpat4 seeds. These results suggest that GPAT4 has a critical role in the development of reproductive organs and the seed of B. napus.

Possible allostery and oligomerization of recombinant plastidial sn-glycerol-3-phosphate acyltransferase

Plastidial acyl-acyl carrier protein:sn-glycerol-3-phosphate acyltransferase (GPAT; EC 2.3.1.15) catalyzes the acyl-acyl carrier protein-dependent sn-1 acylation of sn-glycerol 3-phosphate (G3P) to produce lysophosphatic acid. Functional recombinant Erysimum asperum GPAT (EaGPAT), devoid of transit peptide, was produced in yeast. Analysis of the dependence of EaGPAT activity on increasing G3P concentration resulted in a hyperbolic response. EaGPAT exhibited a preference for 18-carbon unsaturated acyl-CoAs. Assays with concentrations of oleoyl-CoA up to 90μM revealed an exponential response to increasing concentrations of acyl donor, and the introduction of increasing concentrations of unlabeled linoleoyl-CoA into the standard reaction mixture resulted in increased incorporation of radiolabeled oleoyl moieties into lysophosphatidic acid. Collectively, the kinetic results suggest that acyl-CoA may act as both substrate and allosteric effector. EaGPAT was also shown to oligomerize to form higher molecular mass multimers, with the monomer and trimer being the predominant forms of the enzyme. Since most allosteric enzyme exhibit quaternary structure, the self-associating properties of EaGPAT are consistent with those of an allosteric enzyme. These results could have important regulatory implications when plastidial GPAT is introduced into a cytoplasmic environment where acyl-CoA is the acyl donor supporting cytoplasmic glycerolipid assembly.

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.

Cloning and characterization of GPAT gene from Lepidium latifolium L.: a step towards translational research in agri-genomics for food and fuel

Glycerol-3-phosphate acyltransferase (GPAT) catalyzes first and the rate limiting step in glycerolipid synthesis pathway, which in turn contribute to stabilization of plasma membrane structure and oil lipid synthesis in plant cells. Here, we report cloning and characterization of GPAT gene from Lepidium latifolium (LlaGPAT). The cDNA sequence (1,615 bp) of LlaGPAT gene consisted of 1,113 bp ORF encoding a protein of 370 aa residues, with deduced mass of 41.2 kDa and four acyltransferase (AT) motifs having role in catalysis and in glycerol-3-phosphate binding. Southern blot analysis suggested presence of a single copy of the gene in the genome. Tissue specific expression of the gene was seen more abundantly in aerial parts, compared to the roots. Quantitative real-time PCR indicated down-regulation of the gene by cold (4 °C), drought (PEG6000), salt (300 mM NaCl) and ABA (100 μM) treatments. Considering the vitality of the function of encoded enzyme, LlaGPAT can be considered a potential candidate gene for genetic engineering of oil yields and abiotic stress management in food as well as fuel crops.

In search of actionable targets for agrigenomics and microalgal biofuel production: sequence-structural diversity studies on algal and higher plants with a focus on GPAT protein

The triacylglycerol (TAG) pathway provides several targets for genetic engineering to optimize microalgal lipid productivity. GPAT (glycerol-3-phosphate acyltransferase) is a crucial enzyme that catalyzes the initial step of TAG biosynthesis. Despite many recent biochemical studies, a comprehensive sequence-structure analysis of GPAT across diverse lipid-yielding organisms is lacking. Hence, we performed a comparative genomic analysis of plastid-located GPAT proteins from 7 microalgae and 3 higher plants species. The close evolutionary relationship observed between red algae/diatoms and green algae/plant lineages in the phylogenetic tree were further corroborated by motif and gene structure analysis. The predicted molecular weight, amino acid composition, Instability Index, and hydropathicity profile gave an overall representation of the biochemical features of GPAT protein across the species under study. Furthermore, homology models of GPAT from Chlamydomonas reinhardtii, Arabidopsis thaliana, and Glycine max provided deep insights into the protein architecture and substrate binding sites. Despite low sequence identity found between algal and plant GPATs, the developed models exhibited strikingly conserved topology consisting of 14α helices and 9β sheets arranged in two domains. However, subtle variations in amino acids of fatty acyl binding site were identified that might influence the substrate selectivity of GPAT. Together, the results will provide useful resources to understand the functional and evolutionary relationship of GPAT and potentially benefit in development of engineered enzyme for augmenting algal biofuel production.

The multigene family of lysophosphatidate acyltransferase (LPAT)-related enzymes in Ricinus communis: cloning and molecular characterization of two LPAT genes that are expressed in castor seeds

The multigene family encoding proteins related to lysophosphatidyl-acyltransferases (LPATs) has been analyzed in the castor plant Ricinus communis. Among them, two genes designated RcLPAT2 and RcLPATB, encoding proteins with LPAT activity and expressed in the developing seed, have been cloned and characterized in some detail. RcLPAT2 groups with well characterized members of the so-called A-class LPATs and it shows a generalized expression pattern in the plant and along seed development. Enzymatic assays of RcLPAT2 indicate a preference for ricinoleoyl-CoA over other fatty acid thioesters when ricinoleoyl-LPA is used as the acyl acceptor, while oleoyl-CoA is the preferred substrate when oleoyl-LPA is employed. RcLPATB groups with B-class LPAT enzymes described as seed specific and selective for unusual fatty acids. However, RcLPATB exhibit a broad specificity on the acyl-CoAs, with saturated fatty acids (12:0-16:0) being the preferred substrates. RcLPATB is upregulated coinciding with seed triacylglycerol accumulation, but its expression is not restricted to the seed. These results are discussed in the light of a possible role for LPAT isoenzymes in the channelling of ricinoleic acid into castor bean triacylglycerol.

A land-plant-specific glycerol-3-phosphate acyltransferase family in Arabidopsis: substrate specificity, sn-2 preference, and evolution

Arabidopsis (Arabidopsis thaliana) has eight glycerol-3-phosphate acyltransferase (GPAT) genes that are members of a plant-specific family with three distinct clades. Several of these GPATs are required for the synthesis of cutin or suberin. Unlike GPATs with sn-1 regiospecificity involved in membrane or storage lipid synthesis, GPAT4 and -6 are unique bifunctional enzymes with both sn-2 acyltransferase and phosphatase activity resulting in 2-monoacylglycerol products. We present enzymology, pathway organization, and evolutionary analysis of this GPAT family. Within the cutin-associated clade, GPAT8 is demonstrated as a bifunctional sn-2 acyltransferase/phosphatase. GPAT4, -6, and -8 strongly prefer C16:0 and C18:1 ω-oxidized acyl-coenzyme As (CoAs) over unmodified or longer acyl chain substrates. In contrast, suberin-associated GPAT5 can accommodate a broad chain length range of ω-oxidized and unsubstituted acyl-CoAs. These substrate specificities (1) strongly support polyester biosynthetic pathways in which acyl transfer to glycerol occurs after oxidation of the acyl group, (2) implicate GPAT specificities as one major determinant of cutin and suberin composition, and (3) argue against a role of sn-2-GPATs (Enzyme Commission 2.3.1.198) in membrane/storage lipid synthesis. Evidence is presented that GPAT7 is induced by wounding, produces suberin-like monomers when overexpressed, and likely functions in suberin biosynthesis. Within the third clade, we demonstrate that GPAT1 possesses sn-2 acyltransferase but not phosphatase activity and can utilize dicarboxylic acyl-CoA substrates. Thus, sn-2 acyltransferase activity extends to all subbranches of the Arabidopsis GPAT family. Phylogenetic analyses of this family indicate that GPAT4/6/8 arose early in land-plant evolution (bryophytes), whereas the phosphatase-minus GPAT1 to -3 and GPAT5/7 clades diverged later with the appearance of tracheophytes.