Isolation and characterisation of a maize cDNA that complements a 1-acyl sn-glycerol-3-phosphate acyltransferase mutant of Escherichia coli and encodes a protein which has similarities to other acyltransferases

Identification of a 1-acyl-glycerol-3-phosphate acyltransferase from Mycobacterium tuberculosis, a key enzyme involved in triacylglycerol biosynthesis

Latent tuberculosis, caused by dormant Mycobacterium tuberculosis (Mtb), poses a threat to global health through the incubation of undiagnosed infections within the community. Dormant Mtb, which is phenotypically tolerant to antibiotics, accumulates triacylglycerol (TAG) utilizing fatty acids obtained from macrophage lipid droplets. TAG is vital to mycobacteria, serving as a cell envelope component and energy reservoir during latency. TAG synthesis occurs by sequential acylation of glycerol-3-phosphate, wherein the second acylation step is catalyzed by acylglycerol-3-phosphate acyltransferase (AGPAT), resulting in the production of phosphatidic acid (PA), a precursor for the synthesis of TAG and various phospholipids. Here, we have characterized a putative acyltransferase of Mtb encoded by Rv3816c. We found that Rv3816c has all four characteristic motifs of AGPAT, exists as a membrane-bound enzyme, and functions as 1-acylglycerol-3-phosphate acyltransferase. The enzyme could transfer the acyl group to acylglycerol-3-phosphate (LPA) from monounsaturated fatty acyl-coenzyme A of chain length 16 or 18 to produce PA. Complementation of Escherichia coli PlsC mutant in vivo by Rv3816c confirmed that it functions as AGPAT. Its active site mutants, H43A and D48A, were incapable of transferring the acyl group to LPA in vitro and were not able to rescue the growth defect of E. coli PlsC mutant in vivo. Identifying Rv3816c as AGPAT and comparing its properties with other AGPAT homologs is not only a step toward understanding the TAG biosynthesis in mycobacteria but has the potential to explore it as a drug target.

PrLPAAT4, a Putative Lysophosphatidic Acid Acyltransferase from Paeonia rockii, Plays an Important Role in Seed Fatty Acid Biosynthesis

Lysophosphatidic acid acyltransferases (LPAATs) are essential for the acylation of lysophosphatidic acid (LPA) and the synthesis of phosphatidic acid (PA), a key intermediate in the synthesis of membrane phospholipids and storage lipids. Here, a putative lysophosphatidic acid acyltransferase gene, designated PrLPAAT4, was isolated from seed unsaturated fatty acid (UFA)-rich P. rockii. The complete PrLPAAT4 cDNA contained a 1116-bp open reading frame (ORF), encoding a 42.9 kDa protein with 371 amino acid residues. Bioinformatic analysis indicates that PrLPAAT4 is a plasma membrane protein belonging to acyl-CoA:1-acylglycerol-sn-3-phosphate acyltranferases (AGPAT) family. PrLPAAT4 shared high sequence similarity with its homologs from Citrus clementina, Populus trichocarpa, Manihot esculenta, and Ricinus communis. In Arabidopsis, overexpression of PrLPAAT4 resulted in a significant increase in the content of oleic acid (OA) and total fatty acids (FAs) in seeds. AtDGAT1, AtGPAT9, and AtOleosin, involved in TAG assembly, were upregulated in PrLPAAT4-overexpressing lines. These results indicated that PrLPAAT4 functions may be as a positive regulator in seed FA biosynthesis.

Acyl-CoA:Lysophosphatidylethanolamine Acyltransferase Activity Regulates Growth of Arabidopsis

Arabidopsis (Arabidopsis thaliana) contains two enzymes (encoded by the At1g80950 and At2g45670 genes) preferentially acylating lysophosphatidylethanolamine (LPE) with acyl-coenzyme A (CoA), designated LYSOPHOSPHATIDYLETHANOLAMINE ACYLTRANSFERASE1 (LPEAT1) and LPEAT2. The transfer DNA insertion mutant lpeat2 and the double mutant lpeat1 lpeat2 showed impaired growth, smaller leaves, shorter roots, less seed setting, and reduced lipid content per fresh weight in roots and seeds and large increases in LPE and lysophosphatidylcholine (LPC) contents in leaves. Microsomal preparations from leaves of these mutants showed around 70% decrease in acylation activity of LPE with 16:0-CoA compared with wild-type membranes, whereas the acylation with 18:1-CoA was much less affected, demonstrating that other lysophospholipid acyltransferases than the two LPEATs could acylate LPE The above-mentioned effects were less pronounced in the single lpeat1 mutant. Overexpression of either LPEAT1 or LPEAT2 under the control of the 35S promotor led to morphological changes opposite to what was seen in the transfer DNA mutants. Acyl specificity studies showed that LPEAT1 utilized 16:0-CoA at the highest rate of 11 tested acyl-CoAs, whereas LPEAT2 utilized 20:0-CoA as the best acyl donor. Both LPEATs could acylate either sn position of ether analogs of LPC The data show that the activities of LPEAT1 and LPEAT2 are, in a complementary way, involved in growth regulation in Arabidopsis. It is shown that LPEAT activity (especially LPEAT2) is essential for maintaining adequate levels of phosphatidylethanolamine, LPE, and LPC in the cells.

A genome-wide analysis of the lysophosphatidate acyltransferase (LPAAT) gene family in cotton: organization, expression, sequence variation, and association with seed oil content and fiber quality

BACKGROUND

Lysophosphatidic acid acyltransferase (LPAAT) encoded by a multigene family is a rate-limiting enzyme in the Kennedy pathway in higher plants. Cotton is the most important natural fiber crop and one of the most important oilseed crops. However, little is known on genes coding for LPAATs involved in oil biosynthesis with regard to its genome organization, diversity, expression, natural genetic variation, and association with fiber development and oil content in cotton.

RESULTS

In this study, a comprehensive genome-wide analysis in four Gossypium species with genome sequences, i.e., tetraploid G. hirsutum- AD₁ and G. barbadense- AD₂ and its possible ancestral diploids G. raimondii- D₅ and G. arboreum- A₂, identified 13, 10, 8, and 9 LPAAT genes, respectively, that were divided into four subfamilies. RNA-seq analyses of the LPAAT genes in the widely grown G. hirsutum suggest their differential expression at the transcriptional level in developing cottonseeds and fibers. Although 10 LPAAT genes were co-localised with quantitative trait loci (QTL) for cottonseed oil or protein content within a 25-cM region, only one single strand conformation polymorphic (SSCP) marker developed from a synonymous single nucleotide polymorphism (SNP) of the At-Gh13LPAAT5 gene was significantly correlated with cottonseed oil and protein contents in one of the three field tests. Moreover, transformed yeasts using the At-Gh13LPAAT5 gene with the two sequences for the SNP led to similar results, i.e., a 25-31% increase in palmitic acid and oleic acid, and a 16-29% increase in total triacylglycerol (TAG).

CONCLUSIONS

The results in this study demonstrated that the natural variation in the LPAAT genes to improving cottonseed oil content and fiber quality is limited; therefore, traditional cross breeding should not expect much progress in improving cottonseed oil content or fiber quality through a marker-assisted selection for the LPAAT genes. However, enhancing the expression of one of the LPAAT genes such as At-Gh13LPAAT5 can significantly increase the production of total TAG and other fatty acids, providing an incentive for further studies into the use of LPAAT genes to increase cottonseed oil content through biotechnology.

Biosynthesis of Membrane Lipids

The pathways in Escherichia coli and (largely by analogy) S. enterica remain the paradigm of bacterial lipid synthetic pathways, although recently considerable diversity among bacteria in the specific areas of lipid synthesis has been demonstrated. The structural biology of the fatty acid synthetic proteins is essentially complete. However, the membrane-bound enzymes of phospholipid synthesis remain recalcitrant to structural analyses. Recent advances in genetic technology have allowed the essentialgenes of lipid synthesis to be tested with rigor, and as expected most genes are essential under standard growth conditions. Conditionally lethal mutants are available in numerous genes, which facilitates physiological analyses. The array of genetic constructs facilitates analysis of the functions of genes from other organisms. Advances in mass spectroscopy have allowed very accurate and detailed analyses of lipid compositions as well as detection of the interactions of lipid biosynthetic proteins with one another and with proteins outside the lipid pathway. The combination of these advances has resulted in use of E. coli and S. enterica for discovery of new antimicrobials targeted to lipid synthesis and in deciphering the molecular actions of known antimicrobials. Finally,roles for bacterial fatty acids other than as membrane lipid structural components have been uncovered. For example, fatty acid synthesis plays major roles in the synthesis of the essential enzyme cofactors, biotin and lipoic acid. Although other roles for bacterial fatty acids, such as synthesis of acyl-homoserine quorum-sensing molecules, are not native to E. coli introduction of the relevant gene(s) synthesis of these foreign molecules readily proceeds and the sophisticated tools available can used to decipher the mechanisms of synthesis of these molecules.

The Peanut (Arachis hypogaea L.) Gene AhLPAT2 Increases the Lipid Content of Transgenic Arabidopsis Seeds

Lysophosphatidic acid acyltransferase (LPAT), which converts lysophosphatidic acid (LPA) to phosphatidic acid (PA), catalyzes the addition of fatty acyl moieties to the sn-2 position of the LPA glycerol backbone in triacylglycerol (TAG) biosynthesis. We recently reported the cloning and temporal-spatial expression of a peanut (Arachis hypogaea) AhLPAT2gene, showing that an increase in AhLPAT2 transcript levels was closely correlated with an increase in seed oil levels. However, the function of the enzyme encoded by the AhLPAT2 gene remains unclear. Here, we report that AhLPAT2 transcript levels were consistently higher in the seeds of a high-oil cultivar than in those of a low-oil cultivar across different seed developmental stages. Seed-specific overexpression of AhLPAT2 in Arabidopsis results in a higher percentage of oil in the seeds and greater-than-average seed weight in the transgenic plants compared with the wild-type plants, leading to a significant increase in total oil yield per plant. The total fatty acid (FA) content and the proportion of unsaturated FAs also increased. In the developing siliques of AhLPAT2-overexpressing plants, the expression levels of genes encoding crucial enzymes involved in de novo FA synthesis, acetyl-CoA subunit (AtBCCP2) and acyl carrier protein 1 (AtACP1) were elevated. AhLPAT2 overexpression also promoted the expression of several key genes related to TAG assembly, sucrose metabolism, and glycolysis. These results demonstrate that the expression of AhLPAT2 plays an important role in glycerolipid production in peanuts.

Genome-wide identification and evolutionary analysis of algal LPAT genes involved in TAG biosynthesis using bioinformatic approaches

Lysophosphatidyl acyltransferase (LPAT) is one of the major triacylglycerol synthesis enzymes, controlling the metabolic flow of lysophosphatidic acid to phosphatidic acid. Experimental studies in Arabidopsis have shown that LPAT activity is exhibited primarily by three distinct isoforms, namely the plastid-located LPAT1, the endoplasmic reticulum-located LPAT2, and the soluble isoform of LPAT (solLPAT). In this study, 24 putative genes representing all LPAT isoforms were identified from the analysis of 11 complete genomes including green algae, red algae, diatoms and higher plants. We observed LPAT1 and solLPAT genes to be ubiquitously present in nearly all genomes examined, whereas LPAT2 genes to have evolved more recently in the plant lineage. Phylogenetic analysis indicated that LPAT1, LPAT2 and solLPAT have convergently evolved through separate evolutionary paths and belong to three different gene families, which was further evidenced by their wide divergence at gene structure and sequence level. The genome distribution supports the hypothesis that each gene encoding a LPAT is not duplicated. Mapping of exon-intron structure of LPAT genes to the domain structure of proteins across different algal and plant species indicates that exon shuffling plays no role in the evolution of LPAT genes. Besides the previously defined motifs, several conserved consensus sequences were discovered which could be useful to distinguish different LPAT isoforms. Taken together, this study will enable the generation of experimental approximations to better understand the functional role of algal LPAT in lipid accumulation.

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.

Identification of Brassica napus lysophosphatidylcholine acyltransferase genes through yeast functional screening

Acyl-CoA:lysophosphatidylcholine acyltransferase (LPCAT), which acylates lysophosphatidylcholine (LPC) to produce phosphatidylcholine (PC), is a key enzyme in the Lands cycle. There is evidence that acyl exchange involving LPCAT is a prevailing metabolic process during triacylglycerol (TAG) synthesis in seeds. In this study, by complementing the yeast lca1Δ mutant deficient in LPCAT activity with an Arabidopsis seedling cDNA library, it was found that the previously reported lysophospholipid acyltransferases (LPLATs), At1g12640 and At1g63050, were the only two acyltransferase genes that restored hyposensitivity of the lca1Δ mutant to lyso-platelet-activating factor (lyso-PAF). A developing seed cDNA library from Brassica napus L. cv Hero was constructed to further explore the heterologous yeast complementation approach. Three B. napusLPCAT homologs were identified, of which BnLPCAT1-1 and BnLPCAT1-2 are orthologous to ArabidopsisAtLPLAT1 (At1g12640) while BnLPCAT2 is an ortholog of AtLPLAT2 (At1g63050). The proteins encoded by BnLPCAT1-1 and BnLPCAT2 were chosen for further study. Enzymatic assays demonstrated that both proteins exhibited a substrate preference for LPCs and unsaturated fatty acyl-CoAs. In addition to the enzymatic properties of plant lysophosphatidylcholine acyltransferases uncovered in this study, this report describes a useful technique that facilitates subsequent analyses into the role of LPCATs in PC turnover and seed oil biosynthesis.

Metabolic engineering of hydroxy fatty acid production in plants: RcDGAT2 drives dramatic increases in ricinoleate levels in seed oil

SUMMARY

A central goal of green chemistry is to produce industrially useful fatty acids in oilseed crops. Although genes encoding suitable fatty acid-modifying enzymes are available from many wild species, progress has been limited because the expression of these genes in transgenic plants produces low yields of the desired products. For example, Ricinus communis fatty acid hydroxylase 12 (FAH12) produces a maximum of only 17% hydroxy fatty acids (HFAs) when expressed in Arabidopsis. cDNA clones encoding R. communis enzymes for additional steps in the seed oil biosynthetic pathway were identified. Expression of these cDNAs in FAH12 transgenic plants revealed that the R. communis type-2 acyl-coenzyme A:diacylglycerol acyltransferase (RcDGAT2) could increase HFAs from 17% to nearly 30%. Detailed comparisons of seed neutral lipids from the single- and double-transgenic lines indicated that RcDGAT2 substantially modified the triacylglycerol (TAG) pool, with significant increases in most of the major TAG species observed in native castor bean oil. These data suggest that RcDGAT2 prefers acyl-coenzyme A and diacylglycerol substrates containing HFAs, and biochemical analyses of RcDGAT2 expressed in yeast cells confirmed a strong preference for HFA-containing diacylglycerol substrates. Our results demonstrate that pathway engineering approaches can be used successfully to increase the yields of industrial feedstocks in plants, and that members of the DGAT2 gene family probably play a key role in this process.

Rhodobacter capsulatus OlsA is a bifunctional enzyme active in both ornithine lipid and phosphatidic acid biosynthesis

The Rhodobacter capsulatus genome contains three genes (olsA [plsC138], plsC316, and plsC3498) that are annotated as lysophosphatidic acid (1-acyl-sn-glycerol-3-phosphate) acyltransferase (AGPAT). Of these genes, olsA was previously shown to be an O-acyltransferase in the second step of ornithine lipid biosynthesis, which is important for optimal steady-state levels of c-type cytochromes (S. Aygun-Sunar, S. Mandaci, H.-G. Koch, I. V. J. Murray, H. Goldfine, and F. Daldal. Mol. Microbiol. 61:418-435, 2006). The roles of the remaining plsC316 and plsC3498 genes remained unknown. In this work, these genes were cloned, and chromosomal insertion-deletion mutations inactivating them were obtained to define their function. Characterization of these mutants indicated that, unlike the Escherichia coli plsC, neither plsC316 nor plsC3498 was essential in R. capsulatus. In contrast, no plsC316 olsA double mutant could be isolated, indicating that an intact copy of either olsA or plsC316 was required for R. capsulatus growth under the conditions tested. Compared to OlsA null mutants, PlsC316 null mutants contained ornithine lipid and had no c-type cytochrome-related phenotype. However, they exhibited slight growth impairment and highly altered total fatty acid and phospholipid profiles. Heterologous expression in an E. coli plsC(Ts) mutant of either R. capsulatus plsC316 or olsA gene products supported growth at a nonpermissive temperature, exhibited AGPAT activity in vitro, and restored phosphatidic acid biosynthesis. The more vigorous AGPAT activity displayed by PlsC316 suggested that plsC316 encodes the main AGPAT required for glycerophospholipid synthesis in R. capsulatus, while olsA acts as an alternative AGPAT that is specific for ornithine lipid synthesis. This study therefore revealed for the first time that some OlsA enzymes, like the enzyme of R. capsulatus, are bifunctional and involved in both membrane ornithine lipid and glycerophospholipid biosynthesis.

Endophilin and CtBP/BARS are not acyl transferases in endocytosis or Golgi fission

Endophilins have been proposed to have an enzymatic activity (a lysophosphatidic acid acyl transferase or LPAAT activity) that can make phosphatidic acid in membranes. This activity is thought to change the bilayer asymmetry in such a way that negative membrane curvature at the neck of a budding vesicle will be stabilized. An LPAAT activity has also been proposed for CtBP/BARS (carboxy-terminal binding protein/brefeldin A-ribosylated substrate), a transcription co-repressor that is implicated in dynamin-independent endocytosis and fission of the Golgi in mitosis. Here we show that the LPAAT activity associated with endophilin is a contaminant of the purification procedure and can be also found associated with the pleckstrin homology domain of dynamin. Likewise, the LPAAT activity associated with CtBP/BARS is also a co-purification artefact. The proposed locus of activity in endophilins includes the BAR domain, which has no catalytic site but instead senses positive membrane curvature. These data will prompt a re-evaluation of the molecular details of membrane budding.

Loss of plastidic lysophosphatidic acid acyltransferase causes embryo-lethality in Arabidopsis

Phosphatidic acid is a key intermediate for chloroplast membrane lipid biosynthesis. De novo phosphatidic acid biosynthesis in plants occurs in two steps: first the acylation of the sn-1 position of glycerol-3-phosphate giving rise to lysophosphatidic acid; second, the acylation of the sn-2 position of lysophosphatidic acid to form phosphatidic acid. The second step is catalyzed by a lysophosphatidic acid acyltransferase (LPAAT). Here we describe the identification of the ATS2 gene of Arabidopsis encoding the plastidic isoform of this enzyme. Introduction of the ATS2 cDNA into E. coli JC 201, which is temperature-sensitive and carries a mutation in its LPAAT gene plsC, restored this mutant to nearly wild type growth at high temperature. A green-fluorescent protein fusion with ATS2 localized to the chloroplast. Disruption of the ATS2 gene of Arabidopsis by T-DNA insertion caused embryo lethality. The development of the embryos was arrested at the globular stage concomitant with a transient increase in ATS2 gene expression. Apparently, plastidic LPAAT is essential for embryo development in Arabidopsis during the transition from the globular to the heart stage when chloroplasts begin to form.

Plastid lysophosphatidyl acyltransferase is essential for embryo development in Arabidopsis

Lysophosphatidyl acyltransferase (LPAAT) is a pivotal enzyme controlling the metabolic flow of lysophosphatidic acid into different phosphatidic acids in diverse tissues. A search of the Arabidopsis genome database revealed five genes that could encode LPAAT-like proteins. We identified one of them, LPAAT1, to be the lone gene that encodes the plastid LPAAT. LPAAT1 could functionally complement a bacterial mutant that has defective LPAAT. Bacteria transformed with LPAAT1 produced LPAAT that had in vitro enzyme activity much higher on 16:0-coenzyme A than on 18:1-coenzyme A in the presence of 18:1-lysophosphatidic acid. LPAAT1 transcript was present in diverse organs, with the highest level in green leaves. A mutant having a T-DNA inserted into LPAAT1 was identified. The heterozygous mutant has no overt phenotype, and its leaf acyl composition is similar to that of the wild type. Selfing of a heterozygous mutant produced normal-sized and shrunken seeds in the Mendelian ratio of 3:1, and the shrunken seeds could not germinate. The shrunken seeds apparently were homozygous of the T-DNA-inserted LPAAT1, and development of the embryo within them was arrested at the heart-torpedo stage. This embryo lethality could be rescued by transformation of the heterozygous mutant with a 35S:LPAAT1 construct. The current findings of embryo death in the homozygous knockout mutant of the plastid LPAAT contrasts with earlier findings of a normal phenotype in the homozygous mutant deficient of the plastid glycerol-3-phosphate acyltransferase; both mutations block the synthesis of plastid phosphatidic acid. Reasons for the discrepancy between the contrasting phenotypes of the two mutants are discussed.

Bacterial Membrane Lipids: Where Do We Stand?

Phospholipids play multiple roles in bacterial cells. These are the establishment of the permeability barrier, provision of the environment for many enzyme and transporter proteins, and they influence membrane-related processes such as protein export and DNA replication. The lipid synthetic pathway also provides precursors for protein modification and for the synthesis of other molecules. This review concentrates on the phospholipid synthetic pathway and discusses recent data on the synthesis and function of phospholipids mainly in the bacterium Escherichia coli.

Limnanthes douglasii lysophosphatidic acid acyltransferases: immunological quantification, acyl selectivity and functional replacement of the Escherichia coli plsC gene

Antibodies were raised against the two membrane-bound lysophosphatidic acid acyltransferase (LPAAT) enzymes from Limnanthes douglasii (meadowfoam), LAT1 and LAT2, using the predicted soluble portion of each protein as recombinant protein antigens. The antibodies can distinguish between the two acyltransferase proteins and demonstrate that both migrate in an anomalous fashion on SDS/PAGE gels. The antibodies were used to determine that LAT1 is present in both leaf and developing seeds, whereas LAT2 is only detectable in developing seeds later than 22 daf (days after flowering). Both proteins were found exclusively in microsomal fractions and their amount was determined using the recombinant antigens as quantification standards. LAT1 is present at a level of 27 pg/microg of membrane protein in leaf tissue and <or=12.5 pg/microg of membrane protein in developing embryos. The amount of LAT2 reaches a peak at 305 pg/microg of membrane protein 25 daf and is not expressed 20 daf or before. This is the first study to quantify these membrane-bound proteins in a plant tissue. The maximal level of LAT2 protein coincides with the maximal level of erucic acid synthesis in the seeds. Both full-length proteins were expressed in the Escherichia coli LPAAT mutant JC201, and membranes from these strains were used to investigate the substrate selectivity of these two enzymes, demonstrating that they are different. Finally, we report that LAT2 and a maize LPAAT enzyme (MAT1) can functionally replace the E. coli plsC gene after its deletion in the chromosome, whereas LAT1 and a coconut LPAAT (Coco1) cannot. This is probably due to differences in substrate utilization.

Cellular transport of nonesterified fatty acids

Transport of nonesterified fatty acids (NEFA) is an important component of whole-body energy metabolism, and derangements in NEFA transport have been linked to several diseases. NEFA are transferred from their sites of production to cells in hepatic and peripheral tissues by mechanisms that are regulated in part by cell status and as determined by the covalent structure of the NEFA species. Major barriers to physical transport are transfer from the hydrophobic surfaces on cell membranes and NEFA-binding proteins, such as albumin, into the surrounding aqueous phase and translocation across a membrane that contains a very hydrophobic interior; this process could be purely diffusive or require specific protein cofactors. Herein evidence is provided suggesting that this step is driven by intracellular metabolism that supports a NEFA gradient across the cell membrane. According to current models of NEFA transfer, the rate-limiting step is likely to be desorption of NEFA from the inner leaflet of the cell membrane or intracellular metabolism; for very long chain NEFA, the former is more likely.

A consensus sequence for long-chain fatty-acid alcohol oxidases from Candida identifies a family of genes involved in lipid omega-oxidation in yeast with homologues in plants and bacteria

The yeast Candida cloacae is capable of growing on alkanes and fatty acids as sole carbon sources. Transfer of cultures from a glucose medium to one containing oleic acid induced seven proteins of M(r) 102,000, 73,000, 61,000, 54,000, and 46,000 and two in the region of M(r) 45,000 and repressed a protein of M(r) 64,000. The induction of the M(r) 73,000 protein reached a 7-fold maximum 24 h after induction. The protein was confirmed by its enzyme activity to be a long-chain fatty-acid alcohol oxidase (LC-FAO) and purified to homogeneity from microsomes by a rapid procedure involving hydrophobic chromatography. An internal peptide of 30 amino acids was sequenced. A 1100-base pair cDNA fragment containing the LC-FAO peptide coding sequence was used to isolate a single exon genomic clone containing the full-length coding sequence of an LC-FAO (fao1). The fao1 gene product was expressed in Escherichia coli and was translated as a functional long-chain alcohol oxidase, which was present in the membrane fraction. In addition, full-length coding sequences for a Candida tropicalis LC-FAO (faoT) and a second C. cloacae LC-FAO (fao2) were isolated. The DNA sequences obtained had open reading frames of 2094 (fao1), 2091 (fao2), and 2112 (faoT) base pairs. The derived amino acid sequences of fao2 and faoT showed 89.4 and 76.2% similarities to fao1. The fao1 gene is much more highly induced on alkane than is fao2. Although this study describes the first known DNA sequences encoding LC-FAOs from any source, there are unassigned Arabidopsis sequences and an unassigned Mycobacterium sequence in the GenBank(TM) Data Bank that show strong homology to the described LC-FAO sequences. The conservation of sequence between yeast, plants, and bacteria suggests that an as yet undescribed family of long-chain fatty-acid oxidases exists in both eukaryotes and prokaryotes.

Genetic engineering of plant lipids

Vegetable oils are a major component of human diets, comprising as much as 25% of average caloric intake. Until recently, it was not possible to exert significant control over the chemical composition of vegetable oils derived from different plants. However, the advent of genetic engineering has provided novel opportunities to tailor the composition of plant-derived lipids so that they are optimized with respect to food functionality and human dietary needs. In order to exploit this new capability, it is essential for food scientists and nutritionists to define the lipid compositions that would be most desirable for various purposes.

A plastidial lysophosphatidic acid acyltransferase from oilseed rape

The biosynthesis of phosphatidic acid, a key intermediate in the biosynthesis of lipids, is controlled by lysophosphatidic acid (LPA, or 1-acyl-glycerol-3-P) acyltransferase (LPAAT, EC 2.3.1.51). We have isolated a cDNA encoding a novel LPAAT by functional complementation of the Escherichia coli mutant plsC with an immature embryo cDNA library of oilseed rape (Brassica napus). Transformation of the acyltransferase-deficient E. coli strain JC201 with the cDNA sequence BAT2 alleviated the temperature-sensitive phenotype of the plsC mutant and conferred a palmitoyl-coenzyme A-preferring acyltransferase activity to membrane fractions. The BAT2 cDNA encoded a protein of 351 amino acids with a predicted molecular mass of 38 kD and an isoelectric point of 9.7. Chloroplast-import experiments showed processing of a BAT2 precursor protein to a mature protein of approximately 32 kD, which was localized in the membrane fraction. BAT2 is encoded by a minimum of two genes that may be expressed ubiquitously. These data are consistent with the identity of BAT2 as the plastidial enzyme of the prokaryotic glycerol-3-P pathway that uses a palmitoyl-ACP to produce phosphatidic acid with a prokaryotic-type acyl composition. The homologies between the deduced protein sequence of BAT2 with prokaryotic and eukaryotic microsomal LAP acytransferases suggest that seed microsomal forms may have evolved from the plastidial enzyme.

Multiple lysophosphatidic acid acyltransferases in Neisseria meningitidis

Lysophosphatidic acid (LPA) and phosphatidic acid (PA) are critical phospholipid intermediates in the biosynthesis of cell membranes. In Escherichia coli, LPA acyltransferase (1-acyl-sn-glycerol-3-phosphate acyltransferase; EC 2.3.1.51) catalyses the transfer of an acyl chain from either acyl-coenzyme A or acyl-acyl carrier protein onto LPA to produce PA. While E. coli possesses one essential LPA acyltransferase (PlsC), Neisseria meningitidis possesses at least two LPA acyltransferases. This study describes the identification and characterization of nlaB (neisserial LPA acyltransferase B), the second LPA acyltransferase identified in N. meningitidis. The gene was located downstream of the Tn916 insertion in N. meningitidis mutant 469 and differed in nucleotide and predicted amino acid sequence from the previously characterized neisserial LPA acyltransferase homologue nlaA. NlaB has specific LPA acyltransferase activity, as demonstrated by complementation of an E. coli plsC(Ts) mutant in trans, by decreased levels of LPA acyltransferase activity in nlaB mutants and by lack of complementation of E. coli plsB26,X50, a mutant defective in the first acyltransferase step in phospholipid biosynthesis. Meningococcal nlaA mutants accumulated LPA and demonstrated alterations in membrane phospholipid composition, yet retained LPA acyltransferase activity. In contrast, meningococcal nlaB mutants exhibited decreased LPA acyltransferase activity, but did not accumulate LPA or display any other observable membrane changes. We propose that N. meningitidis possesses at least two LPA acyltransferases to provide for the production of a greater diversity of membrane phospholipids.

Nutritional and hormonal regulation of enzymes in fat synthesis: studies of fatty acid synthase and mitochondrial glycerol-3-phosphate acyltransferase gene transcription

The activities of critical enzymes in fatty acid and triacylglycerol biosynthesis are tightly controlled by different nutritional, hormonal, and developmental conditions. Feeding previously fasted animals high-carbohydrate, low-fat diets causes a dramatic induction of enzymes-such as fatty acid synthase (FAS) and mitochondrial glycerol-3-phosphate acyltransferase (GPAT)-involved in fatty acid and triacylglycerol synthesis. During fasting and refeeding, transcription of these two enzymes is coordinately regulated by nutrients and hormones, such as glucose, insulin, glucagon, glucocorticoids, and thyroid hormone. Insulin stimulates transcription of the FAS and mitochondrial GPAT genes, and glucagon antagonizes the insulin effect through the cis-acting elements within the promoters and their bound trans-acting factors. This review discusses advances made in the understanding of the transcriptional regulation of FAS and mitochondrial GPAT genes, with emphasis on elucidation of the mechanisms by which multiple nutrients and hormones achieve their effects.

Alteration of the fatty acid substrate specificity of lysophosphatidate acyltransferase by site-directed mutagenesis

The JC201 strain of Eschericia coli contains a temperature-sensitive lesion in lysophosphatidate acyltransferase (LPAT) activity. The LPAT gene from JC201 was isolated by PCR and a single mutant nucleotide, adenine-440, was identified by DNA sequence analysis. Site-directed mutagenesis converted the mutant adenine-440 back to the native guanine-440 nucleotide. The restored LPAT gene rescued JC201 cells at the non-permissive temperature. The fatty acid substrate specificity of LPAT from Eschericia coli was altered by site-directed mutagenesis of a single amino acid in the restored LPAT gene. Threonine-122 of LPAT was changed to alanine or leucine. A change from threonine-122 to alanine increased the substrate specificity in vitro for oleoyl-CoA and linoleoyl-CoA; whereas a change to leucine increased the substrate specificity for lignoceroyl-CoA.

A conserved histidine is essential for glycerolipid acyltransferase catalysis

Sequence analysis of membrane-bound glycerolipid acyltransferases revealed that proteins from the bacterial, plant, and animal kingdoms share a highly conserved domain containing invariant histidine and aspartic acid residues separated by four less conserved residues in an HX4D configuration. We investigated the role of the invariant histidine residue in acyltransferase catalysis by site-directed mutagenesis of two representative members of this family, the sn-glycerol-3-phosphate acyltransferase (PlsB) and the bifunctional 2-acyl-glycerophosphoethanolamine acyltransferase/acyl-acyl carrier protein synthetase (Aas) of Escherichia coli. Both the PlsB[H306A] and Aas[H36A] mutants lacked acyltransferase activity. However, the Aas[H36A] mutant retained significant acyl-acyl carrier protein synthetase activity, illustrating that the lack of acyltransferase activity was specifically associated with the H36A substitution. The invariant aspartic acid residue in the HX4D pattern was also important. The substitution of aspartic acid 311 with glutamic acid in PlsB resulted in an enzyme with significantly reduced catalytic activity. Substitution of an alanine at this position eliminated acyltransferase activity; however, the PlsB[D311A] mutant protein did not assemble into the membrane, indicating that aspartic acid 311 is also important for the proper folding and membrane insertion of the acyltransferases. These data are consistent with a mechanism for glycerolipid acyltransferase catalysis where the invariant histidine functions as a general base to deprotonate the hydroxyl moiety of the acyl acceptor.

Effect of insulin and prolactin on acyltransferase activities in MAC-T bovine mammary cells

The enzymatic activities of sn-glycerol-3-phosphate acyltransferase and lysophosphatidate acyltransferase were investigated in microsomal fractions prepared from MAC-T cells from bovine mammary gland and from FTO-2B cells from rat liver. In both cell lines, sn-glycerol-3-phosphate acyltransferase exhibited similar rates of palmitate and oleate incorporation. However, lysophosphatidate acyltransferase activity in MAC-T cells had a 2.8-fold greater rate of palmitate incorporation than of oleate incorporation. In FTO-2B cells, there was a 1.4-fold greater rate of oleate incorporation than of palmitate incorporation. FTO-2B and MAC-T cells displayed acyltransferase activities that were consistent with liver and mammary tissues, respectively. The acyltransferases were examined from FTO-2B and MAC-T cells that were treated with insulin and prolactin. Insulin suppressed both acyltransferase activities in FTO-2B cells, and prolactin had a stimulatory effect; however, these effects were very small. In contrast, insulin and prolactin had a stimulatory effect on both acyltransferase activities in MAC-T cells; prolactin elicited the largest effect. Treatment of MAC-T cells with cycloheximide inhibited the stimulatory effect of prolactin on acyltransferase activities.

A gene (plsD) from Clostridium butyricum that functionally substitutes for the sn-glycerol-3-phosphate acyltransferase gene (plsB) of Escherichia coli

The sn-glycerol-3-phosphate acyltransferase (plsB) of Escherichia coli is a key regulatory enzyme that catalyzes the first committed step in phospholipid biosynthesis. We report the initial characterization of a novel gene (termed plsD) from Clostridium butyricum, cloned based on its ability to complement the sn-glycerol-3-phosphate auxotrophic phenotype of a plsB mutant strain of E. coli. Unlike the 83-kDa PlsB acyltransferase from E. coli, the predicted plsD open reading frame encoded a protein of 26.5 kDa. Two regions of strong homology to other lipid acyltransferases, including PlsB and PlsC analogs from mammals, plants, yeast, and bacteria, were identified. PlsD was most closely related to the 1-acyl-sn-glycerol-3-phosphate acyltransferase (plsC) gene family but did not complement the growth of plsC(Ts) mutants. An in vivo metabolic labeling experiment using a plsB plsX plsC(Ts) strain of E. coli confirmed that the plsD expression restored the ability of the cells to synthesize 1-acyl-glycerol-3-phosphate. However, glycerol-3-phosphate acyltransferase activity was not detected in vitro in assays using either acyl-acyl carrier protein or acyl coenzyme A as the substrate.

Glycerol-3-phosphate acyltransferase in plants

Glycerol-3-phosphate acyltransferase (GPAT) catalyzes the transfer of an acyl group from an acyl donor to the sn-1 position of glycerol 3-phosphate. The plant cell contains three types of GPAT, which are located in the chloroplasts, mitochondria and cytoplasm, respectively. The enzyme in chloroplasts is soluble and uses acyl-(acyl-carrier protein) as the acyl donor, whereas the enzymes in the mitochondria and the cytoplasm are bound to membranes and use acyl-CoA as the acyl donor. cDNAs for GPAT of chloroplasts have been cloned from several plants, and the gene for the enzyme has been cloned from Arabidopsis thaliana. The amino acid sequences deduced from the nucleotide sequences of cDNAs indicate that the product of translation is a precursor of about 460 amino acid residues, which consists of a leader sequence of about 70 amino acid residues and a mature protein of about 400 residues, with a molecular mass of about 42 kDa. Genetic engineering of the unsaturation of fatty acids has been achieved by manipulation of the cDNA for the GPAT found in chloroplasts and has allowed modification of the ability of tobacco to tolerate chilling temperatures.

A human cDNA sequence with homology to non-mammalian lysophosphatidic acid acyltransferases

A novel human homologue of Escherichia coli, yeast and plant 1-acylglycerol-3-phosphate acyltransferase has been isolated from U937 cell cDNA. Expression of the cloned sequence in 1-acylglycerol-3-phosphate acyltransferase-deficient E. coli resulted in increased incorporation of oleic acid into cellular phospholipids. Membranes made from COS7 cells transfected with the cDNA exhibited higher acyltransferase activity towards a range of donor fatty acyl-CoAs and lysophosphatidic acid. Northern-blot analysis of the cDNA sequence indicated high levels of expression in immune cells and epithelium. Rapid amplification of cDNA ends revealed differentially expressed splice variants, which suggests regulation of the enzyme by alternative splicing. This cDNA therefore represents the first described sequence of a mammalian gene homologous to non-mammalian lysophosphatidic acid acyltransferases.

Human lysophosphatidic acid acyltransferase. cDNA cloning, expression, and localization to chromosome 9q34.3

Lysophosphatidic acid (1-acyl-sn-glycero-3-phosphate (LPA)) is a phospholipid with diverse biological activities. The mediator serves as an intermediate in membrane phospholipid metabolism but is also produced in acute settings by activated platelets. LPA is converted to phosphatidic acid, itself a lipid mediator, by an LPA acyltransferase (LPAAT). A human expressed sequence tag was identified by homology with a coconut LPAAT and used to isolate a full-length human cDNA from a heart muscle library. The predicted amino acid sequence bears 33% identity with a Caenorhabditis elegans LPAAT homologue and 23-28% identity with plant and prokaryotic LPAATs. Recombinant protein produced in COS 7 cells exhibited LPAAT activity with a preference for LPA as the acceptor phosphoglycerol and arachidonyl coenzyme A as the acyl donor. Northern blotting demonstrated that the mRNA is expressed in most human tissues including a panel of brain subregions; expression is highest in liver and pancreas and lowest in placenta. The human LPAAT gene is contained on six exons that map to chromosome 9, region q34.3.

Cloning and expression of two human lysophosphatidic acid acyltransferase cDNAs that enhance cytokine-induced signaling responses in cells

Lysophosphatidic acid (LPA) and phosphatidic acid (PA) are two phospholipids involved in signal transduction and in lipid biosynthesis in cells. LPA acyltransferase (LPAAT), also known as 1-acyl sn-glycerol-3-phosphate acetyltransferase (EC 2.3.1.51), catalyzes the conversion of LPA to PA. In this study, we describe the isolation and characterization of two human cDNAs that encode proteins possessing LPAAT activities. These two proteins, designated here as LPAAT-alpha and LPAAT-beta, contain extensive sequence sequence similarities to microbial or plant LPAAT sequences. LPAAT-alpha mRNA was detected in all tissues with highest expression in skeletal muscle whereas LPAAT-beta was expressed predominantly in heart and liver tissues. Expression of these two cDNAs in an Escherichia coli strain with a mutated LPAAT gene (plsC) complements its growth defect and shifts the equilibrium of cellular lipid content from LPA to PA and other lipids. Overexpression of these two cDNAs in mammalian cells leads to increased LPAAT activity in cell-free extracts using an in vitro assay that measures the conversion of fluorescently labeled LPA to PA. This increase in LPAAT activity correlates with enhancement of transcription and synthesis of tumor necrosis factor-alpha and interleukin-6 from cells upon stimulation with interleukin-1beta, suggesting LPAAT overexpression may amplify cellular signaling responses from cytokines.

Phosphatidate phosphatases of mammals, yeast, and higher plants

Phosphatidate phosphatase (EC 3.1.3.4) catalyzes the hydrolysis of phosphatidate to yield sn-1,2-diacylglycerol and inorganic phosphate. In mammalian systems, forms of phosphatidate phosphatase involved in glycerolipid synthesis and signal transduction have been identified. Forms of the enzyme involved in signal transduction have been purified and partially characterized. In yeast, phosphatidate phosphatases associated with the endoplasmic reticulum and mitochondria have also been purified and partially characterized. Information on phosphatidate phosphatases from mammals and yeast is useful in characterizing the enzyme from plant systems. This review examines progress on the characterization of phosphatidate phosphatases from mammals, yeast, and higher plants. The purification and characterization of the phosphatidate phosphatase involved in glycerolipid synthesis in developing oilseeds may lead to the identification of the encoding gene. Increasing our understanding of the enzymes of lipid synthesis in developing seeds will aid in the development of biotechnological strategies for seed oil modification.

Cloning of a cDNA encoding the 21.2 kDa oleosin isoform from Arabidopsis thaliana and a study of its expression in a mutant defective in diacylglycerol acyltransferase activity

A full-length cDNA clone (pA23) of 832 bp encoding an oleosin from Arabidopsis thaliana was isolated by differential screening of a silique-specific cDNA library with probes prepared from poly(A)+ RNA isolated from developing seeds of wild-type (WT) Arabidopsis and from mutant AS11 with a lesion affecting diacylglycerol acyltransferase (DGAT) activity during embryo development. The encoded protein has a calculated molecular mass of 21.2 kDa, and its amino acid sequence shows strong sequence homology and structural similarity to other known oleosins. Transcription of the oleosin gene during seed development was both reduced and delayed in AS11 compared to WT. However, the level of oleosin protein did not appear to be down-regulated during seed development, and at maturity, the overall level of oleosin protein was similar in both WT and AS11. These findings indicate that regulation of oleosin gene expression is part of a highly complex, and co-ordinated expression of storage lipid biosynthesis and related (oleosin) genes during oilseed development.

Role of oxidation in the neurotoxic effects of intrastriatal dopamine injections

We have examined the biochemical and histological effects of high concentrations of dopamine (0.05-1.0 micromol) injected into the rat striatum. Twenty-four hours after such injections, the oxidation products of dopamine and dihydroxyphenylacetic acid were detected as both free and protein-bound cysteinyl dopamine and cysteinyl dihydroxyphenylacetic acid. Protein-bound cysteinyl catechols were increased 7- to 20-fold above control tissue levels. By 7 days postinjection, the protein-bound cysteinyl catechols were still detectable, although reduced in concentration, whereas the free forms could no longer be measured. Histological examination of striatum at 7 days revealed a central core of nonspecific damage including neuronal loss and gliosis. This core was surrounded by a region containing a marked reduction in tyrosine hydroxylase immunoreactivity but no apparent loss of serotonin or synaptophysin immunoreactivity. When dopamine was injected with an equimolar concentration of either ascorbic acid or glutathione, the formation of protein-bound cysteinyl catechols was greatly reduced. Moreover, the specific loss of tyrosine hydroxylase immunoreactivity associated with dopamine injections was no longer detectable, although the nonspecific changes in cytoarchitecture were still apparent. Thus, following its oxidation, dopamine in high concentrations binds to protein in the striatum, an event that is correlated with the specific loss of dopaminergic terminals. We suggest that the selective degeneration of dopamine neurons in Parkinson's disease may be caused by an imbalance between the oxidation of dopamine and the availability of antioxidant defenses.

Single-amino acid substitutions eliminate lysine inhibition of maize dihydrodipicolinate synthase

Dihydrodipicolinate synthase (DHPS; EC 4.2.1.52) catalyzes the first step in biosynthesis of lysine in plants and bacteria. DHPS in plants is highly sensitive to end-product inhibition by lysine and, therefore, has an important role in regulating metabolite flux into lysine. To better understand the feedback inhibition properties of the plant enzyme, we transformed a maize cDNA for lysine-sensitive DHPS into an Escherichia coli strain lacking DHPS activity. Cells were mutagenized with ethylmethanesulfonate, and potential DHPS mutants were selected by growth on minimal medium containing the inhibitory lysine analogue S-2-aminoethyl-L-cysteine. DHPS assays identified surviving colonies expressing lysine-insensitive DHPS activity. Ten single-base-pair mutations were identified in the maize DHPS cDNA sequence; these mutations were specific to one of three amino acid residues (amino acids 157, 162, and 166) localized within a short region of the polypeptide. No other mutations were present in the remaining DHPS cDNA sequence, indicating that altering only one of the three residues suffices to eliminate lysine inhibition of maize DHPS. Identification of these specific mutations that change the highly sensitive maize DHPS to a lysine-insensitive isoform will help resolve the lysine-binding mechanism and the resultant conformational changes involved in inhibition of DHPS activity. The plant-derived mutant DHPS genes may also be used to improve nutritional quality of maize or other cereal grains that have inadequate lysine content when fed to animals such as poultry, swine, or humans.

Identification of a cDNA that encodes a 1-acyl-sn-glycerol-3-phosphate acyltransferase from Limnanthes douglasii

Two different techniques were used to isolate potential cDNAs for acyl-CoA: 1-acyl-sn-glycerol-3-phosphate acyltransferase (LPA-AT) enzymes from Limnanthes douglasii. Both heterologous screening with the maize pMAT1 clone and in vivo complementation of the Escherichia coli mutant JC201 which is deficient in LPA-AT activity, were carried out. Clones identified by these procedures were different. Homology searches demonstrated that the clone isolated by heterologous probing, pLAT1, encodes a protein which is most similar to the maize (open reading frame in pMAT1) and yeast SLC1 proteins, which are putative LPA-AT sequences. This L. douglasii sequence shows much lower homology to the E. coli LPA-AT protein PlsC, which is the only LPA-AT sequence confirmed by over-expression studies. The clone isolated by complementation, pLAT2, encodes a protein with homology to both SLC1 and PlsC. It was not possible to over-express the complementing protein encoded by pLAT2 but further experimentation on membranes from complemented JC201 demonstrated that they possess a substrate specificity distinctly different from PlsC and similar to Limnanthes sp. microsome specificity. This data strongly supports the contention that pLAT2 is an LPA-AT clone. Northern blot analysis revealed different expression patterns for the two genes in pLAT1 and pLAT2. Transcription of the gene encoding the insert of pLAT2 occurred almost exclusively in developing seed tissue, whilst the cDNA of pLAT1 hybridised to poly(A)+ mRNA from seed, stem and leaf, demonstrating more widespread expression throughout the plant. Southern blot analysis indicated that the cDNA of pLAT2 was transcribed from a single-copy gene while that for pLAT1 was a member of a small gene family.