Mlg1, a yeast acyltransferase located in ER membranes associated with mitochondria (MAMs), is involved in de novo synthesis and remodelling of phospholipids

In cells, phospholipids contain acyl chains of variable lengths and saturation, features that affect their functions. Their de novo synthesis in the endoplasmic reticulum takes place via the cytidine diphosphate diacylglycerol (CDP-DAG) and Kennedy pathways, which are conserved in eukaryotes. PA is a key intermediate for all phospholipids (PI, PIPs, PS, PE, PC, PG and CL). The de novo synthesis of PA occurs by acylation of glycerophosphate leading to the synthesis of 1-acyl lysoPA and subsequent acylation of 1-acyl lysoPA at the sn-2 position. Using membranes from Escherichia coli overexpressing MLG1, we showed that the yeast gene MLG1 encodes an acyltransferase, leading specifically to the synthesis of PA from 1-acyl lysoPA. Moreover, after their de novo synthesis, phospholipids can be remodelled by acyl exchange with one and/or two acyl chains exchanged at the sn-1 and/or sn-2 position. Based on shotgun lipidomics of the reference and mlg1Δ strains, as well as biochemical assays for acyltransferase activities, we identified an additional remodelling activity for Mlg1p, namely, incorporation of palmitic acid into the sn-1 position of PS and PE. By using confocal microscopy and subcellular fractionation, we also found that this acyltransferase is located in ER membranes associated with mitochondria, a finding that highlights the importance of these organelles in the global cellular metabolism of lipids.

Lysophosphatidic acid acyltransferases: a link with intracellular protein trafficking in Arabidopsis root cells?

Phosphatidic acid (PA) and lysophosphatidic acid acyltransferases (LPAATs) might be critical for the secretory pathway. Four extra-plastidial LPAATs (LPAAT2, 3, 4, and 5) were identified in Arabidopsis thaliana. These AtLPAATs display a specific enzymatic activity converting lysophosphatidic acid to PA and are located in the endomembrane system. We investigate a putative role for AtLPAATs 3, 4, and 5 in the secretory pathway of root cells through genetical (knockout mutants), biochemical (activity inhibitor, lipid analyses), and imaging (live and immuno-confocal microscopy) approaches. Treating a lpaat4;lpaat5 double mutant with the LPAAT inhibitor CI976 produced a significant decrease in primary root growth. The trafficking of the auxin transporter PIN2 was disturbed in this lpaat4;lpaat5 double mutant treated with CI976, whereas trafficking of H+-ATPases was unaffected. The lpaat4;lpaat5 double mutant is sensitive to salt stress, and the trafficking of the aquaporin PIP2;7 to the plasma membrane in the lpaat4;lpaat5 double mutant treated with CI976 was reduced. We measured the amounts of neo-synthesized PA in roots, and found a decrease in PA only in the lpaat4;lpaat5 double mutant treated with CI976, suggesting that the protein trafficking impairment was due to a critical PA concentration threshold.

Phosphoinositides containing stearic acid are required for interaction between Rho GTPases and the exocyst to control the late steps of polarised exocytosis

Cell polarity is achieved by regulators such as small G proteins, exocyst members and phosphoinositides, with the latter playing a key role when bound to the exocyst proteins Sec3p and Exo70p, and Rho GTPases. This ensures asymmetric growth via the routing of proteins and lipids to the cell surface using actin cables. Previously, using a yeast mutant for a lysophosphatidylinositol acyl transferase encoded by the PSI1 gene, we demonstrated the role of stearic acid in the acyl chain of phosphoinositides in cytoskeletal organisation and secretion. Here, we use a genetic approach to characterise the effect on late steps of the secretory pathway. The constitutive overexpression of PSI1 in mutants affecting kinases involved in the phosphoinositide pathway demonstrated the role of molecular species containing stearic acid in bypassing a lack of phosphatidylinositol-4-phosphate PI(4)P at the plasma membrane, which is essential for the function of the Cdc42p module. Decreasing the levels of stearic acid-containing phosphoinositides modifies the environment of the actors involved in the control of late steps in the secretory pathway. This leads to decreased interactions between Exo70p and Sec3p, with Cdc42p, Rho1p and Rho3p, due to disruption of the GTP/GDP ratio of at least Rho1p and Rho3p GTPases, thereby preventing activation of the exocyst.

Impact of diuron and S-metolachlor on the freshwater diatom Gomphonema gracile: Complementarity between fatty acid profiles and different kinds of ecotoxicological impact-endpoints

Fatty acids (FA) are crucial for the maintenance of membrane fluidity and play a central role in metabolic energy storage. Polyunsaturated fatty acids play an essential ecological role since they are key parameters in the nutritional value of algae. Pesticide impacts on fatty acid profiles have been documented in marine microalgae, but remain understudied in freshwater diatoms. The aims of this study were to: 1) investigate the impact of diuron and S-metolachlor on "classical descriptors" (photosynthesis, growth rate, pigment contents, and on the expression levels of target genes in freshwater diatoms), 2) examine the impact of these pesticides on diatom fatty acid profiles and finally, 3) compare fatty acid profiles and "classical descriptor" responses in order to evaluate their complementarity and ecological role. To address this issue, the model freshwater diatom Gomphonema gracile was exposed during seven days to diuron and S-metolachlor at 10 μg.L^-1. G. gracile was mostly composed of the following fatty acids: 20:5n3; 16:1; 16:0; 16:3n4; 14:0 and 20:4n6 and highly unsaturated fatty acids were overall the best represented fatty acid class. S-metolachlor decreased the growth rate and chlorophyll a content of G. gracile and induced the expression of cox1, nad5, d1 and cat genes, while no significant impacts were observed on photosynthesis and carotenoid content. In a more global way, S-metolachlor did not impact the fatty acid profiles of G. gracile. Diuron inhibited photosynthesis, growth rate, chlorophyll a content and induced cat and d1 gene expressions but no significant effect was observed on carotenoid content. Diuron decreased the percentage of highly unsaturated fatty acids but increased the percentage of monounsaturated fatty acids. These results demonstrated that fatty acids responded to diuron conversely to pigment content, suggesting that fatty acids can inform on energy content variation in diatoms subjected to herbicide stress.

Proteomic Analysis of Lipid Droplets from Arabidopsis Aging Leaves Brings New Insight into Their Biogenesis and Functions

Lipid droplets (LDs) are cell compartments specialized for oil storage. Although their role and biogenesis are relatively well documented in seeds, little is known about their composition, structure and function in senescing leaves where they also accumulate. Here, we used a label free quantitative mass spectrometry approach to define the LD proteome of aging Arabidopsis leaves. We found that its composition is highly different from that of seed/cotyledon and identified 28 proteins including 9 enzymes of the secondary metabolism pathways involved in plant defense response. With the exception of the TRIGALACTOSYLDIACYLGLYCEROL2 protein, we did not identify enzymes implicated in lipid metabolism, suggesting that growth of leaf LDs does not occur by local lipid synthesis but rather through contact sites with the endoplasmic reticulum (ER) or other membranes. The two most abundant proteins of the leaf LDs are the CALEOSIN3 and the SMALL RUBBER PARTICLE1 (AtSRP1); both proteins have structural functions and participate in plant response to stress. CALEOSIN3 and AtSRP1 are part of larger protein families, yet no other members were enriched in the LD proteome suggesting a specific role of both proteins in aging leaves. We thus examined the function of AtSRP1 at this developmental stage and found that AtSRP1 modulates the expression of CALEOSIN3 in aging leaves. Furthermore, AtSRP1 overexpression induces the accumulation of triacylglycerol with an unusual composition compared to wild-type. We demonstrate that, although AtSRP1 expression is naturally increased in wild type senescing leaves, its overexpression in senescent transgenic lines induces an over-accumulation of LDs organized in clusters at restricted sites of the ER. Conversely, atsrp1 knock-down mutants displayed fewer but larger LDs. Together our results reveal that the abundancy of AtSRP1 regulates the neo-formation of LDs during senescence. Using electron tomography, we further provide evidence that LDs in leaves share tenuous physical continuity as well as numerous contact sites with the ER membrane. Thus, our data suggest that leaf LDs are functionally distinct from seed LDs and that their biogenesis is strictly controlled by AtSRP1 at restricted sites of the ER.

Trace element bioavailability, yield and seed quality of rapeseed (Brassica napus L.) modulated by biochar incorporation into a contaminated technosol

BACKGROUND AND AIMS

Rapeseed (Brassica napus L.) is a Cd/Zn-accumulator whereas soil conditioners such as biochars may immobilize trace elements. These potentially complementary soil remediation options were trialed, singly and in combination, in a pot experiment with a metal(loid)-contaminated technosol.

METHODS

The technosol [total content in mg kg(-1) Zn 6089, Cd 9.4, Cu 110, and Pb 956] was either amended (2% w/w) or not with a poultry manure-derived biochar. Rapeseed was cultivated for both soil treatments during 24 weeks up to harvest under controlled conditions.

RESULTS

Biochar incorporation into the technosol promoted the As, Cd, Cu, Mo, Ni, Pb and Zn solubility. It decreased foliar B, Cu and Mo concentrations, and Mo concentration in stems, pericarps and seeds. But, it did not impact neither the biomass of aerial rapeseed parts (except a decrease for seeds), nor their C (except a decrease for stems), seed fatty acid, seed starch and soluble sugar contents, and antioxidant capacity in both leaves and seeds. Biochar amendment increased the phytoextraction by aerial plant parts for K, P, and S, reduced it for N, Ca, B, Mo, Ni and Se, whereas it remained steady for Mg, Zn, Fe, Mn, Cu, Cd and Co.

CONCLUSIONS

The biochar incorporation into this technosol did not promote Cd, Cu and Zn phytoextraction by rapeseed and its potential oilseed production, but increased the solubility of several metal(loid)s. Here Zn and Cd concentrations in the soil pore water were decreased by rapeseed, showing the feasibility to strip available soil Zn and Cd in combination with seed production.

Complementation of Seed Maturation Phenotypes by Ectopic Expression of ABSCISIC ACID INSENSITIVE3, FUSCA3 and LEAFY COTYLEDON2 in Arabidopsis

ABSCISIC ACID INSENSITIVE3 (ABI3), FUSCA3 (FUS3) and LEAFY COTYLEDON2 (LEC2), collectively the AFL, are master regulators of seed maturation processes. This study examined the role of AFL in the production of seed reserves in Arabidopsis. Quantification of seed reserves and cytological observations of afl mutant embryos show that protein and lipid but not starch reserves are spatially regulated by AFL. Although AFL contribute to a common regulation of reserves, ABI3 exerts a quantitatively greater control over storage protein content whereas FUS3 controls lipid content to a greater extent. Although ABI3 controls the reserve content throughout the embryo, LEC2 and FUS3 regulate reserves in distinct embryonic territories. By analyzing the ability of an individual ectopically expressed AFL to suppress afl phenotypes genetically, we show that conserved domains common to each component of the AFL are sufficient for the initiation of storage product synthesis and the establishment of embryo morphology. This confirms redundancy among the AFL and indicates a threshold necessary for function within the AFL pool. Since no individual AFL was able to suppress the tolerance to desiccation, mid- and late-maturation programs were uncoupled.

Involvement of the caleosin/peroxygenase RD20 in the control of cell death during Arabidopsis responses to pathogens

Caleosins, mostly found in lipid droplets of seeds and leaves, are believed to play physiological roles through their enzymatic capacities to produce oxylipins. We recently identified the caleosin RD20 as a peroxygenase reducing endogenous fatty acid hydroperoxides into their corresponding alcohols. Such oxylipins confer tolerance to oxidative stress by decreasing reactive oxygen species accumulation and by minimizing cell death. RD20 expression being induced by pathogens, we have examined the mode of action of this caleosin in response to biotic stress. Plants overexpressing RD20 exhibited an alteration of their leaf cuticle wax components and an increased resistance to the fungus Alternaria brassicicola. Conversely, silencing RD20 led to an enhanced propagation of the fungus and to reduced severity of the damages caused by the inoculation of the bacteria Pseudomonas syringae pv tomato. We discuss these findings and propose that the major function of RD20 is to generate oxylipins modulating oxidative status and cell death.

A Fox2-dependent fatty acid ß-oxidation pathway coexists both in peroxisomes and mitochondria of the ascomycete yeast Candida lusitaniae

It is generally admitted that the ascomycete yeasts of the subphylum Saccharomycotina possess a single fatty acid ß-oxidation pathway located exclusively in peroxisomes, and that they lost mitochondrial ß-oxidation early during evolution. In this work, we showed that mutants of the opportunistic pathogenic yeast Candida lusitaniae which lack the multifunctional enzyme Fox2p, a key enzyme of the ß-oxidation pathway, were still able to grow on fatty acids as the sole carbon source, suggesting that C. lusitaniae harbored an alternative pathway for fatty acid catabolism. By assaying 14Cα-palmitoyl-CoA consumption, we demonstrated that fatty acid catabolism takes place in both peroxisomal and mitochondrial subcellular fractions. We then observed that a fox2Δ null mutant was unable to catabolize fatty acids in the mitochondrial fraction, thus indicating that the mitochondrial pathway was Fox2p-dependent. This finding was confirmed by the immunodetection of Fox2p in protein extracts obtained from purified peroxisomal and mitochondrial fractions. Finally, immunoelectron microscopy provided evidence that Fox2p was localized in both peroxisomes and mitochondria. This work constitutes the first demonstration of the existence of a Fox2p-dependent mitochondrial β-oxidation pathway in an ascomycetous yeast, C. lusitaniae. It also points to the existence of an alternative fatty acid catabolism pathway, probably located in peroxisomes, and functioning in a Fox2p-independent manner.

The reductase activity of the Arabidopsis caleosin RESPONSIVE TO DESSICATION20 mediates gibberellin-dependent flowering time, abscisic acid sensitivity, and tolerance to oxidative stress

Contrasting with the wealth of information available on the multiple roles of jasmonates in plant development and defense, knowledge about the functions and the biosynthesis of hydroxylated oxylipins remains scarce. By expressing the caleosin RESPONSIVE TO DESSICATION20 (RD20) in Saccharomyces cerevisiae, we show that the recombinant protein possesses an unusual peroxygenase activity with restricted specificity toward hydroperoxides of unsaturated fatty acid. Accordingly, Arabidopsis (Arabidopsis thaliana) plants overexpressing RD20 accumulate the product 13-hydroxy-9,11,15-octadecatrienoic acid, a linolenate-derived hydroxide. These plants exhibit elevated levels of reactive oxygen species (ROS) associated with early gibberellin-dependent flowering and abscisic acid hypersensitivity at seed germination. These phenotypes are dependent on the presence of active RD20, since they are abolished in the rd20 null mutant and in lines overexpressing RD20, in which peroxygenase was inactivated by a point mutation of a catalytic histidine residue. RD20 also confers tolerance against stress induced by Paraquat, Rose Bengal, heavy metal, and the synthetic auxins 1-naphthaleneacetic acid and 2,4-dichlorophenoxyacetic acid. Under oxidative stress, 13-hydroxy-9,11,15-octadecatrienoic acid still accumulates in RD20-overexpressing lines, but this lipid oxidation is associated with reduced ROS levels, minor cell death, and delayed floral transition. A model is discussed where the interplay between fatty acid hydroxides generated by RD20 and ROS is counteracted by ethylene during development in unstressed environments.

Influence of fine process particles enriched with metals and metalloids on Lactuca sativa L. leaf fatty acid composition following air and/or soil-plant field exposure

We investigate the effect of both foliar and root uptake of a mixture of metal(loid)s on the fatty acid composition of plant leaves. Our objectives are to determine whether both contamination pathways have a similar effect and whether they interact. Lactuca sativa L. were exposed to fine process particles enriched with metal(loid)s in an industrial area. Data from a first experiment were used to conduct an exploratory statistical analysis which findings were successfully cross-validated by using the data from a second one. Both foliar and root pathways impact plant leaf fatty acid composition and do not interact. Z index (dimensionless quantity), weighted product of fatty acid concentration ratios was built up from the statistical analyses. It provides new insights on the mechanisms involved in metal uptake and phytotoxicity. Plant leaf fatty acid composition is a robust and fruitful approach to detect and understand the effects of metal(loid) contamination on plants.

Early changes in the fatty acid composition of photosynthetic membrane lipids from Populus nigra grown on a metallurgical landfill

We compared the fatty acid composition of leaves taken from poplars on a metal-contaminated landfill, and on the uncontaminated roadside bordering this site. For the first time, it is shown that the percentage of linolenic acid, which is mainly associated with thylakoid lipids, was significantly lower in tree species within the landfill than within the control area. A correlation study was carried out to investigate relationships between the C18:3/(C18:0 + C18:1 + C18:2) fatty acid ratios and the metal contents in soils and leaves. Lead and chromium leaf contents were significantly negatively correlated to this fatty acid ratio. The impact of each of these metals remains difficult to evaluate, but chromium in leaf likely plays a major role in toxicity. In addition, the decrease in the C18:3/(C18:0 + C18:1 + C18:2) fatty acid ratio occurred at low leaf metal content, and therefore it is shown that this ratio can be used as an early indicator of the effect of metals.

Lipids of plant membrane rafts

Lipids tend to organize in mono or bilayer phases in a hydrophilic environment. While they have long been thought to be incapable of coherent lateral segregation, it is now clear that spontaneous assembly of these compounds can confer microdomain organization beyond spontaneous fluidity. Membrane raft microdomains have the ability to influence spatiotemporal organization of protein complexes, thereby allowing regulation of cellular processes. In this review, we aim at summarizing briefly: (i) the history of raft discovery in animals and plants, (ii) the main findings about structural and signalling plant lipids involved in raft segregation, (iii) imaging of plant membrane domains, and their biochemical purification through detergent-insoluble membranes, as well as the existing debate on the topic. We also discuss the potential involvement of rafts in the regulation of plant physiological processes, and further discuss the prospects of future research into plant membrane rafts.

N-Acylethanolamines and related compounds: aspects of metabolism and functions

N-Acylethanolamines (NAE) are fatty acid derivates that are linked with an ethanolamine group via an amide bond. NAE can be characterized as lipid mediators in the plant and animal kingdoms owing to the diverse functions throughout the eukaryotic domain. The functions of NAE have been widely investigated in animal tissues in part due to their abilities to interact with the cannabinoid receptors, vanilloid receptors or peroxisome proliferator activated receptors. However, the interest of studying the functions of these lipids in plants is progressively becoming more apparent. The number of publications about the functions related to NAE and to structural analogs (homoserine lactone and alkamides) is greatly increasing, showing the importance of these lipids in various plant physiological processes. This review sheds light on their role in different processes such as seedling development, plant pathogen interaction, phospholipase D alpha inhibition and senescence of cut flowers, and underlines the interaction between NAE and NAE-related molecules with plant hormone signaling. The different metabolic pathways promoting the synthesis and degradation of NAE are also discussed, in particular the oxygenation of polyunsaturated N-acylethanolamines, which leads to NAE-oxylipins, a new family of bioactive lipids.

YPR139c/LOA1 encodes a novel lysophosphatidic acid acyltransferase associated with lipid droplets and involved in TAG homeostasis

LOA1, a yeast member of the glycerolipid acyltransferase family, encodes a novel lysophosphatidic acid acyltransferase associated with lipid droplets (LDs) and involved in triacylglycerol (TAG) accumulation. Loa1p, recruited during LD formation, preferentially directs oleic acid–containing phosphatidic acid species into the TAG biosynthetic pathway.

For many years, lipid droplets (LDs) were considered to be an inert store of lipids. However, recent data showed that LDs are dynamic organelles playing an important role in storage and mobilization of neutral lipids. In this paper, we report the characterization of LOA1 (alias VPS66, alias YPR139c), a yeast member of the glycerolipid acyltransferase family. LOA1 mutants show abnormalities in LD morphology. As previously reported, cells lacking LOA1 contain more LDs. Conversely, we showed that overexpression results in fewer LDs. We then compared the lipidome of loa1Δ mutant and wild-type strains. Steady-state metabolic labeling of loa1Δ revealed a significant reduction in triacylglycerol content, while phospholipid (PL) composition remained unchanged. Interestingly, lipidomic analysis indicates that both PLs and glycerolipids are qualitatively affected by the mutation, suggesting that Loa1p is a lysophosphatidic acid acyltransferase (LPA AT) with a preference for oleoyl-CoA. This hypothesis was tested by in vitro assays using both membranes of Escherichia coli cells expressing LOA1 and purified proteins as enzyme sources. Our results from purification of subcellular compartments and proteomic studies show that Loa1p is associated with LD and active in this compartment. Loa1p is therefore a novel LPA AT and plays a role in LD formation.

Polyphosphoinositides are enriched in plant membrane rafts and form microdomains in the plasma membrane

In this article, we analyzed the lipid composition of detergent-insoluble membranes (DIMs) purified from tobacco (Nicotiana tabacum) plasma membrane (PM), focusing on polyphosphoinositides, lipids known to be involved in various signal transduction events. Polyphosphoinositides were enriched in DIMs compared with whole PM, whereas all structural phospholipids were largely depleted from this fraction. Fatty acid composition analyses suggest that enrichment of polyphosphoinositides in DIMs is accompanied by their association with more saturated fatty acids. Using an immunogold-electron microscopy strategy, we were able to visualize domains of phosphatidylinositol 4,5-bisphosphate in the plane of the PM, with 60% of the epitope found in clusters of approximately 25 nm in diameter and 40% randomly distributed at the surface of the PM. Interestingly, the phosphatidylinositol 4,5-bisphosphate cluster formation was not significantly sensitive to sterol depletion induced by methyl-beta-cyclodextrin. Finally, we measured the activities of various enzymes of polyphosphoinositide metabolism in DIMs and PM and showed that these activities are present in the DIM fraction but not enriched. The putative role of plant membrane rafts as signaling membrane domains or membrane-docking platforms is discussed.

Expression of rapeseed microsomal lysophosphatidic acid acyltransferase isozymes enhances seed oil content in Arabidopsis

In higher plants, lysophosphatidic acid acyltransferase (LPAAT), located in the cytoplasmic endomembrane compartment, plays an essential role in the synthesis of phosphatidic acid, a key intermediate in the biosynthesis of membrane phospholipids in all tissues and storage lipids in developing seeds. In order to assess the contribution of LPAATs to the synthesis of storage lipids, we have characterized two microsomal LPAAT isozymes, the products of homoeologous genes that are expressed in rapeseed (Brassica napus). DNA sequence homologies, complementation of a bacterial LPAAT-deficient mutant, and enzymatic properties confirmed that each of two cDNAs isolated from a Brassica napus immature embryo library encoded a functional LPAAT possessing the properties of a eukaryotic pathway enzyme. Analyses in planta revealed differences in the expression of the two genes, one of which was detected in all rapeseed tissues and during silique and seed development, whereas the expression of the second gene was restricted predominantly to siliques and developing seeds. Expression of each rapeseed LPAAT isozyme in Arabidopsis (Arabidopsis thaliana) resulted in the production of seeds characterized by a greater lipid content and seed mass. These results support the hypothesis that increasing the expression of glycerolipid acyltransferases in seeds leads to a greater flux of intermediates through the Kennedy pathway and results in enhanced triacylglycerol accumulation.

Discovery and characterization of an Arabidopsis thaliana N-acylphosphatidylethanolamine synthase

N-Acylethanolamines (NAEs) are lipids involved in several physiological processes in animal and plant cells. In brain, NAEs are ligands of endocannabinoid receptors, which modulate various signaling pathways. In plant, NAEs regulate seed germination and root development, and they are involved in plant defense against pathogen attack. This signaling activity is started by an enzyme called N-acylphosphatidylethanolamine (NAPE) synthase. This catalyzes the N-acylation of phosphatidylethanolamine to form NAPE, which is most likely hydrolyzed by phospholipase D beta/gamma isoforms to generate NAE. This compound is further catabolized by fatty amide hydrolase. The genes encoding the enzymes involved in NAE metabolism are well characterized except for the NAPE synthase gene(s). By heterologous expression in Escherichia coli and overexpression in plants, we characterized an acyltransferase from Arabidopsis thaliana (At1g78690p) catalyzing the synthesis of lipids identified as NAPEs (two-dimensional TLC, phospholipase D hydrolysis assay, and electrospray ionization-tandem mass spectrometry analyses). The ability of free fatty acid and acyl-CoA to be used as acyl donor was compared in vitro with E. coli membranes and purified enzyme (obtained by immobilized metal ion affinity chromatography). In both cases, NAPE was synthesized only in the presence of acyl-CoA. beta-Glucuronidase promoter experiments revealed a strong expression in roots and young tissues of plants. Using yellow fluorescent protein fusion, we showed that the NAPE synthase is located in the plasmalemma of plant cells.

Remorin, a solanaceae protein resident in membrane rafts and plasmodesmata, impairs potato virus X movement

Remorins (REMs) are proteins of unknown function specific to vascular plants. We have used imaging and biochemical approaches and in situ labeling to demonstrate that REM clusters at plasmodesmata and in approximately 70-nm membrane domains, similar to lipid rafts, in the cytosolic leaflet of the plasma membrane. From a manipulation of REM levels in transgenic tomato (Solanum lycopersicum) plants, we show that Potato virus X (PVX) movement is inversely related to REM accumulation. We show that REM can interact physically with the movement protein TRIPLE GENE BLOCK PROTEIN1 from PVX. Based on the localization of REM and its impact on virus macromolecular trafficking, we discuss the potential for lipid rafts to act as functional components in plasmodesmata and the plasma membrane.

Development of a biomarker for metal bioavailability: the lettuce fatty acid composition

Because Lactuca sativa L. is a plant widely used in ecotoxicological analyses, a study was developed to determine whether the fatty acid composition of lettuce leaves could be used as an additional biomarker of soil contamination by metals such as Pb, Cu, Zn, and Cd. Unlike seed germination or seedling growth, the fatty acid composition of lettuce leaves differed significantly between uncontaminated and field metal-contaminated soils. Hence, this lipid biomarker might provide an early indication of a plant's exposure to metals and the potential bioavailability of metals, and could facilitate or strengthen the diagnosis of soil contamination. Not only is the experimental protocol cheap, rapid, and easy, but the values of the lipid biomarker are highly reproducible when seedlings are grown at the same light intensity. In addition, the values of the biomarker did not vary greatly when 14- to 18-day-old plants were analyzed and when slight differences were introduced in the experimental conditions used to determine the leaf fatty acid composition.

Plant lipid rafts: fluctuat nec mergitur

Lipid rafts in plasma membranes are hypothesized to play key roles in many cellular processes including signal transduction, membrane trafficking and entry of pathogens. We recently documented the biochemical characterization of lipid rafts, isolated as detergent-insoluble membranes, from Medicago truncatula root plasma membranes. We evidenced that the plant-specific lipid steryl-conjugates are among the main lipids of rafts together with free sterols and sphingolipids. An extensive proteomic analysis showed the presence of a specific set of proteins common to other lipid rafts, plus the presence of a redox system around a cytochrome b(561) not previously identified in lipid rafts of either plants or animals. Here, we discuss the similarities and differences between the lipids and proteins of plant and animal lipid rafts. Moreover we describe the potential biochemical functioning of the M. truncatula root lipid raft redox proteins and question whether they may play a physiological role in legume-symbiont interactions.

Characterization of lipid rafts from Medicago truncatula root plasma membranes: a proteomic study reveals the presence of a raft-associated redox system

Several studies have provided new insights into the role of sphingolipid/sterol-rich domains so-called lipid rafts of the plasma membrane (PM) from mammalian cells, and more recently from leaves, cell cultures, and seedlings of higher plants. Here we show that lipid raft domains, defined as Triton X-100-insoluble membranes, can also be prepared from Medicago truncatula root PMs. These domains have been extensively characterized by ultrastructural studies as well as by analysis of their content in lipids and proteins. M. truncatula lipid domains are shown to be enriched in sphingolipids and Delta(7)-sterols, with spinasterol as the major compound, but also in steryl glycosides and acyl-steryl glycosides. A large number of proteins (i.e. 270) have been identified. Among them, receptor kinases and proteins related to signaling, cellular trafficking, and cell wall functioning were well represented whereas those involved in transport and metabolism were poorly represented. Evidence is also given for the presence of a complete PM redox system in the lipid rafts.

Lipid oxidation and autophagy in yeast

Autophagy, a process involved in the degradation and the recycling of long-lived proteins and organelles to survive nitrogen starvation, is generally non-selective. However, recent data suggest that selective forms of autophagy exist, that are able to specifically target several organelles, including mitochondria. Conversely, mitochondrial alterations could trigger autophagy. Such a selective form of autophagy might be involved in the elimination of damaged mitochondria. We reported previously that, mitochondria were early targets of rapamycin-induced autophagy. Here we report that rapamycin-induced autophagy is accompanied by the early production of reactive oxygen species and by the early oxidation of mitochondrial lipid. Inhibition of these oxidative effects by resveratrol largely impaired autophagy of both cytosolic proteins and mitochondria, and delayed subsequent cell death. These results support a role of mitochondrial oxidation events in the activation of autophagy.

Proteomics of plant detergent-resistant membranes

A large body of evidence from the past decade supports the existence, in membrane from animal and yeast cells, of functional microdomains that play important roles in protein sorting, signal transduction, or infection by pathogens. Recent reports demonstrated the presence, in plants, of detergent-resistant fractions isolated from plasma membrane. Analysis of the lipidic composition of this fraction revealed its enrichment in sphingolipids and sterols and depletion in phospho- and glycerolipids as previously observed for animal microdomains. One-dimensional gel electrophoresis experiments indicated that these detergent-resistant fractions are able to recruit a specific set of plasma membrane proteins and exclude others. In the present study, we used mass spectrometry to give an extensive description of a tobacco plasma membrane fraction resistant to solubilization with Triton X-100. This led to the identification of 145 proteins whose functional and physicochemical characteristics were analyzed in silico. Parameters such as isoelectric point, molecular weight, number and length of transmembrane segments, or global hydrophobicity were analyzed and compared with the data available concerning plant plasma membrane proteins. Post-translational modifications, such as myristoylation, palmitoylation, or presence of a glycosylphosphatidylinositol anchor, were examined in relation to the presence of the corresponding proteins in these microdomains. From a functional point of view, this analysis indicated that if a primary function of the plasma membrane, such as transport, seems under-represented in the detergent-resistant fraction, others undergo a significant increase of their relative importance. Among these are signaling and response to biotic and abiotic stress, cellular trafficking, and cell wall metabolism. This suggests that these domains are likely to constitute, as in animal cells, signaling platforms involved in these physiological functions.

Ypr140wp, 'the yeast tafazzin', displays a mitochondrial lysophosphatidylcholine (lyso-PC) acyltransferase activity related to triacylglycerol and mitochondrial lipid synthesis

When the yeast protein Ypr140w was expressed in Escherichia coli, a lyso-PC [lysophosphatidylcholine (1-acylglycerophosphorylcholine)] acyltransferase activity was found associated with the membranes of the bacteria. To our knowledge, this is the first identification of a protein capable of catalysing the acylation of lyso-PC molecules to form PC. Fluorescence microscopy analysis of living yeasts revealed that the fusion protein Ypr140w-green fluorescent protein is targeted to the mitochondria. Moreover, in contrast with wild-type cells, in the absence of acyl-CoA, the yeast mutant deleted for the YPR140w gene has no lyso-PC acyltransferase activity associated with the mitochondrial fraction. When yeast cells were grown in the presence of lactate, the mutant synthesized 2-fold more triacylglycerols when compared with the wild-type. Moreover, its mitochondrial membranes contained a lesser amount of PC and cardiolipin, and the fatty acid composition of these latter was greatly changed. These modifications were accompanied by a 2-fold increase in the respiration rates (states 3 and 4) of the mitochondria. The relationship between the deletion of the YPR140w gene and the lipid composition of the ypr140wDelta cells is discussed.

Chemotaxonomy of the Rubiaceae family based on leaf fatty acid composition

With 10,700 species distributed in 637 genera, the Rubiaceae family is one of the largest of the angiosperms. Since it was previously evidenced that the fatty acid composition of photosynthetic tissues can be a tool for chemotaxonomic studies, the fatty acid composition of leaves from 107 Rubiaceae species highly representative of the diversity of the family was determined. Principal component analysis allowed a clear-cut separation of Coffeae, Psychotrieae and Rubieae. The occurrence of C16:3 fatty acid, a marker of the prokaryotic plastidial lipid biosynthetic pathway, concerned at least two branches: Theligoneae/Rubieae and Anthospermeae-Anthosperminae which appeared to be in close relationship. Additional experiments were carried out to ensure the correlation between the presence of C16:3 fatty acid and the prokaryotic biosynthetic pathway.

The upstream oxylipin profile of Arabidopsis thaliana: a tool to scan for oxidative stresses

Various physiological imbalances lead to reactive oxygen species (ROS) overproduction and/or increases in lipoxygenase (LOX) activities, both events ending in lipid peroxidation of polyunsaturated fatty acids (PUFAs). Besides the quantification of such a process, the development of tools is necessary in order to allow the identification of the primary cause of its development and localization. A biochemical method assessing 9 LOX, 13 LOX and ROS-mediated peroxidation of membrane-bound and free PUFAs has been improved. The assay is based on the analysis of hydroxy fatty acids derived from PUFA hydroperoxides by both the straight and chiral phase high-performance liquid chromatography. Besides the upstream products of peroxidation of the 18:2 and 18:3 PUFAs, products coming from the 16:3 were characterized and their steady-state level quantified. Moreover, the observation that the relative amounts of the ROS-mediated peroxidation isomers of 18:3 were constant in leaves allowed us to circumvent the chiral analyses for the discrimination and quantification of 9 LOX, 13 LOX and ROS-mediated processes in routine experiments. The methodology has been successfully applied to decipher lipid peroxidation in Arabidopsis leaves submitted to biotic and abiotic stresses. We provide evidence of the relative timing of enzymatic and non-enzymatic lipid peroxidation processes. The 13 LOX pathway is activated early whatever the nature of the stress, leading to the peroxidation of chloroplast lipids. Under cadmium stress, the 9 LOX pathway added to the 13 LOX one. ROS-mediated peroxidation was mainly driven by light and always appeared as a late process.

Lipid rafts in higher plant cells: purification and characterization of Triton X-100-insoluble microdomains from tobacco plasma membrane

A large body of evidence from the past decade supports the existence of functional microdomains in membranes of animal and yeast cells, which play important roles in protein sorting, signal transduction, or infection by pathogens. They are based on the dynamic clustering of sphingolipids and cholesterol or ergosterol and are characterized by their insolubility, at low temperature, in nonionic detergents. Here we show that similar microdomains also exist in plant plasma membrane isolated from both tobacco leaves and BY2 cells. Tobacco lipid rafts were found to be greatly enriched in a sphingolipid, identified as glycosylceramide, as well as in a mixture of stigmasterol, sitosterol, 24-methylcholesterol, and cholesterol. Phospho- and glycoglycerolipids of the plasma membrane were largely excluded from lipid rafts. Membrane proteins were separated by one- and two-dimensional gel electrophoresis and identified by tandem mass spectrometry or use of specific antibody. The data clearly indicate that tobacco microdomains are able to recruit a specific set of the plasma membrane proteins and exclude others. We demonstrate the recruitment of the NADPH oxidase after elicitation by cryptogein and the presence of the small G protein NtRac5, a negative regulator of NADPH oxidase, in lipid rafts.

The product of the UTH1 gene, required for Bax-induced cell death in yeast, is involved in the response to rapamycin

A yeast mutant was isolated that was resistant to Bax-induced cell death. It supports a mutation leading to decreased amounts of the protein Uth1p. A strain in which the UTH1 gene is disrupted also exhibits resistance to Bax expression. The absence of Uth1p does not change the mitochondrial localization of Bax, its insertion in the mitochondrial outer membrane or its cytochrome c-releasing activity. On the other hand, the absence of Uth1p does prevent the appearance of other hallmarks related to Bax expression in yeast, such as oxidation of mitochondrial lipid, production of reactive oxygen species and maintenance of plasma membrane properties after ethanol stress. The absence of Uth1p was also found to induce resistance to rapamycin, a specific inducer of autophagy. This resistance only appears when cells are grown under respiratory conditions, but not under fermentative conditions, suggesting that Uth1p acts in an autophagic pathway involving mitochondria, in accordance with its main localization in the outer mitochondrial membrane. Taken together, these data show that Bax is able to activate a death pathway related to autophagy in yeast, which also exhibits typical hallmarks of apoptosis, revealing a possible dual function of Bax in both types of death. This hypothesis is discussed in the light of observations suggesting a co-regulation of apoptosis and autophagy in mammalian cells.

Bax-induced cell death in yeast depends on mitochondrial lipid oxidation

The oxidant function of pro-apoptotic protein Bax was investigated through heterologous expression in yeast. Direct measurements of fatty acid content show that Bax-expression induces oxidation of mitochondrial lipids. This effect is prevented by the coexpression of Bcl-xL. The oxidation actually could be followed on isolated mitochondria as respiration-induced peroxidation of polyunsaturated cis-parinaric acid and on whole cells as the increase in the amount of thiobarbituric acid-reactive products. Treatments that increase the unsaturation ratio of lipids, making them more sensitive to oxidation, increase kinetics of Bax-induced death. Conversely, inhibitors of lipid oxidation and treatments that decrease the unsaturation ratio of fatty acids decrease kinetics of Bax-induced death. Taken together, these results show that Bax-induced mitochondrial lipid oxidation is relevant to Bax-induced cell death. Conversely, lipid oxidation is poorly related to the massive Bax-induced superoxide and hydrogen peroxide accumulation, which occurs at the same time, as chemical or enzymatic scavenging of ROS does not prevent lipid oxidation nor has any effects on kinetics of Bax-induced cell death. Whatever the origin of mitochondrial lipid oxidation, these data show that it represents a major step in the cascade of events leading to Bax-induced cell death. These results are discussed in the light of the role of lipid oxidation both in mammalian apoptosis and in other forms of cell death in other organisms.

Nonenzymatic synthesis of glycerolipids catalyzed by imidazole

Imidazole catalyzed acylations of lysolipids by acyl-CoAs in water at room temperature and at a pH close to neutrality. In the presence of oleoyl-CoA and either lysophosphatidylcholine, 1-palmitoyl-sn-glycero-3-phosphocholine (LPC); lysophosphatidylglycerol, monoacyl-sn-glycero-3-phosphoglycerol; lysophosphatidyl acid, 1-oleoyl-sn-glycero-3-phosphate; lysophosphatidylserine, monoacyl-sn-glycero-3-phosphoserin; or lysophosphatidylethanolamine, monoacyl-sn-glycero-3-phosphoethanolamine, the corresponding phospholipids were synthesized. Similarly, the use of lyso-platelet activating factor, an ether analog of LPC, yielded the formation of 1-O-alkyl-2-oleoyl-sn-glycero-3-phosphocholine. In the presence of LPC, an imidazole-catalyzed synthesis of phosphatidylcholine (PC) occurred when medium, long, and very long chain acyl-CoAs were added. With hydroxyacyl-CoA, a similar PC synthesis was obtained. The process described in the present paper appears to offer several potential applications of interest for the synthesis of glycerophospholipids and triglycerides with labeled and/or an unusual or fragile fatty acid, or when suitable acyltransferases have not yet been described in the literature and/or are not commercially available. The method described is very safe and simple since lipids can be synthesized in tubes containing 0.7% imidazole in water, and left for a few hours at room temperature on the bench.

Inhibition of the plastidial phosphatidylcholine synthesis by silver, copper, lead and mercury induced by formation of mercaptides with the lyso-PC acyltransferase

Plastids greatly rely on the import of extraplastidial precursors for the synthesis of their own lipids, and several studies have shown that a lyso-PC acyltransferase located in the envelope may be involved in the import process. Because the presence of heavy metals in soil or in nutrient solutions induces changes in the lipid composition of plastid membranes (and therefore greatly reduces the photosynthetic capability of plants), we analysed the effect of several metal salts on plastidial lyso-PC acyltransferase activity. Among the 12 heavy metals studied, silver, copper, mercury and lead inhibited this activity. Metal bound to the enzyme was not - or only very slightly - released from the protein except when thiol-reducing agents (and not imidazole) were added. The results strongly suggest that the inhibitory effect is due to a formation of mercaptide between metal and cysteine(s). The relationship between the inhibition of the plastidial lyso-PC acyltransferase activity and the in vivo effects of metal salts on the plastid membranes is discussed.

Taxonomy of gymnospermae: multivariate analyses of leaf fatty acid composition

The fatty acid composition of photosynthetic tissues from 137 species of gymnosperms belonging to 14 families was determined by gas chromatography. Statistical analysis clearly discriminated four groups. Ginkgoaceae, Cycadaceae, Stangeriaceae, Zamiaceae, Sciadopityaceae, Podocarpaceae, Cephalotaxaceae, Taxaceae, Ephedraceae and Welwitschiaceae are in the first group, while Cupressaceae and Araucariaceae are mainly in the second one. The third and the fourth groups composed of Pinaceae species are characterized by the genera Larix, and Abies and Cedrus, respectively. Principal component and discriminant analyses and divisive hierarchical clustering analysis of the 43 Pinaceae species were also performed. A clear-cut separation of the genera Abies, Larix, and Cedrus from the other Pinaceae was evidenced. In addition, a mass analysis of the two main chloroplastic lipids from 14 gymnosperms was performed. The results point to a great originality in gymnosperms since in several species and contrary to the angiosperms, the amount of digalactosyldiacylglycerol exceeds that of monogalactosyldiacylglycerol.

Fatty acid composition of tomato leaves as biomarkers of metal-contaminated soils

The aim of this study was to determine whether fatty acid composition of leaves, cotyledons or roots could be used as an indicator of the bioavailability and of the adverse effects of heavy metals on plants. Tomato seedlings were grown on soils obtained by mixing increasing amounts of a highly metal-contaminated soil with an uncontaminated sandy soil, and the fatty acid composition of plant tissues was analyzed. The fatty acid composition of roots and cotyledons of plants grown on contaminated soils was mostly the same as in the control plants. In contrast, significant changes in the fatty acid composition of primary leaves occurred. Our results clearly indicate a relationship between metal accumulation (Cd, Pb, Zn, and Cu) and the fatty acid composition of primary leaves, with the contribution of 18 C atom fatty acids (as 18:3 fatty acid and precursors) being more closely correlated with the availability of heavy metals in soils.

Solubilization of the plastidial lysophosphatidylcholine acyltransferase from Allium porrum leaves: towards plants devoid of eukaryotic plastid lipids?

To analyse the involvement of the plastidial lysophosphatidylcholine (lyso-PC) acyltransferase in the import of the extraplastidial lipid precursors required for eukaryotic plastid lipid synthesis, we plan to obtain transgenic plants. Since no sequence of lyso-PC acyltransferase is known, the purification of this enzyme has been undertaken to establish its sequence. First we determined the conditions allowing the solubilization of this membrane-bound enzyme. It is shown that by using CHAPS as a detergent, a lyso-PC acyltransferase activity is associated with the solubilized proteins.

Mutagenesis of a plastidial lysophosphatidic acid acyltransferase

A combination of site-directed and random mutagenesis generated sequence variants of a plastidial lysophosphatidic acid acyltransferase. Alanine substitutions of residues present within two conserved motifs including the putative catalytic histidine resulted in a loss of acyltransferase activity assessed as complementation competence. Substitutions at five sites within the central core resulted in reduced or loss of activity. Truncation mutants reveal that sequences in the C-terminal moiety are essential for function.

Import of lyso-phosphatidylcholine into chloroplasts likely at the origin of eukaryotic plastidial lipids

Plastids rely on the import of extraplastidial precursor for the synthesis of their own lipids. This key phenomenon in the formation of plastidial phosphatidylcholine (PC) and of the most abundant lipids on earth, namely galactolipids, is poorly understood. Various suggestions have been made on the nature of the precursor molecule(s) transferred to plastids, but despite general agreement that PC or a close metabolite plays a central role, there is no clear-cut answer to this question because of a lack of conclusive experimental data. We therefore designed experiments to discriminate between a transfer of PC, 1-acylglycero phosphorylcholine (lyso-PC), or glycerophosphorylcholine. After pulse-chase experiments with glycerol and acetate, plastids of leek (Allium porrum L.) seedlings were purified. The labels of the glycerol moiety and the sn-1- and sn-2-bound fatty acids of plastidial lipids were determined and compared with those associated with the extraplastidial PC. After import, plastid lipids contained the glycerol moiety and the fatty acids esterified to the sn-1 position originating from the extraplastidial PC; no import of sn-2-bound fatty acid was detected. These results rule out a transfer of PC or glycerophosphorylcholine, and are totally explained by an import of lyso-PC molecules used subsequently as precursor for the synthesis of eukaryotic plastid lipids.

Involvement of lipoxygenase-dependent production of fatty acid hydroperoxides in the development of the hypersensitive cell death induced by cryptogein on tobacco leaves

Lipid peroxidation was investigated in relation with the hypersensitive reaction in cryptogein-elicited tobacco leaves. A massive production of free polyunsaturated fatty acid (PUFA) hydroperoxides dependent on a 9-lipoxygenase (LOX) activity was characterized during the development of leaf necrosis. The process occurred after a lag phase of 12 h, was accompanied by the concomitant increase of 9-LOX activity, and preceded by a transient accumulation of LOX transcripts. Free radical-mediated lipid peroxidation represented 10% of the process. Inhibition and activation of the LOX pathway was shown to inhibit or to activate cell death, and evidence was provided that fatty acid hydroperoxides are able to mimic leaf necrotic symptoms. Within 24 h, about 50% of leaf PUFAs were consumed, chloroplast lipids being the major source of PUFAs. The results minimize the direct participation of active oxygen species from the oxidative burst in membrane lipid peroxidation. They suggest, furthermore, the involvement of lipase activity to provide the free PUFA substrates for LOX. The LOX-dependent peroxidative pathway, responsible for tissue necrosis, appears as being one of the features of hypersensitive programmed cell death.

Dehydration of 3-hydroxyacyl-CoA in brain very-long-chain fatty acid synthesis

Rat brain microsomes actively dehydrate 3-hydroxyacyl-CoAs. Using chemically synthesized [1-(14)C] (R,S) 3-hydroxyeicosanoyl-CoA, we investigated the biochemical characteristics of the dehydration and reduction steps of stearoyl-CoA elongation. The reaction products, separated and identified as trans2,3-enoyl-CoAs and, in the presence of NADPH, as saturated acyl-CoAs, were released from the enzyme as thioesters which were partly hydrolysed. A kinetic analysis of the two coupled reactions showed that the 3-hydroxyacyl-CoA dehydrase catalysed a reversible reaction with kinetic constants of about 0.045 min(-1) for forward reaction (dehydration) and 0.025 min(-1) for reverse reaction (hydration); Vmax of the dehydration reached 20 nmoles/min/mg and the apparent Km was 44 microM. In the presence of NADPH, the kinetic constants for the dehydrase were unchanged and that for the trans2,3-enoyl-CoA reductase was 0.025 min(-1). The relative proportion of trans2,3-enoyl-CoA and saturated acyl-CoA depended on the protein amount. An inhibition of the reduction step was observed for substrate concentrations above 15 microM. The 3-hydroxyacyl-CoA dehydrase used (R) rather than (S) 3-hydroxyacyl-CoA. Furthermore, the elongation of (R) 3-hydroxyeicosanoyl-CoA yielded saturated very-long-chain acyl-CoA. These results demonstrated that 3-hydroxyacyl-CoAs entered the elongating complex exclusively at the level of the dehydrase and not of the condensing enzyme.

Fatty acid elongation in yeast--biochemical characteristics of the enzyme system and isolation of elongation-defective mutants

Elongation of long-chain fatty acids was investigated in yeast mutants lacking endogenous de novo fatty acid synthesis. In this background, in vitro fatty acid elongation was dependent strictly on the substrates malonyl-CoA, NADPH and a medium-chain or long-chain acyl-CoA primer of 10 or more carbon atoms. Maximal activity was observed with primers containing 12-14 carbon atoms, while shorter-chain-length acyl-CoA were almost (octanoyl-CoA) or completely (hexanoyl-CoA, acetyl-CoA) inactive. In particular, acetyl-CoA was inactive as a primer and as extender unit. The Michaelis constants for octanoyl-CoA (0.33 mM), decanoyl-CoA (0.83 mM) lauroyl-CoA (0.05 mM), myristoyl-CoA (0.4 mM) and palmitoyl-CoA (0.13 mM) were determined and were comparable for fatty acid synthesis and elongation. In contrast, the affinity of malonyl-CoA was 17-fold lower for elongation (Km = 0.13 mM) than for the fatty acid synthase (FAS) system. With increasing chain length of the primer (> or = 12:0), fatty acid elongation becomes increasingly sensitive to substrate inhibition. Due to the activation of endogenous fatty acids, ATP exhibits a stimulatory effect at suboptimal but not at saturating substrate concentrations. In the yeast cell homogenate, the specific activity of fatty acid elongation is about 10-20-fold lower than that of de novo fatty acid synthesis. The same elongation activity is observed in respiratory competent and in mitochondrially defective cells. The products of in vitro fatty acid elongation are fatty acids of 15-17 or 22-26 carbon atoms, depending on whether tridecanoyl-CoA or stearoyl-CoA is used as a primer. In vitro, the elongation products are converted in part, by alpha-oxidation, to their odd-chain-length lower homologues or are hydrolyzed to fatty acids. In contrast, no odd-chain-length elongation products or very-long-chain fatty acids (VLCFA) shorter than 26:0 are observed in vivo. Hence, VLCFA synthesis exhibits a higher processivity in vivo than in the cell homogenate. In addition, the in vivo process appears to be protected against side reactions such as hydrolysis or alpha-oxidation. Yeast mutants defective in 12:0 or 13:0 elongation were derived from fas-mutant strains according to their failure to grow on 13:0-supplemented media. In vivo, 12:0 elongation was reduced to 0-10% of the normal level, while 16:0 elongation and VLCFA synthesis were unimpaired. It is concluded that yeast contains either two different elongation systems, or that the respective mutation interferes differentially with medium-chain and long-chain fatty acid elongation. The yeast gene affected in the elongation-defective mutants was isolated and, upon sequencing, identified as the known ELO1 sequence. It encodes a putative membrane protein of 32-kDa molecular mass with no obvious similarity to any of the known FAS component enzymes.

A re-examination in vivo of the phosphatidylcholine-galactolipid metabolic relationship during plant lipid biosynthesis

It remains unclear how and in what form the lipids synthesized in plant endoplasmic reticulum are exported to chloroplasts and used as precursors for the biosynthesis of plastid galactolipids, which are the most abundant lipids on Earth. Neither the mechanism of transfer nor the nature of the lipids imported into plastids has been elucidated. To characterize events occurring in vivo, the labelling of lipids from 15-day-old leek seedlings (Allium porrum, var. furor) was studied using pulse-chase experiments. During the chase, a substantial decline in the radioactivity incorporated into phosphatidylcholine (and not in other phospholipids) was accompanied by an increase in the label found in galactolipids. The positional distribution of labelled fatty acids in phosphatidylcholine and galactolipids was further studied as a function of the chase time; whereas phosphatidylcholine was preferentially labelled at the sn-2 position, the increase in radioactivity in galactolipids mainly concerned the sn-1 position. These results strongly suggest that the diacylglycerol moiety of phosphatidylcholine might not be integrated as a whole in the galactolipid.

Occurrence of an acyl-CoA:1-acylglycerophosphorylcholine acyltransferase in plant mitochondria

In the presence of oleoyl-CoA, purified and intact mitochondria from potato tuber formed phosphatidylcholine from labeled lysophosphatidylcholine. The labeled oleoyl moiety of the acyl-CoA was also incorporated in the absence of exogenous lysolipids, such incorporation being largely increased by the addition of exogenous lysophosphatidylcholine. In the presence of various other lysophospholipids, no synthesis of the corresponding phospholipids was observed, suggesting a high specificity of the acyltransferase towards the acyl acceptor. This enzyme was chiefly located in the outer membrane of mitochondria. These results indicate that any acylglycerophosphorylcholine transferred from the endoplasmic reticulum to mitochondria may be acylated to phosphatidylcholine.

Pathways of incorporation of fatty acid into glycerolipids of the murine peripheral nervous system in vivo: alterations in the dysmyelinating mutant trembler mouse

In vivo glycerolipid metabolism was studied in sciatic nerves of normal and Trembler mice. The results showed that two kinetically independent pathways were implicated in the labeling of diacylglycerophospholipids from [3H]palmitate: the Kennedy pathway and a 'direct acylation' pathway. In normal nerves, 45% of the glycerophospholipids were labeled, with a rate constant k3 = 3.9 x 10(-3) min-1, from phosphatidic acid and diacylglycerol intermediates, themselves formed with a rate constant of k1 = 0.24 min-1 from a free 3H-fatty acid pool, FFA1, that represents 45% of the total injected label. The remaining 55% of the glycerophospholipids were labeled from a kinetically distinct free 3H-fatty acid pool, FFA2, with a rate constant of k4 = 9.8 x 10(-2) min-1, via a process that does not implicate a detectably labeled metabolic intermediate ('direct acylation'). Glycerophospholipid labeling via the Kennedy pathway in the Trembler mouse sciatic nerves was reduced to 75% of the normal level, while labeling via the 'direct acylation' pathway was increased 1.4-fold. The values of the rate constants for free 3H-fatty acid utilisation (k1 and k4) were both increased about 2.5-fold, while that of glycerophospholipid formation from diacylglycerol (k3) was close to normal.

Acylation of endogenous acyl acceptors by mouse sciatic nerve microsomes

Phospholipid (chiefly phosphatidylcholine) labeling from radioactive acyl-CoAs by mouse sciatic nerve microsomes is observed in the absence of added acyl acceptors. The maximal acylation (ca 10% of administered) for 10 micrograms microsomal proteins is observed at relatively low amounts of oleoyl-CoA (0.2-0.3 nmol) and decreases as the acyl-CoA amount increases. Labeled lysophosphatidylcholine (almost exclusively esterified at position 2) is also observed, particularly when the [1-14C]oleoyl-CoA concentration is higher than 0.2-0.3 nmol/50 microliters. The labeled acyl group is mainly inserted in position 2] of the glycerophosphorylcholine. With 0.15 nmol labeled oleoyl-CoA, phosphatidylcholine acylation increases as a function of the protein amount and reaches 25% of the added label at 40 microgram proteins. It is evaluated that, in the presence of 10 microgram proteins, 2% of the microsomal phosphatidylcholine molecules are acylated from 0.1 nmol acyl-CoA. The acylation mechanism seems to involve an acyl exchange between acyl-CoA and phosphatidylcholine.

Sphingolipid metabolic disorders in Trembler mouse peripheral nerves in vivo result from an abnormal substrate supply

Sphingolipid metabolic pathways in the peripheral nerves of dysmyelinating Trembler mice were studied in vivo, using intraneurally injected [3H]palmitate as the exogenous substrate. The kinetic analysis of the experimental data obtained for the mutant revealed that, as in normal nerves, two metabolically and kinetically independent pathways are implicated in the biosynthesis of the major peripheral nerve sphingolipids: the ceramide pathway and another pathway in which there is no detectable labeled intermediate ("direct amidification"). The results also show that, in the Trembler mouse sciatic nerves: (a) The severely deficient sphingolipid biosynthesis results from the constitution of a qualitatively and quantitatively abnormal fatty acid substrate pool destined for metabolism via the ceramide pathway, which ensures the totality of the galactocerebroside labeling and two-thirds of that of sphingomyelin. The ceramide intermediates of this pathway are labeled only on their fatty acyl moiety, which contains only 16-carbon atom chains. (b) "Direct amidification" events implicated in sphingolipid labeling are decreased compared with normal and account for the remaining sphingomyelin formation.