Acyl-acyl carrier protein and pool sizes during steady-state fatty acid synthesis by isolated spinach chloroplasts

Cloning, heterologous expression and biochemical characterization of plastidial sn-glycerol-3-phosphate acyltransferase from Helianthus annuus

The acyl-[acyl carrier protein]:sn-1-glycerol-3-phosphate acyltransferase (GPAT; E.C. 2.3.1.15) catalyzes the first step of glycerolipid assembly within the stroma of the chloroplast. In the present study, the sunflower (Helianthus annuus, L.) stromal GPAT was cloned, sequenced and characterized. We identified a single ORF of 1344base pairs that encoded a GPAT sharing strong sequence homology with the plastidial GPAT from Arabidopsis thaliana (ATS1, At1g32200). Gene expression studies showed that the highest transcript levels occurred in green tissues in which chloroplasts are abundant. The corresponding mature protein was heterologously overexpressed in Escherichia coli for purification and biochemical characterization. In vitro assays using radiolabelled acyl-ACPs and glycerol-3-phosphate as substrates revealed a strong preference for oleic versus palmitic acid, and weak activity towards stearic acid. The positional fatty acid composition of relevant chloroplast phospholipids from sunflower leaves did not reflect the in vitro GPAT specificity, suggesting a more complex scenario with mixed substrates at different concentrations, competition with other acyl-ACP consuming enzymatic reactions, etc. In summary, this study has confirmed the affinity of this enzyme which would partly explain the resistance to cold temperatures observed in sunflower plants.

Rapid kinetic labeling of Arabidopsis cell suspension cultures: implications for models of lipid export from plastids

Cell cultures allow rapid kinetic labeling experiments that can provide information on precursor-product relationships and intermediate pools. T-87 suspension cells are increasingly used in Arabidopsis (Arabidopsis thaliana) research, but there are no reports describing their lipid composition or biosynthesis. To facilitate application of T-87 cells for analysis of glycerolipid metabolism, including tests of gene functions, we determined composition and accumulation of lipids of light- and dark-grown cultures. Fatty acid synthesis in T-87 cells was 7- to 8-fold higher than in leaves. Similar to other plant tissues, phosphatidylcholine (PC) and phosphatidylethanolamine were major phospholipids, but galactolipid levels were 3- to 4-fold lower than Arabidopsis leaves. Triacylglycerol represented 10% of total acyl chains, a greater percentage than in most nonseed tissues. The initial steps in T-87 cell lipid assembly were evaluated by pulse labeling cultures with [(14)C]acetate and [(14)C]glycerol. [(14)C]acetate was very rapidly incorporated into PC, preferentially at sn-2 and without an apparent precursor-product relationship to diacylglycerol (DAG). By contrast, [(14)C]glycerol most rapidly labeled DAG. These results indicate that acyl editing of PC is the major pathway for initial incorporation of fatty acids into glycerolipids of cells derived from a 16:3 plant. A very short lag time (5.4 s) for [(14)C]acetate labeling of PC implied channeled incorporation of acyl chains without mixing with the bulk acyl-CoA pool. Subcellular fractionation of pea (Pisum sativum) leaf protoplasts indicated that 30% of lysophosphatidylcholine acyltransferase activity colocalized with chloroplasts. Together, these data support a model in which PC participates in trafficking of newly synthesized acyl chains from plastids to the endoplasmic reticulum.

The ferredoxin/thioredoxin system of oxygenic photosynthesis

Forty years ago, ferredoxin (Fdx) was shown to activate fructose 1,6-bisphosphatase in illuminated chloroplast preparations, thereby laying the foundation for the field now known as "redox biology." Enzyme activation was later shown to require the ubiquitous protein thioredoxin (Trx), reduced photosynthetically by Fdx via an enzyme then unknown-ferredoxin:thioredoxin reductase (FTR). These proteins, Fdx, FTR, and Trx, constitute a regulatory ensemble, the "Fdx/Trx system." The redox biology field has since grown beyond all expectations and now embraces a spectrum of processes throughout biology. Progress has been notable with plants that possess not only the plastid Fdx/Trx system, but also the earlier known NADP/Trx system in the cytosol, endoplasmic reticulum, and mitochondria. Plants contain at least 19 types of Trx (nine in chloroplasts). In this review, we focus on the structure and mechanism of action of members of the photosynthetic Fdx/Trx system and on biochemical processes linked to Trx. We also summarize recent evidence that extends the Fdx/Trx system to amyloplasts-heterotrophic plastids functional in the biosynthesis of starch and other cell components. The review highlights the plant as a model system to uncover principles of redox biology that apply to other organisms.

Identification of a plastid acyl-acyl carrier protein synthetase in Arabidopsis and its role in the activation and elongation of exogenous fatty acids

Plant cells are known to elongate exogenously provided fatty acid (FA), but the subcellular sites and mechanisms for this process are not currently understood. When Arabidopsis leaves were incubated with 14C-FAs with or=20 carbons) but not synthesis of 14C-unsaturated 18-carbon or 16-carbon FAs. Isolated pea chloroplasts were also able to elongate 14C-FAs ( Collapse

Key Words Collapse

MESH Headings

• Arabidopsis/drug effects
• Arabidopsis/genetics
• Arabidopsis/metabolism
• Cerulenin/pharmacology
• Chloroplasts/metabolism
• Enzyme Inhibitors/pharmacology
• Fatty Acids/metabolism
• Fatty Acids, Nonesterified/metabolism
• Gene Silencing
• Genes, Plant
• Lauric Acids/metabolism
• Mutagenesis, Insertional
• Pisum sativum/metabolism
• Plant Leaves/metabolism
• Plastids/metabolism
• Transferases (Other Substituted Phosphate Groups)/antagonists & inhibitors
• Transferases (Other Substituted Phosphate Groups)/genetics
• Transferases (Other Substituted Phosphate Groups)/metabolism

Collapse

Grants Collapse

Affiliation(s)

Abraham J K Koo Department of Plant Biology, Michigan State University, East Lansing, MI 48824, USA
Martin Fulda
John Browse
John B Ohlrogge

Collapse

Plant biotin-containing carboxylases

Biotin-containing proteins are found in all forms of life, and they catalyze carboxylation, decarboxylation, or transcarboxylation reactions that are central to metabolism. In plants, five biotin-containing proteins have been characterized. Of these, four are catalysts, namely the two structurally distinct acetyl-CoA carboxylases (heteromeric and homomeric), 3-methylcrotonyl-CoA carboxylase and geranoyl-CoA carboxylase. In addition, plants contain a noncatalytic biotin protein that accumulates in seeds and is thought to play a role in storing biotin. Acetyl-CoA carboxylases generate two pools of malonyl-CoA, one in plastids that is the precursor for de novo fatty acid biosynthesis and the other in the cytosol that is the precursor for fatty acid elongation and a large number of secondary metabolites. 3-Methylcrotonyl-CoA carboxylase catalyzes a reaction in the mitochondrial pathway for leucine catabolism. The exact metabolic function of geranoyl-CoA carboxylase is as yet unknown, but it may be involved in isoprenoid metabolism. This minireview summarizes the recent developments in our understanding of the structure, regulation, and metabolic functions of these proteins in plants.

Squash glycerol-3-phosphate (1)-acyltransferase. Alteration of substrate selectivity and identification of arginine and lysine residues important in catalytic activity

Glycerol-3-phosphate 1-acyltransferase is a soluble chloroplast enzyme involved in glycerol-lipid biosynthesis associated with chilling resistance in plants (). Resistance is associated with higher selectivity for unsaturated acyl substrates over saturated ones. In vitro substrate selectivity assays performed under physiologically relevant conditions have been established that discriminate between selective and non-selective forms of the enzyme. A mutation, L261F, in the squash protein converts it from a non-selective enzyme into a selective one. The mutation lies within 10 A of the predicted acyl binding site and results in a higher K(m) for 16:0 acyl carrier protein (ACP). Site-directed mutagenesis was used to determine the importance of four residues, Arg(235), Arg(237), Lys(193), and His(194), implicated to be involved in binding of the phosphate group of glycerol 3-phosphate to the enzyme. All the proteins were highly homologous in structure to the wild type enzyme. Mutations in Arg(235), Arg(237), and Lys(193) resulted in inactive enzyme, while His(194) had reduced catalytic activity. The mutant proteins retained the ability to bind stoichiometric quantities of acyl-ACPs supporting the potential role of these residues in glycerol 3-phosphate binding.

The biosynthesis of erucic acid in developing embryos of brassica rapa

The prevailing hypothesis on the biosynthesis of erucic acid in developing seeds is that oleic acid, produced in the plastid, is activated to oleoyl-coenzyme A (CoA) for malonyl-CoA-dependent elongation to erucic acid in the cytosol. Several in vivo-labeling experiments designed to probe and extend this hypothesis are reported here. To examine whether newly synthesized oleic acid is directly elongated to erucic acid in developing seeds of Brassica rapa L., embryos were labeled with [14C]acetate, and the ratio of radioactivity of carbon atoms C-5 to C-22 (de novo fatty acid synthesis portion) to carbon atoms C-1 to C-4 (elongated portion) of erucic acid was monitored with time. If newly synthesized 18:1 (oleate) immediately becomes a substrate for elongation to erucic acid, this ratio would be expected to remain constant with incubation time. However, if erucic acid is produced from a pool of preexisting oleic acid, the ratio of 14C in the 4 elongation carbons to 14C in the methyl-terminal 18 carbons would be expected to decrease with time. This labeling ratio decreased with time and, therefore, suggests the existence of an intermediate pool of 18:1, which contributes at least part of the oleoyl precursor for the production of erucic acid. The addition of 2-[3-chloro-5-(trifluromethyl)-2-pyridinyloxyphenoxy] propanoic acid, which inhibits the homodimeric acetyl-CoA carboxylase, severely inhibited the synthesis of [14C]erucic acid, indicating that essentially all malonyl-CoA for elongation of 18:1 to erucate was produced by homodimeric acetyl-CoA carboxylase. Both light and 2-[3-chloro-5-(trifluromethyl)-2-pyridinyloxyphenoxy]-propanoic acid increased the accumulation of [14C]18:1 and the parallel accumulation of [14C]phosphatidylcholine. Taken together, these results show an additional level of complexity in the biosynthesis of erucic acid.

REGULATION OF FATTY ACID SYNTHESIS

All plant cells produce fatty acids from acetyl-CoA by a common pathway localized in plastids. Although the biochemistry of this pathway is now well understood, much less is known about how plants control the very different amounts and types of lipids produced in different tissues. Thus, a central challenge for plant lipid research is to provide a molecular understanding of how plants regulate the major differences in lipid metabolism found, for example, in mesophyll, epidermal, or developing seed cells. Acetyl-CoA carboxylase (ACCase) is one control point that regulates rates of fatty acid synthesis. However, the biochemical modulators that act on ACCase and the factors that in turn control these modulators are poorly understood. In addition, little is known about how the expression of genes involved in fatty acid synthesis is controlled. This review evaluates current knowledge of regulation of plant fatty metabolism and attempts to identify the major unanswered questions.

The gene and the RNA for the precursor to the plastid-located glycerol-3-phosphate acyltransferase of Arabidopsis thaliana

The gene and the RNA from Arabidopsis thaliana for the plastid-located glycerol-3-phosphate acyltransferase (GPAT; EC 2.3.1.15) and their encoded product have been studied. The gene (designated ATS1) was isolated by screening a lambda DASH genomic library for cross-hybridization with a radiolabeled probe prepared from cDNA for GPAT from squash. cDNA clones representing the mRNA were isolated by screening a lambda ZAPII cDNA library for hybridization with a radiolabeled probe prepared from a DNA fragment of ATS1. The nucleotide sequences of the gene and the cDNA were determined, and the 5' end of the RNA was mapped by primer extension. Sequences similar to the TATA box, polyadenylation sequences and intron-splicing sequences were found at the expected locations. The pre-mRNA was 3288 nucleotides long and contained 5' and 3'-untranslated sequences of 57 and 442 nucleotides, respectively. The coding sequence of 1377 nucleotides was interrupted by 11 introns of 1412 nucleotides in total and the 3'-untranslated sequence contained another intron of 94 nucleotides. The open-reading frame encoded a polypeptide of 459 amino acid residues, the amino acid sequence of which was highly homologous to those of precursors to plastid-located GPATs from squash and pea. The enzymatic activity of a gene product that was over-produced in Escherichia coli confirmed the identity of the gene.

The influence of endogenous acyl-acyl carrier protein concentrations on fatty acid compositions of chloroplast glycerolipids

The concentrations of long-chain acyl-acyl carrier proteins (acyl-ACP) occurring during fatty acid synthesis from [1-14C]acetate within chloroplasts isolated from spinach, pea, and amaranthus leaves were manipulated by making minor changes to a basal incubation medium containing sn-glycerol 3-phosphate (G3P). Pools of oleoyl-, stearoyl-, and palmitoyl-ACP were compared with those of the corresponding 1-acyl glycerol 3-phosphates to determine how endogenous acyl-ACP concentrations affected the fatty acid compositions of chloroplast glycerolipids. The 1-acyl G3P synthesized by isolated chloroplasts contained more palmitate than would be expected for the precursor of thylakoid phosphatidylglycerol in the different plant species. However, treatments which increased ratios of oleoyl- to palmitoyl-ACP by about 50% increased synthesis of sn-1-oleoyl G3P to the extent anticipated from known fatty acid compositions of the different phosphatidylglycerols. Since stearate constituted 70-73% of the acyl-ACP and 48-51% of the 1-acyl-G3P pool of spinach and pea chloroplasts incubated in the presence of cyanide, it is transferred to G3P much more efficiently in situ than would be predicted from competition studies using mixtures of acyl donors and purified acyltransferases. Increasing concentrations of G3P in incubation media from 0.1 to 2 mM had relatively little effect on the amounts and proportions of acyl-ACPs but forced the synthesis of palmitoyl-G3P and, ultimately, disaturated glycerolipid. It is concluded that the chloroplast G3P acyltransferases are primarily responsible for determining the fatty acid compositions of procaryotic glycerolipids in plants, but that acyl-ACP concentrations may play a more important role than would be anticipated from the kinetics of the purified enzyme. However, those kinetics may be quite complex; allosteric effectors may influence the affinities of the enzyme for oleoyl-ACP and for G3P.

Concentrations of long-chain acyl-acyl carrier proteins during fatty acid synthesis by chloroplasts isolated from pea (Pisum sativum), safflower (Carthamus tinctoris), and amaranthus (Amaranthus lividus) leaves

Fatty acid synthesis from [1-14C]acetate by chloroplasts isolated from peas and amaranthus was linear for at least 15 min, whereas incorporation of the tracer into long-chain acyl-acyl carrier protein (ACP) did not increase after 2-3 min. When reactions were transferred to the dark after 3-5 min, long-chain acyl-ACPs lost about 90% of their radioactivity and total fatty acids retained all of theirs. Half-lives of the long-chain acyl-ACPs were estimated to be 10-15 s. Concentrations of palmitoyl-, stearoyl-, and oleoyl-ACP as indicated by equilibrium labeling during steady-state fatty acid synthesis, ranged from 0.6-1.1, 0.2-0.7, and 0.4-1.6 microM, respectively, for peas and from 1.6-1.9, 1.3-2.6, and 0.6-1.4 microM, respectively, for amaranthus. These values are based on a chloroplast volume of 47 microliters/mg chlorophyll and varied according to the mode of the incubation. A slow increase in activity of the fatty acid synthetase in safflower chloroplasts resulted in long-chain acyl-ACPs continuing to incorporate labeled acetate for 10 min. Upon re-illumination following a dark break, however, both fatty acid synthetase activity and acyl-ACP concentrations increased very rapidly. Palmitoyl-ACP was present at concentrations up to 2.5 microM in safflower chloroplasts, whereas those of stearoyl- and oleoyl-ACPs were in the lower ranges measured for peas. Acyl-ACPs were routinely separated from extracts of chloroplasts that had been synthesising long-chain fatty acids from labeled acetate by a minor modification of the method of Mancha et al. (Anal. Biochem., 1975, 68, 600-608). The results compared favorably with those obtained using alternative analytical methods such as adsorption to filter paper and partition chromatography on silicic acid columns. The acyl-ACP which coprecipitated with ammonium sulfate was not affected by treatments with neutral hydroxylamine or borohydride, whereas that eluted from silicic acid was relatively easily derivatized. A single radioactive polypeptide of Mr 11,500 from pea and amaranthus chloroplasts was revealed by autoradiography of gels from sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of the silicic acid eluates.

Role of plastidial acyl-acyl carrier protein: Glycerol 3-phosphate acyltransferase and acyl-acyl carrier protein hydrolase in channelling the acyl flux through the prokaryotic and eukaryotic pathway

In order to investigate whether the relative activities of the plastidial acyl-acyl carrier protein (ACP):glycerol 3-phosphate acyltransferase (EC 2.3.1.15) and acyl-ACP hydrolase play a role in controlling the acyl flux through the prokaryotic and eukaryotic pathway, we determined these enzymic activities in stroma fractions from 16:3- and 18:3-plants using glycerol 3-phosphate and labelled acyl-ACP as substrates. Several factors were examined which might influence the activities within plastids, such as leaf development, salts at physiological concentrations, stroma pH and substrates available to the enzymes. An appreciable alteration of the two enzymic activities was only observed with changes in the pH and substrate concentration, especially the concentration of glycerol 3-phosphate. An increase in pH from 7 to 8 resulted in a decreased ratio of acyltransferase versus hydrolase activity in stroma fractions from both pea (Pisum sativum L.) and spinach (Spinacia oleracea L.), whereas exogenously added glycerol 3-phosphate, which only influenced the acyltransferase, raised this ratio. On the other hand, the relative activities of the two enzymes stayed rather constant at oleoyl-ACP concentrations between 1 and 2 μM not only when it was offered alone but also in a mixture with palmitoyl-ACP. At pH 8, the stroma pH of illuminated chloroplasts, and at physiologically relevant substrate concentrations we observed clear differences between the 16:3-plants spinach and mustard (Sinapis alba ssp. alba L.) and the 18:3-plants pea and maize (Zea mays L.). In accordance with the different proportions of prokaryotic glycerolipids in the two groups of plants, pea and maize showed distinctly lower ratios of acyltransferase versus hydrolase activity than spinach and mustard. Consequently the relative activities of the plastidial glycerol 3-phosphate acyltransferase and acyl-ACP hydrolase can play a decisive role in controlling the acyl flux through the different pathways at least in these plants.