Purification and characterization of a basic lysophospholipase in germinating barley

Protein interactors of acyl-CoA-binding protein ACBP2 mediate cadmium tolerance in Arabidopsis

In our recent paper in the Plant Journal, we reported that Arabidopsis thaliana lysophospholipase 2 (lysoPL2) binds acyl-CoA-binding protein 2 (ACBP2) to mediate cadmium [Cd(II)] tolerance in transgenic Arabidopsis. ACBP2 contains ankyrin repeats that have been previously shown to mediate protein-protein interactions with an ethylene-responsive element binding protein (AtEBP) and a farnesylated protein 6 (AtFP6). Transgenic Arabidopsis ACBP2-overexpressors, lysoPL2-overexpressors and AtFP6-overexpressors all display enhanced Cd(II) tolerance, in comparison to wild type, suggesting that ACBP2 and its protein partners work together to mediate Cd(II) tolerance. Given that recombinant ACBP2 and AtFP6 can independently bind Cd(II) in vitro, they may be able to participate in Cd(II) translocation. The binding of recombinant ACBP2 to [(14)C]linoleoyl-CoA and [(14)C]linolenoyl-CoA implies its role in phospholipid repair. In conclusion, ACBP2 can mediate tolerance to Cd(II)-induced oxidative stress by interacting with two protein partners, AtFP6 and lysoPL2. Observations that ACBP2 also binds lysophosphatidylcholine (lysoPC) in vitro and that recombinant lysoPL2 degrades lysoPC, further confirm an interactive role for ACBP2 and lysoPL2 in overcoming Cd(II)-induced stress.

Acyl-CoA-binding protein 2 binds lysophospholipase 2 and lysoPC to promote tolerance to cadmium-induced oxidative stress in transgenic Arabidopsis

Lysophospholipids are intermediates of phospholipid metabolism resulting from stress and lysophospholipases detoxify lysophosphatidylcholine (lysoPC). Many lysophospholipases have been characterized in mammals and bacteria, but few have been reported from plants. Arabidopsis thaliana lysophospholipase 2 (lysoPL2) (At1g52760) was identified as a protein interactor of acyl-CoA-binding protein 2 (ACBP2) in yeast two-hybrid analysis and co-immunoprecipitation assays. BLASTP analysis indicated that lysoPL2 showed approximately 35% amino acid identity to the lysoPL1 family. Co-localization of autofluorescence-tagged lysoPL2 and ACBP2 by confocal microscopy in agroinfiltrated tobacco suggests the plasma membrane as a site for their subcellular interaction. LysoPL2 mRNA was induced by zinc (Zn) and hydrogen peroxide (H(2)O(2)), and lysoPL2 knockout mutants showed enhanced sensitivity to Zn and H(2)O(2) in comparison to wild type. LysoPL2-overexpressing Arabidopsis was more tolerant to H(2)O(2) and cadmium (Cd) than wild type, suggesting involvement of lysoPL2 in phospholipid repair following lipid peroxidation arising from metal-induced stress. Lipid hydroperoxide (LOOH) contents in ACBP2-overexpressors and lysoPL2-overexpressors after Cd-treatment were lower than wild type, indicating that ACBP2 and lysoPL2 confer protection during oxidative stress. A role for lysoPL2 in lysoPC detoxification was demonstrated when recombinant lysoPL2 was observed to degrade lysoPC in vitro. Filter-binding assays and Lipidex competition assays showed that (His)(6)-ACBP2 binds lysoPC in vitro. Binding was disrupted in a (His)(6)-ACBP2 derivative lacking the acyl-CoA-binding domain, confirming that this domain confers lysoPC binding. These results suggest that ACBP2 can bind both lysoPC and lysoPL2 to promote the degradation of lysoPC in response to Cd-induced oxidative stress.

Lipid deacylating enzymes in plants: old activities, new genes

Because lipids are major components of cellular membranes, their degradation under stress conditions compromises compartmentalization. However, in addition to having structural roles, membrane lipids are also implicated in signalling processes involving the activity of lipolytic enzymes. Phospholipases D and C, acting on the polar heads of phospholipids, have been relatively well characterized in plants. In contrast, knowledge of lipid deacylating enzymes remains limited. Lipid acyl hydrolases (LAH) are able to hydrolyse both fatty acid moieties of polar lipids. They differ from phospholipases A(1) or A(2) (PLA) acting on sn-1 or sn-2 positions of phospholipids, respectively, as well as from lipases which de-esterify triacylglycerols. The free polyunsaturated fatty acids generated by deacylating enzymes can be used in the biosynthesis of oxylipins and the lysophospholipids, provided by PLAs, are also bioactive molecules. In the four decades that have passed since the first description of LAH activities in plants some enzymes have been purified. In recent years, the widespread use of molecular approaches together with the attention paid to lipid signalling has contributed to a renewed interest in LAH and has led to the identification of different gene families and the characterization of new enzymes. Additionally, several proteins with putative lipase/esterase signatures have been identified. In the present paper we review currently available data on LAHs, PLAs, triacylglycerol lipases and other putative deacylating enzymes. The roles of lipid deacylating enzymes in plant growth, development and stress responses are discussed in the context of their involvement in membrane deterioration, lipid turnover and cellular signalling.

Characterization of esterases from Cucurbita pepo cv. "Eskandrani"

Two of the six esterases identified in Cucurbita pepo cv. "Eskandrani" were purified to homogeneity using two chromatography steps: anion exchange and gel filtration. The molecular weights of C. pepo esterases EIc and EII were 50,000 +/- 1500 and 68,000 +/- 1900 Da from gel filtration and 47,000 and 66,000 Da from SDS/PAGE, respectively, suggesting a monomeric structure for both enzymes. Esterases EIc and EII had K(m) values of 1.22 and 1.56 mM and pH optima at 9.0 and 8.0, respectively. The substrate specificity of C. pepo esterases EIc and EII were determined for a number of p-nitrophenyl esters, where their affinity toward these substrates were decreased as carbon atom number increased. Esterases EIc and EII had the same temperature optima, 40 degrees C. Thermal stability studies of esterases EIc and EII indicated that half maximal activities of EIc and EII esterases were reached at 55 degrees C and 50 degrees C, while they lost 45%, 51% and 70%, 77% of their activities after 30 and 90 min of incubation at 40 degrees C, respectively. The effect of different metal cations and inhibitors were examined. The inhibition studies revealed that the active sites of the two esterases contain serine and cysteine residues. The characteristics of C. pepo esterases are closely similar to those of microbial esterases used in food processing and food industry.

Glycerophosphocholine metabolism in higher plant cells. Evidence of a new glyceryl-phosphodiester phosphodiesterase

Glycerophosphocholine (GroPCho) is a diester that accumulates in different physiological processes leading to phospholipid remodeling. However, very little is known about its metabolism in higher plant cells. (31)P-Nuclear magnetic resonance spectroscopy and biochemical analyses performed on carrot (Daucus carota) cells fed with GroPCho revealed the existence of an extracellular GroPCho phosphodiesterase. This enzymatic activity splits GroPCho into sn-glycerol-3-phosphate and free choline. In vivo, sn-glycerol-3-phosphate is further hydrolyzed into glycerol and inorganic phosphate by acid phosphatase. We visualized the incorporation and the compartmentation of choline and observed that the major choline pool was phosphorylated and accumulated in the cytosol, whereas a minor fraction was incorporated in the vacuole as free choline. Isolation of plasma membranes, culture medium, and cell wall proteins enabled us to localize this phosphodiesterase activity on the cell wall. We also report the existence of an intracellular glycerophosphodiesterase. This second activity is localized in the vacuole and hydrolyzes GroPCho in a similar fashion to the cell wall phosphodiesterase. Both extra- and intracellular phosphodiesterases are widespread among different plant species and are often enhanced during phosphate deprivation. Finally, competition experiments on the extracellular phosphodiesterase suggested a specificity for glycerophosphodiesters (apparent K(m) of 50 microM), which distinguishes it from other phosphodiesterases previously described in the literature.

PLANT PHOSPHOLIPASES

Phospholipases are a diverse series of enzymes that hydrolyze phospholipids. Multiple forms of phospholipases D, C, and A have been characterized in plants. These enzymes are involved in a broad range of functions in cellular regulation, lipid metabolism, and membrane remodeling. In recent years, increasing attention has been paid to the many roles of phospholipases in signal transduction. This review highlights recent developments in the understanding of biochemical, molecular biological, and functional aspects of various phospholipases in plants.

Purification and characterization of a fatty acyl-ester hydrolase from post-germinated sunflower seeds

Fatty acyl-ester hydrolase was not detectable in dry sunflower seeds using various p-nitrophenyl-acyl-esters, 1,2-O-didodecyl-rac-glycero-3-glutaric acid-resorufin ester or emulsified sunflower oil as substrate. After inhibition of the seeds, acyl-ester hydrolase activity slowly developed in cotyledon extracts and was maximal after 5 days. No activity was directly measurable on oil bodies. The enzyme was purified 615-fold to apparent homogeneity, as determined by performing SDS-PAGE electrophoresis, and biochemically characterized. With p-nitrophenyl-caprylate the optimum pH was around 8.0. The purification procedure involved an acetone powder from 5-day dark-germinated seedlings, chloroform-butanol extraction and three chromatography steps. We obtained 35 micrograms of pure enzyme from 250 g of fresh cotyledons with an activity yield of around 7%. It should be possible to subsequently improve this low recovery as we gave priority here, in the first instance, to purity at the expense of the yield. The enzyme consisted of one glycosylated polypeptide chain with a molecular mass of approx. 45 kDa and, as far as we could tell, it did not seem to require metal ions to be fully active, as it was not inhibited by EDTA or o-phenanthroline and not activated by various mono or bivalent metal ions. The amino acid composition showed the presence of four cysteine and four tryptophan residues. The enzyme was partially inhibited by dithiothreitol, DTNB and PCMB. The fact that high inhibition was observed in the presence of PMSF indicates that a serine residue may possibly be involved in the catalytic mechanism. The hydrophobicity index was about 53.6% which places this enzyme in the class of the soluble proteins in good agreement with the fact that it was mainly present in the soluble part of the crude extract. Partial characterization of glycan chains, using antiglycan antibodies, showed the presence of complex Asn-linked glycans. The enzyme was active on purified sunflower glycerol derivatives. It was also able to hydrolyze monooleyl and dioleyl glycerols, as well as phosphatidylcholine, but not cholesteryl esters. Using p-nitrophenyl-acyl-esters as substrate, the highest activity was observed with middle-chain derivatives (C6 and C8). Its maximum activity was about 0.015 units mg-1 with sunflower oil. It was not activated by an organic solvent such as isooctane. This enzyme probably is involved in acyl-ester hydrolysis which follows triacylglycerol mobilization by true lipases.

Secretion of a lipolytic protein aggregate by barley aleurone and its dissociation by starchy endosperm

Acidic and basic lysophospholipase activities (LPL) have been separated by ion-exchange chromatography of barley extracts. The basic activity predominates in the starchy endosperm of germinating barley and in the medium of hormone-stimulated half-seeds; the acidic activity is the predominant form in the medium of hormone-stimulated aleurone layers. Addition of either starchy endosperm or EDTA to the acidic activity produces the basic activity. The two activities display the same pH optimum and have similar Km values. Inactivation profiles of LPLs with immunoglobulin G (IgG) prepared against the purified basic LPL are the same. The acidic LPL obtained from the incubation medium from stimulated aleurone layers appears in the void volume on gel filtration with Bio-Gel P100. Acid phosphatase and alpha-amylase in the same incubation medium appear at their expected elution volumes on this column. Gel filtration in the presence of EDTA results in the acidic activity eluting in a volume characteristic of the basic LPL (Mr, 40,000). On Bio-Gel P300 the acidic activity peak is centered at Mr, 160,000. SDS-gel electrophoresis of fractions across this peak shows a simple distribution of proteins eluting with Mr greater than or equal to 160,000. The potential role of an aggregate in the secretion of lipolytic proteins is discussed.