Isoforms of soybean seed oil body membrane protein 24 kDa oleosin are encoded by closely related cDNAs

Abscisic acid activation of oleosin gene HvOle3 expression prevents the coalescence of protein storage vacuoles in barley aleurone cells

Protein storage vacuoles (PSVs) in aleurone cells coalesce during germination, and this process is highly coupled with mobilization of PSV reserves, allowing de novo synthesis of various hydrolases in aleurone cells for endosperm degradation. Here we show that in barley (Hordeum vulgare L.) oleosins, the major integral proteins of oleosomes are encoded by four genes (HvOle1 to 4), and the expression of HvOle1 and HvOle3 is strongly up-regulated by abscisic acid (ABA), which shows antagonism to gibberellic acid. In aleurone cells, all HvOLEs were subcellularly targeted to the tonoplast of PSVs. Gain-of-function analyses revealed that HvOLE3 effectively delayed PSV coalescence, whereas HvOLE1 only had a moderate effect, with no notable effect of HvOLE2 and 4. With regard to longevity, HvOLE3 chiefly outperformed other HvOLEs, followed by HvOLE1. Experiments swapping the N- and C-terminal domain between HvOLE3 and other HvOLEs showed that the N-terminal region of HvOLE3 is mainly responsible, with some positive effect by the C-terminal region, for mediating the specific preventive effect of HvOLE3 on PSV coalescence. Three ACGT-core elements and the RY-motif were responsible for ABA induction of HvOle3 promoter activity. Transient expression assays using aleurone protoplasts demonstrated that transcriptional activation of the HvOle3 promoter was mediated by transcription factors HvABI3 and HvABI5, which acted downstream of protein kinase HvPKABA1.

Genome-Wide Analysis of Oleosin Gene Family in 22 Tree Species: An Accelerator for Metabolic Engineering of BioFuel Crops and Agrigenomics Industrial Applications?

Trees contribute to enormous plant oil reserves because many trees contain 50%-80% of oil (triacylglycerols, TAGs) in the fruits and kernels. TAGs accumulate in subcellular structures called oil bodies/droplets, in which TAGs are covered by low-molecular-mass hydrophobic proteins called oleosins (OLEs). The OLEs/TAGs ratio determines the size and shape of intracellular oil bodies. There is a lack of comprehensive sequence analysis and structural information of OLEs among diverse trees. The objectives of this study were to identify OLEs from 22 tree species (e.g., tung tree, tea-oil tree, castor bean), perform genome-wide analysis of OLEs, classify OLEs, identify conserved sequence motifs and amino acid residues, and predict secondary and three-dimensional structures in tree OLEs and OLE subfamilies. Data mining identified 65 OLEs with perfect conservation of the "proline knot" motif (PX5SPX3P) from 19 trees. These OLEs contained >40% hydrophobic amino acid residues. They displayed similar properties and amino acid composition. Genome-wide phylogenetic analysis and multiple sequence alignment demonstrated that these proteins could be classified into five OLE subfamilies. There were distinct patterns of sequence conservation among the OLE subfamilies and within individual tree species. Computational modeling indicated that OLEs were composed of at least three α-helixes connected with short coils without any β-strand and that they exhibited distinct 3D structures and ligand binding sites. These analyses provide fundamental information in the similarity and specificity of diverse OLE isoforms within the same subfamily and among the different species, which should facilitate studying the structure-function relationship and identify critical amino acid residues in OLEs for metabolic engineering of tree TAGs.

The characterization of soybean oil body integral oleosin isoforms and the effects of alkaline pH on them

Oil body, an organelle in seed cell (naturally pre-emulsified oil), has great potentials to be used in food, cosmetics, pharmaceutical and other applications requiring stable oil-in-water emulsions. Researchers have tried to extract oil body by alkaline buffers, which are beneficial for removing contaminated proteins. But it is not clear whether alkaline buffers could remove oil body integral proteins (mainly oleosins), which could keep oil body integrity and stability. In this study, seven oleosin isoforms were identified for soybean oil body (three isoforms, 24 kDa; three isoforms, 18 kDa; one isoform, 16kDa). Oleosins were not glycoproteins and 24 kDa oleosin isoforms possessed less thiol groups than 18 kDa ones. It was found that alkaline pH not only removed contaminated proteins but also oleosins, and more and more oleosins were removed with increasing alkaline pH.

Evolution of oleosin in land plants

Oleosins form a steric barrier surface on lipid droplets in cytoplasm, preventing them from contacting and coalescing with adjacent droplets. Oleosin genes have been detected in numerous plant species. However, the presence of oleosin genes in the most basally diverging lineage of land plants, liverworts, has not been reported previously. Thus we explored whether liverworts have an oleosin gene. In Marchantia polymorpha L., a thalloid liverwort, one predicted sequence was found that could encode oleosin, possessing the hallmark of oleosin, a proline knot (-PX5SPX3P-) motif. The phylogeny of the oleosin gene family in land plants was reconstructed based on both nucleotide and amino acid sequences of oleosins, from 31 representative species covering almost all the main lineages of land plants. Based on our phylogenetic trees, oleosin genes were classified into three groups: M-oleosins (defined here as a novel group distinct from the two previously known groups), low molecular weight isoform (L-oleosin), and high molecular weight isoform (H-oleosin), according to their amino-acid organization, phylogenetic relationships, expression tissues, and immunological characteristics. In liverworts, mosses, lycophytes, and gymnosperms, only M-oleosins have been described. In angiosperms, however, while this isoform remains and is highly expressed in the gametophyte pollen tube, two other isoforms also occur, L-oleosins and H-oleosins. Phylogenetic analyses suggest that the M-oleosin isoform is the precursor to the ancestor of L-oleosins and H-oleosins. The later two isoforms evolved by successive gene duplications in ancestral angiosperms. At the genomic level, most oleosins possess no introns. If introns are present, in both the L-isoform and the M-isoform a single intron inserts behind the central region, while in the H-isoform, a single intron is located at the 5'-terminus. This study fills a major gap in understanding functional gene evolution of oleosin in land plants, shedding new light on evolutionary transitions of lipid storage strategies.

The integral and extrinsic bioactive proteins in the aqueous extracted soybean oil bodies

Soybean oil bodies (OBs), naturally pre-emulsified soybean oil, have been examined by many researchers owing to their great potential utilizations in food, cosmetics, pharmaceutical, and other applications requiring stable oil-in-water emulsions. This study was the first time to confirm that lectin, Gly m Bd 28K (Bd 28K, one soybean allergenic protein), Kunitz trypsin inhibitor (KTI), and Bowman-Birk inhibitor (BBI) were not contained in the extracted soybean OBs even by neutral pH aqueous extraction. It was clarified that the well-known Gly m Bd 30K (Bd 30K), another soybean allergenic protein, was strongly bound to soybean OBs through a disulfide bond with 24 kDa oleosin. One steroleosin isoform (41 kDa) and two caleosin isoforms (27 kDa, 29 kDa), the integral bioactive proteins, were confirmed for the first time in soybean OBs, and a considerable amount of calcium, necessary for the biological activities of caleosin, was strongly bound to OBs. Unexpectedly, it was found that 24 kDa and 18 kDa oleosins could be hydrolyzed by an unknown soybean endoprotease in the extracted soybean OBs, which might give some hints for improving the enzyme-assisted aqueous extraction processing of soybean free oil.

Crop seed oil bodies: from challenges in protein identification to an emerging picture of the oil body proteome

Oleaginous seeds store lipids in specialized structures called oil bodies (OBs). These organelles consist of a core of neutral lipids bound by proteins embedded in a phospholipid monolayer. OB proteins are well conserved in plants and have long been grouped into only two categories: structural proteins or enzymes. Recent work, however, which identified other classes of proteins associated with OBs, clearly shows that this classification is obsolete. Proteomics-mediated OB protein identification is facilitated in plants for which the genome is sequenced and annotated. However, it is not clear whether this knowledge can be dependably transposed to less well-characterized plants, including the well-established commercial sources of seed oil as well as the many others being proposed as novel sources for biodiesel, especially in Africa and Asia. Toward an update of the current data available on OB proteins this review discusses (i) the specific difficulties for proteomic studies of organelles; (ii) a 2012 census of the proteins found in seed OBs from various crops; (iii) the oleosin composition of OBs and their role in organelle stability; (iv) PTM of OB proteins as an emerging field of investigation; and finally we describe the emerging model of the OB proteome from oilseed crops.

Integral Proteins in Plant Oil Bodies

Hydrophobic storage neutral lipids are stably preserved in specialized organelles termed oil bodies in the aqueous cytosolic compartment of plant cells via encapsulation with surfactant molecules including phospholipids and integral proteins. To date, three classes of integral proteins, termed oleosin, caleosin, and steroleosin, have been identified in oil bodies of angiosperm seeds. Proposed structures, targeting traffic routes, and biological functions of these three integral oil-body proteins were summarized and discussed. In the viewpoint of evolution, isoforms of oleosin and caleosin are found in oil bodies of pollens as well as those of more primitive species; moreover, caleosin- and steroleosin-like proteins are also present in other subcellular locations besides oil bodies. Technically, artificial oil bodies of structural stability similar to native ones were successfully constituted and seemed to serve as a useful tool for both basic research studies and biotechnological applications.

Seed oil bodies from Gevuina avellana and Madia sativa

In this study, oil bodies (OBs) from Gevuina avellana (OBs-G) and Madia sativa (OBs-M) were isolated and characterized. Microscopic inspection revealed that the monolayer on OB-G was thinner compared to that on OB-M. Cytometric profiles regarding size, complexity, and staining for the two OB sources were similar. Fatty acid to protein mass ratio in both OBs was near 29, indicating high lipid enrichment. OBs-G and OBs-M showed a strong electrostatic repulsion over wide ranges of pH (5.5-9.5) and NaCl concentration (0-150 mM). Proteins displaying highly conserved sequences (steroleosins and aquaporins) in the plant kingdom were identified. The presence of oleosins was immunologically revealed using antibodies raised against Arabidopsis thaliana oleosins. OBs-G and OBs-M exhibited no significant cytotoxicity against the cells. This is the first report about the isolation and characterization of OBs-G and OBs-M, and this knowledge could be used for novel applications of these raw materials.

Protein and lipid composition analysis of oil bodies from twoBrassica napus cultivars

Oil bodies were purified from mature seed of two Brassica napus crop cultivars, Reston and Westar. Purified oil body proteins were subjected to both 2-DE followed by LC-MS/MS and multidimensional protein identification technology. Besides previously known oil body proteins oleosin, putative embryo specific protein ATS1, (similar to caleosin), and 11-beta-hydroxysteroid dehydrogenase-like protein (steroleosin), several new proteins were identified in this study. One of the identified proteins, a short chain dehydrogenase/reductase, is similar to a triacylglycerol-associated factor from narrow-leafed lupin while the other, a protein annotated as a myrosinase associated protein, shows high similarity to the lipase/hydrolase family of enzymes with GDSL-motifs. These similarities suggest these two proteins could be involved in oil body degradation. Detailed analysis of the two other oil body components, polar lipids (lipid monolayer) and neutral lipids (triacylglycerol matrix) was also performed. Major differences were observed in the fatty acid composition of polar lipid fractions between the two B. napus cultivars. Neutral lipid composition confirmed erucic acid and oleic acid accumulation in Reston and Westar seed oil, respectively.

Purification and cloning of two high molecular mass isoforms of peanut seed oleosin encoded by cDNAs of equal sizes

Oleosins are small plant proteins characterized by a long hydrophobic core flanked by amphipathic N- and C-terminal domains, which act as emulsifiers for the storage of lipids in seeds. They have been sequenced in a number of oilseeds important for the food industry but not in peanuts. We purified the major isoform of peanut oleosin by preparative electrophoresis with continuous elution, in sufficient amounts to raise specific antibodies, perform circular dichroism and N-sequence tryptic fragments. The structure of the purified oleosin was dominated by alpha-helix that may be assigned to the SDS-resistant central hydrophobic stretch. A two-step RT-PCR strategy was developed to determine the cDNA sequence of this oleosin. Two cDNA variants of equal sizes encoding for isoforms of 176 amino acids each were identified. The isoforms differed by seven amino acids mainly located in the N- and C-terminal domains. The corresponding mRNAs were estimated at 0.9 kb by Northern blot and were transcribed from genes without introns. Immunoprecipitation of the in vitro-translated peanut oleosin labeled with [14C]leucine or [35S]methionine produced the full-length protein (17 kDa) and a 6-kDa peptide corresponding to the N/C-terminal domains. This peptide was able to form SDS-PAGE stable oligomers by interacting with the full-length protein. A similar peptide was released after [125I]iodination of the purified oleosin that generated intermediate-sized oligomers also visible by Western blot on a crude oleosin extract. Oligomers reflect the natural ability of oleosins to strongly interact with each other via not only their central domains but also their N- and C-terminal domains.

Free amino acid profiles suggest a possible role for asparagine in the control of storage-product accumulation in developing seeds of low- and high-protein soybean lines

Several approaches were taken to examine the role of N-assimilate supply in the control of soybean (Glycine max) seed composition. In the first study, developing seeds were grown in vitro with D-[U-14C]sucrose (Suc) and different concentrations of Gln. Light stimulated carbon flux into oil and protein, and was required to sustain Suc uptake and anabolic processes under conditions of elevated nitrogen supply. High Gln supply resulted in higher transcript levels of beta-conglycinin and oleosin. In the second study, analyses of soluble amino acid pools in two genetically related lines, NC103 and NC106 (low- and high-seed protein, respectively) showed that, in the light, NC106 accumulated higher levels of Asn and several other amino acids in developing cotyledons compared with NC103, whereas at the seed coat and apoplast levels both lines were similar. In the dark, NC103 accumulated Gln, Arg, and its precursors, suggesting a reduced availability of organic acids required for amino acid interconversions, while NC106 maintained higher levels of the pyruvate-derived amino acids Val, Leu, and Ile. Comparing NC103 and NC106, differences in seed composition were reflected in steady-state transcript levels of storage proteins and the lipogenic enzyme multi-subunit acetyl CoA carboxylase. In the third study, a positive correlation (P < or = 0.05) between free Asn in developing cotyledons and seed protein content at maturity was confirmed in a comparison of five unrelated field-grown cultivars. The findings support the hypothesis that high seed-protein content in soybean is determined by the capacity of the embryo to take up nitrogen sources and to synthesize storage proteins. Asn levels are probably tightly regulated in the embryo of high-protein lines, and may act as a metabolic signal of seed nitrogen status.

Influence of cultivar and fruit ripening on olive (Olea europaea) fruit protein content, composition, and antioxidant activity

Proteins of olive fruit mesocarp are not very well-known at present. However, they have been shown to pass, at least partially, to the olive oil during its elaboration and therefore might be contributing to some of the special characteristics of this vegetable oil. In this study, protein content and composition were determined in olive fruits, cv. Arbequina and Picual, at three stages of ripening: green, spotted, and purple. Mesocarp proteins constituted 1.3-1.8% of the dry weight of the olive fruit, and cultivar and fruit ripening did not produce important changes in mesocarp protein content or composition. In addition, this composition was also similar to the amino acid composition of a 4.6-kDa polypeptide, which is the major protein component of olive oils and of oil bodies of olive fruit mesocarp, suggesting that this polypeptide is likely to be a major component of mesocarp proteins. There was, also, a relationship between the oil content of the olive fruit and the protein content determined, suggesting a stabilizing function of these proteins in the oil bodies of the olive fruit, analogously to the role suggested for oleosins. This stabilizing function does not seem to be extended to olive oils because when the polypeptides isolated were added at 20 ppm to soybean oil, the stability of the oil increased only slightly, suggesting that if these compounds play some role in the stability of the oils, this should be mostly a consequence of the possible interactions among these protein components and other olive oil antioxidant constituents.

Determination of peptides and proteins in fats and oils

A method for the determination of proteins in fats and oils is described. Proteins were sequentially precipitated with acetone and hydrolyzed, and the produced amino acids were fractionated and quantificated. This analysis protocol afforded a method of high sensitivity and specificity which was fully evaluated and validated. The data obtained showed good accuracy and linearity with excellent reproducibility and recovery. When the method was applied to 40 olive oils, all of them contained proteins in the range 10-50 microg/100 g of oil, suggesting that proteins are nonpreviously described minor components of these oils. In addition, the proteins precipitated were almost exclusively composed by one polypeptide of apparent 4600 molecular weight, which was isolated from olive drupes and partially characterized by amino acid analysis. Similar polypeptides were also detected in other seeds, suggesting that they may constitute a new class of polypeptides in plants with oleosin-like characteristics. Furthermore, the method was also applied to different fats and oils, and all the samples analyzed contained proteins, suggesting that natural fats and oils always contain polypeptides and/or proteins as minor components. These results also suggest that some peptides are soluble in lipid matrixes, where they might be playing unknown functions. The developed procedure provides a methodology for the determination of these components.

The upstream domain of soybean oleosin genes contains regulatory elements similar to those of legume storage proteins

Seed reserve storage products consisting of proteins, oil and starch are accumulated in a developmentally coordinated pattern. The control of the vacuolar storage protein expression has been shown to be transcriptionally regulated and involves a series of positive and negative regulatory as well as enhancing gene elements. We have analyzed the upstream sequence of the genes encoding the soybean oleosins, the protein that encases the oil body. We have found that soybean oleosin genes possess regulatory elements in upstream domain that are similar to those found in vacuolar storage protein genes.

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

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

Purification and characterization of oil-bodies (oleosomes) and oil-body boundary proteins (oleosins) from the developing cotyledons of sunflower (Helianthus annuus L.)

Oil-bodies, from the immature cotyledons of sunflower (Helianthus annuus L.), were difficult to purify to homogeneity using conventional techniques. The major protein contaminants were albumin and globulin storage proteins. A protocol has been developed, therefore, based upon the stringent washing of the oil-body fraction in 9 M urea, which effectively removed almost all the contaminating protein as judged by SDS/PAGE. The urea-washed oil-bodies were enriched in two major proteins of M(r) 19000 and 20000. These proteins were oleosins as demonstrated by their amino acid compositions and the sequence analysis of peptides produced by CNBr cleavage. Far-UV CD spectra of the oleosins in trifluoroethanol, trifluoroethanol/water mixtures and as mixed micelles in SDS, were typical of alpha-helical proteins with alpha-helical contents of some 55%. The phospholipid content of the urea-washed preparations was less than 0.1% of that required to form a half-unit membrane surrounding the oil-body. The oil-body surface therefore appears to be an unusual and novel structure, covered largely by an oleosin protein coat or pellicle rather than a conventional fluid membrane, half-unit or otherwise.

Regulation of an Arabidopsis oleosin gene promoter in transgenic Brassica napus

Progressive deletions of the 5'-flanking sequences of an Arabidopsis oleosin gene were fused to beta-glucuronidase (GUS) and introduced into Brassica napus plants using Agrobacterium-mediated transformation. The effect of these deletions on the quantitative level of gene expression, organ specificity and developmental regulation was assessed. In addition, the influence of abscisic acid (ABA), jasmonic acid (JA), sorbitol and a combined ABA/sorbitol treatment on gene expression was investigated. Sequences that positively regulate quantitative levels of gene expression are present between -1100 to -600 and -400 to -200 of the promoter. In addition, sequences present between -600 and -400 down-regulate quantitative levels of expression. In transgenic B. napus plants, the oleosin promoter directs seed-specific expression of GUS which is present at early stages of seed development and increases throughout seed maturation. Sequences present between -2500 and -1100 of the promoter are involved in modulating the levels of expression at early stages of embryo development. Histochemical staining of embryos demonstrated that expression is uniform throughout the tissues of the embryo. Sequences involved in the response to ABA and sorbitol are present between -400 and -200. The induction of GUS activity by a combined ABA/sorbitol treatment is additive suggesting that ABA is not the sole mediator of osmotically induced oleosin gene expression. A response to JA was only observed when the oleosin promoter was truncated to -600 suggesting that the reported effect of JA on oleosin gene expression may be at a post-transcriptional level.