Caleosins: Ca2+-binding proteins associated with lipid bodies

Suppression of soybean oleosin produces micro-oil bodies that aggregate into oil body/ER complexes

Using RNAi, the seed oil body protein 24-kDa oleosin has been suppressed in transgenic soybeans. The endoplasmic reticulum (ER) forms micro-oil bodies about 50 nm in diameter that coalesce with adjacent oil bodies forming a hierarchy of oil body sizes. The oil bodies in the oleosin knockdown form large oil body-ER complexes with the interior dominated by micro-oil bodies and intermediate-sized oil bodies, while the peripheral areas of the complex are dominated by large oil bodies. The complex merges to form giant oil bodies with onset of seed dormancy that disrupts cell structure. The transcriptome of the oleosin knockdown shows few changes compared to wild-type. Proteomic analysis of the isolated oil bodies of the 24-kDa oleosin knockdown shows the absence of the 24-kDa oleosin and the presence of abundant caleosin and lipoxygenase. The formation of the micro-oil bodies in the oleosin knockdown is interpreted to indicate a function of the oleosin as a surfactant.

Elevating Bioavailability of Cyclosporine A via Encapsulation in Artificial Oil Bodies Stabilized by Caleosin

To elevate its bioavailability via oral administration, cyclosporine A (CsA), a hydrophobic drug, was either incorporated into olive oil directly or encapsulated in artificial oil bodies (AOBs) constituted with olive oil and phospholipid in the presence or absence of recombinant caleosin purified from Escherichia coli. The bioavailabilities of CsA in these formulations were assessed in Wistar rats in comparison with the commercial formulation, Sandimmun Neoral. Among these tests, CsA-loaded AOBs stabilized by the recombinant caleosin exhibited better bioavailability than the commercial formulation and possessed the highest maximum whole blood concentration (C(max)), 1247.4 +/- 106.8 ng/mL, in the experimental animals 4.3 +/- 0.7 h (t(max)) after oral administration. C(max) and the area under the plasma concentration-time curve (AUC(0-24)) were individually increased by 50.8% and 71.3% in the rats fed with caleosin-stabilized AOBs when compared with those fed with the reference Sandimmun Neoral. The results suggest that constitution of AOBs stabilized by caleosin may be a suitable technique to encapsulate hydrophobic drugs for oral administration.

A unique caleosin in oil bodies of lily pollen

In view of the recent isolation of stable oil bodies as well as a unique oleosin from lily pollen, this study examined whether other minor proteins were present in this lipid-storage organelle. Immunological cross-recognition using antibodies against three minor oil-body proteins from sesame suggested that a putative caleosin was specifically detected in the oil-body fraction of pollen extract. A cDNA fragment encoding this putative pollen caleosin, obtained by PCR cloning, was confirmed by immunodetection and MALDI-MS analyses of the recombinant protein over-expressed in Escherichia coli and the native form. Caleosin in lily pollen oil bodies seemed to be a unique isoform distinct from that in lily seed oil bodies.

A novel role for oleosins in freezing tolerance of oilseeds in Arabidopsis thaliana

Oil bodies in seeds of higher plants are surrounded with oleosins. Here we demonstrate a novel role for oleosins in protecting oilseeds against freeze/thaw-induced damage of their cells. We detected four oleosins in oil bodies isolated from seeds of Arabidopsis thaliana, and designated them OLE1, OLE2, OLE3 and OLE4 in decreasing order of abundance in the seeds. For reverse genetics, we isolated oleosin-deficient mutants (ole1, ole2, ole3 and ole4) and generated three double mutants (ole1 ole2, ole1 ole3 and ole2 ole3). Electron microscopy showed an inverse relationship between oil body sizes and total oleosin levels. The double mutant ole1 ole2, which had the lowest levels of oleosins, had irregular enlarged oil-containing structures throughout the seed cells. Germination rates were positively associated with oleosin levels, suggesting that defects in germination are related to the expansion of oil bodies due to oleosin deficiency. We found that freezing followed by imbibition at 4 degrees C abolished seed germination of single mutants (ole1, ole2 and ole3), which germinated normally without freezing treatment. The treatment accelerated the fusion of oil bodies and the abnormal-positioning and deformation of nuclei in ole1 seeds, which caused seed mortality. In contrast, ole1 seeds that had undergone freezing treatment germinated normally when incubated at 22 degrees C instead of 4 degrees C, because degradation of oils abolished the acceleration of fusion of oil bodies during imbibition. Taken together, our findings suggest that oleosins increase the viability of over-wintering oilseeds by preventing abnormal fusion of oil bodies during imbibition in the spring.

Fusion of oil bodies in endosperm of oat grains

Few microscopical studies have been made on lipid storage in oat grains, with variable results as to the extent of lipid accumulation in the starchy endosperm. Grains of medium- and high-lipid oat (Avena sativa L.) were studied at two developmental stages and at maturity, by light microscopy using different staining methods, and by scanning and transmission electron microscopy. Discrete oil bodies occurred in the aleurone layer, scutellum and embryo. In contrast, oil bodies in the starchy endosperm often had diffuse boundaries and fused with each other and with protein vacuoles during grain development, forming a continuous oil matrix between the protein and starch components. The different microscopical methods were confirmative to each other regarding the coalescence of oil bodies, a phenomenon probably correlated with the reduced amount of oil-body associated proteins in the endosperm. This was supported experimentally by SDS-PAGE separation of oil-body proteins and immunoblotting and immunolocalization with antibodies against a 16 kD oil-body protein. Much more oil-body proteins per amount of oil occurred in the embryo and scutellum than in the endosperm. Immunolocalization of 14 and 16 kD oil-body associated proteins on sectioned grains resulted in more heavy labeling of the embryo, scutellum and aleurone layer than the rest of the endosperm. Observations on the appearance of oil bodies at an early stage of development pertain to the prevailing hypotheses of oil-body biogenesis.

Subcellular localisation of Medicago truncatula 9/13-hydroperoxide lyase reveals a new localisation pattern and activation mechanism for CYP74C enzymes

BACKGROUND

Hydroperoxide lyase (HPL) is a key enzyme in plant oxylipin metabolism that catalyses the cleavage of polyunsaturated fatty acid hydroperoxides produced by the action of lipoxygenase (LOX) to volatile aldehydes and oxo acids. The synthesis of these volatile aldehydes is rapidly induced in plant tissues upon mechanical wounding and insect or pathogen attack. Together with their direct defence role towards different pathogens, these compounds are believed to play an important role in signalling within and between plants, and in the molecular cross-talk between plants and other organisms surrounding them. We have recently described the targeting of a seed 9-HPL to microsomes and putative lipid bodies and were interested to compare the localisation patterns of both a 13-HPL and a 9/13-HPL from Medicago truncatula, which were known to be expressed in leaves and roots, respectively.

RESULTS

To study the subcellular localisation of plant 9/13-HPLs, a set of YFP-tagged chimeric constructs were prepared using two M. truncatula HPL cDNAs and the localisation of the corresponding chimeras were verified by confocal microscopy in tobacco protoplasts and leaves. Results reported here indicated a distribution of M.truncatula 9/13-HPL (HPLF) between cytosol and lipid droplets (LD) whereas, as expected, M.truncatula 13-HPL (HPLE) was targeted to plastids. Notably, such endocellular localisation has not yet been reported previously for any 9/13-HPL. To verify a possible physiological significance of such association, purified recombinant HPLF was used in activation experiments with purified seed lipid bodies. Our results showed that lipid bodies can fully activate HPLF.

CONCLUSION

We provide evidence for the first CYP74C enzyme, to be targeted to cytosol and LD. We also showed by sedimentation and kinetic analyses that the association with LD or lipid bodies can result in the protein conformational changes required for full activation of the enzyme. This activation mechanism, which supports previous in vitro work with synthetic detergent micelle, fits well with a mechanism for regulating the rate of release of volatile aldehydes that is observed soon after wounding or tissue disruption.

Structural properties of caleosin: A MS and CD study

We have investigated the covalent and secondary solution structure of caleosin, a 27-kDa protein also called ATS1 or AtClo1 (At4g26740) found within Arabidopsis thaliana seed lipid bodies. The native protein was partly phosphorylated at S225. Purified bacterially expressed caleosin (recClo) was not phosphorylated; cysteine residues C221 and C230 were connected by a disulfide bridge. In solution it exists as a mixture of predominant monomers and covalent dimers. We have used recClo as a model for the study of AtClo1 secondary structure. recClo is folded in aqueous solution (16% alpha-helix, 29% beta-sheet), its secondary structure being dramatically influenced by the polarity of media, as deduced from CD spectra measured in the presence of increasing concentrations of various aliphatic alcohols.

Evidence for the probable oil body association of a thiol-protease, leading to oleosin degradation in sunflower seedling cotyledons

The activity of a 65 kDa, cytosolic protease from sunflower seedling cotyledons coincides with the degradation of oleosins during seed germination. Further investigations carried out in this laboratory have demonstrated the probable association of a thiol-protease with oil bodies, leading to gradual degradation of oleosins during seedling growth. Evidence to this effect have been brought out through zymographic detection of protease activity from oil bodies, degradation of oleosins by electrophoretically eluted protease from the seedling cotyledons and inhibition of protease activity by thiol-protease inhibitor, such as N-ethylmaleimide (NEM). In addition to these biochemical evidence, visualization of thiol-protease activity has also been achieved by a novel fluorescence microscopic method and confocal imaging. It involves the uptake and binding of a fluorogenic thiol-protease inhibitor (fluorescein mercuric acetate, FMA) at the intracellular thiol-protease activity sites in protoplasts, leading to fluorescence emission at 523 nm following excitation at 499 nm. Maximum protease activity is observed in 4-d-old seedling cotyledons, coinciding with the phase of active triacylglycerol (TAGs) hydrolysis. All these observations provide evidence for the expression of the said thiol-protease activity on the oil body surface, leading to gradual proteolysis of oleosins during seed germination.

Plant seed peroxygenase is an original heme-oxygenase with an EF-hand calcium binding motif

A growing body of evidence indicates that phytooxylipins play important roles in plant defense responses. However, many enzymes involved in the biosynthesis of these metabolites are still elusive. We have purified one of these enzymes, the peroxygenase (PXG), from oat microsomes and lipid droplets. It is an integral membrane protein requiring detergent for its solubilization. Proteinase K digestion showed that PXG is probably deeply buried in lipid droplets or microsomes with only about 2 kDa at the C-terminal region accessible to proteolytic digestion. Sequencing of the N terminus of the purified protein showed that PXG had no sequence similarity with either a peroxidase or a cytochrome P450 but, rather, with caleosins, i.e. calcium-binding proteins. In agreement with this finding, we demonstrated that recombinant thale cress and rice caleosins, expressed in yeast, catalyze hydroperoxide-dependent mono-oxygenation reactions that are characteristic of PXG. Calcium was also found to be crucial for peroxygenase activity, whereas phosphorylation of the protein had no impact on catalysis. Site-directed mutagenesis studies revealed that PXG catalytic activity is dependent on two highly conserved histidines, the 9 GHz EPR spectrum being consistent with a high spin pentacoordinated ferric heme.

A role for caleosin in degradation of oil-body storage lipid during seed germination

Caleosin is a Ca(2+)-binding oil-body surface protein. To assess its role in the degradation of oil-bodies, two independent insertion mutants lacking caleosin were studied. Both mutants demonstrated significant delay of breakdown of the 20:1 storage lipid at 48 and 60 h of germination. Additionally, although germination rates for seeds were not affected by the mutations, mutant seedlings grew more slowly than wild type when measured at 48 h of germination, a defect that was corrected with continued growth for 72 and 96 h in the light. After 48 h of germination, wild-type central vacuoles had smooth contours and demonstrated internalization of oil bodies and of membrane containing alpha- and delta-tonoplast intrinsic proteins (TIPs), markers for protein storage vacuoles. In contrast, mutant central vacuoles had distorted limiting membranes displaying domains with clumps of the two TIPs, and they contained fewer oil bodies. Thus, during germination caleosin plays a role in the degradation of storage lipid in oil bodies. Its role involves both the normal modification of storage vacuole membrane and the interaction of oil bodies with vacuoles. The results indicate that interaction of oil bodies with vacuoles is one mechanism that contributes to the degradation of storage lipid.

Gene expression patterns during somatic embryo development and germination in maize Hi II callus cultures

Gene expression patterns were profiled during somatic embryogenesis in a regeneration-proficient maize hybrid line, Hi II, in an effort to identify genes that might be used as developmental markers or targets to optimize regeneration steps for recovering maize plants from tissue culture. Gene expression profiles were generated from embryogenic calli induced to undergo embryo maturation and germination. Over 1,000 genes in the 12,060 element arrays showed significant time variation during somatic embryo development. A substantial number of genes were downregulated during embryo maturation, largely histone and ribosomal protein genes, which may result from a slowdown in cell proliferation and growth during embryo maturation. The expression of these genes dramatically recovered at germination. Other genes up-regulated during embryo maturation included genes encoding hydrolytic enzymes (nucleases, glucosidases and proteases) and a few storage genes (an alpha-zein and caleosin), which are good candidates for developmental marker genes. Germination is accompanied by the up-regulation of a number of stress response and membrane transporter genes, and, as expected, greening is associated with the up-regulation of many genes encoding photosynthetic and chloroplast components. Thus, some, but not all genes typically associated with zygotic embryogenesis are significantly up or down-regulated during somatic embryogenesis in Hi II maize line regeneration. Although many genes varied in expression throughout somatic embryo development in this study, no statistically significant gene expression changes were detected between total embryogenic callus and callus enriched for transition stage somatic embryos.

The extracellular pollen coat in members of the Brassicaceae: composition, biosynthesis, and functions in pollination

I have used cellular and molecular genetic and bioinformatic approaches to characterise the components of the pollen coat in plants of the family Brassicaceae, including Arabidopsis thaliana and several brassicas including Brassica napus, B. oleracea, and B. rapa. The pollen coat in these species is mostly made up of a unique mixture of lipids that is highly enriched in acylated compounds, such as sterol esters and phospholipids. These acyl lipids are characterised by an unusually high degree of saturation. The fatty acids typically contain 70-90% saturated acyl residues such as myristate, palmitate, and stearate. The major sterol components of the pollen coat are saturated fatty acyl esters of stigmasterol, campesterol, and campestdienol. In addition to lipids, the second major component of the pollen coat is a specific group of proteins that is dominated by a family of proteins that we term pollenins. Although pollenins are by far the major protein components of the pollen coat of members of the Brassicaceae, proteomic analysis reveals that there are several additional protein components, including lipases, protein kinases, a pectin esterase, and a caleosin. The biosynthesis of these lipids and proteins and their significance for overall pollen function are reviewed and discussed.

Oleosin gene family of Coffea canephora: quantitative expression analysis of five oleosin genes in developing and germinating coffee grain

Coffee grains have an oil content between 10% and 16%, with these values associated with Coffea canephora (robusta) and C. arabica (arabica), respectively. As the majority of the oil stored in oil seeds is contained in specific structures called oil bodies, we were interested in determining whether there are any differences in the expression of the main oil body proteins, the oleosins, between the robusta and arabica varieties. Here, we present the isolation, characterization and quantitative expression analysis of six cDNAs representing five genes of the coffee oleosin family (CcOLE-1 to CcOLE-5) and one gene of the steroleosin family (CcSTO-1). Each coffee oleosin cDNA encodes for the signature structure for oleosins, a long hydrophobic central sequence containing a proline KNOT motif. Sequence analysis also indicates that the C-terminal domain of CcOLE-1, CcOLE-3 and CcOLE-5 contain an 18-residue sequence typical of H-form oleosins. Quantitative RT-PCR showed that the transcripts of all five oleosins were predominantly expressed during grain maturation in robusta and arabica grain, with CcOLE-1 and CcOLE-2 being more highly expressed. While the relative expression levels of the five oleosins were similar for robusta and arabica, significant differences in the absolute levels of expression were found between the two species. Quantitative analysis of oleosin transcripts in germinating arabica grain generally showed that the levels of these transcripts were lower in the grain after drying, and then further decreased during germination, except for a small spike of expression for CcOLE-2 early in germination. In contrast, the levels of CcSTO-1 transcripts remained relatively constant during germination, in agreement with suggestions that this protein is actively involved in the process of oil body turnover. Finally, we discuss the implications of the coffee oleosin expression data presented relative to the predicted roles for the different coffee oleosins during development and germination.

Differentially expressed genes associated with dormancy or germination of Arabidopsis thaliana seeds

Differential display analysis using dormant and non-dormant Arabidopsis thaliana (L.) Heynh seeds resulted in a set of genes that were associated with either dormancy or germination. Expression of the germination-associated genes AtRPL36B and AtRPL27B, encoding two ribosomal proteins, was undetectable in the dry seed, low in dormant seed, and high under conditions that allowed completion of germination. Expression of these genes was also found to be light-regulated and to correlate with germination speed. Expression of the dormancy-associated genes ATS2 and ATS4, encoding a caleosin-like protein and a protein similar to a low-temperature-induced protein respectively, was high in the dry seed and decreased during germination. Expression of ATS2 and ATS4 was high in primary and secondary dormant seed but low in after-ripened or chilled seed. The expression of both genes was also light-regulated, but no relationship with temperature-dependent germination speed was found.

Characterisation and functional analysis of two barley caleosins expressed during barley caryopsis development

Two full-length cDNA sequences homologous to caleosin, a seed-storage oil-body protein from sesame, were identified from a series of barley grain development cDNA libraries and further characterised. The cDNAs, subsequently termed HvClo1 and HvClo2, encode proteins of 34 kDa and 28 kDa, respectively. Real-time RT-PCR indicated that HvClo1 is expressed abundantly during the later stages of embryogenesis and is seed-specific, accumulating in the scutellum of mature embryos. HvClo2 is expressed mainly in the endosperm tissues of the developing grain. We show that HvClo1 and HvClo2 are paralogs that co-segregate on barley chromosome 2HL. Transient expression of HvClo1 in lipid storage and non-storage cells of barley using biolistic particle bombardment indicates that caleosins have different subcellular locations from the structural oil-body protein oleosin, and by inference participate in different sorting pathways. We observe that caleosin sorts via small vesicles, suggesting a likely association with lipid trafficking, membrane expansion and oil-body biogenesis.

Subcellular localization of Arabidopsis 3-hydroxy-3-methylglutaryl-coenzyme A reductase

Plants produce diverse isoprenoids, which are synthesized in plastids, mitochondria, endoplasmic reticulum (ER), and the nonorganellar cytoplasm. 3-Hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR) catalyzes the synthesis of mevalonate, a rate-limiting step in the cytoplasmic pathway. Several branches of the pathway lead to the synthesis of structurally and functionally varied, yet essential, isoprenoids. Several HMGR isoforms have been identified in all plants examined. Studies based on gene expression and on fractionation of enzyme activity suggested that subcellular compartmentalization of HMGR is an important intracellular channeling mechanism for the production of the specific classes of isoprenoids. Plant HMGR has been shown previously to insert in vitro into the membrane of microsomal vesicles, but the final in vivo subcellular localization(s) remains controversial. To address the latter in Arabidopsis (Arabidopsis thaliana) cells, we conducted a multipronged microscopy and cell fractionation approach that included imaging of chimeric HMGR green fluorescent protein localizations in transiently transformed cell leaves, immunofluorescence confocal microscopy in wild-type and stably transformed seedlings, immunogold electron microscopy examinations of endogenous HMGR in seedling cotyledons, and sucrose density gradient analyses of HMGR-containing organelles. Taken together, the results reveal that endogenous Arabidopsis HMGR is localized at steady state within ER as expected, but surprisingly also predominantly within spherical, vesicular structures that range from 0.2- to 0.6-microm diameter, located in the cytoplasm and within the central vacuole in differentiated cotyledon cells. The N-terminal region, including the transmembrane domain of HMGR, was found to be necessary and sufficient for directing HMGR to ER and the spherical structures. It is believed, although not directly demonstrated, that these vesicle-like structures are derived from segments of HMGR-ER. Nevertheless, they represent a previously undescribed subcellular compartment likely capable of synthesizing mevalonate, which provides new evidence for multiorganelle compartmentalization of the isoprenoid biosynthetic pathways in plants.

Role of the hydrophobic domain in targeting caveolin-1 to lipid droplets

Although caveolins normally reside in caveolae, they can accumulate on the surface of cytoplasmic lipid droplets (LDs). Here, we first provided support for our model that overaccumulation of caveolins in the endoplasmic reticulum (ER) diverts the proteins to nascent LDs budding from the ER. Next, we found that a mutant H-Ras, present on the cytoplasmic surface of the ER but lacking a hydrophobic peptide domain, did not accumulate on LDs. We used the fact that wild-type caveolin-1 accumulates in LDs after brefeldin A treatment or when linked to an ER retrieval motif to search for mutants defective in LD targeting. The hydrophobic domain, but no specific sequence therein, was required for LD targeting of caveolin-1. Certain Leu insertions blocked LD targeting, independently of hydrophobic domain length, but dependent on their position in the domain. We propose that proper packing of putative hydrophobic helices may be required for LD targeting of caveolin-1.

Constitution of stable artificial oil bodies with triacylglycerol, phospholipid, and caleosin

Seed oil bodies are lipid storage organelles of 0.5-2 microm in diameter and comprise a triacylglycerol matrix shielded by a monolayer of phospholipids and proteins. These proteins include abundant structural proteins, oleosins, and at least two minor proteins termed caleosin and steroleosin. This study examined if artificial oil bodies (AOBs) composed of triacylglycerol and phospholipid could be stabilized by oleosin, caleosin, or steroleosin. Our results showed that stabilization effects could be realized by oleosin or caleosin but not by steroleosin. The sizes of the AOBs constituted with oleosin (0.5-2 microm) or caleosin (50-200 nm) were similar to or 10 times smaller than those of the native oil bodies. Recombinant caleosin expressed in Escherichia coli also encapsulated AOBs with a size, topology, and stability comparable to those encapsulated with native caleosin. A proteinase K digestion indicated that caleosin anchored the AOBs via its central hydrophobic domain of approximately 4 kDa. Isoelectrofocusing revealed that the isoelectric point of the caleosin-stabilized AOBs was pH 4.0. Aggregation of AOBs was observed at a pH lower than 4.5; thus, their stability and integrity were presumably contributed by surface caleosin via electronegative repulsion and steric hindrance. The caleosin-stabilized AOBs were thermostable up to 70 degrees C and potentially useful for biotechnological applications.

Proteomics of calcium-signaling components in plants

Calcium functions as a versatile messenger in mediating responses to hormones, biotic/abiotic stress signals and a variety of developmental cues in plants. The Ca(2+)-signaling circuit consists of three major "nodes"--generation of a Ca(2+)-signature in response to a signal, recognition of the signature by Ca2+ sensors and transduction of the signature message to targets that participate in producing signal-specific responses. Molecular genetic and protein-protein interaction approaches together with bioinformatic analysis of the Arabidopsis genome have resulted in identification of a large number of proteins at each "node"--approximately 80 at Ca2+ signature, approximately 400 sensors and approximately 200 targets--that form a myriad of Ca2+ signaling networks in a "mix and match" fashion. In parallel, biochemical, cell biological, genetic and transgenic approaches have unraveled functions and regulatory mechanisms of a few of these components. The emerging paradigm from these studies is that plants have many unique Ca2+ signaling proteins. The presence of a large number of proteins, including several families, at each "node" and potential interaction of several targets by a sensor or vice versa are likely to generate highly complex networks that regulate Ca(2+)-mediated processes. Therefore, there is a great demand for high-throughput technologies for identification of signaling networks in the "Ca(2+)-signaling-grid" and their roles in cellular processes. Here we discuss the current status of Ca2+ signaling components, their known functions and potential of emerging high-throughput genomic and proteomic technologies in unraveling complex Ca2+ circuitry.

Protein composition of oil bodies in Arabidopsis thaliana ecotype WS

Till now, only scattered data are available in the literature, which describes the protein content of plant oil bodies. Especially, the proteins closely associated with the model plant Arabidopsis thaliana oil bodies have never been previously purified and characterized. Oil bodies have been purified using flotation techniques, combined with incubations under high salt concentration, in the presence of detergents and urea in order to remove non-specifically trapped proteins. The identity and integrity of the oil bodies have been characterized. Oil bodies exhibited hydrodynamic diameters close to 2.6 microm, and a ratio fatty acid-protein content near 20. The proteins composing these organelles were extracted, separated by SDS-PAGE, digested by trypsin, and their peptides were subsequently analyzed by nano-chromatography-mass spectrometry (nano-LC-MS/MS). This led to the identification of a limited number of proteins: four different oleosins, ATS1, a protein homologous to calcium binding protein, a 11-beta-hydroxysteroid dehydrogenase-like protein, a probable aquaporin and a glycosylphosphatidylinositol-anchored protein with no known function. The two last proteins were till now never identified in plant oil bodies. Structural proteins (oleosins) represented up to 79% of oil body proteins and the 18.5 kDa oleosin was the most abundant among them.

Appearance of Smaller Lipid Bodies and Protein Kinase Activation in the Lipid Body Fraction Are Induced by an Increase in the Nitrogen Source in the Mortierella Fungus

We studied the regulation of lipid body biogenesis in the oleaginous fungus Mortierella ramanniana var. angulispora by investigating culture conditions to modulate lipid body size, which we found was affected by the carbon-to-nitrogen ratio (C/N ratio) in the culture medium. Increasing the nitrogen source or decreasing the C/N ratio from 38 to 9 induced the appearance of lipid bodies with diameters less than 2-3 micro m, which are usually found at a C/N ratio of 38 in this fungus. To determine factors regulating lipid body size, we compared lipid body fractions from fungal cells cultured at different C/N ratios. We found some differences in polypeptide profiles between lipid body fractions from fungal cells cultured at different C/N ratios for 2 days when the lipid bodies were enlarged at a C/N ratio of 38. We then compared the phosphorylation of lipid body proteins, since protein phosphorylation plays a pivotal role in various aspects of signal transduction. In vitro phosphorylation in the lipid body fraction indicated that protein kinase activity toward endogenous and exogenous substrates such as histone IIIS, VIIS, and myelin basic protein increased in the lipid body fraction at a C/N ratio of 9. Further analysis by in-gel protein kinase assay indicated the presence of at least three activated protein kinases with molecular masses of 75, 72, and 42 kDa, which were also autophosphorylated. These results indicate the presence of nutrient-regulated protein kinases and increased phosphorylation in lipid bodies, which correlate with the appearance of smaller lipid bodies in this fungus. Further studies to characterize these protein kinases at the molecular level should provide new insights into the link between nutrient sensing and lipid storage.

Temporal association of Ca(2+)-dependent protein kinase with oil bodies during seed development in Santalum album L.: its biochemical characterization and significance

Calcium-dependent protein kinase (CDPK) is expressed in sandalwood (Santalum album L.) seeds under developmental regulation, and it is localized with spherical storage organelles in the endosperm [Anil et al. (2000) Plant Physiol. 122: 1035]. This study identifies these storage organelles as oil bodies. A 55 kDa protein associated with isolated oil bodies, showed Ca(2+)-dependent autophosphorylation and also cross-reacted with anti-soybean CDPK. The CDPK activity detected in the oil body-protein fraction was calmodulin-independent and sensitive to W7 (N-(6-aminohexyl)-5-chloro-1-naphthalene sulfonamide) inhibition. Differences in Michaelis Menton kinetics, rate of histone phosphorylation and sensitivity to W7 inhibition between a soluble CDPK from embryos and the oil body-associated CDPK of endosperm suggest that these are tissue-specific isozymes. The association of CDPK with oil bodies of endosperm was found to show a temporal pattern during seed development. CDPK protein and activity, and the in vivo phosphorylation of Ser and Thr residues were detected strongly in the oil bodies of endosperm from maturing seed. Since oil body formation occurs during seed maturation, the observations indicate that CDPK and Ca(2+) may have a regulatory role during oil accumulation/oil body biogenesis. The detection of CDPK-protein and activity in oil bodies of groundnut, sesame, cotton, sunflower, soybean and safflower suggests the ubiquity of the association of CDPKs with oil bodies.

Steroleosin, a sterol-binding dehydrogenase in seed oil bodies

Besides abundant oleosin, three minor proteins, Sop 1, 2, and 3, are present in sesame (Sesamum indicum) oil bodies. The gene encoding Sop1, named caleosin for its calcium-binding capacity, has recently been cloned. In this study, Sop2 gene was obtained by immunoscreening, and it was subsequently confirmed by amino acid partial sequencing and immunological recognition of its overexpressed protein in Escherichia coli. Immunological cross recognition implies that Sop2 exists in seed oil bodies of diverse species. Along with oleosin and caleosin genes, Sop2 gene was transcribed in maturing seeds where oil bodies are actively assembled. Sequence analysis reveals that Sop2, tentatively named steroleosin, possesses a hydrophobic anchoring segment preceding a soluble domain homologous to sterol-binding dehydrogenases/reductases involved in signal transduction in diverse organisms. Three-dimensional structure of the soluble domain was predicted via homology modeling. The structure forms a seven-stranded parallel beta-sheet with the active site, S-(12X)-Y-(3X)-K, between an NADPH and a sterol-binding subdomain. Sterol-coupling dehydrogenase activity was demonstrated in the overexpressed soluble domain of steroleosin as well as in purified oil bodies. Southern hybridization suggests that one steroleosin gene and certain homologous genes may be present in the sesame genome. Comparably, eight hypothetical steroleosin-like proteins are present in the Arabidopsis genome with a conserved NADPH-binding subdomain, but a divergent sterol-binding subdomain. It is indicated that steroleosin-like proteins may represent a class of dehydrogenases/reductases that are involved in plant signal transduction regulated by various sterols.

The domains required to direct core proteins of hepatitis C virus and GB virus-B to lipid droplets share common features with plant oleosin proteins

In mammalian tissue culture cells, the core protein of hepatitis C virus (HCV) is located at the surface of lipid droplets, which are cytoplasmic structures that store lipid. The critical amino acid sequences necessary for this localization are in a region of core protein that is absent in flavi- and pestiviruses, which are related to HCV. From our sequence comparisons, this region in HCV core was present in the corresponding protein of GBV-B, another virus whose genomic sequence has significant similarity to HCV. Expression of the putative GBV-B core protein revealed that it also was directed to lipid droplets. By extending the comparisons to cellular proteins, there were amino acid sequence similarities between the domains for lipid droplet association in HCV core and plant oleosin proteins. To determine whether these similarities were related functionally, an oleosin encoded by the Brassica napus bniii gene was expressed in different mammalian cell lines, where it retained the capacity to bind to lipid droplets. Analysis of deletion mutants indicated that the critical region within the protein required for this localization was the same for both plant and mammalian cells. A common feature in the viral and plant sequences was a motif containing proline residues. Mutagenesis of these residues in HCV core and plant oleosin abolished lipid droplet association. Finally, the domain within HCV core required for binding to lipid droplets could substitute for the equivalent domain in oleosin, further indicating the functional relatedness between the viral and plant sequences. These studies identify common features in disparate proteins that are required for lipid droplet localization.

Analysis of EF-hand-containing proteins in Arabidopsis

BACKGROUND

In plants, calcium (Ca2+) has emerged as an important messenger mediating the action of many hormonal and environmental signals, including biotic and abiotic stresses. Many different signals raise cytosolic calcium concentration ([Ca2+]cyt), which in turn is thought to regulate cellular and developmental processes via Ca2+-binding proteins. Three out of the four classes of Ca2+-binding proteins in plants contain Ca2+-binding EF-hand motif(s). This motif is a conserved helix-loop-helix structure that can bind a single Ca2+ ion. To identify all EF-hand-containing proteins in Arabidopsis, we analyzed its completed genome sequence for genes encoding EF-hand-containing proteins.

RESULTS

A maximum of 250 proteins possibly having EF-hands were identified. Diverse proteins, including enzymes, proteins involved in transcription and translation, protein- and nucleic-acid-binding proteins and a large number of unknown proteins, have one or more putative EF-hands. Phylogenetic analysis identified six major groups that contain some families of proteins.

CONCLUSIONS

The presence of EF-hand motif(s) in a diversity of proteins is consistent with the involvement of Ca2+ in regulating many cellular and developmental processes. Thus far, only 47 of the possible 250 EF-hand proteins have been reported in the literature. Various domains that we identified in many of the uncharacterized EF-hand-containing proteins should help in elucidating their cellular role(s). Our analyses suggest that the Ca2+ messenger system is widely used in plants and that EF-hand-containing proteins are likely to be the key transducers mediating Ca2+ action.

Oil bodies and their associated proteins, oleosin and caleosin

Oil bodies are lipid storage organelles which have been analyzed biochemically due to the economic importance of oil seeds. Although oil bodies are structurally simple, the mechanisms involved in their formation and degradation remain controversial. At present, only two proteins associated with oil bodies have been described, oleosin and caleosin. Oleosin is thought to be important for oil body stabilization in the cytosol, although neither the structure nor the function of oleosin has been fully elucidated. Even less is known about caleosin, which has only recently been described [Chen et al. (1999) Plant Cell Physiol 40: 1079-1086; Naested et al. (2000) Plant Mol Biol 44: 463-476]. Caleosin and caleosin-like proteins are not unique to oil bodies and are associated with an endoplasmatic reticulum subdomain in some cell types. Here we review the synthesis and degradation of oil bodies as they relate to structural and functional aspects of oleosin and caleosin.