Targeting and glycosylation of patatin the major potato tuber protein in leaves of transgenic tobacco

Yeast-derived potato patatins: Biochemical and biophysical characterization

Potato patatin is considered a valuable plant protein by the food industry for its exceptional functional properties and nutritional value. Nonetheless, it has not been widely used due to its low abundance in potatoes and high cost. Pichia pastoris was utilized for expression of patatin to overcome agricultural limitations. Biochemical and biophysical characterization of Patatin-B2 (rPatB2) and Patatin-17 (rPat17) is described. rPatB2 and rPat17 had higher zeta potential and superior solubility at various pH conditions in comparison with commercial patatin, whereas particle size distribution was similar. Inflection temperatures were higher than potato isolated patatins. Antioxidant capacity of rPatB2 and rPat17 was similar to that of commercial patatin and the specific enzymatic activity of rPatB2 was 5-fold higher than rPat17 and patatins isolated from potato. Results indicate yeast-derived patatin properties are comparable to patatins from potatoes, suggesting their potential use in various plant-based products such as meat and dairy analogues.

N-glycome profiling of patatins from different potato species of Solanum genus

It is hypothesized that oligosaccharides are another potential source of immunological cross-reaction between different plant allergens. Patatin is the most abundant glycoprotein in potato and has been described to have an oligosaccharide of composition Man3(Xyl)GlcNAc2(Fuc). In this work, N-glycosylation profiles of patatin proteins isolated from tubers of different potato species were investigated and compared. Oligosaccharides were released by enzymatic digestion with PNAGase A and analyzed primarily by matrix-assisted laser desorption ionization mass spectrometry. For glycan labeling, a modified version of on-target derivatization with phenylhydrazine was applied. This study found the presence of glycan structures not described previously in patatins of potato tubers, and their glycan profiles significantly differed. This knowledge about the glycosylation of potato patatins may be helpful for correct choice of potato species to decrease the presence of specific glycan epitopes causing food allergy as well as for utilization of potatoes for the manufacture of therapeutic proteins.

Separation of patatins and protease inhibitors from potato fruit juice with clay minerals as cation exchangers

Potato fruit juice as a by-product of the starch industry contains proteins with interesting functionalities such as protease inhibitors or patatin with its high nutritional value. Due to their functional properties, these proteins are principally of industrial interest. A drawback for the application of these potato proteins is the separation and isolation under maintenance of the biological activity. So far, there are no methods in literature, which are satisfying concerning the costs or the separation performance. In this study, we show a chromatographic approach using natural clay minerals as cation exchangers to separate two protein fractions in potato fruit juice. Additionally, the content of glycoalkaloids naturally occurring in potatoes is significantly reduced in a single step together with the separation of the patatins and the protease inhibitors.

Cultivar variability of patatin biochemical characteristics: table versus processing potatoes (Solanum tuberosum L.)

Biochemical characteristics of patatin proteins purified by ion-exchange and affinity chromatography from tubers of 20 potato cultivars were studied to evaluate their genotype differences with respect to utility groups, table potato cultivars (TPCs) and processing potato cultivars (PPCs). Both groups of cultivars showed similar values of protein content in dry matter (3.98-7.39%) and of patatin relative abundance (5.40-35.40%). Three mass levels (∼40.6, 41.8, and 42.9 kDa) of purified patatins were found by MALDI-TOF MS within all cultivars. Differences among mass levels corresponding with the mass of sugar antenna (∼1.2 kDa) confirmed the previous concept of different glycosylation extentsin patatin proteins. It was showed that the individual types of patatin varying in their masses occur in the patatin family in a ratio specific for each of the cultivars, with the lowest mass type being the major one. Electrophoretic analyses demonstrated wide cultivar variability in number of patatin forms. Especially 2D-PAGE showed 17-23 detected protein spots independently on the utility group. Specific lipid acyl hydrolase (LAH) activity of purified patatins from the individual tested cultivars varied between 0.92 and 5.46 μmol/(min mg). Patatin samples within most of the TPCs exhibited higher values of specific LAH activity than samples of PPCs. It may be supposed that individual patatin forms do not have similar physiological roles.

Reduction of cross-reactive carbohydrate determinants in plant foodstuff: elucidation of clinical relevance and implications for allergy diagnosis

BACKGROUND

A longstanding debate in allergy is whether or not specific immunoglobulin-E antibodies (sIgE), recognizing cross-reactive carbohydrate determinants (CCD), are able to elicit clinical symptoms. In pollen and food allergy, ≥20% of patients display in-vitro CCD reactivity based on presence of α1,3-fucose and/or β1,2-xylose residues on N-glycans of plant (xylose/fucose) and insect (fucose) glycoproteins. Because the allergenicity of tomato glycoallergen Lyc e 2 was ascribed to N-glycan chains alone, this study aimed at evaluating clinical relevance of CCD-reduced foodstuff in patients with carbohydrate-specific IgE (CCD-sIgE).

METHODOLOGY/PRINCIPAL FINDINGS

Tomato and/or potato plants with stable reduction of Lyc e 2 (tomato) or CCD formation in general were obtained via RNA interference, and gene-silencing was confirmed by immunoblot analyses. Two different CCD-positive patient groups were compared: one with tomato and/or potato food allergy and another with hymenoptera-venom allergy (the latter to distinguish between CCD- and peptide-specific reactions in the food-allergic group). Non-allergic and CCD-negative food-allergic patients served as controls for immunoblot, basophil activation, and ImmunoCAP analyses. Basophil activation tests (BAT) revealed that Lyc e 2 is no key player among other tomato (glyco)allergens. CCD-positive patients showed decreased (re)activity with CCD-reduced foodstuff, most obvious in the hymenoptera venom-allergic but less in the food-allergic group, suggesting that in-vivo reactivity is primarily based on peptide- and not CCD-sIgE. Peptide epitopes remained unaffected in CCD-reduced plants, because CCD-negative patient sera showed reactivity similar to wild-type. In-house-made ImmunoCAPs, applied to investigate feasibility in routine diagnosis, confirmed BAT results at the sIgE level.

CONCLUSIONS/SIGNIFICANCE

CCD-positive hymenoptera venom-allergic patients (control group) showed basophil activation despite no allergic symptoms towards tomato and potato. Therefore, this proof-of-principle study demonstrates feasibility of CCD-reduced foodstuff to minimize 'false-positive results' in routine serum tests. Despite confirming low clinical relevance of CCD antibodies, we identified one patient with ambiguous in-vitro results, indicating need for further component-resolved diagnosis.

Rheological properties of patatin gels compared with β-lactoglobulin, ovalbumin, and glycinin

BACKGROUND

The thermal unfolding and rheological properties of patatin gels were compared with those of commonly used proteins (β-lactoglobulin, ovalbumin, glycinin).

RESULTS

A significant difference between these proteins was observed in both the denaturation temperature (59 °C for patatin; about 20 °C lower than the other proteins) and the onset temperature of gel formation (50-60 °C, compared to 70-85 °C for the other proteins). At low ionic strength the minimal concentration was only 6% (w/v) for patatin, compared to 8-11% for the other proteins. This effect was attributed to the relatively high exposed hydrophobicity of patatin as determined by hydrophobic interaction chromatography. For gels compared at 'iso-strength', the frequency dependence was found to be close to identical, while small differences were observed in the strain at fracture.

CONCLUSIONS

Patatin was found to form gels with comparable small-deformational rheological properties as typical food proteins. In addition, at concentrations where the elastic modulus was similar for all proteins, the frequency and strain dependence were also comparable. From this it is concluded that patatin is a promising protein to be used in food applications as a gelling agent.

Analysis of genome expression in the response of Oryza granulata to Xanthomonas oryzae pv oryzae

In order to understand the mechanism of the strong resistance of Oryza granulata to Xanthomonas oryzae pv oryzae (Xoo), cDNA microarrays containing 2,436 cDNA clones of Oryza granulata derived from Suppression subtractive library and cDNA library were constructed and genome expression patterns after inoculating Xoo were investigated. Three hundred and 83 clones were up-regulated, 836 clones were down-regulated after pathogen infection. Approximately 800 clones were sequenced and BLAST search were carried out. There are no homologous sequences for 35 clones of them. The functions of the homologous sequences for most clones are unknown. The known functions of the homologous sequences involved in photosynthesis, respiration, material transport, signal transduction, pathogenesis-related proteins, transcription factors, the active oxygen scavenging system and so on. The putative functions of them in responding to Xoo were discussed.

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.

Covalent structures of potato tuber lipases (patatins) and implications for vacuolar import

Proteome data of potato (Solanum tuberosum) tuber juice and of purified potato tuber vacuoles indicated that mature patatins may perhaps lack a C-terminal propeptide. We have confirmed this by complete mass spectrometric sequencing of a number of patatin variants as well as their N-linked complex-type glycans from the starch-rich cultivar Kuras. For this cultivar full-length patatin cDNAs have also been sequenced, as the patatin locus is highly polymorphous. It is well known that patatins are located in the vacuoles of potato tubers. Furthermore, the complex glycan structures show that the path is via the Golgi apparatus. However, the vacuolar targeting signal has never been identified for this storage and defense protein, which amounts to 25-40% of tuber protein. We propose that a six-residue C-terminal propeptide, -ANKASY-COO(-) comprises this signal. The crystallographic structure of a recombinant patatin (Rydel, T. J., Williams, J. M., Krieger, E., Moshiri, F., Stallings, W. C., Brown, S. M., Pershing, J. C., Prucell, J. P., and Alibhai, M. F. (2003) Biochemistry 42, 6696-6708), which included this propeptide thus, for the first time, shows the structure of a putative ligand of the vacuolar sorting receptor and processing enzyme responsible for patatin import.

Cloning, expression, purification and characterization of patatin, a novel phospholipase A

Patatin is the major protein constituent of potato tubers and displays broad esterase activity. The native enzyme actually belongs to a highly homologous multigene family of vacuolar glycoproteins. From these, the patB2 patatin gene was selected and cloned into pUC19 without its signal sequence but with an N-terminal histidine-tag. This patatin was overexpressed under the control of the lac promotor in Escherichia coli strain DH5alpha. The protein was recovered as inclusion bodies, folded into its native state by solubilization in urea and purified to homogeneity. Starting with one gram of inclusion bodies, 19 mg of pure and active recombinant patatin was isolated, with even higher specific activity than the glycosylated wild-type patatin purified from potato tubers. The purified enzyme showed esterolytic activity with p-nitrophenylesters dissolved in Triton X-100 micelles. The activity of patatin on p-nitrophenylesters with different carbon chain lengths showed an optimum for p-nitrophenylesters with 10 carbon atoms. Besides general esterolytic activity, the pure enzyme was found to display high phospholipase A activity in particular with the substrates 1,2-dioctanoyl-sn-glycero-3-phosphocholine (diC(8)PCho) (127 U.mg(-1)) and 1,2-dinonanoyl-sn-glycero-3-phosphocholine (diC(9)PCho) (109 U.mg(-1)). Recently, the structure of human cytosolic PLA(2) (cPLA(2)) was solved, showing a novel Ser-Asp active site dyad [1]. Based on a partial sequence alignment of patatin with human cPLA(2), we propose that patatin contains a similar active site dyad. To verify this assumption, conserved Ser, Asp and His residues in the family of patatins have been modified in patatin B2. Identification of active site residues was based on the observation of correctly folded but inactive variants. This led to the assignment of Ser54 and Asp192 as the active site serine and aspartate residues in patatin B2, respectively.

Glycosylation of the cationic peanut peroxidase gene expressed in transgenic tobacco

The major cationic peanut (Arachis hypogaea) peroxidase, secreted into the extracellular space, is a glycoprotein with three N-linked glycans (polysaccharides) which are connected to the peptide backbone at Asn-60, Asn-144 and Asn-185. In this report, a C-terminal histidine-tagged cationic peanut peroxidase gene was expressed in transgenic tobacco (Nicotiana tabacum). Tissue of the transgenic tobacco was cultured in suspension culture and the his-tagged peroxidase was purified in large quantities from 14-day-old suspension culture. The number of glycans, glycosylation sites and the chemical nature of glycan moieties attached to cationic peanut peroxidase expressed in transgenic tobacco were examined. Cationic peanut peroxidase isolated from the above transgenic tobacco had the identical number of complex glycans, attached at the same glycosylation sites as on cationic peanut peroxidase isolated from peanut suspension culture. Monosaccharide components of these glycans are N-acetylglucosamine (GlcNAc), mannose (Man), fucose (Fuc), xylose (Xyl) and galactose (Gal), the same sugars as found in native cationic peanut peroxidase.

Isolation and characterization of patatin isoforms

Patatin has, so far, been considered a homogeneous group of proteins. A comparison of the isoforms in terms of structural properties or stability has not been reported. A method to obtain various isoform fractions as well as a comparison of the physicochemical properties of these pools is presented. Patatin could be separated in four isoform pools, denoted A, B, C, and D, representing 62%, 26%, 5%, and 7% of the total amount of patatin, respectively. These isoforms differed in surface charge, resulting in a different behavior on anion exchange chromatography, isoelectric focusing, native polyacrylamide gel, and capillary electrophoresis. All isoforms of the patatin family contained proteins with two molecular masses of approximately 40.3 and 41.6 kDa, respectively. The size of this difference in the molar mass (1300 Da) is on the order of one carbohydrate moiety. Despite the biochemical differences given above, no variations in the structural properties nor in the thermal conformational stability could be observed using far-ultraviolet circular dichroism, infrared, and fluorescence spectroscopy.

Kinetic modeling of the thermal aggregation of patatin

A kinetic model of the thermal aggregation of patatin is presented based on chromatographic analysis of the proportions of nonaggregated and aggregated patatin. It was observed that the decrease of the amount of nonaggregated patatin proceeded initially quickly and was followed by slower aggregation at longer incubation times. It was shown that this behavior was not due to heterogeneity of the starting material. It was noted that overestimation of the amount of native molecules after a heat treatment, caused by refolding of the unfolded protein during the cooling step prior to the analysis, was significant and could not be neglected. Hence, corrections based on information on the structural properties of patatin were applied. Taking this into account, a model was proposed consisting of a first-order formation of reactive particles, followed by a second-order aggregation reaction. This model described the thermal aggregation of patatin rather accurately and was confirmed by experiments at various protein concentrations.

Processing, targeting, and antifungal activity of stinging nettle agglutinin in transgenic tobacco

The gene encoding the precursor to stinging nettle (Urtica dioica L. ) isolectin I was introduced into tobacco (Nicotiana tabacum). In transgenic plants this precursor was processed to mature-sized lectin. The mature isolectin is deposited intracellularly, most likely in the vacuoles. A gene construct lacking the C-terminal 25 amino acids was also introduced in tobacco to study the role of the C terminus in subcellular trafficking. In tobacco plants that expressed this construct, the mutant precursor was correctly processed and the mature isolectin was targeted to the intercellular space. These results indicate the presence of a C-terminal signal for intracellular retention of stinging nettle lectin and most likely for sorting of the lectin to the vacuoles. In addition, correct processing of this lectin did not depend on vacuolar deposition. Isolectin I purified from tobacco displayed identical biological activities as isolectin I isolated from stinging nettle. In vitro antifungal assays on germinated spores of the fungi Botrytis cinerea, Trichoderma viride, and Colletotrichum lindemuthianum revealed that growth inhibition by stinging nettle isolectin I occurs at a specific phase of fungal growth and is temporal, suggesting that the fungi had an adaptation mechanism.

Deposition of storage proteins

Plants store amino acids for longer periods in the form of specific storage proteins. These are deposited in seeds, in root and shoot tubers, in the wood and bark parenchyma of trees and in other vegetative organs. Storage proteins are protected against uncontrolled premature degradation by several mechanisms. The major one is to deposit the storage proteins into specialized membrane-bounded storage organelles, called protein bodies (PB). In the endosperm cells of maize and rice prolamins are sequestered into PBs which are derived from the endoplasmic reticulum (ER). Globulins, the typical storage proteins of dicotyledonous plants, and prolamins of some cereals are transported from the ER through the Golgi apparatus and then into protein storage vacuoles (PSV) which later become transformed into PBs. Sorting and targeting of storage proteins begins during their biosynthesis on membrane-bound polysomes where an N-terminal signal peptide mediates their segregation into the lumen of the ER. After cleavage of the signal peptide, the polypeptides are glycosylated and folded with the aid of chaperones. While still in the ER, disulfide bridges are formed which stabilize the structure and several polypeptides are joined to form an oligomer which has the proper conformation to be either deposited in ER-derived PB or to be further transferred to the PSV. At the trans-Golgi cisternae transport vesicles are sequestered which carry the storage proteins to the PSV. Several storage proteins are also processed after arriving in the PSVs in order to generate a conformation that is capable of final deposition. Some storage protein precursors have short N- or C-terminal targeting sequences which are detached after arrival in the PSV. Others have been shown to have internal sequence regions which could act as targeting information. In some cases positive targeting information is known to mediate sorting into the PSV whereas in other cases aggregation and membrane association seem to be major sorting mechanisms.

Sorting of proteins to vacuoles in plant cells

An individual plant cell may contain at least two functionally and structurally distinct types of vacuoles: protein storage vacuoles and lytic vacuoles. Presumably a cell that stores proteins in vacuoles must maintain these separate compartments to prevent exposure of the storage proteins to an acidified environment with active hydrolytic enzymes where they would be degraded. Thus, the organization of the secretory pathway in plant cells, which includes the vacuoles, has a fascinating complexity not anticipated from the extensive genetic and biochemical studies of the secretory pathway in yeast. Plant cells must generate the membranes to form two separate types of tonoplast, maintain them as separate organelles, and direct soluble proteins from the secretory flow specifically to one or the other via separate vesicular pathways. Individual soluble and membrane proteins must be recognized and sorted into one or the other pathway by distinct, specific mechanisms. Here we review the emerging picture of how separate plant vacuoles are organized structurally and how proteins are recognized and sorted to each type.

A phospholipase A2 is transiently synthesized during seed germination and localized to lipid bodies

A patatin-like protein is present in the storage tissue of cucumber seedlings during the stage of fat mobilization. The cucumber protein is a homologue of a glycoprotein which in potatoes accounts for most of the total protein content of tubers. Following preparation of a cucumber cDNA library representing the developmental stage of cotyledons of 1 day old germinating seeds we isolated and characterized a clone encoding a patatin-like protein. Antibodies raised against the protein expressed in bacteria were used for immunodetection in subcellular fractions of cucumber seedlings. It was shown that the patatin-like protein was virtually exclusively confined to lipid bodies. The protein expressed in bacteria was characterized in vitro by its esterase activity acting on monoacylglycerols and phospholipids. Detailed analysis using various forms of phosphatidyl choline as substrates demonstrated that the patatin-like protein is a phospholipase A2 acting on palmitoyl, linoleoyl and hydroperoxidized linoleoyl groups equally well. Studying the temporal and tissue-specific expression of patatin-like protein mRNA we showed its appearance exclusively during fat catabolism. As maximal amounts of the protein were found at an early stage of mobilization and confined to lipid bodies, we propose that the patatin-like hydrolase is involved in lipid body mobilization.

Isolation and characterisation of cDNA clones representing the genes encoding the major tuber storage protein (dioscorin) of yam (Dioscorea cayenensis Lam.)

cDNA clones encoding dioscorins, the major tuber storage proteins (M(r) 32,000) of yam (Dioscorea cayenesis) have been isolated. Two classes of clone (A and B, based on hybrid release translation product sizes and nucleotide sequence differences) which are 84.1% similar in their protein coding regions, were identified. The protein encoded by the open reading frame of the class A cDNA insert is of M(r) 30,015. The difference in observed and calculated molecular mass might be attributed to glycosylation. Nucleotide sequencing and in vitro transcription/translation suggest that the class A dioscorin proteins are synthesised with signal peptides of 18 amino acid residues which are cleaved from the mature peptide. The class A and class B proteins are 69.6% similar with respect to each other, but show no sequence identity with other plant proteins or with the major tuber storage proteins of potato (patatin) or sweet potato (sporamin). Storage protein gene expression was restricted to developing tubers and was not induced by growth conditions known to induce expression of tuber storage protein genes in other plant species. The codon usage of the dioscorin genes suggests that the Dioscoreaceae are more closely related to dicotyledonous than to monocotyledonous plants.

Sorting of proteins to the vacuoles of plant cells

The secretory system of plant cells sorts a large number of soluble proteins that either are secreted or accumulate in vacuoles. Secretion is a bulk-flow process that requires no information beyond the presence of a signal peptide necessary to enter the endoplasmic reticulum. Many vacuolar proteins are glycoproteins and the glycans are often modified as the proteins pass through the Golgi complex. Vacuolar targeting information is not contained in glycans as it is in animal cells; rather, targeting information is in polypeptide domains as it is in yeast cells. Several such domains have now been identified, but these show little or no amino acid sequence homology. We discuss the possibilities that targeting of protein to plant vacuoles may involve receptors as well as aggregation of protein at low pH.

Further characterization of cytochrome P450 involved in phytoalexin synthesis in soybean: cytochrome P450 cinnamate 4-hydroxylase and 3,9-dihydroxypterocarpan 6a-hydroxylase

Two cytochrome P450 enzymes, cinnamate 4-hydroxylase (C4H) and 3,9-dihydroxypterocarpan 6a-hydroxylase (D6aH), were isolated from elicitor-challenged soybean (Glycine max) cell cultures (G. Kochs and H. Grisebach, 1989, Arch. Biochem. Biophys. 273, 543-553). An earlier purification protocol was improved by the use of new chromatographic media, leading to a higher yield of enzymatic activity. After separation of C4H from D6aH on hydroxyapatite, the C4H was identified using anti-C4H antibody from Jerusalem artichoke (Helianthus tuberosus) (B. Gabriac et al., 1991, Arch. Biochem. Biophys. 288, 302-309). The two proteins show molecular weights of about 58,000 for C4H and about 55,000 for D6aH on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Both enzyme activities are dependent on NADPH:cytochrome P450 reductase and cross-react with their respective antibodies. Both cytochrome P450 subspecies show substrate binding and CO-difference spectra typical for cytochrome P450 and were found to be glycoproteins by their cross-reaction with biotinylated lectins in Western blot. The N-terminal sequence of C4H from soybean shows high similarity to the N-terminus of C4H from Jerusalem artichoke.

Gene expression during tuber development in potato plants

Potato tubers are modified stems that have differentiated into storage organs. Factors such as day-length, nitrogen supply, and levels of the phytohormones cytokinin and gibberellic acid, are known to control tuberization. Morphological changes during tuber initiation are accompanied by the accumulation of a characteristic set of proteins, thought to be involved in N-storage (i.e. patatin) or defense against microbial or insect attack (i.e. proteinase inhibitor II). Additionally, deposition of large amounts of starch occurs during tuber formation, which is paralleled by an increase in sucrose synthase and other enzymes involved in starch biosynthesis (i.e. ADP-glucose pyrophosphorylase, starch synthases, and branching enzyme). Potential controlling mechanisms for genes expressed during tuberization are discussed.