A bacterial expression system revisited for the recombinant production of cystine-rich plant lipid transfer proteins

Critical structural elements for the antigenicity of wheat allergen LTP1 (Tri a 14) revealed by site-directed mutagenesis

Lipid transfer proteins (LTPs) were identified as allergens in a large variety of pollens and foods, including cereals. LTPs belong to the prolamin superfamily and display an α-helical fold, with a bundle of four α-helices held together by four disulfide bonds. Wheat LTP1 is involved in allergic reactions to food. To identify critical structural elements of antibody binding to wheat LTP1, we used site-directed mutagenesis on wheat recombinant LTP1 to target: (i) sequence conservation and/or structure flexibility or (ii) each disulfide bond. We evaluated the modifications induced by these mutations on LTP1 secondary structure by synchrotron radiation circular dichroism and on its antigenicity with patient's sera and with mouse monoclonal antibodies. Disruption of the C28-C73 disulfide bond significantly affected IgE-binding and caused protein denaturation, while removing C13-C27 bond decreased LTP1 antigenicity and slightly modified LTP1 overall folding. In addition, we showed Lys72 to be a key residue; the K72A mutation did not affect global folding but modified the local 3D structure of LTP1 and strongly reduced IgE-binding. This work revealed a cluster of residues (C13, C27, C28, C73 and K72), four of which embedded in disulfide bonds, which play a critical role in LTP1 antigenicity.

Relationships between puroindoline A-prolamin interactions and wheat grain hardness

Grain hardness is an important quality trait of cereal crops. In wheat, it is mainly determined by the Hardness locus that harbors genes encoding puroindoline A (PINA) and puroindoline B (PINB). Any deletion or mutation of these genes leading to the absence of PINA or to single amino acid changes in PINB leads to hard endosperms. Although it is generally acknowledged that hardness is controlled by adhesion strength between the protein matrix and starch granules, the physicochemical mechanisms connecting puroindolines and the starch-protein interactions are unknown as of this time. To explore these mechanisms, we focused on PINA. The overexpression in a hard wheat cultivar (cv. Courtot with the Pina-D1a and Pinb-D1d alleles) decreased grain hardness in a dose-related effect, suggesting an interactive process. When PINA was added to gliadins in solution, large aggregates of up to 13 μm in diameter were formed. Turbidimetry measurements showed that the PINA-gliadin interaction displayed a high cooperativity that increased with a decrease in pH from neutral to acid (pH 4) media, mimicking the pH change during endosperm development. No turbidity was observed in the presence of isolated α– and γ-gliadins, but non-cooperative interactions of PINA with these proteins could be confirmed by surface plasmon resonance. A significant higher interaction of PINA with γ-gliadins than with α–gliadins was observed. Similar binding behavior was observed with a recombinant repeated polypeptide that mimics the repeat domain of gliadins, i.e., (Pro-Gln-Gln-Pro-Tyr)₈. Taken together, these results suggest that the interaction of PINA with a monomeric gliadin creates a nucleation point leading to the aggregation of other gliadins, a phenomenon that could prevent further interaction of the storage prolamins with starch granules. Consequently, the role of puroindoline-prolamin interactions on grain hardness should be addressed on the basis of previous observations that highlight the similar subcellular routing of storage prolamins and puroindolines.

Functional expression and activity of the recombinant antifungal defensin PvD1r from Phaseolus vulgaris L. (common bean) seeds

Background

Defensins are basic, cysteine-rich antimicrobial peptides that are important components of plant defense against pathogens. Previously, we isolated a defensin, PvD₁, from Phaseolus vulgaris L. (common bean) seeds.

Results

The aim of this study was to overexpress PvD₁ in a prokaryotic system, verify the biologic function of recombinant PvD₁ (PvD₁r) by comparing the antimicrobial activity of PvD₁r to that of the natural defensin, PvD₁, and use a mutant Candida albicans strain that lacks the gene for sphingolipid biosynthesis to unravel the target site of the PvD₁r in C. albicans cells. The cDNA encoding PvD₁, which was previously obtained, was cloned into the pET-32 EK/LIC vector, and the resulting construct was used to transform bacterial cells (Rosetta Gami 2 (DE₃) pLysS) leading to recombinant protein expression. After expression had been induced, PvD₁r was purified, cleaved with enterokinase and repurified by chromatographic steps. N-terminal amino acid sequencing showed that the overall process of the recombinant production of PvD₁r, including cleavage with the enterokinase, was successful. Additionally, modeling revealed that PvD₁r had a structure that was similar to the defensin isolated from plants. Purified PvD₁ and PvD₁r possessed inhibitory activity against the growth of the wild-type pathogenic yeast strain C. albicans. Both defensins, however, did not present inhibitory activity against the mutant strain of C. albicans. Antifungal assays with the wild-type C. albicans strains showed morphological changes upon observation by light microscopy following growth assays. PvD₁r was coupled to FITC, and the subsequent treatment of wild type C. albicans with DAPI revealed that the labeled peptide was intracellularly localized. In the mutant strain, no intracellular labeling was detected.

Conclusion

Our results indicate that PvD₁r retains full biological activity after recombinant production, enterokinase cleavage and purification. Additionally, our results from the antimicrobial assay, the microscopic analysis and the PvD₁r-FITC labeling assays corroborate each other and lead us to suggest that the target of PvD₁ in C. albicans cells is the sphingolipid glucosylceramide.

Purification of a defensin isolated from Vigna unguiculata seeds, its functional expression in Escherichia coli, and assessment of its insect alpha-amylase inhibitory activity

Plant defensins make up a family of cationic antimicrobial peptides with a characteristic three-dimensional folding pattern stabilized by four disulfide bridges. The aim of this work was the purification and functional expression of a defensin from cowpea seeds and the assessment of its alpha-amylase inhibitory activity. The cDNA encoding the cowpea defensin was cloned into the pET-32 EK/LIC vector, and the resulting construct was used to transform Escherichia coli cells. The recombinant peptide was purified via affinity chromatography on a Ni Sepharose column and by reverse-phase chromatography on a C2/C18 column using HPLC. N-terminal amino acid sequencing revealed that the recombinant peptide had a similar sequence to that of the defensin isolated from seeds. The natural and recombinant defensins were submitted to the alpha-amylase inhibition assay. The cowpea seed defensin was found to inhibit alpha-amylases from the weevils Callosobruchus maculatus and Zabrotes subfasciatus. alpha-Amylase inhibition assays also showed that the recombinant defensin inhibited alpha-amylase from the weevil C. maculatus. The cowpea seed defensin and its recombinant form were unable to inhibit mammalian alpha-amylases. The three-dimensional structure of the recombinant defensin was modeled, and the resulting structure was found to be similar to those of other plant defensins.

Identification of trypsin-inhibitory site and structure determination of human SPINK2 serine proteinase inhibitor

Human serine proteinase inhibitor Kazal-type 2 (SPINK2) functions as a trypsin/acrosin inhibitor and is synthesized mainly in the testis and seminal vesicle where its activity is engaged in fertility. The SPINK2 protein contains a typical Kazal domain composed by six cysteine residues forming three disulfide bridges. The expression of SPINK2 is closely related to cancer such as lymphomas, in that a high transcript level of SPINK2 in patients with primary cutaneous follicle center cell lymphomas have better prognosis with lower mortality. To clarify the role of SPINK2 in cancer, we performed quantitative real-time PCR and showed that the expression level of SPINK2 is significantly elevated in most leukemia cell lines except B-lymphoblast TK-6 cells. The molecular function and structural features of SPINK2 were also investigated by employing the recombinant active and mutant inactive SPINK2 proteins to determine its key P2-P2' (Pro(23)-Arg(24)-His(25)-Phe(26)) active site. The inhibition assay results demonstrated that Arg(24) at the P1 site is crucial for the specificity of SPINK2 on target enzyme. Although His(25) at the P1' and Phe(26) at the P2' residues are also involved in trypsin-SPINK2 interaction, Pro(23) at the P2 site may not be directly participated in interacting with trypsin. In addition, we determined the 3D solution structure of SPINK2 and used this structure to predict the SPINK2-proteinase complex structure and binding properties. These studies not only provide critical information about the structural properties and biophysical features of the SPINK2 proteinase inhibitor, but also suggest its important role in tumor progression and response to treatment.

Characterization and antifungal properties of wheat nonspecific lipid transfer proteins

This study simultaneously considered the phylogeny, fatty acid binding ability, and fungal toxicity of a large number of monocot nonspecific lipid transfer proteins (ns-LTP). Nine novel full-length wheat ns-LTP1 clones, all possessing coding sequences of 348 bp, isolated from abiotic- and biotic-stressed cDNA libraries from aerial tissues, exhibited highly conserved coding regions with 78 to 99 and 71 to 100% identity at the nucleotide and amino acid levels, respectively. Phylogenetic analyses revealed two major ns-LTP families in wheat. Eight wheat ns-LTP genes from different clades were cloned into the expression vector pPICZalpha and transformed into Pichia pastoris. Sodium dodecyl sulfate polyacrylamide gel electrophoresis, Western blotting, and in vitro lipid binding activity assay confirmed that the eight ns-LTP were all successfully expressed and capable of in vitro binding fatty acid molecules. A comparative in vitro study on the toxicity of eight wheat ns-LTP to mycelium growth or spore germination of eight wheat pathogens and three nonwheat pathogens revealed differential toxicities among different ns-LTP. Values indicating 50% inhibition of fungal growth or spore germination of three selected ns-LTP against six fungi ranged from 1 to 7 microM. In vitro lipid-binding activity of ns-LTP was not correlated with their antifungal activity. Using the fluorescent probe SYTOX Green as an indicator of fungal membrane integrity, the in vitro toxicity of wheat ns-LTP was associated with alteration in permeability of fungal membranes.

Molecular genetics of puroindolines and related genes: regulation of expression, membrane binding properties and applications

Kernel texture of wheat is a primary determinant of its technological properties. Soft kernel texture phenotype results when the Puroindoline a and Puroindoline b genes are present and encode the wild-type puroindolines PINA and PINB, respectively, and various mutations in either or both gene(s) result in hard phenotypes. A wealth of information is now available that furthers our understanding regarding the spatial and temporal regulation of expression of Puroindoline genes. Through the use of model membranes and synthetic peptides we also have a clearer understanding of the significance of the cysteine backbone, the tryptophan-rich domain (TRD) and the helicoid tertiary structures of PIN proteins in relation to their membrane-active properties. Many studies suggest individual yet co-operative modes of action of the PIN proteins in determining kernel texture, and significant evidence is accumulating that the proteins have in vivo and in vitro antimicrobial activities, shedding light on the biological roles of this unique ensemble of proteins. The puroindolines are now being explored for grain kernel texture modifications as well as antimicrobial activities.

Structure-based protein engineering for alpha-amylase inhibitory activity of plant defensin

The structure of a novel plant defensin isolated from the seeds of the mung bean, Vigna radiate, has been determined by (1)H nuclear magnetic resonance spectroscopy. The three-dimensional structure of VrD2, the V. radiate plant defensin 2 protein, comprises an alpha-helix and one triple-stranded anti-parallel beta-sheet stabilized by four disulfide bonds. This protein exhibits neither insecticidal activity nor alpha-amylase inhibitory activity in spite of showing a similar global fold to that of VrD1, an insecticidal plant defensin that has been suggested to function by inhibiting insect alpha-amylase. Our previous study proposed that loop L3 of plant defensins is important for this inhibition. Structural analyses and surface charge comparisons of VrD1 and VrD2 revealed that the charged residues of L3 correlate with the observed difference in inhibitory activities of these proteins. A VrD2 chimera that was produced by transferring the proposed functional loop of VrD1 onto the structurally equivalent loop of VrD2 supported this hypothesis. The VrD2 chimera, which differs by only five residues compared with VrD2, showed obvious activity against Tenebrio molitor alpha-amylase. These results clarify the mode of alpha-amylase inhibition of plant defensins and also represent a possible approach for engineering novel alpha-amylase inhibitors. Plant defensins are important constituents of the innate immune system of plants, and thus the application of protein engineering to this protein family may provide an efficient method for protecting against crop losses.

Ha-DEF1, a sunflower defensin, induces cell death in Orobanche parasitic plants

Plant defensins are small basic peptides of 5-10 kDa and most of them exhibit antifungal activity. In a sunflower resistant to broomrape, among the three defensin encoding cDNA identified, SF18, SD2 and HaDef1, only HaDef1 presented a preferential root expression pattern and was induced upon infection by the root parasitic plant Orobanche cumana. The amino acid sequence deduced from HaDef1 coding sequence was composed of an endoplasmic reticulum signal sequence of 28 amino acids, a standard defensin domain of 50 amino-acid residues and an unusual C-terminal domain of 30 amino acids with a net positive charge. A 5.8 kDa recombinant mature Ha-DEF1 corresponding to the defensin domain was produced in Escherichia coli and was purified by means of a two-step chromatography procedure, Immobilized Metal Affinity Chromatography (IMAC) and Ion Exchange Chromatography. Investigation of in vitro antifungal activity of Ha-DEF1 showed a strong inhibition on Saccharomyces cerevisiae growth linked to a membrane permeabilization, and a morphogenetic activity on Alternaria brassicicola germ tube development, as already reported for some other plant defensins. Bioassays also revealed that Ha-DEF1 rapidly induced browning symptoms at the radicle apex of Orobanche seedlings but not of another parasitic plant, Striga hermonthica, nor of Arabidopsis thaliana. FDA vital staining showed that these browning areas corresponded to dead cells. These results demonstrate for the first time a lethal effect of defensins on plant cells. The potent mode of action of defensin in Orobanche cell death and the possible involvement in sunflower resistance are discussed.

Plant lipid binding proteins: Properties and applications

Lipid transfer proteins (LTP) and puroindolines are abundant lipid binding proteins of plant seeds. While LTP are ubiquitous plant proteins, puroindolines are only found in the seeds of plants from the Triticae and Avenae tribes. These proteins display a similar overall folding pattern but different lipid binding properties. The unique and diverse biological and technological functions of LTPs and puroindolines are closely related to their structural and lipid binding properties. These proteins are attractive to improve the agronomic performances and food quality of crops. Heterologous expression and genetic engineering should allow industrial production and enlarge applications of these lipid binding proteins.

Specific Adduction of Plant Lipid Transfer Protein by an Allene Oxide Generated by 9-Lipoxygenase and Allene Oxide Synthase

Lipid transfer proteins (LTPs) are ubiquitous plant lipid-binding proteins that have been associated with multiple developmental and stress responses. Although LTPs typically bind fatty acids and fatty acid derivatives in a non-covalent way, studies on the LTPs of barley seeds have identified an abundantly occurring covalently modified form, LTP1b, the lipid ligand of which has resisted clarification. In the present study, this adduct was identified as the alpha-ketol 9-hydroxy-10-oxo-12(Z)-octadecenoic acid. Further studies on the formation of LTP1b demonstrated that the ligand was introduced by nucleophilic attack of the free carboxylate group of the Asp-7 residue of the protein at carbon-9 of the allene oxide fatty acid 9(S),10-epoxy-10,12(Z)-octadecadienoic acid. This reactive oxylipin was produced in barley seeds by oxygenation of linoleic acid by 9-lipoxygenase followed by dehydration of the resulting hydroperoxide by allene oxide synthase. The generation of protein-oxylipin adducts represents a new function for plant allene oxide synthases, enzymes that have earlier been implicated mainly in the biosynthesis of the jasmonate family of plant hormones. Additionally, the LTP-allene oxide synthase interaction opens new perspectives regarding the roles of LTPs in the signaling of plant defense and development.

Puroindoline-a and alpha1-purothionin form ion channels in giant liposomes but exert different toxic actions on murine cells

Puroindoline-a (PIN-a) and alpha1-purothionin (alpha1-PTH), isolated from wheat endosperm of Triticum aestivum sp., have been suggested to play a role in plant defence mechanisms against phytopathogenic organisms. We investigated their ability to form pores when incorporated into giant liposomes using the patch-clamp technique. PIN-a formed cationic channels (approximately 15 pS) with the following selectivity K(+) > Na(+) >> Cl(-). Also, alpha1-PTH formed channels of approximately 46 pS and 125 pS at +100 mV, the selectivity of which was Ca(2+) > Na(+) approximately K(+) >> Cl(-) and Cl(-) >> Na(+), respectively. In isolated mouse neuromuscular preparations, alpha1-PTH induced muscle membrane depolarization, leading to blockade of synaptic transmission and directly elicited muscle twitches. Also, alpha1-PTH caused swelling of differentiated neuroblastoma NG108-15 cells, membrane bleb formation, and disorganization of F-actin. In contrast, similar concentrations of PIN-a had no detectable effects. The cytotoxic actions of alpha1-PTH on mammalian cells may be explained by its ability to induce cationic-selective channels.

Modulation of the biological activity of a tobacco LTP1 by lipid complexation

Plant lipid transfer proteins (LTPs) are small, cysteine-rich proteins secreted into the extracellular space. They belong to the pathogenesis-related proteins (PR-14) family and are believed to be involved in several physiological processes including plant disease resistance, although their precise biological function is still unknown. Here, we show that a recombinant tobacco LTP1 is able to load fatty acids and jasmonic acid. This LTP1 binds to specific plasma membrane sites, previously characterized as elicitin receptors, and is shown to be involved in the activation of plant defense. The biological properties of this LTP1 were compared with those of LTP1-linolenic and LTP1-jasmonic acid complexes. The binding curve of the LTP1-linolenic acid complex to purified tobacco plasma membranes is comparable to the curve obtained with LTP1. In contrast, the LTP1-jasmonic acid complex shows a strongly increased interaction with the plasma membrane receptors. Treatment of tobacco plants with LTP1-jasmonic acid resulted in an enhancement of resistance toward Phytophthora parasitica. These effects were absent upon treatment with LTP1 or jasmonic acid alone. This work presents the first evidence for a biological activity of a LTP1 and points out the crucial role of protein-specific lipophilic ligand interaction in the modulation of the protein activity.