Lectins as plant defense proteins

Purification of a lectin from the marine red alga Gracilaria ornata and its effect on the development of the cowpea weevil Callosobruchus maculatus (Coleoptera: Bruchidae)

A lectin from the marine red alga Gracilaria ornata (Gracilariaceae, Rodophyta) was purified and characterized. The purification procedure consisted of extracting soluble proteins in 0.025 M Tris-HCl buffer, pH 7.5, followed by ammonium sulfate precipitation (70% saturation), ion exchange chromatography on DEAE-cellulose and affinity chromatography on mucin-Sepharose 4B. The purified G. ornata lectin (GOL) showed a single protein band with an apparent molecular mass of 17 kDa when submitted to SDS-polyacrylamide gel electrophoresis under reducing conditions. The native molecular mass of GOL determined by gel filtration on a Sephadex G-100 column was 17.4 kDa and its carbohydrate content was estimated to be 2.9%. Therefore, GOL is a monomeric glycoprotein. The purified lectin agglutinated trypsin-treated erythrocytes from rabbit and chicken but not from human. Its activity was not inhibited by any of the mono- and disaccharides tested but by the complex glycoproteins porcine stomach mucin, lactotransferrin, asialofetuin and bovine and porcine thyroglobulins. Isoelectric focusing showed that GOL is an acidic protein with a pI of 5.4 with analysis of its amino acid composition revealing high contents of Asx, Glx, Ser, Glu, Ala and Cys. When incorporated in artificial seeds, GOL significantly affected the development of Callosobruchus maculatus larvae, indicating the possibility of using this lectin in a biotechnological strategy for insect management of stored cowpea seeds.

Isolation and characterization of a new mannose-binding lectin gene from Taxus media

In this paper, we report the cloning and characterization of the first mannose-binding lectin gene from a gymnosperm plant species, Taxus media. The full-length cDNA of T. media agglutinin (TMA) consisted of 676 bp and contained a 432 bp open reading frame (ORF) encoding a 144 amino acid protein. Comparative analysis showed that TMA had high homology with many previously reported plant mannose-binding lectins and that tma encoded a precursor lectin with a 26-aa signal peptide. Molecular modelling revealed that TMA was a new mannosebinding lectin with three typical mannose-binding boxes like lectins from species of angiosperms. Tissue expression pattern analyses revealed that tma is expressed in a tissue-specific manner in leaves and stems, but not in fruits and roots. Phylogenetic tree analyses showed that TMA belonged to the structurally and evolutionarily closely related monocot mannose-binding lectin superfamily. This study provides useful information to understand the molecular evolution of plant lectins.

Cloning and expression of a novel cDNA encoding a mannose-binding lectin from Dendrobium officinale

Using RNA extracted from Dendrobium officinale young leaves and primers designed according to the conservative regions of orchidaceae lectins, the full-length cDNA of Dendrobium officinale agglutinin2 (DOA2) was cloned by rapid amplification of cDNA ends (RACE). The full-length cDNA of doa2 was 777 bp and contained a 513 bp open reading frame (ORF) encoding a lectin precursor of 170 amino acids. Through comparative analysis of doa2 gene and its deduced amino acid sequence with those of other orchidaceae species and Amaryllidaceae species, it was found that DOA2 had many common characters of mannose-binding lectin superfamily including three mannose-binding sites. Semi-Quantitative RT-PCR analysis revealed that doa2 mRNA expression was detected in all tested tissues including root, stem and leaf, however, the expression was higher in stem, and lower in leaf. As the doa2 mRNA was detected in all the tested plant tissues, the doa2 was considered to be a constitutively expressed gene. The recombinant protein was expressed in E. coli and purified. Anti-fungal assay showed that DOA2 has anti-fungal activity towards Gibberella zeae. To our knowledge, this is the first report on cDNA cloning of mannose binding lectin from Dendrobium officinale.

A proximal upstream sequence controls tissue-specific expression of Lem2, a salicylate-inducible barley lectin-like gene

The lemma and palea (lemma/palea), which form the husk of barley (Hordeum vulgare L.) seeds, constitutively express high levels of defense-related genes, relative to leaves [Abebe et al. (2004) Crop Sci 44:942-950]. One of these genes, Lem2, is expressed mainly in the lemma/palea and coleoptile and is strongly upregulated by salicylic acid (SA) and its functional analog 2,6-dichloroisonicotinic acid . Induction by SA was rapid, occurring within 4 h of treatment. However, Lem2 is not responsive to methyl jasmonate (MeJA) or wounding and is downregulated by drought, dehydration, and abscisic acid. These results suggest that Lem2 is involved in systemic acquired resistance. Sequence analysis showed that LEM2 is a jacalin-related lectin (JRL)-like protein with two domains. Consistent with northern and western blot data, transient expression analyses using Lem2::gfp constructs showed strong expression in lemmas and a trace expression in leaves. Successive 5' deletions of the 1,414 bp upstream region gradually weakened promoter strength, as measured by real-time PCR. Promoter deletion studies also revealed that the -75/+70 region (containing the TATA box, 5' UTR, and a SA-response element) determines tissue specificity and that the distal promoter region simply enhances expression. Southern analysis indicated that Morex barley has at least three copies of the Lem2 gene arranged in tandem on chromosome 5(1H) Bin 02, near the short arm telomere. Lem2 is not present in the barley cultivars Steptoe, Harrington, Golden Promise, and Q21861.

Red marine alga Bryothamnion triquetrum lectin induces endothelium-dependent relaxation of the rat aorta via release of nitric oxide

We have investigated the vascular relaxant effects of the lectin from a red marine alga Bryothamnion triquetrum (BTL), in particular, the endothelial-dependency and the participation of a specific glycoprotein-binding site. BTL (1-100 microg mL(-1)) was applied to rat isolated aortic rings, with or without endothelium, tonically precontracted with phenylephrine (0.1 microM). Endothelium-dependent relaxation was assessed in the presence of indometacin (10 microM), L-nitro arginine methyl ester (L-NAME, 100 microM) and tetraethylammonium (TEA, 500 microM). For the involvement of the glycoprotein-binding site, BTL was assayed in presence of mucin (300 microg mL(-1)) or N-acetyl D-glucosamine (GlcNAc; 300 microg mL(-1)), a specific and non-specific lectin-binding sugar, respectively. BTL fully and concentration dependently relaxed preparations that possessed an intact endothelium (IC50 (concn producing 50% contraction) = 12.1 +/- 1.6 microg mL(-1)), whereas no significant relaxation was observed in endothelial-denuded tissue. L-NAME, but not indometacin or TEA, completely inhibited the lectin relaxation, suggesting the involvement of nitric oxide (NO). The lectin in association with mucin, but not with GlcNAc, inhibited BTL-induced relaxation, implicating the involvement of the lectin binding site. Our data suggest that the relaxant effect of the red marine alga Bryothamnion triquetrumlectin on isolated aorta occurs via interaction with a specific lectin-binding site on the endothelium, resulting in a release of NO.

cDNA cloning and characterization of a mannose-binding lectin from Zingiber officinale Roscoe (ginger) rhizomes

Using RNA extracted from Zingiber officinale rhizomes and primers designed according to the conservative regions of monocot mannose-binding lectins, the full-length cDNA of Z. officinale agglutinin (ZOA) was cloned by rapid amplification of cDNA ends (RACE). The full-length cDNA of zoa was 746 bp and contained a 510 bp open reading frame (ORF) encoding a lectin precursor of 169 amino acids with a signal peptide. ZOA was a mannose-binding lectin with three typical mannose-binding sites (QDNY). Semi-quantitative RT-PCR analysis revealed that zoa expressed in all the tested tissues of Z. officinale including leaf, root and rhizome, suggesting it to be a constitutively expressing form. ZOA protein was successfully expressed in Escherichia coli with the molecular weight expected. To our knowledge, this is the first mannose-binding lectin cDNA cloned from the family Zingiberaceae. Our results demonstrate that monocot mannose-binding lectins also occur within the family Zingiberaceae.

Mitogenic and anti-proliferative activity of a lectin from the tubers of Voodoo lily (Sauromatum venosum)

A new lectin with the potent mitogenic and in vitro anti-proliferative activity was isolated from the tubers of a wild monocotyledonous plant Sauromatum venosum (Schott), from the family Araceae, by affinity chromatography on the asialofetuin linked amino-activated silica beads. The apparent native molecular mass of S. venosum lectin (SVL), as determined by gel filtration chromatography, was 54 kDa. In HPLC, size exclusion and cation exchange chromatography, SVL gave a single peak and also a single band of 13.5 kDa in SDS-PAGE, pH 8.3, under reducing and non-reducing conditions, indicating that the lectin is composed of four identical subunits. S. venosum lectin agglutinated rabbit, rat, sheep and guinea pig erythrocytes but reacted with goat erythrocytes after the neuraminidase treatment. However, SVL was unable to agglutinate human ABO blood group erythrocytes even after treatment with neuraminidase. SVL was inhibited by N-acetyl-D-Lactosamine (LacNAc), which is an important marker in various carcinomas and a complex desialylated glycoprotein, asialofetuin. The amino acid composition showed that lectin contained a high amount of aspartic acid and glycine but totally devoid of cysteine. However, trace amounts of methionine was present. The lectin showed a potent mitogenic response towards BALB/c splenocytes and human lymphocytes. As the mitogenic stimulation was more than that of Con A, a standard well-known plant mitogen and the response of this lectin was almost double than that of Con A. This lectin is endowed with proliferation of T cells as revealed by IL-2 bioassay but showed no production of immunoglobulins thus indicating the non-stimulation of B cells. SVL significantly inhibited the proliferation of murine cancer cell-lines, i.e., WEHI-279 to 84.6%, J774 to 81%, P388D1 to 74% and A-20 to 47%. In addition, the in vitro anti-proliferative activity of SVL was also evaluated against nine human cancer cell lines representing different organs and tissues namely, T-47D (breast), SiHa (cervix), SK-N-MC (CNS), SK-N-SH (CNS), SW-620 (colon), HT-29 (colon), HEP-2 (liver), OVCAR-5 (ovary) and PC-3 (prostate). SVL showed a significant inhibition towards the entire cell lines except the cell lines from CNS, which showed partial response in comparison to a standard anticancer drug adriamycin which was used at a concentration of 5 x 10(-5) M. Thus the anti-proliferative ability of SVL may be helpful in identification of new lectin probes that can lead to better understanding in the detection and study of certain types of cancer.

Lectins: from basic science to clinical application in cancer prevention

Many physiological functions are attributable to lectin-carbohydrate interactions. Lectins are currently being studied for their ability to destroy tumour growth by binding to specific carbohydrate motifs on cancer cells. Cell-surface molecules, including growth factor receptors are often glycosylated, and lectins may act by binding to these. Certain lectins effect the proliferation of intestinal epithelial cells. This effect is cell-type and lectin specific and occurs in the intestine of intact animals, in human colonic explants and colorectal cancer cell lines. Lectins present in mammalian tissue are involved in cell-matrix adhesion, differentiation, lymphocyte circulation and immunomodulation. Mammalian lectins contribute to detection, diagnosis and prognosis of tumour cells, and can be targeted for therapy. New lectins of plant and mammalian origin that have one or more of these functions are currently being developed as tools that could be used to target tumour cells.

Structural mechanism governing the quaternary organization of monocot mannose-binding lectin revealed by the novel monomeric structure of an orchid lectin

Two isoforms of an antifungal protein, gastrodianin, were isolated from two subspecies of the orchid Gastrodia elata, belonging to the protein superfamily of monocot mannose-specific lectins. In the context that all available structures in this superfamily are oligomers so far, the crystal structures of the orchid lectins, both at 2.0 A, revealed a novel monomeric structure. It resulted from the rearrangement of the C-terminal peptide inclusive of the 12th beta-strand, which changes from the "C-terminal exchange" into a "C-terminal self-assembly" mode. Thus, the overall tertiary scaffold is stabilized with an intramolecular beta-sheet instead of the hybrid observed on subunit/subunit interface in all known homologous dimeric or tetrameric lectins. In contrast to the constrained extended conformation with a cis peptide bond between residues 98 and 99 commonly occurring in oligomers, a beta-hairpin forms from position 97 to 101 with a normal trans peptide bond at the corresponding site in gastrodianin, which determines the topology of the C-terminal peptide and thereby its unique fold pattern. Sequence and structure comparison shows that residue replacement and insertion at the position where the beta-hairpin occurs in association with cis-trans inter-conversion of the specific peptide bond (97-98) are possibly responsible for such a radical structure switch between monomers and oligomers. Moreover, this seems to be a common melody controlling the quaternary states among bulb lectins through studies on sequence alignment. The observations revealed a structural mechanism by which the quaternary organization of monocot mannose binding lectins could be governed. The mutation experiment performed on maltose-binding protein-gastrodianin fusion protein followed by a few biochemical detections provides direct evidence to support this conclusion. Potential carbohydrate recognition sites and biological implications of the orchid lectin based on its monomeric state are also discussed in this paper.

Crystallization and preliminary X-ray diffraction analysis of a lectin from Canavalia maritima seeds

A lectin from Canavalia maritima seeds (ConM) was purified and submitted to crystallization experiments. The best crystals were obtained using the vapour-diffusion method at a constant temperature of 293 K and grew in 7 d. A complete structural data set was collected to 2.1 A resolution using a synchrotron-radiation source. The ConM crystal belongs to the orthorhombic space group P2(1)2(1)2, with unit-cell parameters a = 67.15, b = 70.90, c = 97.37 A. A molecular-replacement search found a solution with a correlation coefficient of 69.2% and an R factor of 42.5%. Crystallographic refinement is under way.

Mistletoe lectin I in complex with galactose and lactose reveals distinct sugar-binding properties

The structures of mistletoe lectin I (ML-I) from Viscum album complexed with lactose and galactose have been determined at 2.3 A resolution and refined to R factors of 20.9% (Rfree = 23.6%) and 20.9 (Rfree = 24.6%), respectively. ML-I is a heterodimer and belongs to the class of ribosome-inactivating proteins of type II, which consist of two chains. The A-chain has rRNA N-glycosidase activity and irreversibly inhibits eukaryotic ribosomes. The B-chain is a lectin and preferentially binds to galactose-terminated glycolipids and glycoproteins on cell membranes. Saccharide binding is performed by two binding sites in subdomains alpha1 and gamma2 of the ML-I B-chain separated by approximately 62 A from each other. The favoured binding of galactose in subdomain alpha1 is achieved via hydrogen bonds connecting the 4-hydroxyl and 3-hydroxyl groups of the sugar moiety with the side chains of Asp23B, Gln36B and Lys41B and the main chain of 26B. The aromatic ring of Trp38B on top of the preferred binding pocket supports van der Waals packing of the apolar face of galactose and stabilizes the sugar-lectin complex. In the galactose-binding site II of subdomain gamma2, Tyr249B provides the hydrophobic stacking and the side chains of Asp235B, Gln238B and Asn256B are hydrogen-bonding partners for galactose. In the case of the galactose-binding site I, the 2-hydroxyl group also stabilizes the sugar-protein complex, an interaction thus far rarely detected in galactose-specific lectins. Finally, a potential third low-affinity galactose-binding site in subunit beta1 was identified in the present ML-I structures, in which a glycerol molecule from the cryoprotectant buffer has bound, mimicking the sugar compound.

Inhibitory action of a new lectin from Xerocomus chrysenteron on cell-substrate adhesion

Lectins are carbohydrate-binding proteins which potentially link to cell surface glycoconjugates and affect cell proliferation. We investigated the effect of a new lectin from the mushroom Xerocomus chrysenteron (XCL) on cell proliferation using adherent and suspension cell lines. XCL caused a dose-dependent inhibition of proliferation of the adherent cell lines NIH-3T3 and HeLa. Several experiments suggest that disruption of cell-substrate adhesion is the main factor affecting cell growth inhibition. (i) No antiproliferative effect was observed on the SF9 cell line, which does not require to be attached to grow. (ii) XCL was shown to affect the adherence of cells following their suspension by trypsin treatment. (iii) XCL was localized on the cell surface where it would act as a coating agent. (iv) XCL induced morphological changes from well spread to rounded cells and disrupted the actin cytoskeleton. By contrast, flow cytometric analysis showed that XCL does not interfere with the cell cycle, and does not induce apoptosis.

Antinutritional properties of plant lectins

Lectins are carbohydrate binding (glyco)proteins which are ubiquitous in nature. In plants, they are distributed in various families and hence ingested daily in appreciable amounts by both humans and animals. One of the most nutritionally important features of plant lectins is their ability to survive digestion by the gastrointestinal tract of consumers. This allows the lectins to bind to membrane glycosyl groups of the cells lining the digestive tract. As a result of this interaction a series of harmful local and systemic reactions are triggered placing this class of molecules as antinutritive and/or toxic substances. Locally, they can affect the turnover and loss of gut epithelial cells, damage the luminal membranes of the epithelium, interfere with nutrient digestion and absorption, stimulate shifts in the bacterial flora and modulate the immune state of the digestive tract. Systemically, they can disrupt lipid, carbohydrate and protein metabolism, promote enlargement and/or atrophy of key internal organs and tissues and alter the hormonal and immunological status. At high intakes, lectins can seriously threaten the growth and health of consuming animals. They are also detrimental to numerous insect pests of crop plants although less is presently known about their insecticidal mechanisms of action. This current review surveys the recent knowledge on the antinutritional/toxic effects of plant lectins on higher animals and insects.

Cratylia argentea seed lectin, a possible defensive protein against plant-eating organisms: effects on rat metabolism and gut histology

This present work was undertaken to answer two basic questions (a) is C. argentea lectin part of the general defensive strategy of the plant against predation by animals? (b) if so, how does it act on them? To achieve these goals the lectin from C. argentea seeds was purified to homogeneity and included at a 2% level in a diet containing 10% total protein and given to growing rats for 10 days. In vivo it was noted that the lectin from C. argentea is resistant to gut proteolysis, binds to the cells lining the small intestine and induces enlargement in the small intestine, caecum and colon, kidneys and pancreas compared to control rats exposed to the egg-white diet (EW). As the diet containing the purified C. argentea lectin has the same basic composition and protein content of EW diet, the small intestine, kidney and pancreas enlargements are clearly lectin-specific effects. Moreover the animals exposed to the lectin-containing diet presented a significant reduction in the growth rate and lower values of digestibility, NPU and biological value compared to animals fed on a control lectin-free diet. Thus the data from this present study and the report that the C. argentea lectin has insecticidal activity upon Callosobruchus maculatus larvae which attacks cowpea (Vigna unguiculata) seeds reinforce the hypothesis that lectins take part in the mechanisms against herbivory.

A conserved transcript pattern in response to a specialist and a generalist herbivore

Transcript patterns elicited in response to attack reveal, at the molecular level, how plants respond to aggressors. These patterns are fashioned both by inflicted physical damage as well as by biological components displayed or released by the attacker. Different types of attacking organisms might therefore be expected to elicit different transcription programs in the host. Using a large-scale DNA microarray, we characterized gene expression in damaged as well as in distal Arabidopsis thaliana leaves in response to the specialist insect, Pieris rapae. More than 100 insect-responsive genes potentially involved in defense were identified, including genes involved in pathogenesis, indole glucosinolate metabolism, detoxification and cell survival, and signal transduction. Of these 114 genes, 111 were induced in Pieris feeding, and only three were repressed. Expression patterns in distal leaves were markedly similar to those of local leaves. Analysis of wild-type and jasmonate mutant plants, coupled with jasmonate treatment, showed that between 67 and 84% of Pieris-regulated gene expression was controlled, totally or in part, by the jasmonate pathway. This was correlated with increased larval performance on the coronatine insensitive1 glabrous1 (coi1-1 gl1) mutant. Independent mutations in COI1 and GL1 led to a faster larval weight gain, but the gl1 mutation had relatively little effect on the expression of the insect-responsive genes examined. Finally, we compared transcript patterns in Arabidopis in response to larvae of the specialist P. rapae and to a generalist insect, Spodoptera littoralis. Surprisingly, given the complex nature of insect salivary components and reported differences between species, almost identical transcript profiles were observed. This study also provides a robustly characterized gene set for the further investigation of plant-insect interaction.

A single amino acid substitution in soybean VSPalpha increases its acid phosphatase activity nearly 20-fold

Soybean [Glycine max (L.) Merr.] contains two proteins called vegetative storage proteins (VSPs) that function as temporary storage reserves, but are also closely related to plant acid phosphatases of the haloacid dehalogenase (HAD) superfamily. This study examined the biochemical basis for the relatively low catalytic activity previously reported for these VSPs. The specific activity of purified recombinant VSPalpha on GMP was about 40-fold lower than for a related soybean root nodule acid phosphatase (APase), which had a specific activity of 845 U mg(-1) protein. Conversion of Ser106 to Asp increased VSPalpha activity about 20-fold. This Asp residue is present in nodule APase and is a highly conserved nucleophile in the HAD superfamily. Related VSPs from cultivated soybean and from three wild perennial soybeans, as well as a pod storage protein (PSP) from Phaseolus vulgaris L. all lack the catalytic Asp, suggesting they too are catalytically inefficient. Phylogenetic analysis showed the VSPs and PSP are more closely related to each other than to 21 other VSP-like proteins from several plant species, all of which have the nucleophilic Asp. This study suggests that loss of catalytic activity may be a requirement for the VSPs and PSP to function as storage proteins in legumes.

Molecular cloning and characterization of GhlecRK, a novel kinase gene with lectin-like domain from Gossypium hirsutum

A novel gene encoding a lectin-like protein kinase was cloned from the upland cotton (Gossypium hirsutum) through cDNA library screening. This gene (named as Ghlecrk; GenBank accession number: AY487461) had a total length of 2233bp with an open reading frame of 1926bp, and encoded a predicted polypeptide of 641 amino acids with a molecular weight of 71.16kDa. The GhLecRK protein shared 73, 65, 64 and 59% identity with other lectin-like kinase proteins isolated from A. thaliana (At3g53810, At2g37710, At3g55550) and Populus nigra (PnLPK) at amino acid level, respectively. Southern blot analysis showed that GhLecRK belonged to a multi-copy gene family. Expression patterns revealed that GhLecRK was enriched in the developing boll (six days post anthesis, 6DPA) and shoot, but low in the root and stem and no expression in the leaf. The domains analysis showed that GhlecRK protein possessed many activating sites/domains including ATP-binding sites, a transmembrane region, a lectin-like domain and a kinase domain. These results indicate that GhlecRK is a lectin-like membrane protein that may play an important role in the phase of fiber development.

Ultrastructural analysis and immunocytochemical localization of isolectins in Cratylia mollis seeds

Cratylia mollis is a native forage from the semi-arid region of Northeast, State of Pernambuco, Brazil, whose seeds have been considered an important lectin source. Multiple molecular forms of lectins, carbohydrate-binding proteins, have been purified from C. mollis seeds (Cra Iso) allowing several applications of these purified proteins. In this work seeds were processed for ultrastructural analysis and immunocytochemical localization of the two most abundant isolectins, Cra Iso 1 and Cra Iso 3, with glucose/mannose and galactose specificities, respectively. The ultrastructural analysis revealed a typical plant cell: organelles, nucleus and cellular wall were visualized. The localization of isolectins occurred mainly in the amorphous matrix of protein bodies, and in the cellular walls of the embryonic axis. The results showed that the isolectins, which differ in relation to carbohydrate specificity and glycosylation are located in the same cellular compartment suggesting different functions inside the same subcellular organelle. Cra Iso 1 and Cra Iso 3 distribution in the C. mollis seeds was consistent with the subcellular localization of several legume lectins.

Bacterial ghost technology for pesticide delivery

Bacterial ghosts are nondenaturated empty cell envelopes of Gram-negative bacteria produced by E-mediated lysis. Such envelopes from the plant-adhering bacterium Pectobacterium cypripedii were tested for their ability to adhere to plant material and to be used as carriers for pesticide delivery. We show, using fluorescence-labeled P. cypripedii ghosts, that depending on the target plants 55 or 10% (rice or soya, respectively) of the applied bacterial ghosts was retained on the leaves after heavy simulated rain (84 mm). Furthermore, the bacterial ghosts could be loaded with the lipophilic triazole fungicide tebuconazole. In subsequent plant experiments in the glass house, the efficacy of the loaded bacterial ghost for resistance to rainfall and the protective and curative effects against the pathogens Erysiphe graminis, Leptosphaeria nodorum, and Pyrenophora teres on barley and wheat and against Sphaerotheca fuliginea on cucumber were tested. The bacterial ghosts were compared primarily with a commercial tebuconazole formulation, a wettable powder, as it has similar physical characteristics. The comparison revealed similar effects and showed consistently higher or comparable efficacy against the pathogens. The standard operational comparison with the most protective, cereal specific emulsion of oil in water displayed that the bacterial ghosts had equal to or lower efficacy than the emulsion. This study confirmed the potential of bacterial ghost platform technology as a new alternative carrier system for pesticides.

Unfolding and refolding of Leucoagglutinin (PHA-L), an oligomeric lectin from kidney beans (Phaseolus vulgaris)

The unfolding and refolding of Phaseolus vulgaris Leucoagglutinin, a homotetrameric legume lectin, was studied at pH 2.5 and 7.2 using fluorescence, far- and near-UV circular dichroism (CD) spectroscopy, 8-anilino-1-naphthalene sulfonate (ANS) binding and FPLC techniques. This protein was found to refold even at pH 2.5 and also exhibited high refolding yield around 60% at pH 2.5 and 85% at pH 7.2. The refolding at pH 2.5 takes place with the formation of a dimeric intermediate. Although the hydrodynamic radius of the completely renatured protein and the dimer at pH 2.5 was found to be same, the ANS binding as well as far-UV CD spectra of the two were different. The denaturation kinetics at pH 2.5 followed single exponential pattern with the rate of denaturation being independent of protein concentration. The renaturation kinetics on the other hand was dependent on the protein concentration providing further evidence of an intermediate state during refolding. From these experiments the folding pathway of the protein at pH 2.5 was proposed.

Response of turkey poults to soybean lectin levels typically encountered in commercial diets. 1. Effect on growth and nutrient digestibility

Lectins are known to bind to the intestinal brush border membrane and induce antinutritional effects such as disruption of the brush border membrane (BBM) and reduced nutrient digestibility in laboratory rodents. Because soybean lectin (SBL) is usually present in poult starter diets, 2 similar experiments with starting turkey poults were conducted to investigate the effects of purified SBL on growth performance and nutrient digestibility. Experimental diets were a corn starch-casein based control (lectin-free) semipurified diet (PD), semipurified diets containing 0.024 or 0.048% soybean lectin (PDL, PDH), and a corn-soybean meal diet (SBD). Experimental diets were fed from hatch to 14 d. Antibodies specific for soybean lectin were detected in the serum of poults fed the PDL and PDH diets, implying that the SBL in these diets remained active in the digestive tract. Poults fed the control PD or SBD grew equally well. The 0.024% SBL level in PDL had no significant detrimental effect on any parameters assessed in the 2 experiments. In contrast, the 0.048% SBL level in the PDH gave inconsistent results for feed efficiency (FE) and brush border enzyme levels. For instance, on d 6 in experiment 2, poults fed the PDH had poorer FE (P < 0.05) compared with the control PD treatment, but had similar FE to poults fed the PD in experiment 1. In conclusion, SBL present at levels up to 0.024% of the diet would not cause antinutritional effect in turkey poults up to 2 wk of age.

Functional analyses of the chitin-binding domains and the catalytic domain of Brassica juncea chitinase BjCHI1

We previously isolated a Brassica juncea cDNA encoding BjCHI1, a novel chitinase with two chitin-binding domains. Synthesis of its mRNA is induced by wounding, methyl jasmonate treatment, Aspergillus niger infection and caterpillar (Pieris rapae) feeding, suggesting that the protein has a role in defense. In that it possesses two chitin-binding domains, BjCHI1 resembles the precursor of Urtica dioica agglutinin but unlike that protein, BjCHI1 retains its chitinase catalytic domain after post-translational processing. To explore the properties of multi-domain BjCHI1, we have expressed recombinant BjCHI1 and two derivatives, which lack one (BjCHI2) or both (BjCHI3) chitin-binding domains, as secreted proteins in Pichia pastoris. Recombinant BjCHI1 and BjCHI2, showed apparent molecular masses on SDS-PAGE larger than calculated, and could be deglycosylated using alpha-mannosidase. Recombinant BjCHI3, without the proline/threonine-rich linker region containing predicted O-glycosylation sites, did not appear to be processed by alpha-mannosidase. BjCHI1's ability to agglutinate rabbit erythrocytes is unique among known chitinases. Both chitin-binding domains are essential for agglutination; this property is absent in recombinant BjCHI2 and BjCHI3. To identify potential catalytic residues, we generated site-directed mutations in recombinant BjCHI3. Mutation E212A showed the largest effect, exhibiting 0% of wild-type specific activity. H211N and R361A resulted in considerable (>91%) activity loss, implying these charged residues are also important in catalysis. E234A showed 36% retention of activity and substitution Y269D, 50%. The least affected mutants were E349A and D360A, with 73% and 68% retention, respectively. Like Y269, E349 and D360 are possibly involved in substrate binding rather than catalysis.

Xylem sap protein composition is conserved among different plant species

Xylem sap from broccoli (Brassica oleracea L. cv. Calabrais), rape (Brassica napus L. cv. Drakkar), pumpkin (Cucurbita maxima Duch. cv. gelber Zentner) and cucumber (Cucumis sativus L. cv. Hoffmanns Giganta) was collected by root pressure exudation from the surface of cut stems of healthy, adult plants. Total protein concentrations were in the range of 100 microg ml(-1). One-dimensional gel electrophoresis (SDS-PAGE) resulted in 10-20 visible protein bands in a molecular mass range from 10 to 100 kDa. The main bands were cut out, digested with trypsin, and analysed using tandem mass spectrometry. Fifty bands resulted in amino acid sequence information that was used to perform database similarity searches. Sequences from 30 bands showed high homology to proteins present in databases. Among them, we found mostly peroxidases, but could also identify the lectin-like xylem protein XSP30, a glycine-rich protein, serine proteases, an aspartyl protease family protein, chitinases, and a lipid transfer protein-like polypeptide. Sequence analysis predicted apoplastic secretion signals for all database entries similar to the partial xylem protein sequences. This and the lack of cross-reactivity with phloem protein-specific antibodies suggest that the proteins really originate from the xylem and do not result from phloem contamination. Most of the highly similar proteins probably function in repair and defence reactions. Some of the most abundant proteins (peroxidases, chitinases, serine proteases) were present in xylem exudate of all species analysed, often in more than one band. This indicates an important basic role of these proteins in maintaining xylem function.

A novel mitogenic and antiproliferative lectin from a wild cobra lily, Arisaema flavum

A novel lectin having specificity towards a complex glycoprotein asialofetuin was purified from tubers of Arisaema flavum (Schott.) by affinity chromatography on asialofetuin-linked amino-activated silica beads. A. flavum gave a single peak on HPLC size exclusion and a single band on non-denatured PAGE at pH 4.5. The molecular mass of the lectin, as determined by gel filtration chromatography, was 56 kDa. In SDS-PAGE, pH 8.3, the lectin migrated as a single band of 13.5 kDa, under reducing and non-reducing conditions, indicating the homotetrameric nature. A. flavum lectin (AFL) readily agglutinated rabbit, rat, sheep, goat, and guinea pig erythrocytes but not human ABO blood group erythrocytes even after neuraminidase treatment. This lectin is stable up to 55 degrees C and does not require metal ions for its hemagglutination activity. AFL was completely devoid of sulphur containing amino acids and was rich in aspartic acid and glycine. In Oucterlony's double immunodiffusion, the antisera raised against A. flavum lectin showed distinct lines of identity with those of other araceous lectins. AFL showed potent mitogenic activity towards BALB/c splenocytes and human lymphocytes in comparison to Con A, a well-known plant mitogen. AFL also showed significant in vitro antiproliferative activity towards J774 and P388D1 murine cancer cell lines.

Characterization of RNase-like major storage protein from the ginseng root by proteomic approach

The most abundant root proteins of ginseng (Panax ginseng) have been detected and identified by comparative proteome analysis with cultured hairy root of ginseng. Four abundant proteins (28, 26, 21 and 20 kDa) of P. ginseng had isoforms with different pl values on two-dimensional gel electrophoresis (2DE). The results of N-terminal and internal amino acid sequencing, however, showed that all of them originate from a 28 kDa protein, known as ginseng major protein (GMP). The GMP gene was searched for in the expressed sequence tag database of P. ginseng and found to encode a 27.3 kDa protein having 238 amino acid residues. Analysis of the amino acid sequences indicates that GMP exhibits high sequence homology with plant RNases and RNase-like proteins. However, purified GMP had no RNase activity even though it has conserved amino acid residues known to be essential for active sites of RNase. The GMPs present in ginseng main root were not expressed in cultured hairy roots of ginseng. 2DE analysis showed that the amounts of GMPs in main roots change according to seasonal fluctuation. These results suggest that the GMPs are root-specific RNase-like proteins, which function as vegetative storage proteins of ginseng for survival in the natural environment.

A salt-responsive receptor-like kinase gene regulated by the ethylene signaling pathway encodes a plasma membrane serine/threonine kinase

NTHK1 is a salt-inducible ethylene receptor gene in tobacco. Transgenic tobacco plants for this gene show reduced ethylene sensitivity. Using cDNA microarray analysis, we were able to identify those genes that have different expression levels between NTHK1 transgenic plants and wild-type plants under salt stress conditions. One of these, AtLecRK2, which encodes a receptor-like kinase with an extracellular lectin-like domain, was characterized in detail in the present study. AtLecRK2 contains a signal peptide, an extracellular lectin-like domain, a single transmembrane domain and a cytoplasmic protein kinase domain. AtLecRK2 is transcribed in the root, flower and leaf but not in the stem. In wild-type Arabidopsis, salt stress induced the transcription level of AtLecRK2, whereas in the transgenic NTHK1 Arabidopsis induction of the AtLecRK2 transcript was inhibited and retarded. AtLecRK2 was constitutively overexpressed in the ethylene-overproducer mutant, eto1-1, and could be induced by ethylene. However, in the ethylene-insensitive mutant, ein2-1, the salt-induced expression pattern of AtLecRK2 was the same as that in wild-type plants. The results demonstrate that the induction of AtLecRK2 in response to salt stress is regulated by the ethylene signaling pathway. The induction was inhibited by the ethylene receptor, NTHK1, while it was independent of EIN2. The kinase activity of AtLecRK2 was also studied. We found that that AtLecRK2 can be autophosphorylated and has serine/threonine kinase activities. The subcellular localization of AtLecRK2-GFP in onion epidermal cells indicates that AtLecRK2 is localized on the plasma membrane.

Synergistic antifungal activity of two chitin-binding proteins from spindle tree (Euonymus europaeus L.)

Two structurally different chitin-binding proteins were isolated from bark and leaves of the spindle tree (Euonymus europaeus L.). Both the small hevein-like chitin-binding protein (Ee-CBP) and the classical class-I chitinase (Ee-chitinase) possess antifungal properties, Ee-CBP being far more potent than Ee-chitinase. In addition, Ee-CBP and Ee-chitinase display a pronounced synergistic effect when added together in the test medium. Determination of the biological activities indicates that the synergism between Ee-CBP and Ee-chitinase relies on a different mode of action. Cloning and sequencing of the corresponding genes further revealed that Ee-CBP and Ee-chitinase are simultaneously expressed in bark and leaf tissues, and hence can act synergistically in planta. Moreover, analysis of the deduced sequences allowed the exact relationship between the structurally different Ee-CBP and Ee-chitinase to be corroborated. Both proteins are synthesized as similar chimeric precursors consisting of an N-terminal hevein domain linked to a C-terminal chitinase-like domain by a hinge region. However, whereas in the case of Ee-chitinase the C-terminal chitinase domain remains linked to the N-terminal hevein domain, the corresponding domain is cleaved from the Ee-CBP-precursor resulting in the formation of the hevein-type Ee-CBP. Since both precursors are--apart from the hinge region between the hevein and chitinase domains--very similar, the Ee-CBP/Ee-chitinase system offers a unique opportunity to study the importance of sequence and/or structural information comprised in the hinge region for the posttranslational processing of the respective precursor proteins.

Mechanisms of the insecticidal action of TEL (Talisia esculenta lectin) against Callosobruchus maculatus (Coleoptera: Bruchidae)

Plant lectins have insecticidal activity that is probably mediated through their ability to bind carbohydrates. To examine the influence of sugars on the insecticidal activity of a lectin from Talisia esculenta seeds (TEL), the lectin was mixed with mannose, glucose, or mannose plus glucose. Mannose abolished the insecticidal activity. Affinity chromatography showed that TEL bound to midgut proteins of the insect Callosobruchus maculatus. Immunoblotting showed that TEL recognized some proteins, probably glycoproteins, present in the midgut membrane of this insect. The principal proteases responsible for digestive proteolysis in fourth instar larvae of C. maculatus were purified by chromatography on activated thiol-Sepharose. These purified proteases were unable to digest TEL after a 15-h incubation. These results suggest that the insecticidal activity of TEL involves a specific carbohydrate-lectin interaction with glycoconjugates on the surface of digestive tract epithelial cells, as well as binding to assimilatory glycoproteins present in midgut extracts and resistance to enzymatic digestion by cysteine proteinases.

NAI1 gene encodes a basic-helix-loop-helix-type putative transcription factor that regulates the formation of an endoplasmic reticulum-derived structure, the ER body

Plant cells develop various types of endoplasmic reticulum (ER)-derived structures with specific functions. ER body, an ER-derived compartment in Arabidopsis thaliana, is a spindle-shaped structure. The NAI1 gene regulates the development of ER bodies because mutation of NAI1 abolishes the formation of ER bodies. To better understand the role of NAI1, we cloned the NAI1 gene using a positional cloning strategy. The nai1-1 mutant had a single nucleotide change at an intron acceptor site of At2g22770 (NAI1 gene). Because of this mutation, aberrant splicing of NAI1 mRNA occurs in the nai1-1 mutant. NAI1 encodes a transcription factor that has a basic-helix-loop-helix (bHLH) domain. Transient expression of NAI1 induced ER bodies in the nai1-1 mutant. Two-dimensional electrophoresis and RT-PCR analyses showed that a putative lectin was depressed at both the mRNA and protein levels in nai1 mutants, as was a beta-glucosidase (PYK10). Our results provide direct evidence that a bHLH protein plays a role in the formation of ER bodies.

Isolation and characterization of anti-HIV peptides fromDorstenia contrajervaandTreculia obovoidea

Using a high throughput screen based on the interaction of the HIV-1 gp41 ectodomain with the virucidal protein cyanovirin-N (CV-N), we isolated two new peptides which inhibited the binding of CV-N to gp41 and which subsequently showed anti-HIV activity in a whole cell assay. A 5-kDa (contrajervin) and 10 kDa (treculavirin) peptide were isolated from Dorstenia contrajerva and Treculia obovoidea, respectively. Treculavirin was composed of two subunits, each containing 50 amino acid residues, which are covalently linked by at least one disulfide bond between the subunits. Both peptides were shown to bind to gp41 and gp120 and to inhibit the cytopathic effects of HIV-1(RF) infection in a human T-lymphoblastoid cell line (CEM-SS).

Purification and characterization of complex carbohydrate specific isolectins from wild legume seeds: Acacia constricta is (vinorama) highly homologous to Phaseolus vulgaris lectins

Vinorama isolectins (VL2-VL4) were purified from seeds of Acacia constricta (vinorama) using affinity chromatography on a fetuin-fractogel column followed by cationic-exchange chromatography. Each isolectin fraction presented a characteristic isoelectric point range from 5.5 to 8.4. Under native conditions, VL containing fractions migrated as tetramers of 133 kDa, while in SDS-PAGE, in presence of 2-mercaptoethanol, a single subunit band with M(r) of 34 kDa was observed. VL was found to be a glycoprotein with a 7.5% neutral sugar content. Antibodies to Phaseolus vulgaris lectins PHA and other wild legume lectins as Olneya tesota (palo fierro) PF2 and PF3, and Parkinsonia aculeate (palo verde) PV reacted with VL, but not with anti Glycine max agglutinin SBA or anti Lotus tetragonolobus agglutinin LTA. Furthermore, direct analysis of VL peptides showed sequences homologous to those reported in different lectins of the Phaseolus genus. VL2-VL4 did not have ABO serological or simple sugar specificity, but were inhibited by complex carbohydrates from fetuin and thyroglobulin. Asialofetuin carbohydrates strongly interacted with VL4 and VL3. Vinorama isolectins could be classified as "complex lectins".

Characterization of the yam tuber storage proteins from Dioscorea batatas exhibiting unique lectin activities

Four major proteins designated DB1, DB2, DB3, and DB4 were isolated and characterized from the yam tuber Dioscorea batatas. The ratios of their yields were 20:50:20:10. DB1 was a mannose-binding lectin (20 kDa) consisting of 10-kDa subunits and was classified as the monocot mannose-binding lectin family. DB2, accounting for 50% of the total protein, was the storage protein, commonly called dioscorins consisting of a 31-kDa subunit. On the basis of amino acid sequence, DB2 was classified to be dioscorin A. DB3 was a maltose-binding lectin, having an apparent molecular mass of 120 kDa and composed of a 66-kDa subunit and two 31-kDa subunits (DB3S). The 66-kDa subunit was further composed of two 31-kDa subunits (DB3L) cross-linked by disulfide bonds. DB3L and DB3S (242 and 241 amino acid residues, respectively) were homologous with each other with 72% sequence identity. They showed a sequence homology to dioscorin B and dioscorin A from Dioscorea alata, with 90 and 93% identity, respectively, and to carbonic anhydrase from Arabidopsis thaliana with about 45% identity. DB3S had one intrachain disulfide bond located at Cys(28)-Cys(187), whereas DB3L had one interchain disulfide bond (Cys(40)-Cys(40)') in addition to the intrachain disulfide bond (Cys(28)-Cys(188)) to form a 66-kDa subunit. DB1 and DB3 agglutinated rabbit erythrocytes at 2.7 and 3.9 microg/ml, respectively. Despite the structural homology between DB2 and DB3, DB2 had no lectin activity. The 66-kDa subunit itself revealed the full hemagglutinating activity of DB3, indicating that DB3L but not DB3S was responsible for the activity. The hemagglutinating activity of DB3 required Ca(2+) ions and was exclusively inhibited by maltose and oligomaltoses (e.g. maltopentaose and maltohexaose) but not by d-glucose. DB3 could not be classified into any known plant lectin family. DB4 was a chitinase, homologous to an acidic chitinase from Dioscorea japonica. DB1, DB2, and DB3 did not show any activity of carbonic anhydrase, amylase, or trypsin inhibitor activity. These results show that two of the four major proteins isolated from the yam tubers D. batatas have unique lectin activities.

Computational modeling of the sugar-lectin interaction

In the last few years numerous experimental studies have shed light onto the details of the lectin-carbohydrate interaction. X-ray crystallography and NMR spectroscopy have been used to elucidate the structures of lectins, sugars, and their complexes. In addition, an increasing number of experimental methods has been employed to determine the thermodynamic and kinetic parameters of the binding process. Based on this experimental data, computational methods have been developed to model and predict these interactions. A plethora of techniques from Molecular Modeling and Computational Chemistry have been applied to the problem and current models achieve high-quality predictions. These successes are based on both new theoretical approaches and reliable experimental data. The aim of the present article is to outline the most relevant computational and experimental methods applied in the field of lectin-carbohydrate interaction and to give an overview of the current state of the art in the modeling of these interactions with a focus on plant lectins.

Molecular cloning and characterization of a novel lectin gene from Zephyranthes candida

A new lectin gene was cloned from Zephyranthes candida by using RACE-PCR. The full-length cDNA of Zephyranthes candida agglutinin (ZCA) was 647 bp and contained a 477 bp open reading frame encoding a 159 amino acid protein. Zephyranthes candida lectin gene was found to encode a precursor lectin with signal peptide and had extensive homology with those of other plant lectins. Molecular modeling of ZCA indicated that the three-dimensional structure of ZCA strongly resembles that of the snowdrop lectin, implying ZCA may have the similar insecticidal functions with GNA.

The galactose-binding lectin from Vatairea macrocarpa seeds induces in vivo neutrophil migration by indirect mechanism

To explore the pathways by which lectins induce an inflammatory response, the lectin from Vatairea macrocarpa (VML) seeds was used to induce neutrophil migration in rats. The lectin was shown to cause cell migration, with the effect partially blocked when galactose was added to inhibit lectin activity. Neutrophil migration was also reduced when peritoneal cavity of the animals was depleted of their resident cells beforehand, suggesting that neutrophil migration was mediated by an indirect mechanism. Pre-treatment of rats with thioglycollate increased recruitment of neutrophils while depletion of mast cells by the addition of compound 48/80 had little effect on neutrophil infiltration, suggesting the involvement of macrophages in the inflammatory process induced by the lectin. Inhibition of the cyclooxigenase, leukotriene and PAF activities by indomethacin, MK886 and BN50730, respectively, did not modify the pro-inflammatory effect previously observed. However, dexamethasone and thalidomide significantly reduced the population of neutrophils in the peritoneal cavity after lectin injection. The present study suggests that the effects produced by a galactose-binding lectin do not involve lipoxygenase, cyclooxygenase or PAF mediators that are well known to be involved in the inflammatory process. The blocking actions of dexamethasone and thalidimide suggest that as yet unidentified pro-inflammatory mediators are involved.

Mast Cell Degranulation Induced by Lectins: Effect on Neutrophil Recruitment

The mammalian lectin macrophage-derived neutrophil chemotactic factor (MNCF) and the plant lectin KM+ were characterized for their ability to activate and degranulate mast cells. The association between mast cell activation and the induction of neutrophil migration was also investigated. Incubation of rat peritoneal mast cells with these lectins resulted in degranulation and mediator release. By confocal microscopy, both lectins were evenly distributed on the cell surface. MNCF activated RBL-2H3 mast cells only if the cells had been sensitized with IgE. KM+ was able to activate either unsensitized or IgE sensitized RBL-2H3 cells. In microplate assays MNCF, but not KM+, bound to rat IgE. In rats that were depleted of mast cells, neutrophil recruitment by MNCF and KM+ were significantly reduced indicating that mast cell activation provides an amplification loop for the neutrophil recruitment induced by these lectins. The present study supports the concept that mammalian lectins play a fundamental role in innate immunity.

Fungal lectin, XCL, is internalized via clathrin-dependent endocytosis and facilitates uptake of other molecules

The lectin isolated from Xerocomus chrysenteron (XCL) displays a toxic activity towards insects. In order to assess its possible mode of action and to gather useful data for its potential use in insect-resistant transgenic plants, we investigated the effects of XCL at the cellular level. Immunofluorescence microscopy studies revealed that XCL is rapidly internalized into small endocytic vesicles that further coalesce in the perinuclear region. We show that XCL is endocytosed by the clathrin-dependent pathway, and is delivered to late endosome/lysosome compartments. The internalization of XCL seems to be general since it occurs in different cell types such as insect (SF9) or mammalian (NIH-3T3 and Hela) cell lines. In the presence of XCL, the uptake of GFP and BSA is greatly enhanced, demonstrating that XCL facilitates endocytosis. Thus, XCL could serve as a delivery agent to facilitate the endocytosis of proteins that do not enter the cell alone.

Transformation of tobacco with genes encoding Helianthus tuberosus agglutinin (HTA) confers resistance to peach-potato aphid (Myzus persicae)

The effects of the hta gene encoding Helianthus tuberosus agglutinin (HTA) on an insect in the order Homoptera were investigated. Homologous cDNAs of hta-a, hta-b, hta-c and hta-d with CaMV35S as promoter were introduced into tobacco via Agrobacterium tumefaciens. Southern blot results showed that the exogenous hta gene was inserted into the genome of host plants, and northern blot analysis confirmed that hta was expressed in transgenic plants. A bioassay with peach-potato aphid (Myzus persicae) demonstrated that transgenic plants had deleterious effects on the insect. The average population of aphids fed on transgenic T0 plants during an 11-day assay decreased by 70%, compared controls. In transgenic plants of T1 generation, aphid fecundity inhibitions were 53.0% (hta-b) and 64.6% (hta-c), respectively. The development of aphids was notably retarded. We conclude that hta could be a novel and promising candidate for plant transgenic engineering against homopteran insect pests.

Lectin-related resistance factors against bruchids evolved through a number of duplication events

Abundant lectin-related proteins found in common beans ( Phaseolus vulgaris L.) have been shown to confer resistance against the larvae of a number of bruchid species. Genes encoding for these proteins are members of the lectin multigene family, the most representative components being arcelins, phytohemagglutinins and alpha-amylase inhibitors. Arcelins have been described in seven variants, some of which are resistance factors against the Mexican bean weevil ( Zabrotes subfasciatus), a major bean predator. In this study the isolation and sequencing of arcelin genes from wild P. vulgaris genotypes, containing Arc3 and Arc7 variants, is reported, and similarities and evolutionary relationships among the seven known arcelins are described. The evolutionary analysis shows that arcelins 3 and 4 cluster together and are the most-ancient variants. A duplication event gave rise to two additional clusters, one comprising arcelins 1, 2 and 6 and separated from the cluster of arcelins 5 and 7. A multiple number of arcelin genes were found in arcelin 3 and 4 genotypes indicating that more than one type of arcelin gene may be present in the same locus. Some of these sequences are reminiscent of ancient duplication events in arcelin evolution demonstrating that arcelins have evolved through multiple duplications. A further aim of this paper was to better understand and describe the evolution of the entire lectin multigene family. Beside arcelins, a number of other types of sequences, such as putative lectins and sequences not easily classifiable, were found in genotypes containing Arc3 and Arc4. These results, together with the evolutionary analysis, indicate that lectin loci are quite complex and confirm their origin by multiple duplication events.

Molecular cloning and characterization of a novel mannose-binding lectin gene from Amorphophallus konjac

A new lectin gene was cloned from Amorphophallus konjac. The full-length cDNA of Amorphophallus konjac agglutinin (aka) was 736 bp and contained a 474 bp open reading frame encoding a 158 amino acid protein. Homology analysis revealed that the lectin from this Araceae species belonged to the superfamily of monocot mannose-binding proteins. Molecular modeling of AKA indicated that the three-dimensional structure of AKA strongly resembles that of the snowdrop lectin. Southern blot analysis of the genomic DNA revealed that aka belonged to a low-copy gene family. Northern blot analysis demonstrated that aka expression was tissue-specific with the strongest expression being found in root.

The Tn antigen-specific lectin from ground ivy is an insecticidal protein with an unusual physiology

Leaves of ground ivy (Glechoma hederacea) contain a lectin (called Gleheda) that is structurally and evolutionary related to the classical legume lectins. Screening of a population of wild plants revealed that Gleheda accounts for more than one-third of the total leaf protein in some clones, whereas it cannot be detected in other clones growing in the same environment. Gleheda is predominantly expressed in the leaves where it accumulates during early leaf maturation. The lectin is not uniformly distributed over the leaves but exhibits a unique localization pattern characterized by an almost exclusive confinement to a single layer of palisade parenchyma cells. Insect feeding trials demonstrated that Gleheda is a potent insecticidal protein for larvae of the Colorado potato beetle (Leptinotarsa decemlineata). Because Gleheda is not cytotoxic, it is suggested that the insecticidal activity is linked to the carbohydrate-binding specificity of the lectin, which as could be demonstrated by agglutination assays with different types of polyagglutinable human erythrocytes is specifically directed against the Tn antigen structure (N-acetylgalactosamine O-linked to serine or threonine residues of proteins).

Xerocomus chrysenteron lectin: identification of a new pesticidal protein

Xerocomus chrysenteron is an edible mushroom with insecticidal properties. In an earlier work, we found that proteins are responsible for this toxicity. Here we describe the purification of a approximately 15 kDa lectin, named XCL, from the mushroom. Its cDNA and gDNA were cloned by PCR strategies and a recombinant form was expressed in Escherichia coli. Sequence alignments and sugar specificity showed that this protein is the third member of a new saline-soluble lectin family present in fungi. This protein, either purified from mushroom or expressed in vitro in E. coli, was found to be toxic to some insects, such as the dipteran Drosophila melanogaster and the hemipteran, Acyrthosiphon pisum. The lectin possesses a high insecticidal activity compared to lectin isolated from leguminosae (Lathyrus ochrus) or from the snowdrop (Galanthus nivalis).

Characterization of a high-affinity binding site for the pea albumin 1b entomotoxin in the weevil Sitophilus

The toxicity of the pea albumin 1b (PA1b), a 37 amino-acid peptide extracted from pea seeds, for cereal weevils (Sitophilus oryzae, Sitophilus granarius and Sitophilus zeamais) was recently discovered. The mechanism of action of this new entomotoxin is still unknown and potentially involves a target protein in the insect tissues. This work describes the characterization of a high-affinity binding site for PA1b in a microsomal fraction of Sitophilus spp. extracts. Purified PA1b was labeled to a high specific radioactivity (c. 900 Ci.mmol-1) using 125I, and the iodinated ligand was found to be biologically active. Binding of this ligand to the microsomal fraction of S. oryzae extract was found to be saturable and reversible, with an affinity (Kd) of 2.6 nm, and a high maximal binding capacity (Bmax) of 40 pmol.mg-1 of protein. A binding site displaying similar characteristics was detectable in the five susceptible weevils strains tested, as well as in the pea aphid or in the fruit fly. However, no binding activity was detectable in extracts from four S. oryzae strains previously shown to be resistant to the toxin through a recessive monogenic mechanism. Therefore, we suggest that this binding site might be involved in the mechanism of action of PA1b.

Physicochemical and saccharide-binding studies on the galactose-specific seed lectin from Trichosanthes cucumerina

Physicochemical and saccharide-binding studies have been performed on Trichosanthes cucumerina seed lectin (TCSL). The agglutination activity of TCSL is highest in the pH range 8.0-11.0, whereas below pH 7.0 it decreases quite rapidly, which is consistent with the involvement of imidazole side chains of His residues, which titrate in this pH range, in the sugar-binding activity of the lectin. The lectin activity is unaffected between 0 and 60 degrees C, but a sharp decline occurs at higher temperatures. Isoelectric focusing studies show that TCSL has three isoforms with pI values of 5.3, 6.2, and 7.1, with the isoform of pI 6.2 being the most abundant. Circular dichroism spectroscopic studies reveal that TCSL contains about 28.4% beta-sheet, 10.6% beta-turns, 7% polyproline type 2 structure, with the remainder comprising unordered structure; the alpha-helix content is negligible. Binding of 4-methylumbelliferyl-beta-D-galactopyranoside (MeUmbbetaGal) to TCSL results in a significant increase in the fluorescence intensity of the ligand, and this change has been used to obtain the association constant for the interaction. At 25 degrees C, the association constant, K(a), for the TCSL-MeUmbbetaGal interaction was determined as 6.9 x 10(4)M(-1). Binding of nonfluorescent, inhibitory sugars was studied by monitoring their ability to reverse the fluorescence changes observed when MeUmbbetaGal was titrated with TCSL.

Promiscuity of hosting nitrogen fixation in rice: an overview from the legume perspective

The subject area of this review provides extraordinary challenges and opportunities. The challenges relate to the fact that the integration of various fields such as microbiology, biochemistry, plant physiology, eukaryotic as well as bacterial genetics, and applied plant sciences are required to assess the disposition of rice, an alien host, for establishing such a unique phenomenon as biological nitrogen fixation. The opportunities signify that, if successful, the breakthrough will have a significant impact on the global economy and will help improve the environment. This review highlights the literature related to the area of legume-rhizobia interactions, particularly those aspects whose understanding is of particular interest in the perspective of rice. This review also discusses the progress achieved so far in this area of rice research and the possibility of built-in nitrogen fixation in rice in the future. However, it is to be borne in mind that such research does not ensure any success at this point. It provides a unique opportunity to broaden our knowledge and understanding about many aspects of plant growth regulation in general.

Investigating the possibility of monitoring lectin levels in commercial soybean meals intended for poultry feeding using steam-heated soybean meal as a model

Native soybean lectins (SBL) could potentially have deleterious effects on young animals. The objectives of this study were to determine the optimum processing temperature and time at which SBL is inactivated and to investigate the possibility of using urease activity (UA) to predict residual lectin levels in soybean meal (SBM). Raw defatted SBM was steam-heated at incremental temperatures between 90 and 120 degrees C for 5 to 20 min in an autoclave. The processed meals were subjected to native-PAGE and measurement of total carbohydrate-binding lectin (TCBL), agglutinating lectin (AL), UA, and trypsin inhibitor (TI). Processing severity was evaluated by determining protein solubility in 0.2% potassium hydroxide. Results indicated that levels of all antinutrients (TCBL, AL, UA, and TI) decreased with increasing processing temperature (P < 0.05). The intensity of the lectin band on the electrophoresis gel was considerably reduced when meal was heated at 100 degrees C for 5 min. This result implied that lectin inactivation occurred at 100 degrees C. More than 90% of all the original antinutrient levels in the raw meal were destroyed when meals were heated at 100 degrees C for 5 min. Meals processed at 100 degrees C for 5 to 20 min had protein solubility values (80 to 85%) indicative of adequate processing. The denaturation pattern of UA was highly correlated with that of SBL (r > or = 0.73), indicating that UA could be used for monitoring lectin levels in commercial meals. We concluded that UA of 0.03 to 0.09 units of pH change are indicative of adequately processed meals that contain negligible lectin levels.