Lectins as plant defense proteins

A lectin with mycelia differentiation and antiphytovirus activities from the edible mushroom Agrocybe aegerita

A lectin named AAL has been purified from the fruiting bodies of the edible mushroom Agrocybe aegerita. AAL consisted of two identical subunits of 15.8 kDa, its pI was about 3.8 determined by isoelectric focusing, and no carbohydrate was discerned. Being treated by pyrogultamate aminopeptidase, the blocked N-terminus of AAL was sequenced as QGVNIYNI. AAL agglutinated human and animal erythrocytes regardless of blood type or animal species. Its hemagglutinating activity was unaffected by acid or alkali treatment and demetalization or addition of divalent metals Mg(2+), Ca(2+) and Zn(2+). AAL was toxic to mice: its LD50 was 15.85 mg per kilogram body weight by intraperitoneal injection. In this study, two novel activities of AAL were proved. It showed inhibition activity to infection of tobacco mosaic virus on Nicotiana glutinosa. The result of IEF suggested that AAL attached to TMV particles. Mycelia differentiation promotion was the other interesting activity. AAL promoted the differentiation of fruit body primordia from the mycelia of Agrocybe aegerita and Auricularia polytricha. AAL antiserum was prepared and immunologically cross-reactived with several proteins from five other kinds of mushrooms. These results suggested that AAL probably was a representative of a large protein family, which plays important physiological roles in mushroom.

Consumption of snowdrop lectin (Galanthus nivalisagglutinin) causes direct effects on adult parasitic wasps

Honeydew is a common sugar-rich excretion of aphids and other phloem-feeding insects and represents the primary sugar in many agricultural systems. When honeydew-producing insects feed on genetically modified plants, the honeydew can contain amounts of the transgene product. Here we address whether this route of exposure poses a risk for non-target insects. Three species of parasitic wasps were selected: i.e. Aphidius colemani, Trichogramma brassicae and Cotesia glomerata, all of which are known to use honeydew as a carbohydrate source in the field. Wasps were fed sucrose solutions with varying concentrations of snowdrop lectin ( Galanthus nivalisagglutinin, GNA), a protein that has been engineered into crops to confer resistance against homopteran pests and that has been detected in honeydew. Parameters evaluated included gustatory response, longevity, fecundity, progeny emergence and sex ratio. While A. colemani and T. brassicae, but not C. glomerata, were able to detect GNA, this gustatory recognition had no effect on the acceptance of a GNA-sucrose solution. In all three species, GNA ingestion reduced parasitoid survival significantly. However, in respect to fecundity, negative effects were observed for T. brassicae but not for A. colemani. The results suggest that the effect of GNA consumption may depend on the specifics of a parasitoid's biology, especially its longevity and its mode of egg maturation.

Plant lectin-like bacteriocin from a rhizosphere-colonizing Pseudomonas isolate

Rhizosphere isolate Pseudomonas sp. strain BW11M1, which belongs to the Pseudomonas putida cluster, secretes a heat- and protease-sensitive bacteriocin which kills P. putida GR12-2R3. The production of this bacteriocin is enhanced by DNA-damaging treatment of producer cells. We isolated a TnMod mutant of strain BW11M1 that had lost the capacity to inhibit the growth of strain GR12-2R3. A wild-type genomic fragment encompassing the transposon insertion site was shown to confer the bacteriocin phenotype when it was introduced into Escherichia coli cells. The bacteriocin structural gene was identified by defining the minimal region required for expression in E. coli. This gene was designated llpA (lectin-like putidacin) on the basis of significant homology of its 276-amino-acid product with mannose-binding lectins from monocotyledonous plants. LlpA is composed of two monocot mannose-binding lectin (MMBL) domains. Several uncharacterized bacterial genes encoding diverse proteins containing one or two MMBL domains were identified. A phylogenetic analysis of the MMBL domains present in eukaryotic and prokaryotic proteins assigned the putidacin domains to a new bacterial clade within the MMBL-containing protein family. Heterologous expression of the llpA gene also conveyed bacteriocin production to several Pseudomonas fluorescens strains. In addition, we demonstrated that strain BW11M1 and heterologous hosts secrete LlpA into the growth medium without requiring a cleavable signal sequence. Most likely, the mode of action of this lectin-like bacteriocin is different from the modes of action of previously described Pseudomonas bacteriocins.

Stochastic molecular dynamics of peanut lectin PNA complex with T-antigen disaccharide

The stochastic boundary molecular dynamics simulation method was applied to investigate the structure of a complex comprised of a tetrameric peanut lectin and the tumour-associated disaccharide, Galbeta1-3GalNAc (T-antigen). Only a small region encompassing the active site was explicitly included in the calculations, but the electrical contribution of most outer atoms was taken into account by adding to the effective potential a term coming from an electrostatic potential grid that was pre-calculated and used to approximate the electrostatic energy and the force at any point in the interacting site. Results of simulating the intermolecular hydrogen bond network agree fairly well with X-ray experiments. An estimation of the direct and water-mediated interaction mean lifetimes and mean water residence times around the T-antigen oxygen atoms was computed over 400 ps. Monitoring the behaviour of water molecules within the active site revealed that there is a constant exchange of water with the bulk, especially in the proximity of ASN41, ASN127 and GLU129. The temporal evolution of the glycosidic linkage was also investigated and compared with simulations of T-antigen in solution. These studies of peanut lectins-sugar complexes clearly emphasize the importance of bound water molecules in generating carbohydrate specificity.

Plant toxic proteins with insecticidal properties. A review on their potentialities as bioinsecticides

To meet the demands for food of the expanding world population, there is need of new ways for protecting plant crops against predators and pathogens while avoiding the use of environmentally aggressive chemicals. A milestone in this field was the introduction into crop plants of genes expressing Bacillus thuringiensis entomotoxic proteins. In spite of the success of this new technology, however, there are difficulties for acceptance of these 'anti-natural' products by the consumers and some concerns about its biosafety in mammals. An alternative could be exploring the plant's own defense mechanisms, by manipulating the expression of their endogenous defense proteins, or introducing an insect control gene derived from another plant. This review deals with the biochemical features and mechanisms of actions of plant proteins supposedly involved in defense mechanisms against insects, including lectins, ribosome-inactivating proteins, enzymes inhibitors, arcelins, chitinases, ureases, and modified storage proteins. The potentialities of genetic engineering of plants with increased resistance to insect predation relying on the repertoire of genes found in plants are also discussed. Several different genes encoding plant entomotoxic proteins have been introduced into crop genomes and many of these insect resistant plants are now being tested in field conditions or awaiting commercialization.

Mistletoe lectins enhance immune responses to intranasally co-administered herpes simplex virus glycoprotein D2

The mucosal adjuvant properties of the three type 2 ribosome-inactivating proteins (RIPs) from the European mistletoe, Viscum album L., were investigated. Mistletoe lectins were compared with cholera toxin (CT) as adjuvants when delivered nasotracheally together with herpes simplex virus glycoprotein D2 (gD2). All three mistletoe lectins (MLI, MLII, MLIII) were potent mucosal adjuvants. Co-administration of MLI, MLII or MLIII with gD2 led to significantly higher levels of gD2-specific mucosal immunoglobulin A (IgA) and systemic immunoglobulin G (IgG) antibody than when the antigen was delivered alone. The levels of antibodies induced were similar to those generated in mice immunized with gD2 and the potent mucosal adjuvant CT. Administration of ML1 with gD2 enhanced the antigen-specific splenic T-cell proliferative response. Interleukin-5 (IL-5), but not interferon-gamma (IFN-gamma), was detected in supernatants from splenocytes stimulated in vitro with gD2. This indicates that MLI enhanced type 2 T-helper cell (Th2) responses to the bystander antigen, gD2. Analysis of the gD2- and lectin-specific IgG subclass titres in mice immunized with gD2 and MLI, MLII or MLIII revealed a high ratio of IgG1 : IgG2a, which is compatible with the selective induction of Th2-type immune responses.

Two distinct jacalin-related lectins with a different specificity and subcellular location are major vegetative storage proteins in the bark of the black mulberry tree

Using a combination of protein isolation/characterization and molecular cloning, we have demonstrated that the bark of the black mulberry tree (Morus nigra) accumulates large quantities of a galactose-specific (MornigaG) and a mannose (Man)-specific (MornigaM) jacalin-related lectin. MornigaG resembles jacalin with respect to its molecular structure, specificity, and co- and posttranslational processing indicating that it follows the secretory pathway and eventually accumulates in the vacuolar compartment. In contrast, MornigaM represents a novel type of highly active Man-specific jacalin-related lectin that is synthesized without signal peptide or other vacuolar targeting sequences, and accordingly, accumulates in the cytoplasm. The isolation and cloning, and immunocytochemical localization of MornigaG and MornigaM not only demonstrates that jacalin-related lectins act as vegetative storage proteins in bark, but also allows a detailed comparison of a vacuolar galactose-specific and a cytoplasmic Man-specific jacalin-related lectin from a single species. Moreover, the identification of MornigaM provides the first evidence, to our knowledge, that bark cells accumulate large quantities of a cytoplasmic storage protein. In addition, due to its high activity, abundance, and ease of preparation, MornigaM is of great potential value for practical applications as a tool and bioactive protein in biological and biomedical research.

Purification and physicochemical characterization of a cotyledonary lectin from Luetzelburgia auriculata

A lectin was purified from the cotyledons of Luetzelburgia auriculata (Fr. All) Ducke by affinity chromatography on agarose-N-acetyl-D-galactosamine. The lectin is a potent agglutinin for rabbit erythrocytes, reacts with human red cells, but is inactive against cow, sheep, and goat erythrocytes. Hemagglutination of rabbit erythrocytes was inhibited by either 0.39 mM N-acetyl-neuraminic acid or N-acetyl-D-galactosamin, 12.5 mM D-lactose or D-melibiose, 50 mM D-galactose or raffinose. Its hemagglutinating activity was lost at 80 degrees C, 5 min, and the activation energy required for denaturation was 104.75 kJ mol(-1). Chromatography on Sephadex G-100, at pH 7.6, showed that at this hydrogenic ionic concentration the native lectin was a homotetramer (123.5 kDa). By denaturing SDS-PAGE, LAA seemed to be composed of a mixture of 29 and 15 kDa polypeptide subunits. At acidic and basic pHs it assumed different conformations, as demonstrated by exclusion chromatography on Superdex 200 HR 10/30. The N-terminal sequence of the 29 kDa band was SEVVSFSFTKFNPNQKDII and the 15 kDa band contained a mixture of SEVVSFSFTKFNPNQKDII and KFNQIVAVEEDTDXESQPQ sequences, indicating that these bands may represent full-length and its endogenous fragments, respectively. The lectin is a glycoprotein having 3.2% neutral carbohydrate, with a pI of 5.8, containing high levels of Asp+Asn and Glu+Gln and hydroxy amino acids, and low amount or absence of sulfur amino acids. Its absorption spectrum showed a maximum at 280 nm and a epsilon (1%) x (1cm) of 5.2. Its CD spectrum was characterized by minima near 228 nm, maxima near 196 nm and a negative to positive crossover at 210 nm. The secondary structure content was 6% alpha-helix, 8% parallel beta-sheet, 38% antiparallel beta-sheet, 17% beta-turn, 31% unordered and others contribution, and 1% RMS (root mean square). In the fluorescence spectroscopy, excitation of the lectin solution at 280 nm gave an emission spectrum in the 285-445 nm range. The wavelength maximum emission was in 334.5 nm, typical for tryptophan residues buried inside the protein.

Insect feeding mobilizes a unique plant defense protease that disrupts the peritrophic matrix of caterpillars

Plants frequently respond to herbivorous insect attack by synthesizing defense proteins that deter insect feeding and prevent additional herbivory. Maize (Zea mays L.) lines, resistant to feeding by a number of lepidopteran species, rapidly mobilize a unique 33-kDa cysteine protease in response to caterpillar feeding. The accumulation of the 33-kDa cysteine protease in the maize mid-whorl was correlated with a significant reduction in caterpillar growth that resulted from impaired nutrient utilization. Black Mexican Sweetcorn callus transformed with mir1, the gene encoding the 33-kDa cysteine protease, expressed the protease and growth of caterpillars reared on the transgenic callus was reduced 60-80%. Scanning electron microscopy was used to examine the effect of plant material expressing the 33-kDa cysteine protease on the structure of the caterpillar peritrophic matrix. Because the peritrophic matrix surrounds the food bolus, assists in digestive processes, and protects the caterpillar midgut from physical and chemical damage, disruption of peritrophic matrix may reduce caterpillar growth. The results indicated that the peritrophic matrix was severely damaged when caterpillars fed on resistant maize plants or transgenic Black Mexican Sweetcorn. The accumulation of the 33-kDa cysteine protease in response to caterpillar feeding, and its ability to damage the insect peritrophic matrix, represents an unusual host-plant resistance mechanism that may have applications in agricultural biotechnology.

Tobacco-expressed Brassica juncea chitinase BjCHI1 shows antifungal activity in vitro

We have previously isolated a Brassica juncea cDNA encoding BjCHI1, a novel chitinase with two chitin-binding domains, and have shown that its mRNA is induced by wounding and methyl jasmonate treatment (K.-J. Zhao and M.-L. Chye, Plant Mol. Biol. 40 (1999) 1009-1018). By the presence of two chitin-binding domains, BjCHI1 resembles the precursor of UDA (Urtica dioica agglutinin) but, unlike UDA, BjCHI1 retains its chitinase catalytic domain after post-translational processing. Here, we indicate the role of BjCHI1 in plant defense by demonstrating its mRNA induction upon Aspergillus niger infection or caterpillar Pieris rapae (L.) feeding. To further investigate the biological properties of BjCHI1, we transformed tobacco with a construct expressing the BjCHI1 cDNA from the CaMV 35S promoter. Subsequently, we purified BjCHI1 from the resultant transgenic Ro plants using a regenerated chitin column followed by fast protein liquid chromatography (FPLC). Also, the significance of the second chitin-binding domain in BjCHI1 was investigated by raising transgenic tobacco plants expressing BjCHI2, a deletion derivative of BjCHI1 lacking one chitin-binding domain. Colorimetric chitinase assays at 25 degrees C, pH 5, showed no significant differences between the activities of BjCHI1 and BjCHI2, suggesting that chitinase activity, due to the catalytic domain, is not enhanced by the presence of a second chitin-binding domain. Both BjCHI1 and BjCHI2 show in vitro anti-fungal activity toward Trichoderma viride, causing reductions in hyphal diameter, hyphal branching and conidia size.

Allergies to cross-reactive plant proteins. Latex-fruit syndrome is comparable with pollen-food allergy syndrome

Both latex-fruit syndrome and oral allergy syndrome concomitant with pollinosis (pollen-food allergy syndrome) are considered to be caused by cross-reactivity between sensitizers and symptom elicitors. The cross-reactive food allergens relevant to these syndromes are mostly sensitive to heat and digestive enzymes. Such a vulnerable antigen cannot sensitize people perorally but provokes allergic reactions in already sensitized patients based on its cross-reactivity to the corresponding sensitizer. These types of food allergens are often called incomplete food allergens or nonsensitizing elicitors. Their features contrast with those of complete food allergens that have the capacity for peroral sensitization as well as symptom elicitation. Although highly antigenic and cross-reactive, carbohydrate epitopes do not generally elicit allergic reactions and often disturb in vitro IgE tests. Recent research has revealed that some of the cross-reactive allergens responsible for the two syndromes are proteins related to the defense responses of higher plants. Plant defense-related proteins are relatively conserved in the course of evolution and can supply cross-reactive epitopes. It is important to note that various stresses can stimulate the expression of these proteins, which implies that allergens increase in plants under stressful conditions like severe growing situations and exposure to some kinds of chemicals. Because defense-related proteins usually provide a plant with resistance to stresses, varieties that are apt to intensively induce such proteins are agriculturally valuable. Less toxic substances that cause crops to express defensive proteins are being investigated as a new type of agrochemical. Moreover, some defense-related proteins are going to be constantly produced in genetically modified plants. Even though these proteins can be useful agriculturally, their allergenicity should be evaluated carefully.

Talisia esculenta lectin and larval development of Callosobruchus maculatus and Zabrotes subfasciatus (Coleoptera: Bruchidae)

Bruchid larvae cause major losses in grain legume crops throughout the world. Some bruchid species, such as the cowpea weevil and the Mexican bean weevil, are pests that damage stored seeds. Plant lectins have been implicated as antibiosis factors against insects, particularly the cowpea weevil, Callosobruchus maculatus. Talisia esculenta lectin (TEL) was tested for anti-insect activity against C. maculatus and Zabrotes subfasciatus larvae. TEL produced ca. 90% mortality to these bruchids when incorporated in an artificial diet at a level of 2% (w/w). The LD(50) and ED(50) for TEL was ca. 1% (w/w) for both insects. TEL was not digested by midgut preparations of C. maculatus and Z. subfasciatus. The transformation of the genes coding for this lectin could be useful in the development of insect resistance in important agricultural crops.

Calcium modulates protease resistance and carbohydrate binding of a plant defense legume lectin, Griffonia simplicifolia lectin II (GSII)

Site-directed mutagenesis previously identified the residues responsible for the biological activity of the plant defense legume lectin, Griffonia simplicifolia lectin II (GSII) [Proc. Natl. Acad. Sci. USA 95, (1998) 15123-15128]. However, these results were inconclusive as to whether these residues function as direct defense determinants through carbohydrate binding, or whether substantial changes of the protein structure had occurred in mutated proteins, with this structural disruption actually causing the loss of biochemical and biological functions. Evidence shown here supports the former explanation: circular dichroism and fluorescence spectra showed that mutations at carbohydrate-binding residues of GSII do not render it dysfunctional because of substantial secondary or tertiary structure modifications; and trypsin treatment confirmed that rGSII structural integrity is retained in these mutants. Reduced biochemical stability was observed through papain digestion and urea denaturation in mutant versions that had lost carbohydrate-binding ability, and this was correlated with lower Ca(2+) content. Accordingly, the re-addition of Ca(2+) to demetalized proteins could recover resistance to papain in the carbohydrate-binding mutant, but not in the non-binding mutant. Thus, both carbohydrate binding (presumably to targets in the insect gut) and biochemical stability to proteolytic degradation in situ indeed contribute to anti-insect activity, and these activities are Ca(2+)-dependent.

Biochemical characterization of a lectin from Delonix regia seeds

A lectin from Delonix regia (DRL) seeds was purified by gel filtration on Sephadex G-100 followed by ion-exchange chromatography on diethylaminoethyl-Sepharose and reverse-phase high-performance liquid chromatography on a C18 column. Hemagglutinating activity was monitored using rat erythrocytes. DRL showed no specificity for human erythrocytes of ABO blood groups. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) revealed a single protein in the presence of 0.1 M of dithiothreitol (DTT) and in nonreducing conditions. Native-PAGE showed that DRL is a monomer with a molecular mass of about 12 kDa, as determined by denaturing gel electrophoresis and gel filtration chromatography. An amino acid composition revealed the absence of cysteine residues, the presence of 1 mol methionine/mol protein and a high proportion of acidic amino acids and glycine. The N-terminal sequence of DRL was determined by Edman degradation, and up to 16 amino acid residues showed more than 90% homology with other lectins from the Leguminosae family. The optimal pH range for lectin activity was between pH 8.0 and 9.0, and the lectin was active up to 60 degrees C. The lectin required Mn2+ for hemagglutinating activity and remained active after reduction with 0.1 M of DTT, but lost activity in the presence of 8 M of urea. Sodium metaperiodate had no effect on the activity of DRL.

Functional and comparative bioinformatic analysis of expressed genes from wheat spikes infected with Fusarium graminearum

Fusarium head blight, caused by the fungus Fusarium graminearum, is a major disease on wheat (Triticum aestivum L.). Expressed sequence tags (ESTs) were used to identify genes expressed during the wheat-F. graminearum interaction. We generated 4,838 ESTs from a cDNA library prepared from spikes of the partially resistant cultivar Sumai 3 infected with F. graminearum. These ESTs were composed of 2,831 singlet (single-copy transcripts) and 715 contigs (multiple-copy transcripts) for a total of 3,546 non-redundant sequences. Four sets of nonredundant sequences were identified. One set contains numerous, common biotic and abiotic stress-related genes. Many of these stress-related genes were represented by multiple ESTs, indicating that they are abundantly expressed. A second set comprised 16 nonredundant sequences from F. graminearum that may be required for pathogenicity. A subset of these fungal genes encodes proteins associated with plant cell wall degradation. A third set of 326 nonredundant sequences had no DNA or amino acid sequence similarity to almost 1 million plant and over 7 million animal sequences in dbEST (as of 22 June 2001). Thus, these 326 nonredundant sequences have only been found in our F. graminearum-infected 'Sumai 3' cDNA library. A fourth set of 29 nonredundant sequences was found in our F. graminearum-infected wheat and another plant-pathogen interaction cDNA library. Some of these sequences encode proteins that may act in establishing various plant-fungal interactions.

Isolation and characterization of a bioactive mannose-binding protein from the Chinese chive Allium tuberosum

A mannose-binding protein was isolated from two different cultivars of the Chinese chive Allium tuberosum by extraction with 0.2 M NaCl, ammonium sulfate precipitation, and affinity chromatography on mannose agarose and fetuin agarose. It exhibited hemagglutinating activity toward rabbit erythrocytes. The lectin (agglutinin) was adsorbed on the mannose-agarose column, but not on the fetuin-agarose column. This A. tuberosum lectin (ATL) is unglycosylated, and not sialic acid binding. Lectins isolated from the two cultivars exhibited the same molecular mass of 25 kDa on gel filtration (Superose 12) and 12.5 kDa on SDS-polyacrylamide gel electrophoresis, indicating that they might be a dimeric protein composed of two identical subunits. The N-terminal amino acid sequence analysis of the lectin of various cultivars of A. tuberosum revealed that they were identical and showed 50%, or more, homology to the lectins from Galanthus nivalis (family Amaryllidaceae), Narcissus tazetta (family Amaryllidaceae), and Aloe arborescenes (family Liliaceae).

Studies of three genes encoding Cinnamomin (a type II RIP) isolated from the seeds of camphor tree and their expression patterns

Cinnamomin, which has three isoforms, is a type II ribosome-inactivating protein (RIP) purified from the mature seeds of camphor tree (Cinnamomum camphora). In a previous study, an incomplete cDNA that encoded the A- and B-chain of Cinnamomin but lacked signal peptide sequence was cloned. In the present paper, its full-length cDNA was obtained by 5' rapid amplification of cDNA ends (5'RACE). Subsequently, polymerase chain reaction (PCR) amplification of its genomic DNA was performed. Unexpectedly, sequence analysis of the PCR products revealed three cinnamomin genes with >98.0% sequence identity. One of them corresponded to the published cDNA and was designated as cinnamomin I, whereas the other two genes were named as cinnamomin II and cinnamomin III, respectively. RT-PCR amplification of the cDNAs of cinnamomin II and III manifested that these two genes were functional. The three genes have no intron. Three Cinnamomin precursors that were inferred from the cDNA sequence of three cinnamomin genes exhibited relatively high sequence homology with other type II RIPs. Northern blot analysis demonstrated that the cinnamomin genes only expressed in cotyledons of C. camphora seeds and the acmes of expression emerged at 75-90 DAF when seeds were close to maturity. It is proposed that the three cinnamomin genes may encode three isoforms of Cinnamomin. The physiological function of Cinnamomin in C. camphora seeds is briefly discussed.

The effects of Phaseolus vulgaris erythro- and leucoagglutinating isolectins (PHA-E and PHA-L) delivered via artificial diet and transgenic plants on the growth and development of tomato moth (Lacanobia oleracea) larvae; lectin binding to gut glycoproteins in vitro and in vivo

Red kidney bean, Phaseolus vulgaris, contains a lectin phytohemagglutinin (PHA) with toxicity towards higher animals. PHA exists in the isoforms PHA-E and PHA-L, which agglutinate erythrocytes and lymphocytes, respectively. Lacanobia oleracea larvae were reared from hatch on artificial diets containing PHA-E or PHA-L at 2% (w/w) dietary protein, and on transgenic Arabidopsis plants expressing either lectin at 0.4-0.6% of total soluble proteins. In artificial diet bioassays neither lectin affected larval survival, development, growth nor consumption. In transgenic plant bioassays both PHA-E and PHA-L promoted larval growth and development. This effect was greatest for PHA-E. Mean larval biomass of insects fed on plants expressing PHA-E was significantly greater (up to two-fold) than controls during the final two instars and the insects developed at a significantly greater rate so that after 26 days 83% of PHA-E exposed insects were in the final instar compared to 44% for control insects. PHA-E and PHA-L were detected by Western blotting in haemolymph, sampled from insects fed diets or plant material containing the lectins. However, despite the demonstrated potential for both isolectins to bind to gut glycopolypeptides in vitro neither was found to accumulate in vivo in the guts of exposed insects. Since lectin binding to gut polypeptides is thought to be necessary for insecticidal activity the failure of PHA-E and PHA-L to bind in vivo may account for their lack of toxicity to L. oleracea.

Mutational analysis at Asn-41 in peanut agglutinin. A residue critical for the binding of the tumor-associated Thomsen-Friedenreich antigen

Peanut agglutinin is a clinically important lectin due to its application in the screening of mature and immature thymocytes as well as in the detection of cancerous malignancies. The basis for these applications is the remarkably strong affinity of the lectin for the tumor-associated Thomsen-Friedenreich antigen (T-antigen) and more so due to its ability to distinguish T-antigen from its cryptic forms. The crystal structure of the complex of peanut agglutinin with T-antigen reveals the basis of this specificity. Among the contacts involved in providing this specificity toward T-antigen is the water-mediated interaction between the side chain of Asn-41 and the carbonyl oxygen of the acetamido group of the second hexopyranose ring of the sugar molecule. Site-directed mutational changes were introduced at this residue with the objective of probing the role of this residue in T-antigen binding and possibly engineering an altered species with increased specificity for T-antigen. Of the three mutants tested, i.e. N41A, N41D, and N41Q, the last one shows improved potency for recognition of T-antigen. The affinities of the mutants can be readily explained on the basis of the crystal structure of the complex and simple modeling. In particular, the change of asparagine to glutamine could lead to a direct interaction of the side chain with the sugar while at the same time retaining the water bridge. This study strengthens the theory that in lectins the nonprimary contacts generally made through water bridges are involved in imparting exquisite specificity.

Functional mechanics of the plant defensive Griffonia simplicifolia lectin II: resistance to proteolysis is independent of glycoconjugate binding in the insect gut

Griffonia simplicifolia lectin II (GSII) is a plant defensive protein that significantly delays development of the cowpea bruchid Callosobruchus maculatus (F.). Previous structure/function analysis by site-directed mutagenesis indicated that carbohydrate binding and resistance to insect gut proteolysis are required for the anti-insect activity of this lectin. However, whether there is a causal link between carbohydrate binding and resistance to insect metabolism remains unknown. Two proteases principally responsible for digestive proteolysis in third and fourth instar larvae of C. maculatus were purified by activated thiol sepharose chromatography and resolved as cathepsin L-like proteases, based on N-terminal amino acid sequence analysis. Digestion of bacterially expressed recombinant GSII (rGSII) and its mutant protein variants with the purified gut proteases indicates that carbohydrate binding, presumably to a target ligand in insect gut, and proteolytic resistance are independent properties of rGSII, and that both facilitate its efficacy as a plant defensive molecule.

Biologically active proteins from natural product extracts

The term "biologically active proteins" is almost redundant. All proteins produced by living creatures are, by their very nature, biologically active to some extent in their homologous species. In this review, a subset of these proteins will be discussed that are biologically active in heterologous systems. The isolation and characterization of novel proteins from natural product extracts including those derived from microorganisms, plants, insects, terrestrial vertebrates, and marine organisms will be reviewed and grouped into several distinct classes based on their biological activity and their structure.

Structure of a legume lectin from the bark of Robinia pseudoacacia and its complex with N-acetylgalactosamine

The structure of the bark lectin RPbAI (isoform A4) from Robinia pseudoacacia has been determined by protein crystallography both in the free form and complexed with N-acetylgalactosamine. The free form is refined at 1.80 A resolution to an R-factor of 18.9% whereas the complexed structure has an R-factor of 19.7% at 2.05 A resolution. Both structures are compared to each other and to other available legume lectin structures. The polypeptide chains of the two structures exhibit the characteristic legume lectin tertiary fold. The quaternary structure resembles that of the Phaseolus vulgaris lectin, the soybean agglutinin, and the Dolichos biflorus lectin, but displays some unique features leading to the extreme stability of this lectin.

The interaction between wheat germ agglutinin and other plant lectins with prostate cancer cells Du-145

The bioadhesive properties of fluorescein-labeled plant lectins with different carbohydrate specificities were investigated by flow cytometry at 4 and 37 degrees C using Du-145 prostate cancer cells. At both temperatures the lectin association rate increased following the order: Dolichos biflorus agglutinin (DBA)<peanut agglutinin<Ulex europaeus isoagglutinin I<Lens culinaris agglutinin<Solanum tuberosum lectin << wheat germ agglutinin (WGA), reflecting the glycosylation pattern of Du-145 cells. Both, the BSA-binding capacity of the cells referring to nonspecific binding and inhibition studies using the complementary carbohydrate, assured specificity of the lectin-cell interactions except for DBA. The WGA-association rate of Du-145 cells was dependent on temperature indicative for cellular uptake of membrane-bound WGA. Intracellular enrichment of WGA was confirmed by confocal microscopy. As resulted from experiments in presence of ouabain active transport mechanisms were involved in cellular uptake of WGA. Equilibration of the intracellular pH with monensin pointed to accumulation of intracellular located WGA within acidic compartments of Du-145 cells such as the lysosomes or the trans-Golgi complex. Consequently the interaction of WGA with Du-145 cells at 37 degrees C is a one way process due to immediate active transport of membrane-bound lectin into acidic compartments of prostate cancer cells.

Characterization of the sugar-binding specificity of the toxic lectins isolated from Abrus pulchellus seeds

The sugar-binding specificity of the toxic lectins from Abrus pulchellus seeds was investigated by combination of affinity chromatography of glycopeptides and oligosaccharides of well-defined structures on a lectin-Sepharose column and measurement of the kinetic interactions in real time towards immobilized glycoproteins. The lectins showed strong affinity for a series of bi- and triantennary N-acetyllactosamine type glycans. The related asialo-oligosaccharides interact more strongly with the lectins. The best recognized structures were asialo-glycopeptides from fetuin. Accordingly, the kinetic interaction with immobilized asialofetuin was by far the most pronounced. Human and bovine lactotransferrins and human serotransferrin interacted to a lesser extent. The interaction with asialofetuin was inhibited by galactose in a dose dependent manner. Lactose, N-acetyllactosamine and lacto-N-biose exhibited similar degree of inhibition while N-acetylgalactosamine was a poor inhibitor. These results suggested that the carbohydrate-binding site of the Abrus pulchellus lectins was specific for galactose and possess a remarkable affinity for the sequences lactose [beta-D-Gal-(1-->4)-D-Glc], N-acetyllactosamine [beta-D-Gal-(1-->4)-D-GlcNAc] and lacto-N-biose [beta-D-Gal-(1-->3)-D-GlcNAc].

Evolutionary biology of plant defenses against herbivory and their predictive implications for endocrine disruptor susceptibility in vertebrates

Hormone disruption is a major, underappreciated component of the plant chemical arsenal, and the historical coevolution between hormone-disrupting plants and herbivores will have both increased the susceptibility of carnivores and diversified the sensitivities of herbivores to man-made endocrine disruptors. Here I review diverse evidence of the influence of plant secondary compounds on vertebrate reproduction, including human reproduction. Three of the testable hypotheses about the evolutionary responses of vertebrate herbivores to hormone-disrupting challenges from their diet are developed. Specifically, the hypotheses are that a) vertebrate herbivores will express steroid hormone receptors in the buccal cavity and/or the vomeronasal organ; b) absolute sex steroid concentrations will be lower in carnivores than in herbivores; and c) herbivore steroid receptors should be more diverse in their binding affinities than carnivore lineages. The argument developed in this review, if empirically validated by support for the specific hypotheses, suggests that a) carnivores will be more susceptible than herbivores to endocrine-disrupting compounds of anthropogenic origin entering their bodies, and b) diverse herbivore lineages will be variably susceptible to any given natural or synthetic contaminant. As screening methods for hormone-disrupting potential are compared and adopted, comparative endocrine physiology research is urgently needed to develop models that predict the broad applicability of those screening results in diverse vertebrate species.

An N-acetylglucosamine oligomer binding agglutinin (lectin) from ripe Cyphomandra betacea Sendt. fruits

A new agglutinin (lectin), called CBL3, was purified from the juice of ripe Cyphomandra betacea Sendt. fruits until electrophoretically pure to homogeneity. The lectin is a homodimer of M(r)=50800 with subunits of 26200 bound by disulfide linkages with a pI of 4.9. The agglutinating capacity of the lectin is only inhibited by oligomers of N-acetylglucosamine in the following order of potency: tetrasaccharide>trisaccharide>disaccharide. CBL3 is not inhibited by N-acetylglucosamine, the same as all known lectins of the Solanaceae family. The human blood group recognition is non-specific. The lectin is a glycoprotein with 13.6% (w/w) of carbohydrates. The agglutinating activity is not affected by EDTA nor by cations. Mitogenic activity was not detected. Heat and pH stability, amino acid composition, N-terminal amino acid sequence and immunological properties show substantial differences to the reported lectins isolated from the Solanaceae family.

Interaction of a mulberry leaf lectin with a phytopathogenic bacterium, P. syringae pv mori

Two N-glycolylneuraminic acid-specific lectins, MLL 1 and 2, from leaves of Morus alba were studied for their anti-bacterial activity against P. syringae pv mori, which was a specific pathogenic bacterium of the mulberry leaf. MLL 1 but not MLL 2 was found to induce the agglutination of P. syringae pv mori. The MLL 1 can induce the agglutination only at the exponential phase of the bacterial growth in a liquid medium and the agglutination was specifically inhibited by N-glycolylneuraminic acid, N-acetylgalactosamine at 12.5 mM and bovine submaxillary mucin at 0.05 &mgr;g/ml.

Gastrodianin-like mannose-binding proteins: a novel class of plant proteins with antifungal properties

The orchid Gastrodia elata depends on the fungus Armillaria mellea to complete its life cycle. In the interaction, fungal hyphae penetrate older, nutritive corms but not newly formed corms. From these corms, a protein fraction with in vitro activity against plant-pathogenic fungi has previously been purified. Here, the sequence of gastrodianin, the main constituent of the antifungal fraction, is reported. Four isoforms that encoded two different mature proteins were identified at the cDNA level. Another isoform was detected in sequenced peptides. Because the antifungal activity of gastrodianins produced in and purified from Escherichia coli and Nicotiana tabacum was comparable to that of gastrodianin purified from the orchid, gastrodianins are the active component of the antifungal fractions. Gastrodianin accumulation is probably an important part of the mechanism by which the orchid controls Armillaria penetration. Gastrodianin was found to be homologous to monomeric mannose-binding proteins of other orchids, of which at least one (Epipactis helleborine mannose-binding protein) also displayed in vitro antifungal activity. This establishes the gastrodianin-like proteins (GLIPs) as a novel class of antifungal proteins.

Iris bulbs express type 1 and type 2 ribosome-inactivating proteins with unusual properties

Two closely related lectins from bulbs of the Dutch iris (Iris hollandica var. Professor Blaauw) have been isolated and cloned. Both lectins, called Iris agglutinin b and Iris agglutinin r, possess N-glycosidase activity and share a high sequence similarity with previously described type 2 ribosome-inactivating proteins (RIP). However, these lectins show only 57% to 59% sequence identity to a previously characterized type 1 RIP from iris, called IRIP. The identification of the iris lectins as type 2 RIP provides unequivocal evidence for the simultaneous occurrence of type 1 and type 2 RIP in iris bulbs and allowed a detailed comparison of type 1 and type 2 RIP from a single plant, which provides further insight into the molecular evolution of RIP. Binding studies and docking experiments revealed that the lectins exhibit binding activity not only toward Gal/N-acetylgalactosamine, but also toward mannose, demonstrating for the first time that RIP-binding sites can accommodate mannose.

Purification, chemical, and immunochemical properties of a new lectin from Mimosoideae (Parkia discolor)

A glucose/mannose-binding lectin was isolated from seeds of Parkia discolor (Mimosoideae) using affinity chromatography on Sephadex G-100 gel. The protein presented a unique component in SDS-PAGE corresponding to a molecular mass of 58,000 Da, which is very similar to that of a closely related lectin from Parkia platycephala. Among the simple sugars tested, mannose was the best inhibitor, but biantennary glycans, containing the trimannoside core, present in N-glycoproteins, also seem to be powerful inhibitors of the haemagglutinating activity induced by the purified lectin. The protein was characterised by high content of glycine and proline and absence of cysteine. Rabbit antibodies, anti-P. platycephala seed lectin, recognised the P. discolor lectin. However, no cross-reaction was observed when a set of other legume lectins from sub-family Papilionoideae and others from families Moraceae and Euphorbiaceae were assayed with the Parkia lectins. This suggests that Parkia lectins comprise a new group of legume lectins exhibiting distinct characteristics.

Purification of a novel antibacterial and haemagglutinating protein from the purple gland of the sea hare, Aplysia dactylomela rang, 1828

Physicochemical characterisation and antibacterial and haemagglutinating properties of a new protein isolated from purple fluid of the Aplysia dactylomela are reported. The purification procedure consisted basically of ammonium sulphate fractionation, ion exchange, exclusion molecular and hydrophobic interaction chromatography. The highly purified protein, designated dactylomelin-P, is a single chain protein of 60,000 Da by SDS-polyacrylamide gel electrophoresis and 56,200 Da by gel filtration on calibrated Superose column at pH 7.5 and contains less than 0.05% of its weight in neutral carbohydrates. Dactylomelin-P has two biological activities, antibacterial and haemagglutinating. The antibacterial action is bacteriostatic but not bactericidal. The haemagglutinating activity is preferentially against rabbit erythrocytes. The glycoprotein fetuin was able to abolish the haemagglutinating activity but not the antibacterial one even when used at concentrations 10 fold higher. This is the first time that a chimeroprotein is described in the purple fluid of sea hares, which may be involved in the chemical defence mechanism of these organisms.

Complete amino acid sequence of Japanese chestnut agglutinin

The complete amino acid sequence of Japanese chestnut (Castanea crenata Sieb. et Zucc.) agglutinin (CCA) was determined. Analysis by SIMS of the acidic peptide obtained by pepsin digestion revealed that the N-terminal amino acid sequence should be Acetyl-Met-Glu-Glu. Prior to sequence analysis, redetermination of cysteine residues indicated the presence of one cysteine residue per subunit. The complete sequence was determined by endoproteinase Arg-C and Achromobacter protease I digestion, and CNBr cleavage. CCA consists of 309 amino acid residues with a high content of glycine (16.5 mol%) and one cysteine residue. The calculated molecular mass was 33, 387 Da including the N-terminal acetyl group. C-terminal sequence analysis of intact CCA gave only one sequence, HMEYF, indicating that no heterogeneous CCA formed by posttranslational cleavage at the C-terminal region, as occurs in some legume lectins. Analysis of the sequence of CCA itself revealed that CCA could be divided into two structural domains, the N-domain and the C-domain, almost at the center. These domains share about 35% identical residues, so CCA has a repeat sequence. Also, both domains show a homology to jacalin-related lectins with 27-38% identity. These results suggest that the structure of CCA resembles two molecules of jacalin-related lectin.

Accessibility of the high-mannose glycans of glycoprotein gp120 from human immunodeficiency virus type 1 probed by in vitro interaction with mannose-binding lectins

The direct interaction of mannose-specific plant lectins with gp120 of HIV-1 was studied by surface plasmon resonance. Inhibition experiments indicated that exposed high mannose type glycans play a key role in the interaction. Most of the lectins specifically accommodate outer alpha1,2-, alpha1,3-, or alpha1,6-linked di- or trimannosides, and especially legume lectins, also interact with the trimannoside core of the complex type glycans. The unexpected affinity of some lectins towards gp120 presumably results from conformational differences in their binding sites. These results demonstrate that mannose-specific plant lectins are powerful tools to study the accessibility and elucidate the function of the gp120 glycans in the recognition and infection of the host cells by HIV-1.

A new lectin in red kidney beans called PvFRIL stimulates proliferation of NIH 3T3 cells expressing the Flt3 receptor

A new legume lectin has been identified by its ability to specifically stimulate proliferation of NIH 3T3 fibroblasts expressing the Flt3 tyrosine kinase receptor. The lectin was isolated from conditioned medium harvested from human peripheral blood mononuclear cells activated to secrete cytokines by a crude red kidney bean extract containing phytohemagglutinin (PHA). Untransfected 3T3 cells and 3T3 cells transfected with the related Fms tyrosine kinase receptor do not respond to this lectin, which we called PvFRIL (Phaseolus vulgaris Flt3 receptor-interacting lectin). When tested on cord blood mononuclear cells enriched for Flt3-expressing progenitors, purified PvFRIL fractions maintained a small population of cells that continued to express CD34 after 2 weeks in suspension cultures containing IL3. These cultures did not show the effects of IL3's strong induction of proliferation and differentiation (high cell number and exhausted medium); instead, low cell number at the end of the culture period resulted in persistence of cells in the context of cell death. These observations led to the hypothesis that PvFRIL acts in a dominant manner to preserve progenitor viability and prevent proliferation and differentiation.

The structural features of concanavalin A governing non-proline peptide isomerization

The reversible binding of manganese and calcium to concanavalin A determines the carbohydrate binding of the lectin by inducing large conformational changes. These changes are governed by the isomerization of a non-proline peptide bond, Ala-207-Asp-208, positioned in a beta-strand in between the calcium binding site S2 and the carbohydrate specificity-determining loop. The replacement of calcium by manganese allowed us to investigate the structures of the carbohydrate binding, locked state and the inactive, unlocked state of concanavalin A, both with and without metal ions bound. Crystals of unlocked metal-free concanavalin A convert to the locked form with the binding of two Mn(2+) ions. Removal of these ions from the crystals traps metal-free concanavalin A in its locked state, a minority species in solution. The ligation of a metal ion in S2 to unlocked concanavalin A causes bending of the beta-strand foregoing the S2 ligand residues Asp-10 and Tyr-12. This bending disrupts conventional beta-sheet hydrogen bonding and forces the Thr-11 side chain against the Ala-207-Asp-208 peptide bond. The steric strain exerted by Thr-11 is presumed to drive the trans-to-cis isomerization. Upon isomerization, Asp-208 flips into its carbohydrate binding position, and the conformation of the carbohydrate specificity determining loop changes dramatically.

Characterization and molecular cloning of two different type 2 ribosome-inactivating proteins from the monocotyledonous plant Polygonatum multiflorum

Leaves of the monocotyledonous plant Polygonatum multiflorum L. (Solomon's seal) contain besides a monocot mannose-binding lectin two galactose/N-acetylgalactosamine (Gal/GalNAc)-binding type 2 ribosome-inactivating proteins (RIPs). Both RIPs were purified using a combination of classical protein purification techniques and affinity chromatography. Although both RIPs consist of protomers of 65 kDa, the P. multiflorum RIP monomer (PMRIPm) occurs as a monomer of approximately 60 kDa, whereas the tetramer (PMRIPt) is a tetramer of 240 kDa. Both RIPs exhibit similar RNA N-glycosidase activity but differ in their specific agglutination activity and carbohydrate-binding specificity, PMRIPt being a GalNAc-specific lectin whereas PMRIPm is Gal/GalNAc-specific. Toxicity tests indicated that both Polygonatum RIPs exhibit a very low cytotoxicity towards human and animal cells. Analysis of the genomic clones encoding both RIPs revealed a high degree of sequence similarity to other type 2 RIPs. Molecular modelling confirmed that both Polygonatum RIPs have a similar structure to ricin.

Purification, molecular cloning and ethylene-inducible expression of a soluble-type epoxide hydrolase from soybean (Glycine max [L.] Merr.)

A soybean protein was purified from mature dry seeds. Amino-acid sequencing of the nine internal peptides derived from this N-terminally blocked protein showed that it has a significant similarity to the soluble epoxide hydrolases known to date. A degenerate series of 23-mer oligonucleotides with sequences corresponding to an internal region of eight amino-acid residues was synthesized as a probe mixture for detection of a putative epoxide hydrolase cDNA in a developing cotyledon cDNA library. The 1332-bp cDNA obtained was found to have an open-reading frame encoding the seed epoxide hydrolase-like precursor consisting of 341 amino-acid residues, suggesting that 25 amino-acid residues upstream from the second methionine correspond to a transit peptide. Employing an Escherichia coli expression system, the putative mature epoxide hydrolase-like protein was overexpressed and purified to homogeneity. This recombinant protein was confirmed to exhibit its epoxide-diol converting activity using styrene oxide as substrate. The Vmax and Km values for styrene oxide are 1.36 micromol x min-1 x mg-1 and 1500 microM, respectively. Sedimentation equilibrium experiments showed that the active form of this epoxide hydrolase is monomeric in solution. Using the above cDNA as a probe, a 12-kb genomic clone was selected and the sequence of a 1933-bp fragment from this clone was found to cover the entire coding region together with 5'- and 3'-flanking regions of the soybean epoxide hydrolase gene. The coding region of the gene, interrupted by two short introns, was identical to the corresponding regions of the cDNA. Northern blot analyses showed that this epoxide hydrolase gene was expressed strongly at a very early stage (13 days after flowering) and then the level of expression gradually decreased and almost ceased at a very late stage (58 days after flowering) of seed development, whereas its expression was markedly up-regulated by ethylene treatment. In stems (hypocotyl portion), the epoxide hydrolase transcript was detected at significant levels and was also up-regulated in response to ethylene. On the other hand, it is hardly expressed in leaves, even though they were treated with the phytohormone. Overall, the results obtained may indicate that soluble-type epoxide hydrolase mRNA is expressed at the maximum level in an early stage of seed development. Later, oil bodies are formed and subsequently epoxy fatty acids, naturally occurring metabolites, accumulate within those bodies. The temporal induction of this epoxide hydrolase transcript in some tissues in response to ethylene also indicates that this epoxide hydrolase may play a crucial role in self-defense systems of plant.

The effect of arcelin-1 on the structure of the midgut of bruchid larvae and immunolocalization of the arcelin protein

Some wild accessions of the common bean (Phaseolus vulgaris) contain a family of proteins called arcelins, that are toxic to the larvae of certain bruchid species. Among the six allelic variants of arcelin tested so far, arcelin-5 and arcelin-1 confer the highest level of resistance against the Mexican bean weevil, Zabrotes subfasciatus. The same proteins are not toxic to the bean weevil, Acanthoscelides obtectus, which is also a serious pest of cultivated beans. Arcelins belong to the bean lectin family that includes phytohemaggutinins and alpha-amylase inhibitors. Although homologous to lectins, arcelins are themselves only very weak lectins, and their binding properties have not been clearly established. The toxic properties of arcelins may be related to their recognition of and interaction with the glycoproteins and other constituents of the membranes along the digestive tract of insects. Since arcelin-1 was shown to have growth inhibitory effects for the larvae of Z. subfasciatus but not of A. obtectus, we examined the effect of an arcelin-1 containing diet on the structure of the cells that line the intestinal tract of the larvae of these two bruchid species, and used antibodies against arcelin to examine the distribution of arcelin within the cells and tissues. Here we show that dietary arcelin-1 caused an alteration of the gut structure and the penetration of arcelin into the haemolymph in Z. subfasciatus but not in A. obtectus. These results lead us to suggest that arcelins exert their toxic effect by severely damaging the epithelial cells.

The African yam bean seed lectin affects the development of the cowpea weevil but does not affect the development of larvae of the legume pod borer

Artificial feeding assays were used to study the effect of purified galactose-specific lectins from African yam beans (Sphenostylis stenocarpa) on development of larvae of the cowpea weevil, Callosobruchus maculatus (Coleoptera : Bruchidae) and the legume pod-borer, Maruca vitrata (Lepidoptera : Pyrialidae). Inhibition of development of C. maculatus was observed when larvae were fed on artificial cowpea seeds containing 0.2%, 2.0% and 5.0% (wt/wt) of dietary lectin. Larval mortality was between 30% and 88%, while delays in total developmental time ranged between 7 and 13 days. The lectin had no effect on development of larvae of M. vitrala, when tested through topical artificial diet incorporation assays, except at the extremely high dose of 35% dietary level.

Isolation and characterization of Dorin M, a lectin from plasma of the soft tick Ornithodoros moubata

A lectin with high hemagglutinating activity, which we have named Dorin M, was identified in the plasma of the soft tick Ornithodoros moubata. The activity of the plasma lectin could be efficiently inhibited by sialic acid, N-acetyl-D-hexosamines and sialoglycoproteins. Dorin M was purified to homogeneity using two different isolation systems: affinity chromatography on a column of bovine submaxillary mucin conjugated to Sepharose 4B with specific elution by N-acetyl-D-glucosamine and chromatography on Blue-Sepharose followed by anion exchange FPLC on a MonoQ column. The purified lectin is a glycoprotein which, in the native state, forms aggregates with molecular mass of about 640 kDa. Non-reducing SDS PAGE revealed that the lectin consists of two noncovalently bound subunits migrating closely around 37 kDa. Dorin M is a glycoprotein, probably modified by N-type glycosylation. After chemical deglycosylation, only one band of about 32 kDa was detected. Dorin M is the first lectin purified from ticks.

Major protein of resting rhizomes of Calystegia sepium (hedge bindweed) closely resembles plant RNases but has no enzymatic activity

The most abundant protein of resting rhizomes of Calystegia sepium (L.) R.Br. (hedge bindweed) has been isolated and its corresponding cDNA cloned. The native protein consists of a single polypeptide of 212 amino acid residues and occurs as a mixture of glycosylated and unglycosylated isoforms. Both forms are derived from the same preproprotein containing a signal peptide and a C-terminal propeptide. Analysis of the deduced amino acid sequence indicated that the C. sepium protein shows high sequence identity and structural similarity with plant RNases. However, no RNase activity could be detected in highly purified preparations of the protein. This apparent lack of activity results most probably from the replacement of a conserved His residue, which is essential for the catalytic activity of plant RNases. Our findings not only demonstrate the occurrence of a catalytically inactive variant of an S-like RNase, but also provide further evidence that genes encoding storage proteins may have evolved from genes encoding enzymes or other biologically active proteins.

Mucosal immunogenicity of plant lectins in mice

The mucosal immunogenicity of a number of plant lectins with different sugar specificities was investigated in mice. Following intranasal (i.n.) or oral administration, the systemic and mucosal antibody responses elicited were compared with those induced by a potent mucosal immunogen (cholera toxin; CT) and a poorly immunogenic protein (ovalbumin; OVA). After three oral or i.n. doses of CT, high levels of specific serum antibodies were measured and specific IgA was detected in the serum, saliva, vaginal wash, nasal wash and gut wash of mice. Immunization with OVA elicited low titres of serum IgG but specific IgA was not detected in mucosal secretions. Both oral and i.n. delivery of all five plant lectins investigated ¿Viscum album (mistletoe lectin 1; ML-1), Lycospersicum esculentum (tomato lectin; LEA), Phaseolus vulgaris (PHA), Triticum vulgaris (wheat germ agglutinin (WGA), Ulex europaeus I (UEA-1) stimulated the production of specific serum IgG and IgA antibody after three i. n. or oral doses. Immunization with ML-1 induced high titres of serum IgG and IgA in addition to specific IgA in mucosal secretions. The response to orally delivered ML-1 was comparable to that induced by CT, although a 10-fold higher dose was administered. Immunization with LEA also induced high titres of serum IgG, particularly after i. n. delivery. Low specific IgA titres were also detected to LEA in mucosal secretions. Responses to PHA, WGA and UEA-1 were measured at a relatively low level in the serum, and little or no specific mucosal IgA was detected.

Helianthus tuberosus lectin reveals a widespread scaffold for mannose-binding lectins

BACKGROUND

Heltuba, a tuber lectin from the Jerusalem artichoke Helianthus tuberosus, belongs to the mannose-binding subgroup of the family of jacalin-related plant lectins. Heltuba is highly specific for the disaccharides Man alpha 1-3Man or Man alpha 1-2Man, two carbohydrates that are particularly abundant in the glycoconjugates exposed on the surface of viruses, bacteria and fungi, and on the epithelial cells along the gastrointestinal tract of lower animals. Heltuba is therefore a good candidate as a defense protein against plant pathogens or predators.

RESULTS

The 2.0 A resolution structure of Heltuba exhibits a threefold symmetric beta-prism fold made up of three four-stranded beta sheets. The crystal structures of Heltuba in complex with Man alpha 1-3Man and Man alpha 1-2Man, solved at 2.35 A and 2.45 A resolution respectively, reveal the carbohydrate-binding site and the residues required for the specificity towards alpha 1-3 or alpha 1-2 mannose linkages. In addition, the crystal packing reveals a remarkable, donut-shaped, octahedral assembly of subunits with the mannose moieties at the periphery, suggesting possible cross-linking interactions with branched oligomannosides.

CONCLUSIONS

The structure of Heltuba, which is the prototype for an extended family of mannose-binding agglutinins, shares the carbohydrate-binding site and beta-prism topology of its galactose-binding counterparts jacalin and Maclura pomifera lectin. However, the beta-prism elements recruited to form the octameric interface of Heltuba, and the strategy used to forge the mannose-binding site, are unique and markedly dissimilar to those described for jacalin. The present structure highlights a hitherto unrecognized adaptability of the beta-prism building block in the evolution of plant proteins.

Role of lectins (and rhizobial exopolysaccharides) in legume nodulation

The lectin recognition hypothesis proposes that plant lectins mediate specificity in the Rhizobium-legume symbiosis. Although the hypothesis was developed eight years before nod genes were identified in rhizobia and sixteen years before Nod factor was shown to be a major determinant of host specificity, experiments performed recently using transgenic lectin plants support its main tenets.

Purification and characterization of an anti-(A+B) specific lectin from the mushroom Hygrophorus hypothejus

A lectin (HHL) was isolated from the fruiting body of the mushroom Hygrophorus hypothejus by a combination of affinity chromatography on stromas of group B erythrocytes embedded in polyacrylamide gel, and DEAE-trisacryl and gel filtration chromatography. Its molecular mass, as determined by gel filtration, is estimated to be 68000 kDa and its structure is tetrameric with four identical subunits assembled with non-covalent bonds. HHL agglutinates specifically A and B blood group erythrocytes and in hemagglutination inhibition assays, exhibits sugar-binding specificity toward lactose, the anomeric alpha form being more effective than the beta form.

Variability in quaternary association of proteins with the same tertiary fold: a case study and rationalization involving legume lectins

Legume lectins constitute a family of proteins in which small alterations arising from sequence variations in essentially the same tertiary structure lead to large changes in quaternary association. All of them are dimers or tetramers made up of dimers. Dimerization involves side-by-side or back-to-back association of the flat six-membered beta-sheets in the protomers. Variations within these modes of dimerization can be satisfactorily described in terms of angles defining the mutual disposition of the two subunits. In all tetrameric lectins, except peanut lectin, oligomerization involves the back-to-back association of side-by-side dimers. An attempt has been made to rationalize the observed modes of oligomerization in terms of hydrophobic surface area buried on association, interaction energy and shape complementarity, by constructing energy minimised models in each of which the subunit of one legume lectin is fitted in the quaternary structure of another. The results indicate that all the three indices favor and, thus, provide a rationale for the observed arrangements. However, the discrimination provided by buried hydrophobic surface area is marginal in a few instances. The same is true, to a lesser extent, about that provided by shape complementarity. The relative values of interaction energy turns out to be a still better discriminator than the other two indices. Variability in the quaternary association of homologous proteins is a widely observed phenomenon and the present study is relevant to the general problem of protein folding.

cDNA cloning of FRIL, a lectin from Dolichos lablab, that preserves hematopoietic progenitors in suspension culture

Ex vivo culture of hematopoietic stem cells is limited by the inability of cytokines to maintain primitive cells without inducing proliferation, differentiation, and subsequent loss of repopulating capacity. We identified recently in extracts of kidney bean and hyacinth bean a mannose-binding lectin, called FRIL, and provide here evidence that this protein appears to satisfy properties of a stem cell preservation factor. FRIL was first identified based on its ability to stimulate NIH 3T3 cells transfected with Flt3, a tyrosine kinase receptor central to regulation of stem cells. Molecular characterization from polypeptide sequencing and identification of the cDNA of hyacinth bean FRIL shows 78% amino acid identity with a mannose-binding lectin of hyacinth beans. Treatment of primitive hematopoietic progenitors in suspension culture with purified hyacinth FRIL alone is able to preserve cells for 1 month without medium changes. In vitro progenitor assays for human hematopoietic cells cultured 3 weeks in FRIL displayed small blast-like colonies that were capable of serial replating and persisted even in the presence of cytokines known to induce differentiation. These results suggest that FRIL is capable of preserving primitive progenitors in suspension culture for prolonged periods. FRIL's clinical utility involving procedures for stem cell transplantation, tumor cell purging before autologous transplantation, and ex vivo cultures used for expansion and stem cell gene therapy currently are being explored.