The role of lectins in plant defence

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.

On the possible function of Telfairia occidentalis agglutinin in the plant

The fate of Telfairia occidentalis seed Agglutinin, TOA, has been monitored during germination. While the level of the Agglutinin in the cotyledons decreased sharply in the first three days to about half of the initial level, it stabilises at this level for the following twelve days. In this interval, Agglutinin activity becomes manifest in the radicle on the fourth day, peaking on the fifth and decreasing rapidly thereafter. In the plumule, the lectin activity becomes manifest on the sixth day, peaks on the seventh and decreases rapidly thereafter. No lectin activity is detectable in any plant tissue including the rump of the cotyledons twenty-seven days after germination. The implications of these observations on the possible role of lectins in plants are discussed.

Enzymatic synthesis of complex glycosaminotrioses and study of their molecular recognition by hevein domains

Hevein, a protein found in Hevea brasiliensis, has a CRD domain, which is known to bind chitin and GlcNAc-containing oligosaccharides. By using NMR and molecular modeling as major tools we have demonstrated that trisaccharides containing GalNAc and ManNAc residues are also recognized by hevein domains. Thus far unknown trisaccharides GlcNAcbeta(1-->4)GlcNAcbeta(1-->4)ManNAc (1) and GalNAcbeta(1-->4)GlcNAcbeta(1-->4)ManNAc (2) were synthesized with the use of beta-N-acetylhexosaminidase from Aspergillus oryzae. This method is based on the rather unique phenomenon that some fungal beta-N-acetylhexosaminidases cannot hydrolyze disaccharide GlcNAcbeta(1-->4)ManNAc (5) contrary to chitobiose GlcNAcbeta(1-->4)GlcNAc (4) that is cleaved and, therefore, cannot be used as an acceptor for further transglycosylation. Both trisaccharides 1 and 2 were prepared by transglycosylation from disaccharidic acceptor in good yields ranging from 35% to 40%. Our observations strongly indicate that the present nature of the modifications of chitotriose (GlcNAcbeta(1-->lcNAcbeta(1-->4)GlcNAc, 3) at either the non-reducing end (GalNAc instead of GlcNAc) or at the reducing end (ManNAc instead of GlcNAc) do not modify the mode of binding of the trisaccharide to hevein. The association constant values indicate that chitotriose (3) binding is better than that of 1 and 2, and that the binding of (with ManNAc at the reducing end) is favored with respect to that of 2 (with ManNAc at the reducing end with a non-reducing GalNAc moiety).

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.

Seasonal and spatial variation of carbohydrates in mistletoes (Viscum album) and the xylem sap of its hosts (Populus x euamericana and Abies alba)

In the present field study we analysed the seasonal pattern of carbohydrate composition and contents in the xylem sap of Viscum album and the xylem sap of a deciduous (Populusxeuramericana) and a coniferous (Abies alba) host tree species. The results were compared with the soluble carbohydrate composition and contents of mistletoe tissues. On both hosts significant amounts of glucose, fructose, and sucrose were found in the xylem sap of Viscum throughout the seasons. The general seasonal pattern of sugar contents, i.e. high concentrations in spring and lower concentrations in other seasons on Populus, and intermediate concentrations throughout the year on Abies, largely reflected the xylem sap carbohydrate composition and contents of the respective host. These observations provide indirect evidence for carbohydrate flux from the xylem sap of the host into the mistletoe. However, in both hosts xylem sap seems to be deviated into the mistletoe without specific control of carbohydrate flux. Differences observed between the seasonal pattern of xylem sap carbohydrate concentrations in Viscum on Populus and Abies may originate from the different time of leaf development of these species. A clear-cut seasonal pattern of soluble carbohydrates was not observed in the leaves of Viscum on both hosts. Still soluble carbohydrates seem to be reallocated from the senescing to the newly developed leaves of Viscum indicating that the seasonal requirement of carbohydrate for growth and development can only completely be met by carbohydrate acquisition from the host and their own photosynthesis.

Expression and purification of the recombinant SALT lectin from rice (Oryza sativa L.)

The SALT protein is a 14.5 kDa mannose-binding lectin, originally described as preferentially expressed in rice plant roots in response to NaCl stress. Recombinant SALT lectin was produced in Escherichia coli from a cDNA clone encoding protein. After isopropyl-beta-d-thiogalactopyranoside induction, the expression level achieved was 23% of the soluble protein. The recombinant agglutinin was purified by a single-step process by dialyses against a high concentrated salt solution. After purification, hemagglutination assays of rabbit erythrocytes revealed that the recombinant SALT protein is a potent agglutinin (0.078 microg ml(-1) minimal concentration). The purified recombinant lectin was also used for comparative estimation of native protein amounts in protein extracts from rice plants by Western blot assay.

Inflammatory responses induced in mice by lectin from Talisia esculenta seeds

A novel lectin from Talisia esculenta seeds (TEL) has recently been purified and characterized. In this study we investigated the proinflammatory activity of TEL in mice using both the air-pouch and peritoneal cavity as well as paw oedema models. TEL (10-40 microg) induced significant neutrophil and mononuclear cell recruitment when injected into either mouse air-pouch or peritoneal cavity. The neutrophil accumulation into the air-pouch was dose- and time-dependent with a maximal response at 16 h, returning to control levels at 72 h whereas maximal mononuclear cell accumulation was observed at 24 h after TEL injection. The same profile of neutrophil accumulation was observed when this lectin was injected into mouse peritoneal cavity, although the maximal mononuclear cell recruitment was observed 48 h after TEL injection. Additionally, TEL (12.5-200 microg/paw) caused a dose-dependent mice paw, as evaluated at 4 h after the lectin injection. D-mannose, better than D-glucose, significantly inhibited TEL-induced neutrophil migration into the peritoneal cavity or air-pouch. D-galactose had no effect on TEL-induced neutrophil migration in either cavity studied. On the other hand, D-mannose slightly inhibited the TEL-induced paw oedema, whereas neither D-glucose nor D-galactose affected this phenomenon. In conclusion, our data show that TEL induces neutrophil and mononuclear cell accumulation by a mechanism related to their specific sugar-binding properties.

Deletion of a lectin gene does not affect the phenotype of the nematode-trapping fungus Arthrobotrys oligospora

A number of filamentous fungi are known to produce high levels of saline-soluble and low-molecular-mass lectins. The function of these proteins are not clear but it has been proposed that they are involved in storage of nutrients, development, recognition of other organisms, and defense reactions. A gene encoding such a lectin (AOL) was deleted in the nematode-trapping fungus Arthrobotrys oligospora by homologous recombination. The deletion mutants did not express any hemagglutinating activity or protein cross-reacting with AOL antibodies. There were no significant differences between the DeltaAOL and wild-type strains in spore (conidia) germination, saprophytic growth, and pathogenicity. Furthermore, there was no significant difference in the growth and reproduction of collembolan feeding on the various strains of A. oligospora. Thus either the previous proposed functions of AOL are not correct, or the fungus can compensate for the absence of the lectin by expressing other proteins with similar function(s) as AOL.

Complex formation of myrosinase isoenzymes in oilseed rape seeds are dependent on the presence of myrosinase-binding proteins

The enzyme myrosinase (EC 3.2.3.1) degrades the secondary compounds glucosinolates upon wounding and serves as a defense to generalist pests in Capparales. Certain myrosinases are present in complexes together with other proteins such as myrosinase-binding proteins (MBP) in extracts of oilseed rape (Brassica napus) seeds. Immunhistochemical analysis of wild-type seeds showed that MBPs were present in most cells but not in the myrosin cells, indicating that the complex formation observed in extracts is initiated upon tissue disruption. To study the role of MBP in complex formation and defense, oilseed rape antisense plants lacking the seed MBPs were produced. Western blotting and immunohistochemical staining confirmed depletion of MBP in the transgenic seeds. The exclusive expression of myrosinase in idioblasts (myrosin cells) of the seed was not affected by the down-regulation of MBP. Using size-exclusion chromatography, we have shown that myrosinases with subunit molecular masses of 62 to 70 kD were present as free dimers from the antisense seed extract, whereas in the wild type, they formed complexes. In accordance with this, MBPs are necessary for myrosinase complex formation of the 62- to 70-kD myrosinases. The product formed from sinalbin hydrolysis by myrosinase was the same whether MBP was present or not. The performance of a common beetle generalist (Tenebrio molitor) fed with seeds, herbivory by flea beetles (Phyllotreta undulata) on cotyledons, or growth rate of the Brassica fungal pathogens Alternaria brassicae or Lepthosphaeria maculans in the presence of seed extracts were not affected by the down-regulation of MBP, leaving the physiological function of this protein family open.

Carbohydrate-protein recognition: molecular dynamics simulations and free energy analysis of oligosaccharide binding to concanavalin A

Carbohydrate ligands are important mediators of biomolecular recognition. Microcalorimetry has found the complex-type N-linked glycan core pentasaccharide beta-GlcNAc-(1-->2)-alpha-Man-(1-->3)-[beta-GlcNAc-(1-->2)-alpha-Man-(1-->6)]-Man to bind to the lectin, Concanavalin A, with almost the same affinity as the trimannoside, Man-alpha-(1-->6)-[Man-alpha-(1-->3)]-Man. Recent determination of the structure of the pentasaccharide complex found a glycosidic linkage psi torsion angle to be distorted by 50 degrees from the NMR solution value and perturbation of some key mannose-protein interactions observed in the structures of the mono- and trimannoside complexes. To unravel the free energy contributions to binding and to determine the structural basis for this degeneracy, we present the results of a series of nanosecond molecular dynamics simulations, coupled to analysis via the recently developed MM-GB/SA approach (Srinivasan et al., J. Am. Chem. Soc. 1998, 120:9401-9409). These calculations indicate that the strength of key mannose-protein interactions at the monosaccharide site is preserved in both the oligosaccharides. Although distortion of the pentasaccharide is significant, the principal factor in reduced binding is incomplete offset of ligand and protein desolvation due to poorly matched polar interactions. This analysis implies that, although Concanavalin A tolerates the additional 6 arm GlcNAc present in the pentasaccharide, it does not serve as a key recognition determinant.

Classification of plant lectins in families of structurally and evolutionary related proteins

The majority of plant lectins can be classified in seven families of structurally and evolutionary related proteins. Within a given lectin family most but not necessarily all members are built up of protomers with a similar primary structure and overall 3-D fold. The overall structure of the native lectins is not only determined by the structure of the protomers but depends also on the degree of oligomerization and in some cases on the post-translational processing of the lectin precursors. In general, lectin families are fairly homogeneous for what concerns the overall specificity of the individual lectins, which illustrates that the 3-D structure of the binding site has been conserved during evolution. In the case of the jacalin-related lectins the occurrence of a mannose- and galactose-binding subfamily can be explained by the fact that a post-translational cleavage of the protomers (of the galactose-binding subfamily) yields a slightly altered binding site. Unlike the other families, the legume lectins display a wide range of specificites, which is clearly reflected in the occurrence of sugar-binding sites with a different 3-D structure.

Chemically prepared hevein domains: effect of C-terminal truncation and the mutagenesis of aromatic residues on the affinity for chitin

Chemically prepared hevein domains (HDs), N-terminal domain of an antifungal protein from Nicotiana tabacum (CBP20-N) and an antimicrobial peptide from Amaranthus caudatus (Ac-AMP2), were examined for their affinity for chitin, a beta-1,4-linked polymer of N-acetylglucosamine. An intact binding domain, CBP20-N, showed a higher affinity than a C-terminal truncated domain, Ac-AMP2. The formation of a pyroglutamate residue from N-terminal Gln of CBP20-N increased the affinity. The single replacement of any aromatic residue of Ac-AMP2 with Ala resulted in a significant reduction in affinity, suggesting the importance of the complete set of three aromatic residues in the ligand binding site. The mutations of Phe18 of Ac-AMP2 to the residues with larger aromatic rings, i.e. Trp, beta-(1-naphthyl)alanine or beta-(2-naphthyl)alanine, enhanced the affinity, whereas the mutation of Tyr20 to Trp reduced the affinity. The affinity of an HD for chitin might be improved by adjusting the size and substituent group of stacking aromatic rings.

Cloning of the Arabidopsis RTM1 gene, which controls restriction of long-distance movement of tobacco etch virus

The locus RTM1 is necessary for restriction of long-distance movement of tobacco etch virus in Arabidopsis thaliana without causing a hypersensitive response or inducing systemic acquired resistance. The RTM1 gene was isolated by map-based cloning. The deduced gene product is similar to the alpha-chain of the Artocarpus integrifolia lectin, jacalin, and to several proteins that contain multiple repeats of a jacalin-like sequence. These proteins comprise a family with members containing modular organizations of one or more jacalin repeat units and are implicated in defense against viruses, fungi, and insects.

The myrosinase-binding protein from Brassica napus seeds possesses lectin activity and has a highly similar vegetatively expressed wound-inducible counterpart

This communication demonstrates that proteins in the family of myrosinase-binding proteins (MBP) present in seeds of Brassica napus possess lectin activity, binding most efficiently to p-aminophenyl alpha-D-mannopyranoside-agarose, and to some extent to N-acetylglucosamine-agarose. A cDNA encoding a vegetatively expressed, wound-inducible counterpart to these seed MBP was isolated and characterised. Upon wounding, this MBP transcript accumulated in old and young leaves, and was systemically expressed in the young plant. Additionally, the wound-induced MBP transcript increased in abundance after treating the young plants with methyl jasmonate (MeJA), jasmonic acid (JA) or abscisic acid (ABA), and to some extent in response to the ethylene precursor 1-aminocyclopropane-1-carboxylic acid. Expression induced by wounding, ABA or JA was antagonised by simultaneous feeding of the plants with salicylic acid. MBP polypeptides accumulated in MeJA-treated plants. The myrosinases redistributed from the soluble fraction into the insoluble fraction of a tissue extract after induction. The most abundant MBP (94 kDa) partitioned in the insoluble fraction, while two larger MBP (103 kDa and 108 kDa) were present only in the soluble fraction of extracts obtained from the control or MeJA-treated plant tissues.

New folds of plant lectins

Among the crystal structures of lectins determined recently, three--snowdrop lectin, jacalin and amaranthin--represent new lectin families. Their polypeptide folds share remarkably similar features and consist exclusively of beta structure. Autonomously folded beta-sheet subdomains, inter-related by a pseudothreefold symmetry, assemble to form beta-prism or beta-barrel structures which are stabilized by a hydrophobic core.

Ribosome-inactivating lectins with polynucleotide:adenosine glycosidase activity

Lectins from Aegopodium podagraria (APA), Bryonia dioica (BDA), Galanthus nivalis (GNA), Iris hybrid (IRA) and Sambucus nigra (SNAI), and a new lectin-related protein from Sambucus nigra (SNLRP) were studied to ascertain whether they had the properties of ribosome-inactivating proteins (RIP). IRA and SNLRP inhibited protein synthesis by a cell-free system and, at much higher concentrations, by cells and had polynucleotide:adenosine glycosidase activity, thus behaving like non-toxic type 2 (two chain) RIP. APA and SNAI had much less activity, and BDA and GNA did not inhibit protein synthesis.

Myrosinase-binding proteins are derived from a large wound-inducible and repetitive transcript

Several non-myrosinase proteins have been found in association with some of the myrosinases extracted from rape (Brassica napus) seed. Most of these proteins seemed to belong to a large family of proteins ranging in size over approximately 30-110 kDa, namely the myrosinase-binding protein (MBP) family. Potentially all of these MBPs might be derived from a single large precursor, encoded by a 3.3-kb transcript. This transcript coded for a 99-kDa glycine-rich protein with a highly repetitive structure. The mature 50-kDa and 52-kDa MBP amino-terminal was located 255 amino acids from the putative initiation methionine. Also, a more divergently related transcript, the protein product of which was unknown, has been cloned. However, the largest open reading frame suggested a proline-rich protein. While this transcript seemed to be expressed predominantly in seeds, the MBP transcripts were expressed in several tissues and also exhibited a responsiveness to wounding and methyl jasmonate. Both proteins exhibited significant similarities to lectins from Artocarpus integer and from Maclura pomifera.

Purification and characterization of a pod lectin from Great Northern bean, Phaseolus vulgaris L

The pods of the Great Northern bean plant contain a lectin (GNpL) that highly resembles seed lectins (GNLs) of the same plant. Purification of GNpL from pod extracts was achieved by ion-exchange chromatographies on CM- and DEAE-celluloses and gel filtration chromatography on Sephacryl S-300 HR. GNpL has a similar SDS-PAGE pattern to that of GNLs. GNpL and GNLs yield three subunits though each GNpL subunit is 0.5 kDa smaller than the corresponding GNLs subunit (GNpL; pod-alpha-subunit of 34.0 kDa, pod-beta-subunit of 36.5 kDa, and pod-gamma-subunit of 38.5 kDa). GNpL and GNLs display indistinguishable carbohydrate specificities and have similar amino acid compositions. Pod-alpha-subunit cross-reacts with antibodies against GNLs on western blotting. On the other hand, the N-terminal amino acid sequence of pod-alpha-subunit suggests that GNpL is a distinct gene product from those of GNLs genes although they are shown to be homologous proteins.

A lectin and a lectin-related protein are the two most prominent proteins in the bark of yellow wood (Cladrastis lutea)

Using a combination of cDNA cloning and protein purification it is demonstrated that bark of yellow wood (Cladrastis lutea) contains two mannose/glucose binding lectins and a lectin-related protein which is devoid of agglutination activity. One of the lectins (CLAI) is the most prominent bark protein. It is built up of four 32 kDa monomers which are post-translationally cleaved into a 15 kDa and a 17 kDa polypeptide. The second lectin (CLAII) is a minor protein, which strongly resembles CLAI except that its monomers are not cleaved into smaller polypeptides. Molecular cloning of the Cladrastis lectin family revealed also the occurrence of a lectin-related protein (CLLRP) which is the second most prominent bark protein. Although CLLRP shows sequence homology to the true lectins, it is devoid of carbohydrate binding activity. Molecular modelling of the three Cladrastis proteins has shown that their three-dimensional structure is strongly related to the three-dimensional models of other legume lectins and, in addition, revealed that the presumed carbohydrate binding site of CLLRP is disrupted by an insertion of three extra amino acids. Since it is demonstrated for the first time that a lectin and a non-carbohydrate binding lectin-related protein are the two most prominent proteins in the bark of a tree, the biological meaning of their simultaneous occurrence is discussed.