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Evidence is presented that the specificity of jacalin, the seed lectin from jack fruit (Artocarpus integrifolia), is not directed exclusively against the T-antigen disaccharide Galβ1,3GalNAc, lactose and galactose, but also against mannose and oligomannosides. Biochemical analyses based on surface-plasmon-resonance measurements, combined with the X-ray-crystallographic determination of the structure of a jacalin—α-methyl-mannose complex at 2Å resolution, demonstrated clearly that jacalin is fully capable of binding mannose. Besides mannose, jacalin also interacts readily with glucose, N-acetylneuraminic acid and N-acetylmuramic acid. Structural analyses demonstrated that the relatively large size of the carbohydrate-binding site enables jacalin to accommodate monosaccharides with different hydroxyl conformations and provided unambiguous evidence that the β-prism structure of jacalin is a sufficiently flexible structural scaffold to confer different carbohydrate-binding specificities to a single lectin.

In contrast to animal lectins, no evidence has indicated the occurrence of plant lectins, which recognize and bind "endogenous" receptors and accordingly are involved in recognition mechanisms within the organism itself. Here we show that the plant hormone jasmonic acid methyl ester (JAME) induces in leaves of Nicotiana tabacum (var. Samsun NN) the expression of a lectin that is absent from untreated plants. The lectin specifically binds to oligomers of N-acetylglucosamine and is detected exclusively in the cytoplasm and the nucleus. Both the subcellular location and specificity indicate that the Nicotiana tabacum agglutinin (called Nictaba) may be involved in the regulation of gene expression in stressed plants through specific protein-carbohydrate interactions with regulatory cytoplasmic/nuclear glycoproteins. Searches in the databases revealed that many flowering plants contain sequences encoding putative homologues of the tobacco lectin, which suggest that Nictaba is the prototype of a widespread or possibly ubiquitous family of lectins with a specific endogenous role.

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One of the predominant proteins in the pulp of ripe bananas (Musa acuminata L.) and plantains (Musa spp.) has been identified as a lectin. The banana and plantain agglutinins (called BanLec and PlanLec, respectively) were purified in reasonable quantities using a novel isolation procedure, which prevented adsorption of the lectins onto insoluble endogenous polysaccharides. Both BanLec and PlanLec are dimeric proteins composed of two identical subunits of 15 kDa. They readily agglutinate rabbit erythrocytes and exhibit specificity towards mannose. Molecular cloning revealed that BanLec has sequence similarity to previously described lectins of the family of jacalin-related lectins, and according to molecular modelling studies has the same overall fold and three-dimensional structure. The identification of BanLec and PlanLec demonstrates the occurrence of jacalin-related lectins in monocot species, suggesting that these lectins are more widespread among higher plants than is actually believed. The banana and plantain lectins are also the first documented examples of jacalin-related lectins, which are abundantly present in the pulp of mature fruits but are apparently absent from other tissues. However, after treatment of intact plants with methyl jasmonate, BanLec is also clearly induced in leaves. The banana lectin is a powerful murine T-cell mitogen. The relevance of the mitogenicity of the banana lectin is discussed in terms of both the physiological role of the lectin and the impact on food safety.

Three mannose‐binding lectins were assayed in artificial diets for their toxic and growth‐inhibitory effects on nymphal development of the peach‐potato aphid Myzus persicae. The snowdrop (Galanthus nivalis) lectin GNA was the most toxic, with an induced nymphal mortality of 42% at 1500 μg ml−1 (30 μM) and an IC50 (50% growth inhibition) of 630 μg ml−1 (13 μM). The daffodil (Narcissus pseudonarcissus) lectin NPA and a garlic (Allium sativum) lectin ASA induced no significant mortality in the range 10–1500 μg ml−1, but did result in growth inhibition of 59% (NPA) and 26% (ASA) at 1500 μg ml−1 (40 μM for NPA, 63 μM for ASA). All three lectins were responsible for a slight but significant growth stimulation when ingested at 10 μg ml−1, reaching + 26%, + 18% and + 11% over the control values for the garlic lectin, the daffodil lectin and the snowdrop lectin, respectively. GNA, as well as the glucose/mannose binding lectin Concanavalin A, were also provided at sublethal doses throughout the life cycle of the aphids, and effects on adult performance were monitored. Adult survival was not significantly altered, but both lectins adversely affected total fecundity and the dynamics of reproduction, resulting in significant reduction in calculated rm's (population intrinsic rate of natural increase) on lectin‐containing diets. These effects are discussed in relation to the use of transgenic plants expressing these toxic lectins for potential control of aphid populations.

IYXX. Related mannosespecific lectins from different species of the family Amaryllidaceae. -Physiol. Plant. 73: 52-57.Bulbs from three species of the plant family Amaryllidaceae (Narrissus pseudonarrissus L.. Leucojum aestivum L. and Leurojum vernum L.) were found to contain mannose-specific lectins. These lectins were serologically identical to a previously reported Amaryllidaceae lectin from Galanthus nivalis L. bulbs. but had a different molecular structure. The lectins described in this paper are dimeric proteins composed of subunits of 13 kDa, which are not held together by disulphide bridges. In hapten-inhibition assays Amaryllidaceae lectins exhibited exclusive specificity towards mannose. Furthermore, they all had a high specific agglutination activity with trypsin-treated rabbit erythrocytes. whereas human red blood cells were not agglutinated.

Poly(A)-rich RNA isolated from ripening ovaries of snowdrop (Galanthus nivalis L.) yielded a single 17-kDa lectin polypeptide upon translation in a wheat-germ cell-free system. This lectin was purified by affinity chromatography. Translation of the same RNA in Xenopus leavis oocytes revealed a lectin polypeptide which was about 2 kDa smaller than the in vitro synthesized precursor, suggesting that the oocyte system had removed a 2-kDa signal peptide.A second post-translational processing step was likely to be involved since both the in vivo precursor and the Xenopus translation products were about 2 kDa larger than the mature lectin polypeptide. This hypothesis was confirmed by the structural analysis of the amino acid sequence of the mature protein and the cloned mRNA.Edman degradation and carboxypeptidase Y digestion of the mature protein, and structural analysis of the peptides obtained after chemical cleavage and modification, allowed determination of the complete 10.5 amino acid sequence of the snowdrop lectin polypeptide. Comparison of this sequence with the deduced amino acid sequence of a lectin cDNA clone revealed that besides the mature lectin polypeptide, the lectin mRNA also encoded a 23 amino acid signal-sequence and a C-terminal extension of 29 amino acids. which confirms the results from in vitro translation experiments.Lectins are a heterogeneous class of (g1yco)proteins grouped together based on their ability to recognize and bind carbohydrate moieties of glycoconjugates. Although numerous plant lectins have been isolated and characterized in detail, the physiological function of these proteins remains unclear (Etzler, 1986). A study of the biosynthesis of lectins and their subsequent subcellular deposition can be helpful in obtaining a better understanding of the molecular biology of lectins. In combination with physiological studies of the lectins concerned (e.g. occurrence and abundance of lectin in different plant tissues) it is possible to obtain important information in a search for the normal biological function of these proteins.