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A report on the crystal structure of a lectin isolated from Canavalia …

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- Structure of a lectin from Canavalia gladiata seeds: new structural insights for old molecules

Most of the biochemical lectin studies have been based on monochromatic view since almost all the properties of these proteins had been commonly reported in terms of lectin-carbohydrate recognition. For several years, the definition of lectins has been improving focused on the carbohydrate-binding properties. The most recent accepted definition establishes lectins as proteins with at least one non-catalytic domain able to recognize and bind reversibly to specific mono and oligosaccharides. They are subdivided into four types: merolectins, hololectins, chimerolectins and superlectins. This classification was conceived in terms of the carbohydrate-binding domain and another unrelated domain [1]. Several studies have tried to find other binding sites that could possibly recognize plant hormones, secondary metabolites and isolated amino acid residues [2-4].

Over 250 non-protein amino acids have been identified in plants [5]. A number of these compounds are intermediates in the synthesis and catabolism of protein amino acids [6]. However, many of these non-protein amino acids may play a role as defensive agents. They show their toxicity in many ways; some of them block the synthesis and the absorption of protein amino acids or can wrongly incorporated into proteins in organisms that feed on these plants. Plants that synthesize non-protein amino acids are not susceptible to the toxicity of these compounds. Seeds from Canavalia ensiformis, which synthesize high quantities of non-protein amino acids, display a biological system capable of discriminating between these amino acids and the others [7].

Non-protein amino acids are especially abundant in Leguminosae, Liliaceae and in several higher fungi and marine algae. Plant organs rich in these metabolites are seeds (Leguminosae) or rhizomes (Liliaceae). Concentrations in seeds can exceed 10% of dry weight and up to 50% of the nitrogen could be attributed to them. Since non-protein amino acids are often remobilized during germination, they certainly function as N-storage compounds in addition to their role as defense chemicals [8]. If non-protein amino acids are taken up by herbivores, microorganisms or other plants, they may interfere with their metabolism.

Aminobutyric acid (Abu) is a non-protein amino acid that can protect certain plants against pathogens; for instance, when introduced into Arabidopis plants, it has the ability to induce resistance to certain pathogens. Abu protects these plants against pathogens through the activation of natural defense mechanisms of the plant, such as callose deposition, hypersensitive response (HR), and the formation of trailing necroses. Induced resistance is often associated with a process called priming, which is an increased capacity to mobilize cellular defense responses [9].

Most plant lectins not only play a role in the plant itself (e.g., as a store of nitrogen or as a specific recognition factor) but are also capable of interfering with the functioning of foreign organisms through an interaction with glycoconjugates on the surface or in the digestive tract of these organisms [10]. Although this interference has been reported as a specific event of carbohydrate recognition, it has not been elucidated yet. Stress-regulated pathways for rapid and high gene expression are one of the essential elements in stress acclimation. Salicylic acid, jasmonic acid, systemin, ethylene, and aminobutyric acid have been implicated in the potentiation of gene expression [11,12], and other signal molecules have been shown to play a similar role [9].

The content of free protein amino acids in seeds varies among species and increases dramatically after germination. More non-protein amino acids were found in lentil seedlings compared to the seeds [12]. Most plant lectins are probably involved in plant defense [13]. The mechanism of action still remains unclear even though induced response mechanisms are proposed for many pathogen-mediated injuries in plants. The direct interference with viruses and microorganisms is rather exceptional, and the deleterious effects of plant lectins on both predatory invertebrates and animals are well documented [13].

We report here the crystal structure of a native and complexed lectin isolated from Canavalia gladiata seeds, describing a new binding pocket in ConA-like lectins, which may be related to pathogen resistance, a site where a non-protein amino acid, such as α-amino butyric acid, can bind.

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