Plant lectins are non-immunogenic proteins present in the plant kingdom that contain one or more non-catalytic structural domains capable of reversibly binding to specific monosaccharides or polysaccharides in their structure. Lectins recognize complex carbohydrate structures in glycoproteins and glycolipids, as well as glycosyl groups on the surface of cell membranes. Thus, lectins can be used as a favorable tool to study cell membrane structures. In addition, lectins can bind to fluorescein, enzymes, biotin, ferritin and colloidal gold without affecting their biological activity due to their multivalent binding ability, so lectins can also be used for immunocytochemical studies, as well as diagnostic evaluation of tumors.
Fig 1. Schematic representation of lectin applications in agriculture (Tsaneva et al., 2020)
Classification of Plant Lectins
Since there are obvious differences in molecular structure, physicochemical properties, biological activity, and glycosyl-binding specificity among different species and sources of plant lectins, plant lectins can be specifically classified according to these different properties. Based on the specificity of plant lectins to bind different kinds of glycosyl groups, they can be classified as: N-acetylaminogalactose lectin, D-mannose or D-glucose lectin, N-acetylaminoglucose lectin, D-galactose lectin, L-fucose lectin and sialic acid lectin. Depending on the molecular structure of plant lectins, they can be classified as hololectin, partial lectin, chemerolectin and superlectin.
Fig 2. Overview of the main lectin families and lectin receptor kinases at the plant cell surface. (Bellande et al., 2017)
Function of Plant lectin
- Specific sugar binding ability
Plant lectins are a class of proteins with specific sugar-binding activity, which is the hallmark that distinguishes them from all other plant proteins. Due to the conserved nature of the sugar-binding domain, plant lectins generally bind only one monosaccharide or oligosaccharide. In organisms, proteins that generally signal or convert are glycoproteins, and the sugar residue genes at the end of their sugar chains can be specifically recognized and bound by lectins, which leads to specific interactions between lectins and glycoproteins and further triggers a series of downstream biochemical signaling cascades.
- Cell agglutination ability
Agglutination is the binding of lectins to sugar-containing receptors on the cell surface. Since a lectin usually has more than two sugar-binding sites, it can bind to multiple cells simultaneously, causing free single cells to aggregate into clusters.
- Lectin-induced apoptosis and autophagy
Since the 1990s, several lectins have been reported to induce apoptosis in mammalian cells, especially in cancer cells. Presumably, the mechanism is that lectins bind to the corresponding receptors on the cell membrane and activate the caspase pathway or the mitochondrial pathway via endocytosis, thereby inducing apoptosis.
The high affinity of the lectin for its specific sugar makes it more stable in binding to the corresponding sugar-containing receptor, which is the most essential mechanism of lectin antiviral. There are two common pathways: (1) the lectin binds to the protein ligand on the virus and prevents the ligand from binding or exchanging information with the receptor on the animal cell surface; (2) the lectin binds to the glycocontaining receptor on the animal cell surface (especially the receptor recognized by the virus) and hinders the subsequent imbibition process of the virus through competitive inhibition.
The defense function of lectins is mainly reflected in the different ways it protects plants from diseases and insects, etc., during various stages of plant growth and development. Plant lectins recognize and bind the glyco-structural domains of invaders, thus interfering with the possible effects of the invaders on the plant.
Detection of Plant Lectins
- Red cell agglutination method
Plant lectin interacts with the sugar binding sites on the surface of erythrocytes, causing the agglutination of erythrocytes by forming multiple crossover bridges between the lectin molecules. The agglutination activity (potency) of lectins is linearly related to their amount. Currently, the most widely used method for the determination of plant lectin activity is the hemagglutination assay.
The detection of plant lectins using their ability to specifically bind sugars, glycoproteins or glycoconjugates.
- Antigen-antibody immunoassay
Plant lectin is a protein that acts as an antigen to induce specific antibody production and can be quantified by methods such as enzyme-linked immunoassay (ELISA), rocket immunoelectrophoresis, or radioimmunoassay (RIA).
An analytical technique in which mass spectrometry ionizes chemical substances and sorts them according to their mass-to-charge ratio. Qualitative and quantitative analysis of plant lectins can be performed by LC-MS.
- Tsaneva, M., & Van Damme, E. J. (2020). 130 years of plant lectin research. Glycoconjugate Journal, 1-19.
- Bellande, K., Bono, J. J., et al. (2017). Plant lectins and lectin receptor-like kinases: how do they sense the outside?. International Journal of Molecular Sciences, 18(6), 1164.
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