Lectins

A class of proteins of nonimmune origin that bind carbohydrates reversibly and noncovalently without inducing any change in the carbohydrate. Lectins bind a variety of cells having cell-surface glycoproteins (carbohydrate bound proteins) or glycolipids (carbohydrate bound lipids). The presence of two or more binding sites for each lectin molecule allows the agglutination of many cell types, and the agglutination reaction has been used extensively to detect the presence of lectins in extracts from different organisms. Although lectins are ubiquitous in nature, their biological role is not well understood. See Carbohydrate

Hemagglutinating activity has been found in more than 1000 plant taxa. However, knowledge on distribution of well-characterized lectins covers only 4–5% of flowering plant families. Moreover, most of the best-characterized lectins come from a single family, Leguminosae.

Animal lectins have also been characterized in only a small number of species, but they have been found in almost all of the invertebrate and vertebrate phyla. Invertebrate lectins occur in body fluids or secretions, such as fish serum, snake venom, seminal and coelomic fluids, and hemolymph. Vertebrate lectins occur as soluble or integral membrane proteins in embryonic and adult fluids, organs, and tissues.

Microbial lectins have been isolated mainly from bacteria, but they are also found in viruses, slime molds, protozoa, green algae, and fungi.

From the few known examples, it seems clear that lectins are involved in recognition phenomena and their ability to bind particular carbohydrate structures is the key to their biological functions. These recognition functions include their involvement in interactions with cells or extracellular materials from the same organism (self-recognition) and interactions with foreign particles or cells (non-self recognition). Among the best-known examples of self-recognition mediated by lectins are the hepatic lectins involved in the recognition of glycoproteins that must be cleared from circulation. Specific lectins recognize and internalize the target proteins into lysosomes, where they are destroyed. See Lysosome

Examples of lectin-mediated non-self-recognition are seen in the involvement of bacterial surface lectins in infection. Numerous bacterial strains produce surface lectins that recognize specific sugars in the surface of host cells and thus initiate the infection process. Another example is the symbiotic interaction between nitrogen-fixing bacteria and plant roots. The lectins present in the roots of legumes are localized at the surface of root hairs and in the root exudates, where they recognize the appropriate Rhizobium strain and account for part of the specificity in the initiation of nodulation.

Lectins are very useful reagents for the study of complex carbohydrates and cell surfaces, for the separation and identification of particular cells, and for the stimulation of cell proliferation. Lectins covalently attached to insoluble matrices are used to separate glycoproteins or glycopeptides that contain different carbohydrates. Labeled lectins are also used in histochemical and cytochemical studies to localize glyconjugates that carry particular sugars. This technique is particularly interesting, since changes in lectin-binding patterns occur during embryonic differentiation, malignant transformation, aging, and some pathological conditions. See Protein