Overview of Animal Galectins: Proto-Type Subfamily
Lectins recognize and bind carbohydrates covalently linked to proteins and lipids on the cell surface and within the extracellular matrix, and they mediate many cellular functions ranging from cell adhesion to pathogen recognition. Phylogenetically conser
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Lectins: An Overview G.S. Gupta
1.1
Lectins: Characteristics and Diversity
1.1.1
Characteristics
Glycan-recognizing proteins can be broadly classified into two groups: lectins [which typically contain an evolutionarily conserved carbohydrate-recognition domain (CRD)] and sulfated glycosaminoglycan (SGAG)-binding proteins (which appear to have evolved by convergent evolution). The term “lectin” is derived from the Latin word legere, meaning, among other things, “to select”. Lectins are phylogenetically ancient and have specific recognition and binding functions for complex carbohydrates of glycoconjugates, i.e., glycoproteins, proteoglycans/glycosaminoglycans and glycolipids. Although lectins were first discovered more than 100 years ago in plants, they are now known to be present throughout nature and found in most of the organisms, ranging from viruses and bacteria to plants and animals. It is generally believed that the earliest description of a lectin was given by Peter Hermann Stillmark in his doctoral thesis in 1888 to the University of Dorpat. Stillmark isolated ricin, an extremely toxic hemagglutinin, from seeds of the castor plant (Ricinus communis) (Stillmark 1988). The animal hemagglutinins were noted quite early, almost in all invertebrates or lower vertebrates, but until the middle of the 1970s, only three of these (of eel, snail, and horseshoe crab) were isolated and characterized. The first of the animal lectins shown to be specific for a sugar (L-fucose) was from eel (Watkins and Morgan 1952). The isolation of the first mammalian lectin, the galactose-specific hepatic asialoglycoprotein receptor, was achieved by Gilbert Ashwell at the NIH and by Anatol G. Morell at the Albert Einstein Medical School (New York) in 1974 (Hudgin et al. 1974; Stockert et al. 1974). At the same time, Teichberg et al. 1975 isolated a b-galactose-specific lectin from electric eel that was designated galectins (Barondes et al. 1994), of which 15 members so far have been characterized. Since the beginning of 1980s, the number of purified animal lectins started to grow quickly, largely thanks to the advent of recombinant techniques.
Presently, lectins are known as a heterogeneous group of carbohydrate binding proteins of non-immune origin, which agglutinate cells and/or precipitate glycoconjugates without affecting their covalent linkages (Goldstein et al. 1980). This definition implied that each lectin molecule has two or more carbohydrate binding sites to allow cross-linking between cells and between sugar containing macromolecules. During last few decades, however, interest in lectins has been greatly intensified after realization that they act as mediators of cell recognition in biological system (Sharon 2006). Though, lectins are similar to antibodies in their ability to agglutinate red blood cells, they are not the product of immune system. Lectins not only distinguish between different monosaccharides, but also specifically bind to oligosaccharides, detecting subtle differences in complex carbohydrate structure. T
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