Molecular properties

Lectins constitute a heterogeneous group of oligo-meric proteins that vary widely in their molecular properties (Figure 1). Nonetheless, many of them can be grouped into distinct families that share sequence homologies and similar structural properties, and in some cases also in superfamilies. The largest and most thoroughly studied family is that of legume lectins, which includes concanavalin A, PHA, and the lectins from soybean (SBA) and peanut (PN'A). Close to 100 such lectins have been purified, the complete sequences of some 40 of which are known. In addition, the three-dimensional structures of close to ten of these lectins, most in complex with their carbohydrate ligand, have been elucidated.

Typically, legume lectins consist of two or four identical, or almost identical, subunits (or protomers) of 25-30 kDa, each of which contains a single carbohydrate-binding site with the same specificity. A prominent exception is PHA, which occurs as a family of five tetrameric proteins with varying proportions of two classes of subunit, E and L, and different biological properties: E, (E-PHA) is a potent hemagglutinin. I,4 (I.-PHA) has leukoagglu-tinating activity and is a potent mitogen; intermediate forms (e.g. E,L2 or F.,L) possess lower levels of the above activities; E4 and L4 also differ in their specificity for oligosaccharides. Other families of plant lectins are those of cereals (e.g. wheat germ agglutinin, WGA) and of solanaceous plants (e.g. potato and tomato lectins). Members of these

Figure 1 Schematic representation of the overall structure of lectins from different sources. (A) Part of fimbria of E. coli. The fimbriae are an assembly of different types of subunit, only one of which (black) contains a carbohydrate-binding site. (B) Dimeric legume lectin (e.g. from peas or from seeds of Erythrina corallodendron). (C) Mammalian soluble mannose-binding protein, a col-lectin. (D) E-Selectin; the coil represents the membrane-spanning and cytoplasmic domains. COL, collagenous region; CR complement regulatory repeats; EGF, epidermal growth factor-like domain; CRD, carbohydrate recognition domain. The indentations represent binding sites. (Reproduced with permission from Sharon N and Lis H (1995) Essays in Biochemistry 30: 59-75.)

Figure 1 Schematic representation of the overall structure of lectins from different sources. (A) Part of fimbria of E. coli. The fimbriae are an assembly of different types of subunit, only one of which (black) contains a carbohydrate-binding site. (B) Dimeric legume lectin (e.g. from peas or from seeds of Erythrina corallodendron). (C) Mammalian soluble mannose-binding protein, a col-lectin. (D) E-Selectin; the coil represents the membrane-spanning and cytoplasmic domains. COL, collagenous region; CR complement regulatory repeats; EGF, epidermal growth factor-like domain; CRD, carbohydrate recognition domain. The indentations represent binding sites. (Reproduced with permission from Sharon N and Lis H (1995) Essays in Biochemistry 30: 59-75.)

families, too, consist mostly of two identical subunits but differ markedly in other properties from those of the legume lectins.

Animal lectins are frequently composite molecules with a wide range of molecular weights, consisting of multiple types of protein modules or domains, only one of which serves as a carbohydrate-recognition domain (CRD) (Figure 1). Three types of CRDs have been discerned: P-type, S-type and C-type, each of them shares a pattern of invariant and highly conserved amino acid residues at a characteristic spacing. The occurrence of such a domain in a protein provides a novel guideline for the identification and classification of new lectins. The P-type CRD has to date been found only in the mannose-6-phosphate receptors. Proteins containing S-type and C-type CRDs are known as galectins (previously S-lectins) and C-type lectins, respectively. The galectins constitute a family of soluble lectins that bind exclusively p-galactosides, such as lactose (Gal(3(l-4)Glc) and N-acetyllactosamine (Gal(3(l-4)GlcNAc). They have a relatively simple structure (molecular weights between 14 and 35 kDa) and occur predominantly in mammals, but have also been described in other vertebrates. Their three-dimensional structure is similar to that of the legume lectins and they may thus be considered as members of the same superfamily. The C-type lectins include endo-cytic lectins, collectins and selectins. Each of these families shares a common overall architecture, defined by the nature of their domains. In addition to their CRDs, the collectins, for example, contain a collagen-like region, while the selectins - E-selectin (CD62E), P-selectin (CD62P) and L-selectin (CD 62 L) - contain an epidermal growth factor-like domain and several short repeating units related to complement-binding protein. The collectins are similar in design to complement protein Clq, which also contains a collagen-like domain linked to globular domains involved in the binding of immunoglobulin.

The sialoadhesins are a family of sialic acid-specific cell surface lectins with variable numbers of extracellular immunoglobulin-like domains and are thus members of the immunoglobulin superfamily. They include the sheep erythrocyte receptor of macrophages (referred to simply as sialoadhesin), CD22 found only on B cells and MAG, the myelin-associated glycoprotein.

Bacterial lectins are usually in the form of fimbriae (or pili), hair-like submicroscopic appendages, consisting of an assembly of 5-6 different types of subunit, only one of which has carbohydrate-binding activity, e.g. for mannose (in type 1 fimbriae) or gala-biose, Gala(l-4)Gal (in P fimbriae).

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