Lectins and glucanases exhibit the common property of reversibly binding to specific complex carbohydrates. The lectins/glucanases are a diverse group of proteins found in a wide range of species from prokaryotes to humans. The different family members all contain a concanavalin A-like domain, which consists of a sandwich of 12-14 beta strands in two sheets with a complex topology. Members of this family are diverse, and include the lectins: legume lectins, cereal lectins, viral lectins, and animal lectins. Plant lectins function in the storage and transport of carbohydrates in seeds, the binding of nitrogen-fixing bacteria to root hairs, the inhibition of fungal growth or insect feeding, and in hormonally regulated plant growth [1]. Protein members include concanavalin A (Con A), favin, isolectin I, lectin IV, soybean agglutinin and lentil lectin. Animal lectins include the galectins, which are S-type lactose-binding and IgE-binding proteins such as S-lectin, CLC protein, galectin1, galectin2, galectin3 CRD, and Congerin I [2].
Other members with a Con A-like domain include the glucanases and xylanases. Bacterial and fungal beta-glucanases, such as Bacillus 1-3,1-4-beta-glucanse, carry out the acid catalysis of beta-glucans found in microorganisms and plants [3]. Similarly, kappa-Carrageenase degrades kappa-carrageenans from marine red algae cell walls [4]. Xylanase and cellobiohydrolase I degrade hemicellulose and cellulose, respectively [5, 6].
There are many Con A-like domains found in proteins involved in cell recognition and adhesion. For example, several viral and bacterial toxins carry Con A-like domains. Examples include the Clostridium neurotoxins responsible for the neuroparalytic effects of botulism and tetanus [7]. The Pseudomonas exotoxin A, a virulence factor which is highly toxic to eukaryotic cells, causing the arrest of protein synthesis, contains a Con A-like domain involved in receptor binding [8]. Cholerae neuraminidase can bind to cell surfaces, possibly through their Con A-like domains, where they function as part of a mucinase complex to degrade the mucin layer of the gastrointestinal tract [9]. The rotaviral outer capsid protein, VP4, has a Con A-like sialic acid binding domain, which functions in cell attachment and membrane penetration [10].
Con A-like domains also play a role in cell recognition in eukaryotes. Proteins containing a Con A-like domain include the sex hormone-binding globulins which transport sex steroids in blood and regulate their access to target tissues [11], laminins which are large heterotrimeric glycoproteins involved in basement membrane architecture and function [12], neurexins which are expressed in hundreds of isoforms on the neuronal cell surface, where they may function as cell recognition molecules [13] and sialidases that are found in both microorganisms and animals, and function in cell adhesion and signal transduction [14].
Other proteins containing a Con A-like domain include pentraxins and calnexins. The pentraxin PTX3 is a TNFalpha-induced, secreted protein of adipose cells produced during inflammation [15]. The calnexin family of molecular chaperones is conserved among plants, fungi, and animals. Family members include Calnexin, a type-I integral membrane protein in the endoplasmic reticulum which coordinates the processing of newly synthesized N-linked glycoproteins with their productive folding, calmegin, a type-I membrane protein expressed mainly in the spermatids of the testis, and calreticulin, a soluble ER lumenal paralog [16].
1. The crystal structure of Canavalia brasiliensis lectin suggests a correlation between its quaternary conformation and its distinct biological properties from Concanavalin A. FEBS Lett. 405, 114-8
2. Charcot-Leyden crystal protein (galectin-10) is not a dual function galectin with lysophospholipase activity but binds a lysophospholipase inhibitor in a novel structural fashion. J. Biol. Chem. 277, 14859-68
3. Crystal structure of Bacillus licheniformis 1,3-1,4-beta-D-glucan 4-glucanohydrolase at 1.8 A resolution. FEBS Lett. 374, 221-4
4. The kappa-carrageenase of P. carrageenovora features a tunnel-shaped active site: a novel insight in the evolution of Clan-B glycoside hydrolases. Structure 9, 513-25
5. Structure and function of the Bacillus hybrid enzyme GluXyn-1: native-like jellyroll fold preserved after insertion of autonomous globular domain. Proc. Natl. Acad. Sci. U.S.A. 95, 6613-8
6. High-resolution crystal structures reveal how a cellulose chain is bound in the 50 A long tunnel of cellobiohydrolase I from Trichoderma reesei. J. Mol. Biol. 275, 309-25
7. Structural analysis of the catalytic and binding sites of Clostridium botulinum neurotoxin B. Nat. Struct. Biol. 7, 693-9
8. Refined crystallographic structure of Pseudomonas aeruginosa exotoxin A and its implications for the molecular mechanism of toxicity. J. Mol. Biol. 314, 823-37
9. Crystal structure of Vibrio cholerae neuraminidase reveals dual lectin-like domains in addition to the catalytic domain. Structure 2, 535-44
10. The rhesus rotavirus VP4 sialic acid binding domain has a galectin fold with a novel carbohydrate binding site. EMBO J. 21, 885-97
11. Crystal structure of human sex hormone-binding globulin: steroid transport by a laminin G-like domain. EMBO J. 19, 504-12
12. Structure of the C-terminal laminin G-like domain pair of the laminin alpha2 chain harbouring binding sites for alpha-dystroglycan and heparin. EMBO J. 19, 1432-40
13. The structure of the ligand-binding domain of neurexin Ibeta: regulation of LNS domain function by alternative splicing. Cell 99, 93-101
14. The crystal structure and mode of action of trans-sialidase, a key enzyme in Trypanosoma cruzi pathogenesis. Mol. Cell 10, 757-68
15. Characterization of the long pentraxin PTX3 as a TNFalpha-induced secreted protein of adipose cells. J. Lipid Res. 44, 994-1000
16. The Structure of calnexin, an ER chaperone involved in quality control of protein folding. Mol. Cell 8, 633-44
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