title: Structure and ligand binding of carbohydrate-binding module CsCBM6-3 reveals similarities with fucose-specific lectins and "galactose-binding" domains.
abstract: Carbohydrate-binding polypeptides, including carbohydrate-binding modules (CBMs) from polysaccharidases, and lectins, are widespread in nature. Whilst CBMs are classically considered distinct from lectins, in that they are found appended to polysaccharide-degrading enzymes, this distinction is blurring. The crystal structure of CsCBM6-3, a "sequence-family 6" CBM in a xylanase from Clostridium stercorarium, at 2.3 A reveals a similar, all beta-sheet fold to that from MvX56, a module found in a family 33 glycoside hydrolase sialidase from Micromonospora viridifaciens, and the lectin AAA from Anguilla anguilla. Sequence analysis leads to the classification of MvX56 and AAA into a family distinct from that containing CsCBM6-3. Whilst these polypeptides are similar in structure they have quite different carbohydrate-binding specificities. AAA is known to bind fucose; CsCBM6-3 binds cellulose, xylan and other beta-glucans. Here we demonstrate that MvX56 binds galactose, lactose and sialic acid. Crystal structures of CsCBM6-3 in complex with xylotriose, cellobiose, and laminaribiose, 2.0 A, 1.35 A, and 1.0 A resolution, respectively, reveal that the binding site of CsCBM6-3 resides on the same polypeptide face as for MvX56 and AAA. Subtle differences in the ligand-binding surface give rise to the different specificities and biological activities, further blurring the distinction between classical lectins and CBMs.
authors.affiliation: Department of Chemistry, The University of York, Heslington, York, YO10 5YW, UK.
authors.forename: Alisdair B
authors.initials: AB
authors.lastname: Boraston
authors.forename: Valerie
authors.initials: V
authors.lastname: Notenboom
authors.forename: R Antony J
authors.initials: RA
authors.lastname: Warren
authors.forename: Douglas G
authors.initials: DG
authors.lastname: Kilburn
authors.forename: David R
authors.initials: DR
authors.lastname: Rose
authors.forename: Gideon
authors.initials: G
authors.lastname: Davies
mesh_terms: D002241:Carbohydrates / Q000737:chemistry; D002475:Cellobiose / Q000737:chemistry; D003013:Clostridium / Q000201:enzymology; D018360:Crystallography, X-Ray; D004247:DNA / Q000378:metabolism; D004187:Disaccharides / Q000737:chemistry; D004583:Electrons; D005643:Fucose / Q000737:chemistry*; D007700:Kinetics; D037102:Lectins / Q000737:chemistry*; D008024:Ligands; D008956:Models, Chemical; D008958:Models, Molecular; D009844:Oligosaccharides / Q000737:chemistry; D011134:Polysaccharides / Q000737:chemistry; D011485:Protein Binding; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D013816:Thermodynamics; D043325:Xylan Endo-1,3-beta-Xylosidase; D014995:Xylosidases / Q000737:chemistry
chemical_list: D002241:Carbohydrates; D004187:Disaccharides; D037102:Lectins; D008024:Ligands; D009844:Oligosaccharides; D011134:Polysaccharides; C014637:laminaribiose; D002475:Cellobiose; D005643:Fucose; D004247:DNA; D014995:Xylosidases; D043325:Xylan Endo-1,3-beta-Xylosidase