title: Crystal structure at 3 A of mistletoe lectin I, a dimeric type-II ribosome-inactivating protein, complexed with galactose.
abstract: The X-ray structure of mistletoe lectin I (MLI), a type-II ribosome-inactivating protein (RIP), cocrystallized with galactose is described. The model was refined at 3.0 A resolution to an R-factor of 19.9% using 21 899 reflections, with Rfree 24.0%. MLI forms a homodimer (A-B)2 in the crystal, as it does in solution at high concentration. The dimer is formed through contacts between the N-terminal domains of two B-chains involving weak polar and non-polar interactions. Consequently, the overall arrangement of sugar-binding sites in MLI differs from those in monomeric type-II RIPs: two N-terminal sugar-binding sites are 15 A apart on one side of the dimer, and two C-terminal sugar-binding sites are 87 A apart on the other side. Galactose binding is achieved by common hydrogen bonds for the two binding sites via hydroxy groups 3-OH and 4-OH and hydrophobic contact by an aromatic ring. In addition, at the N-terminal site 2-OH forms hydrogen bonds with Asp27 and Lys41, and at the C-terminal site 3-OH and 6-OH undergo water-mediated interactions and C5 has a hydrophobic contact. MLI is a galactose-specific lectin and shows little affinity for N-acetylgalactosamine. The reason for this is discussed. Structural differences among the RIPs investigated in this study (their quaternary structures, location of sugar-binding sites, and fine sugar specificities of their B-chains, which could have diverged through evolution from a two-domain protein) may affect the binding sites, and consequently the cellular transport processes and biological responses of these toxins.
authors.affiliation: School of Crystallography, Birkbeck College, University of London, Malet Street, London WC1E 7HX, UK. h.niwa@mail.cryst.bbk.ac.uk
authors.forename: Hideaki
authors.initials: H
authors.lastname: Niwa
authors.forename: Alexander G
authors.initials: AG
authors.lastname: Tonevitsky
authors.forename: Igor I
authors.initials: II
authors.lastname: Agapov
authors.forename: Steve
authors.initials: S
authors.lastname: Saward
authors.forename: Uwe
authors.initials: U
authors.lastname: Pfüller
authors.forename: Rex A
authors.initials: RA
authors.lastname: Palmer
mesh_terms: D000276:Adjuvants, Immunologic / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D000595:Amino Acid Sequence; D001665:Binding Sites; D018360:Crystallography, X-Ray; D019281:Dimerization; D005690:Galactose / Q000737:chemistry* / Q000378:metabolism; D006860:Hydrogen Bonding; D014784:Mistletoe / Q000737:chemistry*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D037121:Plant Lectins / Q000737:chemistry / Q000378:metabolism; D028321:Plant Preparations / Q000737:chemistry* / Q000378:metabolism; D010940:Plant Proteins; D011487:Protein Conformation; D054790:Ribosome Inactivating Proteins, Type 2; D016415:Sequence Alignment; D014118:Toxins, Biological / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism
chemical_list: D000276:Adjuvants, Immunologic; D037121:Plant Lectins; D028321:Plant Preparations; D010940:Plant Proteins; D054790:Ribosome Inactivating Proteins, Type 2; D014118:Toxins, Biological; C009674:ribosome inactivating protein, Viscum; D005690:Galactose