title: Crystal structure of a symbiosis-related lectin from octocoral.
abstract: D-Galactose-binding lectin from the octocoral, Sinularia lochmodes (SLL-2), distributes densely on the cell surface of microalgae, Symbiodinium sp., an endosymbiotic dinoflagellate of the coral, and is also shown to be a chemical cue that transforms dinoflagellate into a non-motile (coccoid) symbiotic state. SLL-2 binds with high affinity to the Forssman antigen (N-acetylgalactosamine(GalNAc)α1-3GalNAcβ1-3Galα1-4Galβ1-4Glc-ceramide), and the presence of Forssman antigen-like sugar on the surface of Symbiodinium CS-156 cells was previously confirmed. Here we report the crystal structures of SLL-2 and its GalNAc complex as the first crystal structures of a lectin involved in the symbiosis between coral and dinoflagellate. N-Linked sugar chains and a galactose derivative binding site common to H-type lectins were observed in each monomer of the hexameric SLL-2 crystal structure. In addition, unique sugar-binding site-like regions were identified at the top and bottom of the hexameric SLL-2 structure. These structural features suggest a possible binding mode between SLL-2 and Forssman antigen-like pentasaccharide.
authors.affiliation: Research Reactor Institute, Kyoto University, Kumatori, Sennan, Osaka 590-0494, Japan kita@rri.kyoto-u.ac.jp miki@kuchem.kyoto-u.ac.jp.
authors.forename: Akiko
authors.initials: A
authors.lastname: Kita
authors.affiliation: School of Marine Biosciences, Kitasato University, 1-15-1, Kitasato, Minami, Sagamihara, Kanagawa 252-0373, Japan.
authors.forename: Mitsuru
authors.initials: M
authors.lastname: Jimbo
authors.affiliation: Faculty of Fisheries Sciences, Hokkaido University, Hakodate, Hokkaido 041-8611, Japan.
authors.forename: Ryuichi
authors.initials: R
authors.lastname: Sakai
authors.affiliation: Research Reactor Institute, Kyoto University, Kumatori, Sennan, Osaka 590-0494, Japan.
authors.forename: Yukio
authors.initials: Y
authors.lastname: Morimoto
authors.affiliation: Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan kita@rri.kyoto-u.ac.jp miki@kuchem.kyoto-u.ac.jp.
authors.forename: Kunio
authors.initials: K
authors.lastname: Miki
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D003063:Cnidaria / Q000737:chemistry; D004141:Dinoflagellida / Q000737:chemistry; D008969:Molecular Sequence Data; D009004:Monosaccharide Transport Proteins / Q000737:chemistry*; D013559:Symbiosis
chemical_list: D009004:Monosaccharide Transport Proteins; C415124:SLL-2 protein, Sinularia lochmodes
keywords: SLL-2; coral lectin; coral-algae symbiosis; crystal structure; signal transduction