title: Crystal structure of neoculin: insights into its sweetness and taste-modifying activity.
abstract: Although the majority of sweet compounds are of low molecular mass, several proteins are known to elicit sweet taste responses in humans. The fruit of Curculigo latifolia contains a heterodimeric protein, neoculin, which has both sweetness and a taste-modifying activity that converts sourness to sweetness. Here, we report the crystal structure of neoculin at 2.76A resolution. This is the first well-defined tertiary structure of a taste-modifying protein of this kind. The overall structure is quite similar to those of monocot mannose-binding lectins. However, crucial topological differences are observed in the C-terminal regions of both subunits. In both subunits of neoculin, the C-terminal tails turn up to form loops fixed by inter-subunit disulfide bonds that are not observed in the lectins. Indeed, the corresponding regions of the lectins stretch straight over the surface of another subunit. Such a C-terminal structural feature as is observed in neoculin results in a decrease in subunit-subunit interactions. Moreover, distribution of electrostatic potential on the surface of neoculin is unique and significantly different from those of the lectins, particularly in the basic subunit (NBS). We have found that there is a large cluster composed of six basic residues on the surface of NBS, and speculate that it might be involved in the elicitation of sweetness and/or taste-modifying activity of neoculin. Molecular dynamics simulation based on the crystallography results suggests that neoculin may adopt a widely "open" conformation at acidic pH, while unprotonated neoculin at neutral pH is in a "closed" conformation. Based on these simulations and the generation of a docking model between neoculin and the sweet-taste receptor, T1R2-T1R3, we propose the hypothesis that neoculin is in dynamic equilibrium between open and closed states, and that the addition of an acid shifts the equilibrium to the open state, allowing ligand-receptor interaction.
authors.affiliation: Department of Applied Biological Chemistry, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Bunkyo-ku, Japan.
authors.forename: Akiko
authors.initials: A
authors.lastname: Shimizu-Ibuka
authors.forename: Yuji
authors.initials: Y
authors.lastname: Morita
authors.forename: Tohru
authors.initials: T
authors.lastname: Terada
authors.forename: Tomiko
authors.initials: T
authors.lastname: Asakura
authors.forename: Ken-ichiro
authors.initials: K
authors.lastname: Nakajima
authors.forename: So
authors.initials: S
authors.lastname: Iwata
authors.forename: Takumi
authors.initials: T
authors.lastname: Misaka
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Sorimachi
authors.forename: Soichi
authors.initials: S
authors.lastname: Arai
authors.forename: Keiko
authors.initials: K
authors.lastname: Abe
mesh_terms: D000595:Amino Acid Sequence; D018360:Crystallography, X-Ray; D031408:Curculigo / Q000737:chemistry*; D019281:Dimerization; D019143:Evolution, Molecular; D006801:Humans; D006863:Hydrogen-Ion Concentration; D037601:Mannose-Binding Lectin / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010940:Plant Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D020836:Protein Structure, Quaternary; D017434:Protein Structure, Tertiary; D021122:Protein Subunits / Q000737:chemistry / Q000378:metabolism; D016415:Sequence Alignment; D055672:Static Electricity; D013549:Sweetening Agents / Q000737:chemistry / Q000378:metabolism; D013649:Taste
chemical_list: D037601:Mannose-Binding Lectin; D010940:Plant Proteins; D021122:Protein Subunits; D013549:Sweetening Agents; C511659:neoculin protein, Curculigo latifolia