title: Acid-induced sweetness of neoculin is ascribed to its pH-dependent agonistic-antagonistic interaction with human sweet taste receptor.
abstract: Neoculin (NCL) is a sweet protein that also has taste-modifying activity to convert sourness to sweetness. However, it has been unclear how NCL induces this unique sensation. Here we quantitatively evaluated the pH-dependent acid-induced sweetness of NCL using a cell-based assay system. The human sweet taste receptor, hT1R2-hT1R3, was functionally expressed in HEK293T cells together with G alpha protein. When NCL was applied to the cells under different pH conditions, it activated hT1R2-hT1R3 in a pH-dependent manner as the condition changed from pH 8 to 5. The pH-response sigmoidal curve resembled the imidazole titration curve, suggesting that His residues were involved in the taste-modifying activity. We then constructed an NCL variant in which all His residues were replaced with Ala and found that the variant elicited strong sweetness at neutral pH as well as at acidic pH. Since NCL and the variant elicited weak and strong sweetness at the same neutral pH, respectively, we applied different proportions of NCL-variant mixtures to the cells at this pH. As a result, NCL competitively inhibits the variant-induced receptor activation. All these data suggest that NCL acts as an hT1R2-hT1R3 agonist at acidic pH but functionally changes into its antagonist at neutral pH.
authors.affiliation: Department of Applied Biological Chemistry, Graduate School of Agricultural and Life Sciences, the University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan.
authors.forename: Ken-ichiro
authors.initials: K
authors.lastname: Nakajima
authors.forename: Yuji
authors.initials: Y
authors.lastname: Morita
authors.forename: Ayako
authors.initials: A
authors.lastname: Koizumi
authors.forename: Tomiko
authors.initials: T
authors.lastname: Asakura
authors.forename: Tohru
authors.initials: T
authors.lastname: Terada
authors.forename: Keisuke
authors.initials: K
authors.lastname: Ito
authors.forename: Akiko
authors.initials: A
authors.lastname: Shimizu-Ibuka
authors.forename: Jun-ichi
authors.initials: J
authors.lastname: Maruyama
authors.forename: Katsuhiko
authors.initials: K
authors.lastname: Kitamoto
authors.forename: Takumi
authors.initials: T
authors.lastname: Misaka
authors.forename: Keiko
authors.initials: K
authors.lastname: Abe
mesh_terms: D001218:Aspartame; D031408:Curculigo; D006801:Humans; D006863:Hydrogen-Ion Concentration; D010940:Plant Proteins / Q000737:chemistry*; D013549:Sweetening Agents; D013649:Taste / Q000502:physiology*; D013650:Taste Buds / Q000502:physiology
chemical_list: D010940:Plant Proteins; D013549:Sweetening Agents; C511659:neoculin protein, Curculigo latifolia; D001218:Aspartame