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This Article Full Text Full Text (PDF) All Versions of this Article: fj.07-100289v1 22/7/2323    most recent Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Google Scholar Articles by Nakajima, K.-i. Articles by Abe, K. PubMed PubMed Citation Articles by Nakajima, K.-i. Articles by Abe, K.

Acid-induced sweetness of neoculin is ascribed to its pH-dependent agonistic-antagonistic interaction with human sweet taste receptor

Ken-ichiro Nakajima^*, Yuji Morita^*, Ayako Koizumi^*, Tomiko Asakura^*, Tohru Terada^*,, Keisuke Ito^*, Akiko Shimizu-Ibuka, Jun-ichi Maruyama, Katsuhiko Kitamoto, Takumi Misaka^* and Keiko Abe^*,,1

^* Department of Applied Biological Chemistry,

Agricultural Bioinformatics Research Unit, and

Department of Applied Biotechnology, Graduate School of Agricultural and Life Sciences, University of Tokyo, Tokyo, Japan; and

Department of Nutritional Science, Tokyo University of Agriculture, Tokyo, Japan

¹Correspondence: 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. E-mail: aka7308{at}mail.ecc.u-tokyo.ac.jp

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 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.—Nakajima, K., Morita, Y., Koizumi, A., Asakura, T., Terada, T., Ito, K., Shimizu-Ibuka, A., Maruyama, J., Kitamoto, K., Misaka, T., Abe, K. Acid-induced sweetness of neoculin is ascribed to its pH-dependent agonistic-antagonistic interaction with human sweet taste receptor.

Key Words: G protein-coupled receptors • histidine • sweet protein