title: Identification and modulation of the key amino acid residue responsible for the pH sensitivity of neoculin, a taste-modifying protein.
abstract: Neoculin occurring in the tropical fruit of Curculigo latifolia is currently the only protein that possesses both a sweet taste and a taste-modifying activity of converting sourness into sweetness. Structurally, this protein is a heterodimer consisting of a neoculin acidic subunit (NAS) and a neoculin basic subunit (NBS). Recently, we found that a neoculin variant in which all five histidine residues are replaced with alanine elicits intense sweetness at both neutral and acidic pH but has no taste-modifying activity. To identify the critical histidine residue(s) responsible for this activity, we produced a series of His-to-Ala neoculin variants and evaluated their sweetness levels using cell-based calcium imaging and a human sensory test. Our results suggest that NBS His11 functions as a primary pH sensor for neoculin to elicit taste modification. Neoculin variants with substitutions other than His-to-Ala were further analyzed to clarify the role of the NBS position 11 in the taste-modifying activity. We found that the aromatic character of the amino acid side chain is necessary to elicit the pH-dependent sweetness. Interestingly, since the His-to-Tyr variant is a novel taste-modifying protein with alternative pH sensitivity, the position 11 in NBS can be critical to modulate the pH-dependent activity of neoculin. These findings are important for understanding the pH-sensitive functional changes in proteinaceous ligands in general and the interaction of taste receptor-taste substance in particular.
authors.affiliation: Department of Applied Biological Chemistry, The University of Tokyo, Tokyo, Japan.
authors.forename: Ken-ichiro
authors.initials: K
authors.lastname: Nakajima
authors.forename: Kanako
authors.initials: K
authors.lastname: Yokoyama
authors.forename: Taichi
authors.initials: T
authors.lastname: Koizumi
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: Katsuyoshi
authors.initials: K
authors.lastname: Masuda
authors.forename: Keisuke
authors.initials: K
authors.lastname: Ito
authors.forename: Akiko
authors.initials: A
authors.lastname: Shimizu-Ibuka
authors.forename: Takumi
authors.initials: T
authors.lastname: Misaka
authors.forename: Keiko
authors.initials: K
authors.lastname: Abe
mesh_terms: D000596:Amino Acids / Q000737:chemistry*; D000818:Animals; D001236:Aspergillus oryzae / Q000378:metabolism; D002118:Calcium / Q000737:chemistry; D002460:Cell Line; D031408:Curculigo; D004305:Dose-Response Relationship, Drug; D014644:Genetic Variation; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008958:Models, Molecular; D010940:Plant Proteins / Q000737:chemistry* / Q000378:metabolism; D017510:Protein Folding; D017434:Protein Structure, Tertiary; D013649:Taste
chemical_list: D000596:Amino Acids; D010940:Plant Proteins; C511659:neoculin protein, Curculigo latifolia; D002118:Calcium