Design of taste-modifying molecules based on the analysis of the structure and function of taste-modifying protein, curculin.

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Curculin isolated from Curculigo latifolia, a plant grown in Malaysia, has an intriguing property of modifying sour taste into sweet taste. In addition to this taste-modifying activity, curculin itself elicits a sweet taste. Although these activities have been attributed to the heterodimeric isoform and not homodimers of curculin, the underlying mechanisms for the dual action of this protein have been largely unknown.To identify critical sites for these activities, we performed a mutational and structural study of recombinant curculin. Based on the comparison of crystal structures of curculin homo- and heterodimers, a series of mutants was designed and subjected to tasting assays. Mapping of amino acid residues on the three-dimensional structure according to their mutational effects revealed that the curculin heterodimer exhibits sweet-tasting and taste-modifying activities through its partially overlapping but distinct molecular surfaces. These findings suggest-that the two activities of the curculin heterodimer are expressed through its two different modes of interactions with the T1R2-T1R3 heterodimeric sweet taste receptor.The site responsible for the taste-modifying activity includes His-36 in the curculin2 subunit. It is possible that the protonation of this histidine residue triggers the activation of the T1R2-T1R3-mediated signal transduction. Supporting this idea, the pH dependence of the taste-modifying activity of the curculin1-2 heterodimer was in good agreement with that of the protonation of His-36 of the curculin2 subunit examined by NMR spectroscopy.