Thaumatin is an intensely sweet-tasting protein perceived by humans but not rodents. Its threshold value of sweetness in humans is 50nM, the lowest of any sweet-tasting protein. In the present study, the sites where sweet receptors interact with thaumatin were investigated using human embryonic kidney 293 (HEK293) cells expressing the sweet receptors T1R2-T1R3. Chimeric human- mouse sweet receptors were constructed and their responses to sweeteners were investigated. The human (h) T1R2- mouse (m) T1R3 combination responded to sucralose but not to thaumatin, clearly indicating that a T1R3 subunit from humans is necessary for the interaction with thaumatin. Furthermore, results obtained from using chimeric T1R3s showed that the cysteine-rich domain (CRD) of human T1R3 is important for the interaction with thaumatin. The CRD of T1R3 would be a prominent target for designing new sweeteners.
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