RESUMO
The T1R receptors, a family of taste-specific class C G protein-coupled receptors, mediate mammalian sweet and umami tastes. The structure-function relationships of T1R receptors remain largely unknown. In this study, we demonstrate the different functional roles of T1R extracellular and transmembrane domains in ligand recognition and G protein coupling. Similar to other family C G protein-coupled receptors, the N-terminal Venus flytrap domain of T1R2 is required for recognizing sweeteners, such as aspartame and neotame. The G protein coupling requires the transmembrane domain of T1R2. Surprisingly, the C-terminal transmembrane domain of T1R3 is required for recognizing sweetener cyclamate and sweet taste inhibitor lactisole. Because T1R3 is the common subunit in the sweet taste receptor and the umami taste receptor, we tested the interaction of lactisole and cyclamate with the umami taste receptor. Lactisole inhibits the activity of the human T1R1/T1R3 receptor, and, as predicted, blocked the umami taste of l-glutamate in human taste tests. Cyclamate does not activate the T1R1/T1R3 receptor by itself, but potentiates the receptor's response to l-glutamate. Taken together, these findings demonstrate the different functional roles of T1R3 and T1R2 and the presence of multiple ligand binding sites on the sweet taste receptor.
Assuntos
Subunidades Proteicas/fisiologia , Receptores Acoplados a Proteínas G/fisiologia , Papilas Gustativas/fisiologia , Sequência de Aminoácidos , Animais , Aspartame/farmacologia , Derivados de Benzeno/farmacologia , Sítios de Ligação , Linhagem Celular , Ciclamatos/farmacologia , Relação Dose-Resposta a Droga , Aromatizantes/farmacologia , Glutamatos/farmacologia , Proteínas Heterotriméricas de Ligação ao GTP/antagonistas & inibidores , Proteínas Heterotriméricas de Ligação ao GTP/química , Proteínas Heterotriméricas de Ligação ao GTP/fisiologia , Humanos , Dados de Sequência Molecular , Mutação Puntual , Estrutura Terciária de Proteína , Ratos , Receptores Acoplados a Proteínas G/antagonistas & inibidores , Receptores Acoplados a Proteínas G/química , Proteínas Recombinantes/química , Proteínas Recombinantes/genética , Proteínas Recombinantes/metabolismo , Alinhamento de Sequência , Glutamato de Sódio/farmacologia , Relação Estrutura-Atividade , Sacarose/farmacologia , Paladar/efeitos dos fármacos , Paladar/fisiologia , TransfecçãoRESUMO
The three members of the T1R class of taste-specific G protein-coupled receptors have been hypothesized to function in combination as heterodimeric sweet taste receptors. Here we show that human T1R2/T1R3 recognizes diverse natural and synthetic sweeteners. In contrast, human T1R1/T1R3 responds to the umami taste stimulus l-glutamate, and this response is enhanced by 5'-ribonucleotides, a hallmark of umami taste. The ligand specificities of rat T1R2/T1R3 and T1R1/T1R3 correspond to those of their human counterparts. These findings implicate the T1Rs in umami taste and suggest that sweet and umami taste receptors share a common subunit.
