Research in polymorphisms in human taste receptor genes and taste disorder

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T2Rs comprise a G-protein-coupled receptor superfamily that contains functionally defined bitter taste receptors. We found coding single-nucleotide polymorphisms (cSNPs) in human T2R genes (hT2R3, hT2R4, and hT2R5) on chromosome 7q31. We identified six cSNPs within the T2R receptor genes. The hT2R4 and hT2R5 contained four and on cSNPs that cause missense mutations, respectively, while hT2R3 included one silent nucleotide mutation. However, we could not find any nonsense mutations that resulted in a frameshift or a premature stop codon within the open reading frames. T2R receptors and a G-protein α subunit (Gα), gustducin, are believed to be key molecules for its perception, but little is known about the molecular basis for its interaction. We use a heterologous expression system to determine a specific domain of gustducin necessary for T2R coupling. Two chimeric Gα16 proteins harboring 37 and 44 gustducin-specific sequences at their C termini (G16/gust37 and G16/gust44) responded to different T2R receptors with known ligands, but G16/gust23, G16/gust11, and G16/gust5 did not. The former two chimeras contained a predicted β6 sheet, an α5 helix, and an extreme C terminus of gustducin, and all the domains were indispensable to the expression of T2R activity. We also expressed G16 protein chimeras with the corresponding domain from other Gαi proteins, cone-transducin (Gαt2), Gαi2, and Gαz (G16/t2, G16/i2, and G16/z). As a result, G16/t2 and G16/i2 produced specific responses of T2Rs, but G16/z did not. Using Gα16-based chimeras and hT2R16 variants, we examined sensitivity to salicin. Identification of nucleotide diversity and amino acid polymorphisms in human T2R receptors could help clarify individual differences in the acceptability and sensitivity to bitter compounds.