Recent progress in the molecular biology of taste reception has revealed that in mammals, the heteromeric receptors T1R1/3 and T1R2/3 respond to amino acids and sweeteners, respectively, whereas T2Rs are receptors for bitter tastants. Similar taste receptors have also been characterized in fish, but their ligands have not been identified yet. In the present study, we conducted a series of experiments to identify the fish taste receptor ligands. Facial nerve recordings in zebrafish (Danio rerio) demonstrated that the fish perceived amino acids and even denatonium, which is a representative of aversive bitter compounds for mammals andDrosophila. Calcium imaging analysis of T1Rs in zebrafish and medaka fish (Oryzias latipes) using an HEK293T heterologous expression system revealed that both T1R1/3 and a series of T1R2/3 responded to amino acids but not to sugars. A triple-labeling,in situhybridization analysis demonstrated that cells expressing T1R1/3 and T1R2/3s exist in PLCβ2-expressing taste bud cells of medaka fish. Functional analysis using T2Rs showed that zfT2R5 and mfT2R1 responded to denatonium. Behavior observations confirmed that zebrafish prefer amino acids and avoid denatonium. These results suggest that, although there may be some fish-specific way of discriminating ligands, vertebrates could have a conserved gustatory mechanism by which T1Rs and T2Rs respond to attractive and aversive tastants, respectively.

Umami taste perception in mammals is mediated by a heteromeric complex of two G-protein-coupled receptors, T1R1 and T1R3. T1R1/T1R3 exhibits species-dependent differences in ligand specificity; human T1R1/T1R3 specifically responds to l-Glu, whereas mouse T1R1/T1R3 responds more strongly to other l-amino acids than to l-Glu. The mechanism underlying this species difference remains unknown. In this study we analyzed chimeric human-mouse receptors and point mutants of T1R1/T1R3 and identified 12 key residues that modulate amino acid recognition in the human- and mouse-type responses in the extracellular Venus flytrap domain of T1R1. Molecular modeling revealed that the residues critical for human-type acidic amino acid recognition were located at the orthosteric ligand binding site. In contrast, all of the key residues for the mouse-type broad response were located at regions outside of both the orthosteric ligand binding site and the allosteric binding site for inosine-5′-monophosphate (IMP), a known natural umami taste enhancer. Site-directed mutagenesis demonstrated that the newly identified key residues for the mouse-type responses modulated receptor activity in a manner distinct from that of the allosteric modulation via IMP. Analyses of multiple point mutants suggested that the combination of two distinct determinants, amino acid selectivity at the orthosteric site and receptor activity modulation at the non-orthosteric sites, may mediate the ligand specificity of T1R1/T1R3. This hypothesis was supported by the results of studies using nonhuman primate T1R1 receptors. A complex molecular mechanism involving changes in the properties of both the orthosteric and non-orthosteric sites of T1R1 underlies the determination of ligand specificity in mammalian T1R1/T1R3.