Two distinct determinants of ligand specificity in T1R1/T1R3 (the umami taste receptor).
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.
From the Department of Applied Biological Chemistry, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo 113-8657, Japan.
Yasuka
Y
Toda
Tomoya
T
Nakagita
Takashi
T
Hayakawa
Shinji
S
Okada
Masataka
M
Narukawa
Hiroo
H
Imai
Yoshiro
Y
Ishimaru
Takumi
T
Misaka
D000818:Animals; D002460:Cell Line; D000882:Haplorhini; D006801:Humans; D007291:Inosine Monophosphate / Q000235:genetics / Q000378:metabolism; D008024:Ligands; D051379:Mice; D016297:Mutagenesis, Site-Directed; D017354:Point Mutation; D017434:Protein Structure, Tertiary; D043562:Receptors, G-Protein-Coupled / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D013045:Species Specificity
D008024:Ligands; D043562:Receptors, G-Protein-Coupled; C431926:taste receptors, type 1; D007291:Inosine Monophosphate
Amino Acid; G Protein-coupled Receptors (GPCR); Glutamate Receptors Metabotropic; Mutagenesis in Vitro; Receptor Structure-Function; Taste Receptor; Umami