Functional characterization of the ligand binding domain of the cat T1R1 taste receptor

The umami taste receptor is a heterodimeric G-protein coupled receptor (GPCR), composed of two subunits called T1R1 and T1R3. Both subunits are class C GPCRs whose members share a common architecture composed of a large N-terminal domain (NTD) connected to a heptahelical transmembrane domain by a short cysteine-rich domain. Cellular assays combined with molecular docking and site-directed mutagenesis studies have revealed that the NTD of the T1R1 subunit contains the primary binding site for umami stimuli, such as L-glutamate (L-Glu) for humans. Inosine-5’-monophosphate (IMP) binds close to the opening of the NTD, responsible for the characteristic umami synergy between L-Glu and IMP. Functional cellular assays of the T1R1/T1R3 receptor have revealed species-dependent differences. Therefore, the human umami receptor responds specifically to L-Glu and L-Asp whereas mouse T1R1/T1R3 detects various amino acids. Because cats have a strictly carnivorous diet, they have strong preferences for amino acids. To better understand the structural basis of umami stimuli recognition, we measured the binding of selected amino acids to cat T1R1 (cT1R1). For this purpose, we expressed cT1R1-NTD in bacteria. Ligand binding quantified by intrinsic tryptophan fluorescence showed that cT1R1-NTD is capable of binding several L-amino acids with affinities in the micromolar range with enhancement activity of IMP. Our study demonstrates the feasibility to produce large amounts of recombinant cT1R1-NTD for structural studies.