Figure 3: Non-specific amino acid recognition by T1r3LBD.

(a) Close-up view of the T1r3 ligand-binding site of the L-glutamine-bound structure. Simulated annealing-omit electron density map (3.0 σ) is also shown. The residues at LB2 are underlined. (b) The ligand-binding pockets observed on T1r3, with the electrostatic potential at±20 kT e⁻¹ mapped on the surfaces. (c) Responses of the T1r2a–T1r3:S300E mutant to various amino acids, monitored as an elevation of intracellular Ca²⁺ concentration. Data points represent mean and s.e.m. of 11, 12 and 6 technical replicates for the Gln, Arg and Ala responses, respectively. (d) The ΔG difference (ΔΔG) between L-glutamine and other amino-acid binding of T1rLBD. The ΔΔG values estimated by FRET (Fig. 2a and Supplementary Table 2) were plotted on those estimated by the structures of T1r2a (magenta diamonds) and T1r3 (cyan triangles) binding sites.