The extracellular calcium (Ca In this study, we examined whether conserved amino acid residues involved in the binding of glutamate to metabotropic glutamate receptors (mGluRs) also participate in the potentiation of the activity of CaR by L-phenylalanine. Ser-170 corresponding to Thr-188 in rat mGluR1a appears to be important for the modulating actions of phenylalanine. In the presence of phenylalanine, a mutant CaR with a single mutation S170A showed no significant decrease in its EC 50 for stimulation by Ca o metabolism. It has been documented that these two homeostatic systems are intimately related. For instance, a reduction in protein intake below the normal level results in secondary hyperparathyroidism in the context of normocalcemia (4), and high dietary protein intake induces elevated urinary calcium excretion (5, 6).The CaR is a G protein-coupled receptor in the same subfamily C as the metabotropic glutamate receptors, mGluRs1-8 (7-9). The family C receptors all possess unusually large 500 -600-residue extracellular amino-terminal domains (ECDs), possibly with a bilobed Venus flytrap structure similar to that found in bacterial periplasmic binding proteins for nutrients (10, 11). The family C receptors interact with their physiological agonists through their ECDs (11-13).For the rat homolog of mGluR1a, the key residues involved in the binding of glutamate to its ECD have been identified through the determination of its crystal structure when it is complexed with glutamate (11). Ser-164, Ser-165, Thr-188, and Glu-292 form hydrogen bonds with the ␣-carboxyl group in glutamate. Five residues, Ser-186, Thr-188, Asp-208, Tyr-236, and Asp-318, form additional hydrogen bonds with the ␣-amino group of glutamate. Of these residues, Ser-165, Thr-188, Asp-208, Tyr-236, and Asp-318 in the mGluR1a are conserved in the CaR, corresponding to Ser-147, Ser-170, Asp-190, Tyr-218, and Glu-297, respectively. The ␥-carboxyl group in glutamate forms one hydrogen bond each with Lys-409 and Arg-323 in mGluR1a; however, these two positively charged amino acid residues are not conserved in the CaR. Mutations S165A and T188A have been reported to impair glutamate binding and resultant activation of mGluR1a (10), whereas the functional significance of other residues in the glutamate binding site of the mGluRs have not been reported.Previous studies of the CaR have shown that substitutions of amino acids such as Ser-147 and Ser-170, which correspond to Ser-165 and Thr-188 in mGluR1a, markedly reduce activation of the receptor by Ca 2ϩ o (13). In addition, the mutations of