A different molecular interaction of bradykinin and the synthetic agonist FR190997 with the human B2 receptor: evidence from mutational analysis

Abstract

Binding affinity at the [3H]-BK binding site and activity as inositol phosphate (IP) production by the peptide bradykinin (BK) and the nonpeptide FR190997 were studied at wild-type or point-mutated human B2 receptors (hB2R) expressed in CHO cells. The effect of the following mutations were analyzed: E47A (TM1), W86A and T89A (TM2), I110A, L114A and S117A (TM3), T158A, M165T and L166F (TM4), T197A and S211A (TM5), F252A, W256A and F259A (TM6), S291A, F292A, Y295A and Y295F (TM7), and the double mutation W256A/Y295F. As the wild-type receptor-binding affinity of FR190997 was 40-fold lower than BK, whereas their agonist potency was comparable, both agonists produced similar maximal effects (Emax). Mutations were evaluated as affecting the affinity and/or efficacy of FR190997 compared with BK. Two mutations were found to impair the agonist affinity of both agonists drastically: W86A and F259A. BK agonist affinity (pEC50) was reduced by 1400- and 150-fold, and that of FR190997 was reduced by 400- and 25-fold, at the W86A and F259A mutant B2 receptors, respectively. Contrary to BK, the affinity of FR190997 was selectively decreased at I110A, Y295A, and Y295F mutants (>103-fold), and a different efficacy was measured at the Y295 mutants, FR190997 being devoid of the capability to trigger IP production at Y295A mutant. L114A, F252A, and W256A selectively impaired the efficacy of FR190997, whereas its binding affinity was not affected. As a consequence, FR190997 behaved as a high-affinity antagonist in blocking the IP production induced by BK. The lack of capability of FR190997 to activate or to bind the double mutant W256A/Y295F suggests that these residues are part of the same binding site, which is also important for receptor activation by the nonpeptide ligand. Overall, by means of mutational analysis, we indicate an hB2R recognition site for the nonpeptide agonist FR190997 (between TM3, 6, and 7), different from that of BK, and show that in the same binding crevice some mutations (L114, W256, and F252) are selectively responsible for the agonist properties of only FR190997.

Figure 1

Agonist concentration–response curves on IP hydrolysis in cells expressing the human wild-type B₂ receptor. Cells were incubated for 30 min at 37°C with BK and FR190997 in the concentration range from 0.1 nM to 1 μM, as described under Methods. Results are expressed as percentage of the basal (no added drugs) IP production. Each point represents the mean±s.e.m. of three independent experiments, each one performed in triplicate.

Figure 2

Cross-sectional schematic sight of the seven TM domains of the human B₂ receptor. The TM domains (large circles 1–7) bear the investigated amino-acid residues (small circles). Amino-acid residues that lie one helix turn down from each other are filled with grey color. Arrows indicate residues which when mutated affected the affinity of both BK and FR190997 (black); amino acids that are important determinants for FR190997 affinity (open), and those which when mutated reduce the receptor-activating properties of FR190997 or convert it into an antagonist (asterisk).