The docking of Arg² of angiotensin II with Asp²⁸¹ of AT¹ receptor is essential for full agonism

The structural model of AT₁ angiotensin receptor contains seven-transmembrane α-helices with three interhelical loops on either side of the membrane. The angiotensin II binding pocket within the receptor is not clearly defined. We showed earlier that Lys¹⁹⁹ in transmembrane-helix-5 of the AT₁ receptor binds the COOH-terminal α-carboxyl group of angiotensin II (Noda, K., Saad, Y., Kinoshita, A., Boyle, T. P., Graham, R. M., Husain, A., and Karnik, S. S. (1995) J. Biol. Chem. 270, 2284-2289). We now show that His¹⁸³ and Asp²⁸¹, both located in the extracellular domain of the AT₁ receptor, are involved in binding the NH₂-terminal Asp¹ and Arg² residues of angiotensin II, respectively. The ASp¹/His¹⁸³ interaction appears to be weak and is unlikely to be important for agonism. But the loss of Arg²/Asp²⁸¹ interaction leads to partial agonism of the receptor. The action of non-peptide agonists is not affected by Asp²⁸¹ mutations. These results suggest that several independent interactions between angiotensin II and AT₁ receptor are necessary for full agonism. Since L-162,313 the non-peptide agonist of the AT₁ receptor is a partial agonist that does not make contact with Asp²⁸¹, we speculate that the degree of agonism may be increased if it is redesigned to make contacts with Asp²⁸¹.