DIFFERENTIATION BETWEEN BINDING-SITES FOR ANGIOTENSIN-II AND NONPEPTIDE ANTAGONISTS ON THE ANGIOTENSIN-II TYPE-1 RECEPTORS

To characterize binding sites for nonpeptide angiotensin antagonists on the human angiotensin II receptor type 1 (AT(1) receptor) we have systematically exchanged segments of the human receptor with corresponding segments from a homologous Xenopus laevis receptor, which does not bind the nonpeptide compounds. Substitution of transmembrane segment VII of the human AT(1) receptor dramatically reduced the binding affinity of all of the 11 nonpeptide antagonists tested (55- to >2000-fold) with no effect on the binding of angiotensin. The affinity for the nonpeptide compounds decreased additionally one order of magnitude when transmembrane segment VI and the connecting extracellular loop 3 from the Xenopus receptor were also introduced into the human AT(1) receptor. Exchanges of smaller segments and single residues in transmembrane segments VI and VII and extracellular loop 3 revealed that the binding of nonpeptide antagonists was dependent on nonconserved residues located deep within the transmembrane segments VI and VII, in particular Asn(295) in transmembrane segment VII. Surprisingly, all exchanges in transmembrane segment VII, including the Asn(295) to Ser substitution, had a more pronounced effect on the binding of the competitive antagonists relative to the insurmountable antagonists. It is concluded that the binding mode for peptide and nonpeptide ligands an the ATI receptor is rather different and that competitive and insurmountable antagonists presumably bind to overlapping but distinct sites located in transmembrane segments VI and VII.