MUTATIONS IN TRANSMEMBRANE SEGMENT-VII OF THE AT(1) RECEPTOR DIFFERENTIATE BETWEEN CLOSELY-RELATED INSURMOUNTABLE AND COMPETITIVE ANGIOTENSIN ANTAGONISTS

Chimeric constructs between the human and the Xenopus laevis AT(1) rec eptor have demonstrated, that the binding of non-peptide angiotensin a ntagonists is dependent on non-conserved residues located deep in tran smembrane segment VII of the AT(1) receptor. Here we have studied four pairs of closely related antagonists each consisting of a competitive and an insurmountable compound differentiated by one out of three dif ferent types of minor chemical modifications. None of the antagonists bound to the Xenopus receptor and the binding of all of the compounds to the human receptor was severely impaired by the introduction of non -conserved residues from transmembrane segment VII of the Xenopus rece ptor. In all four pairs of antagonists the competitive compound was af fected more by these substitutions than the corresponding insurmountab le one (209 vs. 22, 281 vs. 29, 290 vs. 29 and 992 vs. 325-fold increa se in K-i values). A similar pattern was observed in response to subst itution of a single non-conserved residue in transmembrane segment VII , Asn(295) to Ser. These results indicate that a common molecular mech anism distinguishes the interaction of insurmountable and competitive antagonists with the AT(1) receptor.