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Although mutant receptors are highly useful to dissect the signal transduction pathways of receptors, they are difficult to study in physiological target tissues, due to the presence of endogenous receptors. To study AT1 angiotensin receptors in their physiological environment, we constructed a mutant receptor, which differs only from the AT1A receptor in its reduced affinity for candesartan, a biphenylimidazole antagonist. We have determined that the conserved S109Y substitution of the rat AT1A receptor eliminates its candesartan binding, without exerting any major effect on its angiotensin II and peptide angiotensin receptor antagonist binding, internalization kinetics, β-arrestin binding, and potency or efficacy of the inositol phosphate response. To demonstrate the usefulness of this mutant receptor in signal transduction studies, we combined it with substitution of the highly conserved DRY sequence with AAY, which abolishes G protein activation. In rat C9 hepatocytes the S109Y receptor caused ERK activation with the same mechanism as the endogenous AT1 receptor. After combination with the DRY/AAY mutation G protein-independent ERK activation was detected demonstrating that this approach can be used to study the angiotensin II-stimulated signaling pathways in cells endogenously expressing AT1 receptors.