A noninternalized nondesensitized truncated AT1A receptor transduces an amplified ANG II signal

The structural determinants of the rat angiotensin (ANG) II AT1A receptor involved in receptor internalization, desensitization, and activation are investigated by producing six mutants that had progessively larger deletions of the cytoplasmic tail (−13, −19, −24, −31, −46, and −56 residues, respectively). After stable transfection of the cDNAs into Chinese hamster ovary cells, all mutants, except the most truncated, exhibit normal [Sar1]ANG II affinities [dissociation constant ( K d) = 0.19–0.70 nM] compared with the wild-type (WT) receptor ( K d = 0.62 nM) and are able to activate a Gq/11protein and a phospholipase C as measured by the ANG II-induced inositol phosphate (IP) turnover in the different clones. However, one of these mutants, Δ329 (deletion of 31 residues), exhibits a peculiar phenotype. This mutant shows a reduced ligand-induced internalization as measured by the acid-washing procedure (only 32% of receptors are internalized vs. 83% for WT). Moreover, the Δ329 mutant is less desensitized by a pretreatment with either ANG II (15% desensitization of ANG II-stimulated IP turnover vs. 60% for WT receptor) or the phorbol ester phorbol 12-myristate 13-acetate (no desensitization vs. 29% for WT receptor). These functional modifications of the Δ329 mutant are associated with the transduction of an amplified signal as demonstrated on both IP turnover and an integrated physiological effect of ANG II. Taken together, these data indicate that the sequence329SLSTKMS335of the rat AT1A receptor is involved in both receptor internalization and desensitization. This is the first demonstration that a desensitization- and internalization-defective AT1Areceptor mutant is also hyperreactive and mediates augmented cellular responses.