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

The structural determinants of the rat angiotensin (ANG) II AT(1A) 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 [Sar(1)]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 G(q/11),ill protein and a phospholipase C as measured by the ANG II-induced inositol phosphate (IF) turnover in the different clones. However, one of these mutants, Delta 329 (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 Delta 329 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 Delta 329 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 sequence (SLSTKMS335)-S-329 of the rat AT(1A) receptor is involved in both receptor internalization and desensitization. This is the first demonstration that a desensitization-and internalization-defective AT(1A) receptor mutant is also hyper-reactive and mediates augmented cellular responses.