Several interesting phenotypes of the AT1 receptor produced by site-directed mutagenesis.

The angiotensin II (AngII) AT1 receptor is a seven-transmembrane domain receptor coupled to a Gq/11 protein and phospholipase C, but also to other G proteins and to several tyrosine kinase pathways. These signaling pathways transduce inside the cells the classical actions of AngII (vasoconstriction, aldosterone secretion, etc.), but also the mitogenic action of this vasoactive peptide. In the past 5 yr, site-directed mutagenesis has elucidated the molecular determinants of the AngII and nonpeptidic analogue-binding sites together with those of G protein interaction. In addition, these studies have demonstrated that modifications of the specific interactions between transmembrane domains are responsible for the activation of the receptor. Therefore, several mutations of these domains are able to block the receptor in active or inactive states. Finally, these mutagenesis studies identify two interesting phenotypes of the AT1 receptor. (1) A carboxy-terminal truncation of the AT1 receptor produces a mutant that is unable to be internalized and desensitized and therefore is functionally hyper-reactive. (2) A replacement of the distal part of the third intracellular loop of the AT1 receptor by the homologous segment of the beta2-adrenergic receptor produces a mutant coupled to both Gq and Gs proteins, which is unable to transduce the mitogenic action of AngII.