Single mutations at Asn295 and Leu305 in the cytoplasmic half of transmembrane α-helix domain 7 of the AT1 receptor induce promiscuous agonist specificity for angiotensin II fragments:: A pseudo-constitutive activity

The most striking feature of a G protein-coupled receptor ( GPCR) is its highly exclusive agonist specificity. This feature guarantees that a GPCR recognizes only its specific native agonist(s). In this study, we showed that two point mutations of N295S and L305Q enabled the AT(1) receptors to recognize multiple Ang II fragments. Similar to the well established constitutively active AT(1) mutant receptor N111G, the mutations of N295S and L305Q induced an increased production of basal inositol 1,4,5-phosphates in the absence of exogenous Ang II when expressed in HEK293 cells. Distinct from the N111G, however, is the fact that the increased basal activity disappeared in COS-7 cells because of the lack of endogenous Ang II fragments produced by the cells - a pseudo-constitutive activity. It is surprising that the Ang II analog [Sar(1), Ile(4), Ile(8)] Ang II and the native angiotensin II fragments Ang 1 - 7, Ang IV, and Ang 5 - 8, which are inactive in activating the wild-type receptor, activated N295S and L305Q. Results generated by lowering the Na+ concentration suggest that the mutant N295S and L305Q may be trapped in neutral conformational states ( RN). These data allow us to identify for the first time a novel pattern of GPCR mutations with a broad spectrum of agonist specificity, suggesting possible existence of functional GPCRs in nature that are activated through conformational "selection" rather than "induction" mechanisms.