Role of transmembrane helix IV in G-protein specificity of the angiotensin II type 1 receptor

G-protein activation by G-protein coupled receptors (GPCRs) is accomplished through proper interaction with the cytoplasmic loops rather than through sequence-specific interactions. However, the mechanism by which a specific G-protein is selected by a GPCR is not known. In the current model of GPCR activation, agonist binding modulates helix-helix interactions, which is necessary for fully determining G-protein specificity and stimulation of GDP/GTP exchange. In this study, we report that a single-residue deletion in trans-membrane helix IV leads the angiotensin II type 1 (AT₁) receptor chimera CR17 to retain GTP-sensitive high affinity for the agonist angiotensin II but results in complete inactivation of intracellular inositol phosphate production. The agonist dissociation profile of CR17 in the presence of guanosine 5'-3-O-(thio)triphosphate suggests that the activation- induced conformational changes of the chimeric receptor itself remain intact. Insertion of an alanine at position 149 (CR17 149A) in this chimera rescued the inactive phenotype, restoring intracellular inositol phosphate production by the chimera. This finding suggests that in the wild-type AT₁ receptor the orientation of transmembrane helix IV-residues following Cys¹⁴⁹ is a key determinant for effectively distinguishing among various structurally similar G-proteins. The results emphasize that the contacts within the membrane- embedded portion of transmembrane helix IV in the AT₁ receptor is important for specific G-protein selection.