Hydrophobic amino acid in the i2 loop plays a key role in receptor-G protein coupling

Signal transduction of the heptahelical G protein-coupled receptors (GPCRs) involves multiple receptor domains, but a universal consensus domain for coupling has not yet been defined. Alanine mutagenesis scanning was performed on the intracellular loops and the COOH tail of the human muscarinic cholinergic receptor (Hm1) to identify coupling domains. Stimulation of phosphatidylinositol (PI) turnover was determined after transfection of the alanine mutants into U293 human embryonic kidney cells. Alanine substitutions in four regions (loops i1, i2, and NH2 and COOH junctions of i3) impaired coupling efficiency by approximately 50% or more, but the strongest reduction (> 80%) resulted from alanine replacement of a single amino acid, leucine 131. This residue is located in the middle of the second intracellular loop (i2), within the highly conserved GPCR motif (DRYXXV(I)XXPL). The position equivalent to Leu-131 in Hm1 contains a bulky hydrophobic amino acid (L, I, V, M, or F) in nearly all cloned GPCRs. Substitution of Leu-131 with polar amino acids (aspartate and asparagine) also resulted in strongly defective coupling, whereas phenylalanine (found in the equivalent position in the beta 2 adrenoceptor) can replace leucine without losing PI coupling ability of Hm1. Alanine substitution of the corresponding amino acid in the Hm3 receptor (L174A) also inhibited agonist-stimulated PI turnover, while replacing Phe-139 with alanine in the beta 2 adrenoceptor suppressed stimulation of adenylyl cyclase. We propose that a bulky hydrophobic amino acid in the middle of the i2 loop serves as a general site relevant to G protein coupling, whereas coupling selectivity is governed by other receptor domains.