Two amino acids in the sixth transmembrane segment of the mouse gastrin-releasing peptide receptor are important for receptor activation

The gastrin-releasing peptide receptor (GRP-R) is a G protein-coupled receptor that mediates a variety of cellular responses, including cell growth and modulation of neuronal activity by activation of heterotrimeric GTP-binding proteins in the Gq family. To understand the regulation of GRP-R signaling we have substituted alanine for each of 10 amino acid residues within the transmembrane (TM) helices of the GRP-R predicted to project into the binding pocket of the receptor and analyzed the importance of each of these residues for receptor function. Two mutations showed selective loss of either agonist (Y285A) or antagonist (F313A) affinity for the GRP-R. In addition, we identified two amino acid residues, Phe270 and Asn281, in the sixth TM segment, which are important for receptor-G protein interaction. In a competition-binding assay with an antagonis