TY - JOUR T1 - A Surface-Exposed Region of G<sub>sα</sub> in Which Substitutions Decrease Receptor-Mediated Activation and Increase Receptor Affinity JF - Molecular Pharmacology JO - Mol Pharmacol SP - 1081 LP - 1092 VL - 57 IS - 6 AU - Galina Grishina AU - Catherine H. Berlot Y1 - 2000/06/01 UR - http://molpharm.aspetjournals.org/content/57/6/1081.abstract N2 - The mechanism by which receptors activate G proteins is unclear because a connection between the receptor and the nucleotide binding site has not been established. To investigate this mechanism, we evaluated the roles in receptor interaction of three potential receptor contact sites in αs: the α2/β4, α3/β5, and α4/β6 loops. Substitutions of αi2 homologs for αsresidues in the α2/β4 loop and alanine substitutions of residues in the α4/β6 loop do not affect activation by the β2-adrenergic receptor. However, replacement of five αs residues in the α3/β5 loop region with the homologous αi2 residues decreases receptor-mediated activation of αs and increases the affinity of Gs for this receptor. The substitutions do not alter guanine nucleotide binding or hydrolysis, or activation by aluminum fluoride, indicating that the effects on receptor interaction are not due to a destabilization of the guanine-nucleotide bound state. In a model of the receptor-G protein complex, the α3/β5 loop maps near the second and third intracellular loops of the receptor. The effects of the α3/β5 substitutions suggest that the wild-type residues may be receptor contact sites that are optimized to ensure the reversibility of receptor-G protein interactions. Furthermore, the α3/β5 region corresponds to an exchange factor contact site in both EF-Tu and Ras, suggesting that the mechanisms by which seven-transmembrane receptors and exchange factors catalyze nucleotide exchange may share common elements. The American Society for Pharmacology and Experimental Therapeutics ER -