Mutations of the DRY motif that preserve beta 2-adrenoceptor coupling

The largest family of G protein coupled receptors is characterized by the presence of the DRY motif at the boundary between the third transmembrane region and the second intracellular loop. Inspection of a recent alignment containing 620 receptors reveals that arginine is the best conserved amino acid of this trio. Disease causing receptor mutations have been identified in which this arginine had been replaced by another amino acid. Those receptors were unable to stimulate G protein activity suggesting an obligatory role of this amino acid in the process. One such mutation was identified in the V2 vasopressin receptor (V2R). The R137H V2R binds vasopressin with wild type-like affinity but fails to stimulate Gs. Because interaction with G proteins have been found to modulate agonist binding affinity of adrenergic receptors, arginine 131 was replaced by histidine in the human beta 2-adrenergic receptor to explore the hypothesis that this mutation may dissociate the G protein effect on binding from G protein activation. Surprisingly, this substitution increased agonist binding affinity preserving wild type like coupling of the receptor. Arginine could also be substituted by other amino acids without loss of coupling to Gs demonstrating an unexpected lack of selectivity in the human beta 2-adrenergic receptor protein, and ruling out a requirement for the side chain of arginine in signalling to G proteins.