Truncation of the extended carboxyl-terminal domain increases the expression and regulatory activity of the avian β-adrenergic receptor

A series of mutant avian β-adrenergic receptors with progressively truncated carboxyl termini have been expressed in insect and mammalian cells. Removal of 18-124 amino acid residues caused multiple phenotypic changes in the receptor. Membranes from cells that expressed the truncated receptors displayed elevated basal (2- to 3-fold) and agonist-stimulated adenylylcyclase activities. Adenylylcyclase activity in these membranes also displayed greater stimulation in response to partial agonists. Activity was also markedly stimulated by β-adrenergic ligands that are usually considered to be antagonists (alprenolol, >4-fold; propranolol, ∼2-fold). Wild type receptor did not mediate a response to these classical antagonists. After purification and reconstitution with G., the truncated receptors did not appear to be more active than the wild type. Guanine nucleotides modulated the affinity of agonist for the truncated receptors, whereas the affinity of agonist for the wild type receptor was not altered by guanine nucleotides. The truncated receptors were solubilized from the membrane more efficiently and were more susceptible to amino-terminal proteolysis than was the wild type protein. These results suggest interaction of the carboxyl terminus of the avian β-adrenergic receptor with cellular regulatory or structural elements.