BINDING OF PURIFIED RECOMBINANT BETA-ARRESTIN TO GUANINE-NUCLEOTIDE-BINDING-PROTEIN-COUPLED RECEPTORS

beta-arrestin is a cytosolic protein thought to be responsible for uncoupling agonist-activated beta(2)-adrenergic receptors from their guanine-nucleotide-binding proteins (G-protein) subsequent to receptor phosphorylation by the beta-adrenergic receptor kinase (beta ARK). In order to investigate this intel action, we generated a recombinant baculovirus for the expression of beta-arrestin in Sf9 insect cells. Apparently homogeneous beta-arrestin preparations were obtained in a one-step purification on heparin-Sepharose. Purified beta-arrestin bound to rhodopsin in a phosphorylation-dependent plus light-dependent manner. Binding to beta(2)-adrenergic receptors was investigated using purified receptors reconstituted into lipid vesicles. The accessibility of the reconstituted receptors was determined using the agonist isoproterenol for the ligand-binding site and an antibody binding to an attached myc tag for the C-terminus, the site of receptor phosphorylation. On the basis of these data, the binding of purified beta-arrestin to beta ARK-phosphorylated beta(2)-adrenergic receptors was found to occur with a K-D of 1.8 nM and with a maximum of 1 beta-arrestin/receptor. beta-arrestin also bound to receptors which had been completely dephosphorylated with acid phosphatase, but the affinity was approximate to 30-fold lower. In contrast to regulation by phosphorylation, binding of agonists or antagonists to the receptors had negligible effects on beta-arrestin binding. Finally, beta-arrestin and beta ARK were shown to be capable of producing synergistic inhibition of beta(2)-adrenergic-receptor-stimulated adenylyl cyclase activity of cell membranes. These data show that high-affinity stoichiometric binding of beta-arrestin to beta(2)-adrenergic receptors occurs in a beta ARK-dependent manner and is sufficient to impair adenylyl cyclase stimulation by the receptors.