Agonist/N-ethylmaleimide mediated inactivation of β-adrenergic receptors: Molecular aspects

Abstract

Incubation of turkey erythrocyte and rat lung membranes with the β-adrenergic agonist isoproterenol in the presence of the group specific reagent N-ethylmaleimide (NEM) causes the apparent inactivation of a well defined fraction of the β-adrenergic receptors. This inactivation process requires the coupling between the receptors and the guanine nucleotide binding regulatory component of the adenylate cyclase system. Accordingly, only the isoproterenol/NEM sensitive receptor population is susceptible to undergo this functional coupling. Incubation of the treated membranes in alkaline buffer and/or in the presence of GTP reactivates the receptors. The reactivated receptors show the initial binding properties for both isoproterenol and the radiolabelled antagonist [ 3H]-dihydroalprenolol and are still dithiothreitol sensitive. Moreover the reactivated receptors can again couple to the regulator in the presence of agonists. The present study makes it possible to discriminate between the different models advanced for the N-ethylmaleimide action and is valid for β 1- as well as β 2-adrenergic receptors. The reagent appears indeed to freeze the hormone-receptor-regulator complex by alkylating the latter component, as previously proposed by Korner et al. (1982) J. biol. Chem. 257, 3389–3396. It is also suggested that the existence of a limited fraction of coupling-prone receptors results from differences in the membrane microenvironment or from a structural heterogeneity of the receptors rather than from a stoichiometric limitation of the regulator concentration.