Abstract

β-Adrenergic agonists appear to induce a conformational change in 65% of the β-adrenergic receptors in wild-type S49 lymphoma cell membranes as shown by the increased sensitivity of a subpopulation of receptor to inactivation by the alkylating agent N-ethylmaleimide (NEM). This inactivation is conveniently monitored through the induction with NEM of a biphasic (-)-isoproterenol displacement curve of [¹²⁵I]IHYP binding to the receptors. In marked contrast, NEM alone or NEM plus a β-adrenergic antagonist has no effect on receptor binding. The final percentage (65%) of total receptor sites inactivated with agonist plus NEM is not dependent on the concentration of NEM used or on the time of exposure. Agonist plus NEM also causes inactivation of about 75% of the receptors in the S49 lymphoma β_d clone, which has about 25% of the β-adrenergic receptor density of wild-type cells. However, agonist plus NEM treatment does not affect receptor binding in unc and cyc^- S49 cell membranes, in which the receptors are functionally uncoupled from the adenylate cyclase complex. Further, agonists plus NEM also cause inactivation of 45 to 60% of the β-adrenergic receptors in turkey erythrocyte membranes, but have no effect on solubilized receptors from the same cell. These findings with variant S49 lines and turkey erythrocytes suggest that the ability of agonists to produce a conformational change of β-adrenergic receptors may be dependent on the ability of these receptors to interact with the adenylate cyclase complex and specifically to interact with the guanine nucleotide regulatory protein. The fact that only a given fraction of the total receptor population can undergo a conformational change in the investigated membranes and that the properties of the agonist/NEM-sensitive population of receptors but not the insensitive population are similar to those of receptors which activate adenylate cyclase suggests that, for structural or stoichiometric reasons, only part of the receptor population can be coupled to the cyclase complex in the presence of agonists. Thus agonist appears to induce a quantifiable, functional heterogeneity in an apparently molecularly homogeneous receptor population.