Marc Caron

Reconstitution of the β-adrenergic receptor

Of the several second messenger generating systems which serve to transmit signals across the plasma membrane the best characterized is the hormone-receptor-adenylate cyclase system (Lefkowitz, Stadel & Caron, 1983), Such systems consist of three distinct types of components which are portrayed schematically in Fig. 1. These components are: (i) a specific hormone receptor which may either stimulate or inhibit the enzyme; (ii) two special classes of regulatory proteins termed guanine nucleotide regulatory proteins which bind GTP and thereby mediate the hormone-receptor regulation of adenylate cyclase activity, and (iii) the catalytic moiety of adenylate cyclase itself. The two types of guanine nucleotide regulatory proteins denoted as Ns and Ni refer to distinct proteins which appear to couple respectively the stimulatory and the inhibitory receptors to the enzyme (Gilman, 1984). Many different types of receptors can affect the activity of the adenylate cyclase system. Several dozen stimulatory receptors and at least a dozen inhibitory receptors have been identified to date. Very little is known about the molecular characteristics of these receptors. Only the/3-adrenergic receptor for catecholamines has been purified and characterized to any extent. It is an integral membrane glycoprotein of Mr ~ 64,000 daltons as isolated from mammalian tissues (Lefkowitz et al., 1983). In contrast, the receptor from turkey erythrocytes appears to be of smaller size with two major peptides visualized by SDS-polyacrylamide gel electrophoresis. These peptides are of about 40,000 and 50,000 daltons (Shorr et al., 1982). Such isolated receptor peptides can be shown to bind /3-