Turkey erythrocyte membranes were solubilized in the mild detergent octylpenta(oxyethylene) [CH3(CH2)7-(OCH2CH2)50H], which possesses a high critical micelle concentration (=6 mM) and forms small, dynamic micelles. Both the native enzyme N,(GDP)-C and the p [NH] Receptors for hormones and neurotransmitters interact with an effector system within the plane of the membrane to generate the primary biochemical response. A major class of receptors consists of those that activate adenylate cyclase upon agonist binding (refs. 1 and 2; ref. 3 and references therein). The activation of the catalytic unit of adenylate cyclase depends not only on the occupancy of the receptor by an agonist but also on the simultaneous occupancy of the GTP regulatory protein Ns by GTP (4). A number of mechanisms have been proposed for the sequence of events leading from the step of agonist occupancy on the receptor to the production of cAMP from ATP by the catalytic unit C. Currently three types of mechanisms are being discussed (Fig. 1).Although there is considerable evidence that hormone activation of adenylate cyclase by 8-agonists (5-7), glucagon (8, 9), and neurohypophyseal hormone (10) occurs by the "collision coupling" mechanism, counter arguments have been raised. These arguments are based on two types of observations: (i) guanyl nucleotides reduce the affinity of fadrenergic receptors towards their agonists (11) and, therefore, Ns and R are associated (12), and (ii) the n-receptor in the presence of guanyl-5'-yl-imidodiphosphate (p[NH]ppG) can activate Ns in the total absence of C (13). These observations were taken to mean (12, 13) that Ns "shuttles" between R and C (Fig. 1).The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact. other at all times. This model predicts (i) linear dependence of the rate constant of adenylate cyclase activation on the concentration of receptor R; (ii) the interaction between N, and C is never rate limiting; (iii) noncooperative binding of H. Model 3: N, shuttle. In this model the receptor R, the GTP stimulatory unit N,, and the catalytic unit C are all separate and interact sequentially as depicted. This model predicts noncooperative binding of H. if HR-N'(GTP) does not accumulate, and complex kinetics of adenylate cyclase activation. In the extreme case in which one of the components is in great excess, the kinetics will approach first-order kinetics.In fact, these two observations do not contradict the collision coupling theory. First, although the f3-adrenergic receptor forms a stable complex with Ns, in the presence of a f3-agonist and in the absence of guanyl nucleotide, it readily decomposes upon GTP or p[NH]ppG binding (12). Second, the fact that R and Ns interact with each other in the absence of C does not disprove that in the native situation Ns and C always function as a complex. Indeed, to separate Ns from C, a combination of ...
