The inhibitory guanine nucleotide-binding regulatory component of adenylate cyclase. Properties and function of the purified protein

Treatment of membranes with islet activating protein (IAP), a toxin from Bordetella pertussis, results in abolition of GTP-dependent, receptor-mediated inhibition of adenylate cyclase. This appears to result from IAP-catalyzed ADP-ribosylation of a 41,000-Da membrane-bound protein. A protein with 41,000- and 35,000-Da subunits has been purified from rabbit liver membranes as the predominant substrate for IAP. This protein has now been shown to be capable of regulating membrane-bound adenylate cyclase activity of human platelets under various conditions. The characteristics of the actions of the IAP substrate are as follows. (1) Purified 41,000/35,000-Da dimer is capable of restoring the inhibitory effects of guanine nucleotides and the α₂-adrenergic agonist, epinephrine, on the adenylate cyclase activity of IAP-treated membranes. (2) The subunits of the dimer dissociate in the presence of guanine nucleotide analogs or A1³⁺, Mg²⁺, and F^-. The 41,000-Da subunit has a high affinity binding site for guanine nucleotides. (3) The resolved 35,000-Da subunit of the dimer mimics guanine nucleotide- and epinephrine-induced inhibition of adenylate cyclase. (4) The resolved (unliganded) 41,000-Da subunit stimulates adenylate cyclase activity and relieves guanine nucleotide- ± epinephrine-induced inhibition of the enzyme. In contrast, the GTPγS-bound form of the 41,000-Da subunit inhibits adenylate cyclase activity, although the lower apparent affinity than does the 35,000-Da subunit. (5) The 35,000-Da subunit increases the rate of deactivation of G(s), the stimulatory regulatory protein of adenylate cyclase. In contrast, the 41,000-Da subunit can interact with G(s) and inhibit its deactivation. These data strongly suggest that the IAP substrate is another dimeric, guanine nucleotide-binding regulatory protein and that it is responsible for inhibitory modulation of adenylate cyclase activity.