Catecholamine-stimulated GTPase cycle. Multiple sites of regulation by β-adrenergic receptor and Mg²⁺ studied in reconstituted receptor-G(s) vesicles

The regulation of the intermediary steps of the catecholamine-stimulated GTPase cycle by β-adrenergic agonists and Mg²⁺ was investigated using unilamellar phosphatidylethanolamine-phosphatidylserine vesicles that contained purified β-adrenergic receptor and the stimulatory GTP-binding protein of the adenylate cyclase system, G(s). The steady-state turnover number of the agonist-stimulated GTPase, normalized according to the receptor-responsive pool of G(s), was 0.8 min^-1 for untreated vesicles and 1.7 min^-1 for vesicles that had been treated with dithiothreitol to activate the receptors. The binding and release of [α-³²P]GTP, [³H]GTP, and [γ-³²P]GTP were used to measure the binding and hydrolysis of GTP and the release of GDP. Agonist-liganded receptor stimulated both the binding of GTP and the release of the GDP product, and GDP release per se did not apear to be the mechanism by which receptor stimulated the binding of GTP. Both processes displayed apparent first order rate constants of about 0.5 min^-1 for untreated vesicles and both rates increased about 5-fold after dithiothreitol treatment. Both processes were formally catalytic with respect to receptor, in that several (up to 8) molecules of G(s) were stimulated per molecule of receptor. The hydrolysis of G(s)·GTP to G(s)·GDP was unaltered by agonist and occurred with a rate constant of about 4 min^-1. The rates of these partial reactions were consistent with the overall rate of steady-state hydrolysis and with the ability of the agonist-liganded receptor to promote the formation of sufficient G(s)·GTP to fully stimulate adenylate cyclase in a native membrane. The Mg²⁺ dependence of agonist-stimulated, steady-state GTPase activity appeared to consist of at least two, distinct Mg²⁺-requiring processes. Very low concentrations of Mg²⁺ (~20 nM) were required for hydrolysis of G(s)·GTP, and 10 μM Mg²⁺ was required to maximize the initial rate of agonist-stimulated [α-³²P]GTP binding.