Effect of Al³⁺ plus F^- on the Catecholamine-Stimulated GTPase Activity of Purified and Reconstituted G_s

The effects of Al³⁺ and F^- on the catecholamine-stimulated GTPase cycle were studied by using reconstituted phospholipid vesicles that contained purified β-adrenergic receptor and the stimulatory GTP-binding protein of the adenylate cyclase system, G_s. Al³⁺/F^- activated reconstituted G_s to levels previously reported for detergent-solubilized, purified G_s, although both activation and deactivation were faster in the reconstituted preparation. Under these conditions, Al³⁺/F^- did not inhibit by more than 15% the β-adrenergic agonist-stimulated GTPase activity of the vesicles nor did it significantly inhibit the rates of GTP binding, GTP hydrolysis, or GDP release. When Mg²⁺ (50 mM) was used instead of agonist to promote GTP hydrolysis in the receptor-G_s vesicles, Al³⁺/F^- was found to inhibit GTPγS binding, GDP release, and steady-state GTPase activity to unstimulated levels. These data can be interpreted as indicating that the receptor catalyzes nucleotide exchange by G_s faster or more efficiently than does Mg²⁺.