Effect of Al3+ plus F- on the catecholamine-stimulated GTPase activity of purified and reconstituted G/sub 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/sub s/. Al3+/F- activated reconstituted G/sub s/ to levels previously reported for detergent-solubilized, purified G/sub 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 [32P-] or [35S-GTP] binding, GTP hydrolysis, or GDP release. When Mg2+ (50 mM) was used instead of agonist to promote GTP hydrolysis in the receptor-G/sub 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 catalyze nucleotide exchange by G/sub s/ faster or more efficiently than does Mg2+