Separation and properties of a regulatory GTPase activity associated with the adenylate cyclase system in rat brain synaptic plasma membranes

A GTPase activity having a Km value of 0.5 microM was present in synaptic plasma membranes from rat brain. The activity was inhibited 63% and 24% by NaF and cholera toxin respectively and solubilized from the membranes together with the adenylate cyclase activity. Upon gel chromatography, the solubilized GTPase activity was co-eluted with the GTP-binding regulatory protein of adenylate cyclase. A similar GTPase activity was also associated with the regulatory protein from myelin. The regulatory protein fraction enhanced the activity of the catalytic unit of adenylate cyclase in the presence of 5'-guanylylimidodiphosphate but inhibited the adenylate cyclase activity by 70% in the presence of GTP. These results indicate the possible involvement of the GTPase activity in the regulation of the adenylate cyclase activity.