Inhibition of a Low K_m GTPase Activity in Rat Striatum by Calmodulin

Abstract: In rat striatum, the activation of adenylate cyclase by the endogenous Ca²⁺‐binding protein, calmodulin, is additive with that of GTP but is not additive with that of the nonhydrolyzable GTP analog, guanosine‐5′‐(β, γ‐imido)triphosphate (GppNHp). One possible mechanism for this difference could be an effect of calmodulin on GTPase activity which has been demonstrated to “turn‐off” adenylate cyclase activity. We examined the effects of Ca²⁺ and calmodulin on GTPase activity in EGTA‐washed rat striatal particulate fractions depleted of Ca²⁺ and calmodulin. Calmodulin inhibited GTP hydrolysis at concentrations of 10⁻⁹–10⁻⁶M but had no effect on the hydrolysis of 10⁻⁵ and 10⁻⁶M GTP, suggesting that calmodulin inhibited a low K_m GTPase activity. The inhibition of GTPase activity by calmodulin was Ca²⁺‐dependent and was maximal at 0.12 μM free Ca²⁺. Maximal inhibition by calmodulin was 40% in the presence of 10⁻⁷M GTP. The IC₅₀ for calmodulin was 100 nM. In five tissues tested, calmodulin inhibited GTP hydrolysis only in those tissues where it could also activate adenylate cyclase. Calmodulin could affect the activation of adenylate cyclase by GTP in the presence of 3,4‐dihydroxyphenylethylamine (DA, dopamine). Calmodulin decreased by nearly 10‐fold the concentration of GTP required to provide maximal stimulation of adenylate cyclase activity by DA in the striatal membranes. The characteristics of the effect of calmodulin on GTPase activity with respect to Ca²⁺ and calmodulin dependence and tissue specificity parallel those of the activation of adenylate cyclase by calmodulin, suggesting that the two activities are closely related. Inhibition of a low K_m GTPase activity by calmodulin could represent an action of calmodulin in increasing the association of a GTP‐binding protein with the catalytic subunit activity resulting in a reduction of the “turn‐off” GTPase activity.