Mastoparan inhibits β-adrenoceptor-Gs signaling by changing the localization of Gαs in lipid rafts

Mastoparan, a wasp venom toxin, has various pharmacological activities, the mechanisms of which are still unknown. To clarify the action of mastoparan on G protein-coupled receptor-mediated signaling, we previously examined the effect of mastoparan on Gq-mediated signaling and demonstrated that mastoparan binds to gangliosides causing a decrease in Gαq/11 content in lipid rafts, and resulting in the inhibition of Gq-mediated phosphoinositide hydrolysis (Sugama et al., Mol. Pharmacol., 68, 1466, 2005). In the present study, we examined the effect of mastoparan on β-adrenoceptor-Gs signaling in 1321N1 human astrocytoma cells. Mastoparan inhibited isoproterenol-induced elevation of cyclic AMP in a concentration-dependent manner. Although mastoparan is known to be an activator of Gi, pertussis toxin only slightly attenuated mastoparan-induced inhibition of cyclic AMP elevation, suggesting that a major part of the inhibition of cyclic AMP elevation induced by mastoparan is not mediated by Gαi. By contrast, mastoparan-induced inhibition of cyclic AMP elevation was clearly attenuated by preincubation of the cells with ganglioside mixtures. Moreover, mastoparan changed the localization of Gαs in lipid rafts without disrupting the structure of lipid rafts. Fluorescent staining analysis showed that mastoparan released GFP-Gαs from plasma membranes into the cytosol. These results suggest that the mastoparan-induced suppression of cyclic AMP elevation is mainly caused by changing the localization of Gαs in lipid rafts into a compartment in the cellular interior where it is not available to activate adenylyl cyclase.