Effects of the wasp venom peptide, mastoparan, on GTP hydrolysis in rat brain membranes

The effects of mastoparan, a wasp venom toxin, on GTP hydrolyzing activity were examined in rat brain membranes. Mastoparan inhibited the low‐affinity GTPase activity, defined as the amount of 32 P i released from 0.3 μ M [γ‐ 32 P]‐GTP in the presence of 100 μ M unlabelled GTP, in a concentration‐dependent manner. This inhibitory effect of mastoparan on low‐affinity GTPase activity was diminished by increasing concentrations of UDP and was completely attenuated at 20 m M , indicating that activation of nucleoside diphosphokinase (NDPK) is inolved in the phenomenon. In the presence of 20 m M UDP, mastoparan stimulated the high‐affinity GTPase activity by increasing the V max value without affecting the apparent K M for GTP. Mastoparan‐stimulated high‐affinity GTPase activity was apparent at concentrations higher than 1 μ M , in a concentration‐dependent manner, but without saturation even at 100 μ M . Mastoparan‐induced high‐affinity GTPase activity showed a characteristic sensitivity to MgCl 2 , quite different from that seen in L ‐glutamate‐stimulated activity, a representative of receptor‐mediated G‐protein activation. There appeared to be a simple additive interaction between mastoparan‐ and L ‐glutamate‐stimulated high‐affinity GTPase activities, indicting that distinct pools of G‐proteins are involved in receptor‐independent and receptor‐mediated G‐protein activation. These results suggest that G‐proteins in brain membranes are functionally altered by mastoparan through multiple mechanisms of action and that the mastoparan‐induced, direct G‐protein activating process lacks a synergistic or antagonistic interaction with an agonist‐induced, receptor‐mediated activation of G‐proteins.