Mastoparan is a 14-amino-acid peptide that stimulates secretion from several cell types. Secretion can be partially blocked by pertussis toxin and may be mediated by guanine-nucleotide-binding proteins (G-proteins). Mastoparan can act directly on G-proteins, probably at the hormone receptor-binding site, to stimulate guanosine 5′-[gamma-thio]triphosphate binding and GTPase activities of pertussis-toxin substrates Go and Gi [Higashijima, Uzu, Nakajima & Ross (1988) J. Biol. Chem. 263, 6491-6494]. We now describe a nucleotidase from bovine brain that is not a known G-protein whose GTPase and ATPase activities are stimulated by mastoparan. This nucleotidase hydrolyses ATP faster than GTP, but has similar affinities for both (0.4 microM). Mastoparan maximally stimulates both ATPase and GTPase activities by about 8-fold after insertion of the protein into phospholipid vesicles, but does not affect the EC50 (concentration at which half the maximal effect is observed) for ATP and GTP. The EC50 for mastoparan stimulation of GTPase and ATPase is 6 and 12 microM respectively. The native molecular mass of the partially purified mastoparan-stimulated nucleotidase is 87 kDa. This nucleotidase may be another receptor-activated enzyme, and its identification may be useful for understanding mastoparan-stimulated processes.
