Direct Interactions of Mastoparan and Compound 48/80 with GTP-Binding Proteins1

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Abstract
The effects of mastoparan and compound 48/80 on the activities of αβγ-trimeric GTP-binding proteins (G proteins) were studied with purified G0 and G1−1 which had been reconstituted into phospholipid vesicles. Pertussis toxin-catalyzed ADP-ribosylation of G0 or G1−1 was inhibited by mastoparan or compound 48/80, suggesting that the G proteins were dissociated into their constituent α- and βγ-subunits in the presence of these compounds. The steady-state rate of GTP hydrolysis catalyzed by G0 or G1−1 was stimulated by the two compounds. Both the stimulations were due to increases in the rate of the GDP-GTP exchange reaction occurring on the G proteins. However, the modes stimulation of the GTPase activity depended on the type of G protein used, and the stimulations caused by the two compounds were differently affected by pertussis toxin-catalyzed ADP-ribosylation of G proteins. Moreover, the mastoparan-induced stimulation of the GTPase activity was partially inhibited by compound 48/80. Thus, the two histamine secretagogues mastoparan and compound 48/80 appear to activate G proteins differently, though they interact with the signal-transducing proteins, at least partly, at a common binding site.

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