Microinjection of a 19-kDa Guanine Nucleotide-binding Protein Inhibits Maturation of Xenopus Oocytes

ADP-ribosylation factors (ARFs) are 19–21-kDa proteins purified from bovine brain that bind guanosine 5′-triphosphate (GTP). They exhibit GTP-dependent activity as activators of cholera toxin-catalyzed ADP-ribosylation of the α-subunit of the stimulatory guanine nucleotide-binding protein of the adenylyl cyclase system (Gsα). ARF, which interacts directly with the catalytic subunit of cholera toxin, has no known physiologic role. Intracellular microinjection of ARF was employed to investigate the effect of ARF on progesterone- and insulin-stimulated maturation of Xenopus oocytes. Maturation was inhibited by injection of ARF 3–8 h before exposure of oocytes to progesterone or insulin. ARF inhibition was dependent on progesterone concentration but not on insulin concentration. Inhibition was enhanced by concomitant injection of GTP and to a greater extent by guanosine 5′-O-(thiotriphosphate) (GTPγS) which, in the absence of ARF, inhibited somewhat at early time points. The demonstration of this effect of ARF on both progesterone- and insulin-stimulated oocyte maturation may provide a clue to the physiologic role of this guanine nucleotide-binding protein.