Guanine nucleotide‐binding protein in sea urchin eggs serving as the specific substrate of islet‐activating protein, pertussis toxin

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Abstract
A GTP‐binding protein serving as the specific substrate of islet‐activating protein (IAP), pertussis toxin, was partially purified from Lubrol extract of sea urchin egg membranes. The partially purified protein possessed two polypeptides of 39 and 37 kDa; the 39 kDa polypeptide was specifically ADP‐ribosylated by IAP and the 37 kDa protein cross‐reacted with the antibody prepared against purified βγ‐subunits of αβγ‐heterotrimeric IAP substrates from rat brain. Incubation of this sea urchin IAP substrate with a non‐hydrolyzable GTP analogue resulted in a reduction of the apparent molecular mass on a column of gel filtration as had been the case with purified rat brain IAP substrates, suggesting that the sea urchin IAP substrate was also a heterooligomer dissociable into two polypeptides in the presence of GTP analogues. Thus, the 39 and 37 kDa polypeptides of the sea urchin IAP substrate correspond to the α‐ and β‐subunits, respectively, of mammalian IAP substrates which are involved in the coupling between membrane receptor and effector systems.