A new GTP-binding protein serving as the specific substrate of islet-activating protein (IAP), pertussis toxin, was purified from porcine brain membranes as an αβγ-heterotrimeric structure. The α-subunit of the purified protein (α40βγ) had a molecular mass of 40 kDa and differed from that of Gi(α41βγ) or Go(α39βγ) previously purified from brain tissues. The fragmentation patterns of limited tryptic digestion and immunological cross-reactivities among the three α were different from one another. However, the βγ-subunit resolved from the three IAP substrates similarly inhibited a membrane-bound adenylate cyclase and their β-subunits were immunologically indistinguishable from one another. Thus, the α40βγ is a new IAP substrate protein different from Gi or Go, in the α-subunit only.
