A new GTP‐binding protein in brain tissues serving as the specific substrate of islet‐activating protein, pertussis toxin

Abstract
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 Gi41βγ) or Go39βγ) 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.

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