Cholera toxin (CT) and islet-activating protein (IAP, a Bordetella pertussis toxin) were employed to test the hypothesis that GTP-binding regulatory proteins are released from plasma membranes to a greater extent when ‘activated’ than when ‘inactivated’. CT, which activates Ns (the stimulatory GTP-binding regulatory protein of the adenylate cyclase system), catalyzed the incorporation of radioactivity from [32P]NAD into 45 and 47.5 kDa peptides associated with rat liver plasma membranes. Following ADP-ribosylation and centrifugation at 100000 × g for 1 h, approx. 30–35% of these CT-labelled peptides were no longer associated with the plasma membranes, but were recovered from the supernatant fraction. IAP, which inactivates Ni (the inhibitory GTP-binding regulatory protein of the adenylate cyclase system) catalyzed the incorporation of radioactivity from [32P]NAD into a 41 kDa peptide associated with the membranes. However, in contrast to the CT-labelled peptides, typically less than 5% of the lAP-labelled peptide was found in the 100000 × g supernatant fraction, but rather was almost exclusively associated with the membrane pellet. The data indicate that the α-subunits of Ns are released from the plasma membrane following activation, and support the hypothesis that the βγ-subunits act to anchor the α-subunits to the plasma membrane. Cholera toxin Islet-activating protein GTP-binding protein
