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Abstract

In isolated pancreatic acinar plasma membranes a 40 kDa protein was labeled with the photoreactive GTP-analogue [α³²P]GTP-γ-azidoanilide. Increased incorporation of the photolabel into the 40 kDa protein was obtained in the presence of increasing concentrations of cholecystokinin-octapeptide (10⁻⁸ – 10⁻5 M) but not with carbachol. Adenylyl cyclase activating hormones such as vasoactive intestinal polypeptide and secretin had no effect. Pretreatment of plasma membranes with cholera toxin reduced incorporation of GTP-γ-azidoanilide into the 40 kDa protein by about 30 %. This reduction was reversed if ADP-ribosylation by cholera toxin was performed in the presence of cholecystokinin, whereas carbachol had no effect. The data indicate that a cholera toxin-sensitive 40 kDa GTP-binding protein is involved in functionally coupling cholecystokinin- but not muscarinic acetylcholine-receptors to phospholipase C.