Prevention of the agonist binding to gamma-aminobutyric acid B receptors by guanine nucleotides and islet-activating protein, pertussis toxin, in bovine cerebral cortex. Possible coupling of the toxin-sensitive GTP-binding proteins to receptors.

Using the membranes treated with Triton X-100, we studied the interaction between gamma-aminobutyric acid (GABA)B receptors and the GTP-binding proteins which are the substrates for ADP-ribosylation by the islet-activating protein (IAP), pertussis toxin. The addition of guanine nucleotides to the membranes markedly decreased the binding of GABA to GABAB receptors. Preincubation of the membranes with IAP plus NAD caused ADP-ribosylation of the 41,000- and 39,000-Da proteins selectively and decreased GABA binding to GABAB receptors in a time- and dose-dependent manner. This decrease of binding appeared to be due to the reduction of receptor affinity for agonist. The GTP-binding proteins which are ADP-ribosylated by IAP were purified from the membrane fraction of bovine cerebral cortex. The addition of the purified GTP-binding proteins to IAP-treated membranes restored the high affinity binding of GABA to GABAB receptor. The two GTP-binding proteins which were resolved by octyl-Sepharose column chromatography showed similar efficacy in restoring GABA binding. Thus, GABAB receptors are coupled to GTP-binding proteins, IAP-specific substrates, in the brain membranes.