Purification and properties of the inhibitory guanine nucleotide-binding regulatory component of adenylate cyclase.

Attenuation of GTP-dependent inhibition of adenylate cyclase by islet-activating protein (pertussis toxin) is due to the ability of the toxin to catalyze the ADP-ribosylation of a 41,000/35,000-Da membrane-bound protein, which is thought to be the inhibitory guanine nucleotide-binding regulatory component of adenylate cyclase (Gi). We describe and document the purification of this protein from rabbit liver, and, in conjunction with evidence presented in the subsequent papers of the series, identify it as Gi. Purified Gi serves as an excellent substrate for islet-activating protein and can be ADP-ribosylated to the extent of 1 mol of ADP-ribose/mol of protein. The extent of ADP-ribosylation of Gi correlates with the amount of guanine nucleotide that can be bound to the protein. Studies of the nucleotide binding site on the 41,000-Da subunit of Gi reveal a high affinity site that is specific for guanine nucleotides. Rank order of affinities for various nucleotides is GTP gamma S greater than Gpp(NH)p = GTP = GDP greater than GMP much greater than App(NH)p, ATP. High affinity binding of guanine nucleotides is dependent on Mg2+ and is essentially irreversible in the presence of divalent cation. Bound nucleotide readily dissociates from its site on the 41,000-Da subunit of Gi in the absence of Mg2+. This reversal of binding is markedly enhanced by the presence of the 35,000-Da subunit of Gi. The physical characteristics of Gi are important determinants of its role as the inhibitory guanine nucleotide-binding regulatory component of adenylate cyclase.