ADP-ribosylation of brain synaptosomal proteins correlates with adenylate cyclase activation

ADP-ribosylation of the adenylate cyclase $\text{G}\text{F}$ regulatory subunit by cholera toxin is a major tool for the study of this enzyme. Investigation of the brain enzyme has been hampered up to now by the failure to demonstrate cholera toxin-dependent ADP-ribosylation of membrane-bound proteins. Synaptosomes prepared by flotation from fresh brains homogenized in the presence of protease inhibitors yielded membranes of which several proteins could be ADP-ribosylated by the toxin. The same membranes subjected to mild proteolysis could not be ADP-ribosylated. Adenylate cyclase activation and ADP-ribosylation were simultaneous processes. The major labeled species was of 47,000 M_r. It was solubilized by Lubrol-PX, together with other labeled polypeptides. As analyzed on sucrose gradients, the 47,000 M_r protein was found both in the 3S region, and in the adenylate cyclase containing fraction (9.1S).