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We compared the effects of guanine nucleotides and Mg2+ on ADP-ribosylation of rat brain and liver membrane proteins catalysed by Bordetella pertussis toxin (IAP) and cholera toxin (CT). Labelling of proteins in the presence of [alpha-32P]NAD+, ATP and CT required GTP or guanosine 5'-[gamma-thio]triphosphate (GTP [S]). In contrast, labelling of one (liver) or two (brain) polypeptides by IAP was enhanced by guanosine 5'-[beta-thio]diphosphate (GDP[S]) or GTP, but was blocked by GTP[S] or guanosine 5'-[beta, gamma-imido]triphosphate (p[NH]ppG). The order of labelling intensity was GDP[S] greater than GTP greater than no addition greater than GTP[S] = p [NH]ppG. Mg2+ increased labelling by CT, but decreased labelling by IAP. In addition, Mg2+ potentiated the effects of the guanine nucleotides, increasing the inhibitory effects of GTP[S] and the activatory effects of GDP[S] or GTP. Preincubating liver membranes at 30 degrees C in the presence of 10 mm-MgCl2 inhibited labelling by IAP irreversibly. Pretreatment of liver membranes with 4.95 mM-N-ethylmaleimide decreased labelling by CT by approximately 15%, but almost completely blocked labelling by IAP. These results suggest that the undissociated, GDP-bound, conformation of Ni, the inhibitory GTP-binding protein of adenylate cyclase, is the preferred substrate for ADP-ribosylation by IAP. This conformation, which is prevalent in native membranes, is sensitive to temperature, Mg2+ ions and alkylating agents such as N-ethylmaleimide. At 30 degrees C, Mg2+ may cause dissociation and denaturation of Ni in native membranes.

Pertussis toxin substrate is a guanosine 5′-[β-thio]diphosphate-, N-ethylmaleimide-, Mg2+- and temperature-sensitive GTP-binding protein