Requirement for both choleragen and pertussis toxin to obtain maximal activation of adenylate cyclase in cultured cells.

NG108-15 cells contain both the inhibitory and stimulatory guanyl nucleotide-binding regulatory proteins of the cyclase system. Choleragen activates cyclase directly by ADP-ribosylating the stimulatory guanyl nucleotide-binding protein; prostaglandin E1 does not further increase activity of cells treated with maximally effective concentrations of choleragen. Including pertussis toxin during incubation with this concentration of choleragen, however, further augments both cyclase activity and cAMP accumulation by intact cells. These observations suggest that the inhibitory guanyl nucleotide-binding protein exerts basal inhibition on catalytic activity which cannot be overcome by maximally effective concentrations of choleragen, stimulatory hormones, or both.