Inhibition of bovine rod outer segment GTPase by Bordetella pertussis toxin

Incubation of photolyzed bovine rod outer segment (ROS) membranes with pertussis toxin led to an inhibitionn of GTPase activity in an NAD-dependent reaction. In the presence of [³²P]NAD, pertussis toxin catalyzed the [³²P]ADP-ribosylation of a 39-kDa ROS protein and the 39-kDa α-subunit of transducin purified from ROS membranes. The decrease in GTPase activity was paralleled by an increase in the extent of [³²P]ADP-ribosylation of the 39-kDa ROS protein. [³²P]ADP-ribosylation of the 39-kDa protein occurred in both photolyzed and dark ROS membranes. Neither ATP nor guanylyl imidodiphosphate were required for either ADP-ribosylation or inhibition of GTPase. It was demonstrated previously that choleragen catalyzed the NAD-dependent inhibition of ROS GTPase. Choleragen-catalyzed inhibition of GTPase and [³²P]ADP-ribosylation of the 39-kDa protein required guanylyl imidodiphosphate and did not occur in dark ROS membranes. The effects of choleragen and pertussis toxin were not additive. Incubation of ROS with choleragen or pertussis toxin prevented the subsequent [³²P]ADP-ribosylation by the homologous but not the heterologous toxin, consistent with the hypothesis that the two toxins act at different sites on the 39-kDa protein. Pertussis toxin is known to ADP-ribosylate the inhibitory guanine nucleotide-binding subunit of adenylate cyclase (N(i)) but not the stimulatory subunit (N(s)); choleragen is known to ADP-ribosylate N(s) but not N(i). Since both toxins ADP-ribosylate the same subunit of transducin, it appears that this protein may possess characteristics of both N(s) and N(i).