Inhibition by Pertussis Toxin of Guanyl Nucleotides Exchange on Transducin in Bovine Rod Cell Membranes

The effect of pertussis toxin on GTP-binding protein of bovine rod cell outer segments (transducin) was studied. Pertussis toxin was shown to ADP ribosylate either alpha subunit of free transducin or transducin-GDP complex, whereas GTP and its analogue Gpp(NH)p strongly inhibit ADP ribosylation of transducin. Pertussis toxin inhibits rod outer segment membrane GTPase and GTPase of homogeneous transducin by 40% and 70-80%, respectively. Activation of rod cell cyclic nucleotide phosphodiesterase by transducin is reduced after its preincubation with pertussis toxin. In transducin modified by pertussis toxin, 83% of GDP becomes tightly bound and cannot be exchanged with Gpp(NH)p. The stabilization of complex transducin-GDP after ADP ribosylation can explain the inhibitory effect of pertussis toxin on GTP hydrolysis by transducin, and on phosphodiesterase activation by guanyl nucleotides.