[Inhibitory effect of pertussis toxin on the metabolism of guanine nucleotides in transducin from bovine outer rod segments].

Transducin from bovine retinal rod outer segments possesses two sites responsible for the binding of guanyl nucleotides, one of which is specific only for GTP (GTP-site), while the other one may bind both GTP and GDP (GTP/GDP-site). Pertussis toxin covalently modifies the alpha-subunit of transducin as a result of which 83% of GDP bound at the GTP/GDP site of the protein remain tightly bound and are not displaced by Gpp(NH)p excess. The GTP-site in modified transducin binds Gpp(NH)p at the same rate and reveals the same sensitivity to rhodopsin as does native transducin. Presumably, the GTP/GDP site is localized in the alpha-subunit of transducin. The inhibiting effect of pertussis toxin on GTP hydrolysis by transducin and on stimulation of retinal rod outer segment phosphodiesterase by guanyl nucleotides is due to the tight binding of GDP in the active center of the protein after transducin ADP-ribosylation, which makes impossible the formation of a complex between GTP and the alpha-subunit of transducin.