Altered guanine nucleoside triphosphate binding to transducin by cholera toxin-catalysed ADP-ribosylation

The influence of cholera toxin (CTX)-catalysed ADP-ribosylation on binding of guanine nucleoside triphosphates to transducin was studied by measuring the binding of the GTP analogue, guanosine 5'-[gamma-thio]triphosphate (GTP[gamma S]), to illuminated bovine rod outer segment (ROS) membranes treated with or without CTX. Besides the well-documented inhibition of the transducin GTPase activity, CTX treatment inhibited binding of GTP[gamma S] to illuminated ROS membranes. This inhibition was due to an approximately two-fold lower apparent affinity for the nucleotide, while the density of binding sites was not altered. CTX decreased the association rate of GTP[gamma S] by a factor of about two. Competition experiments with GTP, guanosine 5'-[beta, gamma]iminotriphosphate or GDP showed that the apparent affinities for both guanine nucleoside triphosphates, but not for GDP, were lowered by about two-fold upon CTX treatment. In contrast to CTX, pertussis toxin treatment of ROS membranes reduced the density of binding sites available to GTP[gamma S], while the apparent affinity of the remaining sites was unchanged. It is concluded that ADP-ribosylation of transducin by CTX not only inhibits its GTPase activity but also decreases the affinity for guanine nucleoside triphosphates, data which suggest that the arginine moiety modified by CTX is involved in both binding and hydrolysis of GTP.