Transducin (Gt) is a member of a family of receptor-coupled signal-transducing guanine nucleotide (GN) binding proteins (G-proteins). Light-activated rhodopsin is known to catalyze GN exchange on Gt, resulting in the formation of the active state of the Gt alpha-GTP complex. However, purified preparations of Gt have been shown to exchange GN in the absence of activated receptors [Wessling-Resnick, M., & Johnson, G. L. (1987) Biochemistry 26, 4316-4323]. To evaluate the role of rhodopsin in the activation of Gt, we studied GN-binding characteristics of different preparations of Gt. Gt preparations obtained rom the supernate of GTP-treated bovine rod outer segment (ROS) disks, followed by removal of free GTP on a Sephadex G-25 column, bound GTP gamma S at 30 degrees C in the absence of added exogenous rhodopsin with an activity of 1 mol of GTP gamma S bound/mol of Gt (Gt-I preparations). Binding of GTP gamma S to Gt-I preparations closely correlated with the activation of ROS disk cGMP phosphodiesterase. GN-binding activity of Gt-I preparations was dependent on reaction temperature, and no binding was observed at 4 degrees C. In the presence of 10 microM bleached rhodopsin, Gt-I preparations bound GTP gamma S at 4 degrees C. However, hexylagarose chromatography of Gt-I preparations led to a preparation of Gt that showed less than 0.1 mol/mol binding activity following 60-min incubation at 30 degrees C in the absence of rhodopsin (Gt-II preparations).(ABSTRACT TRUNCATED AT 250 WORDS)