Light‐Induced Interactions between Rhodopsin and the GTP‐Binding Protein

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Abstract
The existence of rapid light-induced changes of light scattering in suspensions of bovine rod outer segment membranes has been described previously [H. Kühn et al. (1981) Proc. Nutl Acud. Sci. USA, 78, 6873-6877]. The signal observed in the presence of GTP has been interpreted as being related to the rhodopsin-catalyzed exchange of GTP for GDP bound to the GTP-binding protein, i.e. to the formation of the activator of the cGMP phosphodiesterase [B. K. K. Fung et al. (1981) Proc. Nutl Acad. Sci. USA, 78, 152- 1561. We have tested this interpretation in the present paper by investigating the relation between the light-scattering signal and the activity of the phosphodiesterase using rapid recording techniques for both processes. All the results obtained are consistent with the above hypothesis. The amplitude of the light-scattering signal and the activity of the phosphodiesterase are shown to present the same dependence upon the flash intensity and upon the concentration of GTP or its analog guanosine 5'-[β,γ-imido]triphosphate (p[NH]ppG). The results suggest that the GTP-binding protein possesses one high-affinity p[NH]ppG-binding site (Kd≪ 0.1 μM). At high concentrations of GTP or p[NH]ppG the phosphodiesterase is activated in the dark and the light-scattering signal is correspondingly reduced; both effects are prevented by previous incubation with guanosine S'-[β-thio]diphosphate (P[S]PG).