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

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. Natl Acad. 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. Natl Acad. Sci. USA, 78, 152-156. 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'-beta, gamma--imidotriphosphate (pNHppG). The results suggest that the GTP-binding protein possesses one high-affinity pNHppG-binding site (Kd much less than 0.1 microM). At high concentrations of GTP or pNHppG the phosphodiesterase is activated in the dark and the light-scattering signal is correspondingly reduced; both effects are prevented by previous incubation with guanosine 5'-beta-thiodiphosphate (pSpG).