title: Light-induced interactions between rhodopsin and the GTP-binding protein. Relation with phosphodiesterase activation.
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. 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--imido]triphosphate (p[NH]ppG). The results suggest that the GTP-binding protein possesses one high-affinity p[NH]ppG-binding site (Kd much less than 0.1 microM). 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 5'-[beta-thio]diphosphate (p[S]pG).
mesh_terms: D015106:3',5'-Cyclic-GMP Phosphodiesterases / Q000378:metabolism*; D000818:Animals; D002417:Cattle; D004789:Enzyme Activation / Q000187:drug effects; D019204:GTP-Binding Proteins; D006153:Guanosine Diphosphate / Q000031:analogs & derivatives / Q000494:pharmacology; D006160:Guanosine Triphosphate / Q000494:pharmacology; D006165:Guanylyl Imidodiphosphate / Q000494:pharmacology; D008027:Light; D011956:Receptors, Cell Surface / Q000378:metabolism*; D012168:Retinal Pigments / Q000528:radiation effects*; D012243:Rhodopsin / Q000528:radiation effects*; D012542:Scattering, Radiation; D013873:Thionucleotides / Q000494:pharmacology
chemical_list: D011956:Receptors, Cell Surface; D012168:Retinal Pigments; D013873:Thionucleotides; D006153:Guanosine Diphosphate; C032829:6-thioguanosine 5'-diphosphate; D006165:Guanylyl Imidodiphosphate; D006160:Guanosine Triphosphate; D012243:Rhodopsin; D015106:3',5'-Cyclic-GMP Phosphodiesterases; D019204:GTP-Binding Proteins