title: The transducin cascade is involved in the light-induced structural changes observed by neutron diffraction on retinal rod outer segments.
abstract: Time-resolved neutron diffraction on retinal rod outer segments are performed to reinvestigate the origin of the light-induced structural change observed by Saibil et al. (Saibil, H., M. Chabre, and D. L. Worcester, 1976, Nature (Lond.), 262:266-270). Photoactivating rhodopsin triggers in rods a cascade of GTP-dependent and transducin-mediated reactions controlling cyclic-GMP hydrolysis. Infrared light-scattering studies (Kühn, H., N. Bennett, M. Michel-Villaz, and M. Chabre, 1981, Proc. Natl. Acad. Sci. USA, 78:6873-6877; Vuong, T. M., M. Chabre, and L. Stryer, 1984, Nature (Lond.), 311:659-661) demonstrated the existence of structural changes that correspond to this cascade rather than to rhodopsin photoactivation. We thus look for neutron diffraction changes of similar origins. With 1-min time resolution, intensity changes are observed mainly for orders 2 and 4. The illumination and GTP dependence of these changes indicates an involvement of transducin. Without GTP, they are linear with the amount of photoexcited rhodopsin, saturate at 10% photolysis, and thus correlate well with the light-scattering "binding signal." With GTP, light sensitivity is higher and saturation occurs below 0.5% photolysis, as for the "dissociation signal" of light scattering. In both cases, lattice compressions of 0.2-0.3% are observed. With 4-s time resolution the intensity change with GTP present precedes the lattice compression. The fast intensity change is probably due to the displacement of transducin alpha-subunits away from the disc membrane and the slower lattice shrinkage to an osmotic readjustment of the rod.
mesh_terms: D000818:Animals; D019204:GTP-Binding Proteins / Q000502:physiology*; D007700:Kinetics; D008027:Light; D008565:Membrane Proteins / Q000502:physiology*; D008722:Methods; D009502:Neutrons; D010786:Photoreceptor Cells / Q000502:physiology*; D012168:Retinal Pigments / Q000502:physiology*; D012243:Rhodopsin / Q000502:physiology*; D012374:Rod Cell Outer Segment / Q000502:physiology* / Q000528:radiation effects; D013057:Spectrum Analysis; D015293:Transducin
chemical_list: D008565:Membrane Proteins; D012168:Retinal Pigments; D012243:Rhodopsin; D019204:GTP-Binding Proteins; D015293:Transducin