Cyclic GMP and photoreceptor function.
A single photon can be detected by a rod photoreceptor cell. The absorption of light by rhodopsin triggers a cascade of reactions that amplifies the photon signal and results in ion channel closure with hyperpolarization of the rod photoreceptor cell. Light-induced conformational changes in rhodopsin facilitate the binding of a guanosine nucleotide-binding protein, transducin, which then undergoes a GTP-GDP exchange reaction and dissociation of the transducin complex. A subunit of transducin then activates a phosphodiesterase complex that hydrolyzes cyclic GMP. In darkness, cyclic GMP binds to cation channels of the photoreceptor plasma membrane, maintaining them in an open configuration. The light-induced reduction in cyclic GMP concentration dissociates the bound cyclic GMP, resulting in channel closure and hyperpolarization. Down-regulation of the cascade involves other proteins that block the interaction of transducin with rhodopsin and another protein that may interfere with transducin recycling. Cone photoreceptors possess a light-activated cascade that follows the rod format, but it is composed of proteins that are homologous to those of rod photoreceptors. Phototransduction in invertebrate photoreceptors uses rhodopsin to activate a cascade that uses phosphoinositides and calcium ion to regulate membrane polarization.
Department of Anatomy and Cell Biology, University of California, Los Angeles 90024.
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D000941:Antigens / Q000378:metabolism; D019393:Arrestin; D006152:Cyclic GMP / Q000378:metabolism / Q000502:physiology*; D003624:Darkness; D005136:Eye Proteins / Q000378:metabolism; D006153:Guanosine Diphosphate / Q000378:metabolism; D006160:Guanosine Triphosphate / Q000378:metabolism; D006162:Guanylate Cyclase / Q000378:metabolism; D006868:Hydrolysis; D015640:Ion Channel Gating; D007473:Ion Channels / Q000378:metabolism; D007536:Isomerism; D008027:Light; D010727:Phosphoric Diester Hydrolases / Q000378:metabolism; D010766:Phosphorylation; D010786:Photoreceptor Cells / Q000502:physiology*; D012243:Rhodopsin / Q000096:biosynthesis; D012374:Rod Cell Outer Segment / Q000378:metabolism / Q000502:physiology*; D015293:Transducin / Q000096:biosynthesis / Q000378:metabolism
D000941:Antigens; D019393:Arrestin; D005136:Eye Proteins; D007473:Ion Channels; D006153:Guanosine Diphosphate; D006160:Guanosine Triphosphate; D012243:Rhodopsin; D010727:Phosphoric Diester Hydrolases; D015293:Transducin; D006162:Guanylate Cyclase; D006152:Cyclic GMP