title: Retinoid cycling proteins redistribute in light-/dark-adapted octopus retinas.
abstract: In cephalopods, the complex rhodopsin-retinochrome system serves to regenerate metarhodopsin and metaretinochrome after illumination. In the dark, a soluble protein, retinal-binding protein (RALBP), shuttles 11-cis retinal released from metaretinochrome located in the photoreceptor inner segments to metarhodopsin present in the rhabdoms. While in the rhabdoms, RALBP delivers 11-cis retinal to regenerate rhodopsin and in turn binds the all-trans isomer released by metarhodopsin. RALBP then returns all-trans retinal to the inner segments to restore retinochrome. The conventional interpretation of retinoid cycling is contradicted by immunocytochemical studies showing that, in addition to rhodopsin, retinochrome is present in the rhabdomal compartment, making possible the direct exchange of chromophores between the metapigments with the potential exclusion of RALBP. By using immunofluorescence and laser scanning confocal microscopy, we have precisely located opsin, aporetinochrome, and RALBP in light-/dark-adapted octopus retinas. We found differences in the distribution of all three proteins throughout the retina. Most significantly, comparison of cross sections though light- and dark-adapted rhabdoms showed a dramatic shift in position of the proteins. In the dark, opsin and retinochrome colocalized at the base of the rhabdomal microvilli. In the light, opsin redistributed along the length of the microvillar membranes, and retinochrome retreated to a location that is perhaps extracellular. RALBP was present in the core cytoplasm of the photoreceptor outer segments in the dark, and RALBP moved to the periphery in the light. Because of the colocalization of opsin and retinochrome in the dark, we believe that the two metapigments participate directly in chromophore exchange. RALBP may serve to transport additional chromophore from the inner segments to the rhabdoms and may not be immediately involved in the exchange process.
authors.affiliation: Department of Biology, California State University, Carson 90747, USA.
authors.forename: L J
authors.initials: LJ
authors.lastname: Robles
authors.forename: J L
authors.initials: JL
authors.lastname: Camacho
authors.forename: S C
authors.initials: SC
authors.lastname: Torres
authors.forename: A
authors.initials: A
authors.lastname: Flores
authors.forename: R N
authors.initials: RN
authors.lastname: Fariss
authors.forename: B
authors.initials: B
authors.lastname: Matsumoto
mesh_terms: D000221:Adaptation, Ocular / Q000502:physiology*; D000818:Animals; D003623:Dark Adaptation / Q000502:physiology*; D005455:Fluorescent Antibody Technique; D006651:Histocytochemistry; D018613:Microscopy, Confocal; D009419:Nerve Tissue Proteins / Q000378:metabolism*; D049831:Octopodiformes / Q000033:anatomy & histology / Q000378:metabolism*; D010786:Photoreceptor Cells / Q000378:metabolism; D012160:Retina / Q000378:metabolism* / Q000648:ultrastructure; D012168:Retinal Pigments / Q000378:metabolism; D012176:Retinoids / Q000378:metabolism*; D012243:Rhodopsin / Q000378:metabolism; D017299:Rod Opsins / Q000378:metabolism
chemical_list: D009419:Nerve Tissue Proteins; D012168:Retinal Pigments; D012176:Retinoids; D017299:Rod Opsins; C023421:retinochrome; D012243:Rhodopsin