Retinal Shuttle Protein and Visual Photoreception

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In Mollusca, vision is maintained by the photoreactions of two photopigments. Rhodopsin is present in the rhabdomeres of the outer segment of the visual cell and retinochrome is in the myeloid bodies of the inner segment. Regeneration of each photopigment is supported by the mutual exchange of retinal between a retinal-binding protein (RALBP) and its metapigment. We have examined the role of RALBP in the rhodopsin-retinochrome conjugate system as follows.Immunocytochemical Localization of RALBP : Detailed localization of RALBP was observed electron-microscopically by using poly- and mono-clonal antibodies. In the outer segment, the most marked anti-RALBP labelling appeared along the envelope of core cytoplasm, corresponding to the basal margins of rhabdomeric microvilli. In the inner segment, the labelling was obvious on the entire surface of the myeloid bodies, but scanty in the mitochondria and the nucleus.Photopigment Regeneration and RALBP in the Eyecup : Based upon the previous in … More vitro experiments, details of the actual movement of RALBP in the intact retina was examined during dark incubation following irradiation of the eyecup. It was clear that RALBP is capable of moving within the visual cell as a shuttle of cis and trans retinals, and of exchanging them for metapigment chromophores to assist in the regeneration of rhodopsin and retinochrome.Mechanism of Retinal Exchange between RALBP and Metapigments : To resolve this problem, we have special interest in comparing protein structures of rhodopsin, retinochrome and RALBP in one species, Todarodes. We have determined amino acid sequence of retinochrome as well as RALBP. Retinochrome consists of 301 amino acids with a M.W. of 33,490. The hydropathicity analysis showed that it is folded into 7 hydrophobic transmembrane segments, just like rhodopsins.Retinol-dehydrogenase : RALBP binds appreciable quantities of 11-cis-retinol in addition to retinal. When incubated with a membrane preparation of the retina 11-cis-retinol ligand of RALBP was changed to 11-cis-retinal without releasing from the protein. The highest activity was found in the distal portion of the inner segment. This was similar to the distribution of the myeloid bodies. Less