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M Kono, RK Crouch; Activation of a Red Cone Pigment . Invest. Ophthalmol. Vis. Sci. 2002;43(13):4546.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: In rhodopsin, specific interactions between the 9-methyl group of the retinal chromophore and the protein are crucial for efficient formation of the active Meta II species. To examine how these interactions might affect photoactivation of the salamander red cone pigment, we measured in vitro properties of this cone pigment regenerated with a retinal analog lacking this methyl group (9-desmethyl retinal). Methods: An epitope tagged salamander red cone opsin was expressed transiently in COS cells and regenerated with either 11-cis retinal or 9-desmethyl-11-cis retinal. The pigments were solublized and immunopurified in 0.1% dodecyl maltoside. Formation of the light-activated Meta II-like intermediate was monitored by spectroscopy and by their ability to activate transducin. Results: The salamander red cone pigment regenerated with 9-desmethyl retinal absorbs maximally at 509 nm, 50 nm blue shifted from its normal absorption maximum. Unlike with 9-desmethyl-regenerated rhodopsin, the red cone pigment readily bleaches with light. The 9-desmethyl regenerated red cone pigment is also able to activate transducin in a light-dependent manner as well as or better than the cone pigment regenerated with 11-cis retinal. This appears to be partly due to increased stabilization of the Meta II-like species with the bound analog. However, both the 11-cis and 9-desmethyl retinals are released from the red cone pigment faster than from rhodopsin. Conclusion: Light-dependent formation of an active intermediate is not coupled to a steric interaction between the 9-methyl group of retinal and the red cone opsin.
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