Photochemical properties of mammalian melanopsin

Melanopsin is the photoreceptor molecule of intrinsically photosensitive retinal ganglion cells, which serve as the input for various nonvisual behavior and physiological functions fundamental to organisms. The retina, therefore, possess a melanopsin-based nonvisual system in addition to the visual system based on the classical visual photoreceptor molecules. To elucidate the molecular properties of melanopsin, we have exogenously expressed mouse melanopsin in cultured cells. We were able to obtain large amounts of purified mouse melanopsin and conducted a comprehensive spectroscopic study of its photochemical properties. Melanopsin has an absorption maximum at 467 nm, and it converts to a meta intermediate having an absorption maximum at 476 nm. The melanopsin photoreaction is similar to that of squid rhodopsin, exhibiting bistability that results in a photosteady mixture of a resting state (melanopsin containing 11-cis-retinal) and an excited state (metamelanopsin containing all-trans-retinal) upon sustained irradiation. The absorption coefficient of melanopsin is 33000 ± 1000 M(-1) cm(-1), and its quantum yield of isomerization is 0.52; these values are also typical of invertebrate bistable pigments. Thus, the nonvisual system in the retina relies on a type of photoreceptor molecule different from that of the visual system. Additionally, we found a new state of melanopsin, containing 7-cis-retinal (extramelanopsin), which forms readily upon long-wavelength irradiation (yellow to red light) and photoconverts to metamelanopsin with short-wavelength (blue light) irradiation. Although it is unclear whether extramelanopsin would have any physiological role, it could potentially allow wavelength-dependent regulation of melanopsin functions.