Melanopsin Forms a Functional Short-Wavelength Photopigment
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- 作者：Lucy A. Newman ; Marquis T. Walker ; R. Lane Brown ; Thomas W. Cronin ; and Phyllis R. Robinson
- 刊名：Biochemistry
- 出版年：2003
- 出版时间：November 11, 2003
- 年：2003
- 卷：42
- 期：44
- 页码：12734 - 12738
- 全文大小：80K
- 年卷期：v.42,no.44(November 11, 2003)
- ISSN：1520-4995
文摘
Recently, melanopsin has emerged as the leading candidate for the elusive photopigment ofthe mammalian circadian system. This novel opsin-like protein is expressed in retinal ganglion cells thatform the retinohypothalamic tract, a neuronal connection between the retina and the suprachiasmatic nucleus.These hypothalamic structures contain the circadian pacemaker, which generates daily rhythms inphysiology and behavior. In mammals, proper synchronization of these rhythms to the environmentallight-dark cycle requires retinal input. Surprisingly, rod and cone photoreceptors are not required. Instead,the melanopsin-containing ganglion cells are intrinsically sensitive to light, perhaps responding via amelanopsin-based signaling pathway. To test this hypothesis, we have characterized melanopsin followingheterologous expression in COS cells. We found that melanopsin absorbed maximally at 424 nm afterreconstitution with 11-cis-retinal. Furthermore, melanopsin activated the photoreceptor G-protein, transducin,in a light-dependent manner. In agreement with the measured absorbance spectrum, melanopsin was mostefficiently excited by blue light (420-440 nm). In contrast, published action spectra suggest that thephotopigment underlying the intrinsic light sensitivity of SCN-projecting RGCs has an absorption maximumnear 484 nm. In summary, our experiments constitute the first direct demonstration that melanopsin formsa photopigment capable of activating a G-protein, but its spectral properties are not consistent with theaction spectrum for circadian entrainment.