Department of Biophysics, Graduate School of Science, Kyoto University, Kyoto, 606-8502, Japan.
Department of Organic Chemistry for Life Science, Kobe Pharmaceutical University, Kobe, 658-8558, Japan.
Sci Rep. 2017 Aug 4;7(1):7349. doi: 10.1038/s41598-017-07461-9.
The genome of Drosophila melanogaster contains seven rhodopsin genes. Rh1-6 proteins are known to have respective absorption spectra and function as visual pigments in ocelli and compound eyes. In contrast, Rh7 protein was recently revealed to function as a circadian photoreceptor in the brain. However, its molecular properties have not been characterized yet. Here we successfully prepared a recombinant protein of Drosophila Rh7 in mammalian cultured cells. Drosophila Rh7 bound both 11-cis-retinal and 11-cis-3-hydroxyretinal to form photo-pigments which can absorb UV light. Irradiation with UV light caused formation of a visible-light absorbing metarhodopsin that activated Gq-type of G protein. This state could be photoconverted back to the original state and, thus Rh7 is a Gq-coupled bistable pigment. Interestingly, Rh7 (lambda max = 350 nm) exhibited an unusual broad spectrum with a longer wavelength tail reaching 500 nm, whose shape is like a composite of spectra of two pigments. In contrast, replacement of lysine at position 90 with glutamic acid caused the formation of a normal-shaped absorption spectrum with maximum at 450 nm. Therefore, Rh7 is a unique photo-sensor that can cover a wide wavelength region by a single pigment to contribute to non-visual photoreception.
果蝇的基因组包含七个视蛋白基因。已知 Rh1-6 蛋白具有各自的吸收光谱，并在小眼和复眼作为视觉色素发挥作用。相比之下，Rh7 蛋白最近被揭示在大脑中作为昼夜节律光感受器发挥作用。然而，其分子特性尚未得到表征。在这里，我们成功地在哺乳动物培养细胞中制备了果蝇 Rh7 的重组蛋白。果蝇 Rh7 结合 11-顺式视黄醛和 11-顺式-3-羟基视黄醛形成可吸收紫外光的光色素。用紫外光照射会形成一种可见吸收的光变视紫红质，激活 Gq 型 G 蛋白。这种状态可以光转化回原来的状态，因此 Rh7 是一种 Gq 偶联的双稳态色素。有趣的是，Rh7（lambda max = 350nm）表现出异常宽的光谱，具有更长波长的尾巴延伸至 500nm，其形状类似于两种色素光谱的复合。相比之下，将位置 90 的赖氨酸替换为谷氨酸会导致形成具有 450nm 最大吸收的正常形状的吸收光谱。因此，Rh7 是一种独特的光传感器，它可以通过单个色素覆盖宽的波长区域，为非视觉光感受做出贡献。
