Abstract
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.
Highlights
The visual system of the fruit fly Drosophila melanogaster has been extensively studied and is one of the model systems for invertebrate vision[1,2,3]
Many vertebrate rhodopsins that function in visual and non-visual photoreception have been expressed in mammalian cultured cells to facilitate analysis of their spectroscopic and biochemical properties, including spectral sensitivity, along with their physiological functions
The absorption spectra of fruit fly rhodopsins were historically investigated by ectopic expression of rhodopsin genes in ommatidia R1–R6 cells of the ninaE mutant of fruit fly[8]
Summary
The visual system of the fruit fly Drosophila melanogaster has been extensively studied and is one of the model systems for invertebrate vision[1,2,3]. Many vertebrate rhodopsins that function in visual and non-visual photoreception have been expressed in mammalian cultured cells to facilitate analysis of their spectroscopic and biochemical properties, including spectral sensitivity, along with their physiological functions. Preparation of recombinant proteins in cultured cells is one of the essential steps to evaluate the detailed molecular properties of rhodopsins by biochemical and biophysical methods. The evaluation of the molecular properties of Rh7 indicated that it is a UV light-sensitive pigment with bistable character. It exhibited an unusual broad absorption band that was not observed previously, suggesting that this property is one of the important clues to the elucidation of the function of Rh7
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