Spectral Tuning in Mammalian Melanopsins

Melanopsin is a light-sensitive G-protein coupled receptor responsible for essential visual and non-visual light-mediated behaviors in mammals. Human melanopsin shows maximal sensitivity (λmax) in the blue region of the spectrum (∼480 nm), and available evidence suggests that this spectral sensitivity may be retained across mammals. However, melanopsin spectral sensitivity has been defined for only a small number of species, and the molecular mechanisms determining this property remain poorly understood. Here, we use heterologous action spectroscopy to determine the spectral sensitivity of melanopsins from 8 mammalian species, selected to cover diversity in retinal physiology, lighting niche, and evolutionary distance, and of engineered mutants of mouse melanopsin designed to explore mechanisms of spectral tuning. We find that melanopsin λmax varies by only 23 nm across tested mammalian species and that, within this range, it is not strongly predicted by phylogeny, retinal physiology, or lighting niche. Mutation of residues predicted to shift the electrostatic environment of the chromophore was successfully applied to produce long and short wavelength shifts in the spectral sensitivity of mouse melanopsins. However, neither natural diversity in melanopsin λmax nor the magnitude of shifts produced by mutagenesis could be adequately predicted by mechanisms of spectral tuning established in vertebrate visual or invertebrate opsins. Our data indicate that melanopsin spectral sensitivity is constrained across mammalian species via molecular mechanisms that are substantially distinct from those defined in other branches of the opsin family.