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Rhodopsins are photoreceptive membrane proteins containing a retinal chromophore in animals and microbes. Animal and microbial rhodopsins possess 11-cis and all-trans retinal, respectively, and undergo isomerization into all-trans and 13-cis retinal by light. While animal rhodopsins are G protein coupled receptors, the function of microbial rhodopsins is highly divergent, including light-driven ion pumps, light-gated ion channels, photosensors, and light-activated enzymes. Microbial rhodopsins have been the main tools in optogenetics.
Function of rhodopsins starts in 10-15 sec, and activation of rhodopsins occurs in the protein environment that has been optimized during evolution (1015 sec). We thus need various methods to understand these events of 30 orders of magnitude in time. We have studied molecular mechanism of rhodopsins by use of spectroscopic methods. Using ultrafast spectroscopy, we showed the primary event in our vision being retinal photoisomerization. In rhodopsins, photoisomerization of retinal, the shape-changing reaction, occurs even at 77 K. Using low-temperature infrared spectroscopy, we detected protein-bound water molecules of rhodopsins before X-ray crystallography. Detailed vibrational analysis provided structural information such as our color discrimination mechanism.
I will talk about our spectroscopic study of animal and microbial rhodopsins. Recent unexpected findings such as unusual isomerization pathways and temperature effects are also presented.