Abstract
Channelrhodopsins (ChR) are light-gated ion-channels heavily used in optogenetics. Upon light excitation an ultrafast all-trans to 13-cis isomerization of the retinal chromophore takes place. It is still uncertain by what means this reaction leads to further protein changes and channel conductivity. Channelrhodopsin-1 in Chlamydomonas augustae exhibits a 100 fs photoisomerization and a protonated counterion complex. By polarization resolved ultrafast spectroscopy in the mid-IR we show that the initial reaction of the retinal is accompanied by changes in the protein backbone and ultrafast protonation changes at the counterion complex comprising Asp299 and Glu169. In combination with homology modelling and quantum mechanics/molecular mechanics (QM/MM) geometry optimization we assign the protonation dynamics to ultrafast deprotonation of Glu169, and transient protonation of the Glu169 backbone, followed by a proton transfer from the backbone to the carboxylate group of Asp299 on a timescale of tens of picoseconds. The second proton transfer is not related to retinal dynamics and reflects pure protein changes in the first photoproduct. We assume these protein dynamics to be the first steps in a cascade of protein-wide changes resulting in channel conductivity.
Highlights
The discovery of channelrhodopsins [1,2], a light-gated cation channel which can be expressed in animal cells, lead to the invention of optogenetics
Regions dominated by retinal modes, which are known from prior resonance Raman studies [12,14,18], are shaded in orange
The C=ND stretching mode of the Schiff base absorbs at 1623 cm−1 in the ground state and shifts to 1602 cm−1 upon photoisomerization
Summary
The discovery of channelrhodopsins [1,2], a light-gated cation channel which can be expressed in animal cells, lead to the invention of optogenetics This new application field renewed the interest in the photo-physics of rhodopsins, in particular how to tailor optical properties of rhodopsins for applications, i.e., the absorption wavelength of the rhodopsin, and the yield for channel opening. A growing number of studies investigate the mechanism of the photo-induced reaction, which starts, like in all rhodopsins, with photoisomerization of the retinal chromophore. How this reaction couples to the protein and transfers the photoisomerization to effective channel opening is still not understood. Since excitation of rhodopsins is connected with a significant charge translocation along the chromophore, the altered electric field is Molecules 2020, 25, 848; doi:10.3390/molecules25040848 www.mdpi.com/journal/molecules
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