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Biochemistry. 2015 Sep 8;54(35):5389-400. doi: 10.1021/acs.biochem.5b00597. Epub 2015 Aug 20.

Light-Dark Adaptation of Channelrhodopsin Involves Photoconversion between the all-trans and 13-cis Retinal Isomers.

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Technische Universität Berlin , Institut für Chemie, Sekr. PC14, Straße des 17. Juni 135, D-10623 Berlin, Germany.
Leibniz-Institut für Molekulare Pharmakologie (FMP) , Robert-Rössle-Strasse 10, 13125 Berlin, Germany.
Humboldt-Universität zu Berlin , Institut für Biologie, Invalidenstrasse 42, D-10115 Berlin, Germany.
Department of Chemistry, Ludwig-Maximilians-Universität München , Butenandtstraße 5-13, 81377 München, Germany.


Channelrhodopsins (ChR) are light-gated ion channels of green algae that are widely used to probe the function of neuronal cells with light. Most ChRs show a substantial reduction in photocurrents during illumination, a process named "light adaptation". The main objective of this spectroscopic study was to elucidate the molecular processes associated with light-dark adaptation. Here we show by liquid and solid-state nuclear magnetic resonance spectroscopy that the retinal chromophore of fully dark-adapted ChR is exclusively in an all-trans configuration. Resonance Raman (RR) spectroscopy, however, revealed that already low light intensities establish a photostationary equilibrium between all-trans,15-anti and 13-cis,15-syn configurations at a ratio of 3:1. The underlying photoreactions involve simultaneous isomerization of the C(13)═C(14) and C(15)═N bonds. Both isomers of this DAapp state may run through photoinduced reaction cycles initiated by photoisomerization of only the C(13)═C(14) bond. RR spectroscopic experiments further demonstrated that photoinduced conversion of the apparent dark-adapted (DAapp) state to the photocycle intermediates P500 and P390 is distinctly more efficient for the all-trans isomer than for the 13-cis isomer, possibly because of different chromophore-water interactions. Our data demonstrating two complementary photocycles of the DAapp isomers are fully consistent with the existence of two conducting states that vary in quantitative relation during light-dark adaptation, as suggested previously by electrical measurements.

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