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

Sara Bruun, Daniel Stoeppler, Anke Keidel, Uwe Kuhlmann, Meike Luck, Anne Diehl, Michel Andreas Geiger, David Woodmansee, Dirk Trauner, Peter Hegemann, Hartmut Oschkinat, Peter Hildebrandt, Katja Stehfest

Research output: Contribution to journalArticlepeer-review

Abstract

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.

Original languageEnglish (US)
Pages (from-to)5389-5400
Number of pages12
JournalBiochemistry
Volume54
Issue number35
DOIs
StatePublished - Sep 8 2015

ASJC Scopus subject areas

  • Biochemistry

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