Channelrhodopsin-1 initiates phototaxis and photophobic responses in Chlamydomonas by immediate light-induced depolarization

P. Berthold1, S.P. Tsunoda1, O.P. Ernst2, D. Gradmann1 and P. Hegemann1

1Institute for Biology, Experimental Biophysics, Humboldt-Universität zu Berlin, 10115 Berlin, Germany 2Institute for Medical Physics and Biophysics, Charité-Universitätsmedizin Berlin, 10117 Berlin, Germany

Channelrhodopsins (ChR1 and ChR2) are light-gated ion channels acting as sensory photoreceptors in Chlamydomonas. In neuroscience they are used to trigger action potentials by light in neuronal cells, tissues or living animals. For Chlamydomonas cells with low ChR2 content we demonstrate that photophobic and phototactic responses strictly depend on the availability of ChR1. Since ChR1 was described as a H+-channel, the ion specificity of ChR1 was reinvestigated in Xenopus oocytes. Our experiments show that ChR1 conducts beyond H+ also Na+, K+ and Ca2+. The kinetic selectivity analysis demonstrates that H+ selectivity is not due to specific translocation but due to selective ion binding. Purified recombinant ChR1 comprises two species with different absorption maxima, ChR1505 and ChR1463 that are in pH-dependent equilibrium. ChR1 is a photochromic and protochromic sensory photoreceptor that functions as a light-activated cation channel mediating phototactic and photophobic responses via depolarizing currents in a wide range of ionic conditions.

e-mail address of presenting author: hegemape@rz.hu-berlin.de