RT Journal Article SR Electronic T1 Channelrhodopsin-1 Initiates Phototaxis and Photophobic Responses in Chlamydomonas by Immediate Light-Induced Depolarization JF The Plant Cell JO Plant Cell FD American Society of Plant Biologists SP 1665 OP 1677 DO 10.1105/tpc.108.057919 VO 20 IS 6 A1 Berthold, Peter A1 Tsunoda, Satoshi P. A1 Ernst, Oliver P. A1 Mages, Wolfgang A1 Gradmann, Dietrich A1 Hegemann, Peter YR 2008 UL http://www.plantcell.org/content/20/6/1665.abstract AB Channelrhodopsins (CHR1 and CHR2) are light-gated ion channels acting as sensory photoreceptors in Chlamydomonas reinhardtii. In neuroscience, they are used to trigger action potentials by light in neuronal cells, tissues, or living animals. Here, we demonstrate that Chlamydomonas cells with low CHR2 content exhibit photophobic and phototactic responses that strictly depend on the availability of CHR1. Since CHR1 was described as a H+-channel, the ion specificity of CHR1 was reinvestigated in Xenopus laevis oocytes. Our experiments show that, in addition to H+, CHR1 also conducts 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 consists of two isoforms with different absorption maxima, CHR1505 and CHR1463, that are in pH-dependent equilibrium. Thus, 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.