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

Channelrhodopsins (CHR1 and CHR2) are light-gated ion channels acting as sensory photoreceptors in Chlamydomonas reinhardtii. Inneuroscience, 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, CHR1(505) and CHR1(463), 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.