Microbial-type rhodopsins are found in archaea, prokaryotes, and eukaryotes. Some of them represent membrane ion transport proteins such as bacteriorhodopsin, a light-driven proton pump, or channelrhodopsin-1 (ChR1), a recently identified light-gated proton channel from the green alga Chlamydomonas reinhardtii. ChR1 and ChR2, a related microbial-type rhodopsin from C. reinhardtii, were shown to be involved in generation of photocurrents of this green alga. We demonstrate by functional expression, both in oocytes of Xenopus laevis and mammalian cells, that ChR2 is a directly light-switched cation-selective ion channel. This channel opens rapidly after absorption of a photon to generate a large permeability for monovalent and divalent cations. ChR2 desensitizes in continuous light to a smaller steady-state conductance. Recovery from desensitization is accelerated by extracellular H ؉ and negative membrane potential, whereas closing of the ChR2 ion channel is decelerated by intracellular H ؉ . ChR2 is expressed mainly in C. reinhardtii under low-light conditions, suggesting involvement in photoreception in dark-adapted cells. The predicted seventransmembrane ␣ helices of ChR2 are characteristic for G proteincoupled receptors but reflect a different motif for a cation-selective ion channel. Finally, we demonstrate that ChR2 may be used to depolarize small or large cells, simply by illumination.voltage clamp ͉ patch clamp ͉ light sensitive ͉ Chlamydomonas reinhardtii ͉ Xenopus laevis oocyte

Phototaxis and photophobic responses of green algae are mediated by rhodopsins with microbial-type chromophores. We report a complementary DNA sequence in the green alga Chlamydomonas reinhardtii that encodes a microbial opsin-related protein, which we term Channelopsin-1. The hydrophobic core region of the protein shows homology to the light-activated proton pump bacteriorhodopsin. Expression of Channelopsin-1, or only the hydrophobic core, in Xenopus laevis oocytes in the presence of all-trans retinal produces a light-gated conductance that shows characteristics of a channel selectively permeable for protons. We suggest that Channelrhodopsins are involved in phototaxis of green algae.

Abstract:The flagellate Euglena gracilis contains a photoactivated adenylyl cyclase (PAC), consisting of the flavoproteins PACa and PACb. Here we report functional expression of PACs in Xenopus laevis oocytes, HEK293 cells and in Drosophila melanogaster, where neuronal expression yields light-induced changes in behavior. The activity of PACs is strongly and reversibly enhanced by blue light, providing a powerful tool for light-induced manipulation of cAMP in animal cells.cAMP is a ubiquitous second messenger across phyla 1 and multiple adenylyl cyclases, and phosphodiesterases are involved in its formation and degradation, respectively. A light-activated adenylyl cyclase that is crucial for photoavoidance has been identified in the unicellular flagellate Euglena gracilis 2 . This adenylyl cyclase is composed of two PACa and two PACb subunits, which exhibit adenylyl cyclase activity that is enhanced by blue light. Each subunit harbors two BLUF-type photoreceptor domains, binding flavin adenine dinucleotide 3,4 , and two catalytic domains that are homologous to