Channelrhodopsin-2 (ChR2) is a directly light-gated cation-selective ion channel found in the green alga Chlamydomonas reinhardtii. It was identified as a related microbial-type rhodopsin to channelrhodopsin-1 (ChR1), which is also a light-gated proton channel. ChR2 was shown to be expressed in C. reinhardtii under low-light conditions and is involved in photoreception in dark-adapted cells. Functional expression in Xenopus laevis oocytes and mammalian cells demonstrated that ChR2 is a directly light-switched cation channel that opens rapidly after absorbing a photon, generating a large permeability for monovalent and divalent cations. ChR2 desensitizes in continuous light, with recovery from desensitization accelerated by extracellular H+ and negative membrane potential, while intracellular H+ slows down channel closure. ChR2 is permeable to various cations, including N-methyl-D-glucamine (NMG+), methyl-ammonium, di-methyl-ammonium, and tetra-methyl-ammonium, but not Mg2+ or Zn2+. ChR2 is also permeable to divalent cations such as Ca2+, and its photocurrents are influenced by membrane potential and pH. ChR2 is a cation channel with a pore size larger than that of voltage-activated Na+ channels. It is not coupled to any signal-acceptor protein or transducer, and its electrical properties are independent of the C-terminal region of the protein. ChR2 may be used to depolarize small or large cells by illumination. The study also showed that ChR2 has a blue-shifted action spectrum compared to ChR1, and that both photoreceptors contribute to photoreceptor currents in C. reinhardtii. The study highlights the importance of ChR2 as a directly light-gated cation channel and its potential use in light signal transduction.Channelrhodopsin-2 (ChR2) is a directly light-gated cation-selective ion channel found in the green alga Chlamydomonas reinhardtii. It was identified as a related microbial-type rhodopsin to channelrhodopsin-1 (ChR1), which is also a light-gated proton channel. ChR2 was shown to be expressed in C. reinhardtii under low-light conditions and is involved in photoreception in dark-adapted cells. Functional expression in Xenopus laevis oocytes and mammalian cells demonstrated that ChR2 is a directly light-switched cation channel that opens rapidly after absorbing a photon, generating a large permeability for monovalent and divalent cations. ChR2 desensitizes in continuous light, with recovery from desensitization accelerated by extracellular H+ and negative membrane potential, while intracellular H+ slows down channel closure. ChR2 is permeable to various cations, including N-methyl-D-glucamine (NMG+), methyl-ammonium, di-methyl-ammonium, and tetra-methyl-ammonium, but not Mg2+ or Zn2+. ChR2 is also permeable to divalent cations such as Ca2+, and its photocurrents are influenced by membrane potential and pH. ChR2 is a cation channel with a pore size larger than that of voltage-activated Na+ channels. It is not coupled to any signal-acceptor protein or transducer, and its electrical properties are independent of the C-terminal region of the protein. ChR2 may be used to depolarize small or large cells by illumination. The study also showed that ChR2 has a blue-shifted action spectrum compared to ChR1, and that both photoreceptors contribute to photoreceptor currents in C. reinhardtii. The study highlights the importance of ChR2 as a directly light-gated cation channel and its potential use in light signal transduction.