Phototaxis and photophobic responses of the green alga Chlamydomonas reinhardtii
are mediated by microbial rhodopsins with the chromophore retinal. Sequence comparison with other microbial rhodopsins from archaea as the light-driven proton pump bacteriorhodopsin and the light-driven chloride pump halorhodopsin showed an overall homology of 15 to 20 % of those two algae chromoproteins. It is equally important that the N-terminal half approximately 300 of 712 and 737 amino acids, respectively, comprises seven hypothetical transmembrane helices as it is typical for rhodopsin-like proteins. Morover, several of the amino acids are conserved, which define the retinal binding site as well as the H+
-transporting pathway in bacteriorhodopsin. Recently, we demonstrated that two of these retinal-binding proteins from the eyespot of the alga, which we named channelrhodopsin-1 and -2 (ChR1 and ChR2), showed channel activity, directly activated by light when expressed in oocytes from Xenopus laevis
or HEK 293 cells. ChR1 is selective for protons, whereas ChR2 is also conductive for monovalent and divalent cations. For both proteins the N-terminal hydrophobic half is sufficient to enable light-gated channel activity, demonstrating that the seven transmembrane helix motif represents a new class of ion channels.