Photosensory responses of green algae are mediated by rhodopsins with microbial-type chromophores. The recently detected cDNA sequences from the green alga Chlamydomonas reinhardtii that encode microbial opsin-related proteins were termed Channelrhodopsin-1 and 2 (Chop1/2) by us (Nagel et al., 2002; 2003) and CSRA/B (Sineshchekov et al., 2002) or Acop1/2 (Suzuki et al., 2003) by others.

The hydrophobic core region of the proteins shows homology to the light-activated proton pump bacteriorhodopsin. Channelrhodopsin-2 (ChR2 = Chop-2 + retinal) is a cation channel, which is directly switched by light, as shown by expression in Xenopus laevis oocytes (Nagel et al., 2003). The action spectrum of ChR2 has its maximum at 460 nm (Sineshchekov et al., 2002; Nagel et al., 2003). We show heterologous expression of Channelopsin-2 in mammalian cells. Whole cell patch clamp studies demonstrate large light-gated ion currents and the capacity of ChR2 to depolarize the membrane by illumination. ChR2 or truncated Channelrhodopsin-2 (ChR2-315), when expressed in mammalian cells, yields light-gated channel activity with no apparent difference from Xenopus laevisexpressed ChR2.