Channelrhodopsin-2 (ChR2) triggers the phototaxis of the green algae Chlamydomonas reinhardtii. Amino acids 1-315 form a Bacteriorhodopsin (BR)-like heptahelical membrane domaine, which comprises the ion permeability and the covalently bound retinal chromophore. Depending on the electrochemical gradients, protons and cations are conducted in both directions upon light activation of ChR2. However, only little is known about the residues that determine the channel function, while proton pumping BR has characterized in almost any detail. In order to ensure a unidirectional charge transfer, BR goes through a photocycle with separated proton uptake and release in which only one proton is pumped. In contrast, Channelrhodopsins conduct hundreds of ions during each cycle. Hence, we deduce that these proteins are approaching a defined photointermediate which forms an open channel pore, i.e. the conducting state. By Two Electrode Voltage Clamp (TEVC) measurments in Xenopus oocytes, we identified an amino acid that plays a crucial role in this process. Mutation of C128 in Helix3 to Thr, Ala or Ser, decelerates the ChR2 kinetics dramatically. For instance, the on-kinetics of C128A is 10 times slower and the off-kinetics even 2000 times slower compared to the wilde type. In addition, we show that cells expressing these mutants are more than 300 times more light-sensitive than ChR2-WT and that they can be used as photo-switches. In BR, a corresponding threonine (BR-Thr90) is located near the 11-12 position of the retinal and is involved in the rearrangement of the α-helices during the photocycle1. We conclude that Cys 128 is critical for both fast opening anf fast closing of the of the ion channelpore.1. Joh, N. H., Min, A., Faham, S., Whitelegge, J. P., Yang, D., Woods, V. L., and Bowie, J. U. (2008) Nature 453, 1266-1270