Multiple photocycles of channelrhodopsin.

Two rhodopsins with intrinsic ion conductance have been identified recently in Chlamydomonas reinhardtii. They were named "channelrhodopsins" ChR1 and ChR2. Both were expressed in Xenopus laevis oocytes, and their properties were studied qualitatively by two electrode voltage clamp techniques. ChR1 is specific for H+, whereas ChR2 conducts Na+, K+, Ca2+, and guanidinium. ChR2 responds to the onset of light with a peak conductance, followed by a smaller steady-state conductance. Upon a second stimulation, the peak is smaller and recovers to full size faster at high external pH. ChR1 was reported to respond with a steady-state conductance only but is demonstrated here to have a peak conductance at high light intensities too. We analyzed quantitatively the light-induced conductance of ChR1 and developed a reaction scheme that describes the photocurrent kinetics at various light conditions. ChR1 exists in two dark states, D1 and D2, that photoisomerize to the conducting states M1 and M2, respectively. Dark-adapted ChR1 is completely arrested in D1. M1 converts into D1 within milliseconds but, in addition, equilibrates with the second conducting state M2 that decays to the second dark state D2. Thus, light-adapted ChR1 represents a mixture of D1 and D2. D2 thermally reconverts to D1 in minutes, i.e., much slower than any reaction of the two photocycles.