Intermediates of the Channel Rhodopsin Photocycle

Channelrhodopsins are sensory photoreceptors that mediate phototaxis in green algae (1, 2). Their function as ion channels renders them useful tools in the field of neuroscience. Due to the prolonged lifetime and light-sensitivity of the conducting state, mutantants of cysteine 128 are especially important. They can be switched on and off by light, and are therefore called step-function rhodopsins (3). Recent photocycle models have been proposed on the basis of visual and vibrational spectroscopy (4, 5), however, details of the photoreactions are still widely unknown. Here we present spectroscopical data on the slow cycling mutant C128T, with 200 fold extended lifetime of the conducting state. During the photocycle, a fraction of the proteins branches off into a sideway consisting of blue-shifted species which accumulate during prolonged illumination and play an important role in dark state regeneration (5). We will show that - depending on the illumination conditions - several structurally different dark states are regenerated. However, although such large structural alterations occur during the photocycle, they are not necessarily connected with on- and off-switching of the channel. A new model that connects chromophore isomerization and structural alterations of the protein will be discussed.(1) Nagel, G. et al. (2002) Science 296, 2395-8.(2) Nagel, G. et al. (2003) Proceedings of the National Academy of Sciences of the United States of America 100, 13940-5.(3) Berndt, A. et al. (2009) Nature neuroscience 12, 229-34.(4) Radu, I. et al. (2009) Journal of the American Chemical Society 131, 7313-9.(5) Stehfest, K. et al. (2010) Journal of molecular biology 398, 690-702.