Charge Transport by Light-Activated Rhodopsins Determined by Electrophysiological Recordings

Electrophysiological experiments are required to determine the ion transport properties of light-activated currents from microbial rhodopsin expressing cells. The recordings set the quantitative basis for correlation with spectroscopic data and for understanding of channel gating, ion transport vectoriality, or ion selectivity. This chapter focuses on voltage-clamp recordings of channelrhodopsin-2-expressing cells, and it will describe different illumination protocols that reveal the kinetic properties of gating. While the opening and closing reaction is determined from a single turnover upon a short laser flash, desensitization of the light-gated currents is studied under continuous illumination. Recovery from the desensitized state is probed after prolonged illumination with a subsequent light activation upon different dark intervals. Compiling the experimental data will define a minimum number of states in kinetic schemes used to describe the light-gated currents in channelrhodopsins, and emphasis will be given on how to correlate the results with the different time-resolved spectroscopic experiments.