Platymonas subcordiformis Channelrhodopsin-2 (PsChR2) Function

Abstract

The photocycle kinetics of PsChR2, among the most highly efficient light-gated cation channels and the most blue-shifted channelrhodopsin, was studied by time-resolved absorption spectroscopy in the 340-650 nm range and in the 100 ns to 3 s time window. Global exponential fitting of the time-dependence of spectral changes revealed six lifetimes: 0.60 μs, 5.3 μs, 170 μs, 1.4 ms, 6.7 ms, and 1.4 s. The sequential intermediates derived for a single cycle scheme based on these lifetimes were found to contain mixtures of K, L, M, O and P molecular states, named in analogy to photointermediates in the bacteriorhodopsin photocycle. The photochemistry is described by the superposition of two independent parallel photocycles. The analysis revealed that 30% of the photoexcited receptor molecules followed Cycle 1 through the K, M, O, and P states, while 70% followed Cycle 2 through the K, L, M, and O states. The recovered state, R, is spectrally close, but not identical to the dark state on the seconds time scale. The two-cycle model of this high efficiency ChR opens new perspectives in understanding the mechanism of channelrhodopsin function.