Channelrhodopsin-2, a directly light-gated cation-selective membrane channel

TLDR

Microbial rhodopsins, including light‑driven ion transport proteins such as bacteriorhodopsin and the green‑alga channelrhodopsins ChR1 and ChR2, generate photocurrents and possess seven‑transmembrane helices characteristic of G‑protein‑coupled receptors but with a distinct cation‑selective motif. The study aims to demonstrate that ChR2 is a directly light‑switched, cation‑selective ion channel. This was shown by functional expression of ChR2 in Xenopus laevis oocytes and mammalian cells. ChR2 rapidly opens upon photon absorption, allowing high permeability for monovalent and divalent cations, desensitizes under continuous light to a lower steady‑state conductance, recovers faster with extracellular H⁺ and hyperpolarization but slower with intracellular H⁺, is mainly expressed under low‑light conditions indicating a role in photoreception, and can depolarize cells simply by illumination.

Abstract

Microbial-type rhodopsins are found in archaea, prokaryotes, and eukaryotes. Some of them represent membrane ion transport proteins such as bacteriorhodopsin, a light-driven proton pump, or channelrhodopsin-1 (ChR1), a recently identified light-gated proton channel from the green alga Chlamydomonas reinhardtii . ChR1 and ChR2, a related microbial-type rhodopsin from C. reinhardtii , were shown to be involved in generation of photocurrents of this green alga. We demonstrate by functional expression, both in oocytes of Xenopus laevis and mammalian cells, that ChR2 is a directly light-switched cation-selective ion channel. This channel opens rapidly after absorption of a photon to generate a large permeability for monovalent and divalent cations. ChR2 desensitizes in continuous light to a smaller steady-state conductance. Recovery from desensitization is accelerated by extracellular H + and negative membrane potential, whereas closing of the ChR2 ion channel is decelerated by intracellular H + . ChR2 is expressed mainly in C. reinhardtii under low-light conditions, suggesting involvement in photoreception in dark-adapted cells. The predicted seven-transmembrane α helices of ChR2 are characteristic for G protein-coupled receptors but reflect a different motif for a cation-selective ion channel. Finally, we demonstrate that ChR2 may be used to depolarize small or large cells, simply by illumination.