Channelrhodopsins: directly light-gated cation channels

Area of Science:

• Biophysics
• Molecular Biology
• Algal Physiology

Background:

• Phototaxis in green algae is mediated by rhodopsins utilizing all-trans-retinal.
• The green alga Chlamydomonas reinhardtii genome sequencing and EST database provide resources for identifying novel proteins.
• Partial cDNA sequences revealed two proteins, termed channelopsins (Chops), with potential roles in light responses.

Purpose of the Study:

• To identify and characterize novel light-sensitive proteins in Chlamydomonas reinhardtii.
• To investigate the function and properties of the identified channelopsins (Chops).
• To explore the potential applications of these light-gated channels in manipulating cellular physiology.

Main Methods:

• Sequence analysis of Chlamydomonas reinhardtii EST database to identify potential channelopsins.
• Heterologous expression of Chop1 and Chop2 (or their hydrophobic cores) in Xenopus laevis oocytes.
• Electrophysiological recordings to characterize light-gated currents and ion permeability.
• Determination of action spectra for light-induced channel activity.

Main Results:

• Identified and characterized two light-gated ion channels, channelrhodopsin-1 (ChR1) and channelrhodopsin-2 (ChR2).
• ChR1 exhibits proton selectivity with a maximum action spectrum around 500 nm.
• ChR2 functions as a cation channel (permeable to univalent and bivalent cations) with a blue-shifted action spectrum (maximum around 460 nm).
• The N-terminal half of both Chops is sufficient for light-sensitive channel activity.
• Heterologous expression of ChR2 in animal cells allows light-induced manipulation of intracellular calcium and membrane potential.

Conclusions:

• Channelrhodopsins (ChRs) are likely involved in the phototaxis of green algae.
• ChRs represent a novel class of light-activated ion channels.
• ChR2 is a valuable tool for optogenetic manipulation of membrane potential and intracellular calcium levels in animal cells.