Sensory photoreceptors called channelrhodopsins, which allow algae to move in response to light, have been harnessed for a powerful technique called optogenetics. By expressing these membrane proteins in genetically targeted neurons, researchers can use light to precisely control the activity of the neurons in freely moving animals.

Researchers in the journal PNAS examined the light-absorbing and photochemical properties of a recently discovered family of natural channelrhodopsins that conduct negatively charged chloride ions, potentially enabling highly sensitive and efficient neuronal silencing.

The authors expressed natural anion channelrhodopsins (ACRs) in yeast and human cells, exposed the proteins to laser flashes, and then measured changes in the absorption spectra and ion conductance. Similar to cation-conducting channelrhodopsins (CCRs) currently used for optogenetics, the ACRs transitioned through a sequence of photocycle stages, from the so-called K state to the O state, each of which was characterized by distinct flash-induced absorption changes.

However, the ACR channels opened and conducted photocurrents during the L state, at an earlier photocycle stage than the CCRs. The findings show that ACRs and CCRs display fundamental differences in ion conduction and photochemistry, confirming that these proteins form distinct families of channelrhodopsins.

According to the authors, analysis of the molecular mechanisms underlying the function of ACRs could help optimize the proteins for optogenetics or gene therapy. 

http://www.pnas.org/site/highlights/highlights.xhtml