Gating mechanisms of a natural anion channelrhodopsin

Anion channelrhodopsins (ACRs) are a class of light-gated channels recently identified in cryptophyte algae that provide unprecedented fast and powerful hyperpolarizing tools for optogenetics. Analysis of photocurrents generated by Guillardia theta ACR 1 (GtACR1) and its mutants in response to laser flashes showed that GtACR1 gating comprises two separate mechanisms with opposite dependencies on the membrane voltage and pH and involving different amino acid residues. The first mechanism, characterized by slow opening and fast closing of the channel, is regulated by Glu-68. Neutralization of this residue (the E68Q mutation) specifically suppressed this first mechanism, but did not eliminate it completely at high pH. Our data indicate the involvement of another, yet-unidentified pH-sensitive group X. Introducing a positive charge at the Glu-68 site (the E68R mutation) inverted the channel gating so that it was open in the dark and closed in the light, without altering its ion selectivity. The second mechanism, characterized by fast opening and slow closing of the channel, was not substantially affected by the E68Q mutation, but was controlled by Cys-102. The C102A mutation reduced the rate of channel closing by the second mechanism by ∼100-fold, whereas it had only a twofold effect on the rate of the first. The results show that anion conductance by ACRs has a fundamentally different structural basis than the relatively well studied conductance by cation channelrhodopsins (CCRs), not attributable to simply a modification of the CCR selectivity filter.