Opposite Charge Movements Within the Photoactive Site Modulate Two-Step Channel Closing in GtACR1.

Guillardia theta anion channelrhodopsin 1 is a light-gated anion channel widely used as an optogenetic inhibitory tool. Our recently published crystal structure of its dark (closed) state revealed that the photoactive retinylidene chromophore is located midmembrane in a full-length intramolecular tunnel through the protein, the radius of which is less than that of a chloride ion. Here we show that acidic (glutamate) substitutions for residues within the inner half-tunnel enhance the fast channel closing and, for residues within the outer half-tunnel, enhance the slow channel closing. The magnitude of these effects was proportional to the distance of the mutated residue from the photoactive site. These data indicate that the local electrical field across the photoactive site controls fast and slow channel closing, involving outward and inward charge displacements. In the purified mutant proteins, we observed corresponding opposite changes in kinetics of the M photocycle intermediate. A correlation between fast closing and M rise and slow closing and M decay observed in the mutants suggests that the Schiff base proton is one of the displaced charges. Opposite signs of the effects indicate that deprotonation and reprotonation of the Schiff base take place on the same (outer) side of the membrane and explains opposite rectification of fast and slow channel closing. Оur comprehensive protein-wide acidic residue substitution screen shows that only mutations of the residues located in the intramolecular tunnel confer strong rectification, which confirms the prediction that the tunnel expands upon photoexcitation to form the anion pathway.