Photochemistry of halorhodopsin

Abstract

Halorhodopsin is a light-driven inward Cl − pump found in the membrane of a halophilic archaeon called Halobacterium salinarum. While the physiological role of halorhodopsin has not been fully resolved, its functional mechanism has been studied as a model system for anion transport. Halorhodopsin has become widely used in optogenetics due to its light-induced neural-silencing ability. Here, we summarize the functional analyses of halorhodopsin since its discovery. Like other microbial rhodopsins, halorhodopsin contains all-trans retinal bound to a specific lysine residue through a protonated Schiff base. Proton-pumping rhodopsins utilize Asp residues as the counter-ions for the protonated Schiff bases. In halorhodopsin, this Asp residue is replaced by Thr, and Cl − becomes the counterion. Photoexcited halorhodopsin undergoes a photocycle including several intermediates where sequential Cl − movements occur. During the formation of the N-intermediate, Cl − moves from its original position to the cytoplasmic channel. During the subsequent N decay, the Cl − is released to the cytoplasmic medium. During the Cl − release, the dissociation constant of Cl − increases significantly compared with that at the dark state. Next, another Cl − is captured from the extracellular medium to complete the net Cl − translocation. This recapture process is not well defined.