Elucidation of Cl--pumping mechanism and significance of trimer formation of halorhodopsin

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Halorhodopsin (HR) is a light-driven Cl^- pump expressed in the membrane of Halobacteria. We succeeded in developing a new method : HR was expressed in Xenopus laevis Oocytes, and photo-induced membrane current was measured due to the Cl^- transport. This method is highly quantitative. We prepared various HR mutants using E. coli expression system. Careful investigation on both the photochemistry and transport activities of the mutants has been done. We found Arg123 that is an essentially important amino acid residue. During the transition from N to O intermediate, Cl is released to cytoplasm while Cl is uptaken from extracellular space during O to HR' transition. We showed that HR forms trimer even in the presence of detergents, and that the trimer is more robust than the monomer.