Structure and function of light driven chloride ion pump

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An extremely haloalkaliphilic archaon Natronomonas pharaonis possesses the light-drivenchloride ion pump, halorhodopsin(pHR) on its membrane. We purified pHR in the form of native membrane, crystallized, and solved its structure at 2. 0A resolution. Furthermore we succeeded to solve the structure of chloride ion free blue form of pHR, which is considered to be O-like intermediate. From the structure comparison between chloride-boud and chloride-free forms of pHR, it was shown that the removal of the chloride ion, which is located between the retinal Schiff base and Thr126, is accompanied by such a deformation of helix C that the side chain of Thr126 moves toward helix G, leading to a significant shrinkage of the chloride ion binding site and the water molecule, which was hydrogen boded with the chloride ion, had moved.