Structure of the light-driven chloride pump halorhodopsin at 1.8 angstrom resolution

Halorhodopsin, an archaeal rhodopsin ubiquitous in Haloarchaea, uses Light energy to pump chloride through biological membranes. Halorhodopsin crystal s were grown in a cubic Lipidic phase, which allowed the x-ray structure de termination of this anion pump at 1.8 angstrom resolution. Halorhodopsin as sembles to trimers around a central patch consisting of palmitic acid. Next to the protonated Schiff base between Lys(242) and the isomerizable retina l chromophore, a single chloride ion occupies the transport site. Energetic calculations on chloride binding reveal a combination of ion-ion and ion-d ipole interactions for stabilizing the anion 18 angstroms below the membran e surface. ion dragging across the protonated Schiff base explains why chlo ride and proton translocation modes are mechanistically equivalent in archa eal rhodopsins.