Structure and Function of Halorhodopsin

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Abstract
A comprehensive review of the physiology, structure, and function of halorhodopsin (HR), the only known light‐driven anion pump, is presented. Beside the well‐studied transport function of HR in intact cells the article focuses on recent results about the molecular properties of HR. Overexpression and in vivo 2‐D crystallization allowed structural investigations at a level of 7 Å resolution. The results demonstrate a very close structural relationship with the proton pump bacteriorhododopsin (BR). The retinal binding site as well as a chloride binding site near the Schiff base in HR can be modeled with the side chains placed into the corresponding positions in the BR structural model. Mechanistically the vectorial catalytic cycle of HR is similar to that of BR, as suggested before (Oesterhelt et al., J. Bioenerg. Biomembr. 1992, 24: 181), and consists of photoisomerization of the retinal moiety which triggers chloride movement within the transport site towards the Schiff base. This is followed by an accessibility change from the extracellular (EC) channel to the cytoplasmic (CP) channel allowing chloride to be released into the cytoplasm. After reisomerization and reversion of the accessibility change, a chloride is rebound into the transport site from the extracellular surface through EC. In the absence of transported ions or the additional presence of azide, photoisomerization to 13‐cis is followed directly by an accessibility change and release of a proton from the Schiff base through CP. Blue light causes photoisomerization back to trans and the accessibility change is reversed. Uptake of a proton through EC completes proton transport from the outside to the inside. Depending on relative concentrations of chloride and azide, both modes of ion translocation operate in parallel and as alternatives during individual cycles of a molecule. Future experimentation will have to fill in the many details of this molecular model of ion transport.