CONVERSION OF BACTERIORHODOPSIN INTO A CHLORIDE-ION PUMP

In the light-driven proton pump bacteriorhodopsin, proton transfer fro m the retinal Schiff base to aspartate-85 is the crucial reaction of t he transport cycle. In halorhodopsin, a light-driven chloride ion pump , the equivalent of residue 85 is threonine. When aspartate-85 was rep laced with threonine, the mutated bacteriorhodopsin became a chloride ion pump when expressed in Halobacterium salinarium and, like halorhod opsin, actively transported chloride ions in the direction opposite fr om the proton pump. Chloride was bound to it, as revealed by large shi fts of the absorption maximum of the chromophore, and its photointerme diates included a red-shifted state in the millisecond time domain, wi th its amplitude and decay rate dependent on chloride concentration. B acteriorhodopsin and halorhodopsin thus share a common transport mecha nism, and the interaction of residue 85 with the retinal Schiff base d etermines the ionic specificity.