Visions & Reflections (Minireview) Xanthorhodopsin: Proton pump with a carotenoid antenna

Retinal proteins function as photoreceptors and ion pumps. Xanthorhodopsin of Salinibacter ruber is a recent addition to this diverse family. Its novel and distinctive feature is a second chromophore, a caro- tenoid, which serves as light-harvesting antenna. Here we discuss the properties of this carotenoid/retinal complex most relevant to its function (such as the specific binding site controlled by the retinal) and its relationship to other retinal proteins (bacteriorho- dopsin, archaerhodopsin, proteorhodopsin and retinal photoreceptors of archaea and eukaryotes). Antenna addition to a retinal protein has not been observed among the archaea and emerged in bacteria appa- rently in response to environmental conditions where light-harvesting becomes a limiting factor in retinal protein functioning.