Xanthorhodopsin: A Proton Pump with a Light-Harvesting Carotenoid Antenna

Energy transfer from light-harvesting carotenoids to chlorophyll is common in photosynthesis, but such antenna pigments have not been observed in retinal-based ion pumps and photoreceptors. Here we describe xanthorhodopsin, a proton-pumping retinal protein/carotenoid complex in the eubacterium Salinibacter ruber. The wavelength dependence of the rate of pumping and difference absorption spectra measured under a variety of conditions indicate that this protein contains two chromophores, retinal and the carotenoid salinixanthin, in a molar ratio of about 1:1. The two chromophores interact strongly, and light energy absorbed by the carotenoid is transferred to the retinal with a quantum efficiency of approximately 40%. The antenna carotenoid extends the wavelength range of the collection of light for uphill transmembrane proton transport.