Xanthorhodopsin: A Proton Pump with a Light-Harvesting Carotenoid Antenna
Sergei P. Balashov,1*
Eleonora S. Imasheva,1
Vladimir A. Boichenko,2
Josefa Antón,3
Jennifer M. Wang,1
Janos K. Lanyi1*
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 
40%. The antenna carotenoid extends the wavelength range of the collection of light for uphill transmembrane proton transport.
1 University of California, Irvine, CA 92697, USA.
2 Institute of Basic Biological Problems, Russian Academy of Sciences, Pushchino, 142290, Russia.
3 University of Alicante, Alicante, E-03080, Spain.
* To whom correspondence should be addressed. E-mail: balashov{at}uci.edu (S.P.B.); jlanyi{at}orion.oac.uci.edu (J.K.L.)
