Interaction of Antenna Carotenoid and Retinal in the Light-Driven Pumps of Salinibacter Ruber and Gloeobacter Violaceus

Abstract

Xanthorhodopsin is a light-driven proton pump like bacteriorhodopsin, but contains a second chromophore, salinixanthin, a carotenoid. Action spectra for transport and fluorescence of the retinal upon excitation of salinixanthin, as well as femtosecond kinetics of carotenoid fluorescence, indicate that the carotenoid functions as an antenna to the retinal, with ca. 50% efficiency for excited-state energy transfer. The resulting charge redistribution in the retinal and the local electric field produced, in the excited state and in the primary stable photoproduct K, in turn, causes distinct electrochromic shifts in the carotenoid spectrum. The close interaction of carotenoid and retinal suggests that the two chromophores are in proximity. Tight binding of the carotenoid, as indicated by its sharpened vibration bands and intense induced circular dichroism in the visible, is removed in the absence of the retinal, and restored upon reconstitution with retinal or retinal analogues. The crystallographic structure of the xanthorhodopsin at 1.9 A resolution identifies the location of the chromophores. The structural model from x-ray diffraction reveals that the ring moieties of the retinal and the carotenoid are at nearly van der Waals distance from one another. Salinixanthin and one of the native carotenoids of Gloeobacter, echinenone, bind to gloeorhodopsin expressed in E. coli, and the complex shows excited-state energy exchange much like xanthorhodopsin. Protein sequences and mutational studies in Gloeobacter indicate that binding of the carotenoid depends on a conserved glycine at the keto-ring, which is a tryptophan in bacteriorhodopsin and other archaeal rhodopsins. On this basis, numerous otherwise unrelated eubacterial rhodopsins are predicted to contain light-harvesting carotenoids.