Energy transfer from antenna pigments to the reaction center is common in chlorophyll-based photosynthesis but never been observed in retinal-based ion pumps and photoreceptors. Here we describe xanthorhodopsin, a retinal protein/carotenoid complex in the eubacterium Salinibacter ruber, a proton pump. Difference absorption spectra measured under a variety of conditions and action spectra for pumping indicate that this protein contains two chromophores: retinal and the carotenoid, salinixanthin, in a molar ratio of about 1:1. The two chromophores strongly interact, and light energy absorbed by the carotenoid is efficiently transferred to the retinal and used for transmembrane proton transport.The extreme halophile Salinibacter ruber isolated from salt crystallizer ponds (1,2) can be grown in aerobic heterotrophic culture in 4 M NaCl. This eubacterium accumulates high concentrations of KCl to adapt to the high ionic strength (3), as the haloarchaea in the same environment. After several days of growth, Salinibacter ruber acquires a deep red color, from salinixanthin which constitutes nearly 100% of its carotenoid content and whose chemical structure was recently established (4). It was proposed to provide protection from photodamage and to stabilize the cell membrane because both the polyene and the fatty acid part of this carotenoid acyl glycoside will be immersed in the lipid bilayer (4). We report here that salinixanthin is not the only pigment in the Salinibacter ruber cell membrane, and heterotrophy is not the only source of energy for this organism. These cells contain an unusual retinal protein, which uses salinixanthin to assist harvesting light energy in a wider spectral range and utilizes it for transmembrane proton transport. Thus, it is a light-driven proton pump similar to bacteriorhodopsin (5) and the archaerhodopsins (6) of the archaea, the proteorhodopsins of planktobacteria (7), and leptosphaeria rhodopsin of an eukaryote (8), but with two chromophores. We term it here xanthorhodopsin. Its novel carotenoid antenna is a feature shared with chlorophyll-based light-harvesting complexes and reaction centers (9,10).Illumination of cell membrane vesicles prepared from Salinibacter produces acidification of the medium, which is abolished by the protonophore carbonyl cyanide m-chlorophenylhydrazone (CCCP) (Fig. 1A). When assayed in 1 M Na 2 SO 4 , these light-dependent pH changes are unaffected by the presence of chloride ions (not shown). Thus, the vesicles contain an outward-directed light-driven proton pump like bacteriorhodopsin, and lack a

The pH dependence of the rate constant of dark adaptation (thermal isomerization from all-trans- to 13-cis-bR) drastically changes when Arg82 of bacteriorhodopsin is replaced by an alanine. In the wild type (WT) the rate decreases sharply between pH 2.5 and pH 5. In R82A the sharp decrease is shifted to pH > 7. This correlates with the shift in the pK of the purple-to-blue transition from pH 2.6 in the wild type to pH 7.2 in the mutant (in 150 mM KCl). We propose that the same group that controls the purple-to-blue transition, namely, Asp85, catalyzes dark adaptation. The rate of dark adaptation in the R82A mutant is proportional to the fraction of protonated Asp85, indicating that dark adaptation occurs when Asp85 is transiently protonated. Thermal isomerization is at least 2 x 10(3) times more likely when Asp85 is protonated (blue membrane) than when it is deprotonated (purple membrane). The pH dependence of dark adaptation in the WT can be explained by a model in which the rate of dark adaptation in the WT is also proportional to the fraction of protonated Asp85 and that the pK of Asp85 depends on some other group, X, which deprotonates (or moves away from Asp85) with pK9 and causes the shift in the pK of Asp85 from 2.6 to 7.2. The quantum yield of light adaptation is at least an order of magnitude less in R82A as compared to the WT. The rise time of M formation is very fast in R82A and, unlike the WT, pH independent (1 microsecond versus 85 and 6 microseconds in the WT at pH 7 and 10, respectively). The activation energy of the L to M transition is 6.9 kcal/mol versus 13.5 kcal/mol in the WT. Thus the loss of a positive charge in the active site greatly increases the rate of light-induced deprotonation of the Schiff base. In the R82A mutant, the M decay at pH > 8.8 is much faster than the recovery of initial bR, which suggests a decrease in the rate of back-reaction from N to M. In a suspension of R82A membranes the rate of proton release as measured by the pH-sensitive dye pyranine is delayed by at least 20-fold (in 2 M KCl), while the uptake of protons did not change much (12 ms in the WT versus 8 ms in R82A).(ABSTRACT TRUNCATED AT 400 WORDS)

Titration of Asp-85, the proton acceptor and part of the counterion in bacteriorhodopsin, over a wide pH range (2-11) leads us to the following conclusions: 1) Asp-85 has a complex titration curve with two values of pKa; in addition to a main transition with pKa = 2.6 it shows a second inflection point at high pH (pKa = 9.7 in 150-mM KCl). This complex titration behavior of Asp-85 is explained by interaction of Asp-85 with an ionizable residue X'. As follows from the fit of the titration curve of Asp-85, deprotonation of X' increases the proton affinity of Asp-85 by shifting its pKa from 2.6 to 7.5. Conversely, protonation of Asp-85 decreases the pKa of X' by 4.9 units, from 9.7 to 4.8. The interaction between Asp-85 and X' has important implications for the mechanism of proton transfer. In the photocycle after the formation of M intermediate (and protonation of Asp-85) the group X' should release a proton. This deprotonated state of X' would stabilize the protonated state of Asp-85.2) Thermal isomerization of the chromophore (dark adaptation) occurs on transient protonation of Asp-85 and formation of the blue membrane. The latter conclusion is based on the observation that the rate constant of dark adaptation is directly proportional to the fraction of blue membrane (in which Asp-85 is protonated) between pH 2 and 11. The rate constant of isomerization is at least 10(4) times faster in the blue membrane than in the purple membrane. The protonated state of Asp-85 probably is important for the catalysis not only of all-trans <=> 13-cis thermal isomerization during dark adaptation but also of the reisomerization of the chromophore from 13-cis to all-trans configuration during N-->O-->bR transition in the photocycle. This would explain why Asp-85 stays protonated in the N and O intermediates.