Purification of photochemically active halorhodopsin

Far-Red Absorbing Rhodopsins, Insights From Heterodimeric Rhodopsin-Cyclases

The recently discovered Rhodopsin-cyclases from Chytridiomycota fungi show completely unexpected properties for microbial rhodopsins. These photoreceptors function exclusively as heterodimers, with the two subunits that have very different retinal chromophores. Among them is the bimodal photoswitchable Neorhodopsin (NeoR), which exhibits a near-infrared absorbing, highly fluorescent state. These are features that have never been described for any retinal photoreceptor. Here these properties are discussed in the context of color-tuning approaches of retinal chromophores, which have been extensively studied since the discovery of the first microbial rhodopsin, bacteriorhodopsin, in 1971 (Oesterhelt et al., Nature New Biology, 1971, 233 (39), 149–152). Further a brief review about the concept of heterodimerization is given, which is widely present in class III cyclases but is unknown for rhodopsins. NIR-sensitive retinal chromophores have greatly expanded our understanding of the spectral range of natural retinal photoreceptors and provide a novel perspective for the development of optogenetic tools.

The photochemical cycle of halorhodopsin: absolute spectra of intermediates obtained by flash photolysis and fast difference spectra measurements

Results of experiments using flash photolysis and fast difference spectroscopy suggest an extended version of the earlier published scheme of the photochemical cycle of halorhodopsin. Detailed experimental verification of the suggested photocycle is given. Due to the high resolution of the time-resolved difference spectra, absolute spectra of the intermediates in the photocycle were derived, allowing the interpretation of complex kinetic absorbance changes.

Temperature and halide dependence of the photocycle of halorhodopsin from Natronobacterium pharaonis

The photocycle kinetics of halorhodopsin from Natronobacterium pharaonis (pHR(575)) was analyzed at different temperatures and chloride concentrations as well as various halides. Over the whole range of modified parameters the kinetics can be adequately modeled with six apparent rate constants. Assuming a model in which the observed rates are assigned to irreversible transitions of a single relaxation chain, six kinetically distinguishable states (P(1-6)) are discernible that are formed from four chromophore states (spectral archetypes S(j): K(570), L(N)(520), O(600), pHR'(575)). Whereas P(1) coincides with K(570) (S(1)), both P(2) and P(3) have identical spectra resembling L(520) (S(2)), thus representing a true spectral silent transition between them. P(4) constitutes a fast temperature-dependent equilibrium between the chromophore states S(2) and S(3) (L(520) and O(600), respectively). The subsequent equilibrium (P(5)) of the same spectral archetypes is only moderately temperature dependent but shows sensitivity toward the type of anion and the chloride concentration. Therefore, S(2) and S(3) occurring in P(4) as well as in P(5) have to be distinguished and are assigned to L(520)<--> O(1)(600) and O(2)(600)<--> N(520) equilibrium, respectively. It is proposed that P(4) and P(5) represent the anion release and uptake steps. Based on the experimental data affinities of the halide binding sites are estimated.

Artificial pigments of halorhodopsin and their chloride pumping activities

Halorhodopsin (HR), the light-driven chloride pump of Halobacterium halobium, was bleached with hydroxylamine and regenerated with all-trans-retinal under several different conditions. The largest recovery of the pigment was found with apoprotein obtained from detergent-free HR [HR(BB)]. To compare the chloride-pumping mechanism of HR with that of bacteriorhodopsin (BR; the light-driven proton pump of the same bacteria), HR pigment analogues were reconstituted with the bleached HR (BB) and retinal analogues. The corresponding BR pigment analogues have previously been shown to have little or no proton-pumping activity, except for retinal2 (3,4-dehydroretinal). Pigment analogues with 13-demethylretinal or retinal2 showed an "opsin shift" similar to that of the all-trans-retinal pigment of both HR and BR. Opsin shifts of the pigments of 9-12-phenylretinal and 3,7-dimethyl-2,4,6,8-decatetraenal and haloopsin are slightly different from those of the corresponding BR pigment analogues, presumably reflecting differences of the chromophoric structures in HR and BR. In addition to the spectral properties, the effect of chloride ion on deprotonation of the Schiff base was measured. These pigment analogues showed the "chloride effect" (a shift of the pK value for deprotonation of the Schiff base), but a smaller one than that seen in HR. For a measurement of the chloride-pumping activity, each retinal analogue was added to a culture of L07 cells (BOP-, HOP+, Ret-), and the activity was measured with the cell suspension. Only cultures with retinal or retinal2 showed chloride-pumping activity, as is true for proton pumping by BR. This suggests that a similar retinal-protein interaction is necessary for both ion pumps.

Halorhodopsin and sensory rhodopsin contain a C6-C7 s-trans retinal chromophore

Halorhodopsin (HR) and sensory rhodopsin (SR) have been regenerated with retinal analogues that are covalently locked in the 6-s-cis or 6-s-trans conformations. Both pigments regenerate more completely with the locked 6-s-trans retinal and produce analogue pigments with absorption maxima (577 nm for HR and 592 nm for SR) nearly identical to those of the native pigments (577 and 587 nm). This indicates that HR and SR bind retinal in the 6-s-trans conformation. The opsin shift for the locked 6-s-trans analogue in HR is 1,200 cm-1 less than that for the native chromophore (5,400 cm-1). The opsin shift for the 6-s-trans analogue in SR is 1,100 cm-1 less than that for the native retinal (5,700 cm-1). This demonstrates that approximately 20% of the opsin shift in these pigments arises from a protein-induced change in the chromophore conformation from twisted 6-s-cis in solution to planar 6-s-trans in the protein. The reduced opsin shift observed for the locked 6-s-cis analogue pigments compared with the locked 6-s-trans pigments may be due to a positive electrostatic perturbation near C7.

Biochemical and spectroscopic characterization of the blue-green photoreceptor in Halobacterium halobium

Spectroscopic evidence indicates the presence of a second sensory receptor sR-II in Halobacterium halobium, which causes a repellent response to blue-green light. Reactions with hydroxylamine and NaCNBH3 and reconstitution of the bleached pigment with retinal show that it is very similar to the other retinylidene pigments bacteriorhodopsin, halorhodopsin, and especially the earlier-discovered phototaxis receptor, sensory rhodopsin, renamed sR-I587. The second sensory receptor, sR-II480, has an absorbance maximum at 480 nm and undergoes a cyclic photoreaction with a half-time of approximately 200 msec. Its predominant photocycle intermediate absorbs maximally near 360 nm. The receptor can be detected spectroscopically in the presence of sR-I587 and quantitated through its transient response to 450-nm excitation. It is selectively bleached by low hydroxylamine concentrations that are insufficient to bleach sR-I587 significantly. Its photochemical and phototactic activities can be restored by addition of retinal. The mobility of the receptor, on NaDodSO4/polyacrylamide gels, was similar or identical to that of sR-I587 and slightly faster than bacteriorhodopsin, yielding an apparent molecular mass of 23-24 kDa.

Properties of a second sensory receptor protein in Halobacterium halobium phototaxis

A second slow-cycling retinylidene protein, in addition to slow-cycling (sensory) rhodopsin (SR), can be bleached with hydroxylamine and regenerated with all-trans retinal in photosensory signaling Halobacterium halobium membranes. Flash photolysis shows this protein undergoes a photochemical reaction cycle characterized by photoconversion of its ground state (lambda max 480 nm) to a species with lambda max less than or equal to 360 nm, which thermally regenerates the 480-nm species with a t1/2 of 260 msec at 25 degrees C, under conditions in which SR photocycles at 650 msec in the same membranes. Mutants characterized with respect to their phototaxis behavior are identified which contain SR and the 480-nm pigment, the latter ranging from undetectable to a concentration equal to that of SR. Receptor mutants lacking all phototaxis sensitivity lack both of the photochemically reactive proteins. The mutant properties contribute to an accumulation of behavioral and spectroscopic evidence that the 480-nm pigment is a second sensory photoreceptor in H. halobium. NaDodSO4-polyacrylamide gel electrophoresis of [3H]retinal-labeled membrane proteins from the mutants indicates SR and the 480-nm pigment contain distinct chromophoric polypeptides differing in their migration rates. The data implicate polypeptides of 25,000 Mr and 23,000 Mr as retinal-binding polypeptides of SR and the 480-nm protein, respectively.

Primary and secondary chloride transport in Halobacterium halobium

Chloride uptake in intact cells of Halobacterium halobium was characterized by rates of influx and efflux of 36Cl- under conditions of light, respiration, or both. Halobacterial mutant strains with and without retinal transport proteins allowed study of the effects of halorhodopsin and bacteriorhodopsin under illumination. Two structurally independent chloride transport systems could be distinguished: halorhodopsin, the already known light-driven chloride pump, and a newly described secondary uptake system, which was energized by respiration or by light via bacteriorhodopsin.

Color discrimination in halobacteria: spectroscopic characterization of a second sensory receptor covering the blue-green region of the spectrum

Halobacterium halobium is attracted by green and red light and repelled by blue-green and shorter wavelength light. a photochromic, rhodopsin-like protein in the cell membrane, sensory rhodopsin sR587, has been identified as the receptor for the long-wavelength and near-UV stimuli. Discrepancies between the action spectrum for the repellent effect of blue light and the absorption spectrum of sR587 and its photocycle intermediate S373 strongly suggest the existence of an additional photoreceptor for the blue region of the spectrum. Transient light-induced absorbance changes in intact cells and cell membranes show, in addition to sR587, the presence of a second photoactive pigment with maximal absorption near 480 nm. It undergoes a cyclic photoreaction with a half-time of 150 msec. One intermediate state with maximal absorption near 360 nm has been resolved. The spectral properties of the new pigment are consistent with a function as the postulated photoreceptor for the repellent effect of blue light. The phototactic reactions and both pigments are absent when retinal synthesis is blocked; both can be restored by the addition of retinal. These results confirm and extend similar observations by Takahashi et al. [Takahashi, T., Tomioka, H., Kamo, N. & Kobatake, Y. (1985) FEMS Microbiol. Lett. 28, 161-164]. The archaeobacterium H. halobium thus uses two different mechanisms for color discrimination; it uses two rhodopsin-like receptors with different spectral sensitivities and also the photochromicity of at least one of these receptors to distinguish between three regions covering the visible and near-UV spectrum.

The opsin family of proteins

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Light-dependent trans to cis isomerization of the retinal in halorhodopsin

Flash-induced absorption changes in the near UV were determined for bacteriorhodopsin and halorhodopsin on a millisecond time scale. The difference spectrum obtained for bacteriorhodopsin was comparable to model difference spectra of tyrosine (aromatic OH deprotonated vs protonated), as found by others. The flash-induced difference spectrum for halorhodopsin, in contrast, resembled a model spectrum obtained for trans to 13-cis isomerization of retinal in bacteriorhodopsin. A model for chloride translocation by halorhodopsin is presented, in which the retinal isomerization moves positive charges, which in turn modulate the affinity of a site to chloride.

Reconstitution of purified halorhodopsin

Asolectin lipid vesicles containing halorhodopsin show light-induced acidification in the presence of proton ionophores. This effect is abolished by triphenyltin chloride, a chloride/hydroxyl antiporter, and is greatly diminished by valinomycin in the presence of potassium ions, which collapse the membrane potential. This indicates that halorhodopsin orients in the lipid vesicles preferentially inside out, pumping chloride into the extravesicular compartment. The absorption maximum of halorhodopsin in asolectin vesicles in 3 M NaCl is at 567 nm, and the action spectrum for the light-induced pH changes followed closely the absorption spectrum. Replacement of chloride by acetate or sulfate causes a shift in the absorption maximum to approximately equal to 559 nm and renders the pump inactive. The different photocycles of the two forms were used to show that 80% of the molecules have the extracellular side exposed to the vesicle interior and that the halide-binding site(s) associated with the spectral transition is accessible from the extracellular side of the molecule. The data presented demonstrate that the purified chromoprotein is the light-driven chloride pump in Halobacterium halobium.