Aspartate 75 mutation in sensory rhodopsin II from Natronobacterium pharaonis does not influence the production of the K-like intermediate, but strongly affects its relaxation pathway

The early steps in the photocycle of the aspartate 75-mutated sensory rhodopsin II from Natrobacterium pharaonis (pSRII-D75N) were studied by time-resolved laser-induced optoacoustic spectroscopy combined with quantum yield determinations by flash photolysis with optical detection. Similar to the case of pSRII-WT, excitation of pSRII-D75N produces in subnanosecond time a K-like intermediate. Different to the case of K in pSRII-WT, in pSRII-D75N there are two K states. K(E) decays into K(L) with a lifetime of 400 ns (independent of temperature in the range 6.5-52 degrees C) which is optically silent under the experimental conditions of our transient absorption experiments. This decay is concomitant with an expansion of 6.5 ml/mol of produced intermediate. This indicates a protein relaxation not affecting the chromophore absorption. For pSRII-D75N reconstituted into polar lipids from purple membrane, the mutation of Asp-75 by the neutral residue Asn affects neither the K(E) production yield (PhiK(e) 0.51 +/- 0.05) nor the energy stored by this intermediate (E(E)K(E) = 91 +/- 11 kJ/mol), nor the expansion upon its production (DeltaV(R,1) = 10 +/- 0.3 ml/mol). All these values are very similar to those previously determined for K with pSRII-WT in the same medium. The millisecond transient species is attributed to K(L) with a lifetime corresponding to that determined by electronic absorption spectroscopy for K(565). The determined energy content of the intermediates as well as the structural volume changes for the various steps afford the calculation of the free energy profile of the phototransformation during the pSRII-D75N photocycle. These data offer insights regarding the photocycle in pSRII-WT. Detergent solubilization of pSRII-D75N affects the sample properties to a larger extent than in the case of pSRII-WT.     

Structural changes of sensory rhodopsin I and its transducer protein are dependent on the protonated state of Asp76

Sensory rhodopsin I (SRI) functions in both positive and negative phototaxis in complex with halobacterial transducer protein I (HtrI). Orange light activation of SRI results in deprotonation of the retinylidene chromophore of SRI to produce the S 373 photocycle intermediate, the signaling state for positive phototaxis. In this study, we observed pH dependence on structural coupling between the two molecules upon the formation of the S 373 intermediate by means of Fourier transform infrared spectroscopy. At alkaline pH, where Asp76 (one of the counterions of the protonated retinylidene Schiff base) is deprotonated, HtrI-dependent alteration of the light-induced difference spectra is limited to reduction of amide I bands at 1661 (+)/ 1647 (-) cm (-1), and perturbation of one of the protonated carboxylic acid bands occurs at 1734 (-) cm (-1) (which appears to become ionized only when complexed with HtrI). However, at acidic pH, HtrI-complexed SRI exhibits not only light-induced reduction of the amide I changes but a wider range of spectral alterations including the appearance of several new amide I bands, perturbation of the chromophore-related vibrational modes, and other additional changes characteristic of tyrosine, glutamate, and aspartate residues. Since such pH dependence of structural changes was not observed in the complex of the D76N mutant of SRI, which behaves much like HtrI-complexed SRI in acidic conditions, we conclude that extensive orange light-induced conformational coupling between SRI and HtrI occurs only when Asp76 is neutralized.     

Different modes of proton translocation by sensory rhodopsin I.

The membrane-bound complex between sensory rhodopsin I (SRI) and its transducer HtrI forms the functional photoreceptor unit that allows transmission of light signals to the flagellar motor. Although being a photosensor, SRI, the mutant SRI-D76N and the HtrI-SRI complex can transport protons, as we demonstrate by using the sensitive and ion-specific black lipid membrane technique. SRI sustains an orange light-driven (one-photon-driven) outward proton transport which is enhanced by additional blue light (two-photon-driven). The vectoriality of the two-photon-driven transport could be reversed at neutral pH from the outward to the inward direction by switching the cut-off wavelength of the long wavelength light from 550 to 630 nm. The cut-off wavelength determining the reversal point decreases with decreasing pH. The currents could be enhanced by azide. A two-photon-driven inward proton transport by SRI-D76N (catalyzed by azide) and by the complex HtrI-SRI is demonstrated. The influence of pH and azide concentration on the rise and decay kinetics of the SRI380 intermediate is analyzed. The different modes of proton translocation of the SRI species are discussed on the basis of a general model of proton translocation of retinal proteins and in the context of signal transduction.     

The residues Leu 93 and Asp 96 act independently in the bacteriorhodopsin photocycle: studies with the leu 93-->Ala, Asp 96-->Asn double mutant.

Previous mutagenesis studies with bacteriorhodopsin have shown that reprotonation of the Schiff's base is the rate-limiting step in the photocycle of the D96N mutant, whereas retinal re-isomerization and return of the protein to the initial state constitute the rate-limiting events in the photocycle of the L93A mutant. Thus, in the D96N mutant, decay of the M intermediate is slowed down by more than 100-fold at pH 7. In the L93A mutant, decay of the O intermediate is slowed down by 250-fold. We report here that in the L93A, D96N double mutant, decay of the M intermediate, as well as the formation and decay of the O intermediate, are slowed down dramatically. The photocycle is completed by the decay of a long-lived O intermediate, as in the L93A mutant. The decay of the M and O intermediates in the double mutant parallels the behavior seen in the single mutants over a wide temperature and pH range, arguing that the observed independence is an intrinsic property of the mutant. The slow decay of the M and O intermediates can be selectively and independently reversed under conditions identical to those used for the corresponding intermediates in the D96N and L93A single mutants. Because the effects of the two individual mutations are preserved in the double mutant and can be independently reversed, we conclude that residues Asp 96 and Leu 93 act independently and at different stages of the bacteriorhodopsin photocycle. These results also show that formation of the O intermediate only requires protonation of the Schiff's base and is independent of the protonation of Asp 96 from the aqueous medium.     

Time-resolved absorption and photothermal measurements with recombinant sensory rhodopsin II from Natronobacterium pharaonis

Purified wild-type sensory rhodopsin II from Natronobacterium pharaonis (pSRII-WT) and its histidine-tagged analog (pSRII-His) were studied by laser-induced optoacoustic spectroscopy (LIOAS) and flash photolysis with optical detection. The samples were either dissolved in detergent or reconstituted into polar lipids from purple membrane (PML). The quantum yield for the formation of the long-lived state M(400) was determined as Phi(M) = 0.5 +/- 0.06 for both proteins. The structural volume change accompanying the production of K(510) as determined with LIOAS was DeltaV(R,1) /= Phi(M), indicating that the His tag does not influence this early step of the photocycle. The medium has no influence on DeltaV(R,1), which is the largest so far measured for a retinal protein in this time range (<10 ns). This confirms the occurrence of conformational movements in pSRII for this step, as previously suggested by Fourier transform infrared spectroscopy. On the contrary, the decay of K(510) is an expansion in the detergent-dissolved sample and a contraction in PML. Assuming an efficiency of 1.0, DeltaV(R,2) = -3 ml/mol for pSRII-WT and -4.6 ml/mol for pSRII-His were calculated in PML, indicative of a small structural difference between the two proteins. The energy content of K(510) is also affected by the tag. It is E(K) = (88 +/- 13) for pSRII-WT and (134 +/- 11) kJ/mol for pSRII-His. A slight difference in the activation parameters for K(510) decay confirms an influence of the C-terminal His on this step. At variance with DeltaV(R,1), the opposite sign of DeltaV(R,2) in detergent and PML suggests the occurrence of solvation effects on the decay of K(510), which are probably due to a different interaction of the active site with the two dissolving media.     

Transducer-binding and transducer-mutations modulate photoactive-site-deprotonation in sensory rhodopsin I.

Sensory rhodopsin I (SRI) is a seven-transmembrane helix retinylidene protein that mediates color-sensitive phototaxis responses through its bound transducer HtrI in the archaeon Halobacterium salinarum. Deprotonation of the Schiff base attachment site of the chromophore accompanies formation of the SRI signaling state, S(373). We measured the rate of laser flash-induced S(373) formation in the presence and absence of HtrI, and the effects of mutations in SRI or HtrI on the kinetics of this process. In the absence of HtrI, deprotonation occurs rapidly (halftime 10 micros) if the proton acceptor Asp76 is ionized (pK(a) = approximately 7), and only very slowly (halftime > 10 ms) when Asp76 is protonated. Transducer-binding, although it increases the pK(a) of Asp76 so that it is protonated throughout the range of pH studied, results in a first order, pH-independent rate of S(373) formation of approximately 300 micros. Therefore, the complexation of HtrI facilitates the proton-transfer reaction, increasing the rate approximately 50-fold at pH6. Arrhenius analysis shows that HtrI-binding accelerates the reaction primarily by an entropic effect, suggesting HtrI constrains the SRI molecule in the complex. Function-perturbing mutations in SRI and HtrI also alter the rate of S(373) formation and the lambda(max) of the parent state as assessed by laser flash-induced kinetic difference spectroscopy, and shifts to longer wavelength are correlated with slower deprotonation. The data indicate that HtrI affects electrostatic interactions of the protonated Schiff base and not only receives the signal from SRI but also optimizes the photochemical reaction process for SRI signaling.     

The photorelease of nitrogen monoxide (NO) from pentacyanonitrosyl coordination compounds of group 8 metals.

The photodetachment of NO from [M(II)(CN)5NO]2- with M = Fe, Ru, and Os, upon laser excitation at various wavelengths (355, 420, and 480 nm) was followed by various techniques. The three complexes showed a wavelength-dependent quantum yield of NO production Phi(NO), as measured with an NO-sensitive electrode, the highest values corresponding to the larger photon energies. For the same excitation wavelength the decrease of Phi(NO) at 20 degrees C in the order Fe > Ru > Os, is explained by the increasing M-N bond strength and inertness of the heavier metals. Transient absorption data at 420 nm indicate the formation of the [M(III)(CN)5H2O]2- species in less than ca. 1 micros for M = Fe and Ru. The enthalpy content of [Fe(III)(CN)5H2O]2- with respect to the parent [Fe(II)(CN)5NO]2- state is (190 +/- 20) kJ mol(-1), as measured by laser-induced optoacoustic spectroscopy (LIOAS) upon excitation at 480 nm. The production of [Fe(III)(CN)5H2O]2- is concomitant with an expansion of (8 +/- 3) ml mol(-1) consistent with an expansion of the water bound through hydrogen bonds to the CN ligands plus the difference between NO release into the bulk and water entrance into the first coordination sphere. The activated process, as indicated by the relatively strong temperature dependence of the Phi(NO) values and by the temperature dependence of the appearance of the [Fe(III)(CN)5H2O]2- species, as determined by LIOAS, is attributed to NO detachment in less than ca. 100 ns from the isonitrosyl (ON) ligand (MS1 state).     

Transducer protein HtrI controls proton movements in sensory rhodopsin I

Sensory rhodopsin I (SR-I lambda(max) 587 nm) is a phototaxis receptor in the archaeon Halobacterium salinarium. Photoisomerization of retinal in SR-I generates a long-lived intermediate with lambda(max) 373 nm which transmits a signal to the membrane-bound transducer protein HtrI. Although SR-I is structurally similar to the electrogenic proton pump bacteriorhodopsin (BR), early studies showed its photoreactions do not pump protons, nor result in membrane hyperpolarization. These studies used functionally active SR-I, that is, SR-I complexed with its transducer HtrI. Using recombinant DNA methods we have expressed SR-I protein containing mutations in ionizable residues near the protonated Schiff base, and studied wild-type and site-specifically mutated SR-I in the presence and absence of the transducer protein. UV-Vis kinetic absorption spectroscopy, FT-IR, and pH and membrane potential probes reveal transducer-free SR-I photoreactions result in vectorial proton translocation across the membrane in the same direction as that of BR. This proton pumping is suppressed by interaction with transducer which diverts the proton movements into an electroneutral path. A key step in this diversion is that transducer interaction raises the pK(a) of the aspartyl residue in SR-I (Asp76) which corresponds to the primary proton-accepting residue in the BR pump (Asp85). In transducer-free SR-I, our evidence indicates the pK(a) of Asp76 is 7.2, and ionized Asp76 functions as the Schiff base proton acceptor in the SR-I pump. In the SR-I/HtrI complex, the pK(a) of Asp76 is 8.5, and therefore at physiological pH (7.4) Asp76 is neutral. Protonation changes on Asp76 are clearly not required for signaling since the SR-I mutants D76N and D76A are active in phototaxis. The latent proton-translocation potential of SR-I may reflect the evolution of the SR-I sensory signaling mechanism from the proton pumping mechanism of BR.     

Volume and enthalpy changes after photoexcitation of bovine rhodopsin: laser-induced optoacoustic studies.

Laser-induced optoacoustic measurements were performed with bovine rhodopsin in the temperature range 5-32 degrees C in its natural environment (i.e., in washed membranes) as well as solubilized in dodecyl-beta-D-maltoside. A signal deconvolution procedure using a simple sequential kinetic scheme for the photobaric time evolution revealed, in the case of the washed membranes, the presence of an intermediate with a 14-ns lifetime at 25 degrees C, of the same order as that reported for the BSI intermediate in solubilized rhodopsin (Hug, S. J., W. J. Lewis, C. M. Einterz, T. E. Thorgeirsson, and D. S. Kliger. 1990. Nanosecond photolysis of rhodopsin: evidence for a new, blue-shifted intermediate. Biochemistry. 29:1475-1485), with an energy content of (85 +/- 20) kJ/mol, and accompanied by an expansion of 26 +/- 3 ml/mol. The difference in energy content between BSI and the next transient lumi was estimated in only -1 +/- 5 kJ/mol, concomitant with an expansion of 9 +/- 3 ml/mol. Thus, this transition, which according to literature involves an equilibrium, should be controlled by an entropic change, rather than by an enthalpic difference. This is supported by the fact that both activation parameters for the decay of batho and BSI decrease upon solubilization. For detergent-solubilized rhodopsin, two time constants were enough to fit the sample signal. A short lifetime ascribable to BSI was not detected in this case. For the first intermediate (probably batho in equilibrium with BSI), an energy content of 50 +/- 20 kJ/mol and an expansion of 20 +/- 1 ml/mol, and for lumi an energy content of 11 +/- 20 kJ/mol and a further expansion of 11 +/- 2 ml/mol were determined. Thus, the intermediates of the membrane-embedded form of rhodopsin (in contrast to solubilized samples) are kept in a higher energy level, although the total expansion from rhodopsin to lumi is similar for both conditions (35 +/- 6 and 31 +/- 3 ml/mol). The expansions are interpreted as protein reorganization processes as a consequence of the photoisomerization of the chromophore. As a result, weak interactions are probably perturbed and the protein gains conformational flexibility.     

Pathways of the rise and decay of the M photointermediate(s) of bacteriorhodopsin

The photocycle of bacteriorhodopsin (BR) was studied at alkaline pH with a gated multichannel analyzer, in order to understand the origins of kinetic complexities in the rise and decay of the M intermediate. The results indicate that the biphasic rise and decay kinetics are unrelated to a photoreaction of the N intermediate of the BR photocycle, proposed earlier by others [Kouyama et al. (1988) Biochemistry 27, 5855-5863]. Rather, under conditions where N did not accumulate in appreciable amounts (high pH, low salt concentration), they were accounted for by conventional kinetic schemes. These contained reversible interconversions, either M in equilibrium with N in one of two parallel photocycles or L in equilibrium with as well as M in equilibrium with N in a single photocycle. Monomeric BR also showed these kinetic complications. Conditions were then created where N accumulated in a photo steady state (high pH, high salt concentration, background illumination). The apparent increase in the proportion of the slow M decay component by the background illumination could be quantitatively accounted for with the single photocycle model, by the mixing of the relaxation of the background light induced photo steady state with the inherent kinetics of the photocycle. Postulating a new M intermediate which is produced by the photoreaction of N was neither necessary nor warranted by the data. The difference spectra suggested instead that absorption of light by N generates only one intermediate, observable between 100 ns and 1 ms, which absorbs near 610 nm. Thus, the photoreaction of N resembles in some respects that of BR containing 13-cis-retinal.     

Demonstration of 2:2 stoichiometry in the functional SRI-HtrI signaling complex in Halobacterium membranes by gene fusion analysis

A fusion protein in which the C-terminus of Halobacterium salinarum sensory rhodopsin I (SRI) is connected by a flexible linker to the N-terminus of its transducer (HtrI) was constructed and expressed in H. salinarum. The fusion protein mediated attractant responses to orange light and repellent responses to UV/violet light that were comparable to those produced by the wild-type SRI-HtrI complex. Immunoblot analysis of H. salinarum membrane proteins demonstrated intact fusion protein and no detectable proteolytic cleavage products. Rapid oxidative cross-linking of a monocysteine mutant in the HtrI domain confirmed that the fusion protein exists as a homodimer in the membrane. HtrI-free SRI and HtrI-complexed SRI have been shown previously to exhibit large differences in the pH dependence of their photocycle kinetics and in the pK(a) of Asp76 that controls a pH-dependent spectral transition in SRI. These differences were used to assess whether only one or both SRI domains in the fusion protein were complexed properly to the HtrI homodimer. Measurement of the photochemical activity, the photocycle kinetics, and the absorption spectra at various pH values established that both SRI domains are complexed to HtrI in the fusion protein, and therefore the stoichiometry is 2:2. Closer examination of the HtrI effect on SRI revealed that Asp76 titration in HtrI-free SRI fits two pK(a) values, with 98% and 2% of the molecules titrating with pK(a)'s of 7 and 9, respectively. The same two pK(a)'s of Asp76 are evident in HtrI-complexed SRI, but with 13% with pK(a) of 7 and 87% with pK(a) of 9 and a similar bias toward the pK(a) of 9 in the fusion protein. Titration of the fusion protein with Ala substitution at Arg73, a residue in the photoactive site, in the SRI domain indicates that a basic residue at Arg73 is necessary for the lower pK(a) to be observed. A model in which Arg73 plays a role in the HtrI effect on SRI is discussed.     

Recording of blue light-induced energy and volume changes within the wild-type and mutated phot-LOV1 domain from Chlamydomonas reinhardtii

The time-resolved thermodynamics of the flavin mononucleotide (FMN)-binding LOV1 domain of Chlamydomonas reinhardtii phot (phototropin homolog) was studied by means of laser-induced optoacoustic spectroscopy. In the wild-type protein the early red-shifted intermediate LOV(715) exhibits a small volume contraction, DeltaV(715) = -1.50 ml/mol, with respect to the parent state. LOV(715) decays within few micro s into the covalent FMN-Cys-57 adduct LOV(390), that shows a larger contraction, DeltaV(390) = -8.8 ml/mol, suggesting a loss of entropy and conformational flexibility. The high energy content of LOV(390), E(390) = 180 kJ/mol, ensures the driving force for the completion of the photocycle and points to a strained photoreceptor conformation. In the LOV-C57S mutated protein the photoadduct is not formed and DeltaV(390) is undetected. Large effects on the measured DeltaVs are observed in the photochemically competent R58K and R58K/D31Q mutated proteins, with DeltaV(390) = -2.0 and -1.9 ml/mol, respectively, and DeltaV(715) approximately 0. The D31Q and D31N substitutions exhibit smaller but well-detectable effects. These results show that the photo-induced volume changes involve the protein region comprising Arg-58, which tightly interacts with the FMN phosphate group.     

Photoinduced volume change and energy storage associated with the early transformations of the photoactive yellow protein from Ectothiorhodospira halophila.

The photocycle of the photoactive yellow protein (PYP) isolated from Ectothiorhodospira halophila was analyzed by flash photolysis with absorption detection at low excitation photon densities and by temperature-dependent laser-induced optoacoustic spectroscopy (LIOAS). The quantum yield for the bleaching recovery of PYP, assumed to be identical to that for the phototransformation of PYP (pG), to the red-shifted intermediate, pR, was phi R = 0.35 +/- 0.05, much lower than the value of 0.64 reported in the literature. With this value and the LIOAS data, an energy content for pR of 120 kJ/mol was obtained, approximately 50% lower than for excited pG. Concomitant with the photochemical process, a volume contraction of 14 ml/photoconverted mol was observed, comparable with the contraction (11 ml/mol) determined for the bacteriorhodopsin monomer. The contraction in both cases is interpreted to arise from a protein reorganization around a phototransformed chromophore with a dipole moment different from that of the initial state. The deviations from linearity of the LIOAS data at photon densities > 0.3 photons per molecule are explained by absorption by pG and pR during the laser pulse duration (i.e., a four-level system, pG, pR, and their respective excited states). The data can be fitted either by a simple saturation process or by a photochromic equilibrium between pG and pR, similar to that established between the parent chromoprotein and the first intermediate(s) in other biological photoreceptors. This nonlinearity has important consequences for the interpretation of the data obtained from in vitro studies with powerful lasers.     

Volume and enthalpy changes in the early steps of bacteriorhodopsin photocycle studied by time-resolved photoacoustics.

We have studied the photoinduced volume changes, energetics, and kinetics in the early steps of the bacteriorhodopsin (BR) photocycle with pulsed, time-resolved photoacoustics. Our data show that there are two volume changes. The fast volume change ( < or = 200 ns) is an expansion (2.5 +/- 0.3 A3/molecule) and is observed exclusively in the purple membrane (PM), vanishing in the 3-[(3-cholamidopropyl)-dimethylammonio] -1-propane-sulfonate-sulfonate-solubilized BR sample; the slow change (approximately 1 micros) is a volume contraction (-3.7 +/- 0.3 A3/molecule). The fast expansion is assigned to the restructuring of the aggregated BR in the PM, and the 1-micros contraction to the change in hydrogen bonding of water at Asp 212 (Kandori et al. 1995. J. Am. Chem. Soc. 117:2118-2119). The formation of the K intermediate releases most of the absorbed energy as heat, with delta Hk = -36 +/- 8 kJ/mol. The activation energy of the K --> L step is 49 +/- 6 kJ/mol, but the enthalpy change is small, -4 +/- 10 kJ/mol. On the time scale we studied, the primary photochemical kinetics, enthalpy, and volume changes are not affected by substituting the solvent D2O for H2O. Comparing data on monomeric and aggregated BR, we conclude that the functional unit for the photocycle is the BR monomer, because both the kinetics (rate constant and activation energy) and the enthalpy changes are independent of its aggregation state.     

Aspartic acids 96 and 85 play a central role in the function of bacteriorhodopsin as a proton pump.

A spectroscopic and functional analysis of two point-mutated bacteriorhodopsins (BRs) from phototrophic negative halobacterial strains is reported. Bacteriorhodopsin from strain 384 contains a glutamic acid instead of an aspartic acid at position 85 and BR from strain 326 contains asparagine instead of aspartic acid at position 96. Compared to wild-type BR, the M formation in BR Asp85---Glu is accwelerated approximately 10-fold, whereas the M decay in BR Asp96---Asn is slowed down approximately 50-fold at pH6. Purple membrane sheets containing the mutated BRs were oriented and immobilized in polyacrylamide gels or adsorbed to planar lipid films. The measured kinetics of the photocurrents under various conditions agree with the observed photocycle kinetics. The ineffectivity of BR Asp85---Glu resides in the dominance of an inactive species absorbing maximally at approximately 610 nm, while BR Asp96---Asn is ineffective due to its slow photocycle. These experimental results suggest that aspartic acid 96 plays a crucial role for the reprotonation of the Schiff base. Both residues are essential for an effective proton pump.     

His166 is critical for active-site proton transfer and phototaxis signaling by sensory rhodopsin I.

Photoinduced deprotonation of the retinylidene Schiff base in the sensory rhodopsin I transducer (SRI-Htrl) complex results in formation of the phototaxis signaling state S373. Here we report identification of a residue, His166, critical to this process, as well as to reprotonation of the Schiff base during the recovery phase of the SRI photocycle. Each of the residue substitutions A, D, G, L, S, V, or Y at position 166 reduces the flash yield of S373, to values ranging from 2% of wild type for H166Y to 23% for H166V. The yield of S373 is restored to wild-type levels in Htrl-free H166L by alkaline deprotonation of Asp76, a Schiff base proton acceptor normally not ionized in the SRI-Htrl complex, showing that proton transfer from the Schiff base in H166L occurs when an acceptor is made available. The flash yield and rate of decay of S373 of the mutants are pH dependent, even when complexed with Htrl, which confers pH insensitivity to wild-type SRI, suggesting that partial disruption of the complex has occurred. The rates of S373 reprotonation at neutral pH are also prolonged in all H166X mutants, with half-times from 5 s to 160 s (wild type, 1 s). All mutations of His166 tested disrupt phototaxis signaling. No response (H166D, H166L), dramatically reduced responses (H166V), or inverted responses to orange light (H166A, H166G, H166S, and H166Y) or to both orange and near-UV light (H166Y) are observed. Our conclusions are that His166 1) plays a role in the pathways of proton transfer both to and from the Schiff base in the SRI-Htrl complex, either as a structurally important residue or possibly as a participant in proton transfers; 2) is involved in the modulation of SRI photoreaction kinetics by Htrl; and 3) is important in phototaxis signaling. Consistent with the involvement of the His imidazole moiety, the addition of 10 mM imidazole to membrane suspensions containing H166A receptors accelerates S373 decay 10-fold at neutral pH, and a negligible effect is seen on wild-type SRI.     

The photochemical reactions of bacterial sensory rhodopsin-I. Flash photolysis study in the one microsecond to eight second time window.

Halobacterium halobium Flx mutants are deficient in bacteriorhodopsin (bR) and halorhodopsin (hR). Such strains are phototactic and the light signal detectors are two additional retinal pigments, sensory rhodopsins I and II (sR-I and sR-II), which absorb maximally at 587 and 480 nm, respectively. A retinal-deficient Flx mutant, Flx5R, overproduces sR-I-opsin and does not show any photochemical activity other than that of sR-I after the pigment is regenerated by addition of all-trans retinal. Using native membrane vesicles from this strain, we have resolved a new photointermediate in the sR-I photocycle between the early bathointermediate S610 and the later intermediate S373. The new form, S560, resembles the L intermediate of bR in its position in the photoreaction cycle, its relatively low extinction, and its moderate blue shift. It forms with a half-time of approximately 90 microseconds at 21 degrees C, concomitant with the decay of S610. Its decay with a half-time of 270 microseconds parallels the appearance of S373. From a data set consisting of laser flash-induced absorbance changes (300 ns, 580-nm excitation) measured at 24 wavelengths from 340 to 720 nm in a time window spanning 1 microsecond to 8 s we have calculated the spectra of the photocycle intermediates assuming a unidirectional, unbranched reaction scheme.     

Cytoplasmic shuttling of protons in anabaena sensory rhodopsin: implications for signaling mechanism

It was found recently that Anabaena sensory rhodopsin (ASR), which possibly serves as a photoreceptor for chromatic adaptation, interacts with a soluble cytoplasmic transducer. The X-ray structure of the transducer-free protein revealed an extensive hydrogen-bonded network of amino acid residues and water molecules in the cytoplasmic half of ASR, in high contrast to its haloarchaeal counterparts. Using time-resolved spectroscopy of the wild-type and mutant ASR in the visible and infrared ranges, we tried to determine whether this hydrogen-bonded network is used to translocate protons and whether those proton transfers are important for interaction with the transducer. We found that the retinal Schiff base deprotonation, which occurs in the M intermediate of the photocycle of all-trans-ASR, results in protonation of Asp217 on the cytoplasmic side of the protein. The deprotonation of the Schiff base induces a conformational change of ASR observed through the perturbation of associated lipids. We suggest that the cytoplasmic shuttling of protons in the photocycle of all-trans-ASR and the ensuing conformational changes might activate the transducer. Consequently, the M intermediate may be the signaling state of ASR.     

Effects of Asp-96----Asn, Asp-85----Asn, and Arg-82----Gln single-site substitutions on the photocycle of bacteriorhodopsin

Bacteriorhodopsin (BR) with the single-site substitutions Arg-82----Gln (R82Q), Asp-85----Asn (D85N), and Asp-96----Asn (D96N) is studied with time-resolved absorption spectroscopy in the time regime from nanoseconds to seconds. Time-resolved spectra are analyzed globally by using multiexponential fitting of the data at multiple wavelengths and times. The photocycle kinetics for BR purified from each mutant are determined for micellar solutions in two detergents, nonyl glucoside and CHAPSO, and are compared to results from studies on delipidated BR (d-BR) in the same detergents. D85N has a red-shifted ground-state absorption spectrum, and the formation of an M intermediate is not observed. R82Q undergoes a pH-dependent transition between a purple and a blue form with different pKa values in the two detergents. The blue form has a photocycle resembling that for D85N, while the purple form of R82Q forms an M intermediate that decays more rapidly than in d-BR. The purple form of R82Q does not light-adapt to the same extent as d-BR, and the spectral changes in the photocycle suggest that the light-adapted purple form of R82Q contains all-trans- and 13-cis-retinal in approximately equal proportions. These results are consistent with the suggestions of others for the roles of Arg-82 and Asp-85 in the photocycle of BR, but results for D96N suggest a more complex role for Asp-96 than previously suggested. In nonyl glucoside, the apparent decay of the M-intermediate is slower in D96N than in d-BR, and the M decay shows biphasic kinetics. However, the role of Asp-96 is not limited to the later steps of the photocycle. In D96N, the decay of the KL intermediate is accelerated, and the rise of the M intermediate has an additional slow phase not observed in the kinetics of d-BR. The results suggest that Asp-96 may play a role in regulating the structure of BR and how it changes during the photocycle.     

Phototaxis of Halobacterium salinarium requires a signalling complex of sensory rhodopsin I and its methyl-accepting transducer HtrI.

Sensory rhodopsin I (SRI) is a photoreceptor that mediates phototaxis in the archaeon Halobacterium salinarium. Receptor excitation is relayed to the motility system of the cell by the methyl-accepting transducer protein HtrI. In membranes prepared from cells that lack HtrI the absorbance difference maximum of SRI was shifted from 587 to 565 nm. The thermal decay of the metastable photocycle intermediate SRI373 was measured as time-dependent recovery of the absorbance at 590 nm. In the absence of HtrI the decay was slowed down by two orders of magnitude. When SRI was overproduced in cells that contained normal levels of HtrI, the decay of SRI373 was biexponential indicating two kinetically distinct species. Spectroscopic measurements on intact cells revealed the same effect of HtrI on SRI photocycling as found in isolated membranes. By transient exposure of membranes from wild-type cells to low ionic strength, the decay of SR373 was slowed to the same value found for untreated membranes in the absence of HtrI. In parallel, the absorbance difference maximum was shifted to 565 nm indicating that a physical interaction of HtrI and SRI had been irreversibly destroyed. Overproduction of SRI in the presence of wild-type amounts of HtrI did not increase the light sensitivity of the cells to orange light step down stimulation. It is concluded that SRI and HtrI form a stable complex in the cell membrane that signals to the flagellar motor and defines absorbance maximum, photocycling rate and photochemical efficiency of SRI.