Specific effects of chloride on the photocycle of E194Q and E204Q mutants of bacteriorhodopsin as measured by FTIR spectroscopy

Low-temperature Fourier transform infrared spectroscopy has been used to study mutants of Glu194 and Glu204, two amino acids that are involved in proton release to the extracellular side of bacteriorhodopsin. Difference spectra of films of E194Q, E204Q, E194Q/E204Q, E9Q/E194Q/E204Q, and E9Q/E74Q/E194Q/E204Q at 243, 277, and 293 K and several pH values were obtained by continuous illumination. A specific effect of Cl(-) ions was found for the mutants, promoting a N-like intermediate at alkaline pH and an O' intermediate at neutral or acid pH. The apparent pK(a) of Asp85 in the M intermediate was found to be decreased for E194Q in the presence of Cl(-) (pK(a) of 7.6), but it was unchanged for E204Q, as compared to wild-type. In the absence of Cl(-) (i.e., in the presence of SO(4)(2)(-)), mutation of Glu194 or of Glu204 produces M- (or M(N), M(G))-like intermediates under all of the conditions examined. The absence of N, O, and O' intermediates suggests a long-range effect of the mutation. Furthermore, it is suggested that Cl(-) acts by reaching the interior of the protein, rather than producing surface effects. The effect of low water content was also examined, in the presence of Cl(-). Similar spectra corresponding to the M(1) intermediate were found for dry samples of both mutants, indicating that the effects of the mutations or of Cl(-) ions are confined to the second part of the photocycle. The water O-H stretching data further confirms altered photocycles and the effect of Cl(-) on the accumulation of the N intermediate.

Photocycle in the M-form in bacteriorhodopsin mutants devoid of primary proton acceptor Asp-85

Photoinduced changes in absorption of the deprotonated M-form in the mutant bacteriorhodopsin without primary proton acceptor Asp-85 were studied and additional evidence in support of the complete transmembrane proton transfer in photocycle was obtained. Measurements of the absorption spectrum were carried out at various pH, temperature, and humidity. The direction of proton transfer was the same as in the normal photocycle of the wild-type bacteriorhodopsin: from the internal to the external side of the membrane. The effect on this process of a terminal acceptor Glu-204 was shown.

Rational design of 'water-soluble' bacteriorhodopsin variants

We have explored the interchangeability of soluble and membrane proteins by attempting to render a helical membrane protein 'water soluble' through mutation of its lipid-exposed residues. Using an atomic resolution structure of bacteriorhodopsin (bR), two different strategies were developed to identify lipid-exposed residues for mutation. In the first strategy all residues in trimeric bR with solvent accessibility >35% were marked for replacement. Replacement residues were chosen so as to map an average surface of helical soluble proteins onto the bR surface, resulting in the mutagenesis of 14.9% of surface residues. The second strategy took into account the observation that accessible residues can be categorized as fully or partially accessible. Consequently, three mutants were designed based on monomeric bR, all with their accessible residues changed and with varying extents of mutagenesis of partially accessible residues. 13.5-24.3% of the wild-type surface was altered in these designs. The construct for the first design was cloned into Escherichia coli. Trace amounts of the mutant protein were expressed with the concurrent overexpression of an endogenous prolyl isomerase. In contrast, all three mutant proteins of the second design expressed well and could be purified to homogeneity. Systematic refolding trials were undertaken with limited success at solubilization in aqueous media. We have discussed the feasibility of applying the 'solubilization strategy' outlined here to membrane proteins.

Mechanism of cell death and disease resistance induction by transgenic expression of bacterio-opsin

One of the earliest signal transduction events that trigger the hypersensitive response (HR) of plants against pathogen attack is thought to be an alteration of proton flux across the plasma membrane (PM). However, no direct genetic evidence for the involvement of PM-localised proton channels or pumps in the induction of this response has been reported. We previously showed that expression of the bacterial proton pump bacterio-opsin (bO) in transgenic plants resulted in the spontaneous activation of the HR. Here we show that the bO protein is likely localised to the PM in transgenic tobacco plants. Furthermore, mutational analysis shows that induction of the HR by bO expression is dependent upon the capability of bO to translocate protons. Although bO functions as a light-driven proton pump in Halobacteria when assembled with retinal, we also show by mutational analysis that this chromophore binding is unnecessary for its in planta activity. Taken together, our results suggest that expression of bO in plants leads to the insertion of a passive proton channel into the PM. The activity of this channel in the PM results in spontaneous activation of cell death and HR-associated phenotypes including enhanced resistance to a broad spectrum of plant pathogens. Our work provides direct molecular evidence to support a working model in which alterations in ionic homeostasis at the level of the PM may work as one of the critical steps in the signalling pathway for the activation of the HR.

Proton translocation by bacteriorhodopsin in the absence of substantial conformational changes

Unlike wild-type bacteriorhodopsin (BR), the BR triple mutant D96G/F171C/F219L has been shown to undergo only minor structural rearrangements during its photocycle. Nonetheless, the mutant is capable of transporting protons at a rate of 125(+/-40) H+/BR per minute under light-saturating conditions. Light adaptation of the triple mutant's retinal proceeds in a pH-dependent manner up to a maximum of 63% all-trans. These two findings imply that the transport activity of the triple mutant comprises 66% of the wild-type activity. Time-resolved spectroscopy reveals that the identity and sequence of intermediates in the photocycle of the triple mutant in the all-trans configuration correspond to that of wild-type BR. The only differences relate to a slower rise and decay of the M and O intermediates, and a significant spectral contribution from a 13-cis component. No indication for accumulation of the N intermediate is found under a variety of conditions that normally favor the formation of this species in wild-type BR. The Fourier transform infrared (FTIR) spectrum of the M intermediate in the triple mutant resembles that of wild type. Minor changes in the amide I region during the photocycle suggest that only small movements of the protein backbone occur. Electron microscopy reveals large differences in conformation between the unilluminated state of the mutant protein and wild-type but no light-induced changes in time-resolved measurements. Evidently, proton transport by the triple mutant does not require the major conformational rearrangements that occur on the same time-scale with wild-type. Thus, we conclude that large conformational changes observed in the photocycle of the wild-type and many BR mutants are not a prerequisite for the change in accessibility of the Schiff base nitrogen atom that must occur during vectorial catalysis to allow proton transport.

Changes in hydrogen bonding and environment of tryptophan residues on helix F of bacteriorhodopsin during the photocycle: a time-resolved ultraviolet resonance Raman study

Protein structural changes during the photocycle of bacteriorhodopsin were examined by time-resolved ultraviolet resonance Raman (UVRR) spectroscopy. Most of the 244-nm UVRR difference signals of Trp were assigned to either Trp182 or Trp189 using the Trp182 --> Phe and Trp189 --> Phe mutants. The W17 mode of Trp182 shows a wavenumber downshift in the M(1) --> M(2) transition, indicating an increase in hydrogen bonding strength at the indole nitrogen. On the other hand, Trp189 shows Raman intensity increases of the W16 and W18 modes ascribable to an increased hydrophobic interaction. These observations suggest that the tilt of helix F, which ensures that reprotonation of the Schiff base is from the cytoplasmic side, occurs in the M(1) --> M(2) transition. In the M(2) --> N transition, the environment of Trp189 returns to the initial state, whereas the hydrophobic interaction of Trp182 decreases drastically. The decrease in hydrophobic interaction of Trp182 in the N state suggests an invasion of water molecules that promote the proton transfer from Asp96 to the Schiff base. Structural reorganization of the protein after the tilt of helix F may be important for efficient reprotonation of the Schiff base.

Time-resolved X-ray diffraction reveals movement of F helix of D96N bacteriorhodopsin during M-MN transition at neutral pH

D96N bacteriorhodopsin has two photointermediates with the deprotonated Schiff base: the M and MN intermediates. We measure the time-resolved x-ray diffraction of the D96N purple membrane after flash photoexcitation (pH 7.0, 25 degrees C). The data clearly show the M-MN transition during the D96N photocycle. Low-resolution projection maps of these states show that the F helix of the MN intermediate shifts from its original position and this shift is much larger than that of the M intermediate. This indicates that the F helix moves in the M-MN transition of the D96N bacteriorhodopsin photocycle. Moreover, the existence of the MN intermediate in the D96N photocycle under neutral pH indicates that the MN intermediate is not peculiar to the alkaline condition. It is notable that the structural transition of M-MN is independent of the protonation state of the Schiff base. Therefore, the F helix movement precedes reprotonation of the Schiff base in the bacteriorhodopsin photocycle. Our previous study showed that the M-MN transition is hydration-dependent and that the MN intermediate is more hydrated than the M intermediate. Considering this together with the present results, we conclude that the movement of the F helix causes hydration of the cytoplasmic side, which promotes the reprotonation of the Schiff base.

Grafting segments from the extracellular surface of CCR5 onto a bacteriorhodopsin transmembrane scaffold confers HIV-1 coreceptor activity

Components from the extracellular surface of CCR5 interact with certain macrophage-tropic strains of human immunodeficiency virus type 1 (HIV-1) to mediate viral fusion and entry. To mimic these viral interacting site(s), the amino-terminal and extracellular loop segments of CCR5 were linked in tandem to form concatenated polypeptides, or grafted onto a seven-transmembrane bacteriorhodopsin scaffold to generate several chimeras. The chimera studies identified specific regions in CCR5 that confer HIV-1 coreceptor function, structural rearrangements in the transmembrane region that may modulate this activity, and a role for the extracellular surface in folding and assembly. Methods developed here may be applicable to the dissection of functional domains from other seven-transmembrane receptors and form a basis for future structural studies.

Conformational change of the E-F interhelical loop in the M photointermediate of bacteriorhodopsin

The conformation of the structured EF interhelical loop of bacteriorhodopsin and its change in the M photointermediate were assessed by measuring the rate of reaction of 16 single engineered cysteine residues along the loop with water-soluble sulfhydryl reagents. The exposure to the bulk in the unilluminated state determined with the cysteine reaction correlated well with the degree of access to water calculated from the crystallographic structure of the loop. The EF-loop should be affected by the well-known outward tilt of helix F in the M and N intermediates of the photocycle. A second mutation in each cysteine mutant, the D96N residue replacement, allowed full conversion to the M state by illumination. The reaction rates measured under these conditions indicated that buried residues tend to become more exposed, and exposed residues become more buried in M. This is to be expected from tilt of helix F. However, the observation of increased exposure of four residues near the middle of the loop, where steric effects are only from other loop residues, indicate that the conformation of the EF-loop itself is changed. Thus, the motion of the loop in M is more complex than expected from simple tilt of helix F, and may include rotation that unwinds its twist.

Interaction of internal water molecules with the schiff base in the L intermediate of the bacteriorhodopsin photocycle

In the photocycle of bacteriorhodopsin (BR), the first proton movement, from the Schiff base to Asp85, occurs after the formation of the L intermediate. In L, the C [double bond] N bond of the Schiff base is strained, and the nitrogen interacts strongly with its counterion. The present study seeks to detect the interaction of internal water molecules with the Schiff base in L using difference FTIR spectroscopy at 170 K. The coupled modes of the hydrogen-out-of plane bending vibrations (HOOPs) of the N-H and C(15)-H of the protonated Schiff base are detected as a broad band centered at 911 cm(-1) for BR. A set of bands at 1073, 1064, and 1056 cm(-1) for L is shown to arise from the coupling of the HOOP with the overtones of interacting water O-H vibrations. Interaction with water was shown by the decreased intensity of the HOOPs of L in H(2)(18)O and by the influence of mutants that have been shown to perturb specific internal water molecules in BR. In contrast, the HOOP band of initial BR was not affected by these mutations. In D85N, the coupled HOOP of BR is depleted, while the coupled HOOPs of L are shifted. The results indicate that the Schiff base interacts with water in the L state but in a different manner than in the BR state. Moreover, the effects of mutations suggest that cytoplasmic water close to Thr46 (Wat46) either interacts stronger with the Schiff base in L or that it is important in stabilizing another water that does.

Buffer effects on electric signals of light-excited bacteriorhodopsin mutants

The effects of glycyl-glycine and bis-trispropane buffers on the light-excited electric signals due to proton motion in the molecule were studied for the bacteriorhodopsin (bR) mutants D38R, D96N, E204Q, R227Q, D85N, D85T, R82Q/D85N, and D85N/D96N in purple membranes and for delipidated purple membrane containing the wild-type bR. The results show additional charge motion caused by the buffers in all cases. Arrhenius parameters calculated from the temperature dependence of the difference signals (with buffer minus without buffer) are similar to the parameters found for the wild-type bR in the case of these buffers: the values of the activation enthalpies are mostly in the range 25-50 kJ/mol; all the activation entropies are negative. The results are evaluated with the cluster hypothesis outlined previously.

Electric signals of light excited bacteriorhodopsin mutant D96N

The study of mutant D96N played an important role in understanding proton translocation by light driven bacteriorhodopsin. Our measurement of photoelectric current for single and double flash illumination revealed new details of the photocycle of this mutant. With double flash excitation we found an intermediate absorbing near the wavelength of the ground state of bacteriorhodopsin (bR) but pumping in the opposite direction. This intermediate has the same lifetime as the species described by Zimányi et al. [Proc. Natl. Acad. Sci. USA 96 (1999) 4414-4419] and was assigned to early recovery of a fraction of the ground state after excitation. Because the electric response does not reconcile with that of the ground state, we tentatively assign it to the L intermediate or to an intermediate similar in absorption to bR (bR').

Thr90 is a key residue of the bacteriorhodopsin proton pumping mechanism

Mutation of Thr90 to Ala has a profound effect on bacteriorhodopsin properties. T90A shows about 20% of the proton pumping efficiency of wild type, once reconstituted into liposomes. Mutation of Thr90 influences greatly the Schiff base/Asp85 environment, as demonstrated by altered lambda(max) of 555 nm and pK(a) of Asp85 (about 1.3 pH units higher than wild type). Hydroxylamine accessibility is increased in both dark and light and differential scanning calorimetry and visible spectrophotometry show decreased thermal stability. These results suggest that Thr90 has an important structural role in both the unphotolysed bacteriorhodopsin and in the proton pumping mechanism.

Exploring the function of Tyr83 in bacteriorhodopsin: features of the Y83F and Y83N mutants

Tyrosine-83, a residue which is conserved in all halobacterial retinal proteins, is located at the extracellular side in helix C of bacteriorhodopsin. Structural studies indicate that its hydroxyl group is hydrogen bonded to Trp189 and possibly to Glu194, a residue which is part of the proton release complex (PRC) in bacteriorhodopsin. To elucidate the role of Tyr83 in proton transport, we studied the Y83F and Y83N mutants. The Y83F mutation causes an 11 nm blue shift of the absorption spectrum and decreases the size of the absorption changes seen upon dark adaptation. The light-induced fast proton release, which accompanies formation of the M intermediate, is observed only at pH above 7 in Y83F. The pK(a) of the PRC in M is elevated in Y83F to about 7.3 (compared to 5.8 in WT). The rate of the recovery of the initial state (the rate of the O --> BR transition) and light-induced proton release at pH below 7 is very slow in Y83F (ca. 30 ms at pH 6). The amount of the O intermediate is decreased in Y83F despite the longer lifetime of O. The Y83N mutant shows a similar phenotype in respect to proton release. As in Y83F, the recovery of the initial state is slowed several fold in Y83N. The O intermediate is not seen in this mutant. The data indicate that the PRC is functional in Y83F and Y83N but its pK(a) in M is increased by about 1.5 pK units compared to the WT. This suggests that Tyr83 is not the main source for the proton released upon M formation in the WT; however, Tyr83 is involved in the proton release affecting the pK(a) of the PRC in M and the rate of proton transport from Asp85 to PRC during the O --> bR transition. Both the Y83F and the Y83N mutations lead to a greatly decreased functionality of the pigment at high pH because most of the pigment is converted into the inactive P480 species, with a pK(a) 8-9.

Contribution of extracellular Glu residues to the structure and function of bacteriorhodopsin. Presence of specific cation-binding sites

Single and multiple mutants of extracellular Glu side chains of bacteriorhodopsin were analyzed by acid and calcium titration, differential scanning calorimetry, and thermal difference spectrophotometry. Acid titration spectra show that the second group protonating with Asp(85) is revealed in E204Q in the absence of Cl(-) but is not observed in the triple mutant E9Q/E194Q/E204Q or in the quadruple mutant E9Q/E74Q/E194Q/E204Q. The results point to Glu(9) as the second group protonating cooperatively with Asp(85). Comparison of the apparent pK(a) of Asp(85) protonation in water and in the deionized forms and results of calcium titration suggest that cation-binding sites are of low affinity in the multiple Glu mutants. Like for deionized wild type bacteriorhodopsin, differential scanning calorimetry reveals a lack of the pretransition in the multiple mutants, whereas in E9Q it appears at lower temperature and with lower cooperativity. Additionally, at neutral pH the band at 630 nm arising from cation release upon temperature increase is absent for the multiple mutants. Based on these results, we propose the presence of two cation-binding sites in the extracellular region of bacteriorhodopsin having as ligands Glu(9), Glu(194), Glu(204), and water molecules.

Proton transport mechanism of bacteriorhodopsin as revealed by site-specific mutagenesis and protein sequence variability

A large share of the current ideas about the mechanism of proton transport by bacteriorhodopsin has emerged from studies of site-specific mutants. This review is an attempt to check some of these ideas against the natural variability in the primary structure of the protein.

Crystal structure of the D85S mutant of bacteriorhodopsin: model of an O-like photocycle intermediate

Crystal structures are reported for the D85S and D85S/F219L mutants of the light-driven proton/hydroxyl-pump bacteriorhodopsin. These mutants crystallize in the orthorhombic C222(1) spacegroup, and provide the first demonstration that monoolein-based cubic lipid phase crystallization can support the growth of well-diffracting crystals in non-hexagonal spacegroups. Both structures exhibit similar and substantial differences relative to wild-type bacteriorhodopsin, suggesting that they represent inherent features resulting from neutralization of the Schiff base counterion Asp85. We argue that these structures provide a model for the last photocycle intermediate (O) of bacteriorhodopsin, in which Asp85 is protonated, the proton release group is deprotonated, and the retinal has reisomerized to all-trans. Unlike for the M and N photointermediates, where structural changes occur mainly on the cytoplasmic side, here the large-scale changes are confined to the extracellular side. As in the M intermediate, the side-chain of Arg82 is in a downward configuration, and in addition, a pi-cloud hydrogen bond forms between Trp189 NE1 and Trp138. On the cytoplasmic side, there is increased hydration near the surface, suggesting how Asp96 might communicate with the bulk during the rise of the O intermediate.

The back photoreaction of the M intermediate in the photocycle of bacteriorhodopsin: mechanism and evidence for two M species

The back photoreaction of the M intermediate in the photocycle of bacteriorhodopsin is investigated both for the native pigment and its D96N mutant. The experimental setup is based on creating the M intermediate by a first pulse, followed by a (blue) laser pulse which drives the back photoreaction of M. Experiments are carried out varying the delay between the two pulses, as well as the temperature over the -25 degrees C-20 degrees C range. It is found that the kinetic patterns of the M back photoreaction change with time after the generation of this intermediate. The data provide independent evidence for the suggestion of a photocycle mechanism based on two distinct M intermediates. They are thus in keeping with the consecutive model of Varo and Lanyi (Biochemistry 30, 5016-5022; 1991), although they cannot exclude other models such as those based on branched or parallel cycles. More generally, we offer a "photochemical" approach to discriminating between intermediate stages in the photocycle which does not depend on spectroscopic and/or kinetic data. While markedly affecting the rate of the M --> N transition in the photocycle, the rate of the thermal step in back photoreaction of M, at both room and low temperatures, is not significantly affected by the D96N mutation. It is proposed that while Asp 96 is the Schiff-base protonating moiety in the M --> N transition, another residue (most probably Asp 85) reprotonates the Schiff base following light absorption by M.

X-ray structure of sensory rhodopsin II at 2.1-A resolution

Sensory rhodopsins (SRs) belong to a subfamily of heptahelical transmembrane proteins containing a retinal chromophore. These photoreceptors mediate the cascade of vision in animal eyes and phototaxis in archaebacteria and unicellular flagellated algae. Signal transduction by these photoreceptors occurs by means of transducer proteins. The two archaebacterial sensory rhodopsins SRI and SRII are coupled to the membrane-bound HtrI and HtrII transducer proteins. Activation of these proteins initiates phosphorylation cascades that modulate the flagellar motors, resulting in either attractant (SRI) or repellent (SRII) phototaxis. In addition, transducer-free SRI and SRII were shown to operate as proton pumps, analogous to bacteriorhodopsin. Here, we present the x-ray structure of SRII from Natronobacterium pharaonis (pSRII) at 2.1-A resolution, revealing a unique molecular architecture of the retinal-binding pocket. In particular, the structure of pSRII exhibits a largely unbent conformation of the retinal (as compared with bacteriorhodopsin and halorhodopsin), a hydroxyl group of Thr-204 in the vicinity of the Schiff base, and an outward orientation of the guanidinium group of Arg-72. Furthermore, the structure reveals a putative chloride ion that is coupled to the Schiff base by means of a hydrogen-bond network and a unique, positively charged surface patch for a probable interaction with HtrII. The high-resolution structure of pSRII provides a structural basis to elucidate the mechanisms of phototransduction and color tuning.

Time-resolved detection of transient movement of helices F and G in doubly spin-labeled bacteriorhodopsin

Photo-excited structural changes of the light-driven proton pump bacteriorhodopsin were monitored using double-site-directed spin labeling combined with electron paramagnetic resonance (EPR) spectroscopy. The inter-spin distances between nitroxides attached at residue positions 100 and 226, 101 and 160, and 101 and 168 were determined for the BR initial state and the trapped M photo-intermediate. Distance changes that occur during the photocycle were followed with millisecond time resolution under physiological conditions at 293 K. The kinetic analysis of the EPR data and comparison with the absorbance changes in the visible spectrum reveal an outward movement of helix F during the late M intermediate and a subsequent approach of helix G toward the proton channel. The displacements of the cytoplasmic moieties of these helices amount to 0.1-0.2 nm. We propose that the resulting opening of the proton channel decreases the pK of the proton donor D96 and facilitates proton transfer to the Schiff base during the M-to-N transition.

Specific binding sites for cations in bacteriorhodopsin

The Asp-85 residue, located in the vicinity of the retinal chromophore, plays a key role in the function of bacteriorhodopsin (bR) as a light-driven proton pump. In the unphotolyzed pigment the protonation of Asp-85 is responsible for the transition from the purple form (lambda(max) = 570 nm) to the blue form (lambda(max) = 605 nm) of bR. This transition can also be induced by deionization (cation removal). It was previously proposed that the cations bind to the bR surface and raise the surface pH, or bind to a specific site in the protein, probably in the retinal vicinity. We have reexamined these possibilities by evaluating the interaction between Mn(2+) and a nitroxyl radical probe covalently bound to several mutants in which protein residues were substituted by cystein. We have found that Mn(2+), which binds to the highest-affinity binding site, significantly affects the EPR spectrum of a spin label attached to residue 74C. Therefore, it is concluded that the highest-affinity binding site is located in the extracellular side of the protein and its distance from the spin label at 74C is estimated to be approximately 9.8 +/- 0.7 A. At least part of the three to four low-affinity cation binding sites are located in the cytoplasmic side, because Mn(2+) bound to these binding sites affects spin labels attached to residues 103C and 163C located in the cytoplasmic side of the protein. The results indicate specific binding sites for the color-controlling cations, and suggest that the binding sites involve negatively charged lipids located on the exterior of the bR trimer structure.

Inducible expression of bacterio-opsin in transgenic tobacco and tomato plants

The development of new strategies to enhance resistance of plants to pathogens is instrumental in preventing agricultural losses. Lesion mimic, the spontaneous formation of lesions resembling hypersensitive response lesions in the absence of a pathogen, is a dramatic phenotype occasionally induced upon expression of certain transgenes in plants. These transgenes simulate the presence of a pathogen and, therefore, activate the plant anti-pathogen defense mechanisms and induce a state of systemic resistance. Lesion mimic genes have been successfully used to enhance the resistance of a number of different plants to pathogen attack. However, constitutive expression of these genes in plants is associated with the spontaneous formation of lesions on leaves and stems, reduced growth, and lower yield. We tested the possibility of using a wound-inducible promoter to control the expression of bacterio-opsin (bO), a transgene that confers a lesion mimic phenotype in tobacco and tomato plants when constitutively expressed. We found that plants with inducible expression of bO did not develop spontaneous lesions. Nevertheless. under controlled laboratory conditions, they were found to be resistant to infection by pathogens. The activation of defense mechanisms by the bO gene was not constitutive, and occurred in response to wounding or pathogen infection. Furthermore, wounding of transgenic tobacco plants resulted in the induction of systemic resistance to pathogen attack within 48 h. Our findings provide a promising initial assessment for the use of wound-inducible promoters as a new strategy to enhance pathogen resistance in transgenic crops by means of lesion mimic genes.

Roles of cytoplasmic arginine and threonine in chloride transport by the bacteriorhodopsin mutant D85T

In the light-driven anion pump halorhodopsin (HR), the residues arginine 200 and threonine 203 are involved in anion release at the cytoplasmic side of the membrane. Because of large sequence homology and great structural similarities between HR and bacteriorhodopsin (BR), it has been suggested that anion translocation by HR and by the chloride-pumping BR mutant BR-D85T occurs by the same mechanism. Consequently, the functions of the R200/T203 pair in HR should be the same as those of the corresponding pair in BR-D85T (R175/T178). We have put this hypothesis to a test by creating two mutants of BR-D85T in which R175 and T178 were replaced by glutamine and valine, respectively. Chloride transport activities were essentially the same for all three mutants, whereas chloride binding and the kinetics of parts of the photocycle were markedly affected by the replacement of T178. In contrast, the consequences of mutating R175 proved to be less significant. These findings are consistent with evidence obtained on HR and therefore support the idea that the respective mechanistic roles of the cytoplasmic arginine/threonine pairs in HR and BR-D85T are equal.

Time-resolved FT-IR spectroscopic investigation of the pH-dependent proton transfer reactions in the E194Q mutant of bacteriorhodopsin

The photoreaction of the E194Q mutant of bacteriorhodopsin has been investigated at various pH values by time-resolved step-scan Fourier-transform infrared difference spectroscopy employing the attenuated total reflection technique. The difference spectrum at pH 8.4 is comparable to the N-BR difference spectra of the wild type with the remarkable exception that D85 is deprotonated. Since the retinal configuration is not perturbed by the E194Q mutation, it is concluded that there is no interaction of D85 with retinal during the lifetime of the N state. At pH 6, a consecutive state to the O intermediate is detected in which D212 is transiently protonated. The comparison with wild-type bacteriorhodopsin reveals that protonation of D212 represents an intermediate step during proton transfer from D85 to the proton release group in the final stage of the reaction cycle. The described effects are more pronounced in the E194Q mutant than in the E204Q mutant demonstrating different roles of these two glutamates/glutamic acids at least in the final stages of the catalytic cycle of bacteriorhodopsin.

Structure and function in bacteriorhodopsin: the role of the interhelical loops in the folding and stability of bacteriorhodopsin

Bacteriorhodopsin functions as a light-driven proton pump in Halobacterium salinarium. The functional protein consists of an apoprotein, bacterioopsin, with seven transmembrane alpha helices together with a covalently bound all-trans retinal chromophore. In order to study the role of the interhelical loop conformations in the structure and function of bacteriorhodopsin, we have constructed bacterioopsin genes where each loop is replaced, one at a time, by a peptide linker consisting of Gly-Gly-Ser- repeat sequences, which are believed to have flexible conformations. These mutant proteins have been expressed in Escherichia coli, purified and reconstituted with all-trans retinal in l-alpha-1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC)/3-(3-cholamidopropyl)dimethylammonio-1-propane sulfonate (CHAPS)/SDS and l-alpha-1,2-dihexanoylphosphatidylcholine (DHPC)/DMPC/SDS micelles. Wild-type-like chromophore formation was observed in all the mutants containing single loop replacements. In the BC and FG mutants, an additional chromophore band with an absorption band at about 480 nm was observed, which was in equilibrium with the 550 nm, wild-type band. The position of the equilibrium depended on temperature, SDS and relative DMPC concentration. The proton pumping activity of all of the mutants was comparable to that of wild-type bR except for the BC and FG mutants, which had lower activity. All of the loop mutants were more sensitive to denaturation by SDS than the wild-type protein, except the mutant where the DE loop was replaced. These results suggest that a specific conformation of all the loops of bR, except the DE loop, contributes to bR stability and is required for the correct folding and function of the protein. An increase in the relative proportion of DHPC in DHPC/DMPC micelles, which reduces the micelle rigidity and alters the micelle shape, resulted in lower folding yields of all loop mutants except the BC and DE mutants. This effect of micelle rigidity on the bR folding yield correlated with a loss in stability of a partially folded, seven-transmembrane apoprotein intermediate state in SDS/DMPC/CHAPS micelles. The folding yield and stability of the apoprotein intermediate state both decreased for the loop mutants in the order WT approximately BC approximately DE>FG>AB>EF> or =CD, where the EF and CD loop mutants were the least stable.