Chloride and proton transport in bacteriorhodopsin mutant D85T: different modes of ion translocation in a retinal protein

Hydrogen-Bonding and Hydrophobic Interaction Networks as Structural Determinants of Microbial Rhodopsin Function

Microbial pump rhodopsins are highly versatile light-driven membrane proteins that couple protein conformational dynamics with ion translocation across the cell membranes. Understanding how microbial pump rhodopsins use specific amino acid residues at key functional sites to control ion selectivity and ion pumping direction is of general interest for membrane transporters, and could guide site-directed mutagenesis for optogenetics applications. To enable direct comparisons between proteins with different sequences we implement, for the first time, a unique numbering scheme for the microbial pump rhodopsin residues, NS-mrho. We use NS-mrho to show that distinct microbial pump rhodopsins typically have hydrogen-bond networks that are less conserved than anticipated from the amino acid residue conservation, whereas their hydrophobic interaction networks are largely conserved. To illustrate the role of the hydrogen-bond networks as structural elements that determine the functionality of microbial pump rhodopsins, we performed experiments, atomic-level simulations, and hydrogen bond network analyses on GR, the outward proton pump from Gloeobacter violaceus, and KR2, the outward sodium pump from Krokinobacter eikastus. The experiments indicate that multiple mutations that recover KR2 amino acid residues in GR not only fail to convert it into a sodium pump, but completely inactivate GR by abolishing photoisomerization of the retinal chromophore. This observation could be attributed to the drastically altered hydrogen-bond interaction network identified with simulations and network analyses. Taken together, our findings suggest that functional specificity could be encoded in the collective hydrogen-bond network of microbial pump rhodopsins.

Converting a Natural-Light-Driven Outward Proton Pump Rhodopsin into an Artificial Inward Proton Pump

Microbial rhodopsins are a large family of photoreceptive membrane proteins with diverse light-regulated functions. While the most ubiquitous microbial rhodopsins are light-driven outward proton (H⁺) pumps, new subfamilies of microbial rhodopsins transporting H⁺ inwardly, i.e., light-driven inward H⁺ pumps, have been discovered recently. Although structural and spectroscopic studies provide insights into their ion transport mechanisms, the minimum key element(s) that determine the direction of H⁺ transport have not yet been clarified. Here, we conducted the first functional conversion study by substituting key amino acids in a natural outward H⁺-pumping rhodopsin (PspR) with those in inward H⁺-pumping rhodopsins. Consequently, an artificial inward H⁺ pump was constructed by mutating only three residues of PspR. This result indicates that these residues govern the key processes that discriminate between outward and inward H⁺ pumps. Spectroscopic studies revealed the presence of an inward H⁺-accepting residue in the H⁺ transport pathway and direct H⁺ uptake from the extracellular solvent. This finding of the simple element for determining H⁺ transport would provide a new basis for understanding the concept of ion transport not only by microbial rhodopsins but also by other ion-pumping proteins.

Low pH structure of heliorhodopsin reveals chloride binding site and intramolecular signaling pathway

Within the microbial rhodopsin family, heliorhodopsins (HeRs) form a phylogenetically distinct group of light-harvesting retinal proteins with largely unknown functions. We have determined the 1.97 Å resolution X-ray crystal structure of Thermoplasmatales archaeon SG8-52-1 heliorhodopsin (TaHeR) in the presence of NaCl under acidic conditions (pH 4.5), which complements the known 2.4 Å TaHeR structure acquired at pH 8.0. The low pH structure revealed that the hydrophilic Schiff base cavity (SBC) accommodates a chloride anion to stabilize the protonated retinal Schiff base when its primary counterion (Glu-108) is neutralized. Comparison of the two structures at different pH revealed conformational changes connecting the SBC and the extracellular loop linking helices A-B. We corroborated this intramolecular signaling transduction pathway with computational studies, which revealed allosteric network changes propagating from the perturbed SBC to the intracellular and extracellular space, suggesting TaHeR may function as a sensory rhodopsin. This intramolecular signaling mechanism may be conserved among HeRs, as similar changes were observed for HeR 48C12 between its pH 8.8 and pH 4.3 structures. We additionally performed DEER experiments, which suggests that TaHeR forms possible dimer-of-dimer associations which may be integral to its putative functionality as a light sensor in binding a transducer protein.

Light-driven proton transfers and proton transport by microbial rhodopsins - A biophysical perspective

In the last twenty years, our understanding of the rules and mechanisms for the outward light-driven proton transport (and underlying proton transfers) by microbial rhodopsins has been changing dramatically. It transitioned from a very detailed atomic-level understanding of proton transport by bacteriorhodopsin, the prototypical proton pump, to a confounding variety of sequence motifs, mechanisms, directions, and modes of transport in its newly found homologs. In this review, we will summarize and discuss experimental data obtained on new microbial rhodopsin variants, highlighting their contribution to the refinement and generalization of the ideas crystallized in the previous century. In particular, we will focus on the proton transport (and transfers) vectoriality and their structural determinants, which, in many cases, remain unidentified.

A single point mutation converts a proton-pumping rhodopsin into a red-shifted, turn-on fluorescent sensor for chloride

The visualization of chloride in living cells with fluorescent sensors is linked to our ability to design hosts that can overcome the energetic penalty of desolvation to bind chloride in water. Fluorescent proteins can be used as biological supramolecular hosts to address this fundamental challenge. Here, we showcase the power of protein engineering to convert the fluorescent proton-pumping rhodopsin GR from Gloeobacter violaceus into GR1, a red-shifted, turn-on fluorescent sensor for chloride in detergent micelles and in live Escherichia coli. This non-natural function was unlocked by mutating D121, which serves as the counterion to the protonated retinylidene Schiff base chromophore. Substitution from aspartate to valine at this position (D121V) creates a binding site for chloride. The binding of chloride tunes the pK a of the chromophore towards the protonated, fluorescent state to generate a pH-dependent response. Moreover, ion pumping assays combined with bulk fluorescence and single-cell fluorescence microscopy experiments with E. coli, expressing a GR1 fusion with a cyan fluorescent protein, show that GR1 does not pump ions nor sense membrane potential but instead provides a reversible, ratiometric readout of changes in extracellular chloride at the membrane. This discovery sets the stage to use natural and laboratory-guided evolution to build a family of rhodopsin-based fluorescent chloride sensors with improved properties for cellular applications and learn how proteins can evolve and adapt to bind anions in water.

Functional Mechanism of Cl--Pump Rhodopsin and Its Conversion into H+ Pump

Cl^--pump rhodopsin is the second discovered microbial rhodopsin. Although its physiological role has not been fully clarified, its functional mechanism has been studied as a model for anion transporters. After the success of neural activation by channel rhodopsin, the first Cl^--pump halorhodopsin (HR) had become widely used as a neural silencer. The emergence of artificial and natural anion channel rhodopsins lowered the importance of HRs. However, the longer absorption maxima of approximately 585-600 nm for HRs are still advantageous for applications in mammalian brains and collaborations with neural activators possessing shorter absorption maxima. In this chapter, the variation and functional mechanisms of Cl^- pumps are summarized. After the discovery of HR, Cl^--pump rhodopsins were confined to only extremely halophilic haloarchaea. However, after 2014, two Cl^--pump groups were newly discovered in marine and terrestrial bacteria. These Cl^- pumps are phylogenetically distinct from HRs and have unique characteristics. In particular, the most recently identified Cl^- pump has close similarity with the H⁺ pump bacteriorhodopsin and was converted into the H⁺ pump by a single amino acid replacement.

The crystal structures of a chloride-pumping microbial rhodopsin and its proton-pumping mutant illuminate proton transfer determinants

Microbial rhodopsins are versatile and ubiquitous retinal-binding proteins that function as light-driven ion pumps, light-gated ion channels, and photosensors, with potential utility as optogenetic tools for altering membrane potential in target cells. Insights from crystal structures have been central for understanding proton, sodium, and chloride transport mechanisms of microbial rhodopsins. Two of three known groups of anion pumps, the archaeal halorhodopsins (HRs) and bacterial chloride-pumping rhodopsins, have been structurally characterized. Here we report the structure of a representative of a recently discovered third group consisting of cyanobacterial chloride and sulfate ion-pumping rhodopsins, the Mastigocladopsis repens rhodopsin (MastR). Chloride-pumping MastR contains in its ion transport pathway a unique Thr-Ser-Asp (TSD) motif, which is involved in the binding of a chloride ion. The structure reveals that the chloride-binding mode is more similar to HRs than chloride-pumping rhodopsins, but the overall structure most closely resembles bacteriorhodopsin (BR), an archaeal proton pump. The MastR structure shows a trimer arrangement reminiscent of BR-like proton pumps and shows features at the extracellular side more similar to BR than the other chloride pumps. We further solved the structure of the MastR-T74D mutant, which contains a single amino acid replacement in the TSD motif. We provide insights into why this point mutation can convert the MastR chloride pump into a proton pump but cannot in HRs. Our study points at the importance of precise coordination and exact location of the water molecule in the active center of proton pumps, which serves as a bridge for the key proton transfer.

Collective exchange processes reveal an active site proton cage in bacteriorhodopsin

Proton translocation across membranes is vital to all kingdoms of life. Mechanistically, it relies on characteristic proton flows and modifications of hydrogen bonding patterns, termed protonation dynamics, which can be directly observed by fast magic angle spinning (MAS) NMR. Here, we demonstrate that reversible proton displacement in the active site of bacteriorhodopsin already takes place in its equilibrated dark-state, providing new information on the underlying hydrogen exchange processes. In particular, MAS NMR reveals proton exchange at D85 and the retinal Schiff base, suggesting a tautomeric equilibrium and thus partial ionization of D85. We provide evidence for a proton cage and detect a preformed proton path between D85 and the proton shuttle R82. The protons at D96 and D85 exchange with water, in line with ab initio molecular dynamics simulations. We propose that retinal isomerization makes the observed proton exchange processes irreversible and delivers a proton towards the extracellular release site.

Daniel Friedrich et al. show that reversible proton translocation occurs in the dark–state of bacteriorhodopsin, involving the retinal Schiff base and D85 exchanging protons with H₂O. They find evidence of an active site proton cage and possible proton transfer via R82.

Anion binding to mutants of the Schiff base counterion in heliorhodopsin 48C12

The anion binds as the direct H-bonding acceptor of the Schiff base in E107A, while E107Q indirectly accommodates an anion.

Molecular details of the unique mechanism of chloride transport by a cyanobacterial rhodopsin

Microbial rhodopsins are well known as versatile and ubiquitous light-driven ion transporters and photosensors. While the proton transport mechanism has been studied in great detail, much less is known about various modes of anion transport. Until recently, only two main groups of light-driven anion pumps were known, archaeal halorhodopsins (HRs) and bacterial chloride pumps (known as ClRs or NTQs). Last year, another group of cyanobacterial anion pumps with a very distinct primary structure was reported. Here, we studied the chloride-transporting photocycle of a representative of this new group, Mastigocladopsis repens rhodopsin (MastR), using time-resolved spectroscopy in the infrared and visible ranges and site-directed mutagenesis. We found that, in accordance with its unique amino acid sequence containing many polar residues in the transmembrane region of the protein, its photocycle features a number of unusual molecular events not known for other anion-pumping rhodopsins. It appears that light-driven chloride ion transfers by MastR are coupled with translocation of protons and water molecules as well as perturbation of several polar sidechains. Of particular interest is transient deprotonation of Asp-85, homologous to the cytoplasmic proton donor of light-driven proton pumps (such as Asp-96 of bacteriorhodopsin), which may serve as a regulatory mechanism.

Low-temperature FTIR spectroscopy provides evidence for protein-bound water molecules in eubacterial light-driven ion pumps

The present FTIR study showed that eubacterial light-driven H⁺, Na⁺ and Cl⁻ pump rhodopsins contain strongly hydrogen-bonded water molecule, the functional determinant of light-driven proton pump. This explains well the asymmetric functional conversions of light-driven ion pumps.

Conversion of microbial rhodopsins: insights into functionally essential elements and rational protein engineering

Technological progress has enabled the successful application of functional conversion to a variety of biological molecules, such as nucleotides and proteins. Such studies have revealed the functionally essential elements of these engineered molecules, which are difficult to characterize at the level of an individual molecule. The functional conversion of biological molecules has also provided a strategy for their rational and atomistic design. The engineered molecules can be used in studies to improve our understanding of their biological functions and to develop protein-based tools. In this review, we introduce the functional conversion of membrane-embedded photoreceptive retinylidene proteins (also called rhodopsins) and discuss these proteins mainly on the basis of results obtained from our own studies. This information provides insights into the molecular mechanism of light-induced protein functions and their use in optogenetics, a technology which involves the use of light to control biological activities.

The light-driven sodium ion pump: A new player in rhodopsin research

Rhodopsins are one of the most studied photoreceptor protein families, and ion-translocating rhodopsins, both pumps and channels, have recently attracted broad attention because of the development of optogenetics. Recently, a new functional class of ion-pumping rhodopsins, an outward Na⁺ pump, was discovered, and following structural and functional studies enable us to compare three functionally different ion-pumping rhodopsins: outward proton pump, inward Cl^- pump, and outward Na⁺ pump. Here, we review the current knowledge on structure-function relationships in these three light-driven pumps, mainly focusing on Na⁺ pumps. A structural and functional comparison reveals both unique and conserved features of these ion pumps, and enhances our understanding about how the structurally similar microbial rhodopsins acquired such diverse functions. We also discuss some unresolved questions and future perspectives in research of ion-pumping rhodopsins, including optogenetics application and engineering of novel rhodopsins.

Single-Amino Acid Modifications Reveal Additional Controls on the Proton Pathway of [FeFe]-Hydrogenase

The proton pathway of [FeFe]-hydrogenase is essential for enzymatic H2 production and oxidation and is composed of four residues and a water molecule. A computational analysis of this pathway in the [FeFe]-hydrogenase from Clostridium pasteurianum revealed that the solvent-exposed residue of the pathway (Glu282) forms hydrogen bonds to two residues outside of the pathway (Arg286 and Ser320), implying that these residues could function in regulating proton transfer. In this study, we show that substituting Arg286 with leucine eliminates hydrogen bonding with Glu282 and results in an ∼3-fold enhancement of H2 production activity when methyl viologen is used as an electron donor, suggesting that Arg286 may help control the rate of proton delivery. In contrast, substitution of Ser320 with alanine reduces the rate ∼5-fold, implying that it either acts as a member of the pathway or influences Glu282 to permit proton transfer. Interestingly, quantum mechanics/molecular mechanics and molecular dynamics calculations indicate that Ser320 does not play a structural role or indirectly influence the barrier for proton movement at the entrance of the channel. Rather, it may act as an additional proton acceptor for the pathway or serve in a regulatory role. While further studies are needed to elucidate the role of Ser320, collectively these data provide insights into the complex proton transport process.

Asymmetric Functional Conversion of Eubacterial Light-driven Ion Pumps

In addition to the well-known light-driven outward proton pumps, novel ion-pumping rhodopsins functioning as outward Na(+) and inward Cl(-) pumps have been recently found in eubacteria. They convert light energy into transmembrane electrochemical potential difference, similar to the prototypical archaeal H(+) pump bacteriorhodopsin (BR) and Cl(-) pump halorhodopsin (HR). The H(+), Na(+), and Cl(-) pumps possess the conserved respective DTE, NDQ, and NTQ motifs in the helix C, which likely serve as their functional determinants. To verify this hypothesis, we attempted functional interconversion between selected pumps from each category by mutagenesis. Introduction of the proton-pumping motif resulted in successful Na(+) → H(+) functional conversion. Introduction of the respective characteristic motifs with several additional mutations leads to successful Na(+) → Cl(-) and Cl(-) → H(+) functional conversions, whereas remaining conversions (H(+) → Na(+), H(+) → Cl(-), Cl(-) → Na(+)) were unsuccessful when mutagenesis of 4-6 residues was used. Phylogenetic analysis suggests that a H(+) pump is the common ancestor of all of these rhodopsins, from which Cl(-) pumps emerged followed by Na(+) pumps. We propose that successful functional conversions of these ion pumps are achieved exclusively when mutagenesis reverses the evolutionary amino acid sequence changes. Dependence of the observed functional conversions on the direction of evolution strongly suggests that the essential structural mechanism of an ancestral function is retained even after the gain of a new function during natural evolution, which can be evoked by a few mutations. By contrast, the gain of a new function needs accumulation of multiple mutations, which may not be easily reproduced by limited mutagenesis in vitro.

Ion-pumping microbial rhodopsins

Rhodopsins are light-sensing proteins used in optogenetics. The word "rhodopsin" originates from the Greek words "rhodo" and "opsis," indicating rose and sight, respectively. Although the classical meaning of rhodopsin is the red-colored pigment in our eyes, the modern meaning of rhodopsin encompasses photoactive proteins containing a retinal chromophore in animals and microbes. Animal and microbial rhodopsins possess 11-cis and all-trans retinal, respectively, to capture light in seven transmembrane α-helices, and photoisomerizations into all-trans and 13-cis forms, respectively, initiate each function. Ion-transporting proteins can be found in microbial rhodopsins, such as light-gated channels and light-driven pumps, which are the main tools in optogenetics. Light-driven pumps, such as archaeal H(+) pump bacteriorhodopsin (BR) and Cl(-) pump halorhodopsin (HR), were discovered in the 1970s, and their mechanism has been extensively studied. On the other hand, different kinds of H(+) and Cl(-) pumps have been found in marine bacteria, such as proteorhodopsin (PR) and Fulvimarina pelagi rhodopsin (FR), respectively. In addition, a light-driven Na(+) pump was found, Krokinobacter eikastus rhodopsin 2 (KR2). These light-driven ion-pumping microbial rhodopsins are classified as DTD, TSA, DTE, NTQ, and NDQ rhodopsins for BR, HR, PR, FR, and KR2, respectively. Recent understanding of ion-pumping microbial rhodopsins is reviewed in this paper.

Pre-gating conformational changes in the ChETA variant of channelrhodopsin-2 monitored by nanosecond IR spectroscopy

Light-gated ion permeation by channelrhodopsin-2 (ChR2) relies on the photoisomerization of the retinal chromophore and the subsequent photocycle, leading to the formation (on-gating) and decay (off-gating) of the conductive state. Here, we have analyzed the photocycle of a fast-cycling ChR2 variant (E123T mutation, also known as ChETA), by time-resolved UV/vis, step-scan FT-IR, and tunable quantum cascade laser IR spectroscopies with nanosecond resolution. Pre-gating conformational changes rise with a half-life of 200 ns, silent to UV/vis but detected by IR spectroscopy. They involve changes in the peptide backbone and in the H-bond of the side chain of the critical residue D156. Thus, the P1(500) intermediate must be separated into early and late states. Light-adapted ChR2 contains a mixture of all-trans and 13-cis retinal in a 70:30 ratio which are both photoactive. Analysis of ethylenic and fingerprint vibrations of retinal provides evidence that the 13-cis photocycle recovers in 1 ms. This recovery is faster than channel off-gating and most of the proton transfer reactions, implying that the 13-cis photocycle is of minor functional relevance for ChR2.

Directed evolution of Gloeobacter violaceus rhodopsin spectral properties

Proton-pumping rhodopsins (PPRs) are photoactive retinal-binding proteins that transport ions across biological membranes in response to light. These proteins are interesting for light-harvesting applications in bioenergy production, in optogenetics applications in neuroscience, and as fluorescent sensors of membrane potential. Little is known, however, about how the protein sequence determines the considerable variation in spectral properties of PPRs from different biological niches or how to engineer these properties in a given PPR. Here we report a comprehensive study of amino acid substitutions in the retinal-binding pocket of Gloeobacter violaceus rhodopsin (GR) that tune its spectral properties. Directed evolution generated 70 GR variants with absorption maxima shifted by up to ±80nm, extending the protein's light absorption significantly beyond the range of known natural PPRs. While proton-pumping activity was disrupted in many of the spectrally shifted variants, we identified single tuning mutations that incurred blue and red shifts of 42nm and 22nm, respectively, that did not disrupt proton pumping. Blue-shifting mutations were distributed evenly along the retinal molecule while red-shifting mutations were clustered near the residue K257, which forms a covalent bond with retinal through a Schiff base linkage. Thirty eight of the identified tuning mutations are not found in known microbial rhodopsins. We discovered a subset of red-shifted GRs that exhibit high levels of fluorescence relative to the WT (wild-type) protein.

The role of protein-bound water molecules in microbial rhodopsins

Protein-bound internal water molecules are essential features of the structure and function of microbial rhodopsins. Besides structural stabilization, they act as proton conductors and even proton storage sites. Currently, the most understood model system exhibiting such features is bacteriorhodopsin (bR). During the last 20 years, the importance of water molecules for proton transport has been revealed through this protein. It has been shown that water molecules are as essential as amino acids for proton transport and biological function. In this review, we present an overview of the historical development of this research on bR. We furthermore summarize the recently discovered protein-bound water features associated with proton transport. Specifically, we discuss a pentameric water/amino acid arrangement close to the protonated Schiff base as central proton-binding site, a protonated water cluster as proton storage site at the proton-release site, and a transient linear water chain at the proton uptake site. We highlight how protein conformational changes reposition or reorient internal water molecules, thereby guiding proton transport. Last, we compare the water positions in bR with those in other microbial rhodopsins to elucidate how protein-bound water molecules guide the function of microbial rhodopsins. This article is part of a Special Issue entitled: Retinal Proteins - You can teach an old dog new tricks.

Eubacterial rhodopsins - unique photosensors and diverse ion pumps

Since the discovery of proteorhodopsins, the ubiquitous marine light-driven proton pumps of eubacteria, a large number of other eubacterial rhodopsins with diverse structures and functions have been characterized. Here, we review the body of knowledge accumulated on the four major groups of eubacterial rhodopsins, with the focus on their biophysical characterization. We discuss advances and controversies on the unique eubacterial sensory rhodopsins (as represented by Anabaena sensory rhodopsin), proton-pumping proteorhodopsins and xanthorhodopsins, as well as novel non-proton ion pumps. This article is part of a Special Issue entitled: Retinal Proteins - You can teach an old dog new tricks.

Directed evolution of bacteriorhodopsin for applications in bioelectronics

In nature, biological systems gradually evolve through complex, algorithmic processes involving mutation and differential selection. Evolution has optimized biological macromolecules for a variety of functions to provide a comparative advantage. However, nature does not optimize molecules for use in human-made devices, as it would gain no survival advantage in such cooperation. Recent advancements in genetic engineering, most notably directed evolution, have allowed for the stepwise manipulation of the properties of living organisms, promoting the expansion of protein-based devices in nanotechnology. In this review, we highlight the use of directed evolution to optimize photoactive proteins, with an emphasis on bacteriorhodopsin (BR), for device applications. BR, a highly stable light-activated proton pump, has shown great promise in three-dimensional optical memories, real-time holographic processors and artificial retinas.

Kinetics of proton release and uptake by channelrhodopsin-2

Electrophysiological experiments showed that the light-activated cation channel channelrhodopsin-2 (ChR2) pumps protons in the absence of a membrane potential. We determined here the kinetics of transient pH change using a water-soluble pH-indicator. It is shown that ChR2 released protons prior to uptake with a stoichiometry of 0.3 protons per ChR2. Comparison to the photocycle kinetics revealed that proton release and uptake match rise and decay of the P(3)(520) intermediate. As the P(3)(520) state also represents the conductive state of cation channeling, the concurrence of proton pumping and channel gating implies an intimate mechanistic link of the two functional modes. Studies on the E123T and S245E mutants show that these residues are not critically involved in proton translocation.

Genetically encoded molecular tools for light-driven silencing of targeted neurons

The ability to silence, in a temporally precise fashion, the electrical activity of specific neurons embedded within intact brain tissue, is important for understanding the role that those neurons play in behaviors, brain disorders, and neural computations. "Optogenetic" silencers, genetically encoded molecules that, when expressed in targeted cells within neural networks, enable their electrical activity to be quieted in response to pulses of light, are enabling these kinds of causal circuit analyses studies. Two major classes of optogenetic silencer are in broad use in species ranging from worm to monkey: light-driven inward chloride pumps, or halorhodopsins, and light-driven outward proton pumps, such as archaerhodopsins and fungal light-driven proton pumps. Both classes of molecule, when expressed in neurons via viral or other transgenic means, enable the targeted neurons to be hyperpolarized by light. We here review the current status of these sets of molecules, and discuss how they are being discovered and engineered. We also discuss their expression properties, ionic properties, spectral characteristics, and kinetics. Such tools may not only find many uses in the quieting of electrical activity for basic science studies but may also, in the future, find clinical uses for their ability to safely and transiently shut down cellular electrical activity in a precise fashion.

Curvature of purple membranes comprising permanently wedge-shaped bacteriorhodopsin molecules is regulated by lipid content

Purple membrane (PM) from Halobacterium salinarum has been studied by many groups and is commonly described as a flat 2-D crystalline membrane microdomain which contains a hexagonal crystalline lattice of bacteriorhodopsin (BR) trimers in a stoichiometric ratio of 10:1 between lipids and BR. BR is the key protein in the halobacterial photosynthetic system and acts as a light-driven proton pump. Upon absorption of a photon, BR undergoes a cyclic series of intramolecular changes, among them a transient "wedge-like" geometrical change of the protein due to a tilt in helix F, one of the seven alpha-helical domains of BR. Due to the strong coupling between the BRs in the crystalline lattice, this may affect membrane topography. In nature, only low light levels occur and the total number of BRs in the "wedge-shaped" state is negligible. For mutated PMs like PM-D85T and PM-D85N (PM-D85X, X = neutral residue), the change of the membrane topography can be triggered in a pH-dependent manner. PMs containing BR-D85X look like "cups" at certain pH values. How does nature deal with a mutated PM like PM-D96G/F171C/F219L (PM-Tri) which comprises permanently "wedge-shaped" BRs and how does this influence membrane assembly? Astonishingly, we observed that PM-Tri is flat. Obviously, the morphology of Halobacterium salinarum is highly conserved and requires flat PMs to be assembled. We found that the lipid content of PM-Tri is specifically altered to assemble a hexagonal crystalline PM-Tri lattice of flat topography.

Influence of the charge at D85 on the initial steps in the photocycle of bacteriorhodopsin

Studies have shown that trans-cis isomerization of retinal is the primary photoreaction in the photocycle of the light-driven proton pump bacteriorhodopsin (BR) from Halobacterium salinarum, as well as in the photocycle of the chloride pump halorhodopsin (HR). The transmembrane proteins HR and BR show extensive structural similarities, but differ in the electrostatic surroundings of the retinal chromophore near the protonated Schiff base. Point mutation of BR of the negatively charged aspartate D85 to a threonine T (D85T) in combination with variation of the pH value and anion concentration is used to study the ultrafast photoisomerization of BR and HR for well-defined electrostatic surroundings of the retinal chromophore. Variations of the pH value and salt concentration allow a switch in the isomerization dynamics of the BR mutant D85T between BR-like and HR-like behaviors. At low salt concentrations or a high pH value (pH 8), the mutant D85T shows a biexponential initial reaction similar to that of HR. The combination of high salt concentration and a low pH value (pH 6) leads to a subpopulation of 25% of the mutant D85T whose stationary and dynamic absorption properties are similar to those of native BR. In this sample, the combination of low pH and high salt concentration reestablishes the electrostatic surroundings originally present in native BR, but only a minor fraction of the D85T molecules have the charge located exactly at the position required for the BR-like fast isomerization reaction. The results suggest that the electrostatics in the native BR protein is optimized by evolution. The accurate location of the fixed charge at the aspartate D85 near the Schiff base in BR is essential for the high efficiency of the primary reaction.

Influence of proline on the thermostability of the active site and membrane arrangement of transmembrane proteins

Proline residues play a fundamental and subtle role in the dynamics, structure, and function in many membrane proteins. Temperature derivative spectroscopy and differential scanning calorimetry have been used to determine the effect of proline substitution in the structural stability of the active site and transmembrane arrangement of bacteriorhodopsin. We have analyzed the Pro-to-Ala mutation for the helix-embedded prolines Pro50, Pro91, and Pro186 in the native membrane environment. This information has been complemented with the analysis of the respective crystallographic structures by the FoldX force field. Differential scanning calorimetry allowed us to determine distorted membrane arrangement for P50A and P186A. The protein stability was severely affected for P186A and P91A. In the case of Pro91, a single point mutation is capable of strongly slowing down the conformational diffusion along the denaturation coordinate, becoming a barrier-free downhill process above 371 K. Temperature derivative spectroscopy, applied for first time to study thermal stability of proteins, has been used to monitor the stability of the active site of bacteriorhodopsin. The mutation of Pro91 and Pro186 showed the most striking effects on the retinal binding pocket. These residues are the Pro in closer contact to the active site (activation energies for retinal release of 60.1 and 76.8 kcal/mol, respectively, compared to 115.8 kcal/mol for WT). FoldX analysis of the protein crystal structures indicates that the Pro-to-Ala mutations have both local and long-range effects on the structural stability of residues involved in the architecture of the protein and the active site and in the proton pumping function. Thus, this study provides a complete overview of the substitution effect of helix-embedded prolines in the thermodynamic and dynamic stability of a membrane protein, also related to its structure and function.

Initial reaction dynamics of proteorhodopsin observed by femtosecond infrared and visible spectroscopy

We present a comparative study using femtosecond pump/probe spectroscopy in the visible and infrared of the early photodynamics of solubilized proteorhodopsin (green absorbing variant) in D(2)O with deprotonated (pD 9.2) and protonated (pD 6.4) primary proton acceptor Asp-97. The vis-pump/vis-probe experiments show a kinetic isotope effect that is more pronounced for alkaline conditions, thus decreasing the previously reported pH-dependence of the primary reaction of proteorhodopsin in H(2)O. This points to a pH dependent H-bonding network in the binding pocket of proteorhodopsin, that directly influences the primary photo-induced dynamics. The vis-pump/IR-probe experiments were carried out in two different spectral regions and allowed to monitor the retinal C=C (1500 cm(-1)-1580 cm(-1)) and C=N stretching vibration as well as the amide I mode of the protein (1590 cm(-1)-1680 cm(-1)). Like the FTIR spectra of the K intermediate (PR(K)-PR difference spectra) in this spectral range, the kinetic parameters and also the quantum efficiency of photo-intermediate formation are found to be virtually independent of the pD value.

Microbial rhodopsins: scaffolds for ion pumps, channels, and sensors

Microbial rhodopsins have been intensively researched for the last three decades. Since the discovery of bacteriorhodopsin, the scope of microbial rhodopsins has been considerably extended, not only in view of the large number of family members, but also their functional properties as pumps, sensors, and channels. In this review, we give a short overview of old and newly discovered microbial rhodopsins, the mechanism of signal transfer and ion transfer, and we discuss structural and mechanistic aspects of phototaxis.

Halide Binding by the D212N Mutant of Bacteriorhodopsin Affects Hydrogen Bonding of Water in the Active Site

Bacteriorhodopsin (BR), a membrane protein found in Halobacterium salinarum, functions as a light-driven proton pump. The Schiff base region has a quadrupolar structure with positive charges located at the protonated Schiff base and Arg82, and the counterbalancing negative charges located at Asp85 and Asp212. The quadropole inside the protein is stabilized by three water molecules, forming a roughly planar pentagonal cluster composed of these waters and two oxygens of Asp85 and Asp212 (one from each carboxylate side chain). It is known that BR lacks proton-pumping activity if Asp85 or Asp212 is neutralized by mutation, but binding of Cl- has different functional effects in mutants at these positions. Binding of Cl- to D85T converts into a chloride ion pump (Sasaki, J., Brown, L. S., Chon, Y.-S., Kandori, H., Maeda, A., Needleman, R., and Lanyi, J. K. (1995) Science 269, 73-75). On the other hand, photovoltage measurements suggested that binding of Cl- to D212N restores the proton-pumping activity at low pH (Moltke, S., Krebs, M. P., Mollaaghababa, R., Khorana, H. G., and Heyn, M. P. (1995) Biophys. J. 69, 2074-2083). In this paper, we studied halide-bound D212N mutant BR in detail. Light-induced pH changes in a suspension of proteoliposomes containing D212N(Cl-) at pH 5 clearly showed that Cl- restores the proton-pumping activity. Spectral blue-shift induced by halide binding to D212N indicates that halides affect the counterion of the protonated Schiff base, whereas much smaller halide dependence of the lambdamax than in D85T suggests that the binding site is distant from the chromophore. In fact, the K minus BR difference Fourier-transform infrared (FTIR) spectra of D212N at 77 K exhibit little halide dependence for vibrational bands of retinal and protein. The only halide-dependent bands were the C=N stretch of Arg82 and some water O-D stretches, suggesting that these groups constitute a halide-binding pocket. A strongly hydrogen-bonded water molecule is observed for halide-bound D212N, but not for halide-free D212N, which is consistent with our hypothesis that such a water molecule is a prerequisite for proton-pumping activity of rhodopsins. We concluded that halide binding near Arg82 in D212N restores the water-containing hydrogen-bonding network in the Schiff base region. In particular, the ion pair formed by the Schiff base and Asp85 through a strongly hydrogen-bonded water is essential for the proton-pumping activity of this mutant and may be controlled by the halide binding to the distant site.

Hydrogen-bonding interaction of the protonated schiff base with halides in a chloride-pumping bacteriorhodopsin mutant

Bacteriorhodopsin (BR) and halorhodopsin (HR) are light-driven proton and chloride ion pumps, respectively, in Halobacterium salinarum. The amino acid identity of these proteins is about 25%, suggesting that each has been optimized for their own functions during evolution. However, it is known that the BR mutants, D85T and D85S, can pump chloride ions. This fact implies that the Schiff base region is important in determining ionic selectivity. The X-ray crystallographic structure of D85S(Br(-)) showed the presence of a bromide ion in the Schiff base region (Facciotti, M. T., Cheung, V. S., Nguyen, D., Rouhani, S., and Glaeser, R. M. (2003) Biophys. J. 85, 451-458). In this article, we report on the study of hydrogen bonds of the Schiff base and water molecules in D85S in the absence and presence of various halides, assigning their N-D and O-D stretching vibrations in D(2)O, respectively, in low-temperature Fourier-transform infrared (FTIR) spectroscopy. We found that the hydrogen bond of the Schiff base in D85S(Cl(-)) is much stronger than that in HR, being as strong as that in wild-type BR. Similar halide dependence in D85S and in solution implies that the Schiff base forms a direct hydrogen bond with a halide, consistent with the X-ray structure. Photoisomerization causes a weakened hydrogen bond of the Schiff base, and halide dependence on the stretching frequency is lost. These spectral features are similar to those in the photocycle of proton-pumping BR, though the weakened hydrogen bond is more significant for BR. However, the spectral features of water bands in D85S are closer to chloride-pumping HR because O-D stretching vibrations of water are observed only at >2500 cm(-)(1). Unlike in BR, we did not observe strongly hydrogen-bonded water molecules for halide-pumping D85S mutants. This observation agrees with our recent hypothesis that strongly hydrogen-bonded water molecules are required for the proton-pumping activity of archaeal rhodopsins. Hydrogen-bonding conditions in the Schiff base region of D85S are discussed on the basis of the spectral comparison with those of wild-type BR and HR.

Specificity of anion binding in the substrate pocket of bacteriorhodopsin

The structure of the D85S mutant of bacteriorhodopsin with a nitrate anion bound in the Schiff base binding site and the structure of the anion-free protein have been obtained in the same crystal form. Together with the previously solved structures of this anion pump, in both the anion-free state and bromide-bound state, these new structures provide insight into how this mutant of bacteriorhodopsin is able to bind a variety of different anions in the same binding pocket. The structural analysis reveals that the main structural change that accommodates different anions is the repositioning of the polar side chain of S85. On the basis of these X-ray crystal structures, the prediction is then made that the D85S/D212N double mutant might bind similar anions and do so over a broader pH range than does the single mutant. Experimental comparison of the dissociation constants, K(d), for a variety of anions confirms this prediction and demonstrates, in addition, that the binding affinity is dramatically improved by the D212N substitution.

Crystal structure of the bromide-bound D85S mutant of bacteriorhodopsin: principles of ion pumping

We report the crystal structure of a bromide-bound form of the D85S mutant of bacteriorhodopsin, bR(D85S), a protein that uses light energy rather than ATP to pump halide ions across the cell membrane. Comparison of the structure of the halide-bound and halide-free states reveals that both displacements of individual side-chain positions and concerted helical movements occur on the extracellular side of the protein. Analysis of these structural changes reveals how this ion pump first facilitates ion uptake deep within the cell membrane and then prevents the backward escape of ions later in the pumping cycle. Together with the information provided by structures of intermediate states in the bacteriorhodopsin photocycle, this study also suggests the overall design principles that are necessary for ion pumping.

Proteorhodopsin is a light-driven proton pump with variable vectoriality

Proteorhodopsin, a homologue of archaeal bacteriorhodopsin (BR), belongs to a newly identified family of retinal proteins from marine bacteria, which could play an important role in the energy balance of the biosphere. We cloned the cDNA sequence of proteorhodopsin by chemical gene synthesis, expressed the protein in Escherichia coli cells, purified and reconstituted the protein in its functional active state. The photocycle characteristics were determined by time-resolved absorption and Fourier transform infrared (FT-IR) spectroscopy. The pH-dependence of the absorption spectrum indicates that the pK(a) of the primary acceptor of the Schiff base proton (Asp97) is 7.68. Generally, the photocycle of proteorhodopsin is similar to that of BR, although an L-like photocycle intermediate was not detectable. Whereas at pH>7 an M-like intermediate is formed upon illumination, at pH 5 no M-like intermediate could be detected. As the photocycle kinetics do not change between the acidic and alkaline state of proteorhodopsin, the only difference between these two forms is the protonation status of Asp97. This is corroborated by time-resolved FT-IR spectroscopy, which demonstrates that proton transfer from the retinal Schiff base to Asp97 is observed at alkaline pH, but the other vibrational changes are essentially pH-independent.After reconstitution into proteoliposomes, light-induced proton currents of proteorhodopsin were measured in a compound membrane system where proteoliposomes were adsorbed to planar lipid bilayers. Our results show that proteorhodopsin is a light-driven proton pump with characteristics similar to those of BR at alkaline pH. However, at acidic pH, the direction of proton pumping is inverted. Complementary experiments were carried out on proteorhodopsin expressed heterologously in Xenopus laevis oocytes under voltage clamp conditions. The following results were obtained. (1) At alkaline pH, proteorhodopsin mediates outwardly directed proton pumping like BR. (2) The direction of proton pumping can be inverted, when Asp97 is protonated. (3) The current can be inverted by changes of the polarity of the applied voltage. (4) The light intensity-dependence of the photocurrents leads to the conclusion that the alkaline form of proteorhodopsin shows efficient proton pumping after sequential excitation by two photons.

Halorhodopsin: light-driven ion pumping made simple?

Halorhodopsin, a light-driven halide pump, is the second archaeal rhodopsin involved in ion pumping to be studied at high resolution by X-ray crystallography. Like its cousin bacteriorhodopsin, halorhodopsin couples vectorial ion transport to the isomerisation state of a covalently linked retinal. Given the similarity and interconvertability of these two ion pumps, a unified mechanism for ion translocation by archaeal rhodopsins is now emerging.

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.

Static and time-resolved step-scan Fourier transform infrared investigations of the photoreaction of halorhodopsin from Natronobacterium pharaonis: consequences for models of the anion translocation mechanism

The molecular changes during the photoreaction of halorhodopsin from Natronobacterium pharaonis have been monitored by low-temperature static and by time-resolved step-scan Fourier transform infrared difference spectroscopy. In the low-temperature L spectrum anions only influence a band around 1650 cm(-1), tentatively assigned to the C=N stretch of the protonated Schiff base of L. The analysis of the time-resolved spectra allows to identify the four states: K, L(1), L(2), and O. Between L(1) and L(2), only the apoprotein undergoes alterations. The O state is characterized by an all-trans chromophore and by rather large amide I spectral changes. Because in our analysis the intermediate containing O is in equilibrium with a state indistinguishable from L(2), we are unable to identify an N-like state. At very high chloride concentrations (>5 M), we observe a branching of the photocycle from L(2) directly back to the dark state, and we provide evidence for direct back-isomerization from L(2). This branching leads to the reported reduction of transport activity at such high chloride concentrations. We interpret the L(1) to L(2) transition as an accessibility change of the anion from the extracellular to the cytosolic side, and the large amide I bands in O as an indication for opening of the cytosolic channel from the Schiff base toward the cytosolic surface and/or as indication for changes of the binding constant of the release site.

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.