Unraveling photoexcited conformational changes of bacteriorhodopsin by time resolved electron paramagnetic resonance spectroscopy

Interdisciplinary biophysical studies of membrane proteins bacteriorhodopsin and rhodopsin

The centenary of the birth of H. Gobind Khorana provides an auspicious opportunity to review the origins and evolution of parallel advances in biophysical methodology and molecular genetics technology used to study membrane proteins. Interdisciplinary work in the Khorana laboratory in the late 1970s and for the next three decades led to productive collaborations and fostered three subsequent scientific generations whose biophysical work on membrane proteins has led to detailed elucidation of the molecular mechanisms of energy transduction by the light-driven proton pump bacteriorhodopsin (bR) and signal transduction by the G protein-coupled receptor (GPCR) rhodopsin. This review will highlight the origins and advances of biophysical studies of membrane proteins made possible by the application of molecular genetics approaches to engineer site-specific alterations of membrane protein structures.

Structural characterization of proton-pumping rhodopsin lacking a cytoplasmic proton donor residue by X-ray crystallography

DTG/DTS rhodopsin, which was named based on a three-residue motif (DTG or DTS) that is important for its function, is a light-driven proton-pumping microbial rhodopsin using a retinal chromophore. In contrast to other light-driven ion-pumping rhodopsins, DTG/DTS rhodopsin does not have a cytoplasmic proton donor residue, such as Asp, Glu, or Lys. Because of the lack of cytoplasmic proton donor residue, proton directly binds to the retinal chromophore from the cytoplasmic solvent. However, mutational experiments that showed the complicated effects of mutations were not able to clarify the roles played by each residue, and the detail of proton uptake pathway is unclear because of the lack of structural information. To understand the proton transport mechanism of DTG/DTS rhodopsin, here we report the three-dimensional structure of one of the DTG/DTS rhodopsins, PspR from Pseudomonas putida, by X-ray crystallography. We show that the structure of the cytoplasmic side of the protein is significantly different from that of bacteriorhodopsin, the best-characterized proton-pumping rhodopsin, and large cytoplasmic cavities were observed. We propose that these hydrophilic cytoplasmic cavities enable direct proton uptake from the cytoplasmic solvent without the need for a specialized cytoplasmic donor residue. The introduction of carboxylic residues homologous to the cytoplasmic donors in other proton-pumping rhodopsins resulted in higher pumping activity with less pH dependence, suggesting that DTG/DTS rhodopsins are advantageous for producing energy and avoiding intracellular alkalization in soil and plant-associated bacteria.

Recent advances in biophysical studies of rhodopsins - Oligomerization, folding, and structure

Retinal-binding proteins, mainly known as rhodopsins, function as photosensors and ion transporters in a wide range of organisms. From halobacterial light-driven proton pump, bacteriorhodopsin, to bovine photoreceptor, visual rhodopsin, they have served as prototypical α-helical membrane proteins in a large number of biophysical studies and aided in the development of many cutting-edge techniques of structural biology and biospectroscopy. In the last decade, microbial and animal rhodopsin families have expanded significantly, bringing into play a number of new interesting structures and functions. In this review, we will discuss recent advances in biophysical approaches to retinal-binding proteins, primarily microbial rhodopsins, including those in optical spectroscopy, X-ray crystallography, nuclear magnetic resonance, and electron paramagnetic resonance, as applied to such fundamental biological aspects as protein oligomerization, folding, and structure.

Light-driven ion-translocating rhodopsins in marine bacteria

Microbial rhodopsins are the photoreceptive membrane proteins found in diverse microorganisms from within Archaea, Eubacteria, and eukaryotes. They have a hep-tahelical transmembrane structure that binds to an all-trans retinal chromophore. Since 2000, thousands of proteorhodopsins, genes of light-driven proton pump rhodopsins, have been identified from various species of marine bacteria. This suggests that they are used for the conversion of light into chemical energy, contribut-ing to carbon circulation related to ATP synthesis in the ocean. Furthermore, novel types of rhodopsin (sodium and chloride pumps) have recently been discovered. Here, we review recent progress in our understanding of ion-transporting rhodopsins of marine bacteria, based mainly on biophysical and biochemical research.

Chimeric proton-pumping rhodopsins containing the cytoplasmic loop of bovine rhodopsin

G-protein-coupled receptors (GPCRs) transmit stimuli to intracellular signaling systems. Rhodopsin (Rh), which is a prototypical GPCR, possesses an 11-cis retinal. Photoisomerization of 11-cis to all-trans leads to structural changes in the protein of cytoplasmic loops, activating G-protein. Microbial rhodopsins are similar heptahelical membrane proteins that function as bacterial sensors, light-driven ion-pumps, or light-gated channels. They possess an all-trans retinal, and photoisomerization to 13-cis triggers structural changes in protein. Despite these similarities, there is no sequence homology between visual and microbial rhodopsins, and microbial rhodopsins do not activate G-proteins. In this study, new chimeric proton-pumping rhodopsins, proteorhodopsin (PR) and Gloeobacter rhodopsin (GR) were designed by replacing cytoplasmic loops with bovine Rh loops. Although G-protein was not activated by the PR chimeras, all 12 GR chimeras activated G-protein. The GR chimera containing the second cytoplasmic loop of bovine Rh did not activate G-protein. However, the chimera with a second and third double-loop further enhanced G-protein activation. Introduction of an E132Q mutation slowed the photocycle 30-fold and enhanced activation. The highest catalytic activity of the GR chimera was still 3,200 times lower than bovine Rh but only 64 times lower than amphioxus Go-rhodopsin. This GR chimera showed a strong absorption change of the amide-I band on a light-minus-dark difference FTIR spectrum which could represent a larger helical opening, important for G-protein activation. The light-dependent catalytic activity of this GR chimera makes it a potential optogenetic tool for enzymatic activation by light.

Site-directed spin labeling EPR spectroscopy in protein research

Site-directed spin labeling (SDSL) in combination with electron paramagnetic resonance (EPR) spectroscopy has emerged as an efficient tool to elucidate the structure and the conformational dynamics of proteins under conditions close to the native state. This review article summarizes the basics as well as the recent progress in SDSL and EPR methods, especially for investigations on protein structure, protein function, and interaction of proteins with other proteins or nucleic acids. Labeling techniques as well as EPR methods are introduced and exemplified with applications to systems that have been studied in the author’s laboratory in the past 15 years, headmost the sensory rhodopsin-transducer complex mediating the photophobic response of the halophilic archaeum Natronomonas pharaonis. Further examples underline the application of SDSL EPR spectroscopy to answer specific questions about the system under investigation, such as the nature and influence of interactions of proteins with other proteins or nucleic acids. Finally, it is discussed how SDSL EPR can be combined with other biophysical techniques to combine the strengths of the different methodologies.

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.

Structural insight into proteorhodopsin oligomers

Oligomerization has important functional implications for many membrane proteins. However, obtaining structural insight into oligomeric assemblies is challenging, as they are large and resist crystallization. We focus on proteorhodopsin (PR), a protein with seven transmembrane α-helices that was found to assemble to hexamers in densely packed lipid membrane, or detergent-solubilized environments. Yet, the structural organization and the subunit interface of these PR oligomers were unknown. We used site-directed spin-labeling together with electron spin-resonance lineshape and Overhauser dynamic nuclear polarization analysis to construct a model for the specific orientation of PR subunits within the hexameric complex. We found intersubunit distances to average 16 Å between neighboring 55 residues and that residues 177 are >20 Å apart from each other. These distance constraints show that PR has a defined and radial orientation within a hexamer, with the 55-site of the A-B loop facing the hexamer core and the 177-site of the E-F loop facing the hexamer exterior. Dynamic nuclear polarization measurements of the local solvent dynamics complement the electron spin-resonance-based distance analysis, by resolving whether protein surfaces at positions 55, 58, and 177 are exposed to solvent, or covered by protein-protein or protein-detergent contacts.

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.

Molecular and evolutionary aspects of microbial sensory rhodopsins

Retinal proteins (~rhodopsins) are photochemically reactive membrane-embedded proteins, with seven transmembrane α-helices which bind the chromophore retinal (vitamin A aldehyde). They are widely distributed through all three biological kingdoms, eukarya, bacteria and archaea, indicating the biological significance of the retinal proteins. Light absorption by the retinal proteins triggers a photoisomerization of the chromophore, leading to the biological function, light-energy conversion or light-signal transduction. This article reviews molecular and evolutionary aspects of the light-signal transduction by microbial sensory receptors and their related proteins. This article is part of a Special Issue entitled: Retinal Proteins - You can teach an old dog new tricks.

Transmembrane signal transduction in archaeal phototaxis: the sensory rhodopsin II-transducer complex studied by electron paramagnetic resonance spectroscopy

Archaeal photoreceptors, together with their cognate transducer proteins, mediate phototaxis by regulating cell motility through two-component signal transduction pathways. This sensory pathway is closely related to the bacterial chemotactic system, which has been studied in detail during the past 40 years. Structural and functional studies applying site-directed spin labelling and electron paramagnetic resonance spectroscopy on the sensory rhodopsin II/transducer (NpSRII/NpHtrII) complex of Natronomonas pharaonis have yielded insights into the structure, the mechanisms of signal perception, the signal transduction across the membrane and provided information about the subsequent information transfer within the transducer protein towards the components of the intracellular signalling pathway. Here, we provide an overview about the findings of the last decade, which, combined with the wealth of data from research on the Escherichia coli chemotaxis system, served to understand the basic principles microorganisms use to adapt to their environment. We document the time course of a signal being perceived at the membrane, transferred across the membrane and, for the first time, how this signal modulates the dynamic properties of a HAMP domain, a ubiquitous signal transduction module found in various protein classes.

Chimeric microbial rhodopsins containing the third cytoplasmic loop of bovine rhodopsin

G-protein-coupled receptors transmit stimuli (light, taste, hormone, neurotransmitter, etc.) to the intracellular signaling systems, and rhodopsin (Rh) is the most-studied G-protein-coupled receptor. Rh possesses an 11-cis retinal as the chromophore, and 11-cis to all-trans photoisomerization leads to the protein structural changes in the cytoplasmic loops to activate G-protein. Microbial rhodopsins are similar heptahelical membrane proteins that function as bacterial sensors, light-driven ion-pumps, or light-gated channels. Microbial rhodopsins possess an all-trans retinal, and all-trans to 13-cis photoisomerization triggers protein structural changes for each function. Despite these similarities, there is no sequence homology between visual and microbial rhodopsins, and microbial rhodopsins do not activate G-proteins. However, it was reported that bacteriorhodopsin (BR) chimeras containing the third cytoplasmic loop of bovine Rh are able to activate G-protein, suggesting a common mechanism of protein structural changes. Here we design chimeric proteins for Natronomonas pharaonis sensory rhodopsin II (SRII, also called pharaonis phoborhodopsin), which has a two-orders-of-magnitude slower photocycle than BR. Light-dependent transducin activation was observed for most of the nine SRII chimeras containing the third cytoplasmic loop of bovine Rh (from Y223, G224, Q225 to T251, R252, and M253), but the activation level was 30,000-140,000 times lower than that of bovine Rh. The BR chimera, BR/Rh223-253, activates a G-protein transducin, whereas the activation level was 37,000 times lower than that of bovine Rh. We interpret the low activation by the chimeric proteins as reasonable, because bovine Rh must have been optimized for activating a G-protein transducin during its evolution. On the other hand, similar activation level of the SRII and BR chimeras suggests that the lifetime of the M intermediates is not the simple determinant of activation, because SRII chimeras have two-orders-of-magnitude's slower photocycle than the BR chimera. Activation mechanism of visual and microbial rhodopsins is discussed on the basis of these results.

Proton transfer via a transient linear water-molecule chain in a membrane protein

High-resolution protein ground-state structures of proton pumps and channels have revealed internal protein-bound water molecules. Their possible active involvement in protein function has recently come into focus. An illustration of the formation of a protonated protein-bound water cluster that is actively involved in proton transfer was described for the membrane protein bacteriorhodopsin (bR) [Garczarek F, Gerwert K (2006) Nature 439:109-112]. Here we show through a combination of time-resolved FTIR spectroscopy and molecular dynamics simulations that three protein-bound water molecules are rearranged by a protein conformational change that resulted in a transient Grotthuss-type proton-transfer chain extending through a hydrophobic protein region of bR. This transient linear water chain facilitates proton transfer at an intermediate conformation only, thereby directing proton transfer within the protein. The rearrangement of protein-bound water molecules that we describe, from inactive positions in the ground state to an active chain in an intermediate state, appears to be energetically favored relative to transient incorporation of water molecules from the bulk. Our discovery provides insight into proton-transfer mechanisms through hydrophobic core regions of ubiquitous membrane spanning proteins such as G-protein coupled receptors or cytochrome C oxidases.

Structural changes in bacteriorhodopsin during in vitro refolding from a partially denatured state

We report on the formation of the secondary and tertiary structure of bacteriorhodopsin during its in vitro refolding from an SDS-denatured state. We used the mobility of single spin labels in seven samples, attached at various locations to six of the seven helical segments to engineered cysteine residues, to follow coil-to-helix formation. Distance measurements obtained by spin dipolar quenching in six samples labeled at either the cytoplasmic or extracellular ends of pairs of helices revealed the time dependence of the recovery of the transmembrane helical bundle. The secondary structure in the majority of the helical segments refolds with a time constant of <100-140 ms. Recovery of the tertiary structure is achieved by sequential association of the helices and occurs in at least three distinct steps with time constants of 1), well below 1 s; 2), 3-4 s; and 3), 60-130 s (the latter depending on the helical pair). The slowest of these processes occurs in concert with recovery of the retinal chromophore.

Structural snapshots of conformational changes in a seven-helix membrane protein: lessons from bacteriorhodopsin

Recent advances in crystallizing integral membrane proteins have led to atomic models for the structures of several seven-helix membrane proteins, including those in the G-protein-coupled receptor family. Further steps toward exploring structure-function relationships will undoubtedly involve determination of the structural changes that occur during the various stages of receptor activation and deactivation. We expect that these efforts will bear many parallels to the studies of conformational changes in bacteriorhodopsin, which still remains the best-studied seven-helix membrane protein. Here, we provide a brief review of some of the lessons learned, the challenges faced, and the controversies over the last decade with determining conformational changes in bacteriorhodopsin. Our hope is that this analysis will be instructive for similar structural studies, especially of other seven-helix membrane proteins, in the coming decade.

Structural changes in the N and N' states of the bacteriorhodopsin photocycle

The bacteriorhodopsin transport cycle includes protonation of the retinal Schiff base by Asp96 (M-->N reaction) and reprotonation of Asp96 from the cytoplasmic surface (N-->N' reaction). We measured distance changes between pairs of spin-labeled structural elements of interest, and in general observed larger overall structural changes in the N state compared with the N' state. The distance between the C-D loop and E-F interhelical loops in A103R1/M163R1 increased approximately 6 A in the N state and approximately 3 A in the N' state. The opposite trend of distance changes in V101R1/A168R1 and L100R1/T170R1 supports counterclockwise rotation of helix F in the N but not the N' state. Small distance increases were observed in S169R1/S226R1, but little change was seen in G106R1/G155R1. Taking earlier published EPR data into account, we suggest that structural changes of the E-F loop occur first, and then helices F and G begin to move together in the late M state. These motions then reach their maximum amplitude in the N state, evidently to facilitate the release of a proton from Asp96 and the formation of a proton-conduction pathway from Asp96 to the Schiff base. The structural changes reverse their directions and decay in the N' state.

Color-changing mutation in the E-F loop of proteorhodopsin

It is usually assumed that only amino acids located near the retinal chromophore are responsible for color tuning of rhodopsins. However, we recently found that replacement of Ala178 with Arg in the E-F loop of proteorhodopsin (PR), an archaeal-type rhodopsin in marine bacteria, shifts the lambda(max) from 525 to 545 nm at neutral pH [Yoshitsugu, M., Shibata, M., Ikeda, D., Furutani, Y., and Kandori, H. (2008) Angew. Chem., Int. Ed. 47, 3923-3926]. Since the location of Ala178 is distant from the retinal chromophore (approximately 25 A), the molecular mechanism of the unusual mutation effect on color tuning is intriguing. Here we studied this mechanism by using additional mutations and some analytical methods. Introduction of Arg into the corresponding amino acid in bacteriorhodopsin (BR, M163R mutant) does not change the absorption spectra, indicating that the effect is specific to PR. Introduction of Arg into the A-B or C-D loop yields little (3 nm) or no color change, respectively. T177R and P180R mutants exhibited absorption spectra identical to that of the wild type, while N176R and S179R mutants exhibit lambda(max) values of 528 and 535 nm, respectively. Therefore, the observed color change is position-specific, being fully effective at position 178 and half-effective at position 179. Salt affects the absorption spectra of wild-type and A178R PR similarly. FTIR spectroscopy at 77 K indicated similar chromophore structures for wild-type and A178R PR, and A178R PR pumps protons normally. We infer that the E-F loop has a unique structure in PR and the mutation of Ala178 disrupts the structure that includes the transmembrane region, leading to the observed changes in color and pK(a).

Studies of the Bacteriorhodopsin Photocycle without the Use of Light: Clues to Proton Transfer Coupled Reactions

In the photochemical cycle of bacteriorhodopsin, the light-driven proton pump of halobacteria, only the first step, the isomerization of the all-trans retinal to 13-cis, is dependent on illumination. Because the steps that accomplish the translocation of a proton during the ensuing reaction sequence of intermediate states are thermal reactions, they have direct analogies with such steps in other ion pumps. In a surprisingly large number of cases, the reactions of the photocycle could be studied without using light. This review recounts experiments of this kind, and what they contribute to understanding the transport mechanism of this pump, and perhaps indirectly other ion pumps as well.

Analysis of Light-Induced Conformational Changes of Natronomonas pharaonis Sensory Rhodopsin II by Time Resolved Electron Paramagnetic Resonance Spectroscopy†

The nature and kinetics of the conformational changes leading to the activated state of NpSRII/NpHtrII157 were investigated by time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy in combination with site-directed spin labeling (SDSL) on a series of spin labeled mutants of NpSRII. A structural rearrangement of the cytoplasmic moiety of NpSRII upon light activation was detected (helices B, C, F and G). The increase in distance between helices C and F in the M-trapped state of the complex observed in one double mutant is in line with the notion that an outward movement of helix F occurs upon receptor activation. The data obtained from the NpSRII/NpHtrII157 complex reconstituted in purple membrane lipids are compared with those obtained from the X-ray structure of the late M-state of the complex which shows some discrepancies. The results are discussed in the context also of other biophysical and EPR experimental evidences.

Laser-induced transient grating analysis of dynamics of interaction between sensory rhodopsin II D75N and the HtrII transducer

The interaction between sensory rhodopsin II (SRII) and its transducer HtrII was studied by the time-resolved laser-induced transient grating method using the D75N mutant of SRII, which exhibits minimal visible light absorption changes during its photocycle, but mediates normal phototaxis responses. Flash-induced transient absorption spectra of transducer-free D75N and D75N joined to 120 amino-acid residues of the N-terminal part of the SRII transducer protein HtrII (DeltaHtrII) showed only one spectrally distinct K-like intermediate in their photocycles, but the transient grating method resolved four intermediates (K(1)-K(4)) distinct in their volumes. D75N bound to HtrII exhibited one additional slower kinetic species, which persists after complete recovery of the initial state as assessed by absorption changes in the UV-visible region. The kinetics indicate a conformationally changed form of the transducer portion (designated Tr*), which persists after the photoreceptor returns to the unphotolyzed state. The largest conformational change in the DeltaHtrII portion was found to cause a DeltaHtrII-dependent increase in volume rising in 8 micros in the K(4) state and a drastic decrease in the diffusion coefficient (D) of K(4) relatively to those of the unphotolyzed state and Tr*. The magnitude of the decrease in D indicates a large structural change, presumably in the solvent-exposed HAMP domain of DeltaHtrII, where rearrangement of interacting molecules in the solvent would substantially change friction between the protein and the solvent.

Electron paramagnetic resonance study of structural changes in the O photointermediate of bacteriorhodopsin

The structural changes of bacteriorhodopsin during its photochemical cycle, as revealed by crystal structures of trapped intermediates, have provided insights to the proton translocation mechanism. Because accumulation of the last photointermediate, O, appears to be hindered by lattice forces in the crystals, the only information about the structure of this state is from suggested analogies with the determined structures of the non-illuminated D85S mutant and wild-type bacteriorhodopsin at low pH. We used electron paramagnetic resonance spectroscopy of site-directed spin labels at the extracellular protein surface in membranes to test these models. Spin-spin dipolar interactions in the authentic O state compared to the non-illuminated state revealed that the distance between helices C and F increases by ca 4 Angstroms, there is no distance change between helices D and F, and the distance between helix D and helix B of the adjacent monomer increases. Further, the mobility changes of single labels indicate that helices E and F move outward from the proton channel at the center of the protein, and helix D tilts inward. The overall pattern of movements suggests that the model at acid pH is a better representation of the O state than D85S. However, the mobility analysis of spin-labels on the B-C interhelical loop indicates that the antiparallel beta-sheet maintains its ordered secondary structure in O, instead of the predicted disorder in the two structural models. During decay of the O state, the last step of the photocycle, a proton is transferred from Asp85 to proton release complex in the extracellular proton channel. The structural changes in O suggest the need of large conformational changes to drive the Arg82 side-chain back to its initial orientation towards Asp85, and to rearrange the numerous water molecules in this region in order to conduct the proton away from Asp85.

Structural changes in the L photointermediate of bacteriorhodopsin

The L to M reaction of the bacteriorhodopsin photocycle includes the crucial proton transfer from the retinal Schiff base to Asp85. In spite of the importance of the L state in deciding central issues of the transport mechanism in this pump, the serious disagreements among the three published crystallographic structures of L have remained unresolved. Here, we report on the X-ray diffraction structure of the L state, to 1.53-1.73 A resolutions, from replicate data sets collected from six independent crystals. Unlike earlier studies, the partial occupancy refinement uses diffraction intensities from the same crystals before and after the illumination to produce the trapped L state. The high reproducibility of inter-atomic distances, and bond angles and torsions of the retinal, lends credibility to the structural model. The photoisomerized 13-cis retinal in L is twisted at the C(13)=C(14) and C(15)=NZ double-bonds, and the Schiff base does not lose its connection to Wat402 and, therefore, to the proton acceptor Asp85. The protonation of Asp85 by the Schiff base in the L-->M reaction is likely to occur, therefore, via Wat402. It is evident from the structure of the L state that various conformational changes involving hydrogen-bonding residues and bound water molecules begin to propagate from the retinal to the protein at this stage already, and in both extracellular and cytoplasmic directions. Their rationales in the transport can be deduced from the way their amplitudes increase in the intermediates that follow L in the reaction cycle, and from the proton transfer reactions with which they are associated.

A structure-based simulation approach for electron paramagnetic resonance spectra using molecular and stochastic dynamics simulations

Electron paramagnetic resonance (EPR) spectroscopy using site-directed spin-labeling is an appropriate technique to analyze the structure and dynamics of flexible protein regions as well as protein-protein interactions under native conditions. The analysis of a set of protein mutants with consecutive spin-label positions leads to the identification of secondary and tertiary structure elements. In the first place, continuous-wave EPR spectra reflect the motional freedom of the spin-label specifically linked to a desired site within the protein. EPR spectra calculations based on molecular dynamics (MD) and stochastic dynamics simulations facilitate verification or refinement of predicted computer-aided models of local protein conformations. The presented spectra simulation algorithm implies a specialized in vacuo MD simulation at 600 K with additional restrictions to sample the entire accessible space of the bound spin-label without large temporal effort. It is shown that the distribution of spin-label orientations obtained from such MD simulations at 600 K agrees well with the extrapolated motion behavior during a long timescale MD at 300 K with explicit water. The following potential-dependent stochastic dynamics simulation combines the MD data about the site-specific orientation probabilities of the spin-label with a realistic rotational diffusion coefficient yielding a set of trajectories, each more than 700 ns long, essential to calculate the EPR spectrum. Analyses of a structural model of the loop between helices E and F of bacteriorhodopsin are illustrated to demonstrate the applicability and potentials of the reported simulation approach. Furthermore, effects on the motional freedom of bound spin-labels induced by solubilization of bacteriorhodopsin with Triton X-100 are examined.

Can the low-resolution structures of photointermediates of bacteriorhodopsin explain their crystal structures?

To understand the molecular mechanism of light-driven proton pumps, the structures of the photointermediates of bacteriorhodopsin have been intensively investigated. Low-resolution diffraction techniques have demonstrated substantial conformational changes at the helix level in the M and N intermediates, between which there are noticeable differences. The intermediate structures at atomic resolution have also been solved by x-ray crystallography. Although the crystal structures have demonstrated local structural changes, such as hydrogen bond network rearrangements including water molecules, the large conformational changes at the helix level are not necessarily observed. Furthermore, the two reported crystal structures of an intermediate accumulated using a common method were distinct. To reconcile these apparent discrepancies, low-resolution projection maps were calculated from the crystal structures and compared to the low-resolution intermediate structures obtained using native membranes. The crystal structures can be categorized into three groups, which qualitatively correspond to the low-resolution structures of the M1-type, M2-type, and N-type determined in the native membrane. Based on these results, we conclude that at least three types of intermediate structures play a role during the photocycle.

Interresidual distance determination by four-pulse double electron-electron resonance in an integral membrane protein: the Na+/proline transporter PutP of Escherichia coli

Proximity relationships within three doubly spin-labeled variants of the Na+/proline transporter PutP of Escherichia coli were studied by means of four-pulse double electron-electron resonance spectroscopy. The large value of 4.8 nm for the interspin distance determined between positions 107 in loop 4 and 223 in loop 7 strongly supports the idea of these positions being located on opposite sides of the membrane. Significant smaller values of between 1.8 and 2.5 nm were found for the average interspin distances between spin labels attached to the cytoplasmic loops 2 and 4 (position 37 and 107) and loops 2 and 6 (position 37 and 187). The large distance distribution widths visible in the pair correlation functions reveal a high flexibility of the studied loop regions. An increase of the distance between positions 37 and 187 upon Na+ binding suggests ligand-induced structural alterations of PutP. The results demonstrate that four-pulse double electron-electron resonance spectroscopy is a powerful means to investigate the structure and conformational changes of integral membrane proteins reconstituted in proteoliposomes.

The Cytoplasmic Membrane-proximal Domain of the HtrII Transducer Interacts with the E-F Loop of Photoactivated Natronomonas pharaonis Sensory Rhodopsin II

The structures of the cytoplasmic loops of the phototaxis receptor sensory rhodopsin II (SRII) and the membrane-proximal cytoplasmic domain of its bound transducer HtrII were examined in the dark and in the light-activated state by fluorescent probes and cysteine cross-linking. Light decreased the accessibility of E-F loop position 154 in the SRII-HtrII complex, but not in free SRII, consistent with HtrII proximity, which was confirmed by tryptophans placed within a 5-residue region identified in the HtrII membrane-proximal domain that exhibited Forster resonance energy transfer to a fluorescent probe at position 154 in SRII. The Forster resonance energy transfer was eliminated in the signaling deficient HtrII mutant G83F without loss of affinity for SRII. Finally, the presence of SRII and HtrII reciprocally inhibit homodimer disulfide cross-linking reactions in their membrane-proximal domains, showing that each interferes with the others self-interaction in this region. The results demonstrate close proximity between SRII-HtrII in the membrane-proximal domain, and in addition, light stimulation of the SRII inhibition of HtrII cross-linking was observed, indicating that the contact is enhanced in the photoactivated complex. A mechanism is proposed in which photoactivation alters the SRII-HtrII interaction in the membrane-proximal region during the signal relay process.

The archaeal sensory rhodopsin II/transducer complex: a model for transmembrane signal transfer

Archaebacterial photoreceptors mediate phototaxis by regulating cell motility through two-component signalling cascades. Homologs of this sensory pathway occur in all three kingdoms of life, most notably in enteric bacteria in which the chemotaxis has been extensively studied. Recent structural and functional studies on the sensory rhodopsin II/transducer complex mediating the photophobic response of Natronomonas pharaonis have yielded new insights into the mechanisms of signal transfer across the membrane. Electron paramagnetic resonance data and the atomic resolution structure of the receptor molecule in complex with the transmembrane segment of its cognate transducer provided a model for signal transfer from the receptor to the cytoplasmic side of the transducer. This mechanism might also be relevant for eubacterial chemoreceptor signalling.

Bacteriorhodopsin

Fourier transform infrared and Raman spectroscopy, solid-state NMR, and X-ray crystallography have contributed detailed information about the structural changes in the proton transport cycle of the light-driven pump, bacteriorhodopsin. The results over the past few years add up to a step-by-step description of the configurational changes of the photoisomerized retinal, how these changes result in internal proton transfers and the release of a proton to the extracellular surface and uptake on the other side, as well as the conservation and transformation of excess free energy during the cycle.

Crystallographic structures of the M and N intermediates of bacteriorhodopsin: assembly of a hydrogen-bonded chain of water molecules between Asp-96 and the retinal Schiff base

An M intermediate of wild-type bacteriorhodopsin and an N intermediate of the V49A mutant were accumulated in photostationary states at pH 5.6 and 295 K, and their crystal structures determined to 1.52A and 1.62A resolution, respectively. They appear to be M(1) and N' in the sequence, M(1)<-->M(2)<-->M'(2)<-->N<-->N'-->O-->BR, where M(1), M(2), and M'(2) contain an unprotonated retinal Schiff base before and after a reorientation switch and after proton release to the extracellular surface, while N and N' contain a reprotonated Schiff base, before and after reprotonation of Asp96 from the cytoplasmic surface. In M(1), we detect a cluster of three hydrogen-bonded water molecules at Asp96, not present in the BR state. In M(2), whose structure we reported earlier, one of these water molecules intercalates between Asp96 and Thr46. In N', the cluster is transformed into a single-file hydrogen-bonded chain of four water molecules that connects Asp96 to the Schiff base. We find a network of three water molecules near residue 219 in the crystal structure of the non-illuminated F219L mutant, where the residue replacement creates a cavity. This suggests that the hydration of the cytoplasmic region we observe in N' might have occurred spontaneously, beginning at an existing water molecule as nucleus, in the cavities from residue rearrangements in the photocycle.

FTIR spectroscopy of the M photointermediate in pharaonis rhoborhodopsin

pharaonis phoborhodopsin (ppR; also called pharaonis sensory rhodopsin II, psR-II) is a photoreceptor for negative phototaxis in Natronobacterium pharaonis. During the photocycle of ppR, the Schiff base of the retinal chromophore is deprotonated upon formation of the M intermediate (ppR(M)). The present FTIR spectroscopy of ppR(M) revealed that the Schiff base proton is transferred to Asp-75, which corresponds to Asp-85 in a light-driven proton-pump bacteriorhodopsin (BR). In addition, the C==O stretching vibrations of Asn-105 were assigned for ppR and ppR(M). The common hydrogen-bonding alterations in Asn-105 of ppR and Asp-115 of BR were found in the process from photoisomerization (K intermediate) to the primary proton transfer (M intermediate). These results implicate similar protein structural changes between ppR and BR. However, BR(M) decays to BR(N) accompanying a proton transfer from Asp-96 to the Schiff base and largely changed protein structure. In the D96N mutant protein of BR that lacks a proton donor to the Schiff base, the N-like protein structure was observed with the deprotonated Schiff base (called M(N)) at alkaline pH. In ppR, such an N-like (M(N)-like) structure was not observed at alkaline pH, suggesting that the protein structure of the M state activates its transducer protein.

Subdomains in the F and G helices of bacteriorhodopsin regulate the conformational transitions of the reprotonation mechanism

We have performed cysteine scanning mutagenesis of the bacteriorhodopsin mutant D85N to explore the role of individual amino acids in the conformational transitions of the reprotonation mechanism. We have used whole-cell reflectance spectroscopy to evaluate the spectral properties of the 59 mutants generated during a scan of the entire F and G helices and the intervening loop region. Cys mutants were grouped into one of six phenotypes based on the spectral changes associated with their M <--> N <--> O intermediate-state transitions. Mutations that produced similar phenotypes were found to cluster in discrete molecular domains and indicate that M, N, and O possess distinct structures and that unique molecular interactions regulate the transitions between them. The distribution of these domains suggests that 1) the extramembranous loop region is involved in the stabilization of the N and M intermediates, 2) lipid-protein interactions play a key role in the accumulation of N, and 3) the amino acid side-chain interactions in the extracellular portion of the interface between helices G and A participate in the accumulation of M.

Magnetic resonance studies of the bacteriorhodopsin pump cycle

Active transport requires the alternation of substrate uptake and release with a switch in the access of the substrate binding site to the two sides of the membrane. Both the transfer and switch aspects of the photocycle have been subjects of magnetic resonance studies in bacteriorhodopsin. The results for ion transfer indicate that the Schiff base of the chromophore is hydrogen bonded before, during, and after its deprotonation. This suggests that the initial complex counterion of the Schiff base decomposes in such a way that the Schiff base carries its immediate hydrogen-bonding partner with it as it rotates during the first half of the photocycle. If so, bacteriorhodopsin acts as an inward-directed hydroxide pump rather than as an outward-directed proton pump. The studies of the access switch explore both protein-based and chromophore-based mechanisms. Combined with evidence from functional studies of mutants and other forms of spectroscopy, the results suggest that maintaining access to the extracellular side of the protein after photoisomerization involves twisting of the chromophore and that the decisive switch in access to the cytoplasmic side results from relaxation of the chromophore when the constraints on the Schiff base are released by decomposition of the complex counterion.

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.

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.

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.

Coupling of the reisomerization of the retinal, proton uptake, and reprotonation of Asp-96 in the N photointermediate of bacteriorhodopsin

In the N to O reaction of the bacteriorhodopsin photocycle, Asp-96 is protonated from the cytoplasmic surface, and coupled to this, the retinal isomerizes from 13-cis,15-anti back to the initial all-trans configuration. To dissect the two steps, and to better understand how and why they occur, we describe the properties of two groups of site-specific mutants in which the N intermediate has greatly increased lifetime. In the first group, with the mutations near the retinal, an unusual N state is produced in which the retinal is 13-cis,15-anti but Asp-96 has a protonated carboxyl group. The apparent pK(a) for the protonation is 7.5, as in the wild-type. It is likely that here the interference with N decay is the result of steric conflict of side-chains with the retinal or with the side-chain of Lys-216 connected to the retinal, which delays the reisomerization after protonation of Asp-96. In the second group, with the mutations located near Asp-96 or between Asp-96 and the cytoplasmic surface, reprotonation of Asp-96 is strongly perturbed. The reisomerization of the retinal occurs only after recovery from a long-living protein conformation in which reprotonation of Asp-96 is either entirely blocked or blocked at low pH.

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.