Projection structure of the cytochrome bo ubiquinol oxidase from Escherichia coli at 6 A resolution

The CydDC Family of Transporters and Their Roles in Oxidase Assembly and Homeostasis

The CydDC complex of Escherichia coli is a heterodimeric ATP-binding cassette type transporter (ABC transporter) that exports the thiol-containing redox-active molecules cysteine and glutathione. These reductants are thought to aid redox homeostasis of the periplasm, permitting correct disulphide folding of periplasmic and secreted proteins. Loss of CydDC results in the periplasm becoming more oxidising and abolishes the assembly of functional bd-type respiratory oxidases that couple the oxidation of ubiquinol to the reduction of oxygen to water. In addition, CydDC-mediated redox control is important for haem ligation during cytochrome c assembly. Given the diverse roles for CydDC in redox homeostasis, respiratory metabolism and the maturation of virulence factors, this ABC transporter is an intriguing system for researchers interested in both the physiology of redox perturbations and the role of low-molecular-weight thiols during infection.

A eukaryotic-type signalling system of Pseudomonas aeruginosa contributes to oxidative stress resistance, intracellular survival and virulence

BACKGROUND

The genome of Pseudomonas aeruginosa contains at least three genes encoding eukaryotic-type Ser/Thr protein kinases, one of which, ppkA, has been implicated in P. aeruginosa virulence. Together with the adjacent pppA phosphatase gene, they belong to the type VI secretion system (H1-T6SS) locus, which is important for bacterial pathogenesis. To determine the biological function of this protein pair, we prepared a pppA-ppkA double mutant and characterised its phenotype and transcriptomic profiles.

RESULTS

Phenotypic studies revealed that the mutant grew slower than the wild-type strain in minimal media and exhibited reduced secretion of pyoverdine. In addition, the mutant had altered sensitivity to oxidative and hyperosmotic stress conditions. Consequently, mutant cells had an impaired ability to survive in murine macrophages and an attenuated virulence in the plant model of infection. Whole-genome transcriptome analysis revealed that pppA-ppkA deletion affects the expression of oxidative stress-responsive genes, stationary phase σ-factor RpoS-regulated genes, and quorum-sensing regulons. The transcriptome of the pppA-ppkA mutant was also analysed under conditions of oxidative stress and showed an impaired response to the stress, manifested by a weaker induction of stress adaptation genes as well as the genes of the SOS regulon. In addition, expression of either RpoS-regulated genes or quorum-sensing-dependent genes was also affected. Complementation analysis confirmed that the transcription levels of the differentially expressed genes were specifically restored when the pppA and ppkA genes were expressed ectopically.

CONCLUSIONS

Our results suggest that in addition to its crucial role in controlling the activity of P. aeruginosa H1-T6SS at the post-translational level, the PppA-PpkA pair also affects the transcription of stress-responsive genes. Based on these data, it is likely that the reduced virulence of the mutant strain results from an impaired ability to survive in the host due to the limited response to stress conditions.

Inhibition of bacterial oxidases by formamide and analogs

The enzymatic activity of Paracoccus denitrificans cytochrome c oxidase (COX) and Escherichia coli cytochrome b(o) ubiquinol oxidase (QOX) was determined in the presence of formamide, N,N-dimethyl formamide and N,N-dimethyl acetamide. Formamide was found to inhibit the enzyme activity of the oxidases most significantly, whereas the other two compounds inhibited the activity to a lesser extent. The effects of formamide and analogs on enzyme activity were very similar for COX and QOX, indicating that the mechanism of inhibition might be the same for both of these oxidases. The inhibition kinetics followed a non-competitive mechanism. Studies using proteoliposomes of COX and QOX containing the electron entry site of the enzyme directed towards the outside of the vesicles showed that the effect of inhibition by formamide was higher when the inhibitor was present on the outside of the proteoliposome compared to when it was present only in the aqueous core. This indicates that inhibition of enzyme activity by formamide possibly predominantly involves blocking of the water exit pathway in the oxidases.

Functional genomics of stress response in Pseudomonas putida KT2440

The metabolically versatile soil bacterium Pseudomonas putida has to cope with numerous abiotic stresses in its habitats. The stress responses of P. putida KT2440 to 4 degrees C, pH 4.5, 0.8 M urea, and 45 mM sodium benzoate were analyzed by determining the global mRNA expression profiles and screening for stress-intolerant nonauxotrophic Tn5 transposon mutants. In 392 regulated genes or operons, 36 gene regions were differentially expressed by more than 2.5-fold, and 32 genes in 23 operons were found to be indispensable for growth during exposure to one of the abiotic stresses. The transcriptomes of the responses to urea, benzoate, and 4 degrees C correlated positively with each other but negatively with the transcriptome of the mineral acid response. The CbrAB sensor kinase, the cysteine synthase CysM, PcnB and VacB, which control mRNA stability, and BipA, which exerts transcript-specific translational control, were essential to cope with cold stress. The cyo operon was required to cope with acid stress. A functional PhoP, PtsP, RelA/SpoT modulon, and adhesion protein LapA were necessary for growth in the presence of urea, and the outer membrane proteins OmlA and FepA and the phosphate transporter PstBACS were indispensable for growth in the presence of benzoate. A lipid A acyltransferase (PP0063) was a mandatory component of the stress responses to cold, mineral acid, and benzoate. Adaptation of the membrane barrier, uptake of phosphate, maintenance of the intracellular pH and redox status, and translational control of metabolism are key mechanisms of the response of P. putida to abiotic stresses.

Two Different Conformational States of the KirBac3.1 Potassium Channel Revealed by Electron Crystallography

Potassium channels allow the selective flow of K(+) ions across membranes. In response to external gating signals, the potassium channel can move reversibly through a series of structural conformations from a closed to an open state. 2D crystals of the inwardly rectifying K(+) channel KirBac3.1 from Magnetospirillum magnetotacticum have been captured in two distinct conformations, providing "snap shots" of the gating process. Analysis by electron cryomicroscopy of these KirBac3.1 crystals has resulted in reconstructed images in projection at 9 A resolution. Kir channels are tetramers of four subunits arranged as dimers of dimers. Each subunit has two transmembrane helices (inner and outer). In one crystal form, the pore is blocked; in the other crystal form, the pore appears open. Modeling based on the KirBac1.1 (closed) crystal structure shows that opening of the ion conduction pathway could be achieved by bending of the inner helices and significant movements of the outer helices.

Genome-wide transcriptional profiling of the steady-state response of Pseudomonas aeruginosa to hydrogen peroxide

The steady-state response of Pseudomonas aeruginosa to the oxidative-stress-generating agent hydrogen peroxide was analyzed by PAO1 transcriptome profiling. In total, 694, 411, and 237 genes were upregulated and 668, 576, and 468 genes were downregulated in P. aeruginosa strains TB, 892, and PAO1, respectively. The expression profiles of the two variants of the TB clone were significantly more related to each other than the expression profile of either strain was to that of PAO1. Exposure to H(2)O(2) activated by more than 10-fold the expression of the cyoABCD operon, which is key for aerobic respiration, and of oxidative-stress response elements such as the catalase KatB, the alkyl hydroperoxide reductase AhpF, and the thioredoxin reductase 2 operon. Genes for iron and sulfur homeostasis were upregulated. Most enzymes necessary for the conversion of amino acids into the citric acid cycle were globally downregulated at the transcriptional level. Nitrate respiration and arginine fermentation were shut off in the clone TB strains and attenuated in the PAO strain. The transcriptional profiles indicate that the two clone TB strains are more proficient in coping with H(2)O(2)-mediated oxidative stress than the reference strain PAO. According to this data, we recommend study of the transcriptome of strain PAO1 in parallel with those of at least two strains of another clone in order to differentiate common responses from clone- and strain-specific responses and to minimize overinterpretations of microarray data.

Two-dimensional crystallogenesis of transmembrane proteins

Two-dimensional crystallogenesis is a crucial step in the long road that leads to the determination of macromolecules structure via electron crystallography. The necessity of having large and highly ordered samples can hold back the resolution of structural works for a long time, and this, despite improvements made in electron microscopes or image processing. Today, finding good conditions for growing two-dimensional crystals still rely on either "biocrystallo-cooks" or on lucky ones. The present review presents the field by first describing the different crystals that one can encounter and the different crystallisation methods used. Then, the effects of different components (such as protein, lipids, detergent, buffer, and temperature) and the different methods (dialysis, hydrophobic adsorption) are discussed. This discussion is punctuated by correspondences made to the world of three-dimensional crystallogenesis. Finally, a guide for setting up 2D crystallogenesis experiments, built on the discussion mentioned before, is proposed to the reader. More than giving recipes, this review is meant to open up the discussions in this field.

Use of detergents in two-dimensional crystallization of membrane proteins

Structure determination at high resolution is actually a difficult challenge for membrane proteins and the number of membrane proteins that have been crystallized is still small and far behind that of soluble proteins. Because of their amphiphilic character, membrane proteins need to be isolated, purified and crystallized in detergent solutions. This makes it difficult to grow the well-ordered three-dimensional crystals that are required for high resolution structure analysis by X-ray crystallography. In this difficult context, growing crystals confined to two dimensions (2D crystals) and their structural analysis by electron crystallography has opened a new way to solve the structure of membrane proteins. However, 2D crystallization is one of the major bottlenecks in the structural studies of membrane proteins. Advances in our understanding of the interaction between proteins, lipids and detergents as well as development and improvement of new strategies will facilitate the success rate of 2D crystallization. This review deals with the various available strategies for obtaining 2D crystals from detergent-solubilized intrinsic membrane proteins. It gives an overview of the methods that have been applied and gives details and suggestions of the physical processes leading to the formation of the ordered arrays which may be of help for getting more proteins crystallized in a form suitable for high resolution structural analysis by electron crystallography.

ENDOR spectroscopic studies of stable semiquinone radicals bound to the Escherichia coli cytochrome bo3 quinol oxidase

The putative oxidation of ubiquinol by the cytochrome bo3 terminal oxidase of Escherichia coli in sequential one-electron steps requires stabilization of the semiquinone. ENDOR spectroscopy has recently been used to study the native ubisemiquinone radical formed in the cytochrome bo3 quinone-binding site [Veselov, A.V., Osborne, J.P., Gennis, R.B. & Scholes, C.P. (2000) Biochemistry 39, 3169-3175]. Comparison of these spectra with those from the decyl-ubisemiquinone radical in vitro indicated that the protein induced large changes in the electronic structure of the ubisemiquinone radical. We have used quinone-substitution experiments to obtain ENDOR spectra of ubisemiquinone, phyllosemiquinone and plastosemiquinone anion radicals bound at the cytochrome bo3 quinone-binding site. Large changes in the electronic structures of these semiquinone anion radicals are induced on binding to the cytochrome bo3 oxidase. The changes in electronic structure are, however, independent of the electronic structures of these semiquinones in vitro. Thus it is shown to be the structure of this binding site in the protein, not the covalent structure of the bound quinone, that determines the electronic structure of the protein-bound semiquinone.

Tracing the D-pathway in reconstituted site-directed mutants of cytochrome c oxidase from Paracoccus denitrificans

Heme-copper terminal oxidases use the free energy of oxygen reduction to establish a transmembrane proton gradient. While the molecular mechanism of coupling electron transfer to proton pumping is still under debate, recent structure determinations and mutagenesis studies have provided evidence for two pathways for protons within subunit I of this class of enzymes. Here, we probe the D-pathway by mutagenesis of the cytochrome c oxidase of the bacterium Paracoccus denitrificans; amino acid replacements were selected with the rationale of interfering with the hydrophilic lining of the pathway, in particular its assumed chain of water molecules. Proton pumping was assayed in the reconstituted vesicle system by a stopped-flow spectroscopic approach, allowing a reliable assessment of proton translocation efficiency even at low turnover rates. Several mutations at positions above the cytoplasmic pathway entrance (Asn 131, Asn 199) and at the periplasmic exit region (Asp 399) led to complete inhibition of proton pumping; one of these mutants, N131D, exhibited an ideal decoupled phenotype, with a turnover comparable to that of the wild-type enzyme. Since sets of mutations in other positions along the presumed course of the pathway showed normal proton translocation stoichiometries, we conclude that the D-pathway is too wide in most areas above positions 131/199 to be disturbed by single amino acid replacements.

Multiple posttranslational modifications at distinct sites contribute to heterogeneity of the lipoprotein cytochrome bo(3)

The heme-copper cytochrome oxidase of Escherichia coli (cytochrome bo(3)) was tagged with oligohistidine at the C-terminus of the small noncatalytic subunit IV. After detergent solubilization, the enzyme was purified by a one-step procedure with immobilized metal affinity chromatography. Using different cytochrome bo(3) constructs as reference, the products were investigated by mass spectroscopical and immunological methods. Several posttranslational modifications of subunits II, III, and IV were observed: (1) N-terminal methionines of subunits III and IV are split off. (2) Fifty percent of subunit III polypeptides are acetylated, presumably at the N-terminal alanine. (3) Lipoprotein processing of subunit II involves cleavage of the signal peptide. (4) Maturation of subunit II [Ma, J., Katsonouri, A., and Gennis, R. B. (1997) Biochemistry 36, 11298-11303] alters the structure of the N-terminal cysteine by N-palmitoylation and S-glyceryldipalmitoylation. (5) A hexapeptide is split off from the C-terminus of subunit II. This happens subsequently to the N-terminal lipoprotein processing step and is dependent on the growth state of cells.

Conformational changes in the cytochrome b6f complex induced by inhibitor binding

Binding of stigmatellin, an inhibitor of the Q(o) site of the bc-type complexes, has been shown to induce large conformational changes of the Rieske protein in the respiratory bc(1) complex (Kim, H., Xia, D., Yu, C. A., Xia, J. Z., Kachurin, A. M., Zhang, L., Yu, L., and Deisenhofer, J. (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 8026-8033; Iwata, S., Lee, J. W., Okada, K., Lee, J. K., Iwata, M., Rasmussen, B., Link, T. A., Ramaswamy, S., and Jap, B. K. (1998) Science 281, 64-71; Zhang, Z., Huang, L., Shulmeister, V. M., Chi, Y. I., Kim, K. K., Hung, L. W., Crofts, A. R., Berry, E. A., and Kim, S. H. (1998) Nature 392, 677-684). Such a movement seems necessary to shuttle electrons from the membrane-soluble quinol to the extramembrane heme of cytochrome c(1). To see whether similar changes occur in the related photosynthetic b(6)f complex, we have studied the effect of the binding of stigmatellin to the eukaryotic b(6)f complex by electron crystallography. Comparison of projection maps of thin three-dimensional crystals prepared with or without stigmatellin, and either negatively stained or embedded in glucose, reveals a similar type of movement to that observed in the bc(1) complex and suggests also the occurrence of conformational changes in the transmembrane region.

Detergent-solubilized Escherichia coli cytochrome bo3 ubiquinol oxidase: a monomeric, not a dimeric complex

The protein molecular weight, M(r), and hydrodynamic radius, R(s), of Triton X-100-solubilized Escherichia coli cytochrome bo3 were evaluated by computer fitting of sedimentation velocity data with finite element solutions to the Lamm equation. Detergent-solubilized cytochrome bo3 sediments as a homogeneous species with an S20,w of 6.75 s and a D20,w of 3.71 x 10(-7) cm2/s, corresponding to a R(s) of 5.8 nm and a M(r) of 144,000 +/- 3500. The protein molecular weight agrees very well with the value of 143,929 calculated from the four known subunit sequences and the value of 143,025 measured by MALDI mass spectrometry for the histidine-tagged enzyme. We conclude that detergent-solubilized E. coli ubiquinol oxidase is a monomeric complex of the four known subunits.

Coordination of CuB in reduced and CO-liganded states of cytochrome bo3 from Escherichia coli. Is chloride ion a cofactor?

The ubiquinol oxidase cytochrome bo3 from Escherichia coli is one of the respiratory heme-copper oxidases which catalyze the reduction of O2 to water linked to translocation of protons across the bacterial or mitochondrial membrane. We have studied the structure of the CuB site in the binuclear heme-copper center of O2 reduction by EXAFS spectroscopy in the fully reduced state of this enzyme, as well as in the reduced CO-liganded states where CO is bound either to the heme iron or to CuB. We find that, in the reduced enzyme, CuB is coordinated by one weakly bound and two strongly bound histidine imidazoles at Cu-N distances of 2.10 and 1.92 A, respectively, and that an additional feature at 2.54 A is due to a highly ordered water molecule that might be weakly associated with the copper. Unexpectedly, the binding of CO to heme iron is found to result in a major conformational change at CuB, which now binds only two equidistant histidine imidazoles at 1.95 A and a chloride ion at 2. 25 A, with elimination of the water molecule and one of the histidines. Attempts to remove the chloride from the enzyme by extensive dialysis did not change this finding, nor did substitution of chloride with bromide. Photolysis of CO bound to the heme iron is known to cause the CO to bind to CuB in a very fast reaction and to remain bound to CuB at low temperatures. In this state, we indeed find the CO to be bound to CuB at a Cu-C distance of 1.85 A, with chloride still bound at 2.25 A and the two histidine imidazoles at a Cu-N distance of 2.01 A. These results suggest that reduction of the binuclear site weakens the bond between CuB and one of its three histidine imidazole ligands, and that binding of CO to the reduced binuclear site causes a major structural change in CuB in which one histidine ligand is lost and replaced by a chloride ion. Whether chloride is a cofactor in this enzyme is discussed.

Molecular genetics of the genus Paracoccus: metabolically versatile bacteria with bioenergetic flexibility

Paracoccus denitrificans and its near relative Paracoccus versutus (formerly known as Thiobacilllus versutus) have been attracting increasing attention because the aerobic respiratory system of P. denitrificans has long been regarded as a model for that of the mitochondrion, with which there are many components (e.g., cytochrome aa3 oxidase) in common. Members of the genus exhibit a great range of metabolic flexibility, particularly with respect to processes involving respiration. Prominent examples of flexibility are the use in denitrification of nitrate, nitrite, nitrous oxide, and nitric oxide as alternative electron acceptors to oxygen and the ability to use C1 compounds (e.g., methanol and methylamine) as electron donors to the respiratory chains. The proteins required for these respiratory processes are not constitutive, and the underlying complex regulatory systems that regulate their expression are beginning to be unraveled. There has been uncertainty about whether transcription in a member of the alpha-3 Proteobacteria such as P. denitrificans involves a conventional sigma70-type RNA polymerase, especially since canonical -35 and -10 DNA binding sites have not been readily identified. In this review, we argue that many genes, in particular those encoding constitutive proteins, may be under the control of a sigma70 RNA polymerase very closely related to that of Rhodobacter capsulatus. While the main focus is on the structure and regulation of genes coding for products involved in respiratory processes in Paracoccus, the current state of knowledge of the components of such respiratory pathways, and their biogenesis, is also reviewed.

The active site of the bacterial nitric oxide reductase is a dinuclear iron center

A novel, improved method for purification of nitric oxide reductase (NOR) from membranes of Paracoccus denitrificans has been developed. The purified enzyme is a cytochrome bc complex which, according to protein chemical and hydrodynamic data, contains two subunits in a 1:1 stoichiometry. The purified NorBC complex binds 0.87 g of dodecyl maltoside/g of protein and forms a dimer in solution. Similarly, it is dimeric in two-dimensional crystals. Images of these crystals have been processed at 8 A resolution in projection to the membrane. The NorB subunit is homologous to the main catalytic subunit of cytochrome oxidase and is predicted to contain the active bimetallic center in which two NO molecules are turned over to N2O. Metal analysis and heme composition implies that it binds two B-type hemes and a nonheme iron but no copper. NorC is a membrane-anchored cytochrome c. Fourier transform infrared spectroscopy shows that carbon monoxide dissociates from the reduced heme in light and associates with another metal center which is distinct from the copper site of heme/copper oxidases. Electron paramagnetic resonance spectroscopy reveals that NO binds to the reduced enzyme under turnover conditions giving rise to signals near g = 2 and g = 4. The former represents a typical nitrosyl-ferroheme signal whereas the latter is a fingerprint of a nonheme iron/NO adduct. We conclude that the active site of NOR is a dinuclear iron center.

Tryptophan-136 in subunit II of cytochrome bo3 from Escherichia coli may participate in the binding of ubiquinol

In the cytochrome c oxidases, the role of subunit II is to provide the electron entry site into the enzyme. This subunit contains both the binding site for the substrate, cytochrome c, and the CuA redox center, which is initially reduced by cytochrome c. Cytochrome bo3 and other quinol oxidases that are members of the heme-copper oxidase superfamily have a homologous subunit II, but the CuA site is absent, as is the docking site for cytochrome c. Speculation that subunit II in the quinol oxidases may also be important as an electron entry site is supported by the demonstration several years ago that a photoreactive substrate analogue, azido-Q, covalently labeled subunit II in cytochrome bo3. In the current work, a sequence alignment of subunit II of heme-copper quinol oxidases is used as a guide to select conserved residues that might be important for the binding of ubiquinol to cytochrome bo3. Results are presented for point mutants in 24 different residue positions in subunit II. The membrane-bound enzymes were examined by optical spectroscopy and by determining the activity of ubiquinol-1 oxidase. In each case, the Km for ubiquinol-1 was determined as a measure of possible perturbation to a quinol binding site. The only mutant that had a noticeably altered Km for ubiquinol-1 was W136A, in which the Km was about sixfold increased. Thus, W136 may be at or close to a substrate (ubiquinol)-binding site in cytochrome bo3. In the cytochrome c oxidases, the equivalent tryptophan (W121 in Paracoccus denitrificans) has been identified as the "electron entry site".

Proton translocation by bacteriorhodopsin and heme-copper oxidases

Proton translocation is the means by which free energy is conserved from oxygen reduction by the respiratory heme-copper oxidases and from sunlight by bacteriorhodopsin. Three-dimensional structures at atomic resolution of both proteins have aided functional studies of the proton translocation mechanism. A comparison reveals common structural and functional features that may be unique to the primary proton pumps in biology.

Using matrix-assisted laser desorption ionization mass spectrometry to map the quinol binding site of cytochrome bo3 from Escherichia coli

The cytochrome bo3 ubiquinol oxidase contains at least one and possibly two binding sites for ubiquinol/ubiquinone. Previous studies used the photoreactive affinity label 3-[3H]azido-2-methyl-5-methoxy-6-geranyl-1,4-benzoquinone (azido-Q), a substrate analogue, to demonstrate that subunit II contributes to at least one of the quinol binding sites. In the current work, mass spectroscopy is used to identify a peptide within subunit II that is photolabeled by the azido-Q. Purified cytochrome bo3 was photolabeled as previously described using azido-Q that was not tritiated (i.e., not radiolabeled). Subunit II was then isolated from an SDS-PAGE gel and proteolyzed in situ with trypsin. The resulting peptides were eluted from the gel and then identified using matrix-assisted laser desorption ionization mass spectrometry. The resulting mass spectrum was compared to that obtained by analysis of subunit II that had not been exposed to the photolabel. Using the amino acid sequence, each peak in the mass spectrum of the unlabeled subunit II could be assigned to an expected trypsin fragment. Two additional peaks were observed in the mass spectrum of the photolabeled subunit with m/z 1931.9 and 2287.7. Subtraction of the mass of azido-Q from the peak at m/z 1931.9 results in a mass equivalent to that of a peptide consisting of amino acids 165-178. The assignment of the peak at m/z 2287.7 cannot be made unequivocally and may correspond either to the covalent attachment of azido-Q to peptide 254-270 or to a peptide resulting from incomplete proteolysis. The labeled peptide, 165-178, is within the water-soluble domain of subunit II, whose X-ray structure is known. This peptide is located near the site where CuA is located in the homologous cytochrome c oxidases and can be placed near the interface between subunits I and II.

Cytochrome c oxidase (heme aa3) from Paracoccus denitrificans: analysis of mutations in putative proton channels of subunit I

One of the challenging features of energy-transducing terminal oxidases, like the aa3 cytochrome c oxidase of Paracoccus denitrificans, is the translocation of protons across the cytoplasmic membrane, which is coupled to the transfer of electrons to oxygen. As a prerequisite for a more advanced examination of the enzymatic properties, several amino acid residues, selected on the basis of recent three-dimensional structure determinations, were exchanged in subunit I of the Paracoccus enzyme by site-directed mutagenesis. The properties of the mutated oxidases were analyzed by different methods to elucidate whether they are involved in the coupled and coordinated transfer of protons via two different pathways either to the site of oxygen reduction or through the enzyme from the cytoplasm to the periplasmic side.

Electron and atomic force microscopy of membrane proteins

Electron crystallography is becoming a powerful tool for the resolution of membrane protein structures. The past year has seen the production of a bacteriorhodopsin model at 3.5 A and the structure of aquaporin 1 approaching atomic resolution. Determination of surface topographies of 2D crystals using the atomic force microscope is similarly advancing to a level that reveals submolecular details. As the latter is operated in solution, membrane proteins can be observed at work.