pH-Induced Conformational Change of the β-Barrel-Forming Protein OmpG Reconstituted into Native E. coli Lipids

Protein engineering of pores for separation, sensing, and sequencing

Proteins are critical to cellular function and survival. They are complex molecules with precise structures and chemistries, which allow them to serve diverse functions for maintaining overall cell homeostasis. Since the discovery of the first enzyme in 1833, a gamut of advanced experimental and computational tools has been developed and deployed for understanding protein structure and function. Recent studies have demonstrated the ability to redesign/alter natural proteins for applications in industrial processes of interest and to make customized, novel synthetic proteins in the laboratory through protein engineering. We comprehensively review the successes in engineering pore-forming proteins and correlate the amino acid-level biochemistry of different pore modification strategies to the intended applications limited to nucleotide/peptide sequencing, single-molecule sensing, and precise molecular separations.

Investigations of pH-dependent dynamic properties of OmpG-16SL, an outer membrane protein G mutant by ATR-FTIR spectroscopy

In this paper, the dynamic properties of outer membrane protein G mutant (OmpG-16SL) are investigated with ATR-FTIR spectroscopy. While OmpG-WT has 14 β-strands in its structure, the mutant is designed to have 16 β-strands with the intention of creating an enlarged pore. Loop L6 is elongated by introducing six residues, two of which are negatively charged. The solvent accessibility of the OmpG-16SL mutant is compared with WT and a previously reported mutant OmpG-16S by tracking the ¹H/²H exchange kinetics in acidic and neutral buffer conditions. The exchange kinetics and dynamics in the fast and slow exchange phases are separately investigated using the 2DCOS technique, which enables the tracking of the structural changes at each phase of the exchange process. The results suggest that the mutant OmpG-16SL is equally exposed to buffer in both acidic and neutral pH conditions. Additionally, the time range in the fast phase is very short - one-tenth of that for WT - and most of the exchange is completed in this phase. This fast exchange within minutes is also indicative of the presence of highly flexible and/or unstructured regions. In all, the fast exchange rates independent of the buffer pH justify the assumption that there is an altered interaction among the charged residues, which leads to a steadily-open pore. The role of the side-chain interactions within the pore and between the loops involving the loop L6 is also discussed.

Correlation of membrane protein conformational and functional dynamics

Conformational changes in ion channels lead to gating of an ion-conductive pore. Ion flux has been measured with high temporal resolution by single-channel electrophysiology for decades. However, correlation between functional and conformational dynamics remained difficult, lacking experimental techniques to monitor sub-millisecond conformational changes. Here, we use the outer membrane protein G (OmpG) as a model system where loop-6 opens and closes the β-barrel pore like a lid in a pH-dependent manner. Functionally, single-channel electrophysiology shows that while closed states are favored at acidic pH and open states are favored at physiological pH, both states coexist and rapidly interchange in all conditions. Using HS-AFM height spectroscopy (HS-AFM-HS), we monitor sub-millisecond loop-6 conformational dynamics, and compare them to the functional dynamics from single-channel recordings, while MD simulations provide atomistic details and energy landscapes of the pH-dependent loop-6 fluctuations. HS-AFM-HS offers new opportunities to analyze conformational dynamics at timescales of domain and loop fluctuations.

A pH-independent quiet OmpG pore with enhanced electrostatic repulsion among the extracellular loops

Membrane protein pores have emerged as powerful nanopore sensors for single-molecule detection. OmpG, a monomeric nanopore, is comprised of fourteen β-strands connected by seven flexible extracellular loops. The OmpG nanopore exhibits pH-dependent gating as revealed by planar lipid bilayer studies. Current evidence strongly suggests that the dynamic movement of loop 6 is responsible for the gating mechanism. In this work, we have shown that enhancing the electrostatic repulsion forces between extracellular loops suppressed the pH-dependent gating. Our mutant containing additional negative charges in loop 6 and loop 1 exhibited minimal spontaneous gating and reduced sensitivity to pH changes compared to the wild type OmpG. These results provide new evidence to support the mechanism of OmpG gating controlled by the complex electrostatic network around the gating loop 6. The pH-independent quiet OmpG pores could potentially be used as a sensing platform that operates at a broad range of pH conditions.

The role of bacterial cell envelope structures in acid stress resistance in E. coli

Acid resistance (AR) is an indispensable mechanism for the survival of neutralophilic bacteria, such as Escherichia coli (E. coli) strains that survive in the gastrointestinal tract. E. coli acid tolerance has been extensively studied during past decades, with most studies focused on gene regulation and mechanisms. However, the role of cell membrane structure in the context of acid stress resistance has not been discussed in depth. Here, we provide a comprehensive review of the roles and mechanisms of the E. coli cell envelope from different membrane components, such as membrane proteins, fatty acids, chaperones, and proton-consuming systems, and particularly focus on the innovative effects revealed by recent studies. We hope that the information guides us to understand the bacterial survival strategies under acid stress and to further explore the AR regulatory mechanisms to prevent or treat E. coli and other related Gram-negative bacteria infection, or to enhance the AR of engineering E. coli.

High-Resolution Imaging of Maltoporin LamB while Quantifying the Free-Energy Landscape and Asymmetry of Sugar Binding

Maltoporins are a family of membrane proteins that facilitate the diffusion of hydrophilic molecules and maltosaccharides across the outer membrane of Gram-negative bacteria. Two contradicting models propose the sugar binding, uptake, and transport by maltoporins to be either symmetric or asymmetric. Here, we address this contradiction and introduce force-distance-based atomic force microscopy to image single maltoporin LamB trimers in the membrane at sub-nanometer resolution and simultaneously quantify the binding of different malto-oligosaccharides. We assay subtle differences of the binding free-energy landscape of maltotriose, maltotetraose, and maltopentaose, which quantifies how binding strength and affinity increase with the malto-oligosaccharide chain length. The ligand-binding parameters change considerably by mutating the extracellular loop 3, which folds into and constricts the transmembrane pore of LamB. By recording LamB topographs and structurally mapping binding events at sub-nanometer resolution, we observe LamB to preferentially bind maltodextrin from the periplasmic side, which shows sugar binding and uptake to be asymmetric. The study introduces atomic force microscopy as an analytical nanoscopic tool that can differentiate among the factors modulating and models describing the binding and uptake of substrates by membrane proteins.

Manipulation of charge distribution in the arginine and glutamate clusters of the OmpG pore alters sugar specificity and ion selectivity

OmpG is a general diffusion pore in the E. coli outer membrane with a molecular architecture comprising a 14-stranded β-barrel scaffold and unique structural features. In contrast to other non-specific porins, OmpG lacks a central constriction zone and has an exceptionally wide pore diameter of about 13 Å. The equatorial plane of OmpG harbors an annulus of four alternating basic and acidic patches whose function is only poorly characterized. We have investigated the role of charge distribution for ion selectivity and sugar transport with the help of OmpG variants mutated in the annulus. Substituting the glutamate residues of the annulus for histidines or alanines led to a strong reduction in cation selectivity. Replacement of the glutamates in the annulus by histidine residues also disfavored the passage of pentoses and hexoses relative to disaccharides. Our results demonstrate that despite the wide pore diameter, an annulus only consisting of two opposing basic patches confers reduced cation and monosaccharide transport compared to OmpG wild type. Furthermore, randomization of charged residues in the annulus had the potential to abolish pH-dependency of sugar transport. Our results indicate that E¹⁵, E³¹, R⁹², R¹¹¹ and R²¹¹ in the annulus form electrostatic interactions with R²²⁸, E²²⁹ and D²³² in loop L6 that influence pH-dependency of sugar transport.

Structural properties of an engineered outer membrane protein G mutant, OmpG-16SL, investigated with infrared spectroscopy

The structural and functional differences between wild type (WT) outer membrane protein G and its two mutants are investigated with Fourier transform infrared spectroscopy. Both mutants have a long extension to the primary sequence to increase the number of β-strands from 14 (wild type) to 16 in an attempt to enlarge the pore diameter. The comparison among proteins is made in terms of pH-dependent conformational changes and thermal stability. Results show that all proteins respond to pH change but at different degrees. At acidic environment, all proteins contain the same number of residues participated in β-sheet structure. However, at neutral pH, the mutants have less ordered structure compared to WT porin. Thermal stability tests show that mutants differ significantly from WT porin at neutral pH. Although the transition temperature is directly proportional with the amount of β-sheet content, the changes in the pre-transition phase that pave the way to structural breakdown are shown to involve interactions among charged residues by two-dimensional correlation spectroscopy analysis. Results of the analysis show that side chain interactions play an active role under increasing temperature. Both mutants have more unordered secondary structure but they respond to pH change in tertiary structure level. Findings of this study provided deeper insight on the active players in structural stability of the WT porin.Communicated by Ramaswamy H. Sarma [Formula: see text].

Modified cantilever arrays improve sensitivity and reproducibility of nanomechanical sensing in living cells

Mechanical signaling involved in molecular interactions lies at the heart of materials science and biological systems, but the mechanisms involved are poorly understood. Here we use nanomechanical sensors and intact human cells to provide unique insights into the signaling pathways of connectivity networks, which deliver the ability to probe cells to produce biologically relevant, quantifiable and reproducible signals. We quantify the mechanical signals from malignant cancer cells, with 10 cells per ml in 1000-fold excess of non-neoplastic human epithelial cells. Moreover, we demonstrate that a direct link between cells and molecules creates a continuous connectivity which acts like a percolating network to propagate mechanical forces over both short and long length-scales. The findings provide mechanistic insights into how cancer cells interact with one another and with their microenvironments, enabling them to invade the surrounding tissues. Further, with this system it is possible to understand how cancer clusters are able to co-ordinate their migration through narrow blood capillaries.

Samadhan Patil et al. report a new method for improving the sensitivity and reproducibility of mechanobiological measurements in malignant cancer cells. Their findings provide insight into the interaction of cells with each other and the microenvironment and may impact our understanding of metastasis.

Properties of Omp2a-Based Supported Lipid Bilayers: Comparison with Polymeric Bioinspired Membranes

Omp2a β-barrel outer membrane protein has been reconstituted into supported lipid bilayers (SLBs) to compare the nanomechanical properties (elastic modulus, adhesion forces, and deformation) and functionality of the resulting bioinspired system with those of Omp2a-based polymeric nanomembranes (NMs). Protein reconstitution into lipid bilayers has been performed using different strategies, the most successful one consisting of a detergent-mediated process into preformed liposomes. The elastic modulus obtained for the lipid bilayer and Omp2a are ∼19 and 10.5 ± 1.7 MPa, respectively. Accordingly, the protein is softer than the lipid bilayer, whereas the latter exhibits less mechanical strength than polymeric NMs. Besides, the function of Omp2a in the SLB is similar to that observed for Omp2a-based polymeric NMs. Results open the door to hybrid bioinspired substrates based on the integration of Omp2a-proteoliposomes and nanoperforated polymeric freestanding NMs.

Reversible Cation-Selective Attachment and Self-Assembly of Human Tau on Supported Brain Lipid Membranes

Misfolding and aggregation of the neuronal, microtubule-associated protein tau is involved in the pathogenesis of Alzheimer's disease and tauopathies. It has been proposed that neuronal membranes could play a role in tau release, internalization, and aggregation and that tau aggregates could exert toxicity via membrane permeabilization. Whether and how tau interacts with lipid membranes remains a matter of discussion. Here, we characterize the interaction of full-length human tau (htau40) with supported lipid membranes (SLMs) made from brain total lipid extract by time-lapse high-resolution atomic force microscopy (AFM). We observe that tau attaches to brain lipid membranes where it self-assembles in a cation-dependent manner. Sodium triggers the attachment, self-assembly, and growth, whereas potassium inhibits these processes. Moreover, tau assemblies are stable in the presence of sodium and lithium but disassemble in the presence of potassium and rubidium. Whereas the pseudorepeat domains (R1-R4) of htau40 promote the sodium-dependent attachment to the membrane and stabilize the tau assemblies, the N-terminal region promotes tau self-assembly and growth.

Mechanism of OmpG pH-Dependent Gating from Loop Ensemble and Single Channel Studies

Outer membrane protein G (OmpG) from Escherichia coli has exhibited pH-dependent gating that can be employed by bacteria to alter the permeability of their outer membranes in response to environmental changes. We developed a computational model, Protein Topology of Zoetic Loops (Pretzel), to investigate the roles of OmpG extracellular loops implicated in gating. The key interactions predicted by our model were verified by single-channel recording data. Our results indicate that the gating equilibrium is primarily controlled by an electrostatic interaction network formed between the gating loop and charged residues in the lumen. The results shed light on the mechanism of OmpG gating and will provide a fundamental basis for the engineering of OmpG as a nanopore sensor. Our computational Pretzel model could be applied to other outer membrane proteins that contain intricate dynamic loops that are functionally important.

The structure and function of cell membranes studied by atomic force microscopy

The cell membrane, involved in almost all communications of cells and surrounding matrix, is one of the most complicated components of cells. Lack of suitable methods for the detection of cell membranes in vivo has sparked debates on the biochemical composition and structure of cell membranes over half a century. The development of single molecule techniques, such as AFM, SMFS, and TREC, provides a versatile platform for imaging and manipulating cell membranes in biological relevant environments. Here, we discuss the latest developments in AFM and the progress made in cell membrane research. In particular, we highlight novel structure models and dynamic processes, including the mechanical properties of the cell membranes.

Outer membrane phospholipase A's roles in Helicobacter pylori acid adaptation

Background

The pH of the human gastric mucosa varies around 2.5 so that only bacteria with strong acidic stress tolerance can colonize it. The ulcer causing Helicobacter pylori thrives in the gastric mucosa. We analyse the roles of the key outer membrane protein OMPLA in its roles in acid tolerance.

Results

The homology model of Helicobacter pylori outer membrane phospholipase A (OMPLA) reveals a twelve stranded β-barrel with a pore that allows molecules to pass with a diameter up to 4 Å. Structure based multiple sequence alignments revealed the functional roles of many amino acids, and led to the suggestion that OMPLA has multiple functions. Besides its role as phospholipase it lets urea enter and ammonium exit the periplasm. Combined with an extensive literature study, our work leads to a comprehensive model for H. pylori’s acid tolerance. This model is based on the conversion of urea into ammonium, and it includes multiple roles for OMPLA and involves two hitherto little studied membrane channels in the OMPLA operon.

Conclusion

The three-dimensional model of OMPLA predicts a transmembrane pore that can aid H. pylori’s acid tolerance through urea influx and ammonium efflux. After urea passes through OMPLA into the periplasm, it passes through the pH-gated inner membrane channel UreI into the cytoplasm where urease hydrolyses it into NH₃ and CO₂. Most of the NH₃ becomes NH₄⁺ that is likely to need an inner membrane channel to reach the periplasm. Two genes that are co-regulated with OMPLA in gastric Helicobacter operons could aid this transport. The NH₄⁺ that might leave the cell through the OMPLA pore has been implicated in H. pylor’s pathogenesis.

Electronic supplementary material

The online version of this article (doi:10.1186/s13099-017-0184-y) contains supplementary material, which is available to authorized users.

In Silico Structure and Sequence Analysis of Bacterial Porins and Specific Diffusion Channels for Hydrophilic Molecules: Conservation, Multimericity and Multifunctionality

Diffusion channels are involved in the selective uptake of nutrients and form the largest outer membrane protein (OMP) family in Gram-negative bacteria. Differences in pore size and amino acid composition contribute to the specificity. Structure-based multiple sequence alignments shed light on the structure-function relations for all eight subclasses. Entropy-variability analysis results are correlated to known structural and functional aspects, such as structural integrity, multimericity, specificity and biological niche adaptation. The high mutation rate in their surface-exposed loops is likely an important mechanism for host immune system evasion. Multiple sequence alignments for each subclass revealed conserved residue positions that are involved in substrate recognition and specificity. An analysis of monomeric protein channels revealed particular sequence patterns of amino acids that were observed in other classes at multimeric interfaces. This adds to the emerging evidence that all members of the family exist in a multimeric state. Our findings are important for understanding the role of members of this family in a wide range of bacterial processes, including bacterial food uptake, survival and adaptation mechanisms.

Purification, Refolding, and Crystallization of the Outer Membrane Protein OmpG from Escherichia coli

OmpG is a pore-forming protein from E. coli outer membranes. Unlike the classical outer membrane porins, which are trimers, the OmpG channel is a monomeric β-barrel made of 14 antiparallel β-strands with short periplasmic turns and longer extracellular loops. The channel activity of OmpG is pH dependent and the channel is gated by the extracellular loop L6. At neutral/high pH, the channel is open and permeable for substrate molecules with a size up to 900 Da. At acidic pH, loop L6 folds across the channel and blocks the pore. The channel blockage at acidic pH appears to be triggered by the protonation of a histidine pair on neighboring β-strands, which repel one another, resulting in the rearrangement of loop L6 and channel closure. OmpG was purified by refolding from inclusion bodies and crystallized in two and three dimensions. Crystallization and analysis by electron microscopy and X-ray crystallography revealed the fundamental mechanisms essential for the channel activity.

High-resolution atomic force microscopy imaging of rhodopsin in rod outer segment disk membranes

Atomic force microscopy (AFM) is a powerful imaging technique that allows recording topographical information of membrane proteins under near-physiological conditions. Remarkable results have been obtained on membrane proteins that were reconstituted into lipid bilayers. High-resolution AFM imaging of native disk membranes from vertebrate rod outer segments has unveiled the higher-order oligomeric state of the G protein-coupled receptor rhodopsin, which is highly expressed in disk membranes. Based on AFM imaging, it has been demonstrated that rhodopsin assembles in rows of dimers and paracrystals and that the rhodopsin dimer is the fundamental building block of higher-order structures.

Theoretical analysis of ion conductance and gating transitions in the OpdK (OccK1) channel

Molecular simulations have been performed on the pore OpdK elucidating molecular details of ion conductance and a possible gating mechanism.

Mechanistic studies of the biogenesis and folding of outer membrane proteins in vitro and in vivo: what have we learned to date?

Research into the mechanisms by which proteins fold into their native structures has been on-going since the work of Anfinsen in the 1960s. Since that time, the folding mechanisms of small, water-soluble proteins have been well characterised. By contrast, progress in understanding the biogenesis and folding mechanisms of integral membrane proteins has lagged significantly because of the need to create a membrane mimetic environment for folding studies in vitro and the difficulties in finding suitable conditions in which reversible folding can be achieved. Improved knowledge of the factors that promote membrane protein folding and disfavour aggregation now allows studies of folding into lipid bilayers in vitro to be performed. Consequently, mechanistic details and structural information about membrane protein folding are now emerging at an ever increasing pace. Using the panoply of methods developed for studies of the folding of water-soluble proteins. This review summarises current knowledge of the mechanisms of outer membrane protein biogenesis and folding into lipid bilayers in vivo and in vitro and discusses the experimental techniques utilised to gain this information. The emerging knowledge is beginning to allow comparisons to be made between the folding of membrane proteins with current understanding of the mechanisms of folding of water-soluble proteins.

AFM images of the dark biocidal action of cationic conjugated polyelectrolytes and oligomers on Escherichia coli

Polymers and oligomers with conjugated phenylene ethynylene or thiophene ethynylene backbones have been shown to be potent antimicrobials. The mechanisms by which they act have been unclear, though AFM imaging of Escherichia coli cells before and after exposure to two such biocides, PPE-Th polymer and EO-OPE-1(C3), shows their effects on cell surface structure. Dried, unexposed E. coli cells could be imaged at resolution high enough to discern the physical structure of the cell surfaces, including individual porin proteins and their distribution on the cell. Exposure to 30 μg/mL PPE-Th polymer caused major cell surface disruption due to either emulsification of the outer membrane or the formation of polymer aggregates or both. In contrast, exposure to 30 μg/mL EO-OPE-1(C3) oligomer did not cause large-scale membrane disruption but did cause apparent reorganization of the surface proteins into linear arrays or protein-lipid-OPE complexes that dominate on a small scale. E. coli cells were also successfully imaged underwater, allowing a real-time AFM image series as cells were exposed to 30 μg/mL EO-OPE-1(C3). Solution exposure caused the cell surfaces to noticeably increase their roughness over time. These results agree with proposed mechanisms for cell killing by PPE-Th and EO-OPE-1(C3) put forth by Wang et al.1 in which PPE-Th kills by large-scale disruption of the outer membrane and EO-OPE-1(C3) kills by membrane reorganization with possible pore formation.

Structure-based engineering of a minimal porin reveals loop-independent channel closure

Porins, like outer membrane protein G (OmpG) of Escherichia coli, are ideal templates among ion channels for protein and chemical engineering because of their robustness and simple architecture. OmpG shows fast transitions between open and closed states, which were attributed to loop 6 (L6). As flickering limits single-channel-based applications, we pruned L6 by either 8 or 12 amino acids. While the open probabilities of both L6 variants resemble that of native OmpG, their gating frequencies were reduced by 63 and 81%, respectively. Using the 3.2 Å structure of the shorter L6 variant in the open state, we engineered a minimal porin (220 amino acids), where all remaining extramembranous loops were truncated. Unexpectedly, this minimized porin still exhibited gating, but it was 5-fold less frequent than in OmpG. The residual gating of the minimal pore is hence independent of L6 rearrangements and involves narrowing of the ion conductance pathway most probably driven by global stretching-flexing deformations of the membrane-embedded β-barrel.

Quantitative imaging of the electrostatic field and potential generated by a transmembrane protein pore at subnanometer resolution

Elucidating the mechanisms by which proteins translocate small molecules and ions through transmembrane pores and channels is of great interest in biology, medicine, and nanotechnology. However, the characterization of pore forming proteins in their native state lacks suitable methods that are capable of high-resolution imaging (~1 nm) while simultaneously mapping physical and chemical properties. Here we report how force-distance (FD) curve-based atomic force microscopy (AFM) imaging can be applied to image the native pore forming outer membrane protein F (OmpF) at subnanometer resolution and to quantify the electrostatic field and potential generated by the transmembrane pore. We further observe the electrostatic field and potential of the OmpF pore switching "on" and "off" in dependence of the electrolyte concentration. Because electrostatic field and potential select for charged molecules and ions and guide them to the transmembrane pore the insights are of fundamental importance to understand the pore function. These experimental results establish FD-based AFM as a unique tool to image biological systems to subnanometer resolution and to quantify their electrostatic properties.

NMR-based conformational ensembles explain pH-gated opening and closing of OmpG channel

The outer membrane protein G (OmpG) is a monomeric 33 kDa 14-stranded β-barrel membrane protein functioning as a nonspecific porin for the uptake of oligosaccharides in Escherichia coli. Two different crystal structures of OmpG obtained at different values of pH suggest a pH-gated pore opening mechanism. In these structures, extracellular loop 6 extends away from the barrel wall at neutral pH but is folded back into the pore lumen at low pH, blocking transport through the pore. Loop 6 was invisible in a previously published solution NMR structure of OmpG in n-dodecylphosphocholine micelles, presumably due to conformational exchange on an intermediate NMR time scale. Here we present an NMR paramagnetic relaxation enhancement (PRE)-based approach to visualize the conformational dynamics of loop 6 and to calculate conformational ensembles that explain the pH-gated opening and closing of the OmpG channel. The different loop conformers detected by the PRE ensemble calculations were validated by disulfide cross-linking of strategically engineered cysteines and electrophysiological single channel recordings. The results indicate a more dynamically regulated channel opening and closing than previously thought and reveal additional membrane-associated conformational ensembles at pH 6.3 and 7.0. We anticipate this approach to be generally applicable to detect and characterize functionally important conformational ensembles of membrane proteins.

Atomic force microscopy: a multifaceted tool to study membrane proteins and their interactions with ligands

Membrane proteins are embedded in lipid bilayers and facilitate the communication between the external environment and the interior of the cell. This communication is often mediated by the binding of ligands to the membrane protein. Understanding the nature of the interaction between a ligand and a membrane protein is required to both understand the mechanism of action of these proteins and for the development of novel pharmacological drugs. The highly hydrophobic nature of membrane proteins and the requirement of a lipid bilayer for native function have hampered the structural and molecular characterizations of these proteins under physiologically relevant conditions. Atomic force microscopy offers a solution to studying membrane proteins and their interactions with ligands under physiologically relevant conditions and can provide novel insights about the nature of these critical molecular interactions that facilitate cellular communication. In this review, we provide an overview of the atomic force microscopy technique and discuss its application in the study of a variety of questions related to the interaction between a membrane protein and a ligand. This article is part of a Special Issue entitled: Structural and biophysical characterization of membrane protein-ligand binding.

An outer membrane protein undergoes enthalpy- and entropy-driven transitions

β-Barrel membrane proteins often fluctuate among various open substates, yet the nature of these transitions is not fully understood. Using temperature-dependent, single-molecule electrophysiology analysis, along with rational protein design, we show that OccK1, a member of the outer membrane carboxylate channel from Pseudomonas aeruginosa, features a discrete gating dynamics comprising both enthalpy-driven and entropy-driven current transitions. OccK1 was chosen for the analysis of these transitions, because it is a monomeric transmembrane β-barrel of a known high-resolution crystal structure and displays three distinguishable, time-resolvable open substates. Native and loop-deletion OccK1 proteins showed substantial changes in the activation enthalpies and entropies of the channel transitions, but modest alterations in the equilibrium free energies, confirming that the system never departs from equilibrium. Moreover, some current fluctuations of OccK1 indicated a counterintuitive, negative activation enthalpy, which was compensated by a significant decrease in the activation entropy. Temperature scanning of the single-channel properties of OccK1 exhibited a thermally induced switch of the energetically most favorable open substate at the lowest examined temperature of 4 °C. Therefore, such a semiquantitative assessment of the current fluctuation dynamics not only demonstrates the complexity of channel gating but also reveals distinct functional traits of a β-barrel outer membrane protein under different temperature circumstances.

In situ opening/closing of OmpG from E. coli and the splitting of β-sheet signals in ATR-FTIR spectroscopy

The pH dependent opening and closure of Escherichia coli OmpG is driven by the formation and breaking of hydrogen bridges in β-strands S11-S13. We have investigated the in situ secondary structural changes of OmpG with ATR-FTIR difference spectroscopy in order to detect the signals associated with the newly established interactions. Curve-fitting of OmpG in two pH conditions revealed the splitting and shifting of β-sheet signals upon opening of the channel. Besides secondary structure changes, there are also amino acid side chain signals that play active role in opening/closing of the channel. An interaction among positively charged arginines and negatively charged aspartic and glutamic acid residues is suggested upon closure of the channel while this interaction is abolished when the channel opens at higher pH.

Gating of the MlotiK1 potassium channel involves large rearrangements of the cyclic nucleotide-binding domains

Cyclic nucleotide-regulated ion channels are present in bacteria, plants, vertebrates, and humans. In higher organisms, they are closely involved in signaling networks of vision and olfaction. Binding of cAMP or cGMP favors the activation of these ion channels. Despite a wealth of structural and studies, there is a lack of structural data describing the gating process in a full-length cyclic nucleotide-regulated channel. We used high-resolution atomic force microscopy (AFM) to directly observe the conformational change of the membrane embedded bacterial cyclic nucleotide-regulated channel MlotiK1. In the nucleotide-bound conformation, the cytoplasmic cyclic nucleotide-binding (CNB) domains of MlotiK1 are disposed in a fourfold symmetric arrangement forming a pore-like vestibule. Upon nucleotide-unbinding, the four CNB domains undergo a large rearrangement, stand up by ∼1.7 nm, and adopt a structurally variable grouped conformation that closes the cytoplasmic vestibule. This fully reversible conformational change provides insight into how CNB domains rearrange when regulating the potassium channel.

Analysis of gating transitions among the three major open states of the OpdK channel

OpdK is an outer membrane protein of the pathogenic bacterium Pseudomonas aeruginosa. The recent crystal structure of this protein revealed a monomeric, 18-stranded β-barrel with a kidney-shaped pore, whose constriction features a diameter of 8 Å. Using systematic single-channel electrical recordings of this protein pore reconstituted into planar lipid bilayers under a broad range of ion concentrations, we were able to probe its discrete gating kinetics involving three major and functionally distinct conformations, in which a dominant open substate O(2) is accompanied by less thermodynamically stable substates O(1) and O(3). Single-channel electrical data enabled us to determine the alterations in the energetics and kinetics of the OpdK protein when experimental conditions were changed. In the future, such a semiquantitative analysis might provide a better understanding on the dynamics of current fluctuations of other β-barrel membrane protein channels.

Dual energy landscape: the functional state of the β-barrel outer membrane protein G molds its unfolding energy landscape

We applied dynamic single-molecule force spectroscopy to quantify the parameters (free energy of activation and distance of the transition state from the folded state) characterizing the energy barriers in the unfolding energy landscape of the outer membrane protein G (OmpG) from Escherichia coli. The pH-dependent functional switching of OmpG directs the protein along different regions on the unfolding energy landscape. The two functional states of OmpG take the same unfolding pathway during the sequential unfolding of β-hairpins I-IV. After the initial unfolding events, the unfolding pathways diverge. In the open state, the unfolding of β-hairpin V in one step precedes the unfolding of β-hairpin VI. In the closed state, β-hairpin V and β-strand S11 with a part of extracellular loop L6 unfold cooperatively, and subsequently β-strand S12 unfolds with the remaining loop L6. These two unfolding pathways in the open and closed states join again in the last unfolding step of β-hairpin VII. Also, the conformational change from the open to the closed state witnesses a rigidified extracellular gating loop L6. Thus, a change in the conformational state of OmpG not only bifurcates its unfolding pathways but also tunes its mechanical properties for optimum function.

Imaging and interrogating native membrane proteins using the atomic force microscope

Membrane proteins exist in a lipid bilayer and provide for cell-cell communication, transport solutes, and convert energies. Detergents are used to extract membrane proteins and keep them in solution for purification and subsequent analyses. The atomic force microscope (AFM) is a powerful tool for imaging and manipulating membrane proteins in their native state without the necessity to solubilize them. It allows membranes that are adsorbed to flat solid supports to be raster-scanned in physiological solutions with an atomically sharp tip. Therefore, AFM is capable of observing biological molecular machines at work. Superb images of native membranes have been recorded, and a quantitative interpretation of the data acquired using the AFM tip has become possible. In addition, multifunctional probes to simultaneously acquire information on the topography and electrical properties of membrane proteins have been produced. This progress is discussed here and fosters expectations for future developments and applications of AFM and single-molecule force spectroscopy.

Correlation between the OmpG secondary structure and its pH-dependent alterations monitored by FTIR

The channel activity of the outer-membrane protein G (OmpG) from Escherichia coli is pH-dependent. To investigate the role of the histidine pair His231/His261 in triggering channel opening and closing, we mutated both histidines to alanines and cysteines. Fourier transform infrared spectra revealed that the OmpG mutants stay-independent of pH-in an open conformation. Temperature ramp experiments indicate that the mutants are as stable as the open state of wild-type OmpG. The X-ray structure of the alanine-substituted OmpG mutant obtained at pH 6.5 confirms the constitutively open conformation. Compared to previous structures of the wild-type protein in the open and closed conformation, the mutant structure shows a difference in the extracellular loop L6 connecting beta-strands S12 and S13. A deletion of amino acids 220-228, which are thought to block the channel at low pH in wild-type OmpG, indicates conformational changes, which might be triggered by His231/His261.

pH-dependent interactions guide the folding and gate the transmembrane pore of the beta-barrel membrane protein OmpG

The physical interactions that switch the functional state of membrane proteins are poorly understood. Previously, the pH-gating conformations of the beta-barrel forming outer membrane protein G (OmpG) from Escherichia coli have been solved. When the pH changes from neutral to acidic the flexible extracellular loop L6 folds into and closes the OmpG pore. Here, we used single-molecule force spectroscopy to structurally localize and quantify the interactions that are associated with the pH-dependent closure. At acidic pH, we detected a pH-dependent interaction at loop L6. This interaction changed the (un)folding of loop L6 and of beta-strands 11 and 12, which connect loop L6. All other interactions detected within OmpG were unaffected by changes in pH. These results provide a quantitative and mechanistic explanation of how pH-dependent interactions change the folding of a peptide loop to gate the transmembrane pore. They further demonstrate how the stability of OmpG is optimized so that pH changes modify only those interactions necessary to gate the transmembrane pore.