Introduction to solid supported membrane based electrophysiology

Transporter function characterization via continuous-exchange cell-free synthesis and solid supported membrane-based electrophysiology

Functional characterization of transporters is impeded by the high cost and technical challenges of current transporter assays. Thus, in this work, we developed a new characterization workflow that combines cell-free protein synthesis (CFPS) and solid supported membrane-based electrophysiology (SSME). For this, membrane protein synthesis was accomplished in a continuous exchange cell-free system (CECF) in the presence of nanodiscs. The resulting transporters expressed in nanodiscs were incorporated into proteoliposomes and assayed in the presence of different substrates using the surface electrogenic event reader. As a proof of concept, we validated this workflow to express and characterize five diverse transporters: the drug/H+-coupled antiporters EmrE and SugE, the lactose permease LacY, the Na+/H+ antiporter NhaA from Escherichia coli, and the mitochondrial carrier AAC2 from Saccharomyces cerevisiae. For all transporters kinetic parameters, such as KM, IMAX, and pH dependency, were evaluated. This robust and expedite workflow (e.g., can be executed within only five workdays) offers a convenient direct functional assessment of transporter protein activity and has the ability to facilitate applications of transporters in medical and biotechnological research.

From Average Transient Transporter Currents to Microscopic Mechanism─A Bayesian Analysis

Electrophysiology studies of secondary active transporters have revealed quantitative mechanistic insights over many decades of research. However, the emergence of new experimental and analytical approaches calls for investigation of the capabilities and limitations of the newer methods. We examine the ability of solid-supported membrane electrophysiology (SSME) to characterize discrete-state kinetic models with >10 rate constants. We use a Bayesian framework applied to synthetic data for three tasks: to quantify and check (i) the precision of parameter estimates under different assumptions, (ii) the ability of computation to guide the selection of experimental conditions, and (iii) the ability of our approach to distinguish among mechanisms based on SSME data. When the general mechanism, i.e., event order, is known in advance, we show that a subset of kinetic parameters can be "practically identified" within ∼1 order of magnitude, based on SSME current traces that visually appear to exhibit simple exponential behavior. This remains true even when accounting for systematic measurement bias and realistic uncertainties in experimental inputs (concentrations) are incorporated into the analysis. When experimental conditions are optimized or different experiments are combined, the number of practically identifiable parameters can be increased substantially. Some parameters remain intrinsically difficult to estimate through SSME data alone, suggesting that additional experiments are required to fully characterize parameters. We also demonstrate the ability to perform model selection and determine the order of events when that is not known in advance, comparing Bayesian and maximum-likelihood approaches. Finally, our studies elucidate good practices for the increasingly popular but subtly challenging Bayesian calculations for structural and systems biology.

Solid-Supported Membrane (SSM)-Based Electrophysiology Assays Using Surface Electrogenic Event Reader Technology (SURFE²R) in Early Drug Discovery

This article presents detailed descriptions of procedures and troubleshooting tips for solid-supported membrane (SSM)-based electrophysiology assays (SURFE²R) to measure electrogenic solute carrier transporter proteins (SLCs) and assess the effects of compounds that modulate their activity. SURFE²R allows the use of the standard 96-well format, making it an ideal platform for tertiary assays in a drug-discovery campaign. The assays are performed with cell-line-derived membrane fractions or proteoliposomes containing the transporter of interest. Three main protocols are described for the isolation of membrane fractions from cell culture and the generation of proteoliposomes containing the transporter of interest. Additionally, detailed protocols for SURFE²R single concentration and dose-response experiments are included to measure the potencies of test compounds in stimulating or inhibiting transporter function (EC₅₀ or IC₅₀ values, respectively) and kinetic functional assays to calculate apparent affinity (k_M ) and maximal velocity (V_max ) of substrate uptake. © 2023 Sanofi. Current Protocols published by Wiley Periodicals LLC. PROTOCOL GROUP 1: Sample preparation for SSM-based electrophysiology assays Support Protocol 1: Production of cell batches Support Protocol 2: Simple isolation of cell membranes Alternate Protocol 1: Isolation of cell membranes with sucrose gradient pre-purification Support Protocol 3: Production and isolation of liposomes Support Protocol 4: Preparation of sensor with isolated cell membranes Alternate Protocol 2: Preparation of sensor with isolated proteoliposomes PROTOCOL GROUP 2: Determination of assay parameters for SSM-based electrophysiology assay Support Protocol 5: Assay with stable buffer Alternate Protocol 3: Assay with ion gradient Support Protocol 6: Determination of membrane/liposome concentration Support Protocol 7: Determination of substrate dependency k_M PROTOCOL GROUP 3: Determination of advanced assay parameters for SSM-based electrophysiology assays Support Protocol 8: Assessment of ion concentration dependency Support Protocol 9: Assessment of pH dependency Support Protocol 10: Assessment of DMSO dependency Support Protocol 11: Assessment of signal stability with multiple activations PROTOCOL GROUP 4: Compound testing through SSM-based electrophysiology assays using SURFE²R apparatus Support Protocol 12: Assessment of signal specificity of a published inhibitor or unknown compound(s) Support Protocol 13: Compound wash-out Support Protocol 14: Statistical analysis.

Current Methods to Unravel the Functional Properties of Lysosomal Ion Channels and Transporters

A distinct set of channels and transporters regulates the ion fluxes across the lysosomal membrane. Malfunctioning of these transport proteins and the resulting ionic imbalance is involved in various human diseases, such as lysosomal storage disorders, cancer, as well as metabolic and neurodegenerative diseases. As a consequence, these proteins have stimulated strong interest for their suitability as possible drug targets. A detailed functional characterization of many lysosomal channels and transporters is lacking, mainly due to technical difficulties in applying the standard patch-clamp technique to these small intracellular compartments. In this review, we focus on current methods used to unravel the functional properties of lysosomal ion channels and transporters, stressing their advantages and disadvantages and evaluating their fields of applicability.

The CopA2-Type P1B-Type ATPase CcoI Serves as Central Hub for cbb 3-Type Cytochrome Oxidase Biogenesis

Copper (Cu)-transporting P_1B-type ATPases are ubiquitous metal transporters and crucial for maintaining Cu homeostasis in all domains of life. In bacteria, the P_1B-type ATPase CopA is required for Cu-detoxification and exports excess Cu(I) in an ATP-dependent reaction from the cytosol into the periplasm. CopA is a member of the CopA1-type ATPase family and has been biochemically and structurally characterized in detail. In contrast, less is known about members of the CopA2-type ATPase family, which are predicted to transport Cu(I) into the periplasm for cuproprotein maturation. One example is CcoI, which is required for the maturation of cbb ₃-type cytochrome oxidase (cbb ₃-Cox) in different species. Here, we reconstituted purified CcoI of Rhodobacter capsulatus into liposomes and determined Cu transport using solid-supported membrane electrophysiology. The data demonstrate ATP-dependent Cu(I) translocation by CcoI, while no transport is observed in the presence of a non-hydrolysable ATP analog. CcoI contains two cytosolically exposed N-terminal metal binding sites (N-MBSs), which are both important, but not essential for Cu delivery to cbb ₃-Cox. CcoI and cbb ₃-Cox activity assays in the presence of different Cu concentrations suggest that the glutaredoxin-like N-MBS1 is primarily involved in regulating the ATPase activity of CcoI, while the CopZ-like N-MBS2 is involved in Cu(I) acquisition. The interaction of CcoI with periplasmic Cu chaperones was analyzed by genetically fusing CcoI to the chaperone SenC. The CcoI-SenC fusion protein was fully functional in vivo and sufficient to provide Cu for cbb ₃-Cox maturation. In summary, our data demonstrate that CcoI provides the link between the cytosolic and periplasmic Cu chaperone networks during cbb ₃-Cox assembly.

An Overview of Cell-Based Assay Platforms for the Solute Carrier Family of Transporters

The solute carrier (SLC) superfamily represents the biggest family of transporters with important roles in health and disease. Despite being attractive and druggable targets, the majority of SLCs remains understudied. One major hurdle in research on SLCs is the lack of tools, such as cell-based assays to investigate their biological role and for drug discovery. Another challenge is the disperse and anecdotal information on assay strategies that are suitable for SLCs. This review provides a comprehensive overview of state-of-the-art cellular assay technologies for SLC research and discusses relevant SLC characteristics enabling the choice of an optimal assay technology. The Innovative Medicines Initiative consortium RESOLUTE intends to accelerate research on SLCs by providing the scientific community with high-quality reagents, assay technologies and data sets, and to ultimately unlock SLCs for drug discovery.

Functional Characterization of SLC Transporters Using Solid Supported Membranes

Here, we present a protocol for the functional characterization of the H⁺-coupled human peptide transporter PepT1 and sufficient notes to transfer the protocol to the Na⁺-coupled sugar transporter SGLT1, the organic cation transporter OCT2, the Na⁺/Ca²⁺ exchanger NCX, and the neuronal glutamate transporter EAAT3.The assay was developed for the commercially available SURFE²R N1 instrument (Nanion Technologies GmbH) which applies solid supported membrane (SSM)-based electrophysiology. This technique is widely used for the functional characterization of membrane transporters with more than 100 different transporters characterized so far. The technique is cost-effective, easy to use, and capable of high-throughput measurements.SSM-based electrophysiology utilizes SSM-coated gold sensors to physically adsorb membrane vesicles containing the protein of interest. A fast solution exchange provides the substrate and activates transport. For the measurement of PepT1 activity, we applied a peptide concentration jump to activate H⁺/peptide symport. Proton influx charges the sensor. A capacitive current is measured reflecting the transport activity of PepT1 . Multiple measurements on the same sensor allow for comparison of transport activity under different conditions. Here, we determine EC₅₀ for PepT1-mediated glycylglycine transport and perform an inhibition experiment using the specific peptide inhibitor Lys[Z(NO₂)]-Val.

Mutation of two key aspartate residues alters stoichiometry of the NhaB Na+/H+ exchanger from Klebsiella pneumoniae

Bacterial NhaB Na⁺/H⁺ exchangers belonging to the Ion Transporter superfamily are poorly characterized in contrast to Na⁺/H⁺ exchangers of the Cation Proton Antiporter superfamily which have NhaA from Escherichia coli as a prominent member. For a more detailed understanding of the intricacies of the exchanger’s transport mechanism, mutational studies are essential. Therefore, we mutated two protonatable residues present in the putative transmembrane region of NhaB from Klebsiella pneumoniae (KpNhaB), which could serve as substrate binding sites, Asp146 and Asp404, to either glutamate or alanine and analyzed transport function and stability of the mutants using electrophysiological and fluorimetric techniques. While mutation of either Asp residue to Glu only had slight to moderate effects on the transport activity of the exchanger, the mutations D404A and D146A, in particular, had more profound effects on the transport function. Furthermore, a double mutant, D146A/D404A, exhibited a remarkable behavior at alkaline pH, where recorded electrical currents changed polarity, showing steady-state transport with a stoichiometry of H⁺:Na⁺ < 1, as opposed to the H⁺:Na⁺ > 1 stoichiometry of the WT. Thus, we showed that Asp146 and Asp404 are part of the substrate binding site(s) of KpNhaB and engineered a Na⁺/H⁺ exchanger with a variable stoichiometry.

A channel profile report of the unusual K+ channel KtrB

KtrAB is a key player in bacterial K⁺ uptake required for K⁺ homeostasis and osmoadaptation. The system is unique in structure and function. It consists of the K⁺-translocating channel subunit KtrB, which forms a dimer in the membrane, and the soluble regulatory subunit KtrA, which attaches to the cytoplasmic side of the dimer as an octameric ring conferring Na⁺ and ATP dependency to the system. Unlike most K⁺ channels, KtrB lacks the highly conserved T(X)GYG selectivity filter sequence. Instead, only a single glycine residue is found in each pore loop, which raises the question of how selective the ion channel is. Here, we characterized the KtrB subunit from the Gram-negative pathogen Vibrio alginolyticus by isothermal titration calorimetry, solid-supported membrane-based electrophysiology, whole-cell K⁺ uptake, and ACMA-based transport assays. We found that, despite its simple selectivity filter, KtrB selectively binds K⁺ with micromolar affinity. Rb⁺ and Cs⁺ bind with millimolar affinities. However, only K⁺ and the poorly binding Na⁺ are efficiently translocated, based on size exclusion by the gating loop. Importantly, the physiologically required K⁺ over Na⁺ selectivity is provided by the channel's high affinity for potassium, which interestingly results from the presence of the sodium ions themselves. In the presence of the KtrA subunit, sodium ions further decrease the Michaelis-Menten constant for K⁺ uptake from milli- to micromolar concentrations and increase the V_max, suggesting that Na⁺ also facilitates channel gating. In conclusion, high binding affinity and facilitated K⁺ gating allow KtrAB to function as a selective K⁺ channel.

Functional Reconstitution of Membrane Proteins Derived From Eukaryotic Cell-Free Systems

Cell-free protein synthesis (CFPS) based on eukaryotic Sf21 lysate is gaining interest among researchers due to its ability to handle the synthesis of complex human membrane proteins (MPs). Additionally Sf21 cell-free systems contain endogenous microsomal vesicles originally derived from the endoplasmic reticulum (ER). After CFPS, MPs will be translocated into the microsomal vesicles membranes present in the lysates. Thus microsomal membranes offer a natural environment for de novo synthesized MPs. Despite the advantage of synthesizing complex MPs with post translational modifications directly into the microsomal membranes without any additional solubilization supplements, batch based Sf21 cell-free synthesis suffers from low yields. The bottleneck for MPs in particular after the synthesis and incorporation into the microsomal membranes is to analyze their functionality. Apart from low yields of the synthesized MPs with batch based cell-free synthesis, the challenges arise in the form of cytoskeleton elements and peripheral endogenous proteins surrounding the microsomes which may impede the functional analysis of the synthesized proteins. So careful sample processing after the synthesis is particularly important for developing the appropriate functional assays. Here we demonstrate how MPs (native and batch synthesized) from ER derived microsomes can be processed for functional analysis by electrophysiology and radioactive uptake assay methods. Treatment of the microsomal membranes either with a sucrose washing step in the case of human serotonin transporter (hSERT) and sarco/endoplasmic reticulum Ca2+/ATPase (SERCA) pump or with mild detergents followed by the preparation of proteoliposomes in the case of the human voltage dependent anionic channel (hVDAC1) helps to analyze the functional properties of MPs.

Evaluation of select biocompatible markers for labelling peripheral nerves on 11.7 T MRI

BACKGROUND

Peripheral nerve injury is often followed by a highly variable recovery process with respect to both rapidity and efficacy. Identifying post-nerve injury phenomena is key to assessing the merit and timing of surgery as well as to tracking nerve recovery postoperatively. Diffusion Tensor Imaging (DTI) has been investigated in the clinical and research settings as a noninvasive technique to both assess and monitor each patient's unique case of peripheral nerve damage.

NEW METHOD

We identify a MRI-suitable marker for tracking the exact site of either nerve injury or coaptation following surgical repair to aid with DTI analysis.

RESULTS

Due to artefact and disruption of tractography, silver wire and microvascular clips were not suitable markers. AxoGuard®, 4-0 vicryl suture, and 10-0 polyamide suture, although detectable, did not produce a signal easily distinguished from post-surgical changes. Silicone was easily identifiable and stable in both the acute and delayed time points, exhibited negligible impact on DTI parameters, and possessed geometry to prevent nerve strangulation.

COMPARISON WITH EXISTING METHOD

Prior studies have not assessed the efficacy of other markers nor have they assessed silicone for potential artefact with DTI parameter analysis. Furthermore, this work demonstrates the reliability and compatibility of silicone in the delayed postoperative time period and includes its unique imaging appearance on high-resolution 11.7 MRI.

CONCLUSION

Semi-cylindrical silicone tubing can be used as a safe, reliable, and readily available radiological marker to visualize and monitor a region of interest on a rodent's peripheral nerve for aiding assessments with diffusion tensor imaging.

The lipid environment determines the activity of the Escherichia coli ammonium transporter AmtB

The movement of ammonium across biologic membranes is a fundamental process in all living organisms and is mediated by the ubiquitous ammonium transporter/methylammonium permease/rhesus protein (Amt/Mep/Rh) family of transporters. Recent structural analysis and coupled mass spectrometry studies have shown that the Escherichia coli ammonium transporter AmtB specifically binds 1-palmitoyl-2-oleoyl phosphatidylglycerol (POPG). Upon POPG binding, several residues of AmtB undergo a small conformational change, which stabilizes the protein against unfolding. However, no studies have so far been conducted, to our knowledge, to explore whether POPG binding to AmtB has functional consequences. Here, we used an in vitro experimental assay with purified components, together with molecular dynamics simulations, to characterize the relation between POPG binding and AmtB activity. We show that the AmtB activity is electrogenic. Our results indicate that the activity, at the molecular level, of Amt in archaebacteria and eubacteria may differ. We also show that POPG is an important cofactor for AmtB activity and that, in the absence of POPG, AmtB cannot complete the full translocation cycle. Furthermore, our simulations reveal previously undiscovered POPG binding sites on the intracellular side of the lipid bilayer between the AmtB subunits. Possible molecular mechanisms explaining the functional role of POPG are discussed.-Mirandela, G. D., Tamburrino, G., Hoskisson, P. A., Zachariae, U., Javelle, A. The lipid environment determines the activity of the Escherichia coli ammonium transporter AmtB.

Spherical-supported membranes as platforms for screening against membrane protein targets

Screening assays performed against membrane protein targets (e.g. phage display) are hampered by issues arising from protein expression and purification, protein stability in detergent solutions and epitope concealment by detergent micelles. Here, we have studied a fast and simple method to improve screening against membrane proteins: spherical-supported bilayer lipid membranes (“SSBLM”). SSBLMs can be quickly isolated via low-speed centrifugation and redispersed in liquid solutions while presenting the target protein in a native-like lipid environment. To provide proof-of-concept, SSBLMs embedding the polytopic bacterial nucleoside transporter NupC were assembled on 100- and 200 nm silica particles. To test specific binding of antibodies, NupC was tagged with a poly-histidine epitope in one of its central loops between two transmembrane helices. Fluorescent labelling, small angle X-ray scattering (SAXS) and cryo-electron microscopy (cryo-EM) were used to monitor formation of the SSBLMs. Specific binding of an anti-his antibody and a gold-nitrilotriacetic acid (NTA) conjugate probe was confirmed with ELISAs and cryo-EM. SSBLMs for screening could be made with purified and lipid reconstituted NupC, as well as crude bacterial membrane extracts. We conclude that SSBLMs are a promising new means of presenting membrane protein targets for (biomimetic) antibody screening in a native-like lipid environment.

Signaling ammonium across membranes through an ammonium sensor histidine kinase

Sensing and uptake of external ammonium is essential for anaerobic ammonium-oxidizing (anammox) bacteria, and is typically the domain of the ubiquitous Amt/Rh ammonium transporters. Here, we report on the structure and function of an ammonium sensor/transducer from the anammox bacterium "Candidatus Kuenenia stuttgartiensis" that combines a membrane-integral ammonium transporter domain with a fused histidine kinase. It contains a high-affinity ammonium binding site not present in assimilatory Amt proteins. The levels of phosphorylated histidine in the kinase are coupled to the presence of ammonium, as conformational changes during signal recognition by the Amt module are transduced internally to modulate the kinase activity. The structural analysis of this ammonium sensor by X-ray crystallography and small-angle X-ray-scattering reveals a flexible, bipartite system that recruits a large uptake transporter as a sensory module and modulates its functionality to achieve a mechanistic coupling to a kinase domain in order to trigger downstream signaling events.

A Loose Relationship: Incomplete H+/Sugar Coupling in the MFS Sugar Transporter GlcP

The glucose transporter from Staphylococcus epidermidis, GlcP_Se, is a homolog of the human GLUT sugar transporters of the major facilitator superfamily. Together with the xylose transporter from Escherichia coli, XylE_Ec, the other prominent prokaryotic GLUT homolog, GlcP_Se, is equipped with a conserved proton-binding site arguing for an electrogenic transport mode. However, the electrophysiological analysis of GlcP_Se presented here reveals important differences between the two GLUT homologs. GlcP_Se, unlike XylE_Ec, does not perform steady-state electrogenic transport at symmetrical pH conditions. Furthermore, when a pH gradient is applied, partially uncoupled transport modes can be generated. In contrast to other bacterial sugar transporters analyzed so far, in GlcP_Se sugar binding, translocation and release are also accomplished by the deprotonated transporter. Based on these experimental results, we conclude that coupling of sugar and H⁺ transport is incomplete in GlcP_Se. To verify the viability of the observed partially coupled GlcP_Se transport modes, we propose a universal eight-state kinetic model in which any degree of coupling is realized and H⁺/sugar symport represents only a specific instance. Furthermore, using sequence comparison with strictly coupled XylE_Ec and similar sugar transporters, we identify an additional charged residue that may be essential for effective H⁺/sugar symport.

Electrophysiological characterization of the archaeal transporter NCX_Mj using solid supported membrane technology

NCX_Mj is a sodium–calcium exchanger from the archaebacterium Methanococcus jannaschii, whose crystal structure has been solved. Barthmes et al. use solid supported membrane–based electrophysiology to characterize NCX_Mj and reveal its functional similarity to eukaryotic isoforms.

Sodium–calcium exchangers (NCXs) are membrane transporters that play an important role in Ca²⁺ homeostasis and Ca²⁺ signaling. The recent crystal structure of NCX_Mj, a member of the NCX family from the archaebacterium Methanococcus jannaschii, provided insight into the atomistic details of sodium–calcium exchange. Here, we extend these findings by providing detailed functional data on purified NCX_Mj using solid supported membrane (SSM)–based electrophysiology, a powerful but unexploited tool for functional studies of electrogenic transporter proteins. We show that NCX_Mj is highly selective for Na⁺, whereas Ca²⁺ can be replaced by Mg²⁺ and Sr²⁺ and that NCX_Mj can be inhibited by divalent ions, particularly Cd²⁺. By directly comparing the apparent affinities of Na⁺ and Ca²⁺ for NCX_Mj with those for human NCX1, we show excellent agreement, indicating a strong functional similarity between NCX_Mj and its eukaryotic isoforms. We also provide detailed instructions to facilitate the adaption of this method to other electrogenic transporter proteins. Our findings demonstrate that NCX_Mj can serve as a model for the NCX family and highlight several possible applications for SSM-based electrophysiology.

Competition is the basis of the transport mechanism of the NhaB Na+/H+ exchanger from Klebsiella pneumoniae

Na⁺/H⁺ exchange is essential for survival of all organisms, having a role in the regulation of the intracellular Na⁺ concentration, pH and cell volume. Furthermore, Na⁺/H⁺ exchangers were shown to be involved in the virulence of the bacterium Yersinia pestis, indicating they might be potential targets for novel antibiotic treatments. The model system for Na⁺/H⁺ exchangers is the NhaA transporter from Escherichia coli, EcNhaA. Therefore, the general transport mechanism of NhaA exchangers is currently well characterized. However, much less is known about NhaB exchangers, with only a limited number of studies available. The pathogen Klebsiella pneumoniae, which is a major source of nosocomial infection, possesses three electrogenic Na⁺/H⁺ exchangers, KpNhaA1, KpNhaA2 and KpNhaB, none of which have been previously investigated. Our aim in this study was to functionally characterize KpNhaB using solid supported membrane-based electrophysiology as the main investigation technique, and thus provide the first electrophysiological investigation of an NhaB Na⁺/H⁺ exchanger. We found that NhaB can be described by the same competition-based mechanism that was shown to be valid for electrogenic NhaA and NapA, and for electroneutral NhaP Na⁺/H⁺ exchangers. For comparison we also characterized the activity of KpNhaA1 and KpNhaA2 and found that the three exchangers have complementary activity profiles, which is likely a survival advantage for K. pneumoniae when faced with environments of different salinity and pH. This underlines their importance as potential antibiotic drug targets.

Discovery of Compounds that Positively Modulate the High Affinity Choline Transporter

Cholinergic hypofunction is associated with decreased attention and cognitive deficits in the central nervous system in addition to compromised motor function. Consequently, stimulation of cholinergic neurotransmission is a rational therapeutic approach for the potential treatment of a variety of neurological conditions. High affinity choline uptake (HACU) into acetylcholine (ACh)-synthesizing neurons is critically mediated by the sodium- and pH-dependent high-affinity choline transporter (CHT, encoded by the SLC5A7 gene). This transporter is comparatively well-characterized but otherwise unexplored as a potential drug target. We therefore sought to identify small molecules that would enable testing of the hypothesis that positive modulation of CHT mediated transport would enhance activity-dependent cholinergic signaling. We utilized existing and novel screening techniques for their ability to reveal both positive and negative modulation of CHT using literature tools. A screening campaign was initiated with a bespoke compound library comprising both the Pfizer Chemogenomic Library (CGL) of 2,753 molecules designed specifically to help enable the elucidation of new mechanisms in phenotypic screens and 887 compounds from a virtual screening campaign to select molecules with field-based similarities to reported negative and positive allosteric modulators. We identified a number of previously unknown active and structurally distinct molecules that could be used as tools to further explore CHT biology or as a starting point for further medicinal chemistry.

Electrogenic Cation Binding in the Electroneutral Na+/H+ Antiporter of Pyrococcus abyssi

Na⁺/H⁺ antiporters in the CPA1 branch of the cation proton antiporter family drive the electroneutral exchange of H⁺ against Na⁺ ions and ensure pH homeostasis in eukaryotic and prokaryotic organisms. Although their transport cycle is overall electroneutral, specific partial reactions are electrogenic. Here, we present an electrophysiological study of the PaNhaP Na⁺/H⁺ antiporter from Pyrococcus abyssi reconstituted into liposomes. Positive transient currents were recorded upon addition of Na⁺ to PaNhaP proteoliposomes, indicating a reaction where positive charge is rapidly displaced into the proteoliposomes with a rate constant of k >200 s^-1 We attribute the recorded currents to an electrogenic reaction that includes Na⁺ binding and possibly occlusion. Subsequently, positive charge is transported out of the cell associated with H⁺ binding, so that the overall reaction is electroneutral. We show that the differences in pH profile and Na⁺ affinity of PaNhaP and the related MjNhaP1 from Methanocaldococcus jannaschii can be attributed to an additional negatively charged glutamate residue in PaNhaP. The results are discussed in the context of the physiological function of PaNhaP and other microbial Na⁺/H⁺ exchangers. We propose that both, electroneutral and electrogenic Na⁺/H⁺ antiporters, represent a carefully tuned self-regulatory system, which drives the cytoplasmic pH back to neutral after any deviation.

pH Regulation of Electrogenic Sugar/H+ Symport in MFS Sugar Permeases

Bacterial sugar symporters in the Major Facilitator Superfamily (MFS) use the H+ (and in a few cases Na+) electrochemical gradients to achieve active transport of sugar into the cell. Because a number of structures of MFS sugar symporters have been solved recently, molecular insight into the transport mechanism is possible from detailed functional analysis. We present here a comparative electrophysiological study of the lactose permease (LacY), the fucose permease (FucP) and the xylose permease (XylE), which reveals common mechanistic principles and differences. In all three symporters energetically downhill electrogenic sugar/H+ symport is observed. Comparison of the pH dependence of symport at symmetrical pH exhibits broad bell-shaped pH profiles extending over 3 to 6 pH units and a decrease at extremely alkaline pH ≥ 9.4 and at acidic to neutral pH = 4.6-7.5. The pH dependence can be described by an acidic to neutral apparent pK (pKapp) and an alkaline pKapp. Experimental evidence suggests that the alkaline pKapp is due to H+ depletion at the protonation site, while the acidic pKapp is due to inhibition of deprotonation. Since previous studies suggest that a single carboxyl group in LacY (Glu325) may be the only side chain directly involved in H+ translocation and a carboxyl side chain with similar properties has been identified in FucP (Asp46) and XylE (Asp27), the present results imply that the pK of this residue is switched during H+/sugar symport in all three symporters.

Keeping it simple, transport mechanism and pH regulation in Na+/H+ exchangers

Na⁺/H⁺ exchangers are essential for regulation of intracellular proton and sodium concentrations in all living organisms. We examined and experimentally verified a kinetic model for Na⁺/H⁺ exchangers, where a single binding site is alternatively occupied by Na⁺ or one or two H⁺ ions. The proposed transport mechanism inherently down-regulates Na⁺/H⁺ exchangers at extreme pH, preventing excessive cytoplasmic acidification or alkalinization. As an experimental test system we present the first electrophysiological investigation of an electroneutral Na⁺/H⁺ exchanger, NhaP1 from Methanocaldococcus jannaschii (MjNhaP1), a close homologue of the medically important eukaryotic NHE Na⁺/H⁺ exchangers. The kinetic model describes the experimentally observed substrate dependences of MjNhaP1, and the transport mechanism explains alkaline down-regulation of MjNhaP1. Because this model also accounts for acidic down-regulation of the electrogenic NhaA Na⁺/H⁺ exchanger from Escherichia coli (EcNhaA, shown in a previous publication) we conclude that it applies generally to all Na⁺/H⁺ exchangers, electrogenic as well as electroneutral, and elegantly explains their pH regulation. Furthermore, the electrophysiological analysis allows insight into the electrostatic structure of the translocation complex in electroneutral and electrogenic Na⁺/H⁺ exchangers.