Lipid bilayer electrostatic energy, curvature stress, and assembly of gramicidin channels

Gramicidin A as a mechanical sensor for mixed nonideal lipid membranes

Gramicidin A (gA) is a short hydrophobic β-helical peptide that forms cation-selective channels in lipid membranes in the course of transbilayer dimerization. The length of the gA helix is smaller than the thickness of a typical lipid monolayer. Consequently, elastic deformations of the membrane arise in the configurations of gA monomers, conducting dimer, and the intermediate state of coaxial pair, where gA monomers from opposing membrane monolayers are located one on top of the other. The gA channel is characterized by the average lifetime of the conducting state. The elastic properties of the membrane influence the average lifetime, thus making gA a convenient sensor of membrane elasticity. However, the utilization of gA to investigate the elastic properties of mixed membranes comprising two or more components frequently relies on the assumption of ideality, namely that the elastic parameters of mixed-lipid bilayers depend linearly on the concentrations of the components. Here, we developed a general approach that does not rely on the aforementioned assumption. Instead, we explicitly accounted for the possibility of inhomogeneous lateral distribution of all lipid components, as well as for membrane-mediated lateral interactions of gA monomers, dimer, coaxial pair, and minor lipid components. This approach enabled us to derive unknown elastic parameters of lipid monolayer from experimentally determined lifetimes of gA channel in mixed-lipid bilayers. A general algorithm was formulated that allows the unknown elastic parameters of a lipid monolayer to be obtained using gA as a mechanical sensor.

Intrinsic Lipid Curvature and Bilayer Elasticity as Regulators of Channel Function: A Comparative Single-Molecule Study

Perturbations in bilayer material properties (thickness, lipid intrinsic curvature and elastic moduli) modulate the free energy difference between different membrane protein conformations, thereby leading to changes in the conformational preferences of bilayer-spanning proteins. To further explore the relative importance of curvature and elasticity in determining the changes in bilayer properties that underlie the modulation of channel function, we investigated how the micelle-forming amphiphiles Triton X-100, reduced Triton X-100 and the HII lipid phase promoter capsaicin modulate the function of alamethicin and gramicidin channels. Whether the amphiphile-induced changes in intrinsic curvature were negative or positive, amphiphile addition increased gramicidin channel appearance rates and lifetimes and stabilized the higher conductance states in alamethicin channels. When the intrinsic curvature was modulated by altering phospholipid head group interactions, however, maneuvers that promote a negative-going curvature stabilized the higher conductance states in alamethicin channels but destabilized gramicidin channels. Using gramicidin channels of different lengths to probe for changes in bilayer elasticity, we found that amphiphile adsorption increases bilayer elasticity, whereas altering head group interactions does not. We draw the following conclusions: first, confirming previous studies, both alamethicin and gramicidin channels are modulated by changes in lipid bilayer material properties, the changes occurring in parallel yet differing dependent on the property that is being changed; second, isolated, negative-going changes in curvature stabilize the higher current levels in alamethicin channels and destabilize gramicidin channels; third, increases in bilayer elasticity stabilize the higher current levels in alamethicin channels and stabilize gramicidin channels; and fourth, the energetic consequences of changes in elasticity tend to dominate over changes in curvature.

Proton Logic Gate Based on a Gramicidin-ATP Synthase Integrated Biotransducer

Ion channels are membrane proteins that allow ionic signals to pass through channel pores for biofunctional modulations. However, biodevices that integrate bidirectional biological signal transmission between a device and biological converter through supported lipid bilayers (SLBs) while simultaneously controlling the process are lacking. Therefore, in this study, we aimed to develop a hybrid biotransducer composed of ATP synthase and proton channel gramicidin A (gA), controlled by a sulfonated polyaniline (SPA) conducting polymer layer deposited on a microelectrode, and to simulate a model circuit for this system. We controlled proton transport across the gA channel using both electrical and chemical input signals by applying voltage to the SPA or introducing calcium ions (inhibitor) and ethylenediaminetetraacetic acid molecules (inhibitor remover). The insertion of gA and ATP synthase into SLBs on microelectrodes resulted in an integrated biotransducer, in which the proton current was controlled by the flux of adenosine diphosphate molecules and calcium ions. Lastly, we created an XOR logic gate as an enzymatic logic system where the output proton current was controlled by Input A (ATP synthase) and Input B (calcium ions), making use of the unidirectional and bidirectional transmission of protons in ATP synthase and gA, respectively. We combined gA, ATP synthase, and SPA as a hybrid bioiontronics system to control bidirectional or unidirectional ion transport across SLBs in biotransducers. Thus, our findings are potentially relevant for a range of advanced biological and medical applications.

Modulation of a rapid neurotransmitter receptor-ion channel by membrane lipids

Membrane lipids modulate the proteins embedded in the bilayer matrix by two non-exclusive mechanisms: direct or indirect. The latter comprise those effects mediated by the physicochemical state of the membrane bilayer, whereas direct modulation entails the more specific regulatory effects transduced via recognition sites on the target membrane protein. The nicotinic acetylcholine receptor (nAChR), the paradigm member of the pentameric ligand-gated ion channel (pLGIC) superfamily of rapid neurotransmitter receptors, is modulated by both mechanisms. Reciprocally, the nAChR protein exerts influence on its surrounding interstitial lipids. Folding, conformational equilibria, ligand binding, ion permeation, topography, and diffusion of the nAChR are modulated by membrane lipids. The knowledge gained from biophysical studies of this prototypic membrane protein can be applied to other neurotransmitter receptors and most other integral membrane proteins.

Active cortical networks promote shunting fast synaptic inhibition in vivo

Fast synaptic inhibition determines neuronal response properties in the mammalian brain and is mediated by chloride-permeable ionotropic GABA-A receptors (GABAARs). Despite their fundamental role, it is still not known how GABAARs signal in the intact brain. Here, we use in vivo gramicidin recordings to investigate synaptic GABAAR signaling in mouse cortical pyramidal neurons under conditions that preserve native transmembrane chloride gradients. In anesthetized cortex, synaptic GABAARs exert classic hyperpolarizing effects. In contrast, GABAAR-mediated synaptic signaling in awake cortex is found to be predominantly shunting. This is due to more depolarized GABAAR equilibrium potentials (EGABAAR), which are shown to result from the high levels of synaptic activity that characterize awake cortical networks. Synaptic EGABAAR observed in awake cortex facilitates the desynchronizing effects of inhibitory inputs upon local networks, which increases the flexibility of spiking responses to external inputs. Our findings therefore suggest that GABAAR signaling adapts to optimize cortical functions.

Chromone-Containing Allylmorpholines Influence Ion Channels in Lipid Membranes via Dipole Potential and Packing Stress

Herein, we report that chromone-containing allylmorpholines can affect ion channels formed by pore-forming antibiotics in model lipid membranes, which correlates with their ability to influence membrane boundary potential and lipid-packing stress. At 100 µg/mL, allylmorpholines 1, 6, 7, and 8 decrease the boundary potential of the bilayers composed of palmitoyloleoylphosphocholine (POPC) by about 100 mV. At the same time, the compounds do not affect the zeta-potential of POPC liposomes, but reduce the membrane dipole potential by 80-120 mV. The allylmorpholine-induced drop in the dipole potential produce 10-30% enhancement in the conductance of gramicidin A channels. Chromone-containing allylmorpholines also affect the thermotropic behavior of dipalmytoylphosphocholine (DPPC), abolishing the pretransition, lowering melting cooperativity, and turning the main phase transition peak into a multicomponent profile. Compounds 4, 6, 7, and 8 are able to decrease DPPC's melting temperature by about 0.5-1.9 °C. Moreover, derivative 7 is shown to increase the temperature of transition of palmitoyloleoylphosphoethanolamine from lamellar to inverted hexagonal phase. The effects on lipid-phase transitions are attributed to the changes in the spontaneous curvature stress. Alterations in lipid packing induced by allylmorpholines are believed to potentiate the pore-forming ability of amphotericin B and gramicidin A by several times.

Regulation of Gramicidin Channel Function Solely by Changes in Lipid Intrinsic Curvature

Membrane protein function is regulated by the lipid bilayer composition. In many cases the changes in function correlate with changes in the lipid intrinsic curvature (c 0), and c 0 is considered a determinant of protein function. Yet, water-soluble amphiphiles that cause either negative or positive changes in curvature have similar effects on membrane protein function, showing that changes in lipid bilayer properties other than c 0 are important-and may be dominant. To further investigate the mechanisms underlying the bilayer regulation of protein function, we examined how maneuvers that alter phospholipid head groups effective "size"-and thereby c 0-alter gramicidin (gA) channel function. Using dioleoylphospholipids and planar bilayers, we varied the head groups' physical volume and the electrostatic repulsion among head groups (and thus their effective size). When 1,2-dioleyol-sn-glycero-3-phosphocholine (DOPC), was replaced by 1,2-dioleyol-sn-glycero-3-phosphoethanolamine (DOPE) with a smaller head group (causing a more negative c 0), the channel lifetime (τ) is decreased. When the pH of the solution bathing a 1,2-dioleyol-sn-glycero-3-phosphoserine (DOPS) bilayer is decreased from 7 to 3 (causing decreased head group repulsion and a more negative c 0), τ is decreased. When some DOPS head groups are replaced by zwitterionic head groups, τ is similarly decreased. These effects do not depend on the sign of the change in surface charge. In DOPE:DOPC (3:1) bilayers, pH changes from 5→9 to 5→0 (both increasing head group electrostatic repulsion, thereby causing a less negative c 0) both increase τ. Nor do the effects depend on the use of planar, hydrocarbon-containing bilayers, as similar changes were observed in hydrocarbon-free lipid vesicles. Altering the interactions among phospholipid head groups may alter also other bilayer properties such as thickness or elastic moduli. Such changes could be excluded using capacitance measurements and single channel measurements on gA channels of different lengths. We conclude that changes gA channel function caused by changes in head group effective size can be predicted from the expected changes in c 0.

Peptide-induced membrane elastic deformations decelerate gramicidin dimer-monomer equilibration

Gramicidin A (gA) is a hydrophobic pentadecapeptide readily incorporating into a planar bilayer lipid membrane (BLM), thereby inducing a large macroscopic current across the BLM. This current results from ion-channel formation due to head-to-head transbilayer dimerization of gA monomers with rapidly established monomer-dimer equilibrium. Any disturbance of the equilibrium, e.g., by sensitized photoinactivation of a portion of gA monomers, causes relaxation toward a new equilibrium state. According to previous studies, the characteristic relaxation time of the gA-mediated electric current decreases as the current increases upon elevating the gA concentration in the membrane. Here, we report data on the current relaxation kinetics for gA analogs with N-terminal valine replaced by glycine or tyrosine. Surprisingly, the relaxation time increased rather than decreased upon elevation of the total membrane conductance induced by these gA analogs, thus contradicting the classical kinetic scheme. We developed a general theoretical model that accounts for lateral interaction of monomers and dimers mediated by membrane elastic deformations. The modified kinetic scheme of the gramicidin dimerization predicts the reverse dependence of the relaxation time on membrane conductance for gA analogs, with a decreased dimerization constant that is in a good agreement with our experimental data. The equilibration process may be also modulated by incorporation of other peptides ("impurities") into the lipid membrane.

Mechanosensitive Regulation of Fibrosis

Cells in the human body experience and integrate a wide variety of environmental cues. A growing interest in tissue mechanics in the past four decades has shown that the mechanical properties of tissue drive key biological processes and facilitate disease development. However, tissue stiffness is not only a potent behavioral cue, but also a product of cellular signaling activity. This review explores both roles of tissue stiffness in the context of inflammation and fibrosis, and the important molecular players driving such processes. During inflammation, proinflammatory cytokines upregulate tissue stiffness by increasing hydrostatic pressure, ECM deposition, and ECM remodeling. As the ECM stiffens, cells involved in the immune response employ intricate molecular sensors to probe and alter their mechanical environment, thereby facilitating immune cell recruitment and potentiating the fibrotic phenotype. This powerful feedforward loop raises numerous possibilities for drug development and warrants further investigation into the mechanisms specific to different fibrotic diseases.

Curvature Energetics Determined by Alchemical Simulation on Four Topologically Distinct Lipid Phases

The relative curvature energetics of two lipids are tested using thermodynamic integration (TI) on four topologically distinct lipid phases. Simulations use TI to switch between choline headgroup lipids (POPC; that prefers to be flat) and ethanolamine headgroup lipids (POPE; that prefer, for example, the inner monolayer of vesicles). The thermodynamical moving of the lipids between planar, inverse hexagonal (HII), cubic (QII; Pn3m space group), and vesicle topologies reveals differences in material parameters that were previously challenging to access. The methodology allows for predictions of two important lipid material properties: the difference in POPC/POPE monolayer intrinsic curvature (ΔJ0) and the difference in POPC/POPE monolayer Gaussian curvature modulus (Δκ̅m), both of which are connected to the energetics of topological variation. Analysis of the TI data indicates that, consistent with previous experiment and simulation, the J0 of POPE is more negative than POPC (ΔJ0 = -0.018 ± 0.001 Å-1). The theoretical framework extracts significant differences in κ̅m of which POPE is less negative than POPC by 2.0 to 4.0 kcal/mol. The range of these values is determined by considering subsets of the simulations, and disagreement between these subsets suggests separate mechanical parameters at very high curvature. Finally, the fit of the TI data to the model indicates that the position of the pivotal plane of curvature is not constant across topologies at high curvature. Overall, the results offer insights into lipid material properties, the limits of a single HC model, and how to test them using simulation.

Membrane-Mediated Lateral Interactions Regulate the Lifetime of Gramicidin Channels

The lipid matrix of cellular membranes is an elastic liquid crystalline medium. Its deformations regulate the functionality and interactions of membrane proteins,f membrane-bound peptides, lipid and protein-lipid domains. Gramicidin A (gA) is a peptide, which incorporates into membrane leaflets as a monomer and may form a transmembrane dimer. In both configurations, gA deforms the membrane. The transmembrane dimer of gA is a cation-selective ion channel. Its electrical response strongly depends on the elastic properties of the membrane. The gA monomer and dimer deform the membrane differently; therefore, the elastic energy contributes to the activation barriers of the dimerization and dissociation of the conducting state. It is shown experimentally that channel characteristics alter if gA molecules have been located in the vicinity of the conducting dimer. Here, based on the theory of elasticity of lipid membranes, we developed a quantitative theoretical model which allows explaining experimentally observed phenomena under conditions of high surface density of gA or its analogues, i.e., in the regime of strong lateral interactions of gA molecules, mediated by elastic deformations of the membrane. The model would be useful for the analysis and prediction of the gA electrical response in various experimental conditions. This potentially widens the possible applications of gA as a convenient molecular sensor of membrane elasticity.

How do cyclic antibiotics with activity against Gram-negative bacteria permeate membranes? A machine learning informed experimental study

All antibiotics have to engage bacterial amphiphilic barriers such as the lipopolysaccharide-rich outer membrane or the phospholipid-based inner membrane in some manner, either by disrupting them outright and/or permeating them and thereby allow the antibiotic to get into bacteria. There is a growing class of cyclic antibiotics, many of which are of bacterial origin, that exhibit activity against Gram-negative bacteria, which constitute an urgent problem in human health. We examine a diverse collection of these cyclic antibiotics, both natural and synthetic, which include bactenecin, polymyxin B, octapeptin, capreomycin, and Kirshenbaum peptoids, in order to identify what they have in common when they interact with bacterial lipid membranes. We find that they virtually all have the ability to induce negative Gaussian curvature (NGC) in bacterial membranes, the type of curvature geometrically required for permeation mechanisms such as pore formation, blebbing, and budding. This is interesting since permeation of membranes is a function usually ascribed to antimicrobial peptides (AMPs) from innate immunity. As prototypical test cases of cyclic antibiotics, we analyzed amino acid sequences of bactenecin, polymyxin B, and capreomycin using our recently developed machine-learning classifier trained on α-helical AMP sequences. Although the original classifier was not trained on cyclic antibiotics, a modified classifier approach correctly predicted that bactenecin and polymyxin B have the ability to induce NGC in membranes, while capreomycin does not. Moreover, the classifier was able to recapitulate empirical structure-activity relationships from alanine scans in polymyxin B surprisingly well. These results suggest that there exists some common ground in the sequence design of hybrid cyclic antibiotics and linear AMPs.

Membrane Elastic Deformations Modulate Gramicidin A Transbilayer Dimerization and Lateral Clustering

Gramicidin A (gA) is a short β-helical peptide known to form conducting channels in lipid membranes because of transbilayer dimerization. The gA conducting dimer, being shorter than the lipid bilayer thickness, deforms the membrane in its vicinity, and the bilayer elastic energy contributes to the gA dimer formation energy. Likewise, membrane incorporation of a gA monomer, which is shorter than the lipid monolayer thickness, creates a void, thereby forcing surrounding lipid molecules to tilt to fill it. The energy of membrane deformation was calculated in the framework of the continuum elasticity theory, taking into account splay, tilt, lateral stretching/compression, Gaussian splay deformations, and external membrane tension. We obtained the interaction energy profiles for two gA monomers located either in the same or in the opposite monolayers. The profiles demonstrated the long-range attraction and short-range repulsion behavior of the monomers resulting from the membrane deformation. Analysis of the profile features revealed conditions under which clusters of gA monomers would not dissipate because of diffusion. The calculated dependence of the dimer formation and decay energy barriers on the membrane elastic properties was in good agreement with the available experimental data and suggested an explanation for a hitherto contentious phenomenon.

Lipid-mediated mode of action of local anesthetics on lipid pores induced by polyenes, peptides and lipopeptides

The effects of local anesthetics (LAs), namely, lidocaine (LDC), prilocaine (PLC), mepivacaine (MPV), bupivacaine (BPV), procaine (PC), and tetracaine (TTC), on the steady-state transmembrane conductance induced by the cis-side addition of the antifungal polyene macrolide antibiotic, nystatin (NYS), in planar lipid bilayers were studied. The addition of TTC to model membranes comprising DOPC and cholesterol (33 mol%) led to a nearly twenty-fold increase in the steady-state NYS-induced membrane conductance. BPV slightly enhanced the channel-forming activity of polyene. LDC, PLC, MPV, and PC did not affect the NYS-induced transmembrane current. We concluded that the effects of LAs on the channel-forming activity of NYS were in agreement with their effects on the elastic properties of model membranes. The ability of aminoamide LAs to promote calcein leakage from large unilamellar DOPC-vesicles was decreased in the following order: BPV >> LDC ≈ PLC ≈ MPV. LDC, PLC, and MPV produced a graded leakage of fluorescent marker from liposomes, up to 10-13%. A initial sharp jump in fluorescence after the introduction of BPV was attributed to the solubilization of liposomes and the formation of mixed DOPC:BPV-micelles. Differential scanning microcalorimetry (DSC) of large unilamellar DPPC-vesicles showed that the main transition temperature (Tm) is continuously decreased upon increasing concentrations of TTC. A sharp drop in the enthalpy of the transition at higher TTC concentrations indicated a formation of anesthetic/lipid mixed micelles. In contrast to TTC, PC slightly decreased Tm, broadened the DSC signal and did not provoke vesicle-to-micelle transition. Both the calcein leakage and DSC data together with the results of measurements of threshold voltages that are required to cause the lipid bilayer breakdown might indicate an alteration in the curvature lipid packing stress, induced by BPV and TTC. The data presented here lend support to a lipid-mediated mode of LAs action on NYS pores via an alteration in curvature stress near the trans-mouth. Similar results were obtained for several lipid pores, formed by polyene amphotericin B, lipopeptide syringomycin E, and the peptides magainin and melittin. This finding further developed the concept of non-specific regulation of lipid pores by LAs. In conclusion, the combination of nystatin with LAs could be a novel treatment for efficient therapy of superficial and mucosal candidiasis.

Integrated microfluidic biosensing platform for simultaneous confocal microscopy and electrophysiological measurements on bilayer lipid membranes and ion channels

Combining high-resolution imaging and electrophysiological recordings is key for various types of experimentation on lipid bilayers and ion channels. Here, we propose an integrated biosensing platform consisting of a microfluidic cartridge and a dedicated chip-holder to conduct such dual measurements on suspended lipid bilayers, in a user-friendly manner. To illustrate the potential of the integrated platform, we characterize lipid bilayers in terms of thickness and fluidity while simultaneously monitoring single ion channel currents. For that purpose, POPC lipid bilayers are supplemented with a fluorescently-tagged phospholipid (NBD-PE, 1% mol) for Fluorescence Recovery After Photobleaching (FRAP) measurements and a model ion channel (gramicidin, 1 nM). These combined measurements reveal that NBD-PE has no effect on the lipid bilayer thickness while gramicidin induces thinning of the membrane. Furthermore, the presence of gramicidin does not alter the lipid bilayer fluidity. Surprisingly, in lipid bilayers supplemented with both probes, a reduction in gramicidin open probability and lifetime is observed compared to lipid bilayers with gramicidin only, suggesting an influence of NBD-PE on the gramicidin ion function. Altogether, our proposed microfluidic biosensing platform in combination with the herein presented multi-parametric measurement scheme paves the way to explore the interdependent relationship between lipid bilayer properties and ion channel function.

Divergent effects of anesthetics on lipid bilayer properties and sodium channel function

General anesthetics revolutionized medicine by allowing surgeons to perform more complex and much longer procedures. This widely used class of drugs is essential to patient care, yet their exact molecular mechanism(s) are incompletely understood. One early hypothesis over a century ago proposed that nonspecific interactions of anesthetics with the lipid bilayer lead to changes in neuronal function via effects on membrane properties. This model was supported by the Meyer-Overton correlation between anesthetic potency and lipid solubility and despite more recent evidence for specific protein targets, in particular ion-channels, lipid bilayer-mediated effects of anesthetics is still under debate. We therefore tested a wide range of chemically diverse general anesthetics on lipid bilayer properties using a sensitive and functional gramicidin-based assay. None of the tested anesthetics altered lipid bilayer properties at clinically relevant concentrations. Some anesthetics did affect the bilayer, though only at high supratherapeutic concentrations, which are unlikely relevant for clinical anesthesia. These results suggest that anesthetics directly interact with membrane proteins without altering lipid bilayer properties at clinically relevant concentrations. Voltage-gated Na⁺ channels are potential anesthetic targets and various isoforms are inhibited by a wide range of volatile anesthetics. They inhibit channel function by reducing peak Na⁺ current and shifting steady-state inactivation toward more hyperpolarized potentials. Recent advances in crystallography of prokaryotic Na⁺ channels, which are sensitive to volatile anesthetics, together with molecular dynamics simulations and electrophysiological studies will help identify potential anesthetic interaction sites within the channel protein itself.

Gramicidin A Channel Formation Induces Local Lipid Redistribution II: A 3D Continuum Elastic Model

To change conformation, a protein must deform the surrounding bilayer. In this work, a three-dimensional continuum elastic model for gramicidin A in a lipid bilayer is shown to describe the sensitivity to thickness, curvature stress, and the mechanical properties of the lipid bilayer. A method is demonstrated to extract the gramicidin-lipid boundary condition from all-atom simulations that can be used in the three-dimensional continuum model. The boundary condition affects the deformation dramatically, potentially much more than typical variations in the material stiffness do as lipid composition is changed. Moreover, it directly controls the sensitivity to curvature stress. The curvature stress and hydrophobic surfaces of the all-atom and continuum models are found to be in excellent agreement. The continuum model is applied to estimate the enrichment of hydrophobically matched lipids near the channel in a mixture, and the results agree with single-channel experiments and extended molecular dynamics simulations from the companion article by Beaven et al. in this issue of Biophysical Journal.

Investigation of membrane condensation induced by CaCO3 nanoparticles and its effect on membrane protein function

Investigation of membrane condensation induced by calcium ions released from nano-CaCO₃.

A general mechanism for drug promiscuity: Studies with amiodarone and other antiarrhythmics

Amiodarone is a widely prescribed antiarrhythmic drug used to treat the most prevalent type of arrhythmia, atrial fibrillation (AF). At therapeutic concentrations, amiodarone alters the function of many diverse membrane proteins, which results in complex therapeutic and toxicity profiles. Other antiarrhythmics, such as dronedarone, similarly alter the function of multiple membrane proteins, suggesting that a multipronged mechanism may be beneficial for treating AF, but raising questions about how these antiarrhythmics regulate a diverse range of membrane proteins at similar concentrations. One possible mechanism is that these molecules regulate membrane protein function by altering the common environment provided by the host lipid bilayer. We took advantage of the gramicidin (gA) channels' sensitivity to changes in bilayer properties to determine whether commonly used antiarrhythmics--amiodarone, dronedarone, propranolol, and pindolol, whose pharmacological modes of action range from multi-target to specific--perturb lipid bilayer properties at therapeutic concentrations. Using a gA-based fluorescence assay, we found that amiodarone and dronedarone are potent bilayer modifiers at therapeutic concentrations; propranolol alters bilayer properties only at supratherapeutic concentration, and pindolol has little effect. Using single-channel electrophysiology, we found that amiodarone and dronedarone, but not propranolol or pindolol, increase bilayer elasticity. The overlap between therapeutic and bilayer-altering concentrations, which is observed also using plasma membrane-like lipid mixtures, underscores the need to explore the role of the bilayer in therapeutic as well as toxic effects of antiarrhythmic agents.

Thermosensing via transmembrane protein-lipid interactions

Cell membranes are composed of a lipid bilayer containing proteins that cross and/or interact with lipids on either side of the two leaflets. The basic structure of cell membranes is this bilayer, composed of two opposing lipid monolayers with fascinating properties designed to perform all the functions the cell requires. To coordinate these functions, lipid composition of cellular membranes is tailored to suit their specialized tasks. In this review, we describe the general mechanisms of membrane-protein interactions and relate them to some of the molecular strategies organisms use to adjust the membrane lipid composition in response to a decrease in environmental temperature. While the activities of all biomolecules are altered as a function of temperature, the thermosensors we focus on here are molecules whose temperature sensitivity appears to be linked to changes in the biophysical properties of membrane lipids. This article is part of a Special Issue entitled: Lipid-protein interactions.

Modifiers of membrane dipole potentials as tools for investigating ion channel formation and functioning

Electrostatic fields generated on and within biological membranes play a fundamental role in key processes in cell functions. The role of the membrane dipole potential is of particular interest because of its powerful impact on membrane permeability and lipid-protein interactions, including protein insertion, oligomerization, and function. The membrane dipole potential is defined by the orientation of electric dipoles of lipid headgroups, fatty acid carbonyl groups, and membrane-adsorbed water. As a result, the membrane interior is several hundred millivolts more positive than the external aqueous phase. This potential decrease depends on the lipid, and especially sterol, composition of the membrane. The adsorption of certain electroneutral molecules known as dipole modifiers may also lead to significant changes in the magnitude of the potential decrease. These agents are widely used to study the effects of the dipole potential on membrane transport. This review presents a critical analysis of a variety of data from studies dedicated to ion channel formation and functioning in membranes with different dipole potentials. The types of ion channels found in cellular membranes and pores formed by antimicrobial agents and toxins in artificial lipid membranes are summarized. The mechanisms underlying the influence of the membrane dipole potential on ion channel activity, including dipole-dipole and charge-dipole interactions in the pores and in membranes, are discussed. A hypothesis, in which lipid rafts in both model and cellular membranes also modulate ion channel activity by virtue of an increased or decreased dipole potential, is also considered.

Volatile anesthetics inhibit sodium channels without altering bulk lipid bilayer properties

Although general anesthetics are clinically important and widely used, their molecular mechanisms of action remain poorly understood. Volatile anesthetics such as isoflurane (ISO) are thought to alter neuronal function by depressing excitatory and facilitating inhibitory neurotransmission through direct interactions with specific protein targets, including voltage-gated sodium channels (Na(v)). Many anesthetics alter lipid bilayer properties, suggesting that ion channel function might also be altered indirectly through effects on the lipid bilayer. We compared the effects of ISO and of a series of fluorobenzene (FB) model volatile anesthetics on Na(v) function and lipid bilayer properties. We examined the effects of these agents on Na(v) in neuronal cells using whole-cell electrophysiology, and on lipid bilayer properties using a gramicidin-based fluorescence assay, which is a functional assay for detecting changes in lipid bilayer properties sensed by a bilayer-spanning ion channel. At clinically relevant concentrations (defined by the minimum alveolar concentration), both the FBs and ISO produced prepulse-dependent inhibition of Na(v) and shifted the voltage dependence of inactivation toward more hyperpolarized potentials without affecting lipid bilayer properties, as sensed by gramicidin channels. Only at supra-anesthetic (toxic) concentrations did ISO alter lipid bilayer properties. These results suggest that clinically relevant concentrations of volatile anesthetics alter Na(v) function through direct interactions with the channel protein with little, if any, contribution from changes in bulk lipid bilayer properties. Our findings further suggest that changes in lipid bilayer properties are not involved in clinical anesthesia.

The effects of lipids and surfactants on TLR5-proteoliposome functionality for flagellin detection using surface plasmon resonance biosensing

The use of proteoliposomes as affinity elements in conjunction with a surface plasmon resonance sensor is a high-sensitivity alternative for the detection of multiple analytes. However, one of the most important aspects of these conformations is maintaining the functionality of the immobilized protein, which is determined by the choice of lipids and surfactants employed in the reconstitutions. Previously, we demonstrated the functionality of TLR5-proteoliposomes as screening affinity elements of bacterial flagellin. In this new study we change the conditions of immobilization of TLR5 and evaluate how the fluidity of the membrane and the final size of the liposomes affect the functionality of the construct and thus increase their utility as an affinity element for design of new biosensors. In particular, we used reconstructions into preformed liposomes composed of the lipids POPC, POPC-DMPC and POPC-POPE mediated by the use of surfactants OG, Triton X100, and DDM, respectively. The affinity results were evaluated by SPR technology proteoliposomes and were correlated with the anisotropic change in the membrane status; the final sizes of the proteoliposomes were estimated. Our results clearly show the dependence of fluidity and final size of the proteoliposomes with surface plasmon resonance affinity measurements.

Bending free energy from simulation: correspondence of planar and inverse hexagonal lipid phases

Simulations of two distinct systems, one a planar bilayer, the other the inverse hexagonal phase, indicate consistent mechanical properties and curvature preferences, with single DOPE leaflets having a spontaneous curvature, R0 = -26 Å (experimentally ~ -29.2 Å) and DOPC leaflets preferring to be approximately flat (R0= -65 Å, experimentally ~ -87.3 Å). Additionally, a well-defined pivotal plane, where a DOPE leaflet bends at constant area, has been determined to be near the glycerol region of the lipid, consistent with the experimentally predicted plane. By examining the curvature frustration of both high and low curvature, the transferability of experimentally determined bending constants is supported. The techniques herein can be applied to predict the effect of biologically active molecules on the mechanical properties of lipid bilayers under well-controlled conditions.

Phosphoinositides alter lipid bilayer properties

Phosphatidylinositol-4,5-bisphosphate (PIP₂), which constitutes ∼1% of the plasma membrane phospholipid, plays a key role in membrane-delimited signaling. PIP₂ regulates structurally and functionally diverse membrane proteins, including voltage- and ligand-gated ion channels, inwardly rectifying ion channels, transporters, and receptors. In some cases, the regulation is known to involve specific lipid–protein interactions, but the mechanisms by which PIP₂ regulates many of its various targets remain to be fully elucidated. Because many PIP₂ targets are membrane-spanning proteins, we explored whether the phosphoinositides might alter bilayer physical properties such as curvature and elasticity, which would alter the equilibrium between membrane protein conformational states—and thereby protein function. Taking advantage of the gramicidin A (gA) channels’ sensitivity to changes in lipid bilayer properties, we used gA-based fluorescence quenching and single-channel assays to examine the effects of long-chain PIP₂s (brain PIP₂, which is predominantly 1-stearyl-2-arachidonyl-PIP₂, and dioleoyl-PIP₂) on bilayer properties. When premixed with dioleoyl-phosphocholine at 2 mol %, both long-chain PIP₂s produced similar changes in gA channel function (bilayer properties); when applied through the aqueous solution, however, brain PIP₂ was a more potent modifier than dioleoyl-PIP₂. Given the widespread use of short-chain dioctanoyl-phosphoinositides, we also examined the effects of diC₈-phosphoinositol (PI), PI(4,5)P₂, PI(3,5)P₂, PI(3,4)P₂, and PI(3,4,5)P₃. The diC₈ phosphoinositides, except for PI(3,5)P₂, altered bilayer properties with potencies that decreased with increasing head group charge. Nonphosphoinositide diC₈ phospholipids generally were more potent bilayer modifiers than the polyphosphoinositides. These results show that physiological increases or decreases in plasma membrane PIP₂ levels, as a result of activation of PI kinases or phosphatases, are likely to alter lipid bilayer properties, in addition to any other effects they may have. The results further show that exogenous PIP₂, as well as structural analogues that differ in acyl chain length or phosphorylation state, alters lipid bilayer properties at the concentrations used in many cell physiological experiments.

Assessing smectic liquid-crystal continuum models for elastic bilayer deformations

For four decades, since W. Helfrich's pioneering study of smectic A liquid crystals in 1973, continuum elastic models (CEMs) have been employed as tools to understand the energetics of protein-induced lipid bilayer deformations. Among the assumptions underlying this use is that all relevant protein-lipid interactions can be included in the continuum representation of the protein-bilayer interactions through the physical parameters determined for protein-free bilayers and the choice of boundary conditions at the protein/bilayer interface. To better understand this assumption, we review the general structure of CEMs, examine how different choices of boundary conditions and physical moduli profiles alter the predicted bilayer thickness profiles around gramicidin A (gA) and mitochondrial voltage-dependent anion channels (VDAC), respectively, and compare these profiles with those obtained from all-atom molecular dynamics simulations. We find that the profiles differ qualitatively in the first lipid shell around the channels, indicating that the CEMs do not capture accurately the consequences of the protein-induced local changes in lipid bilayer dynamics. Therefore, one needs to be careful when interpreting the results of CEM-based analyses of lipid bilayer-membrane protein interactions.

Interactions of drugs and amphiphiles with membranes: modulation of lipid bilayer elastic properties by changes in acyl chain unsaturation and protonation

Poly-unsaturated fatty acids (PUFAs) alter the function of many membrane proteins, whereas monounsatured fatty acids generally are inert. We previously showed that docosahexaenoic acid (DHA) at pH 7 decreases the bilayer stiffness, consistent with an amphiphile-induced increase in elasticity, but not with a negative change in curvature; oleic acid (OA) was inert (Bruno, Koeppe and Andersen, Proc. Natl. Acad. Sci., 2007, 104, 9638-9643). To further explore how PUFAs and other amphiphiles may alter lipid bilayer properties, and thus membrane protein function, we examined how changes in acyl chain unsaturation and head group charge and size alter bilayer properties, as sensed by bilayer-spanning gramicidin A (gA) channels of different lengths. Compared to DHA, the neutral DHA-methyl ester has reduced effects on bilayer properties and 1-palmitoyl-2-docosahexaenoyl-phosphatidylcholine (PDPC) forms bilayers that are softer than dioleoylphosphatidylcholine (DOPC). The changes in channel function are larger for the short gA channels, indicating that changes in elasticity dominate over changes in curvature. We altered the fatty acid protonation by titration: docosahexaenoic acid (DHA) is more potent at pH 9 (relative to pH 7) and is inert at pH 4; OA, which was inert at pH 7, becomes a potent modifier of bilayer properties at pH 9. At both pH 7 and 9, DHA and OA produced larger changes in the lifetimes of the short gA channels, demonstrating that they increase lipid bilayer elasticity when deprotonated--though OA promotes the formation of inverted hexagonal phases at pH 7. The positively charged oleylamine (OAm), which has a small head-group and therefore should be a negative curvature promoter, inhibited gA channel function with similar reductions in the lifetimes of the short and long gA channels, indicating a curvature-dominated effect. Monitoring the single-channel conductance, we find that the negatively charged fatty acids increase the conductance by increasing the local negative charge around the channel, whereas the positively charged OAm has no effect. These results suggest that deprotonated fatty acids increase bilayer elasticity by reversibly adsorbing at the bilayer/solution interface.

The tension of a curved surface from simulation

This paper demonstrates a method for calculating the tension of a system with a curved interface from a molecular dynamics simulation. To do so, the pressure of a subset of the system is determined by applying a local (virtual) mechanical deformation, fitting the response to that of a bulk fluid, and then using the Young-Laplace equation to infer the tension of the interface. The accuracy of the method is tested by calculating the local pressure of a series of water simulations at various external pressures. The tension of a simulated curved octane-water interface is computed with the method and compares well with the planar tension (≈ 46.7 dyn/cm). Finally, an ambiguity is resolved between the Harasima and Irving-Kirkwood methods of calculating the local pressure as a means for computing the tension.

Curvature forces in membrane lipid-protein interactions

Membrane biochemists are becoming increasingly aware of the role of lipid-protein interactions in diverse cellular functions. This review describes how conformational changes in membrane proteins, involving folding, stability, and membrane shape transitions, potentially involve elastic remodeling of the lipid bilayer. Evidence suggests that membrane lipids affect proteins through interactions of a relatively long-range nature, extending beyond a single annulus of next-neighbor boundary lipids. It is assumed the distance scale of the forces is large compared to the molecular range of action. Application of the theory of elasticity to flexible soft surfaces derives from classical physics and explains the polymorphism of both detergents and membrane phospholipids. A flexible surface model (FSM) describes the balance of curvature and hydrophobic forces in lipid-protein interactions. Chemically nonspecific properties of the lipid bilayer modulate the conformational energetics of membrane proteins. The new biomembrane model challenges the standard model (the fluid mosaic model) found in biochemistry texts. The idea of a curvature force field based on data first introduced for rhodopsin gives a bridge between theory and experiment. Influences of bilayer thickness, nonlamellar-forming lipids, detergents, and osmotic stress are all explained by the FSM. An increased awareness of curvature forces suggests that research will accelerate as structural biology becomes more closely entwined with the physical chemistry of lipids in explaining membrane structure and function.

The determinants of hydrophobic mismatch response for transmembrane helices

Hydrophobic mismatch arises from a difference in the hydrophobic thickness of a lipid membrane and a transmembrane protein segment, and is thought to play an important role in the folding, stability and function of membrane proteins. We have investigated the possible adaptations that lipid bilayers and transmembrane α-helices undergo in response to mismatch, using fully-atomistic molecular dynamics simulations totaling 1.4 μs. We have created 25 different tryptophan-alanine-leucine transmembrane α-helical peptide systems, each composed of a hydrophobic alanine-leucine stretch, flanked by 1-4 tryptophan side chains, as well as the β-helical peptide dimer, gramicidin A. Membrane responses to mismatch include changes in local bilayer thickness and lipid order, varying systematically with peptide length. Adding more flanking tryptophan side chains led to an increase in bilayer thinning for negatively mismatched peptides, though it was also associated with a spreading of the bilayer interface. Peptide tilting, bending and stretching were systematic, with tilting dominating the responses, with values of up to ~45° for the most positively mismatched peptides. Peptide responses were modulated by the number of tryptophan side chains due to their anchoring roles and distributions around the helices. Potential of mean force calculations for local membrane thickness changes, helix tilting, bending and stretching revealed that membrane deformation is the least energetically costly of all mismatch responses, except for positively mismatched peptides where helix tilting also contributes substantially. This comparison of energetic driving forces of mismatch responses allows for deeper insight into protein stability and conformational changes in lipid membranes.

Hyaluronan synthase mediates dye translocation across liposomal membranes

Background

Hyaluronan (HA) is made at the plasma membrane and secreted into the extracellular medium or matrix by phospolipid-dependent hyaluronan synthase (HAS), which is active as a monomer. Since the mechanism by which HA is translocated across membranes is still unresolved, we assessed the presence of an intraprotein pore within HAS by adding purified Streptococcus equisimilis HAS (SeHAS) to liposomes preloaded with the fluorophore Cascade Blue (CB).

Results

CB translocation (efflux) was not observed with mock-purified material from empty vector control E. coli membranes, but was induced by SeHAS, purified from membranes, in a time- and dose-dependent manner. CB efflux was eliminated or greatly reduced when purified SeHAS was first treated under conditions that inhibit enzyme activity: heating, oxidization or cysteine modification with N-ethylmaleimide. Reduced CB efflux also occurred with SeHAS K48E or K48F mutants, in which alteration of K48 within membrane domain 2 causes decreased activity and HA product size. The above results used liposomes containing bovine cardiolipin (BCL). An earlier study testing many synthetic lipids found that the best activating lipid for SeHAS is tetraoleoyl cardiolipin (TO-CL) and that, in contrast, tetramyristoyl cardiolipin (TM-CL) is an inactivating lipid (Weigel et al, J. Biol. Chem. 281, 36542, 2006). Consistent with the effects of these CL species on SeHAS activity, CB efflux was more than 2-fold greater in liposomes made with TO-CL compared to TM-CL.

Conclusions

The results indicate the presence of an intraprotein pore in HAS and support a model in which HA is translocated to the exterior by HAS itself.

The role of membrane thickness in charged protein-lipid interactions

Charged amino acids are known to be important in controlling the actions of integral and peripheral membrane proteins and cell disrupting peptides. Atomistic molecular dynamics studies have shed much light on the mechanisms of membrane binding and translocation of charged protein groups, yet the impact of the full diversity of membrane physico-chemical properties and topologies has yet to be explored. Here we have performed a systematic study of an arginine (Arg) side chain analog moving across saturated phosphatidylcholine (PC) bilayers of variable hydrocarbon tail length from 10 to 18 carbons. For all bilayers we observe similar ion-induced defects, where Arg draws water molecules and lipid head groups into the bilayers to avoid large dehydration energy costs. The free energy profiles all exhibit sharp climbs with increasing penetration into the hydrocarbon core, with predictable shifts between bilayers of different thickness, leading to barrier reduction from 26 kcal/mol for 18 carbons to 6 kcal/mol for 10 carbons. For lipids of 10 and 12 carbons we observe narrow transmembrane pores and corresponding plateaus in the free energy profiles. Allowing for movements of the protein and side chain snorkeling, we argue that the energetic cost for burying Arg inside a thin bilayer will be small, consistent with recent experiments, also leading to a dramatic reduction in pK(a) shifts for Arg. We provide evidence that Arg translocation occurs via an ion-induced defect mechanism, except in thick bilayers (of at least 18 carbons) where solubility-diffusion becomes energetically favored. Our findings shed light on the mechanisms of ion movement through membranes of varying composition, with implications for a range of charged protein-lipid interactions and the actions of cell-perturbing peptides. This article is part of a Special Issue entitled: Membrane protein structure and function.

Cholesterol regulates prokaryotic Kir channel by direct binding to channel protein

Cholesterol is a major regulator of a variety of ion channels but the mechanisms underlying cholesterol sensitivity of ion channels are still poorly understood. The key question is whether cholesterol regulates ion channels by direct binding to the channel protein or by altering the physical environment of lipid bilayer. In this study, we provide the first direct evidence that cholesterol binds to prokaryotic Kir channels, KirBac1.1, and that cholesterol binding is essential for its regulatory effect. Specifically, we show that cholesterol is eluted together with the KirBac1.1 protein when separated on an affinity column and that the amount of bound cholesterol is proportional to the amount of the protein. We also show that cholesterol binding to KirBac1.1 is saturable with a K(D) of 390μM. Moreover, there is clear competition between radioactive and non-radioactive cholesterol for the binding site. There is no competition, however, between cholesterol and 5-Androsten 3β-17 β-diol, a sterol that we showed previously to have no effect on KirBac1.1 function. Finally, we show that cholesterol-KirBac1.1 binding is significantly inhibited by trifluoperazine, known to inhibit cholesterol binding to other proteins, and that inhibition of cholesterol-KirBac1.1 binding results in full recovery of the channel activity. Collectively, results from this study indicate that cholesterol-induced suppression of KirBac1.1 activity is mediated by direct interaction between cholesterol and the channel protein.

Lipid bilayer regulation of membrane protein function: gramicidin channels as molecular force probes

Membrane protein function is regulated by the host lipid bilayer composition. This regulation may depend on specific chemical interactions between proteins and individual molecules in the bilayer, as well as on non-specific interactions between proteins and the bilayer behaving as a physical entity with collective physical properties (e.g. thickness, intrinsic monolayer curvature or elastic moduli). Studies in physico-chemical model systems have demonstrated that changes in bilayer physical properties can regulate membrane protein function by altering the energetic cost of the bilayer deformation associated with a protein conformational change. This type of regulation is well characterized, and its mechanistic elucidation is an interdisciplinary field bordering on physics, chemistry and biology. Changes in lipid composition that alter bilayer physical properties (including cholesterol, polyunsaturated fatty acids, other lipid metabolites and amphiphiles) regulate a wide range of membrane proteins in a seemingly non-specific manner. The commonality of the changes in protein function suggests an underlying physical mechanism, and recent studies show that at least some of the changes are caused by altered bilayer physical properties. This advance is because of the introduction of new tools for studying lipid bilayer regulation of protein function. The present review provides an introduction to the regulation of membrane protein function by the bilayer physical properties. We further describe the use of gramicidin channels as molecular force probes for studying this mechanism, with a unique ability to discriminate between consequences of changes in monolayer curvature and bilayer elastic moduli.

The role of cell cholesterol and the cytoskeleton in the interaction between IK1 and maxi-K channels

Recently, we demonstrated a novel interaction between large-conductance (maxi-K or K(Ca)1.1) and intermediate-conductance (IK1 or K(Ca)3.1) Ca(2+)-activated K channels: activation of IK1 channels causes the inhibition of maxi-K activity (Thompson J and Begenisich T. J Gen Physiol 127: 159-169, 2006). Here we show that the interaction between these two channels can be regulated by the membrane cholesterol level in parotid acinar cells. Depletion of cholesterol using methyl-beta-cyclodextrin weakened, while cholesterol enrichment increased, the ability of IK1 activation to inhibit maxi-K channels. Cholesterol's stereoisomer, epicholesterol, was unable to substitute for cholesterol in the interaction between the two K channels, suggesting a specific cholesterol-protein interaction. This suggestion was strengthened by the results of experiments in which cholesterol was replaced by coprostanol and epicoprostanol. These two sterols have nearly identical effects on membrane physical properties and cholesterol-rich microdomain stability, but had very different effects on the IK1/maxi-K interaction. In addition, the IK1/maxi-K interaction was unaltered in cells lacking caveolin, the protein essential for formation and stability of caveolae. Finally, disruption of the actin cytoskeleton restored the IK1-induced maxi-K inhibition that was lost with cell cholesterol depletion, demonstrating the importance of an intact cytoskeleton for the cholesterol-dependent regulation of the IK1/maxi-K interaction.

pH modulation of transport properties of alamethicin oligomers inserted in zwitterionic-based artificial lipid membranes

Electric features of biological membranes are major determinants of the function and physiological manifestation of membrane-penetrating peptides, and such features are prone to be modulated by the properties of the surrounding aqueous medium. In this work, we demonstrate that pH plays crucial roles in modulating electric characteristics of zwitterionic-based artificial lipid membranes. The effect of pH on electrical properties of such membranes was probed by evaluating the transport properties of embedded alamethicin oligomers over a wide range of pH values (i.e., 0.65, 2.08, 2.94, 7 and 10.1). Our data strongly support the paradigm of a pH-dependent variation of the surface and membrane dipole potential which, in conjunction with possible lateral pressure effects within the lipid membrane, lead to a non-monotonic modulation of the electrical conductance of alamethicin oligomers. As expected, pH modulation of transport properties through the alamethicin oligomer is more visible for narrower pores (that is, the 1st conductive state) with slightly better cation selectivity as compared to larger oligomers.

The gramicidin ion channel: A model membrane protein

The linear peptide gramicidin forms prototypical ion channels specific for monovalent cations and has been extensively used to study the organization, dynamics and function of membrane-spanning channels. In recent times, the availability of crystal structures of complex ion channels has challenged the role of gramicidin as a model membrane protein and ion channel. This review focuses on the suitability of gramicidin as a model membrane protein in general, and the information gained from gramicidin to understand lipid-protein interactions in particular. Special emphasis is given to the role and orientation of tryptophan residues in channel structure and function and recent spectroscopic approaches that have highlighted the organization and dynamics of the channel in membrane and membrane-mimetic media.

Effect of structural transition of the host assembly on dynamics of a membrane-bound tryptophan analogue

Tryptophan octyl ester (TOE) represents an important model for membrane-bound tryptophan residues. In this article, we have explored the effect of sphere-to-rod transition of sodium dodecyl sulfate micelles on the dynamics of the membrane-bound tryptophan analogue, TOE, utilizing a combination of fluorescence spectroscopic approaches which include red edge excitation shift (REES). Our results show that REES and fluorescence spectroscopic parameters such as lifetime, anisotropy and acrylamide quenching of micelle-bound TOE are sensitive to the change in micellar organization accompanied by the sphere-to-rod transition.

Bilayer thickness and membrane protein function: an energetic perspective

The lipid bilayer component of biological membranes is important for the distribution, organization, and function of bilayer-spanning proteins. This regulation is due to both specific lipid-protein interactions and general bilayer-protein interactions, which modulate the energetics and kinetics of protein conformational transitions, as well as the protein distribution between different membrane compartments. The bilayer regulation of membrane protein function arises from the hydrophobic coupling between the protein's hydrophobic domains and the bilayer hydrophobic core, which causes protein conformational changes that involve the protein/bilayer boundary to perturb the adjacent bilayer. Such bilayer perturbations, or deformations, incur an energetic cost, which for a given conformational change varies as a function of the bilayer material properties (bilayer thickness, intrinsic lipid curvature, and the elastic compression and bending moduli). Protein function therefore is regulated by changes in bilayer material properties, which determine the free-energy changes caused by the protein-induced bilayer deformation. The lipid bilayer thus becomes an allosteric regulator of membrane function.

Biophysical effects of electric fields on membrane water interfaces: a mini review

Lipid-water interfaces are dielectric transition regions. Their local organizations are highly sophisticated. They are sensitive to electric field with dramatic consequences on the global membrane organization and function. The importance of using local values of parameters (e.g. dielectric constant) near water-solution interface due to hydration and different electrostatic effects is often neglected in the description of cellular functions. Structural changes in the lipid layer are induced by minute changes in the electric properties of the interface. They bring alterations in the structure and oligomerization of membrane proteins.

Single-molecule methods for monitoring changes in bilayer elastic properties

Membrane-spanning proteins perturb the organization and dynamics of the adjacent bilayer lipids. For example, when the hydrophobic length (l) of a bilayer-spanning protein differs from the average thickness (d0) of the host bilayer, the bilayer thickness will vary locally in the vicinity of the protein in order to "match" the length of the protein's hydrophobic exterior to the thickness of the bilayer hydrophobic core. Such bilayer deformations incur an energetic cost, the bilayer deformation energy (DeltaG0def), which will vary as a function of the protein shape, the protein-bilayer hydrophobic mismatch (d0 - l), the lipid bilayer elastic properties, and the lipid intrinsic curvature (c0). Thus, if the membrane protein conformational changes underlying protein function involve the protein/bilayer interface, the ensuing changes in DeltaG0def (DeltaDeltaG0def) will contribute to the overall free-energy change of the conformational changes (DeltaG0tot)-meaning that the host lipid bilayer will modulate protein function. For a given protein, (DeltaDeltaG0def) varies as a function of the bilayer geometric properties (thickness and intrinsic curvature) and the elastic (bending and compression) moduli, which vary as a function of changes in lipid composition or with the adsorption of amphiphiles at the bilayer/solution interface. To understand how changes in bilayer properties modulate the function of bilayer-spanning proteins, single-molecule methods have been developed to probe changes in bilayer elastic properties using gramicidins as molecular force transducers. Different approaches to measuring the deformation energy are described: (1) measurements of changes in channel lifetimes and appearance rates as the lipid bilayer thickness or channel length are varied, (2) measurements of the equilibrium distribution among channels of different lengths, formed by homo- and heterodimers between gramicidin subunits of different lengths, and (3) measurements of the ratio of the appearance rates of heterodimer channels relative to parent homodimer channels formed by gramicidin subunits of different lengths.