Tuning the membrane surface potential for efficient toxin import

Expanding MPEx Hydropathy Analysis to Account for Electrostatic Contributions to Protein Interactions with Anionic Membranes

Hydropathy plots are a crucial tool to guide experimental design, as they generate predictions of protein-membrane interactions and their bilayer topology. The predictions are based on experimentally determined hydrophobicity scales, which provide an estimate for the propensity and stability of these interactions. A significant improvement to the accuracy of hydropathy analyses was provided by the development of the popular Wimley-White interfacial and octanol hydrophobicity scales. These scales have been previously incorporated into the freely available MPEx (Membrane Protein Explorer) online application. Here, we introduce a substantial update to MPEx that allows for the consideration of electrostatic contributions to the bilayer partitioning free energy. This component originates from the Coulombic attraction or repulsion of charges between proteins and membranes. Its inclusion in hydropathy calculations increases the accuracy of hydropathy plot predictions and extends their use to more complex systems (i.e., anionic membranes). We illustrate the application of this analysis to studies on the membrane selectivity of antimicrobial peptides, the membrane partitioning of ion-channel gating modifiers, and the amyloid proteins α-synuclein and Tau, as well as pH-dependent bilayer interactions of diphtheria toxin and apoptotic inhibitor Bcl-xL.

Investigating Ions at Amphiphilic Monolayers with X-ray Fluorescence

Amphiphilic monolayers formed at the soft air/liquid interface are easy-to-handle and versatile model systems for material and life sciences. Helmuth Möhwald was one of the pioneers in this field. Over the last few decades, total-reflection X-ray fluorescence (TRXF) has become an important analytical tool for the investigation of monolayer interactions with ions. Here, the theoretical background of TRXF is described, and practical aspects are discussed. The experimentally determined fluorescence intensity from the adsorbed ions can be interpreted quantitatively either by a calibration procedure utilizing monolayers with known charge density or by calibration with respect to the bare aqueous surface. Both calibration approaches yield quantitatively consistent results within <10% accuracy. Some examples demonstrating the power of TRXF for the study of ion adsorption to charged and noncharged monolayers as well as for the characterization of the physicochemical properties of novel cationic lipids used for improved gene delivery are given.

Lipid charge regulation of non-specific biological ion channels

Lipid charge regulation effects in different protein–lipid conformations highlight the role of electrostatic interactions in conductance and selectivity of non-specific biological ion channels.

Rotavirus disrupts calcium homeostasis by NSP4 viroporin activity

Many viruses alter intracellular calcium homeostasis. The rotavirus nonstructural protein 4 (NSP4), an endoplasmic reticulum (ER) transmembrane glycoprotein, increases intracellular levels of cytoplasmic Ca²⁺ ([Ca²⁺]cyto) through a phospholipase C-independent pathway, which is required for virus replication and morphogenesis. However, the NSP4 domain and mechanism that increases [Ca²⁺]cyto are unknown. We identified an NSP4 domain (amino acids [aa] 47 to 90) that inserts into membranes and has structural characteristics of viroporins, a class of small hydrophobic viral proteins that disrupt membrane integrity and ion homeostasis to facilitate virus entry, assembly, or release. Mutational analysis showed that NSP4 viroporin activity was mediated by an amphipathic α-helical domain downstream of a conserved lysine cluster. The lysine cluster directed integral membrane insertion of the viroporin domain and was critical for viroporin activity. In epithelial cells, expression of wild-type NSP4 increased the levels of free cytoplasmic Ca²⁺ by 3.7-fold, but NSP4 viroporin mutants maintained low levels of [Ca²⁺]cyto, were retained in the ER, and failed to form cytoplasmic vesicular structures, called puncta, which surround viral replication and assembly sites in rotavirus-infected cells. When [Ca²⁺]cyto was increased pharmacologically with thapsigargin, viroporin mutants formed puncta, showing that elevation of calcium levels and puncta formation are distinct functions of NSP4 and indicating that NSP4 directly or indirectly responds to elevated cytoplasmic calcium levels. NSP4 viroporin activity establishes the mechanism for NSP4-mediated elevation of [Ca²⁺]cyto, a critical event that regulates rotavirus replication and virion assembly.

Rotavirus is the leading cause of viral gastroenteritis in children and young animals. Rotavirus infection and expression of nonstructural protein 4 (NSP4) alone dramatically increase cytosolic calcium, which is essential for replication and assembly of infectious virions. This work identifies the intracellular mechanism by which NSP4 disrupts calcium homeostasis by showing that NSP4 is a viroporin, a class of virus-encoded transmembrane pores. Mutational analyses identified residues critical for viroporin activity. Viroporin mutants did not elevate the levels of cytoplasmic calcium in mammalian cells and were maintained in the endoplasmic reticulum rather than forming punctate vesicular structures that are critical for virus replication and morphogenesis. Pharmacological elevation of cytoplasmic calcium levels rescued puncta formation in viroporin mutants, demonstrating that elevation of calcium levels and puncta formation are distinct NSP4 functions. While viroporins typically function in virus entry or release, elevation of calcium levels by NSP4 viroporin activity may serve as a regulatory function to facilitate virus replication and assembly.

Test of the Gouy-Chapman theory for a charged lipid membrane against explicit-solvent molecular dynamics simulations

A wealth of experimental data has verified the applicability of the Gouy-Chapman (GC) theory to charged lipid membranes. Surprisingly, a validation of GC by molecular dynamics (MD) simulations has been elusive. Here, we report a test of GC against extensive MD simulations of an anionic lipid bilayer solvated by water at different concentrations of NaCl or KCl. We demonstrate that the ion distributions from the simulations agree remarkably well with GC predictions when information on the adsorption of counterions to the bilayer is incorporated.

Breakdown of the Gouy-Chapman model for highly charged Langmuir monolayers: counterion size effect

Deviations from the classic Gouy-Chapman (GC) model due to the finite size of hydrated counterions were tested for negatively charged Langmuir monolayers with different surface charge densities. Monolayers with the largest charge density (>0.6 C.m(-2)) show an increase of the surface potential for a series of alkali metal cations from Li(+) to Cs(+) by 200-250 mV. The increase is similar for different monolayers and suggests that this effect is independent of the particular type of headgroup. The magnitude of variation is comparable with model estimations of the electrical double layer (EDL) potential implying that the deviation from the GC model is drastic. Deviations from the GC model rapidly vanish with decreasing monolayer charge density and become hardly observable below 0.3 C.m(-2). For monolayers with a high charge density on subphases containing different sized counterions, preferential participation of the smallest ions in the EDL should be favorable in terms of electrostatic free energy because of packing density limitations. This effect was demonstrated for behenyl sulfate (BS) monolayers (0.64 C.m(-2)) with the X-ray reflectivity technique. For the Cs(+)-Li(+) system, the fraction of Cs(+) in the EDL is 50-60% compared with only 10% of Cs(+) in the subphase. Providing high surface charge density, a small univalent Cs(+) is capable to compete even with a bulky divalent Mg(2+). For equal concentrations of Cs(+) and Mg(2+) in the subphase, the Cs(+)/Mg(2+) ratio in EDL of BS monolayer is 1.3 to 2.0 (in contrast to 0.04, predicted by the GC model). All experimental results of this study are described in terms of packing density limitations for hydrated counterions in the EDL.

Properties of mixed lipid monolayers assembled on hydrophobic surfaces through vesicle adsorption

Supported lipid films are becoming increasingly important tools for the study of membrane protein function because of the availability of high-sensitivity surface analytical and patterning techniques. In this study, we have characterized the physical chemical properties of lipid films assembled on hydrophobic surfaces through the spontaneous adsorption of large unilamellar lipid vesicles composed of dioleoylphosphatidylglycerol (DOPG) and dioleoylphosphatidylcholine (DOPC). The density of the lipid films was measured with surface plasmon resonance spectroscopy as the lipid composition of the vesicles and ionic concentration were varied. As expected, monolayer films were formed, but the density of the monolayers was found to be weakly dependent on the lipid composition of the vesicles and strongly dependent on the ionic concentration of the solution in contact with the monolayer. Atomic force microscopy (AFM) images of the lipid films indicate that they are composed of a homogeneous monolayer. Surface force measurements were used to determine the surface charge and DOPG density of the monolayers. The DOPG content of the films was found to be weakly dependent on the DOPG composition of the vesicles and strongly dependent on the salt concentration of the environment. A model has been developed to describe the behavior of the lipid composition of the films in terms of the hydrophobic, electrostatic, and steric forces acting on the lipid monolayer on the hydrophobic surface.

Lipid Dependence of the Channel Properties of a Colicin E1-Lipid Toroidal Pore

Colicin E1 belongs to a group of bacteriocins whose cytotoxicity toward Escherichia coli is exerted through formation of ion channels that depolarize the cytoplasmic membrane. The lipid dependence of colicin single-channel conductance demonstrated intimate involvement of lipid in the structure of this channel. The colicin formed "small" conductance 60-picosiemens (pS) channels, with properties similar to those previously characterized, in 1,2-dieicosenoyl-sn-glycero-3-phosphocholine (C20) or thinner membranes, whereas it formed a novel "large" conductance 600-pS state in thicker 1,2-dierucoyl-sn-glycero-3-phosphocholine (C22) bilayers. Both channel states were anion-selective and voltage-gated and displayed a requirement for acidic pH. Lipids having negative spontaneous curvature inhibited the formation of both channels but increased the ratio of open 600 pS to 60 pS conductance states. Different diameters of small and large channels, 12 and 16 A, were determined from the dependence of single-channel conductance on the size of nonelectrolyte solute probes. Colicin-induced lipid "flip-flop" and the decrease in anion selectivity of the channel in the presence of negatively charged lipids implied a significant contribution of lipid to the structure of the channel, most readily described as toroidal organization of lipid and protein to form the channel pore.

Electroinsertion and activation of the C-terminal domain of colicin A, a voltage gated bacterial toxin, into mammalian cell membranes

The C-terminal fragment of colicin, a protein that is highly soluble in aqueous solution, is spontaneously and irreversibly inserted into the membranes of mammalian cells, which are locally permeabilized by a transmembrane voltage increase. Insertion is detected by immunodetection. This is obtained by mixing the protein with electropermeabilized cells. The same result is observed by pulsing the colicin/cell mixture. Electroinsertion is therefore obtained for the first time with a multi-fragment spanning protein. The cell viability is not affected beyond the effect of electropermeabilization. A train of low voltage repetitive transmembrane modulation, which cannot trigger membrane permeabilization, is applied a day after the electroinsertion. This induces no effect on unmodified cells but triggers the lysis of cells in which colicin has been inserted by the first electropulsation. The low-level electrical treatment is high enough to trigger the voltage gated opening of colicin and to induce the associated toxicity. A transmembrane configuration of colicin is therefore obtained by electroinsertion. The toxic effect of their voltage gating is only obtained when a critical number of voltage gated channels are activated.

Effect of lipids with different spontaneous curvature on the channel activity of colicin E1: evidence in favor of a toroidal pore

The channel activity of colicin E1 was studied in planar lipid bilayers and liposomes. Colicin E1 pore-forming activity was found to depend on the curvature of the lipid bilayer, as judged by the effect on channel activity of curvature-modulating agents. In particular, the colicin-induced trans-membrane current was augmented by lysophosphatidylcholine and reduced by oleic acid, agents promoting positive and negative membrane curvature, respectively. The data obtained imply direct involvement of lipids in the formation of colicin E1-induced pore walls. It is inferred that the toroidal pore model previously validated for small antimicrobial peptides is applicable to colicin E1, a large protein that contains ten alpha-helices in its pore-forming domain.

On the role of lipid in colicin pore formation

Insights into the protein-membrane interactions by which the C-terminal pore-forming domain of colicins inserts into membranes and forms voltage-gated channels, and the nature of the colicin channel, are provided by data on: (i) the flexible helix-elongated state of the colicin pore-forming domain in the fluid anionic membrane interfacial layer, the optimum anionic surface charge for channel formation, and voltage-gated translocation of charged regions of the colicin domain across the membrane; (ii) structure-function data on the voltage-gated K(+) channel showing translocation of an arginine-rich helical segment through the membrane; (iii) toroidal channels formed by small peptides that involve local participation of anionic lipids in an inverted phase. It is proposed that translocation of the colicin across the membrane occurs through minimization of the Born charging energy for translocation of positively charged basic residues across the lipid bilayer by neutralization with anionic lipid head groups. The resulting pore structure may consist of somewhat short, ca. 16 residues, trans-membrane helices, in a locally thinned membrane, together with surface elements of inverted phase lipid micelles.

Electrochemical and photon polarization modulation infrared reflection absorption spectroscopy study of the electric field driven transformations of a phospholipid bilayer supported at a gold electrode surface

Electrochemistry and polarization modulation Fourier transform infrared reflection absorption spectroscopy (PM-FTIRRAS) was employed to investigate fusion of small unilamellar vesicles of 1,2dioyl-sn-glycero-3-phosphatidyl choline (DOPC) onto the Au(111) electrode. Electrochemical studies demonstrated that the DOPC vesicles fuse and spread onto the gold electrode surface at small charge densities -8 microC cm(-2)<sigmaM<0 microC cm(-2)(if the static electric field is <2 x 10(8) V/m) to form a bilayer. At sigmaM<-8 microC cm(-2), the film is detached from the electrode surface; however, the film remains in close proximity to the surface. The PM-FTIRRAS experiments demonstrated that the field-driven transformation of the film involves changes in hydration, orientation, and conformation in the polar headgroup region and that changes in the packing and tilt of the acyl chains are consequences of the headgroup rearrangements.

Chemical and Photochemical Modification of Colicin E1 and Gramicidin A in Bilayer Lipid Membranes

Chemical modification and photodynamic treatment of the colicin E1 channel-forming domain (P178) in vesicular and planar bilayer lipid membranes (BLMs) was used to elucidate the role of tryptophan residues in colicin E1 channel activity. Modification of colicin tryptophan residues by N-bromosuccinimide (NBS), as judged by the loss of tryptophan fluorescence, resulted in complete suppression of wild-type P178 channel activity in BLMs formed from fully saturated (diphytanoyl) phospholipids, both at the macroscopic-current and single-channel levels. The similar effect on both the tryptophan fluorescence and the electric current across BLM was observed also after NBS treatment of gramicidin channels. Of the single-tryptophan P178 mutants studied, W460 showed the highest sensitivity to NBS treatment, pointing to the importance of the water-exposed Trp460 in colicin channel activity. In line with previous work, the photodynamic treatment (illumination with visible light in the presence of a photosensitizer) led to suppression of P178 channel activity in diphytanoyl-phospholipid membranes concomitant with the damage to tryptophan residues detected here by a decrease in tryptophan fluorescence. The present work revealed novel effects: activation of P178 channels as a result of both NBS and photodynamic treatments was observed with BLMs formed from unsaturated (dioleoyl) phospholipids. These phenomena are ascribed to the effect of oxidative modification of double-bond-containing lipids on P178 channel formation. The pronounced stimulation of the colicin-mediated ionic current observed after both pretreatment with NBS and sensitized photomodification of the BLMs support the idea that distortion of membrane structure can facilitate channel formation.

Mechanisms of colicin binding and transport through outer membrane porins

To kill Escherichia coli, toxic proteins, called colicins, pass through the permeability barrier created by the outer membrane (OM) of the bacterial cell envelope. We consider a variety of different colicins, including A, B, D, E1, E3, Ia, M and N, that penetrate through the porins OmpF, FepA, BtuB, Cir and FhuA, to subsequently interact with a few targets in the periplasm, including TolA, TolB, TolC and TonB. We review the mechanisms, demonstrated and postulated, by which such toxins enter bacterial cells, from the initial binding stage on the cell surface to the internalization reaction through the OM bilayer. Our discussions endeavor to answer two main questions: what is the origin of colicin-binding affinity and specificity, and after adsorption to OM porins, do colicin polypeptides translocate through porin channels, or enter by another, currently unknown pathway?

Insertion intermediates of pore-forming colicins in membrane two-dimensional space

The formation of integral membrane voltage-gated ion channels by the initially soluble C-terminal channel polypeptide (CP) of the pore-forming colicins is a fruitful area for studies on membrane protein import. The dependence of CP import on specific membrane parameters can be better understood using liposomes and planar membranes of defined lipid composition. The membrane surface and interfacial layer provide special conditions for the transition of a pore-forming colicin from the soluble to the integral membrane state. The colicin E1 CP is arranged in the membrane interfacial layer as a conformationally mobile helical array that is extended far more in the two dimensions parallel to the membrane surface than in the third dimension perpendicular to it. The alpha-helical content of CP(E1) increases by approximately 30% upon binding to the membrane. The sequence of kinetically distinguishable events in the CP(E1)-membrane interaction is binding, unfolding to a subtended area of 4200 A(2), helix extension, and insertion, the last three events overlapping in their time course ( approximately 10 s(-1)). The extension into two dimensions and the interaction with the membrane surface may explain the reversible denaturation and refolding of secondary structure that occurs after boiling of the CP-membrane complex. Although DSC showed the presence of helix-helix interactions in the membrane-bound state, the change in secondary structure and the extended surface area argue against a molten-globule intermediate in the CP-membrane interaction. However, the surface-bound state is mobile, as surface conformational mobility is a necessary prerequisite for insertion of CP trans-membrane helices into the bilayer. The requirement for this surface protein mobility, described by "thermal melting" FRET experiments, may provide the explanation for the precipitous decrease in the voltage-gated CP channel formation at high values of surface potential of planar bilayer membranes. Thus, the membrane interfacial layer, with the CP backbone situated near the acyl chain carbonyls, provides a favorable environment for the structure changes necessary for the transition from the soluble to the membrane-inserted state.