A differential scanning calorimetric and 31P NMR spectroscopic study of the effect of transmembrane alpha-helical peptides on the lamellar-reversed hexagonal phase transition of phosphatidylethanolamine model membranes

Influenza M2 Transmembrane Domain Senses Membrane Heterogeneity and Enhances Membrane Curvature

Targeting host cell membranes by M2 of influenza A virus is important for virus invasion and replication. We study the transmembrane domain of M2 (M2TM) interacting with mica-supported planar bilayers and free-standing giant unilamellar vesicles (GUVs). Using solution atomic force microscopy (AFM), we show that the size of M2TM oligomers is dependent on lipid composition. The addition of M2TM to lipid bilayers containing liquid-ordered (Lo) and liquid-disordered (Ld) phases reveals that M2TM preferentially partitions into the Ld phase; phase-dependent partitioning results in a larger rigidity of the Ld phase. We next use fluorescence microscopy to study the effects of M2TM on phase-coexisting GUVs. In particular, M2TM is found to increase GUVs' miscibility transition temperature Tmix. The augmented thermodynamic stability can be accounted for by considering an enhanced energy barrier of lipid mixing between coexisting phases. Our GUV study also shows that M2TM can elicit an array of vesicle shapes mimicking virus budding. M2TM enhanced membrane curvature is consistent with our AFM data, which show altered membrane rigidity and consequently line tension at domain edges. Together, our results highlight that in addition to conducting protons, M2TM can actively regulate membrane heterogeneity and augment membrane curvature.

Analyzing the effects of hydrophobic mismatch on transmembrane α-helices using tryptophan fluorescence spectroscopy

Hydrophobic matching between transmembrane protein segments and the lipid bilayer in which they are embedded is a significant factor in the behavior and orientation of such transmembrane segments. The condition of hydrophobic mismatch occurs when the hydrophobic thickness of a lipid bilayer is significantly different than the length of the membrane spanning segment of a protein, resulting in a mismatch. This mismatch can result in altered function of proteins as well as nonnative structural arrangements including effects on transmembrane α-helix tilt angles, oligomerization state, and/or the formation of non-transmembrane topographies. Here, a fluorescence-based protocol is described for testing model transmembrane α-helices and their sensitivity to hydrophobic mismatch by measuring the propensity of these helices to form non-transmembrane structures. Overall, good hydrophobic matching between the bilayer and transmembrane segments is an important factor that must be considered when designing membrane proteins or peptides.

The thermodynamics of simple biomembrane mimetic systems

Insight into the forces governing a system is essential for understanding its behavior and function. Thermodynamic investigations provide a wealth of information that is not, or is hardly, available from other methods. This article reviews thermodynamic approaches and assays to measure collective properties such as heat adsorption / emission and volume variations. These methods can be successfully applied to the study of lipid vesicles (liposomes) and biological membranes. With respect to instrumentation, differential scanning calorimetry, pressure perturbation calorimetry, isothermal titration calorimetry, dilatometry, and acoustic techniques aimed at measuring the isothermal and adiabatic processes, two- and three-dimensional compressibilities are considered. Applications of these techniques to lipid systems include the measurement of different thermodynamic parameters and a detailed characterization of thermotropic, barotropic, and lyotropic phase behavior. The membrane binding and / or partitioning of solutes (proteins, peptides, drugs, surfactants, ions, etc.) can also be quantified and modeled. Many thermodynamic assays are available for studying the effect of proteins and other additives on membranes, characterizing non-ideal mixing, domain formation, bilayer stability, curvature strain, permeability, solubilization, and fusion. Studies of membrane proteins in lipid environments elucidate lipid-protein interactions in membranes. Finally, a plethora of relaxation phenomena toward equilibrium thermodynamic structures can be also investigated. The systems are described in terms of enthalpic and entropic forces, equilibrium constants, heat capacities, partial volume changes, volume and area compressibility, and so on, also shedding light on the stability of the structures and the molecular origin and mechanism of the structural changes.

Protein–lipid interactions studied with designed transmembrane peptides: role of hydrophobic matching and interfacial anchoring (Review)

Biological membranes are characterized by a heterogeneous composition, which is not only manifested in the wide variety of their components, but also in aspects like the lateral organization, topology, and conformation of proteins and lipids. In bringing about the correct membrane structure, protein-lipid interactions can be expected to play a prominent role. The extent of hydrophobic matching between transmembrane protein segments and lipids potentially constitutes a versatile director of membrane organization, because a tendency to avoid hydrophobic mismatch could result in compensating adaptations such as tilt of the transmembrane segment or segregation into distinct domains. Also, interfacial interactions between lipid headgroups and the aromatic and charged residues that typically flank transmembrane domains may act as an organizing element. In this review, we discuss the numerous model studies that have systematically explored the influence of hydrophobic matching and interfacial anchoring on membrane structure. Designed peptides consisting of a polyleucine or polyleucine/alanine hydrophobic stretch, which is flanked on both sides by tryptophan or lysine residues, reflect the general layout of transmembrane protein segments. It is shown for phosphatidylcholine bilayers and for other model membranes that these peptides adapt a transmembrane topology without extensive peptide or lipid adaptations under conditions of hydrophobic matching, but that significant rearrangements can result from hydrophobic mismatch. Moreover, these effects depend on the nature of the flanking residues, implying a modulation of the mismatch response by interfacial interactions of the flanking residues. The implications of these model studies for the organization of biomembranes are discussed in the context of recent experiments with more complex systems.

The interaction of amyloid Abeta(1-40) with lipid bilayers and ganglioside as studied by 31P solid-state NMR

Amyloid beta-peptide (Abeta) is a major component of plaques in Alzheimer's disease, and formation of senile plaques has been suggested to originate from regions of neuronal membrane rich in gangliosides. We analyzed the mode of interaction of Abeta with lipid bilayers by multinuclear NMR using (31)P nuclei. We found that Abeta (1-40) strongly perturbed the bilayer structure of dimyristoylphosphatidylcholine (DMPC), to form a non-lamellar phase (most likely micellar). The ganglioside GM1 potentiated the effect of Abeta (1-40), as viewed from (31)P NMR. The difference of the isotropic peak intensity between DMPC/Abeta and DMPC/GM1/Abeta suggests a specific interaction between Abeta and GM1. We show that in the DMPC/GM1/Abeta system there are three lipid phases, namely a lamellar phase, a hexagonal phase and non-oriented lipids. The latter two phases are induced by the presence of the Abeta peptide, and facilitated by GM1.

The activity of the amphipathic peptide delta-lysin correlates with phospholipid acyl chain structure and bilayer elastic properties

Release of lipid vesicle content induced by the amphipathic peptide delta-lysin was investigated as a function of lipid acyl chain length and degree of unsaturation for a series of phosphatidylcholines. Dye efflux and peptide binding were examined for three homologous lipid series: di-monounsaturated, di-polyunsaturated, and asymmetric phosphatidylcholines, with one saturated and one monounsaturated acyl chain. Except for the third series, peptide activity correlated with the first moment of the lateral pressure profile, which is a function of lipid acyl chain structure. In vesicles composed of asymmetric phosphatidylcholines, peptide binding and dye efflux are enhanced compared to symmetric, unsaturated lipids with similar pressure profiles. We attribute this to the entropically more favorable interaction of delta-lysin with partially saturated phospholipids. We find that lipid acyl chain structure has a major impact on the activity of delta-lysin and is likely to be an important factor contributing to the target specificity of amphipathic peptides.

Structural features of distinctin affecting peptide biological and biochemical properties

The antimicrobial peptide distinctin consists of two peptide chains linked by a disulfide bridge; it presents a peculiar fold in water resulting from noncovalent dimerization of two heterodimeric molecules. To investigate the contribution of each peptide chain and the S-S bond to distinctin biochemical properties, different monomeric and homodimeric peptide analogues were synthesized and comparatively evaluated with respect to the native molecule. Our experiments demonstrate that the simultaneous occurrence of both peptide chains and the disulfide bond is essential for the formation of the quaternary structure of distinctin in aqueous media, able to resist protease action. In contrast, distinctin and monomeric and homodimeric analogues exhibited comparable antimicrobial activities, suggesting only a partial contribution of the S-S bond to peptide killing effectiveness. Relative bactericidal properties paralleled liposome permeabilization results, definitively demonstrating that microbial membranes are the main target of distinctin activity. Various biophysical experiments performed in membrane-mimicking media, before and after peptide addition, provided information about peptide secondary structure, lipid bilayer organization, and lipid-peptide orientation with respect to membrane surface. These data were instrumental in the generation of putative models of peptide-lipid supramolecular pore complexes.

Mode of Membrane Interaction and Fusogenic Properties of a de Novo Transmembrane Model Peptide Depend on the Length of the Hydrophobic Core

Model peptides composed of alanine and leucine residues are often used to mimic single helical transmembrane domains. Many studies have been carried out to determine how they interact with membranes. However, few studies have investigated their lipid-destabilizing effect. We designed three peptides designated KALRs containing a hydrophobic stretch of 14, 18, or 22 alanines/leucines surrounded by charged amino acids. Molecular modeling simulations in an implicit membrane model as well as attenuated total reflection-Fourier transform infrared analyses show that KALR is a good model of a transmembrane helix. However, tryptophan fluorescence and attenuated total reflection-Fourier transform infrared spectroscopy indicate that the extent of binding and insertion into lipids increases with the length of the peptide hydrophobic core. Although binding can be directly correlated to peptide hydrophobicity, we show that insertion of peptides into a membrane is determined by the length of the peptide hydrophobic core. Functional studies were performed by measuring the ability of peptides to induce lipid mixing and leakage of liposomes. The data reveal that whereas KALR14 does not destabilize liposomal membranes, KALR18 and KALR22 induce 40 and 50% of lipid-mixing, and 65 and 80% of leakage, respectively. These results indicate that a transmembrane model peptide can induce liposome fusion in vitro if it is long enough. The reasons for the link between length and fusogenicity are discussed in relation to studies of transmembrane domains of viral fusion proteins. We propose that fusogenicity depends not only on peptide insertion but also on the ability of peptides to destabilize the two leaflets of the liposome membrane.

Tuning curvature and stability of monoolein bilayers by designer lipid-like peptide surfactants

This study reports the effect of loading four different charged designer lipid-like short anionic and cationic peptide surfactants on the fully hydrated monoolein (MO)-based Pn3m phase (Q(224)). The studied peptide surfactants comprise seven amino acid residues, namely A(6)D, DA(6), A(6)K, and KA(6). D (aspartic acid) bears two negative charges, K (lysine) bears one positive charge, and A (alanine) constitutes the hydrophobic tail. To elucidate the impact of these peptide surfactants, the ternary MO/peptide/water system has been investigated using small-angle X-ray scattering (SAXS), within a certain range of peptide concentrations (R Collapse

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MESH Headings

• Anions
• Lipid Bilayers
• Lipids/chemistry
• Models, Molecular
• Peptides/chemistry
• Scattering, Radiation
• Surface-Active Agents/chemistry
• Temperature

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Affiliation(s)

Anan Yaghmur Institute of Biophysics and Nanosystems Research (IBN), Austrian Academy of Sciences, Graz, Austria.
Peter Laggner
Shuguang Zhang
Michael Rappolt

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Studies of the minimum hydrophobicity of alpha-helical peptides required to maintain a stable transmembrane association with phospholipid bilayer membranes

The effects of the hydrophobicity and the distribution of hydrophobic residues on the surfaces of some designed alpha-helical transmembrane peptides (acetyl-K2-L(m)-A(n)-K2-amide, where m + n = 24) on their solution behavior and interactions with phospholipids were examined. We find that although these peptides exhibit strong alpha-helix forming propensities in water, membrane-mimetic media, and lipid model membranes, the stability of the helices decreases as the Leu content decreases. Also, their binding to reversed phase high-performance liquid chromatography columns is largely determined by their hydrophobicity and generally decreases with decreases in the Leu/Ala ratio. However, the retention of these peptides by such columns is also affected by the distribution of hydrophobic residues on their helical surfaces, being further enhanced when peptide helical hydrophobic moments are increased by clustering hydrophobic residues on one side of the helix. This clustering of hydrophobic residues also increases peptide propensity for self-aggregation in aqueous media and enhances partitioning of the peptide into lipid bilayer membranes. We also find that the peptides LA3LA2 [acetyl-K2-(LAAALAA)3LAA-K2-amide] and particularly LA6 [acetyl-K2-(LAAAAAA)3LAA-K2-amide] associate less strongly with and perturb the thermotropic phase behavior of phosphatidylcholine bilayers much less than peptides with higher L/A ratios. These results are consistent with free energies calculated for the partitioning of these peptides between water and phospholipid bilayers, which suggest that LA3LA2 has an equal tendency to partition into water and into the hydrophobic core of phospholipid model membranes, whereas LA6 should strongly prefer the aqueous phase. We conclude that for alpha-helical peptides of this type, Leu/Ala ratios of greater than 7/17 are required for stable transmembrane associations with phospholipid bilayers.

Determining the ratio of the Gaussian curvature and bending elastic moduli of phospholipids from Q(II) phase unit cell dimensions

A method is presented for measuring M, the ratio of the Gaussian (saddle splay) elastic modulus to the bending elastic modulus of a lipid monolayer. The ratio M is determined from measurements of the equilibrium bicontinuous inverted cubic (Q(II)) phase unit cell size in excess water as a function of temperature. The analysis includes the effect of a curvature elastic term that is second-order in the Gaussian curvature, K. Preliminary results using data on DOPE-Me validate the method. The fitted value of M is within 8% of the value estimated in an earlier treatment. The method can be used to measure changes in M due to addition of exogenous lipids and peptides to a host lipid system. The Gaussian elastic modulus has a substantial effect on the stability of fusion intermediates (stalks, hemifusion diaphragms, and fusion pores). Studying the effects of peptides and different lipids on M via this method may yield insights into how fusion protein moieties stabilize intermediates in membrane fusion in vivo. The contribution of the K2 curvature elastic term to the free energy of Q(II) phase and fusion pores explains some features of fusion pore stability and dynamics, and some peculiar observations concerning the mechanism of L(alpha)/Q(II) phase transitions.

Changes in molecular order across the lamellar-to-inverted hexagonal phase transition depend on the position of the double-bond in mono-unsaturated phospholipid dispersions

A series of mono-unsaturated phosphatidylethanolamine (PE) model membranes is studied using deuterium nuclear magnetic resonance (NMR) and differential scanning calorimetry. As the position of the double bond is systematically changed, the internal conformational motions are monitored through the bilayer-to-inverted-hexagonal phase transition. The order parameter profiles extracted from the NMR spectra report on the conformational order of the lipid and on the way this order is changed by structural reorganizations of the membrane. The calculation of a ratio of renormalized order parameter profiles is presented here as an attempt to distill the essential features of these changes into dimensionless descriptions of "shape" functions. This variation of the extent of molecular disorder along the long molecular axis of the phospholipids appears to be a recurring motif, modulated by temperature, structural rearrangement, and chemical composition of the membrane. The reported experimentally measured changes in the shape of the order parameter profile can be compared to those obtained during molecular dynamics simulation studies.

Molecular dynamics simulations of model trans-membrane peptides in lipid bilayers: a systematic investigation of hydrophobic mismatch

Hydrophobic mismatch, which is the difference between the hydrophobic length of trans-membrane segments of a protein and the hydrophobic width of the surrounding lipid bilayer, is known to play a role in membrane protein function. We have performed molecular dynamics simulations of trans-membrane KALP peptides (sequence: GKK(LA)nLKKA) in phospholipid bilayers to investigate hydrophobic mismatch alleviation mechanisms. By varying systematically the length of the peptide (KALP15, KALP19, KALP23, KALP27, and KALP31) and the lipid hydrophobic length (DLPC, DMPC, and DPPC), a wide range of mismatch conditions were studied. Simulations of durations of 50-200 ns show that under positive mismatch, the system alleviates the mismatch predominantly by tilting the peptide and to a smaller extent by increased lipid ordering in the immediate vicinity of the peptide. Under negative mismatch, alleviation takes place by a combination of local bilayer bending and the snorkeling of the lysine residues of the peptide. Simulations performed at a higher peptide/lipid molar ratio (1:25) reveal slower dynamics of both the peptide and lipid relative to those at a lower peptide/lipid ratio (1:128). The lysine residues have favorable interactions with specific oxygen atoms of the phospholipid headgroups, indicating the preferred localization of these residues at the lipid/water interface.

Effects of pH-induced variations of the charge of the transmembrane α-helical peptide Ac-K2(LA)12K2-amide on the organization and dynamics of the host dimyristoylphosphatidylcholine bilayer membrane

The effects of the transmembrane alpha-helical peptide Ac-K(2)(LA)(12)K(2)-amide ((LA)(12)) on the phase transition and dynamics of saturated dimyristoylphosphatidylcholine (DMPC) membranes were investigated at different pH using conventional and saturation-recovery EPR observations of phosphatidylcholine spin labels. At a peptide-to-DMPC ratio of 1/10, the main phase-transition temperature of the DMPC bilayer is decreased by 4.0 degrees C when measured at pH 7.0, by 1.6 degrees C when measured at pH 9.5, and not affected when measured at pH 11.5. This reversible pH effect is due to the subsequent neutralization of the positive charges of lysine side chains at both ends of (LA)(12). Apparent pK(a)s of the lysine side chain amino groups of (LA)(12) in DMPC bilayer are 8.6 and approximately 10.9, as compared with the pK(a) value of 10.5 for these groups when lysine is dissolved in water. Saturation-recovery curves as a function of oxygen concentration using phosphatidylcholine spin labels in DMPC bilayer containing (LA)(12) are always mono-exponential when measured at pH 7.0 and 9.5. This observation is consistent with the hypothesis that the lipid exchange rates among the bulk, boundary, and (LA)(12)-rich regions are faster than 0.5 micros, the electron spin-lattice relaxation time in the presence of molecular oxygen, suggesting that stable oligomers of (LA)(12) do not form. Neutralization of one lysine side chain positive charge on each end of the peptide significantly decreases the ordering effect of (LA)(12) on the lipid hydrocarbon chains, while its effect on the reorientational motion of terminal groups of lipid hydrocarbon chains is rather moderate. It does not affect the local diffusion-solubility product of oxygen measured in the DMPC-(LA)(12) membrane interior.

Structural changes in a binary mixed phospholipid bilayer of DOPG and DOPS upon saposin C interaction at acidic pH utilizing 31P and 2H solid-state NMR spectroscopy

Saposin C (Sap C) is known to stimulate the catalytic activity of the lysosomal enzyme glucosylceramidase (GCase) that facilitates the hydrolysis of glucosylceramide to ceramide and glucose. Both Sap C and acidic phospholipids are required for full activity of GCase. In order to better understand this interaction, mixed bilayer samples prepared from dioleoylphosphatidylglycerol (DOPG) and dioleoylphosphatidylserine (DOPS) (5:3 ratio) and Sap C were investigated using (2)H and (31)P solid-state NMR spectroscopy at temperatures ranging from 25 to 50 degrees C at pH 4.7. The Sap C concentrations used to carry out these experiments were 0 mol%, 1 mol% and 3 mol% with respect to the phospholipids. The molecular order parameters (S(CD)) were calculated from the dePaked (2)H solid-state NMR spectra of Distearoyl-d70-phosphatidylglycerol (DSPG-d70) incorporated with DOPG and DOPS binary mixed bilayers. The S(CD) profiles indicate that the addition of Sap C to the negatively charged phospholipids is concentration dependent. S(CD) profiles of 1 mol% of the Sap C protein show only a very slight decrease in the acyl chain order. However, the S(CD) profiles of the 3 mol% of Sap C protein indicate that the interaction is predominantly increasing the disorder in the first half of the acyl chain near the head group (C1-C8) indicating that the amino and the carboxyl termini of Sap C are not inserting deep into the DOPG and DOPS mixed bilayers. The (31)P solid-state NMR spectra show that the chemical shift anisotropy (CSA) for both phospholipids decrease and the spectral broadening increases upon addition of Sap C to the mixed bilayers. The data indicate that Sap C interacts similarly with the head groups of both acidic phospholipids and that Sap C has no preference to DOPS over DOPG. Moreover, our solid-state NMR spectroscopic data agree with the structural model previously proposed in the literature [X. Qi, G.A. Grabowski, Differential membrane interactions of saposins A and C. Implication for the functional specificity, J. Biol. Chem. 276 (2001) 27010-27017] [1].

Transmembrane peptides stabilize inverted cubic phases in a biphasic length-dependent manner: implications for protein-induced membrane fusion

WALP peptides consist of repeating alanine-leucine sequences of different lengths, flanked with tryptophan "anchors" at each end. They form membrane-spanning alpha-helices in lipid membranes, and mimic protein transmembrane domains. WALP peptides of increasing length, from 19 to 31 amino acids, were incorporated into N-monomethylated dioleoylphosphatidylethanolamine (DOPE-Me) at concentrations up to 0.5 mol % peptide. When pure DOPE-Me is heated slowly, the lamellar liquid crystalline (L(alpha)) phase first forms an inverted cubic (Q(II)) phase, and the inverted hexagonal (H(II)) phase at higher temperatures. Using time-resolved x-ray diffraction and slow temperature scans (1.5 degrees C/h), WALP peptides were shown to decrease the temperatures of Q(II) and H(II) phase formation (T(Q) and T(H), respectively) as a function of peptide concentration. The shortest and longest peptides reduced T(Q) the most, whereas intermediate lengths had weaker effects. These findings are relevant to membrane fusion because the first step in the L(alpha)/Q(II) phase transition is believed to be the formation of fusion pores between pure lipid membranes. These results imply that physiologically relevant concentrations of these peptides could increase the susceptibility of biomembrane lipids to fusion through an effect on lipid phase behavior, and may explain one role of the membrane-spanning domains in the proteins that mediate membrane fusion.

Tilt angles of transmembrane model peptides in oriented and non-oriented lipid bilayers as determined by 2H solid-state NMR

Solid-state NMR methods employing (2)H NMR and geometric analysis of labeled alanines (GALA) were used to study the structure and orientation of the transmembrane alpha-helical peptide acetyl-GWW(LA)(8)LWWA-amide (WALP23) in phosphatidylcholine (PC) bilayers of varying thickness. In all lipids the peptide was found to adopt a transmembrane alpha-helical conformation. A small tilt angle of 4.5 degrees was observed in di-18:1-PC, which has a hydrophobic bilayer thickness that approximately matches the hydrophobic length of the peptide. This tilt angle increased slightly but systematically with increasing positive mismatch to 8.2 degrees in di-C12:0-PC, the shortest lipid used. This small increase in tilt angle is insufficient to significantly change the effective hydrophobic length of the peptide and thereby to compensate for the increasing hydrophobic mismatch, suggesting that tilt of these peptides in a lipid bilayer is energetically unfavorable. The tilt and also the orientation around the peptide axis were found to be very similar to the values previously reported for a shorter WALP19 peptide (GWW(LA)(6)LWWA). As also observed in this previous study, the peptide rotates rapidly around the bilayer normal, but not around its helix axis. Here we show that these properties allow application of the GALA method not only to macroscopically aligned samples but also to randomly oriented samples, which has important practical advantages. A minimum of four labeled alanine residues in the hydrophobic transmembrane sequence was found to be required to obtain accurate tilt values using the GALA method.

Investigating structural changes in the lipid bilayer upon insertion of the transmembrane domain of the membrane-bound protein phospholamban utilizing 31P and 2H solid-state NMR spectroscopy

Phospholamban (PLB) is a 52-amino acid integral membrane protein that regulates the flow of Ca(2+) ions in cardiac muscle cells. In the present study, the transmembrane domain of PLB (24-52) was incorporated into phospholipid bilayers prepared from 1-palmitoyl-2-oleoyl-sn-glycero-phosphocholine (POPC). Solid-state (31)P and (2)H NMR experiments were carried out to study the behavior of POPC bilayers in the presence of the hydrophobic peptide PLB at temperatures ranging from 30 degrees C to 60 degrees C. The PLB peptide concentration varied from 0 mol % to 6 mol % with respect to POPC. Solid-state (31)P NMR spectroscopy is a valuable technique to study the different phases formed by phospholipid membranes. (31)P NMR results suggest that the transmembrane protein phospholamban is incorporated successfully into the bilayer and the effects are observed in the lipid lamellar phase. Simulations of the (31)P NMR spectra were carried out to reveal the formation of different vesicle sizes upon PLB insertion. The bilayer vesicles fragmented into smaller sizes by increasing the concentration of PLB with respect to POPC. Finally, molecular order parameters (S(CD)) were calculated by performing (2)H solid-state NMR studies on deuterated POPC (sn-1 chain) phospholipid bilayers when the PLB peptide was inserted into the membrane.

Effect of variations in the structure of a polyleucine-based alpha-helical transmembrane peptide on its interaction with phosphatidylglycerol bilayers

High-sensitivity differential scanning calorimetry and Fourier transform infrared spectroscopy were used to study the interaction of a cationic alpha-helical transmembrane peptide, acetyl-Lys(2)-Leu(24)-Lys(2)-amide (L(24)), and members of the homologous series of anionic n-saturated diacyl phosphatidylglycerols (PGs). Analogues of L(24), in which the lysine residues were replaced by 2,3-diaminopropionic acid (L(24)DAP), or in which a leucine residue at each end of the polyleucine sequence was replaced by a tryptophan (WL(22)W), were also studied to investigate the roles of lysine side-chain snorkeling and aromatic side-chain interactions with the interfacial region of phospholipid bilayers. The gel/liquid-crystalline phase transition temperature of the host PG bilayers is altered by these peptides in a hydrophobic mismatch-dependent manner, as previously found with zwitterionic phosphatidylcholine (PC) bilayers. However, all three peptides reduce the phase transition temperature and enthalpy to a greater extent in anionic PG bilayers than in zwitterionic PC bilayers, with WL(22)W having the largest effect. All three peptides form very stable alpha-helices in PG bilayers, but small conformational changes are induced in response to a mismatch between peptide hydrophobic length and gel-state lipid bilayer hydrophobic thickness. Moreover, electrostatic and hydrogen-bonding interactions occur between the terminal lysine residues of L(24) and L(24)DAP and the polar headgroups of PG bilayers. However, such interactions were not observed in PG/WL(22)W bilayers, suggesting that the cation-pi interactions between the tryptophan and lysine residues predominate. These results indicate that the lipid-peptide interactions are affected not only by the hydrophobic mismatch between these peptides and the host lipid bilayer, but also by the tryptophan-modulated electrostatic and hydrogen-bonding interactions between the positively charged lysine residues at the termini of these peptides and the negatively charged polar headgroups of the PG bilayers.

The gaussian curvature elastic modulus of N-monomethylated dioleoylphosphatidylethanolamine: relevance to membrane fusion and lipid phase behavior

The energy of intermediates in fusion of phospholipid bilayers is sensitive to kappa(m), the saddle splay (Gaussian curvature) elastic modulus of the lipid monolayers. The value kappa(m) is also important in understanding the stability of inverted cubic (Q(II)) and rhombohedral (R) phases relative to the lamellar (L(alpha)) and inverted hexagonal (H(II)) phases in phospholipids. However, kappa(m) cannot be measured directly. It was previously measured by observing changes in Q(II) phase lattice dimensions as a function of water content. Here we use observations of the phase behavior of N-mono-methylated dioleoylphosphatidylethanolamine (DOPE-Me) to determine kappa(m). At the temperature of the L(alpha)/Q(II) phase transition, T(Q), the partial energies of the two phases are equal, and we can express kappa(m) in terms of known lipid monolayer parameters: the spontaneous curvature of DOPE-Me, the monolayer bending modulus kappa(m), and the distance of the monolayer neutral surface from the bilayer midplane, delta. The calculated ratio kappa(m)/kappa(m) is -0.83 +/- 0.08 at T(Q) approximately 55 degrees C. The uncertainty is due primarily to uncertainty in the value of delta for the L(alpha) phase. This value of kappa(m)/kappa(m) is in accord with theoretical expectations, including recent estimates of the value required to rationalize observations of rhombohedral (R) phase stability in phospholipids. The value kappa(m) substantially affects the free energy of formation of fusion intermediates: more energy (tens of k(B)T) is required to form stalks and fusion pores (ILAs) than estimated solely on the basis of the bending elastic energy. In particular, ILAs are much higher in energy than previously estimated. This rationalizes the action of fusion-catalyzing proteins in stabilizing nascent fusion pores in biomembranes; a function inferred from recent experiments in viral systems. These results change predictions of earlier work on ILA and Q(II) phase stability and L(alpha)/Q(II) phase transition mechanisms. To our knowledge, this is the first determination of the saddle splay (Gaussian) modulus in a lipid system consisting only of phospholipids.

Pore formation by equinatoxin II, a eukaryotic protein toxin, occurs by induction of nonlamellar lipid structures

Pore formation in the target cell membranes is a common mechanism used by many toxins in order to kill cells. Among various described mechanisms, a toroidal pore concept was described recently in the course of action of small antimicrobial peptides. Here we provide evidence that such mechanism may be used also by larger toxins. Membrane-destabilizing effects of equinatoxin II, a sea anemone cytolysin, were studied by various biophysical techniques. 31P NMR showed an occurrence of an isotropic component when toxin was added to multilamellar vesicles and heated. This component was not observed with melittin, alpha-staphylococcal toxin, or myoglobin. It does not originate from isolated small lipid structures, since the size of the vesicles after the experiment was similar to the control without toxin. Electron microscopy shows occurrence of a honeycomb structure, previously observed only for some particular lipid mixtures. The analysis of FTIR spectra of the equinatoxin II-lipid complex showed lipid disordering that is consistent with isotropic component observed in NMR. Finally, the cation selectivity of the toxin-induced pores increased in the presence of negatively charged phosphatidic acid, indicating the presence of lipids in the conductive channel. The results are compatible with the toroidal pore concept that might be a general mechanism of pore formation for various membrane-interacting proteins or peptides.

MSI-78, an analogue of the magainin antimicrobial peptides, disrupts lipid bilayer structure via positive curvature strain

In this work, we present the first characterization of the cell lysing mechanism of MSI-78, an antimicrobial peptide. MSI-78 is an amphipathic alpha-helical peptide designed by Genaera Corporation as a synthetic analog to peptides from the magainin family. (31)P-NMR of mechanically aligned samples and differential scanning calorimetry (DSC) were used to study peptide-containing lipid bilayers. DSC showed that MSI-78 increased the fluid lamellar to inverted hexagonal phase transition temperature of 1,2-dipalmitoleoyl-phosphatidylethanolamine indicating the peptide induces positive curvature strain in lipid bilayers. (31)P-NMR of lipid bilayers composed of MSI-78 and 1-palmitoyl-2-oleoyl-phosphatidylethanolamine demonstrated that the peptide inhibited the fluid lamellar to inverted hexagonal phase transition of 1-palmitoyl-2-oleoyl-phosphatidylethanolamine, supporting the DSC results, and the peptide did not induce the formation of nonlamellar phases, even at very high peptide concentrations (15 mol %). (31)P-NMR of samples containing 1-palmitoyl-2-oleoyl-phosphatidylcholine and MSI-78 revealed that MSI-78 induces significant changes in the bilayer structure, particularly at high peptide concentrations. At lower concentrations (1-5%), the peptide altered the morphology of the bilayer in a way consistent with the formation of a toroidal pore. Higher concentrations of peptide (10-15%) led to the formation of a mixture of normal hexagonal phase and lamellar phase lipids. This work shows that MSI-78 induces significant changes in lipid bilayers via positive curvature strain and presents a model consistent with both the observed spectral changes and previously published work.

Membrane composition determines pardaxin's mechanism of lipid bilayer disruption

Pardaxin is a membrane-lysing peptide originally isolated from the fish Pardachirus marmoratus. The effect of the carboxy-amide of pardaxin (P1a) on bilayers of varying composition was studied using (15)N and (31)P solid-state NMR of mechanically aligned samples and differential scanning calorimetry (DSC). (15)N NMR spectroscopy of [(15)N-Leu(19)]P1a found that the orientation of the peptide's C-terminal helix depends on membrane composition. It is located on the surface of lipid bilayers composed of 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) and is inserted in lipid bilayers composed of 1,2-dimyristoyl-phosphatidylcholine (DMPC). The former suggests a carpet mechanism for bilayer disruption whereas the latter is consistent with a barrel-stave mechanism. The (31)P chemical shift NMR spectra showed that the peptide significantly disrupts lipid bilayers composed solely of zwitterionic lipids, particularly bilayers composed of POPC, in agreement with a carpet mechanism. P1a caused the formation of an isotropic phase in 1-palmitoyl-2-oleoyl-phosphatidylethanolamine (POPE) lipid bilayers. This, combined with DSC data that found P1a reduced the fluid lamellar-to-inverted hexagonal phase transition temperature at very low concentrations (1:50,000), is interpreted as the formation of a cubic phase and not micellization of the membrane. Experiments exploring the effect of P1a on lipid bilayers composed of 4:1 POPC:cholesterol, 4:1 POPE:cholesterol, 3:1 POPC:1-palmitoyl-2-oleoyl-phosphatidylglycerol (POPG), and 3:1 POPE:POPG were also conducted, and the presence of anionic lipids or cholesterol was found to reduce the peptide's ability to disrupt bilayers. Considered together, these data demonstrate that the mechanism of P1a is dependent on membrane composition.

Mechanisms of the interaction of alpha-helical transmembrane peptides with phospholipid bilayers

The synthetic peptide acetyl-K(2)-G-L(24)-K(2)-A-amide (P(24)) and its analogs have been successfully utilized as models of the hydrophobic transmembrane alpha-helical segments of integral membrane proteins. The central polyleucine region of these peptides was designed to form a maximally stable, very hydrophobic alpha-helix which will partition strongly into the hydrophobic environment of the lipid bilayer core, while the dilysine caps were designed to anchor the ends of these peptides to the polar surface of the lipid bilayer and to inhibit the lateral aggregation of these peptides. Moreover, the normally positively charged N-terminus and the negatively charged C-terminus have both been blocked in order to provide a symmetrical tetracationic peptide, which will more faithfully mimic the transbilayer region of natural membrane proteins and preclude favorable electrostatic interactions. In fact, P(24) adopts a very stable alpha-helical conformation and transbilayer orientation in lipid model membranes. The results of our recent studies of the interaction of this family of alpha-helical transmembrane peptides with phospholipid bilayers are summarized here.

An innovative procedure using a sublimable solid to align lipid bilayers for solid-state NMR studies

Uniaxially aligned phospholipid bilayers are often used as model membranes to obtain structural details of membrane-associated molecules, such as peptides, proteins, drugs, and cholesterol. Well-aligned bilayer samples can be difficult to prepare and no universal procedure has been reported that orients all combinations of membrane-embedded components. In this study, a new method for producing mechanically aligned phospholipid bilayer samples using naphthalene, a sublimable solid, was developed. Using (31)P-NMR spectroscopy, comparison of a conventional method of preparing mechanically aligned samples with the new naphthalene procedure found that the use of naphthalene significantly enhanced the alignment of 3:1 1-palmitoyl-2-oleoyl-phosphatidylethanolamine to 1-palmitoyl-2-oleoyl-phosphatidylglycerol. The utility of the naphthalene procedure is also demonstrated on bilayers of many different compositions, including bilayers containing peptides such as pardaxin and gramicidin. These results show that the naphthalene procedure is a generally applicable method for producing mechanically aligned samples for use in NMR spectroscopy. The increase in bilayer alignment implies that this procedure will improve the sensitivity of solid-state NMR experiments, in particular those techniques that detect low-sensitivity nuclei, such as 15N and 13C.

Characterization of the thermotropic behavior and lateral organization of lipid-peptide mixtures by a combined experimental and theoretical approach: effects of hydrophobic mismatch and role of flanking residues

A combined experimental and theoretical study was performed on a series of mixtures of dipalmitoylphosphatidylcholine (DPPC) and synthetic peptides to investigate their thermotropic behavior and lateral organization. The experimental study was based on differential scanning calorimetry (DSC) and phosphorous nuclear magnetic resonance ((31)P-NMR) techniques; the theoretical study was based on calculations on a microscopic molecular interaction model, where the lipid-peptide interaction is built on the hydrophobic matching principle. The chosen peptides, WALP and KALP, consist of a hydrophobic stretch, of variable length, of alternating leucine and alanine residues, flanked on both ends with tryptophan and lysine residues, respectively. By systematically varying the peptide hydrophobic length it was thus possible to explore different matching conditions between the peptide's hydrophobic length and the lipid bilayer hydrophobic thickness, and to investigate the potential role of flanking residues. The results show that both the WALP and the KALP peptides tend to favor the liquid-crystalline (or fluid) phase of the system; i.e., they tend to depress the main-transition temperature, T(m), of pure DPPC. However, the detailed effects of both peptides on the lateral phase behavior of the lipid-peptide system are dependent on the peptide length and the type of flanking residues. The results suggest that below T(m), the shortest among the WALP and KALP peptides induce gel-fluid phase separation in the system within an extensive temperature-composition region. The longer the hydrophobic length of the peptides is, the more narrow this region appears to become.