Comparative effects of melittin and its hydrophobic and hydrophilic fragments on bilayer organization by Raman spectroscopy

The impact of transmembrane peptides on lipid bilayer structure and mechanics: A study of the transmembrane domain of the influenza A virus M2 protein

Transmembrane peptides play important roles in many biological processes by interacting with lipid membranes. This study investigates how the transmembrane domain of the influenza A virus M2 protein, M2TM, affects the structure and mechanics of model lipid bilayers. Atomic force microscopy (AFM) imaging revealed small decreases in bilayer thickness with increasing peptide concentrations. AFM-based force spectroscopy experiments complemented by theoretical model analysis demonstrated significant decreases in bilayer's Young's modulus (E) and lateral area compressibility modulus (KA). This suggests that M2TM disrupts the cohesive interactions between neighboring lipid molecules, leading to a decrease in both the bilayer's resistance to indentation (E) and its ability to resist lateral compression/expansion (KA). The large decreases in bilayer elastic parameters (i.e., E and KA) contrast with small changes in bilayer thickness, implying that bilayer mechanics are not solely dictated by bilayer thickness in the presence of transmembrane peptides. The observed significant reduction in bilayer mechanical properties suggests a softening effect on the bilayer, potentially facilitating membrane curvature generation, a crucial step for M2-mediated viral budding. In parallel, our Raman spectroscopy revealed small but statistically significant changes in hydrocarbon chain vibrational dynamics, indicative of minor disordering in lipid chain conformation. Our findings provide useful insights into the complex interplay between transmembrane peptides and lipid bilayers, highlighting the significance of peptide-lipid interactions in modulating membrane structure, mechanics, and molecular dynamics.

Raman Microscopy Investigation of GLP-1 Peptide Association with Supported Phospholipid Bilayers

A wide range of important biological processes occur at phospholipid membranes including cell signaling, where a peptide or small molecule targets a membrane-localized receptor protein. In this work, we report the adaptation of confocal Raman microscopy to quantify populations of unlabeled glucagon-like peptide-1 (GLP-1), a membrane-active 30-residue incretin peptide, in supported phospholipid bilayers deposited on the interior surfaces of wide-pore porous silica particles. Quantification of lipid bilayer-associated peptide is achieved by measuring the Raman scattering intensity of the peptide relative to that of the supported lipid bilayer, which serves as an internal standard. The dependence of the bilayer-associated GLP-1 population on the solution concentration of GLP-1 produces an isotherm used to determine the equilibrium constant for peptide-bilayer association and the maximum peptide surface coverage. The maximum coverage of GLP-1 in the lipid bilayer was found to be only 1/5th of a full monolayer based on its hydrodynamic radius. The saturation coverage, therefore, is not limited by the size of GLP-1 but by the ability of the bilayer to accommodate the peptide at high concentrations within the bilayer. Raman spectra show that GLP-1 association with the supported bilayer is accompanied by structural changes consistent with the intercalation of the peptide into the bilayer, where the observed increase in acyl-chain order would increase the lipid density and provide free volume needed to accommodate the peptide. These results were compared with previous measurements of the association of fluorescently labeled GLP-1 with a planar-supported bilayer; the unlabeled peptide exhibits a 3-fold greater affinity for the lipid bilayer on the porous silica support, suggesting that the fluorescent label alters the GLP-1 lipid bilayer association.

Structural changes in the model of the outer cell membrane of Gram-negative bacteria interacting with melittin: an in situ spectroelectrochemical study

The cell membrane of Gram-negative bacteria interacting with an antimicrobial peptide presents a complex supramolecular assembly. Fabrication of models of bacterial cell membranes remains a large experimental challenge. Langmuir-Blodgett and Langmuir-Schaefer (LS-LB) transfer makes possible the deposition of multicomponent asymmetric lipid bilayers onto a gold surface. Two lipids: 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) and di[3-deoxy-D-manno-octulosonyl]-lipid A (KLA) were used to deposit a model of the outer membrane of Gram-negative bacteria on the Au(111) substrate. The use of gold as the solid substrate enables control of the membrane potential. Molecular scale changes in the model membrane exposed to physiological electric fields and interacting with melittin antimicrobial peptide are discussed in this paper. The interaction of the outer membrane with melittin leads to an increase in the membrane capacitance and permeability to ions and water. The stability of the outer membrane with bound melittin decreases at positive membrane potentials. In situ polarization modulation infrared reflection absorption spectroscopy is used to investigate membrane potential-dependent changes in the structure of the outer membrane interacting with melittin. The hydration of the ester carbonyl groups is not affected by the interaction with melittin. However, the orientation and hydrogen bond network with the carboxylate groups in KLA changes drastically after POPE-KLA bilayer interacts with melittin. We propose that the positively charged groups in the amino acids present at the C-terminus of the peptide interact directly with the polar head group of KLA. Simultaneously, the packing order in hydrocarbon chains in the membrane with bound melittin increases. A hydrophobic match between the chains in the lipids and the peptide, which spans the membrane, seems to be responsible for the ordering of the hydrocarbon chains region of the bilayer. The N-terminus enters into the hydrophobic region of the membrane and forms a channel to the hydrophilic head groups in POPE.

Membrane Disruption Mechanism of a Prion Peptide (106-126) Investigated by Atomic Force Microscopy, Raman and Electron Paramagnetic Resonance Spectroscopy

A fragment of the human prion protein spanning residues 106-126 (PrP106-126) recapitulates many essential properties of the disease-causing protein such as amyloidogenicity and cytotoxicity. PrP106-126 has an amphipathic characteristic that resembles many antimicrobial peptides (AMPs). Therefore, the toxic effect of PrP106-126 could arise from a direct association of monomeric peptides with the membrane matrix. Several experimental approaches are employed to scrutinize the impacts of monomeric PrP106-126 on model lipid membranes. Porous defects in planar bilayers are observed by using solution atomic force microscopy. Adding cholesterol does not impede defect formation. A force spectroscopy experiment shows that PrP106-126 reduces Young's modulus of planar lipid bilayers. We use Raman microspectroscopy to study the effect of PrP106-126 on lipid atomic vibrational dynamics. For phosphatidylcholine lipids, PrP106-126 disorders the intrachain conformation, while the interchain interaction is not altered; for phosphatidylethanolamine lipids, PrP106-126 increases the interchain interaction, while the intrachain conformational order remains similar. We explain the observed differences by considering different modes of peptide insertion. Finally, electron paramagnetic resonance spectroscopy shows that PrP106-126 progressively decreases the orientational order of lipid acyl chains in magnetically aligned bicelles. Together, our experimental data support the proposition that monomeric PrP106-126 can disrupt lipid membranes by using similar mechanisms found in AMPs.

Raman spectroscopy of model membrane monolayers of dipalmitoylphosphatidic acid at the air-water interface using surface enhancement from buoyant thin silver films

A novel method for the acquisition of surface enhanced Raman (SER) spectra of model membranes of dipalmitoylphosphatidic acid (DPPA) in Langmuir layers at the air-water interface is reported. The approach is based on the electrochemical formation of a buoyant thin layer of coalesced silver colloids in the vicinity of the phosphatidic acid head groups at the interface. This Ag layer is an excellent platform for SER scattering, which shows the spectral features from all parts of the molecule and water between the Ag surface and the DPPA layer. The observation of the spectral response from the phosphatidic acid head groups is of particular significance, allowing insight into their chemical state and orientation at the air-water interface.

The differential miscibility of lipids as the basis for the formation of functional membrane rafts

The formation of sphingolipid-cholesterol microdomains in cellular membranes has been proposed to function in sorting and transport of lipids and proteins as well as in signal transduction. An increasing number of cell biological and biochemical studies now supports this concept. Here we discuss the structural properties of lipids in a cell biological context. The sphingolipid-cholesterol microdomains or rafts are described as dispersed liquid ordered phase domains. These domains are dynamic assemblies to which specific proteins are selectively sequestered while others are excluded. The proteins associated to rafts can act as organizers and can modulate raft size and function.

Synergism between mellitin and phospholipase A2 from bee venom: apparent activation by intervesicle exchange of phospholipids

Mellitin, a cationic amphiphilic peptide, has an apparent activating effect on interfacial catalysis by phospholipase A2 (PLA2) of bee venom on zwitterionic vesicles of 1-palmitoyl-2-oleoylglycero-sn-3-phosphocholine (POPC) and on anionic vesicles of 1,2-dimyristoylglycero-sn-3-phosphomethanol (DMPM), as well as on covesicles of POPC/DMPM (3:7). On the other hand, mellitin-induced increase in the rate of pig pancreatic PLA2 is seen only on anionic vesicles. Interfacial kinetic protocols and spectroscopic methods show that the activation is due to enhanced substrate replenishment resulting from intervesicle exchange of zwitterionic or anionic phospholipids through vesicle-vesicle contacts established by mellitin. It is shown that as the hydrolysis on POPC vesicles progresses, due to a high propensity of bee PLA2 for binding to the product containing zwitterionic vesicles, most of the enzyme in the reaction mixture is trapped on few vesicles that are initially hydrolyzed, and thus reaction ceases. Under these conditions, mellitin promotes substrate replenishment by direct exchange of the products of hydrolysis from the enzyme-containing vesicles with the substrate present in excess vesicles which have not been hydrolyzed. Pig PLA2 has poor affinity for POPC vesicles, and the affinity is only modestly higher in the presence of low mole fractions of the products of hydrolysis; therefore, the enzyme is not trapped on those vesicles. Biophysical studies confirm that the phospholipid exchange occurs through stable intervesicle contacts formed by low mole fractions of mellitin, without transbilayer movement of phospholipids or fusion of vesicles. At high mole fraction (> 1.5%) mellitin induces leakage in POPC vesicles and does not form additional contacts. In POPC/DMPM vesicles, the contacts are formed even at high mole fractions of mellitin. Changes in intrinsic tryptophan fluorescence of mellitin indicate that bound mellitin exists in at least two different functional forms depending on the lipid composition and on the lipid:peptide ratio. A model is proposed to accommodate amphiphilic mellitin as a transmembrane channel or an intervesicle contact.

Cell-lytic and antibacterial peptides that act by perturbing the barrier function of membranes: facets of their conformational features, structure-function correlations and membrane-perturbing abilities

Almost all hemolytic and antimicrobial peptides form part of the defense mechanism of species widely distributed across the evolutionary scale. Although these peptides are of varying lengths and composition, they form amphiphilic structures in a hydrophobic environment. They also have the ability to form channels in natural and model membranes. Hemolytic peptides have proven to be very useful in studying the mechanism of hemolysis and the permeability properties of red blood cells. Preliminary investigations indicate that these peptides may also be useful in the investigation of complex cellular phenomena like exocytosis and neurotransmission. Although molecules like vancomycin, bacitracin and penicillins have been extensively used as antibiotics for therapeutic purposes, most species throughout the evolutionary scale use peptides as antimicrobial agents. These peptides exert their activity by altering the permeability properties of the bacterial plasma membrane and do not interfere with macro molecular synthesis like the other antibiotics that are presently used in therapies. Hence it is likely that resistance to peptide antibacterial agents may not develop easily. Since the problem of antibiotic resistance is presently a particularly severe one, peptide antibiotics may be the drugs of choice in the future.

Fluorescence-quenching-resolved spectra of melittin in lipid bilayers

The interaction of bee venom melittin with dimyristoylphosphatidylcholine (DMPC) unilamellar vesicles has been studied by means of fluorescence quenching of the single tryptophan residue of the protein, at lipid-to-peptide ratio, Ri = 50 and at high ionic strength (2 M NaCl). The method of fluorescence-quenching-resolved spectra (FQRS), applied in this study with potassium iodide as a quencher, enabled us to decompose the tryptophan emission spectrum of liposome-bound melittin into components, at temperatures above as well as below the main phase transition temperature (Tt) of DMPC. One of the two resolved spectra exhibits maximum at 342 and 338 nm for experiments above and below Tt, respectively, and is similar to the maximum of tryptophan emission found for tetrameric melittin in solution (340 nm). This spectrum is characterized by the Stern-Volmer quenching constant, Ksv, of about 4 M-1 and it represents the fraction of melittin molecules whose tryptophan residues are exposed to the solvent to a degree comparable with tetrameric species in solution. The other spectrum component, corresponding to the quencher-inaccessible fraction of tryptophan molecules (Ksv = 0 M-1) has its maximum blue-shifted up to 15 nm, indicating a decrease in polarity of the environment. For experiments above Tt, the blue spectrum component revealed the excitation-wavelength dependence, originating probably from the relaxation processes between the excited tryptophan molecules and lipid polar head groups. We conclude that melittin bound to DMPC liposomes exists in two lipid-associated forms; one, with tryptophan residues exposed to the solvent and the other, penetrating the membrane interior, with tryptophan residues located in close proximity to the phospholipid polar head groups of the outer vesicle lipid layer. We also discuss our data with current models of melittin-bilayer interactions.

Attenuated total reflectance Fourier transform infrared studies of the interaction of melittin, two fragments of melittin, and delta-hemolysin with phosphatidylcholines

Attenuated total reflectance Fourier transform infrared spectroscopy (ATR FT-IR) has been used to monitor alterations in phospholipid organization in thin layers of 1,2-dipalmitoylphosphatidylcholine (DPPC) and 1-palmitoyl-2-oleoylphosphatidylcholine (POPC), induced by the membrane lytic peptide melittin, its fragments 1-15 (hydrophobic fragment) and 16-26 (hydrophilic fragment), and delta-hemolysin. In addition, the secondary structures of the peptides and the orientation of helical fragments were determined with respect to the bilayer. The insertion of melittin into POPC caused large perturbations in the order and increased rates of motion of the acyl chains, as monitored by the frequency and half-width of the symmetric CH2 stretching vibration near 2850 cm-1, as well as by the ATR dichroic ratio for this mode. Changes in DPPC organization were less and were consistent with peptide-induced static disordering (gauche rotamer formation) in the acyl chains. Melittin adopted primarily an alpha-helical secondary structure, although varying small proportions of beta and/or aggregated forms were noted. The helical segments were preferentially oriented perpendicular to the bilayer plane. Several modes of melittin/lipid interaction were considered in an attempt to semiquantitatively understand the observed dichroic ratios. By considering the peptide as a bent rigid rod, a plausible model for its lytic properties has been developed. The hydrophilic fragment in DPPC showed a secondary structure with little alpha-helix present. As judged by its effect on phospholipid acyl chain organizational parameters, the fragment did not penetrate the bilayer substantially. The hydrophobic fragment in DPPC gave amide I spectral patterns consistent with a mixture of predominantly beta-antiparallel pleated sheet with a smaller fraction of alpha-helix.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of bee venom melittin on the order and dynamics of dimyristoylphosphatidylcholine unilamellar and multilamellar vesicles

The effects of bee venom melittin on the order and dynamics of dimyristoylphosphatidylcholine unilamellar and multilamellar vesicles at a protein-to-lipid molar ratio of 1:60 have been investigated by employing the techniques of nanosecond emission anisotropy with 1,6-diphenyl-1,3,5-hexatriene as the fluorescent probe, enhancement by polar groups of the weakly allowed 0-0 vibronic transition in the fluorescence spectrum of pyrene, and Raman spectroscopy. The emission anisotropy results, which are found to be consistent with the wobble-in-cone model, show that the protein induces an increase in the order parameter, S, of the acyl chains of unilamellar vesicles below, at, and above their phase transition temperature, Tt, and it decreases strongly the diffusion rate, Dw, only below Tt. On the other hand, for multilamellar vesicles, the protein induces a decrease in S only at Tt and does not affect Dw. These effects are consistent with the observed changes in the degree of enhancement of the 0-0 vibronic transition of pyrene. Moreover, the protein broadens the thermal transition profile of multilamellar vesicles but sharpens dramatically that of unilamellar vesicles and fuses them without changing significantly the Tt in either case. On the other hand, the Raman data detect a decrease in the inter- and intramolecular order of the acyl chains of multilamellar vesicles below Tt and a decrease of only the former Tt. This disparity between the Raman and the nanosecond emission anisotropy data is discussed in terms of differences in the time scales of the two techniques and in the state of aggregation of the lipid-bound melittin. The data for the enhancement of the 0-0 vibronic transition of pyrene suggest that, for a melittin-to-lipid ratio of 1:60, the size or structure of channels formed in the bilayer by melittin does not allow the penetration of a neutral molecule the size of pyrene deeply into the bilayer.

Signal recognition particle causes a transient arrest in the biosynthesis of prepromelittin and mediates its translocation across mammalian endoplasmic reticulum

The translocation of prepromelittin (pPM) across mammalian endoplasmic reticulum was studied in both wheat germ and reticulocyte lysate. In the wheat germ system, signal recognition particle (SRP) caused a transient arrest in the synthesis of pPM. This was indicated by a slowdown in the rate of synthesis of pPM in the presence of SRP. The arrest was specific, dependent on the concentration of SRP, and more effective at early incubation time. In a tightly synchronized translation system, SRP had no apparent effect on the elongation of pPM, indicating that the effect of SRP on pPM chain synthesis might be at the final stages of chain elongation and release from the ribosome. This was reflected in a transient accumulation of pPM as peptidyl tRNA. Because pPM is composed of only 70 amino acids, arrest by SRP may be very close to chain termination. Arrest at this stage of chain synthesis seems to be unstable and the nascent chain gets terminated and released from the ribosome after a transient delay. The translocation of pPM was shown to be dependent on both SRP and docking protein. The difference in the translocation efficiency of pPM in reticulocyte and wheat germ lysates may reflect a difference in the targeting process in the two systems.

Role of peptide structure in lipid-peptide interactions: high-sensitivity differential scanning calorimetry and electron spin resonance studies of the structural properties of dimyristoylphosphatidylcholine membranes interacting with pentagastrin-related pentapeptides

The effects of amino acid substitutions in the pentapeptide pentagastrin on the nature of its interactions with dimyristoylphosphatidylcholine (DMPC) are assessed by differential scanning calorimetry and electron spin resonance. In two peptide analogues, the Asp at position 4 in pentagastrin (N-t-Boc-beta-Ala-Trp-Met-Asp-Phe-NH2) is replaced by Gly or Phe. These uncharged, more hydrophobic peptides have little effect on the transition temperature of DMPC, but they broaden the transition and lower the transition enthalpy as do integral membrane proteins. These peptides also mimic the behavior of integral membrane proteins in decreasing the order of a 5-doxylstearic acid spin probe below the transition temperature and in exhibiting a second immobilized lipid component using a 16-doxylstearic acid spin probe in DMPC. Three charged peptides were studied: pentagastrin, an analogue with positions 4 and 5 reversed (i.e., ending in Phe-Asp-NH2), and one with Asp replaced by Arg at position 4. All three of these charged peptides altered the phase transition behavior of DMPC to give two components, one above and one below the transition temperature of the pure lipid. With increasing peptide concentration, the higher melting transition became more prominent. The arginine-containing peptide produced the largest shifts in melting temperature followed by pentagastrin and then the "reversed" peptide. The arginine-containing peptide also increased the enthalpy of the transition. These peptides also increased the ordering of DMPC below the phase transition as measured with both 5- and 16-doxylstearic acid. The ordering effect was most pronounced with the arginine-containing peptide using the 5-doxylstearic acid probe. The results demonstrate that even the zwitterionic DMPC can interact more strongly with positively charged peptides than with negatively charged ones. In addition, peptide sequence as well as composition is important in determining the nature of peptide-lipid interactions. The markedly different effects of these pentagastrin peptides on the phase transition and motional properties of DMPC occur despite the similar depth of burial of these peptides with DMPC.

A restatement of melittin-induced effects on the thermotropism of zwitterionic phospholipids

Perturbations induced by melittin on the thermotropism of dimyristoyl-, dipalmitoyl-, distearoylphosphatidylcholine and natural sphingomyelin are investigated and rationalized from data obtained by fluorescence polarization, differential scanning calorimetry and Raman spectroscopy. Depending on the technique and/or experimental conditions used, the observed effects differ at the same lipid to protein molar ratio, due to partial binding of melittin. The binding is more efficient for tetrameric than for monomeric melittin, but in both cases its affinity is weaker for phosphatidylcholine dispersions in the gel phase than for sonicated vesicles. For temperatures T greater than or equal to Tm efficient binding occurs whatever the initial state of the lipids is. One can summarize the effects induced by melittin on the transition temperature as follows: No upward shift is observed on synthetic phosphatidylcholines when lipid degradation is avoided. This is achieved by using highly purified melittin, phospholipase inhibitors, and/or non-hydrolysable lipids. Melittin monomer does not change Tm. When melittin tetramer is stabilized, it decreases Tm by 10-15 deg. C. The transition broadens, and is finally abolished for Ri less than or equal to 2. Very similar results are found for natural sphingomyelin. Fluorescence polarization indicates similar changes in order and dynamics of the acyl chains for all lipid studied. For T less than or equal to Tm, fluorescence and Raman show that melittin decreases the amount of CH2 groups in 'trans' conformation and the intermolecular order of the chains. According to fluorescence data, there is an increase of the rigid-body orientational order at T greater than or equal to Tm, while from Raman the positional intermolecular order decreases without significant change in the CH2 groups 'trans'/'gauche' ratio.

Studies of the kinetics of Na+ gradient-coupled glucose transport as found in brush-border membrane vesicles from rabbit jejunum

The Na+-dependent D-glucose transport reaction in rabbit jejunal brush-border vesicles was studied. Initial rate data were obtained by fitting a polynomial equation to progress curves at different D-glucose concentrations and extracting the slope of the tangent at zero-time. Kinetic replots of the initial rate values produced biphasic Hofstee patterns indicative of two pathways for transport distinguished by their Km values for glucose. Neither was dependent on the presence of a membrane potential. Both were dependent on Na+ and both were inhibited by phlorizin. Increasing external sodium was found to elevate the apparent Vmax for both pathways. Internal sodium was inhibitory. Pulsed progress curve analysis indicated that the effect of internal sodium was best characterized as carrier sequestration by a sodium-carrier binary complex. Inhibition by internal sodium was completely reversed by the presence, internally, of D-glucose. The presence of two pathways and the kinetic constants for these pathways do not agree with the conclusions of Hopfer and Groseclose (1980) J. Biol. Chem. 255, 4453-4462). Experiments are presented which bear on the reason for the disagreement.