Interactions of membrane active peptides with planar supported bilayers: an impedance spectroscopy study

Membrane active peptides exert their biological effects by interacting directly with a cell's lipid bilayer membrane. These therapeutically promising peptides have demonstrated a variety of activities including antimicrobial, cytolytic, membrane translocating, and cell penetrating activities. Here, we use electrochemical impedance spectroscopy (EIS) on polymer-cushioned supported lipid bilayers constructed on single crystal silicon to study two pairs of closely related membrane active peptides selected from rationally designed, combinatorial libraries to have different activities in lipid bilayers: translocation, permeabilization, or no activity. Using EIS, we observed that binding of a membrane translocating peptide to the lipid bilayer resulted in a small decrease in membrane resistance followed by a recovery back to the original value. The recovery may be directly attributable to peptide translocation. A nontranslocating peptide did not decrease the resistance. The other pair, two membrane permeabilizing peptides, caused an exponential decrease of membrane resistance in a concentration-dependent manner. This permeabilization of the supported bilayer occurs at peptide to lipid ratios as much as 1000-fold lower than that needed to observe effects in vesicle leakage assays and gives new insights into the fundamental peptide-bilayer interactions involved in membrane permeabilization.     

Activation, exposure and penetration of virally encoded, membrane-active polypeptides during non-enveloped virus entry

Host cell entry by influenza and other enveloped viruses is well characterized, however, the manner in which non-enveloped viruses deliver their genome across host cell membranes in the absence of membrane fusion remains unresolved. The discovery of short, membrane altering, amphipathic or hydrophobic sequences in several non-enveloped virus capsid proteins such as the gamma (gamma) peptide of nodaviruses and tetraviruses, VP4 and the N-terminal region of VP1 of picornaviruses, micro1N of reoviruses, and protein VI of adenoviruses suggests that these small peptides facilitate breaching of the host membrane and the delivery of the viral genome into the host cell. In spite of conspicuous differences in entry among non-enveloped virions, the short stretches of membrane active regions are associated with similar, entry-related events including: I) proteolytic cleavage of a precursor capsid protein resulting in increased dynamic character and/or accessibility of these peptides; II) structural changes in the virus capsid triggered by receptor binding and/or low pH in entry compartments, resulting in peptide exposure; III) externalized peptides interact with host membranes and disrupt them, facilitating delivery of the viral genome inside the host cell. Here we discuss the membrane alteration activity in non-enveloped viruses with reference to the gamma peptide of nodaviruses. Virtually all of the characteristics of gamma are shared by analogous peptides in other non-enveloped viruses, making it a simple prototype for comparative purposes.     

How far can a sodium ion travel within a lipid bilayer?

Analogues of a synthetic ion channel made from a helical peptide were used to study the mechanism of cation translocation within bilayer membranes. Derivatives bearing two, three, four, and six crown ethers used as ion relays were synthesized, and their transport abilities across lipid bilayers were measured. The results showed that the maximum distance a sodium ion is permitted to travel between two binding sites within a lipid bilayer environment is 11 ?.     

Unraveling dendrimer translocation across cell membrane mimics

Poly(amidoamine) (PAMAM) dendrimers are promising candidates in several applications within the medical field. However, it is still to date not fully understood whether they are able to passively translocate across lipid bilayers. Recently, we used fluorescence microscopy to show that PAMAM dendrimers induced changes in the permeability of lipid membranes but the dendrimers themselves could not translocate to be released into the vesicle lumen. Because of the lack of resolution, these experiments could not assess whether the dendrimers were able to translocate but remained attached to the membrane. Using quartz crystal microbalance with dissipation monitoring and neutron reflectivity, a structural investigation was performed to determine how dendrimers interact with zwitterionic and negatively charged lipid bilayers. We hereby show that dendrimers adsorb on top of lipid bilayers without significant dendrimer translocation, regardless of the lipid membrane surface charge. Thus, most likely dendrimers are actively transported through cell membranes by protein-mediated endocytosis in agreement with previous cell studies. Finally, the higher activity of PAMAM dendrimers for phosphoglycerol-containing membranes is in line with their high antimicrobial activity against Gram-negative bacteria.     

Phase behavior and the partitioning of caveolin-1 scaffolding domain peptides in model lipid bilayers

The membrane binding and model lipid raft interaction of synthetic peptides derived from the caveolin scaffolding domain (CSD) of the protein caveolin-1 have been investigated. CSD peptides bind preferentially to liquid-disordered domains in model lipid bilayers composed of cholesterol and an equimolar ratio of dioleoylphosphatidylcholine (DOPC) and brain sphingomyelin. Three caveolin-1 peptides were studied: the scaffolding domain (residues 83-101), a water-insoluble construct containing residues 89-101, and a water-soluble construct containing residues 89-101. Confocal and fluorescence microscopy investigation shows that the caveolin-1 peptides bind to the more fluid cholesterol-poor phase. The binding of the water-soluble peptide to lipid bilayers was measured using fluorescence correlation spectroscopy (FCS). We measured molar partition coefficients of 10(4) M(-1) between the soluble peptide and phase-separated lipid bilayers and 10(3) M(-1) between the soluble peptide and bilayers with a single liquid phase. Partial phase diagrams for our phase-separating lipid mixture with added caveolin-1 peptides were measured using fluorescence microscopy. The water-soluble peptide did not change the phase morphology or the miscibility transition in giant unilamellar vesicles (GUVs); however, the water-insoluble and full-length CSD peptides lowered the liquid-liquid melting temperature.     

Creation of lipid partitions by deposition of amphipathic viral peptides

Phospholipid vesicles exhibit a natural characteristic to fuse and reform into a continuous single bilayer membrane on hydrophilic solid substrates such as glass, mica, and silica. The resulting solid-supported bilayer mimics physiological tendencies such as lipid flip-flop and lateral mobility. The lateral mobility of fluorescently labeled lipids fused into solid-supported bilayers is found to change upon deposition on the membrane surface of an amphipathic alpha-helical peptide (AH) derived from the hepatitis C virus (HCV) NS5A protein. The binding of the AH peptide to a phospholipid bilayer, with the helical axis parallel to the bilayer, leads to immobilization of the bilayer. We used AFM to better understand the mechanistic details of this specific interaction, and determined that the diminished fluidity of the bilayer is due to membrane thinning. Utilizing this specific interaction between AH peptides and lipid molecules, we demonstrate a novel process for the creation of lipid partition by employing AH peptides as agents to immobilize lipid molecules, thus creating a patterned solid support with partition-defined areas of freely mobile lipid bilayers. This architecture could have a wide range of applications in novel sensing, biotechnology, high-throughput screening, and biomimetic strategies.     

分子自组装脂类微管的螺旋带特征分析

利用联乙炔基甘油磷脂酰胆碱分子的自组装特性,制备得到脂类微管结构,并在大量观察基础上,对脂类微管的螺旋带特征进行分析归纳．观察表明脂类微管是由螺旋带紧密缠绕形成的稳定结构,具有明显的螺旋缠绕特征;同时体系中存在少量不同形态的松驰螺旋带．体系中存在有单层脂膜包埋的螺旋带,这是一种新的结构形态．螺旋带边缘有明显的错位和分层现象,端部具有不同于其它区域的松驰现象．脂类微管的这些特征对其表面纳米颗粒的沉积产生很大的影响,金属钯和镍纳米颗粒在螺旋带边缘的分层沉积可清楚地显示脂类微管螺旋带的错位和分层特征．金属钯的分层沉积特征可尝试用于标志自组装膜的脂类双层数;同时,自组装脂类螺旋带的研究可用于揭示脂类分子自组装的本质,并在生物矿化和生物膜力学研究方面有重要意义．     

Fusogenic tilted peptides induce nanoscale holes in supported phosphatidylcholine bilayers

Tilted peptides are known to insert in lipid bilayers with an oblique orientation, thereby destabilizing membranes and facilitating membrane fusion processes. Here, we report the first direct visualization of the interaction of tilted peptides with lipid membranes using in situ atomic force microscopy (AFM) imaging. Phase-separated supported dioleoylphosphatidylcholine/dipalmitoylphosphatidylcholine (DOPC/DPPC) bilayers were prepared by fusion of small unilamellar vesicles and imaged in buffer solution, in the absence and in the presence of the simian immunodeficiency virus (SIV) peptide. The SIV peptide was shown to induce the rapid appearance of nanometer scale bilayer holes within the DPPC gel domains, while keeping the domain shape unaltered. We attribute this behavior to a local weakening and destabilization of the DPPC domains due to the oblique insertion of the peptide molecules. These results were directly correlated with the fusogenic activity of the peptide as determined using fluorescently labeled DOPC/DPPC liposomes. By contrast, the nontilted ApoE peptide did not promote liposome fusion and did not induce bilayer holes but caused slight erosion of the DPPC domains. In conclusion, this work provides the first direct evidence for the production of stable, well-defined nanoholes in lipid bilayer domains by the SIV peptide, a behavior that we have shown to be specifically related to the tilted character of the peptide. A molecular mechanism underlying spontaneous insertion of the SIV peptide within lipid bilayers and the subsequent removal of bilayer patches is proposed, and its relevance to membrane fusion processes is discussed.     

Molecular dynamics simulations of the helical antimicrobial peptide ovispirin-1 in a zwitterionic dodecylphosphocholine micelle: insights into host-cell toxicity

We have carried out a 40-ns all-atom molecular dynamics simulation of the helical antimicrobial peptide ovispirin-1 (OVIS) in a zwitterionic diphosphocholine (DPC) micelle. The DPC micelle serves as an economical and effective model for a cellular membrane owing to the presence of a choline headgroup, which resembles those of membrane phospholipids. OVIS, which was initially placed along a micelle diameter, diffuses out to the water-DPC interface, and the simulation stabilizes to an interface-bound steady state in 40 ns. The helical content of the peptide marginally increases in the process. The final conformation, orientation, and the structure of OVIS are in excellent agreement with the experimentally observed properties of the peptide in the presence of lipid bilayers composed of 75% zwitterionic lipids. The amphipathic peptide binds to the micelle with its hydrophobic face buried in the micellar core and the polar side chains protruding into the aqueous phase. There is overwhelming evidence that points to the significant and indispensable participation of hydrophobic residues in binding to the zwitterionic interface. The simulation starts with a conformation that is unbiased toward the final experimentally known binding state of the peptide. The ability of the model to reproduce experimental binding states despite this starting conformation is encouraging.     

Directional Potassium Transport through a Unimolecular Peptide Channel

Three unimolecular peptide channels have been designed and prepared by using the β‐helical conformation of gramicidin A (gA). The new peptides bear one to three NH₃⁺ groups at the N‐end and one to three CO₂^? groups at the C‐end. These zwitterionic peptides were inserted into lipid bilayers in an orientation‐selective manner. Conductance experiments on planar lipid bilayers showed that this orientation bias could lead to observable directional K⁺ transport under multi‐channel conditions. This directional transport behavior can further cause the generation of a current across a planar bilayer without applying a voltage. More importantly, in vesicles with identical external and internal KCl concentrations, the channels can pump K⁺ across the lipid bilayer and cause a membrane potential.     

Determining the topology of integral membrane peptides using EPR spectroscopy

This paper reports on the development of a new structural biology technique for determining the membrane topology of an integral membrane protein inserted into magnetically aligned phospholipid bilayers (bicelles) using EPR spectroscopy. The nitroxide spin probe, 2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid (TOAC), was attached to the pore-lining transmembrane domain (M2delta) of the nicotinic acetylcholine receptor (AChR) and incorporated into a bicelle. The corresponding EPR spectra revealed hyperfine splittings that were highly dependent on the macroscopic orientation of the bicelles with respect to the static magnetic field. The helical tilt of the peptide can be easily calculated using the hyperfine splittings gleaned from the orientational dependent EPR spectra. A helical tilt of 14 degrees was calculated for the M2delta peptide with respect to the bilayer normal of the membrane, which agrees well with previous 15N solid-state NMR studies. The helical tilt of the peptide was verified by simulating the corresponding EPR spectra using the standardized MOMD approach. This new method is advantageous because: (1) bicelle samples are easy to prepare, (2) the helical tilt can be directly calculated from the orientational-dependent hyperfine splitting in the EPR spectra, and (3) EPR spectroscopy is approximately 1000-fold more sensitive than 15N solid-state NMR spectroscopy; thus, the helical tilt of an integral membrane peptide can be determined with only 100 microg of peptide. The helical tilt can be determined more accurately by placing TOAC spin labels at several positions with this technique.     

Asymmetric insertion of membrane proteins in lipid bilayers by solid-state NMR paramagnetic relaxation enhancement: a cell-penetrating Peptide example

A novel solid-state NMR technique for identifying the asymmetric insertion depths of membrane proteins in lipid bilayers is introduced. By applying Mn (2+) ions on the outer but not the inner leaflet of lipid bilayers, the sidedness of protein residues in the lipid bilayer can be determined through paramagnetic relaxation enhancement (PRE) effects. Protein-free lipid membranes with one-side Mn (2+)-bound surfaces exhibit significant residual (31)P and lipid headgroup (13)C intensities, in contrast to two-side Mn (2+)-bound membranes, where lipid headgroup signals are mostly suppressed. Applying this method to a cell-penetrating peptide, penetratin, we found that at low peptide concentrations, penetratin is distributed in both leaflets of the bilayer, in contrast to the prediction of the electroporation model, which predicts that penetratin binds to only the outer lipid leaflet at low peptide concentrations to cause an electric field that drives subsequent peptide translocation. The invalidation of the electroporation model suggests an alternative mechanism for intracellular import of penetratin, which may involve guanidinium-phosphate complexation between the peptide and the lipids.     

Phosphate-mediated arginine insertion into lipid membranes and pore formation by a cationic membrane peptide from solid-state NMR

The insertion of charged amino acid residues into the hydrophobic part of lipid bilayers is energetically unfavorable yet found in many cationic membrane peptides and protein domains. To understand the mechanism of this translocation, we measured the (13)C-(31)P distances for an Arg-rich beta-hairpin antimicrobial peptide, PG-1, in the lipid membrane using solid-state NMR. Four residues, including two Arg's, scattered through the peptide were chosen for the distance measurements. Surprisingly, all residues show short distances to the lipid (31)P: 4.0-6.5 A in anionic POPE/POPG membranes and 6.5-8.0 A in zwitterionic POPC membranes. The shortest distance of 4.0 A, found for a guanidinium Czeta at the beta-turn, suggests N-H...O-P hydrogen bond formation. Torsion angle measurements of the two Arg's quantitatively confirm that the peptide adopts a beta-hairpin conformation in the lipid bilayer, and gel-phase 1H spin diffusion from water to the peptide indicates that PG-1 remains transmembrane in the gel phase of the membrane. For this transmembrane beta-hairpin peptide to have short (13)C-(31)P distances for multiple residues in the molecule, some phosphate groups must be embedded in the hydrophobic part of the membrane, with the local (31)P plane parallel to the beta-strand. This provides direct evidence for toroidal pores, where some lipid molecules change their orientation to merge the two monolayers. We propose that the driving force for this toroidal pore formation is guanidinium-phosphate complexation, where the cationic Arg residues drag the anionic phosphate groups along as they insert into the hydrophobic part of the membrane. This phosphate-mediated translocation of guanidinium ions may underlie the activity of other Arg-rich antimocrobial peptides and may be common among cationic membrane proteins.     

Responses of Transmembrane Peptide and Lipid Chains to Hydrophobic Mismatch

Hydrophobic mismatch between the hydrophobic length of membrane proteins and hydrophobic thickness of membranes is a crucial factor in controlling protein function and assembly. We combined fluorescence with circular dichroism(CD) and attenuated total reflection infrared(ATR-IR) spectroscopic methods to investigate the behaviors of the peptide and lipids under hydrophobic mismatch using a model peptide from the fourth transmembrane domain of natural resistance-associated macrophage protein 1(Nramp1), the phosphatidylcholines(PCs) and phosphatidylglycerols(PGs) with different lengths of acyl chains(14:0, 16:0 and 18:0). In all PG lipid membranes, the peptide forms stable a-helix structure, and the helix axis is parallel to lipid chains. The helical span and orientation hardly change in varying thickness of PG membranes, while the lipid chains can deform to accommodate to the hydrophobic surface of embedded peptide. By comparison, the helical structures of the model peptide in PC lipid membranes are less stable. Upon incorporation with PC lipid membranes, the peptide can deform itself to accommodate to the hydrophobic thickness of lipid membranes in response to hydrophobic mismatch. In addition, hydrophobic mismatch can increase the aggregation propensity of the peptide in both PC and PG lipid membranes and the peptide in PC membranes has more aggregation tendency than that in PG membranes.     

Visualizing the solubilization of supported lipid bilayers by an amphiphilic peptide

The effect of the presequence peptide of cytochrome c oxidase subunit IV (p25) on supported phospholipid bilayers (SPBs) was visualized using atomic force microscopy (AFM). The presequence was found to cause the complete disruption of supported bilayers containing neutral lipids. At relatively low concentrations of presequence, the peptide was found to bind to the membrane, coalescing to form microdomains within the liquid-crystalline bilayer that were located predominantly at bilayer-mica boundaries. Further increases in peptide concentration resulted in the formation of holes within the SPB that were spanned by an interpenetrating network of narrower regions of the bilayer, which, at higher applied peptide concentrations, were observed to disappear through a budding process, ultimately leading to the formation of spherical structures at yet higher peptide concentrations. Within this paper, the impact the presequence has upon the structure and order of the membrane is discussed, as is the potential implication of this apparent solubilization process on the translocation of cytochrome c oxidase into the inner mitochondrial membrane.     

Solid-state NMR spectroscopy of aligned lipid bilayers at low temperatures

This study, for the first time, demonstrates that it is possible to mechanically align lipid bilayers at a very low temperature (as low as the gel-to-liquid crystalline phase transition temperature). Performing NMR experiments on mechanically aligned lipid bilayers at a low temperature increases the signal-to-noise ratio, the resolution, and the span of NMR parameters. The increased lifetime of the alignment and the nature of the bilayer sample would enhance the application of solid-state NMR techniques to study membrane proteins.     

Kinetics and thermodynamics of chlorpromazine interaction with lipid bilayers: effect of charge and cholesterol

Passive transport across cell membranes is the major route for the permeation of xenobiotics through tight endothelia such as the blood–brain barrier. The rate of passive permeation through lipid bilayers for a given drug is therefore a critical step in the prediction of its pharmacodynamics. We describe a detailed study on the kinetics and thermodynamics for the interaction of chlorpromazine (CPZ), an antipsychotic drug used in the treatment of schizophrenia, with neutral and negatively charged lipid bilayers. Isothermal titration calorimetry was used to study the partition and translocation of CPZ in lipid membranes composed of pure POPC, POPC:POPS (9:1), and POPC:Chol:POPS (6:3:1). The membrane charge due to the presence of POPS as well as the additional charge resulting from the introduction of CPZ in the membrane were taken into account, allowing the calculation of the intrinsic partition coefficients (K(P)) and the enthalpy change (ΔH) associated with the process. The enthalpy change upon partition to all lipid bilayers studied is negative, but a significant entropy contribution was also observed for partition to the neutral membrane. Because of the positive charge of CPZ, the presence of negatively charged lipids in the bilayer increases both the observed amount of CPZ that partitions to the membrane (KP(obs)) and the magnitude of ΔH. However, when the electrostatic effects are discounted, the intrinsic partition coefficient was smaller, indicating that the hydrophobic contribution was less significant for the negatively charged membrane. The presence of cholesterol strongly decreases the affinity of CPZ for the bilayer in terms of both the amount of CPZ that associates with the membrane and the interaction enthalpy. A quantitative characterization of the rate of CPZ translocation through membranes composed of pure POPC and POPC:POPS (9:1) was also performed using an innovative methodology developed in this work based on the kinetics of the heat evolved due to the interaction of CPZ with the membranes.     

Synthesis and physicochemical study of the laminin active sequence: SIKVAV

The synthesis, physicochemical characterization, and interaction with membrane model systems of a peptide derived from the PA22-2 region of laminin are described. Surface activity studies indicate that this peptide is able to spread at the air-water interface being the maximal spreading pressure 20 mN/m at subphase concentrations around 10 micro M. Besides, these peptide molecules are also able to form stable monolayers. Physicochemical studies concerning the interaction of this peptide with lipids, organized in mono and bilayers, were carried out using Langmuir balance experiments and polarization fluorescence techniques. The peptide penetrates better in monolayers of DPPC than in those of PC and forms condensed mixed monolayers with DPPC. Energies of mixing are small thus indicating that deviations from ideality were almost negligible. Interactions with bilayers were studied through microviscosity changes (DPH and TMA-DPH probes), membrane permeability alterations (CF, NBD-PE/dithionite), and fusion promotion (NBD-PE/Rh-PE, resonance energy transfer). Results indicate that this sequence interacts very softly with bilayers without promoting changes in their organization. These data as well as the lack of interaction with erythrocytes suggest that coating liposomes with this peptide through chemical amide bonds can render stable inmunoliposomes for further biological applications.     

Thermotropic phase transition in soluble nanoscale lipid bilayers

The role of lipid domain size and protein-lipid interfaces in the thermotropic phase transition of dipalmitoylphosphatidylcholine (DPPC) and dimyristoylphosphatidylcholine (DMPC) bilayers in Nanodiscs was studied using small-angle X-ray scattering (SAXS), differential scanning calorimetry (DSC), and generalized polarization (GP) of the lipophilic probe Laurdan. Nanodiscs are water-soluble, monodisperse, self-assembled lipid bilayers encompassed by a helical membrane scaffold protein (MSP). MSPs of different lengths were used to define the diameter of the Nanodisc lipid bilayer from 76 to 108 A and the number of DPPC molecules from 164 to 335 per discoidal structure. In Nanodiscs of all sizes, the phase transitions were broader and shifted to higher temperatures relative to those observed in vesicle preparations. The size dependences of the transition enthalpies and structural parameters of Nanodiscs reveal the presence of a boundary lipid layer in contact with the scaffold protein encircling the perimeter of the disc. The thickness of this annular layer was estimated to be approximately 15 A, or two lipid molecules. SAXS was used to measure the lateral thermal expansion of Nanodiscs, and a steep decrease of bilayer thickness during the main lipid phase transition was observed. These results provide the basis for the quantitative understanding of cooperative phase transitions in membrane bilayers in confined geometries at the nanoscale.     

Freezing point depression of water in phospholipid membranes: a solid-state NMR study

Lipid-water interaction plays an important role in the properties of lipid bilayers, cryoprotectants, and membrane-associated peptides and proteins. The temperature at which water bound to lipid bilayers freezes is lower than that of free water. Here, we report a solid-state NMR investigation on the freezing point depression of water in phospholipid bilayers in the presence and absence of cholesterol. Deuterium NMR spectra at different temperatures ranging from -75 to + 10 degrees C were obtained from fully (2)H2O-hydrated POPC (1-palmitoyl-2-oleoylphosphatidylcholine) multilamellar vesicles (MLVs), prepared with and without cholesterol, to determine the freezing temperature of water and the effect of cholesterol on the freezing temperature of water in POPC bilayers. Our 2H NMR experiments reveal the motional behavior of unfrozen water molecules in POPC bilayers even at temperatures significantly below 0 degrees C and show that the presence of cholesterol further lowered the freezing temperature of water in POPC bilayers. These results suggest that in the presence of cholesterol the fluidity and dynamics of lipid bilayers can be retained even at very low temperatures as exist in the liquid crystalline phase of the lipid. Therefore, bilayer samples prepared with a cryoprotectant like cholesterol should enable the performance of multidimensional solid-state NMR experiments to investigate the structure, dynamics, and topology of membrane proteins at a very low temperature with enhanced sample stability and possibly a better sensitivity. Phosphorus-31 NMR data suggest that lipid bilayers can be aligned at low temperatures, while 15N NMR experiments demonstrate that such aligned samples can be used to enhance the signal-to-noise ratio of is 15N chemical shift spectra of a 37-residue human antimicrobial peptide, LL-37.