Surface plasmon resonance spectroscopy: an emerging tool for the study of peptide-membrane interactions

Cancer-Associated Gangliosides as a Therapeutic Target for Host Defense Peptide Mimics

Aberrant levels of glycolipids expressed on cellular surfaces are characteristic of different types of cancers. The oligomer of acylated lysine (OAK) mimicking antimicrobial peptides displays in vitro activity against human and murine melanoma cell lines with upregulated GD3 and GM3 gangliosides. Herein, we demonstrate the capability of OAK to intercalate into the sialo-oligosaccharides of DPPC/GD3 and DPPC/GM3 lipid monolayers using X-ray scattering. The lack of insertion into monolayers containing phosphatidylserine suggests that the mechanism of action by OAKs against glycosylated lipid membranes is not merely driven by charge effects. The fluorescence microscopy data demonstrates the membrane-lytic activity of OAK. Understanding the molecular basis for selectivity toward GD3 and GM3 gangliosides by antimicrobial lipopeptides will contribute to the development of novel therapies to cure melanoma and other malignancies.

Mechanisms of Influenza Virus HA2 Peptide Interaction with Liposomes Studied by Dual-Wavelength MP-SPR

A phospholipid-based liposome layer was used as an effective biomimetic membrane model to study the binding of the pH-dependent fusogenic peptide (E4-GGYC) from the influenza virus hemagglutinin HA2 subunit. To this end, a multiparameter surface plasmon resonance approach (MP-SPR) was used for monitoring peptide-liposome interactions at two pH values (4.5 and 8) by means of recording sensorgrams in real time without the need for labeling. Biotinylated liposomes were first immobilized as a monolayer onto the surface of an SPR gold chip coated with a streptavidin layer. Multiple sets of sensorgrams with different HA2 peptide concentrations were generated at both pHs. Dual-wavelength Fresnel layer modeling was applied to calculate the thickness (d) and the refractive index (n) of the liposome layer to monitor the change in its optical parameters upon interaction with the peptide. At acidic pH, the peptide, in its α helix form, entered the lipid bilayer of liposomes, inducing vesicle swelling and increasing membrane robustness. Conversely, a contraction of liposomes was observed at pH 8, associated with noninsertion of the peptide in the double layer of phospholipids. The equilibrium dissociation constant KD = 4.7 × 10-7 M of the peptide/liposome interaction at pH 4.5 was determined by fitting the "OneToOne" model to the experimental sensorgrams using Trace Drawer software. Our experimental approach showed that the HA2 peptide at a concentration up to 100 μM produced no disruption of liposomes at pH 4.5.

Surface Plasmon Resonance Monitoring of Mono-Rhamnolipid Interaction with Phospholipid-Based Liposomes

The interactions of mono-rhamnolipids (mono-RLs) with model membranes were investigated through a biomimetic approach using phospholipid-based liposomes immobilized on a gold substrate and also by the multiparametric surface plasmon resonance (MP-SPR) technique. Biotinylated liposomes were bound onto an SPR gold chip surface coated with a streptavidin layer. The resulting MP-SPR signal proved the efficient binding of the liposomes. The thickness of the liposome layer calculated by modeling the MP-SPR signal was about 80 nm, which matched the average diameter of the liposomes. The mono-RL binding to the film of the phospholipid liposomes was monitored by SPR and the morphological changes of the liposome layer were assessed by modeling the SPR signal. We demonstrated the capacity of the MP-SPR technique to characterize the different steps of the liposome architecture evolution, i.e., from a monolayer of phospholipid liposomes to a single phospholipid bilayer induced by the interaction with mono-RLs. Further washing treatment with Triton X-100 detergent left a monolayer of phospholipid on the surface. As a possible practical application, our method based on a biomimetic membrane coupled to an SPR measurement proved to be a robust and sensitive analytical tool for the detection of mono-RLs with a limit of detection of 2 μg mL^-1.

Structural analysis and mode of action of BMAP-27, a cathelicidin-derived antimicrobial peptide

BMAP-27, a member of cathelicidin family, plays an important role against microorganisms, including bacteria and fungi. BMAP-27 may exert antimicrobial effects through membrane integrity disruption, but the exact molecular mechanism remains unclear. To identify the structural features important for antimicrobial activity and propose a mechanism underlying antibacterial effects, we determined the nuclear magnetic resonance structure of BMAP-27 in a membrane-mimetic environment and investigated its interactions with lipid membranes. BMAP-27 exhibited a long N-terminal α-helix with faces patterned into aromatic and cationic regions, central kink, and short hydrophobic C-terminal helix. While the N-terminal 18-residue peptide (BMAP-18) exerted only antibacterial activity, BMAP-27 showed potent activity against bacteria and cancer cells. Both peptides inhibited bacterial growth, but BMAP-18 showed delayed bactericidal activity and BMAP-27 completely killed bacteria within 20 min. The differences in antimicrobial activities and microbicidal kinetics may be associated with membrane permeabilisation; BMAP-27 rapidly and largely disrupted membrane integrity, whereas BMAP-18 showed low membrane disruption activity. Thus, the N-terminal helix is sufficient to inhibit bacterial growth and the C-terminal helix is involved in membrane permeabilisation for rapid bactericidal and efficient anticancer activities. The structural and functional characterisation of BMAP-27 may encourage the development of novel antimicrobial/anticancer agents.

Biophysical Characterization of Cationic Antibacterial Oligothioetheramides

Biophysical analysis into the mechanism of action of membrane-disrupting antibiotics such as antimicrobial peptides (AMPs) and AMP mimetics is necessary to improve our understanding of this promising but relatively untapped class of antibiotics. We evaluate the impact of cationic nature, specifically the presence of guanidine versus amine functional groups using sequence-defined oligothioetheramides (oligoTEAs). Relative to amines, guanidine groups demonstrated improved antibacterial activity against methicillin-resistant Staphylococcus aureus (MRSA). To understand the mechanism of action, we evaluated membrane interactions by performing a propidium iodide assay and fluorescence microscopy of supported MRSA mimetic bilayers treated with oligoTEAs. Both studies demonstrated membrane disruption, while fluorescence microscopy showed the formation of lipid aggregates. We further analyzed the mechanism using surface plasmon resonance with a recently developed two-state binding model with loss. Our biophysical analysis points to the importance of lipid aggregation for antibacterial activity and suggests that guanidine groups improve antibacterial activity by increasing the extent of lipid aggregation. Altogether, these results verify and rationalize the importance of guanidines for enhanced antibacterial activity of oligoTEAs, and present biophysical phenomena for the design and analysis of additional membrane-active antibiotics.

Computational insights into lipid assisted peptide misfolding and aggregation in neurodegeneration

An overview of recent advances in computational investigation of peptide–lipid interactions in neurodegeneration – Alzheimer's, Parkinson's and Huntington's disease.

Antibacterial isoamphipathic oligomers highlight the importance of multimeric lipid aggregation for antibacterial potency

Cationic charge and hydrophobicity have long been understood to drive the potency and selectivity of antimicrobial peptides (AMPs). However, these properties alone struggle to guide broad success in vivo, where AMPs must differentiate bacterial and mammalian cells, while avoiding complex barriers. New parameters describing the biophysical processes of membrane disruption could provide new opportunities for antimicrobial optimization. In this work, we utilize oligothioetheramides (oligoTEAs) to explore the membrane-targeting mechanism of oligomers, which have the same cationic charge and hydrophobicity, yet show a unique ~ 10-fold difference in antibacterial potency. Solution-phase characterization reveals little difference in structure and dynamics. However, fluorescence microscopy of oligomer-treated Staphylococcus aureus mimetic membranes shows multimeric lipid aggregation that correlates with biological activity and helps establish a framework for the kinetic mechanism of action. Surface plasmon resonance supports the kinetic framework and supports lipid aggregation as a driver of antimicrobial function.

Joseph Brown et al. use oligothioetheramides (oligo TEAs) to show that multimeric lipid aggregation in Staphylococcus aureus mimetic membranes correlates with the biological activity of oligoTEAs. These results may explain why antimicrobial peptides with identical cationic charge and hydrophobicity show different biological activity.

Role of the Ebola membrane in the protection conferred by the three-mAb cocktail MIL77

MIL77, which has a higher manufacturing capacity than ZMapp, comprises MIL77-1, MIL77-2, and MIL77-3. The mechanisms by which these antibodies inhibit glycoprotein are unclear. Infection by viruses with lipid-bilayer envelopes occurs via the fusion of the viral membrane with the membrane of the target cell. Therefore, the interaction between the antibodies and the EBOV membrane is crucial. We examined the interactions between MIL77 and the viral membrane using SPR. MIL77-1 selectively binds to viral membranes, while MIL77-2 and MIL77-3 do not. MIL77-1’s ability to screen the more rigid domains of the membranes results in a locally increased concentration of the drug at the fusion site. Although MIL77-2 recognizes an epitope of GP, it is not necessary in the MIL77 cocktail. These results highlight the importance of EBOV membrane interactions in improving the efficiency of a neutralizing antibody. Furthermore, the viral membrane may be an important target of antibodies against EBOV.

Sensitivity of Antibacterial Activity to Backbone Sequence in Constitutionally Isomeric OligoTEAs

Antimicrobial peptides are promising alternatives to traditional antibiotics but their translational potential is limited due to rapid degradation by serum proteases. Recently, a number of peptidomimetics with backbones resistant to proteolysis have been synthesized and their antimicrobial potential evaluated as a function of their hydrophobic to cationic ratio. However, these mimetics also have a fixed backbone thus making it difficult to isolate the effect of backbone hydrophobic composition and sequence. In this work, advantage is taken of the oligothioetheramide (oligoTEA) synthetic strategy that allows for precise control over backbone and pendant group placement to systematically study the effect of backbone hydrophobic sequence while keeping pendant group constant. Biophysical data acquired with a set of constitutional oligoTEA isomers show that backbone hydrophobic sequence, that is, local hydrophobicity, affects the mode of oligoTEA interaction with lipid bilayers. This differential interaction among the constitutionally isomeric oligoTEAs is manifested in their antibacterial activities and points to the possibility of using backbone hydrophobic sequence to tune antibacterial potency and selectivity.

Exploring Molecular-Biomembrane Interactions with Surface Plasmon Resonance and Dual Polarization Interferometry Technology: Expanding the Spotlight onto Biomembrane Structure

The molecular analysis of biomolecular-membrane interactions is central to understanding most cellular systems but has emerged as a complex technical challenge given the complexities of membrane structure and composition across all living cells. We present a review of the application of surface plasmon resonance and dual polarization interferometry-based biosensors to the study of biomembrane-based systems using both planar mono- or bilayers or liposomes. We first describe the optical principals and instrumentation of surface plasmon resonance, including both linear and extraordinary transmission modes and dual polarization interferometry. We then describe the wide range of model membrane systems that have been developed for deposition on the chips surfaces that include planar, polymer cushioned, tethered bilayers, and liposomes. This is followed by a description of the different chemical immobilization or physisorption techniques. The application of this broad range of engineered membrane surfaces to biomolecular-membrane interactions is then overviewed and how the information obtained using these techniques enhance our molecular understanding of membrane-mediated peptide and protein function. We first discuss experiments where SPR alone has been used to characterize membrane binding and describe how these studies yielded novel insight into the molecular events associated with membrane interactions and how they provided a significant impetus to more recent studies that focus on coincident membrane structure changes during binding of peptides and proteins. We then discuss the emerging limitations of not monitoring the effects on membrane structure and how SPR data can be combined with DPI to provide significant new information on how a membrane responds to the binding of peptides and proteins.

Interaction of Soybean 7S Globulin Peptide with Cell Membrane Model via Isothermal Titration Calorimetry, Quartz Crystal Microbalance with Dissipation, and Langmuir Monolayer Study

To understand the underlying molecular mechanism of the cholesterol-lowering effect of soybean 7S globulins, the interactions of their pepsin-released peptides (7S-peptides) with cell membrane models consisting of dipalmitoylphosphatidylcholine (DPPC), dioleoylphosphatidylcholine (DOPC), and cholesterol (CHOL) were systematically studied. The results showed that 7S-peptides were bound to DPPC/DOPC/CHOL liposomes mainly through van der Waals forces and hydrogen bonds, and the presence of higher CHOL concentrations enhanced the binding affinity (e.g., DPPC/DOPC/CHOL = 1:1:0, binding ratio = 0.114; DPPC/DOPC/CHOL = 1:1:1, binding ratio = 2.02). Compression isotherms indicated that the incorporation of 7S-peptides increased the DPPC/DOPC/CHOL monolayer fluidity and the lipid raft size. The presence of CHOL accelerated the 7S-peptide accumulation on lipid rafts, which could serve as platforms for peptides to develop into β-sheet rich structures. These results allow us to hypothesize that 7S-peptides may indirectly influence membrane protein functions via altering the membrane organization in the enterocytes.

The interaction of antimicrobial peptides with membranes

The interaction of antimicrobial peptides (AMPs) with biological membranes is in the focus of research since several years, and the most important features and modes of action of AMPs are described in this review. Different model systems can be used to understand such interactions on a molecular level. As a special example, we use 2D and 3D model membranes to investigate the interaction of the natural cyclic (Ar-1) and the synthetic linear molecule arenicin with selected amphiphiles and phospholipids. A panoply of sophisticated methods has been used to analyze these interactions on a molecular level. As a general trend, one observes that cationic antimicrobial peptides do not interact with cationic amphiphiles due to electrostatic repulsion, whereas with non-ionic amphiphiles, the peptide interacts only with aggregated systems and not with monomers. The interaction is weak (hydrophobic interaction) and requires an aggregated state with a large surface (cylindrical micelles). Anionic amphiphiles (as monomers or micelles) exhibit strong electrostatic interactions with the AMPs leading to changes in the peptide conformation. Both types of peptides interact strongly with anionic phospholipid monolayers with a preference for fluid layers. The interaction with a zwitterionic layer is almost absent for the linear derivative but measurable for the cyclic arenicin Ar-1. This is in accordance with biological experiments showing that Ar-1 forms well defined stable pores in phospholipid and lipopolysaccharide (LPS) membranes (cytotoxicity). The synthetic linear arenicin, which is less cytotoxic, does not affect the mammalian lipids to such an extent. The interaction of arenicin with bacterial membrane lipids is dominated by hydrogen bonding together with electrostatic and hydrophobic interactions.

Quantitative analysis of molecular partition towards lipid membranes using surface plasmon resonance

Understanding the interplay between molecules and lipid membranes is fundamental when studying cellular and biotechnological phenomena. Partition between aqueous media and lipid membranes is key to the mechanism of action of many biomolecules and drugs. Quantifying membrane partition, through adequate and robust parameters, is thus essential. Surface Plasmon Resonance (SPR) is a powerful technique for studying 1:1 stoichiometric interactions but has limited application to lipid membrane partition data. We have developed and applied a novel mathematical model for SPR data treatment that enables determination of kinetic and equilibrium partition constants. The method uses two complementary fitting models for association and dissociation sensorgram data. The SPR partition data obtained for the antibody fragment F63, the HIV fusion inhibitor enfuvirtide, and the endogenous drug kyotorphin towards POPC membranes were compared against data from independent techniques. The comprehensive kinetic and partition models were applied to the membrane interaction data of HRC4, a measles virus entry inhibitor peptide, revealing its increased affinity for, and retention in, cholesterol-rich membranes. Overall, our work extends the application of SPR beyond the realm of 1:1 stoichiometric ligand-receptor binding into a new and immense field of applications: the interaction of solutes such as biomolecules and drugs with lipids.

Simple direct formation of self-assembled N-heterocyclic carbene monolayers on gold and their application in biosensing

The formation of organic films on gold employing N-heterocyclic carbenes (NHCs) has been previously shown to be a useful strategy for generating stable organic films. However, NHCs or NHC precursors typically require inert atmosphere and harsh conditions for their generation and use. Herein we describe the use of benzimidazolium hydrogen carbonates as bench stable solid precursors for the preparation of NHC films in solution or by vapour-phase deposition from the solid state. The ability to prepare these films by vapour-phase deposition permitted the analysis of the films by a variety of surface science techniques, resulting in the first measurement of NHC desorption energy (158±10 kJ mol⁻¹) and confirmation that the NHC sits upright on the surface. The use of these films in surface plasmon resonance-type biosensing is described, where they provide specific advantages versus traditional thiol-based films.

Self-assembled monolayers (SAMs) have shown tremendous number of applications but can suffer from low stability. Here, the authors report air and bench stable carbene precursors allowing facile SAM formation, and furthermore demonstrate an application in biosensing

Anionic Lipid Content Presents a Barrier to the Activity of ROMP-Based Synthetic Mimics of Protein Transduction Domains (PTDMs)

Many biophysical studies of protein transduction domains (PTDs) and their synthetic mimics (PTDMs) focus on the interaction between the polycationic PTD(M) and anionic phospholipid surfaces. Most, but not all, of these studies suggest that these cation-anion interactions are vital for membrane activity. In this study, the effect of anionic lipid content on PTDM performance was examined for three ring-opening metathesis (ROMP)-based PTDMs with varying hydrophobicity. Using a series of dye-loaded vesicles with gradually increasing anionic lipid content, we saw that increased anionic lipid content inhibited dye release caused by these PTDMs. This result is the opposite of what was found in studies with poly- and oligo-arginine. While the effect is reduced for more hydrophobic PTDMs, it is observable even with the most hydrophobic PTDMs of our test panel. Additional experiments included dynamic light scattering and zeta potential measurements to measure size as a function of vesicle surface charge in the presence of increasing PTDM concentration and surface plasmon resonance spectroscopy to quantify binding between PTDMs and surface-bound lipid layers with varying anion content. The results from these measurements suggested that PTDM hydrophobicity, not cation-anion interactions, is the main driving force of the interaction between our PTDMs and the model membranes investigated. This suggests a model of interaction where surface association and membrane insertion are driven by PTDM hydrophobicity, while anionic lipid content serves primarily to "pin" the PTDM to the membrane surface and limit insertion.

New insights into the molecular mechanisms of biomembrane structural changes and interactions by optical biosensor technology

Biomolecular-membrane interactions play a critical role in the regulation of many important biological processes such as protein trafficking, cellular signalling and ion channel formation. Peptide/protein-membrane interactions can also destabilise and damage the membrane which can lead to cell death. Characterisation of the molecular details of these binding-mediated membrane destabilisation processes is therefore central to understanding cellular events such as antimicrobial action, membrane-mediated amyloid aggregation, and apoptotic protein induced mitochondrial membrane permeabilisation. Optical biosensors have provided a unique approach to characterising membrane interactions allowing quantitation of binding events and new insight into the kinetic mechanism of these interactions. One of the most commonly used optical biosensor technologies is surface plasmon resonance (SPR) and there have been an increasing number of studies reporting the use of this technique for investigating biophysical analysis of membrane-mediated events. More recently, a number of new optical biosensors based on waveguide techniques have been developed, allowing membrane structure changes to be measured simultaneously with mass binding measurements. These techniques include dual polarisation interferometry (DPI), plasmon waveguide resonance spectroscopy (PWR) and optical waveguide light mode spectroscopy (OWLS). These techniques have expanded the application of optical biosensors to allow the analysis of membrane structure changes during peptide and protein binding. This review provides a theoretical and practical overview of the application of biosensor technology with a specific focus on DPI, PWR and OWLS to study biomembrane-mediated events and the mechanism of biomembrane disruption. This article is part of a Special Issue entitled: Lipid-protein interactions.

Membrane interactions of S100A12 (Calgranulin C)

S100A12 (Calgranulin C) is a small acidic calcium-binding peripheral membrane protein with two EF-hand structural motifs. It is expressed in macrophages and lymphocytes and highly up-regulated in several human inflammatory diseases. In pigs, S100A12 is abundant in the cytosol of granulocytes, where it is believed to be involved in signal modulation of inflammatory process. In this study, we investigated the interaction of the porcine S100A12 with phospholipid bilayers and the effect that ions (Ca²⁺, Zn²⁺ or both together) have in modifying protein-lipid interactions. More specifically, we intended to address issues such as: (1) is the protein-membrane interaction modulated by the presence of ions? (2) is the protein overall structure affected by the presence of the ions and membrane models simultaneously? (3) what are the specific conformational changes taking place when ions and membranes are both present? (4) does the protein have any kind of molecular preferences for a specific lipid component? To provide insight into membrane interactions and answer those questions, synchrotron radiation circular dichroism spectroscopy, fluorescence spectroscopy, and surface plasmon resonance were used. The use of these combined techniques demonstrated that this protein was capable of interacting both with lipids and with ions in solution, and enabled examination of changes that occur at different levels of structure organization. The presence of both Ca²⁺ and Zn²⁺ ions modify the binding, conformation and thermal stability of the protein in the presence of lipids. Hence, these studies examining molecular interactions of porcine S100A12 in solution complement the previously determined crystal structure information on this family of proteins, enhancing our understanding of its dynamics of interaction with membranes.

Interaction between the marine sponge cyclic peptide theonellamide A and sterols in lipid bilayers as viewed by surface plasmon resonance and solid-state (2)H nuclear magnetic resonance

Theonellamides (TNMs) are members of a distinctive family of antifungal and cytotoxic bicyclic dodecapeptides isolated from the marine sponge Theonella sp. Recently, it has been shown that TNMs recognize 3β-hydroxysterol-containing membranes, induce glucan overproduction, and damage cellular membranes. However, to date, the detailed mode of sterol binding at a molecular level has not been determined. In this study, to gain insight into the mechanism of sterol recognition of TNM in lipid bilayers, surface plasmon resonance (SPR) experiments and solid-state deuterium nuclear magnetic resonance ((2)H NMR) measurements were performed on theonellamide A (TNM-A). SPR results revealed that the incorporation of 10 mol % cholesterol or ergosterol into 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) membranes significantly enhances the affinity of the peptide for the membrane, particularly in the initial binding to the membrane surface. These findings, together with the fact that binding of TNM-A to epicholesterol (3α-cholesterol)-containing liposomes and pure POPC liposomes was comparably weak, confirmed the preference of the peptide for the 3β-hydroxysterol-containing membranes. To further establish the formation of the complex of TNM-A with 3β-hydroxysterols in lipid bilayers, solid-state (2)H NMR measurements were conducted using deuterium-labeled cholesterol, ergosterol, or epicholesterol. The (2)H NMR spectra showed that TNM-A significantly inhibits the fast rotational motion of cholesterol and ergosterol, but not epicholesterol, therefore verifying the direct complexation between TNM-A and 3β-hydroxysterols in lipid bilayers. This study demonstrates that TNM-A directly recognizes the 3β-OH moiety of sterols, which greatly facilitates its binding to bilayer membranes.

Emerging methodologies to investigate lipid-protein interactions

Cellular membranes are composed of hundreds of different lipids, ion channels, receptors and scaffolding complexes that act as signalling and trafficking platforms for processes fundamental to life. Cellular signalling and membrane trafficking are often regulated by peripheral proteins, which reversibly interact with lipid molecules in highly regulated spatial and temporal fashions. In most cases, one or more modular lipid-binding domain(s) mediate recruitment of peripheral proteins to specific cellular membranes. These domains, of which more than 10 have been identified since 1989, harbour structurally selective lipid-binding sites. Traditional in vitro and in vivo studies have elucidated how these domains coordinate their cognate lipids and thus how the parent proteins associate with membranes. Cellular activities of peripheral proteins and subsequent physiological processes depend upon lipid binding affinities and selectivity. Thus, the development of novel sensitive and quantitative tools is essential in furthering our understanding of the function and regulation of these proteins. As this field expands into new areas such as computational biology, cellular lipid mapping, single molecule imaging, and lipidomics, there is an urgent need to integrate technologies to detail the molecular architecture and mechanisms of lipid signalling. This review surveys emerging cellular and in vitro approaches for studying protein-lipid interactions and provides perspective on how integration of methodologies directs the future development of the field.

Molecular interactions of proteins and peptides at interfaces studied by sum frequency generation vibrational spectroscopy

Interfacial peptides and proteins are critical in many biological processes and thus are of interest to various research fields. To study these processes, surface sensitive techniques are required to completely describe different interfacial interactions intrinsic to many complicated processes. Sum frequency generation (SFG) spectroscopy has been developed into a powerful tool to investigate these interactions and mechanisms of a variety of interfacial peptides and proteins. It has been shown that SFG has intrinsic surface sensitivity and the ability to acquire conformation, orientation, and ordering information about these systems. This paper reviews recent studies on peptide/protein-substrate interactions, peptide/protein-membrane interactions, and protein complexes at interfaces and demonstrates the ability of SFG on unveiling the molecular pictures of complicated interfacial biological processes.

A Synthetic mirror image of kalata B1 reveals that cyclotide activity is independent of a protein receptor

Featuring a circular, knotted structure and diverse bioactivities, cyclotides are a fascinating family of peptides that have inspired applications in drug design. Most likely evolved to protect plants against pests and herbivores, cyclotides also exhibit anti-cancer, anti-HIV, and hemolytic activities. In all of these activities, cell membranes appear to play an important role. However, the question of whether the activity of cyclotides depends on the recognition of chiral receptors or is primarily modulated by the lipid-bilayer environment has remained unknown. To determine the importance of lipid membranes on the activity of the prototypic cyclotide, kalata B1, we synthesized its all-D enantiomer and assessed its bioactivities. After the all-D enantiomer had been confirmed by (1)H NMR to be the structural mirror image of the native kalata B1, it was tested for anti-HIV activity, cytotoxicity, and hemolytic properties. The all-D peptide is active in these assays, albeit with less efficiency; this reveals that kalata B1 does not require chiral recognition to be active. The lower activity than the native peptide correlates with a lower affinity for phospholipid bilayers in model membranes. These results exclude a chiral receptor mechanism and support the idea that interaction with phospholipid membranes plays a role in the activity of kalata B1. In addition, studies with mixtures of L and D enantiomers of kalata B1 suggested that biological activity depends on peptide oligomerization at the membrane surface, which determines affinity for membranes by modulating the association-dissociation equilibrium.

The role of electrostatic interactions in the membrane binding of melittin

The binding of melittin and the C-terminally truncated analogue of melittin (21Q) to a range of phospholipid bilayers was studied using surface plasmon resonance (SPR). The phospholipid model membranes included zwitterionic dimyristylphosphatidylcholine (DMPC) and dimyristylphosphatidylethanolamine (DMPE), together with mixtures DMPC/dimyristylphosphatidylglycerol (DMPG), DMPC/DMPG/cholesterol and DMPE/DMPG. Melittin bound rapidly to all membrane mixtures, whereas 21Q, which has a reduced charge, bound much more slowly on the DMPC and DMPC/DMPG mixtures reflecting the role of the initial electrostatic interaction. The loss of the cationic residues also significantly decreased the binding of 21Q with DMPC/DMPG/Cholesterol, DMPE and DMPE/DMPG. The role of electrostatics was also highlighted with NaCl in the buffer, which affected the way melittin bound to the different membranes, causing a more uniform, concentration dependant increase in response. The biosensor results were correlated with the conformation of the peptides determined by circular dichroism analysis, which indicated that high α-helicity was associated with high binding affinity. Overall, the results demonstrate that the positively charged residues at the C-terminus of melittin play an essential role in membrane binding, that modulation of peptide charge influences selectivity of binding to different phospholipids and that manipulation of the cationic regions of antimicrobial peptides can be used to modulate membrane selectivity.

Exploring the membrane mechanism of the bioactive peptaibol ampullosporin a using lipid monolayers and supported biomimetic membranes

Ampullosporin A is an antimicrobial, neuroleptic peptaibol, the behavior of which was investigated in different membrane mimetic environments made of egg yolk L-α-phosphatidylcholine. In monolayers, the peptaibol adopted a mixed α/3(10)-helical structure with an in-plane orientation. The binding step was followed by the peptide insertion into the lipid monolayer core. The relevance of the inner lipid leaflet nature was studied by comparing ampullosporin binding on a hybrid bilayer, in which this leaflet was a rigid alkane layer, and on supported fluid lipid bilayers. The membrane binding was examined by surface plasmon resonance spectroscopy and the effect on lipid dynamics was explored using fluorescence recovery after photobleaching. In the absence of voltage and at low concentration, ampullosporin A substantially adsorbed onto lipid surfaces and its interaction with biomimetic models was strongly modified depending on the inner leaflet structure. At high concentration, ampullosporin A addition led to the lipid bilayers disruption.

Effect of acyl chain structure and bilayer phase state on binding and penetration of a supported lipid bilayer by HPA3

The effect of acyl chain structure and bilayer phase state on binding and penetration by the peptide HPA3 was studied using dual polarisation interferometry. This peptide is an analogue of Hp(2-20) derived from the N-terminus of Helicobacter pylori ribosomal protein L1 (RpL1) which has been shown to have antimicrobial and cell-penetrating properties. The binding of HPA3 to zwitterionic 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) or 1-palmitolyl-2-oleyl-sn-glycero-3-phosphocholine (POPC) and negatively charged membranes composed of DMPC and 1,2-dimyristoyl-sn-glycero-3-(phosphor-rac-(1-glycerol)) (DMPG) or POPC and 1-palmitolyl-2-oleyl-sn-glycero-3-(phosphor-rac-(1-glycerol)) (POPG) was determined using dual polarisation interferometry (DPI). Mass and birefringence were measured in real time, enabling the creation of birefringence-mass plots for detailed analysis of the changes in lipid bilayer order during the peptide-binding process. HPA3 bound to all four lipids and the binding progressed as a single phase for the saturated gel phase bilayers DMPC and DMPC-DMPG. However, the binding process involved two or more phases, with penetration of the unsaturated fluid phase POPC and POPC-POPG bilayers. Structural changes in the saturated bilayer were partially reversible whereas binding to the unsaturated bilayer resulted in irreversible changes in membrane structure. These results demonstrate that more disordered unsaturated bilayers are more susceptible to further disorganisation and have a lower capacity to recover from peptide-induced structural changes than saturated ordered bilayers. In addition, this study further establishes DPI as powerful tool for analysis of multiphase peptide-insertion processes associated with complex structural changes in the liquid-crystalline membrane.

Surface plasmon resonance spectroscopy: a new lead in studying the membrane binding of amyloidogenic transthyretin

Surface plasmon resonance (SPR) employs the optical principle of SPR to measure changes in mass on a sensor chip surface in real time. Surface chemistry has been developed which enables the immoblization of lipid bilayers and determination of protein-membrane interactions in real time. In the last decade, the plasma membrane has been demonstrated to play an important role in amyloidogenesis and cytotoxicity induced by amyloidogenic proteins. SPR provides an ideal way to study the membrane binding of amyloidogenic proteins. In this chapter, we describe the application of SPR to the study of amyloidogenic transthyretin binding to the plasma membrane and artificial lipid bilayers.

Preparation of lipid membrane surfaces for molecular interaction studies by surface plasmon resonance biosensors

Surface plasmon resonance has become one of the most important techniques for studying bimolecular interactions. Most of the researchers are using it to study protein-protein interactions, but in recent years membrane model systems have also become available and this makes it possible to study protein-membrane interactions as well. In this review chapter we describe possible ways to prepare lipid membrane surfaces on various sensor chips and some of the experimental considerations one has to take into account when performing such experiments.

Capture of intact liposomes on biacore sensor chips for protein-membrane interaction studies

Qualitative and quantitative aspects of protein interactions with membranes may be studied by optical sensors. Biacore offers two dedicated chips for working with lipids and membranes: the L1 and HPA sensor chips. The L1 chip is the most frequently used in protein-membrane interaction studies and it allows the capture of intact liposomes. This chapter describes the protocol for immobilization of liposomes on L1 sensor chips and discusses some of the experimental considerations. An alternative approach that utilizes a streptavidin-coated sensor chip (SA sensor chip) is described for cases when it is not possible to use an L1 chip.

Fast membrane association is a crucial factor in the peptide pep-1 translocation mechanism: a kinetic study followed by surface plasmon resonance

The use of peptide carriers, termed "cell-penetrating peptides (CPPs)" has attracted much attention due to their potential for cellular delivery of hydrophilic molecules with pharmacological interest, overcoming the membrane barrier. These peptides are able to deliver attached cargos in a nontoxic manner, with the uptake mechanisms being either endosomally or physically driven. Pep-1 is a CPP of particular interest, not only due to outstanding delivery rates but also because its mechanism of membrane translocation is exclusively physically driven which appears to be dependent on a very high affinity for the phospholipid bilayer in the cell membrane. In this study, pep-1-lipid interactions were further explored by characterization of the pep-1-lipid association/dissociation by surface plasmon resonance. Although a high affinity of pep-1 for lipid bilayers was observed in all conditions tested, negatively charged phospholipids resulted in a larger peptide/lipid ratio. We also show that pep-1-membrane interaction is a fast process described by a multistep model initiated by peptide adsorption, primarily governed by electrostatic attractions, and followed by peptide insertion in the hydrophobic membrane core. In the context of a cell-based process, the translocation of pep-1 is a physical mechanism promoted by peptide primary amphipathicity and asymmetric properties of the membrane. This explains the high efficiency rates of pep-1 when compared with other CPPs.

Role of membranotropic sequences from herpes simplex virus type I glycoproteins B and H in the fusion process

The entry of enveloped viruses involves attachment followed by close apposition of the viral and plasma membranes. Then, either on the cell surface or in an endocytotic vesicle, the two membranes fuse by an energetically unfavourable process requiring the destabilisation of membrane microenvironment in order to release the viral nucleocapsid into the cytoplasm. The core fusion machinery, conserved throughout the herpesvirus family, involves glycoprotein B (gB) and the non-covalently associated complex of glycoproteins H and L (gH/gL). Both gB and gH possess several hydrophobic domains necessary for efficient induction of fusion, and synthetic peptides corresponding to these regions are able to associate to membranes and induce fusion of artificial liposomes. Here, we describe the first application of surface plasmon resonance (SPR) to the study of the interaction of viral membranotropic peptides with model membranes in order to enhance our molecular understanding of the mechanism of membrane fusion. SPR spectroscopy data are supported by tryptophan fluorescence, circular dichroism and electron spin resonance spectroscopy (ESR). We selected peptides from gB and gH and also analysed the behaviour of HIV gp41 fusion peptide and the cationic antimicrobial peptide melittin. The combined results of SPR and ESR showed a marked difference between the mode of action of the HSV peptides and the HIV fusion peptide compared to melittin, suggesting that viral-derived membrane interacting peptides all act via a similar mechanism, which is substantially different from that of the non-cell selective lytic peptide melittin.

Surface plasmon resonance spectroscopy in determination of the interactions between amyloid beta proteins (Abeta) and lipid membranes

Surface plasmon resonance (SPR) spectroscopy is emerging as a useful tool for determination of molecular interactions in real time. Studies on the molecular pathogenesis of amyloidoses have shown that the plasma membrane plays an important role in amyloidogenesis and cytotoxicity induced by amyloidogenic proteins. By immobilizing lipid bilayers on a sensor chip surface, SPR spectroscopy has been employed to examine the binding of amyloidogenic proteins, such as amyloid beta protein (Abeta), to a variety of lipid membranes, and it provided new insights into the molecular interactions between these amyloidogenic proteins and membranes. In this chapter, we describe the application of SPR spectroscopy to the determination of the binding of Abeta to lipid membranes.

Surface plasmon resonance spectroscopy for studying the membrane binding of antimicrobial peptides

Surface plasmon resonance (SPR) employs the optical principle of SPR to measure changes in mass on a sensor chip surface in real time. Surface chemistry has been developed which enables the immobilization of lipid bilayers and determination of protein-membrane interactions in real time. Antimicrobial peptides are being increasingly recognized as potential candidate antibacterial drugs in the face of the rapidly emerging bacterial resistance to conventional antibiotics in recent years. However, a precise understanding of the relationship between antimicrobial peptide structure and their cytolytic function in a range of organisms is still lacking. This is a result of the complex nature of the interactions of antimicrobial peptides with the cell membrane, the mechanism of which can vary considerably between different classes of antimicrobial peptides. SPR has recently been applied to the study of biomembrane-based systems which has allowed a real-time analysis of binding affinity and kinetics. This chapter describes an SPR method to study the membrane interactions of melittin, a well-known antimicrobial peptide.

Real-time monitoring of peptide cleavage using a nanopore probe

Here we report a rapid, label-free method for monitoring peptide cleavage. Monitoring peptide translocation through an engineered ion channel in the absence and the presence of an enzyme allowed quantitative chemical kinetics information on enzymatic processes to be obtained. In addition to its potential application in disease diagnostics and drug discovery, this peptide/protein cleavage approach is envisioned for further development as a novel rapid, label-free protein sequencing technique.

Interactions of HIV-1 antibodies 2F5 and 4E10 with a gp41 epitope prebound to host and viral membrane model systems

Two HIV-1 recognition domains for the human monoclonal antibodies (MAb) 2F5, which recognises the core sequence ELDKWA, and 4E10, which recognises the core sequence NWFNIT, serve as promising models for immunogens in vaccine development against HIV-1. However, the failure of these recognition domains to generate broadly reactive neutralizing antibodies, and the putative membrane-binding properties of the antibodies raised to these recognition domains, suggest that additional features or recognition motifs are required to form an efficient immunogen, which could possibly include the membrane components. In this study we used an extended peptide epitope sequence derived from the gp41 native sequence (H-NEQELLELDKWASLWNWFNITNWLWYIK-NH), which contains the two recognition domains for 2F5 and 4E10, to examine the role of model cell (POPC) and viral (POPC/cholesterol/sphingomyelin) membranes in the recognition of these two antibodies. By using a surface plasmon resonance biosensor, the binding of 2F5 and 4E10 to membranes was compared and contrasted in the presence and absence of prebound peptide epitope. The recognition of the peptide epitope by each MAb was found to be distinct; 2F5 exhibited strong and almost irreversible binding to both membranes in the presence of the peptide, but bound weakly in the absence of the peptide epitope. In contrast, 4E10 exhibited strong membrane binding in the presence or absence of the peptide epitope, and the binding was essentially irreversible in the presence of the peptide epitope. Overall, these results demonstrate that both 2F5 and 4E10 can bind to membranes prior to epitope recognition, but that high-affinity recognition of gp41-derived epitope sequences by 2F5 and 4E10 occurs in a membrane context. Moreover, 4E10 might utilise the membrane to access and bind to gp41; such membrane properties of 2F5 and 4E10 could be exploited in immunogen design.