The amphipathic helix concept: length effects on ideally amphipathic LiKj(i=2j) peptides to acquire optimal hemolytic activity

Erythrocyte Membrane Biophysical Changes Mediated by Pooled Immunoglobulin G and Hematin: Electrokinetic and Lipid Peroxidation Studies

Pooled Immunoglobulin G (IgG), hematin and the membrane-disruptive amphipathic peptide melittin have received attention as powerful biomacromolecules for biomedical and pharmacology applications. Their action on surface properties, oxidation status and epifluorescence properties measured in vitro provide useful information about the functional activity of upper biomacromolecules in erythrocytes in vivo. The hemolysis of erythrocyte membranes, as well as changes in hematocrit and the morphology of erythrocytes, was investigated here via fluorescence microscopy using FITC-concanavalin A binding to cells. The effect of melittin on the membrane capacitance and resistance of model lipid bilayers was probed via electrochemical impedance spectroscopy. Lipid bilayer capacitance was higher in the presence of 0.10 g/L melittin compared to that in the control, which is likely related to bilayer thinning and alterations of the dielectric permittivity of melittin-treated membranes. The biomolecule interactions with red blood cells were probed in physiological media in which the surface of erythrocyte membranes was negatively charged. Surface parameters of erythrocytes are reported upon IgG/hematin and IgG/melittin treatment. Pooled IgG in the presence of melittin, preincubated IgG/hematin preparations promoted a significant decrease in the electrokinetic potential of erythrocytes (Rh-positive). A malondialdehyde (MDA) assay revealed a high rate of lipid peroxidation in erythrocytes treated with IgG/hematin or IgG/melittin preparations. This finding might be a result of pooled IgG interactions with the hematin molecule and the subsequent conformational changes in the protein molecule altering the electrokinetic properties of the erythrocyte membrane related to the Rh group type of erythrocytes. The pooled IgG and hematin are reported to have important consequences for the biophysical understanding of the immunopathological mechanisms of inflammatory, autoimmune and antibody-mediated pathological processes.

Prediction of amphipathic helix-membrane interactions with Rosetta

Amphipathic helices have hydrophobic and hydrophilic/charged residues situated on opposite faces of the helix. They can anchor peripheral membrane proteins to the membrane, be attached to integral membrane proteins, or exist as independent peptides. Despite the widespread presence of membrane-interacting amphipathic helices, there is no computational tool within Rosetta to model their interactions with membranes. In order to address this need, we developed the AmphiScan protocol with PyRosetta, which runs a grid search to find the most favorable position of an amphipathic helix with respect to the membrane. The performance of the algorithm was tested in benchmarks with the RosettaMembrane, ref2015_memb, and franklin2019 score functions on six engineered and 44 naturally-occurring amphipathic helices using membrane coordinates from the OPM and PDBTM databases, OREMPRO server, and MD simulations for comparison. The AmphiScan protocol predicted the coordinates of amphipathic helices within less than 3Å of the reference structures and identified membrane-embedded residues with a Matthews Correlation Constant (MCC) of up to 0.57. Overall, AmphiScan stands as fast, accurate, and highly-customizable protocol that can be pipelined with other Rosetta and Python applications.

Amphipathic helices are important targets as antibacterial peptides and as domains of membrane proteins that play a role in sensing the membrane environment. Understanding how amphipathic helices interact with membrane enables us to design better peptides and understand how membrane proteins use them to interact with their environment. However, there is a limited number of tools available for the modeling of amphipathic helices in membranes. Implicit membrane models can be used for this purpose as simplistic representations of the membrane environment. In this work, we developed the AmphiScan protocol that can be used to predict membrane coordinates of amphipathic helices starting with a helix structure in an implicit membrane environment. We benchmarked the performance of AmphiScan on engineered LK peptides, naturally-occurring amphipathic helices, and hydrophobic and hydrophilic peptides. Our approach provides a reliable and customizable tool to model amphipathic helix–membrane interactions, and pose a platform for the screening of amphipathic helix properties in silico.

Antibacterial and leishmanicidal activities of temporin-SHd, a 17-residue long membrane-damaging peptide

Temporins are a family of short antimicrobial peptides (8-17 residues) that mostly show potent activity against Gram-positive bacteria. Herein, we demonstrate that temporin-SHd, a 17-residue peptide with a net charge of +2 (FLPAALAGIGGILGKLF(amide)), expressed a broad spectrum of antimicrobial activity. This peptide displayed potent antibacterial activities against Gram-negative and Gram-positive bacteria, including multi-drug resistant Staphylococcus aureus strains, as well as antiparasitic activity against promastigote and the intracellular stage (amastigote) of Leishmania infantum, at concentration not toxic for the macrophages. Temporin-SHd that is structured in a non-amphipathic α-helix in anionic membrane-mimetic environments, strongly and selectively perturbs anionic bilayer membranes by interacting with the polar head groups and acyl region of the phospholipids, with formation of regions of two coexisting phases: one phase rich in peptide and the other lipid-rich. The disruption of lipid packing within the bilayer may lead to the formation of transient pores and membrane permeation/disruption once a threshold peptide accumulation is reached. To our knowledge, Temporin-SHd represents the first known 17-residue long temporin expressing such broad spectrum of antimicrobial activity including members of the trypanosomatidae family. Additionally, since only a few shorter members (13 residues) of the temporin family are known to display antileishmanial activity (temporins-TA, -TB and -SHa), SHd is an interesting tool to analyze the antiparasitic mechanism of action of temporins.

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.

Limiting an antimicrobial peptide to the lipid-water interface enhances its bacterial membrane selectivity: a case study of MSI-367

In a minimalist design approach, a synthetic peptide MSI-367 [(KFAKKFA)(3)-NH(2)] was designed and synthesized with the objective of generating cell-selective nonlytic peptides, which have a significant bearing on cell targeting. The peptide exhibited potent activity against both bacteria and fungi, but no toxicity to human cells at micromolar concentrations. Bacterial versus human cell membrane selectivity of the peptide was determined via membrane permeabilization assays. Circular dichroism investigations revealed the intrinsic helix propensity of the peptide, β-turn structure in aqueous buffer and extended and turn conformations upon binding to lipid vesicles. Differential scanning calorimetry experiments with 1,2-dipalmitoleoyl-sn-glycero-3-phosphatidylethanolamine bilayers indicated the induction of positive curvature strain and repression of the fluid lamellar to inverted hexagonal phase transition by MSI-367. Results of isothermal titration calorimetry (ITC) experiments suggested the possibility of formation of specific lipid-peptide complexes leading to aggregation. (2)H nuclear magnetic resonance (NMR) of deuterated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (POPC) multilamellar vesicles confirmed the limited effect of the membrane-embedded peptide at the lipid-water interface. (31)P NMR data indicated changes in the lipid headgroup orientation of POPC, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylglycerol, and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylethanolamine lipid bilayers upon peptide binding. Membrane-embedded and membrane-inserted states of the peptide were observed via sum frequency generation vibrational spectroscopy. Circular dichroism, ITC, and (31)P NMR data for Escherichia coli lipids agree with the hypothesis that strong electrostatic lipid-peptide interactions embrace the peptide at the lipid-water interface and provide the basis for bacterial cell selectivity.

A peptide fragment derived from the T-cell antigen receptor protein alpha-chain adopts beta-sheet structure and shows potent antimicrobial activity

A 9-residue peptide, CP-1 (GLRILLLKV-NH(2)), is synthesized by solid-phase synthesis method. CP-1 is a C-terminal amidated derivative of a hydrophobic transmembrane segment (CP) of the T-cell antigen receptor (TCR) alpha-chain. CP-1 shows broad-spectrum antimicrobial activities against Gram-positive and Gram-negative bacteria with the minimal inhibitory concentration (MIC) values between 3 and 77microM. Circular dichroism (CD) spectral data shows that CP-1 adopts a well-defined beta-sheet structure in membrane-mimicking environments. CP-1 kills E. coli without lysing the cell membrane or forming transmembrane pores. However, CP-1 can penetrate the bacterial cell membranes and accumulate in the cytoplasm in both Gram-positive S. aureus and Gram-negative E. coli. Moreover CP-1 shows binding affinity for plasmid DNA. These results indicate that the killing mechanism of CP-1 likely involves the penetration into the cytoplasm and binding to intracellular components such as DNA.

Monitoring orientation and dynamics of membrane-bound melittin utilizing dansyl fluorescence

Melittin is a cationic hemolytic peptide isolated from the European honey bee, Apis mellifera. In spite of a number of studies, there is no consensus regarding the orientation of melittin in membranes. In this study, we used a melittin analogue that is covalently labeled at its amino terminal (Gly-1) with the environment-sensitive 1-dimethylamino-5-sulfonylnaphthalene (dansyl) group to obtain information regarding the orientation and dynamics of the amino terminal region of membrane-bound melittin. Our results show that the dansyl group in Dns-melittin exhibits red edge excitation shift in vesicles of 1,2-dioleoyl-sn-glycero-3-phosphocholine, implying its localization in a motionally restricted region of the membrane. This is further supported by wavelength-dependent anisotropy and lifetime changes and time-resolved emission spectra characterized by dynamic Stokes shift, which indicates relatively slow solvent relaxation in the excited state. Membrane penetration depth analysis using the parallax method shows that the dansyl group is localized at a depth of approximately 18 A from the center of the bilayer in membrane-bound Dns-melittin. Further analysis of dansyl and tryptophan depths in Dns-melittin shows that the tilt angle between the helix axis of membrane-bound melittin and the bilayer normal is approximately 70 degrees. Our results therefore suggest that melittin adopts a pseudoparallel orientation in DOPC membranes at low concentration.

Dynamic surface activity by folding and unfolding an amphiphilic alpha-helix

We describe a rationally designed peptide with tunable surface activity, where the dynamics of surface activity are an outcome of helical folding. Our rationally designed model peptide is surface-active only as an alpha-helix. We apply circular dichroism to show that the folded population can be controlled with changes in electrolyte concentration, and we apply pendant bubble tensiometry to explore dynamic surfactant activity. This study shows a peptide that responds to environmental stimuli with dynamic folding and surface activity. Extending this concept to selective binding peptides will lead to new tools, where dynamic surface activity is coupled to targeted binding.

Characterization of an ideal amphipathic peptide as a procoagulant agent

On the basis of previous evidence that amphipathic helical peptides accelerate Factor IXa activation of Factor X [Blostein, Rigby, Furie, Furie and Gilbert (2000) Biochemistry 39, 12000–12006], the present study was designed to assess the procoagulant activity of an IAP (ideal amphipathic peptide) of Lys7Leu15 composition. The results show that IAP accelerates Factor X activation by Factor IXa in a concentration-dependent manner and accelerates thrombin generation by Factor Xa with a comparable peptide- and substrate-concentration-dependence. A scrambled helical peptide with the same amino acid composition as IAP, but with its amphipathicity abolished, eliminated most of the aforementioned effects. The Gla (γ-carboxyglutamic acid)-rich domain of Factor X is required for IAP activity, suggesting that this peptide behaves as a phospholipid membrane. This hypothesis was confirmed, using fluorescence spectroscopy, by demonstrating direct binding between IAP and the Gla-rich domain of Factor X. In addition, the catalytic efficiencies of the tenase and prothrombinase enzymatic complexes, containing cofactors Factor VIIIa and Factor Va respectively, are enhanced by IAP. Finally, we show that IAP delays clot lysis in vitro. In summary, these observations demonstrate that IAP not only enhances essential procoagulant reactions required for fibrin generation, but also inhibits fibrinolysis, suggesting a potential role for IAP as a haemostatic agent.

Design and synthesis of novel antibacterial peptide-resin conjugates

We synthesized a novel peptide-resin conjugate by immobilizing beta-sheet antibacterial peptide on PEG-PS resin. The peptide-resin conjugate, similar to cationic antimicrobial peptides, demonstrated unique properties such as potent antibacterial activity, no hemolytic activity, lipid membrane perturbation activity, and potent synergism with vancomycin. Specially, the peptide-resin conjugate showed a more increased lipid membrane perturbation activity in comparison to unbound beta-sheet antibacterial peptide.

Melittin: a membrane-active peptide with diverse functions

Melittin is the principal toxic component in the venom of the European honey bee Apis mellifera and is a cationic, hemolytic peptide. It is a small linear peptide composed of 26 amino acid residues in which the amino-terminal region is predominantly hydrophobic whereas the carboxy-terminal region is hydrophilic due to the presence of a stretch of positively charged amino acids. This amphiphilic property of melittin has resulted in melittin being used as a suitable model peptide for monitoring lipid-protein interactions in membranes. In this review, the solution and membrane properties of melittin are highlighted, with an emphasis on melittin-membrane interaction using biophysical approaches. The recent applications of melittin in various cellular processes are discussed.

Synthesis, characterization and cytolytic activity of α-helical amphiphilic peptide nanostructures containing crown ethers

Many natural alpha-helical amphiphilic peptides are known to have lytic activity toward different cells. Herein, we describe the synthesis and the characterization of synthetic alpha-helical amphiphilic peptide nanostructures containing crown ethers, as well as the modulation of their cytolytic activity by adding different acidic dipeptide chains at the N- or C-terminus.

Orientation and dynamics of melittin in membranes of varying composition utilizing NBD fluorescence

Melittin is a cationic hemolytic peptide isolated from the European honey bee, Apis mellifera. The organization of membrane-bound melittin has earlier been shown to be dependent on the physical state and composition of membranes. In this study, we covalently labeled the N-terminal (Gly-1) and Lys-7 of melittin with an environment-sensitive fluorescent probe, the NBD group, to monitor the influence of negatively charged lipids and cholesterol on the organization and dynamics of membrane-bound melittin. Our results show that the NBD group of melittin labeled at its N-terminal end does not exhibit red edge excitation shift in DOPC and DOPC/DOPG membranes, whereas the NBD group of melittin labeled at Lys-7 exhibits REES of approximately 8 nm. This could be attributed to difference in membrane microenvironment experienced by the NBD groups in these analogs. Interestingly, the membrane environment of the NBD groups is sensitive to the presence of cholesterol, which is supported by time-resolved fluorescence measurements. Importantly, the orientation of melittin is found to be parallel to the membrane surface as determined by membrane penetration depth analysis using the parallax method in all cases. Our results constitute the first report to our knowledge describing the orientation of melittin in cholesterol-containing membranes. These results assume significance in the overall context of the role of membrane lipids in the orientation and function of membrane proteins and peptides.

Secondary conformation of short lysine- and leucine-rich peptides assessed by optical spectroscopies: effect of chain length, concentration, solvent, and time

Solution secondary structures of three synthetic cationic peptides, currently used in antisense oligonucleotide delivery into living cells, have been analyzed by means of circular dichroism (CD) and Raman scattering in different buffers as a function of concentration and time. All three peptides are of minimalist conception, i.e., formed by only two types of amino acids (leucine: L and lysine: K). Two of these peptides contain 15 aminoacids: N(ter)- KLLKLLLKLLLKLLK (L(10)K(5)), N(ter)-KLKLKLKLKLKLKLK (L(7)K(8)), and the third one has only 9 residues: N(ter)-KLKLKLKLK (L(4)K(5)). The conformational behavior of the 15-mers in pure water differs considerably one from another. Although both of them are initially disordered in the 50-350 microM range, L(10)K(5) gradually undergoes a disordered to alpha-helix transition for molecular concentrations above 100 microM. In all other solvents used, L(10)K(5) adopts a stable alpha-helical conformation. In methanol and methanol/Tris mixture, nonnative alpha-helices can be induced in both KL-alternating peptides, i.e., L(7)K(8) and L(4)K(5). However, in major cases and with a time delay depending on peptide concentration, beta-like structures can be gradually formed in both solutions. In PBS and methanol/PBS mixture, the tendency for L(7)K(8) and L(4)K(5) is to form structures belonging to beta-family. A discussion has been undertaken on the effect of counterions as well as their nature in the stabilization of ordered structures in both KL-alternating peptides.

Revisited and large-scale synthesis and purification of the mutated and wild type neu/erbB-2 membrane-spanning segment

Solid-phase syntheses of the hydrophobic peptides Neu(TM35) ((1)EQRASPVTFIIATVVGVLLFLILVVVVGILIKRRR(35)) and Neu*(TM35) ((1)EQRASPVTFIIATVEGVLLFLILVVVVGILIKRRR(35)), corresponding to the native and mutated (V15E) transmembrane domain of the neu/erbB-2 tyrosine kinase receptor, respectively, were accomplished using Fmoc chemistry. The use of a new resin and cleavage and purification conditions led to large increases in yields and peptide purity. Two (15)N-labelled versions of both wild type and mutated peptides were also synthesized. Approximately 20-40 mg of peptide was obtained using a small-scale synthesis, whereas ca 100 mg of pure peptide was collected on a medium scale. Peptide purity, as monitored by HPLC and mass spectrometry, ranged from 95 to 98% for the six peptides synthesized. Secondary structure as determined by UV circular dichroism (CD) in trifluoroethanol (TFE) showed ca 74% alpha-helical content for the native peptide and ca 63% for that bearing the mutation. Secondary structure of Neu(TM35) was retained in DMPC (dimyristoylphosphatidylcholine)/DCPC (dicaproylphosphatidylcholine) membrane bicelles, and evidences for dimers/oligomers in the lipid bilayer were found.

Study of structure and orientation of mesentericin Y105, a bacteriocin from Gram-positive Leuconostoc mesenteroides, and its Trp-substituted analogues in phospholipid environments

Mesentericin Y105 (Mes-Y105) is a bacteriocin secreted by Leuconostoc mesenteroides which is particularly active on Listeria. It is constituted by 37 residues and reticulated by one disulfide bridge. It has two W residues, W18 and W37, which can be studied by fluorescence. Two single substituted W/F analogues were synthesized (Mes-Y105/W18 and Mes-Y105/W37) to differentiate the local environment around each W and to study their changes in the presence of lipid vesicles. Fluorescence experiments show that, for the pure Trp-analogues, W18 and W37 are fully exposed to solvent whatever pH and buffer conditions. In the presence of lipid vesicles, both became buried. Lipid affinities were estimated: they are weak for zwitterionic phospholipids but an order of magnitude higher for negatively charged phosphatidylserine (PS) and phosphatidylglycerol (PG) lipids. On negatively charged PG lipids, Mes-Y105 and Mes-Y105/W37 display comparable lipid affinities. A decrease in lipid affinity is observed for Mes-Y105/W18 compared to Mes-Y105, which means that W37 would seem to be required for increased lipid selectivity. In the lipid-bound state W18 is strongly dehydrated, probably embedded into the acyl chains, while W37 stands more at the interface. Mes-Y105 was also studied by polarization modulation infrared reflection absorption spectroscopy (PMIRRAS), alone and in various phospholipid environments, to obtain structural information and to assess lipid perturbations. At nanomolar concentrations close to those required for anti-Listeria activity, Mes-Y105 forms films at the air/water interface and inserts into negatively charged lipid monolayers. In situ infrared data show that Mes-Y105 binding only affects the polar head group vibrations while the lipid order of the acyl chains remains unaffected. The PMIRRAS show that Mes-Y105 folds into an N-terminal antiparallel beta-sheet followed by an alpha-helix, both structures being tilted (40 degrees) compared to the normal at the interface, which is in agreement with the thickness estimated by Brewster angle microscopy (BAM). All these data support the proposal of a new model for Mes-Y105 at the membrane interface.

Infrared reflection absorption spectroscopy of amphipathic model peptides at the air/water interface

The linear sequence KLAL (KLALKLALKALKAALKLA-NH(2)) and its corresponding d,l-isomers k(9)a(10)-KLAL (KLALKLALkaLKAALKLA-NH(2)) and l(11)k(12)-KLAL (KLALKLALKAlkAALKLA-NH(2)) are model compounds for potentially amphipathic alpha-helical peptides which are able to bind to membranes and to increase the membrane permeability in a structure- and target-dependent manner (Dathe and Wieprecht, 1999) We first studied the secondary structure of KLAL and its analogs bound to the air/water using infrared reflection absorption spectroscopy. For the peptide films the shape and position of the amide I and amide II bands indicate that the KLAL adopts at large areas per molecule an alpha-helical secondary structure, whereas at higher surface pressures or smaller areas it converts into a beta-sheet structure. This transition could be observed in the compression isotherm as well as during the adsorption at the air/water interface from the subphase as a function of time. The secondary structures are essentially orientated parallel to the air/water interface. The analogs with d-amino acids in two different positions of the sequence, k(9)a(10)-KLAL and l(11)k(12)-KLAL, form only beta-sheet structures at all surface pressures. The observed results are interpreted using a comparison of hydrophobic moments calculated for alpha-helices and beta-sheets. The differences between the hydrophobic moments calculated using the consensus scale are not large. Using the optimal matching hydrophobicity scale or the whole-residue hydrophobicity scale the beta-sheet even has the larger hydrophobic moment.

Complex formation and vectorization of a phosphorothioate oligonucleotide with an amphipathic leucine- and lysine-rich peptide: study at molecular and cellular levels

Optical spectroscopic techniques such as CD, Raman scattering, and fluorescence imaging allowed us to analyze the complex formation and vectorization of a single-stranded 20-mer phosphorothioate oligodeoxynucleotide with a 15-mer amphipathic peptide at molecular and cellular levels. Different solvent mixtures (methanol and water) and molecular ratios of peptide/oligodeoxynucleotide complexes were tested in order to overcome the problems related to solubility. Optimal conditions for both spectroscopic and cellular experiments were obtained with the molecular ratio peptide/oligodeoxynucleotide equal to 21:4, corresponding to a 7:5 ratio for their respective +/- charge ratio. At the molecular level, CD and Raman spectra were consistent with a alpha-helix conformation of the peptide in water or in a methanol-water mixture. The presence of methanol increased considerably the solubility of the peptide without altering its alpha-helix conformation, as evidenced by CD and Raman spectroscopies. UV absorption melting profile of the oligodeoxynucleotide gave rise to a flat melting profile, corresponding to its random structure in solution. Raman spectra of oligodeoxynucleotide/peptide complexes could only be studied in methanol/water mixture solutions. Drastic changes observed in Raman spectra have undoubtedly shown: (a) the perturbation occurred in the peptide secondary structure, and (b) possible interaction between the lysine residues of the peptide and the oligodeoxynucleotide. At the cellular level, the complex was prepared in a mixture of 10% methanol and 90% cell medium. Cellular uptake in optimal conditions for the oligodeoxynucleotide delivery with low cytotoxicity was controlled by fluorescence imaging allowing to specifically locate the compacted oligonucleotide labeled with fluorescein at its 5'-terminus with the peptide into human glioma cells after 1 h of incubation at 37 degrees C.

Stabilisation of mixed peptide/lipid complexes in selective antifungal hexapeptides

The design of antimicrobial peptides could have benefited from structural studies of known peptides having specific activity against target microbes, but not toward other microorganisms. We have previously reported the identification of a series of peptides (PAF-series) active against certain postharvest fungal phytopathogens, and devoid of toxicity towards E. coli and S. cerevisiae [López-García et al. Appl. Environ. Microbiol. 68 (2002) 2453]. The peptides inhibited the conidia germination and hyphal growth. Here, we present a comparative structural characterisation of selected PAF peptides obtained by single-amino-acid replacement, which differ in biological activity. The peptides were characterised in solution using fluorescence and circular dichroism (CD) spectroscopies. Membrane and membrane mimetic-peptide interactions and the lipid-bound structures were studied using fluorescence with the aid of extrinsic fluorescent probes that allowed the identification of mixed peptide/lipid complexes. A direct correlation was found between the capability of complex formation and antifungal activity. These studies provide a putative structural basis for the mechanism of action of selective antifungal peptides.

The NMR-derived solution structure of a new cationic antimicrobial peptide from the skin secretion of the anuran Hyla punctata

Amphibian skin secretions constitute an important source of molecules for antimicrobial drug research in order to combat the increasing resistance of pathogens to conventional antibiotics. Among the various types of substances secreted by the dermal granular amphibian glands, there is a wide range of peptides and proteins, often displaying potent antimicrobial activities and providing an effective defense system against parasite infection. In the present work, we report the NMR solution structure and the biological activity of a cationic 14-residue amphiphilic alpha-helical polypeptide named Hylaseptin P1 (HSP1), isolated from the skin secretion of the hylid frog Hyla punctata. The peptide antimicrobial activity was verified against Candida albicans, Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa, whereas no significant lytic effect was detected toward red or white blood cells.

Direct comparison of membrane interactions of model peptides composed of only Leu and Lys residues

We compared the properties of two peptides of identical size and amino acid composition, Ac-(LKKL)(5)-NHEt and Ac-(KL)(10)-NHEt. Both are amphipathic, but only Ac-(LKKL)(5)-NHEt is a potent promoter of negative curvature. CD studies performed in the presence of lipids confirmed that under these conditions Ac-(LKKL)(5)-NHEt forms an alpha-helix, and Ac-(KL)(10)-NHEt adopts a beta structure. We studied their binding affinity by centrifugation and isothermal titration calorimetry techniques. The Ac-(LKKL)(5)-NHEt bound to zwitterionic and anionic liposomes, while Ac-(KL)(10)-NHEt interacted mainly with anionic liposomes. Ac-(LKKL)(5)-NHEt was more lytic than Ac-(KL)(10)-NHEt for zwitterionic palmitoyloleoylphosphatidylcholine (POPC) liposomes, and for liposomes composed of lipids extracted from either sheep or human erythrocytes (RBC). Both peptides had similar lytic and lipid mixing activities for liposomes containing anionic lipids. Both peptides were highly hemolytic, with Ac-(LKKL)(5)-NHEt active against sheep RBC and Ac-(KL)(10)-NHEt more active against human RBC. From their respective minimal effective concentrations (MECs) as antimicrobial agents, we judged Ac-(KL)(10)-NHEt to be 2 to 5-fold more potent than Ac-(LKKL)(5)-NHEt in media that contained physiological concentrations of NaCl. Notwithstanding, both peptides had MECs <1 microg/mL for Escherichia coli and Pseudomonas aeruginosa and <4 microg/mL for Staphylococcus aureus and methicillin-resistant Staphylococcus aureus. Although selectivity of antimicrobial peptides for bacterial membranes may result, in part, from the preferential display of anionic residues in these membranes, inability to interact with or bind to zwitterionic phospholipids offers no guarantee that the peptide will lack appreciable cytotoxicity for host cells.

The antibiotic activity of cationic linear amphipathic peptides: lessons from the action of leucine/lysine copolymers on bacteria of the class Mollicutes

Peptides composed of leucyl and lysyl residues ('LK peptides') with different compositions and sequences were compared for their antibacterial activities using cell wall-less bacteria of the class Mollicutes (acholeplasmas, mycoplasmas and spiroplasmas) as targets. The antibacterial activity of the amphipathic alpha-helical peptides varied with their size, 15 residues being the optimal length, independent of the membrane hydrophobic core thickness and the amount of cholesterol. The 15-residue ideally amphipathic alpha helix with a +5 positive net charge (KLLKLLLKLLLKLLK) had the strongest antibacterial activity, similar to that of melittin. In contrast, scrambled peptides devoid of amphipathy and the less hydrophobic beta-sheeted peptides [(LK)nK], even those 15-residue long, were far less potent than the helical ones. Furthermore, the growth inhibitory activity of the peptides was correlated with their ability to abolish membrane potential. These data are fully consistent with a predominantly flat orientation of LK peptides at the lipid/water interface and strongly supports that these peptides and probably the linear polycationic amphipathic defence peptides act on bacterial membranes in four main steps according to the 'carpet' model: (a) interfacial partitioning with accumulation of monomers on the target membrane (limiting step); (b) peptide structural changes (conformation, aggregation, and orientation) induced by interactions with the lipid bilayer (as already shown with liposomes and erythrocytes); (c) plasma membrane permeabilization/depolarization via a detergent-like effect; and (d) rapid bacterial cell death if the extent of depolarization is maintained above a critical threshold.

Selective cytotoxicity following Arg-to-Lys substitution in tritrpticin adopting a unique amphipathic turn structure

In antimicrobial peptides, the cationic property due to basic amino acids has been widely recognized as an important factor to promote electrostatic interaction with negatively charged phospholipids. However, little is known about the differences between two basic residues, Arg and Lys, in membrane binding affinity. Tritrpticin is an Arg- or Trp-rich antimicrobial peptide with a broad spectrum of antibacterial and antifungal activity. To investigate the structural and functional differences between Arg and Lys residues, here we designed and synthesized Arg-containing peptides, tritrpticin and SYM11, and their counterpart Lys-substituted peptides, TRK and SYM11KK, respectively. Although there were no remarkable conformational differences between Arg-containing and Lys-substituted peptides, TRK and SYM11KK exhibited almost two-fold enhanced antibacterial activity but significantly reduced hemolytic activity as compared to tritrpticin and SYM11, respectively. Furthermore, Arg-containing peptides showed strong binding affinity to both zwitterionic and anionic liposomes, whereas Lys-substituted peptides interacted weakly with zwitterionic liposomes but strongly with anionic liposomes. These results suggest that the primary amine of Lys interacts less electrostatically with zwitterionic phospholipids than the guanidinium group of Arg. Our results obtained in this study may be helpful in the design of drugs that target negatively charged phospholipids.

General aspects of peptide selectivity towards lipid bilayers and cell membranes studied by variation of the structural parameters of amphipathic helical model peptides

Model compounds of modified hydrophobicity (Eta), hydrophobic moment (mu) and angle subtended by charged residues (Phi) were synthesized to define the general roles of structural motifs of cationic helical peptides for membrane activity and selectivity. The peptide sets were based on a highly hydrophobic, non-selective KLA model peptide with high antimicrobial and hemolytic activity. Variation of the investigated parameters was found to be a suitable method for modifying peptide selectivity towards either neutral or highly negatively charged lipid bilayers. Eta and mu influenced selectivity preferentially via modification of activity on 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (POPC) bilayers, while the size of the polar/hydrophobic angle affected the activity against 1-palmitoyl-2-oleoylphosphatidyl-DL-glycerol (POPG). The influence of the parameters on the activity determining step was modest in both lipid systems and the activity profiles were the result of the parameters' influence on the second less pronounced permeabilization step. Thus, the activity towards POPC vesicles was determined by the high permeabilizing efficiency, however, changes in the structural parameters preferentially influenced the relatively moderate affinity. In contrast, intensive peptide accumulation via electrostatic interactions was sufficient for the destabilization of highly negatively charged POPG lipid membranes, but changes in the activity profile, as revealed by the modification of Phi, seem to be preferentially caused by variation of the low permeabilizing efficiency. The parameters proved very effective also in modifying antimicrobial and hemolytic activity. However, their influence on cell selectivity was limited. A threshold value of hydrophobicity seems to exist which restricted the activity modifying potential of mu and Phi on both lipid bilayers and cell membranes.

Molecular dynamics simulations of the insertion of two ideally amphipathic lytic peptides LK(15) and LK(9) in a 1,2-dimyristoylphosphatidylcholine monolayer

We present here the results of 1-ns molecular dynamics (MD) simulations of two ideally amphipathic lytic peptides, namely LK(15) and LK(9), in a 1,2-dimyristoylphosphatidylcholine monolayer with two different cross-sectional areas per lipid of 80 A(2) (loose film) and 63 A(2) (tight standard film). These peptides are lytic, ideally amphipathic with a minimalist composition L(i)K(j) and the following sequences: H(2)N-KLLKLLLKLLLKLLK-CO-Ph for LK(15) and H(2)N-KLKLKLKLK-CO-Ph for LK(9). From experimental data, LK(15) exhibits an alpha-helical secondary structure, whereas LK(9) was found to form antiparallel beta-sheets at the interface of a DMPC monolayer. Whatever the specific lipid surface is, the two peptides exhibit very different behavior: the alpha-helix inserts deeply into the monolayer whereas the beta-sheeted peptide stays at the surface within the upper polar part of the monolayer. In all cases, a loose monolayer (80 A(2)) results in noticeable artifacts whereas a monolayer with standard specific surface leads to very reliable behavior well in accordance with experimental data. Despite their different insertion depth, the two peptides exhibit identical lytic efficiency. This is very likely a direct consequence of the very strong Van der Waals interactions between the fatty alkyl chains of the lipids and the highly lipophilic lower part of the peptide, resulting in an identical thinning of the two monolayers.

Combinatorial libraries: a tool to design antimicrobial and antifungal peptide analogues having lytic specificities for structure-activity relationship studies

In the race for supremacy, microbes are sprinting ahead. This warning by the World Health Organization clearly demonstrates that the spread of antibiotic-resistant bacteria leads to a global health problem and that antibiotics never seen before by bacteria are urgently needed. Antimicrobial peptides represent such a source for novel antibiotics due to their rapid lytic activity (within minutes) through disruption of cell membranes. However, due to the similarities between bacterial, fungal, and mammalian plasma cell membranes, a large number of antimicrobial peptides have low lytic specificities and exhibit a broad activity spectrum and/or significant toxic effect toward mammalian cells. Mutation strategies have allowed the development of analogues of existing antimicrobial peptides having greater lytic specificities, although such methods are lengthy and would be more efficient if the molecular mechanisms of action of antimicrobial peptides were clearly elucidated. Synthetic combinatorial library approaches have brought a new dimension to the design of novel biologically active compounds. Thus, a set of peptide analogues were generated based on the screening of a library built around an existing lytic peptide, and on a deconvolution strategy directed toward activity specificity. These peptide analogues also served as model systems to further study the effect of biomembrane mimetic systems on the peptides structural behavior relevant to their biological activities.

The effect of lipid composition and physical state of phospholipid monolayer on the binding and incorporation of a basic amphipathic peptide from the C-terminal region of the HIV envelope protein gp41

The interaction of a peptide identical to the carboxy terminal region of the envelope glycoprotein gp41(828) of HIV with negatively charged phospholipids in a monolayer was studied by a Wilhelmy film balance. No significant interaction of the peptide with a monolayer composed of pure neutral but a strong affinity to negatively charged phospholipids could be observed. In mixed phospholipid monolayers the binding of the gp41(828) is primarily limited by the amount of acidic phospholipids. The physical state of the monolayer is another important parameter for binding. Clustering of negatively charged phospholipids and the surface pressure are crucial. Ca(2+) ions strongly interfere with the peptide-lipid interaction up to complete abolishment. The effects observed are dependent on the nature of the acidic lipid. Phosphatidylglycerol was found to be more sensitive than phosphatidylserine. The significance of the results for processes like virus assembly and budding will be discussed.

Ideally amphipathic beta-sheeted peptides at interfaces: structure, orientation, affinities for lipids and hemolytic activity of (KL)(m)K peptides

Designed to model ideally amphipathic beta-sheets, the minimalist linear (KL)(m)K peptides (m=4-7) were synthesized and proved to form stable films at the air/water interface, they insert into compressed dimyristoylphosphatidylcholine monolayers and interact with egg phosphatidylcholine vesicles. Whatever the interface or the lateral pressure applied to the films, FT-IR and polarization-modulated IRRAS spectroscopy developed in situ on the films indicated that all the peptides totally fold into intermolecular antiparallel beta-sheets. Calculated spectra of the amide region allowed us to define the orientation of the beta-strands compared to the interface. It is concluded that such beta-sheets remain flat-oriented without deep perturbation of zwitterionic phospholipids. Dansyl labelling at the N-terminus indicates that all the peptides are monomeric at a low concentration in aqueous buffer and bind to lipids with similar Dns burying. The affinities for zwitterionic lecithin mono- and bilayers, quantitatively estimated from buffer to lipid partition constants, monotonically increased with peptide length, indicating that hydrophobicity is a limiting parameter for lipid and membrane affinities. Peptides induced permeability increases on zwitterionic liposomes, they are strongly hemolytic towards human erythrocytes and their activity increases concurrently with length. Taking into account the lipid affinity, a hemolytic efficiency can be defined: at the same amount of peptide bound, this efficiency strongly increases with the peptide length. It is proposed that the first determinant step of membrane disturbance is the invasion of the outer membrane leaflet by these ideally amphipathic beta-sheeted structures lying flat at the interface, like large rafts depending on the number of beta-strands.

Lipid-induced conformation and lipid-binding properties of cytolytic and antimicrobial peptides: determination and biological specificity

While antimicrobial and cytolytic peptides exert their effects on cells largely by interacting with the lipid bilayers of their membranes, the influence of the cell membrane lipid composition on the specificity of these peptides towards a given organism is not yet understood. The lack of experimental model systems that mimic the complexity of natural cell membranes has hampered efforts to establish a direct correlation between the induced conformation of these peptides upon binding to cell membranes and their biological specificities. Nevertheless, studies using model membranes reconstituted from lipids and a few membrane-associated proteins, combined with spectroscopic techniques (i.e. circular dichroism, fluorescence spectroscopy, Fourier transform infra red spectroscopy, etc.), have provided information on specific structure-function relationships of peptide-membrane interactions at the molecular level. Reversed phase-high performance chromatography (RP-HPLC) and surface plasmon resonance (SPR) are emerging techniques for the study of the dynamics of the interactions between cytolytic and antimicrobial peptides and lipid surfaces. Thus, the immobilization of lipid moieties onto RP-HPLC sorbent now allows the investigation of peptide conformational transition upon interaction with membrane surfaces, while SPR allows the observation of the time course of peptide binding to membrane surfaces. Such studies have clearly demonstrated the complexity of peptide-membrane interactions in terms of the mutual changes in peptide binding, conformation, orientation, and lipid organization, and have, to a certain extent, allowed correlations to be drawn between peptide conformational properties and lytic activity.

Structure, orientation and affinity for interfaces and lipids of ideally amphipathic lytic LiKj(i=2j) peptides

The behavior of lytic ideally amphipathic peptides of generic composition LiKj(i=2j) and named LKn, n=i+j, is investigated in situ by the monolayer technique combined with the recently developed polarization modulation IR spectroscopy (PMIRRAS). A change in the secondary structure occurs versus peptide length. Peptides longer than 12 residues fold into alpha-helices at interfaces as expected from their design, while enough shorter peptides, from 9 down to 5 residues, form intermolecular antiparallel beta-sheets. Analysis of experimental and calculated PMIRRAS spectra in the amide I and II regions show that peptides are flat oriented at the interfaces. Structures and orientation are preserved whatever the nature of the interface, air/water or DMPC monolayer, and the lateral pressure. Peptide partition constants, KaffPi, are estimated from isobar surface increases of DMPC monolayers. They strongly increase when Pi decreases from 30 mN/m to 8 mN/m and they vary with peptide length with an optimum for 12 residues. This non-monotonous dependence fits with data obtained in bilayers and follows the hemolytic activity of the peptides. Lipid perturbations due to peptide insertion essentially detected on the PO4- and CO bands indicate disorder of the lipid head groups. Lysis induced on membranes by such peptides is proposed to first result from their flat asymmetric insertion.