Change in membrane permeability induced by protegrin 1: implication of disulphide bridges for pore formation

Enhanced Antibacterial Activity of Substituted Derivatives of NCR169C Peptide

Medicago truncatula in symbiosis with its rhizobial bacterium partner produces more than 700 nodule-specific cysteine-rich (NCR) peptides with diverse physicochemical properties. Most of the cationic NCR peptides have antimicrobial activity and the potential to tackle antimicrobial resistance with their novel modes of action. This work focuses on the antibacterial activity of the NCR169 peptide derivatives as we previously demonstrated that the C-terminal sequence of NCR169 (NCR169C_17-38) has antifungal activity, affecting the viability, morphology, and biofilm formation of various Candida species. Here, we show that NCR169C_17-38 and its various substituted derivatives are also able to kill ESKAPE pathogens such as Enterococcus faecalis, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Escherichia coli. The replacement of the two cysteines with serines enhanced the antimicrobial activity against most of the tested bacteria, indicating that the formation of a disulfide bridge is not required. As tryptophan can play role in the interaction with bacterial membranes and thus in antibacterial activity, we replaced the tryptophans in the NCR169C_17-38C_12,17/S sequence with various modified tryptophans, namely 5-methyl tryptophan, 5-fluoro tryptophan, 6-fluoro tryptophan, 7-aza tryptophan, and 5-methoxy tryptophan, in the synthesis of NCR169C_17-38C_12,17/S analogs. The results demonstrate that the presence of modified fluorotryptophans can significantly enhance the antimicrobial activity without notable hemolytic effect, and this finding could be beneficial for the further development of new AMPs from the members of the NCR peptide family.

Structure-activity trend analysis between amino-acids and minimal inhibitory concentration of antimicrobial peptides

Antimicrobial peptides (AMPs) provide large structural libraries of molecules with high variability of constitutional amino-acids (AAs). Highlighting structural organization and structure-activity trends in such molecular systems provide key information on structural associations and functional conditions that could usefully help for drug design. This work presents link analyses between minimal inhibitory concentration (MIC) and different types of constitutional AAs of anti-Pseudomonas aeruginosa AMPs. This scope was based on a dataset of 328 published molecules. Regulation levels of AAs in AMPs were statistically ordinated by correspondence analysis helping for classification of the 328 AMPs into nine structurally homogeneous peptide clusters (PCs 1-9) characterized by high/low relative occurrences of different AAs. Within each PC, negative trends between MIC and AAs were highlighted by iterated multiple linear regression models built by bootstrap processes (bagging). MIC-decrease was linked to different AAs that varied with PCs: alcohol type AAs (Thr, Ser) in Cys-rich and low Arg PCs (PCs 1-3); basic AAs (Lys, Arg) in Pro-rich and low Val PCs (PCs 4-8); Trp (heterocyclic AA) in Arg-rich PCs (PCs 6, 7, 9). Aliphatic AAs (more particularly Gly) showed MIC-reduction effects in different PCs essentially under interactive forms.

Promiscuous Installation of d-Amino Acids in Gene-Encoded Peptides

d-Amino acids can have major effects on the structure, proteolytic stability, and bioactivity of peptides. Proteusin radical S-adenosyl methionine epimerases regioselectively install such residues in ribosomal peptides to generate peptides with the largest number of d-residues currently known in biomolecules. To study their utility in synthetic biology, we investigated the substrate tolerance and substrate-product relationships of the cyanobacterial model epimerase OspD using libraries of point mutants as well as distinct extended peptides that were fused to an N-terminal leader sequence. OspD was found to exhibit exceptional substrate promiscuity in E. coli, accepting 15 different amino acids and converting peptides with a broad range of compositions, secondary structures, and polarities. Diverse single and multiple epimerization patterns were identified that were dictated by the peptide sequence. The data suggest major potential in creating genetically encoded products previously inaccessible by synthetic biology.

Experimental and Computational Characterization of Oxidized and Reduced Protegrin Pores in Lipid Bilayers

Protegrin-1 (PG-1), an 18-residue β-hairpin stabilized by two disulfide bonds, is a member of a family of powerful antimicrobial peptides which are believed to act through membrane permeabilization. Here we used a combination of experimental and computational approaches to characterize possible structural arrangements of PG-1 in lipid bilayers mimicking bacterial membranes. We have measured the dose-response function of the PG-1-induced leakage of markers of various sizes from vesicles and found it to be consistent with the formation of pores of two different sizes. The first one allows the release of small dyes and occurs at peptide:lipid ratios < 0.006. Above this ratio, larger pores are observed through which the smallest of dextrans FD4 can be released. In parallel with pore formation, we observe a general large-scale destabilization of vesicles which is probably related to complete rupture of some vesicles. The population of vesicles that are completely ruptured depends linearly on PG-1:lipid ratio. Neither pore size, nor vesicle rupture are influenced by the formation of disulfide bonds. Previous computational work on oxidized protegrin is complemented here by all-atom MD simulations of PG-1 with reduced disulfide bonds both in solution (monomer) and in a bilayer (dimer and octamer). The simulations provide molecular insights into the influence of disulfide bonds on peptide conformation, aggregation, and oligomeric structure.

Characterization of the antimicrobial peptide family defensins in the Tasmanian devil (Sarcophilus harrisii), koala (Phascolarctos cinereus), and tammar wallaby (Macropus eugenii)

Defensins comprise a family of cysteine-rich antimicrobial peptides with important roles in innate and adaptive immune defense in vertebrates. We characterized alpha and beta defensin genes in three Australian marsupials: the Tasmanian devil (Sarcophilus harrisii), koala (Phascolarctos cinereus), and tammar wallaby (Macropus eugenii) and identified 48, 34, and 39 defensins, respectively. One hundred and twelve have the classical antimicrobial peptides characteristics required for pathogen membrane targeting, including cationic charge (between 1+ and 15+) and a high proportion of hydrophobic residues (>30%). Phylogenetic analysis shows that gene duplication has driven unique and species-specific expansions of devil, koala, and tammar wallaby beta defensins and devil alpha defensins. Defensin genes are arranged in three genomic clusters in marsupials, whereas further duplications and translocations have occurred in eutherians resulting in four and five gene clusters in mice and humans, respectively. Marsupial defensins are generally under purifying selection, particularly residues essential for defensin structural stability. Certain hydrophobic or positively charged sites, predominantly found in the defensin loop, are positively selected, which may have functional significance in defensin-target interaction and membrane insertion.

The Disulfide Bond of the Peptide Thanatin Is Dispensible for Its Antimicrobial Activity In Vivo and In Vitro

Thanatin (THA) displays potent antibiotic activity, especially against extended-spectrum-β-lactamase (ESBL)-producing Escherichia coli both in vitro and in vivo, with minimal hemolytic toxicity and satisfactory stability in plasma. However, the high cost of thanatin significantly limits its development and clinical application. To reduce the cost of peptide synthesis, a formulation of cyclic thanatin (C-thanatin) called linear thanatin (L-thanatin) was synthesized and its activity was evaluated in vivo and in vitro Results showed that C-thanatin and L-thanatin MICs did not differ against eight Gram-negative and two Gram-positive bacterial strains. Furthermore, the survival rates of ESBL-producing-E. coli-infected mice were consistent after C-thanatin or L-thanatin treatment at 5 or 10 mg/kg of body weight. Neither C-thanatin nor L-thanatin showed toxicity for human red blood cells (hRBCs) and human umbilical vein endothelial cells (HUVECs) at a concentration as high as 256 μg/ml. Results of circular dichroism spectroscopy indicated that the secondary structure of L-thanatin is extremely similar to that of C-thanatin. Membrane permeabilization and depolarization assays showed that C-thanatin and L-thanatin have similar abilities to permeabilize the outer and inner membranes and to induce membrane depolarization in ESBL-producing E. coli However, neither of them caused significant HUVEC membrane permeability. These findings indicate that the two peptides have similar effects on bacterial cell membranes and that the disulfide bond in thanatin is not essential for its antimicrobial activities in vivo and in vitro L-thanatin is thus a promising low-cost peptide candidate for treating ESBL-producing E. coli infections.

Structure-Dependent Immune Modulatory Activity of Protegrin-1 Analogs

Protegrins are porcine antimicrobial peptides (AMPs) that belong to the cathelicidin family of host defense peptides. Protegrin-1 (PG-1), the most investigated member of the protegrin family, is an arginine-rich peptide consisting of 18 amino acid residues, its main chain adopting a β-hairpin structure that is linked by two disulfide bridges. We report on the immune modulatory activity of PG-1 and its analogs in neutralizing bacterial endotoxin and capsular polysaccharides, consequently inhibiting inflammatory mediators’ release from macrophages. We demonstrate that the β-hairpin structure motif stabilized with at least one disulfide bridge is a prerequisite for the immune modulatory activity of this type of AMP.

Peptides and Peptidomimetics for Antimicrobial Drug Design

The purpose of this paper is to introduce and highlight a few classes of traditional antimicrobial peptides with a focus on structure-activity relationship studies. After first dissecting the important physiochemical properties that influence the antimicrobial and toxic properties of antimicrobial peptides, the contributions of individual amino acids with respect to the peptides antibacterial properties are presented. A brief discussion of the mechanisms of action of different antimicrobials as well as the development of bacterial resistance towards antimicrobial peptides follows. Finally, current efforts on novel design strategies and peptidomimetics are introduced to illustrate the importance of antimicrobial peptide research in the development of future antibiotics.

The Antimicrobial and Antiviral Applications of Cell-Penetrating Peptides

Over the past two decades, cell-penetrating peptides (CPPs) have become increasingly popular both in research and in application. There have been numerous studies on the physiochemical characteristics and behavior of CPPs in various environments; likewise, the mechanisms of entry and delivery capabilities of these peptides have also been extensively researched. Besides the fundamental issues, there is an enormous interest in the delivery capabilities of the peptides as the family of CPPs is a promising and mostly non-toxic delivery vector candidate for numerous medical applications such as gene silencing, transgene delivery, and splice correction. Lately, however, there has been an emerging field of study besides the high-profile gene therapy applications-the use of peptides and CPPs to combat various infections caused by harmful bacteria, fungi, and viruses.In this chapter, we aim to provide a short overview of the history and properties of CPPs which is followed by more thorough descriptions of antimicrobial and antiviral peptides. To achieve this, we analyze the origin of such peptides, give an overview of the mechanisms of action and discuss the various practical applications which are ongoing or have been suggested based on research.

Implicit Membrane Investigation of the Stability of Antimicrobial Peptide β-Barrels and Arcs

Previous simulations showed that the β-hairpin antimicrobial peptide (AMP) protegrin-1 can form stable octameric β-barrels and tetrameric arcs (half barrels) in both implicit and explicit membranes. Here, we extend this investigation to several AMPs of similar structure: tachyplesin, androctonin, polyphemusin, gomesin, and the retrocyclin θ-defensin. These peptides form short β-hairpins stabilized by 2-3 disulfide bonds. We also examine synthetic β-sheet peptides selected from a combinatorial library for their ability or inability to form pores in lipid membranes. When heptameric, octameric, and decameric β-barrels and tetrameric arcs of these peptides were embedded in pre-formed neutral or anionic lipid pores (i.e., pores in neutral or anionic membranes, respectively), a variety of behaviors and membrane binding energies were observed. Due to the cationic charge of the peptides, more favorable transfer energies and more stable binding were observed in anionic than neutral pores. The synthetic peptides bound very strongly and formed stable barrels and arcs in both neutral and anionic pores. The natural AMPs exhibited unfavorable or marginally favorable binding energy and kinetic stability in neutral pores, consistent with the lower hemolytic activity of some of them compared with protegrin-1. Binding to anionic pores was more favorable, but significant distortions of the barrel or arc structures were sometimes noted. These results are discussed in light of the available experimental data. The diversity of behaviors obtained makes it unlikely that the barrel and arc mechanisms are valid for the entire family of β-hairpin AMPs.

Cell-based peptide screening to access the undruggable target space

Only 20-30% of drug target proteins can be accessed by common drug classes, like small molecules or therapeutic antibodies. The vast majority of the remaining proteins are considered "undruggable" and include drug target proteins, like transcription factors, scaffold or adapter proteins, which play important roles in disease. However over the last years innovative compound classes including nucleotide derived drugs (e.g. siRNA, antisense), macrocyclic compounds and cell-permeable peptides matured significantly and hold now the potential to modulate these hard to access target proteins for therapeutic use. This article will focus on the discovery of cell-permeable peptides and discuss intracellular screening systems for peptides, which yield highly relevant peptides, because peptide selection takes place in eukaryotic cells, under conditions, which are very similar to the later therapeutic use.

Protein arcs may form stable pores in lipid membranes

Electron microscopy and atomic force microscopy images of cholesterol-dependent cytolysins and related proteins that form large pores in lipid membranes have revealed the presence of incomplete rings, or arcs. Some evidence indicates that these arcs are inserted into the membrane and induce membrane leakage, but other experiments seem to refute that. Could such pores, only partially lined by protein, be kinetically and thermodynamically stable? How would the lipids be structured in such a pore? Using the antimicrobial peptide protegrin-1 as a model, we test the stability of pores only partially lined by peptide using all-atom molecular dynamics simulations in POPC and POPE/POPG membranes. The data show that, whereas pure lipid pores close rapidly, pores partially lined by protegrin arcs are stable for at least 300 ns. Estimates of the thermodynamic stability of these arcs using line tension data and implicit solvent calculations show that these arcs can be marginally stable in both zwitterionic and anionic membranes. Arcs provide an explanation for the observed ion selectivity in protegrin electrophysiology experiments and could possibly be involved in other membrane permeabilization processes where lipids are thought to participate, such as those induced by antimicrobial peptides and colicins, as well as the Bax apoptotic pore.

Cysteine deleted protegrin-1 (CDP-1): anti-bacterial activity, outer-membrane disruption and selectivity

BACKGROUND

Protegin-1 (PG-1: RGGRLCYCRRRFCVCVGR-amide) assumes a rigid β-hairpin like structure that is stabilized by two disulfide bridges between Cys6-Cys15 and Cys8-Cys13. Previous studies, employing linear analogs of PG-1, with Cys to Ala mutations or modified Cys, have demonstrated that the disulfide bridges are critical for the broad spectrum and salt resistant antimicrobial activity of PG-1.

METHODS

In order to understand structural and functional roles of disulfide bonds in protegrins, we have synthesized a Cys deleted variant of PG-1 or CDP-1, RGGRLYRRRFVVGR-amide, and two of its analogs, RR11, RLYRRRFVVGR-amide, and LR10, LYRRRFVVGR-amide, containing deletion of residues at the N-terminus. These peptides have been characterized for bactericidal activity and mode of action in lipopolysaccharide (LPS) using optical spectroscopy, ITC and NMR.

RESULTS

Antibacterial activity, against Gram-negative and Gram-positive strains, of the three peptides follows the order: CDP-1>RR11>LR10. LR10 displays only limited activity toward Gram-negative strains. CDP-1 demonstrates efficient membrane permeabilization and high-affinity interactions with LPS. CDP-1 and RR11 both assume β-hairpin like compact structures in complex with LPS, whereas LR10 adopts an extended conformation in LPS. In zwitterionic DPC micelles CDP-1 and the truncated analog peptides do not adopt folded conformations.

MAJOR CONCLUSIONS

Despite the absence of stabilizing disulfide bridges CDP-1 shows broad-spectrum antibacterial activity and assumes β-hairpin like structure in complex with LPS. The β-hairpin structure may be essential for outer membrane permeabilization and cell killing.

Strategies to deliver peptide drugs to the brain

Neurological diseases such as neurodegeneration, pain, psychiatric disorders, stroke, and brain cancers would greatly benefit from the use of highly potent and specific peptide pharmaceuticals. Peptides are especially desirable because of their low inherent toxicity. The presence of the blood brain barrier (BBB), their short duration of action, and their need for parenteral administration limits their clinical use. However, over the past decade there have been significant advances in delivering peptides to the central nervous system. Angiopep peptides developed by Angiochem (Montreal, Canada), transferrin antibodies developed by ArmaGen (Santa Monica, USA), and cell penetrating peptides have all shown promise in delivering therapeutic peptides across the BBB after intravenous administration. Noninvasive methods of delivering peptides to the brain include the use of chitosan amphiphile nanoparticles for oral delivery and nose to brain strategies. The uptake of the chitosan amphiphile nanoparticles by the gastrointestinal epithelium is important for oral peptide delivery. Finally protecting peptides from plasma degradation is integral to the success of most of these peptide delivery strategies.

Antiviral potential of cathelicidins

ABSTRACT: The global burden of morbidity and mortality arising from viral infections is high; however, the development of effective therapeutics has been slow. As our understanding of innate immunity has expanded over recent years, knowledge of natural host defenses against viral infections has started to offer potential for novel therapeutic strategies. An area of current research interest is in understanding the roles played by naturally occurring cationic host defense peptides, such as the cathelicidins, in these innate antiviral host defenses across different species. This research also has the potential to inform the design of novel synthetic antiviral peptide analogs and/or provide rationale for therapies aimed at boosting the natural production of these peptides. In this review, we will discuss our knowledge of the antiviral activities of cathelicidins, an important family of cationic host defense peptides, and consider the implications for novel antiviral therapeutic approaches.

Antimicrobial peptides

The rapid increase in drug-resistant infections has presented a serious challenge to antimicrobial therapies. The failure of the most potent antibiotics to kill “superbugs” emphasizes the urgent need to develop other control agents. Here we review the history and new development of antimicrobial peptides (AMPs), a growing class of natural and synthetic peptides with a wide spectrum of targets including viruses, bacteria, fungi, and parasites. We summarize the major types of AMPs, their modes of action, and the common mechanisms of AMP resistance. In addition, we discuss the principles for designing effective AMPs and the potential of using AMPs to control biofilms (multicellular structures of bacteria embedded in extracellular matrixes) and persister cells (dormant phenotypic variants of bacterial cells that are highly tolerant to antibiotics).

Membrane interactions and pore formation by the antimicrobial peptide protegrin

Protegrin is an antimicrobial peptide with a β-hairpin structure stabilized by a pair of disulfide bonds. It has been extensively studied by solid-state NMR and computational methods. Here we use implicit membrane models to examine the binding of monomers on the surface and in the interior of the membrane, the energetics of dimerization, the binding to membrane pores, and the stability of different membrane barrel structures in pores. Our results challenge a number of conclusions based on previous experimental and theoretical work. The burial of monomers into the membrane interior is found to be unfavorable for any membrane thickness. Because of its imperfect amphipathicity, protegrin binds weakly, at most, on the surface of zwitterionic membranes. However, it binds more favorably onto toroidal pores. Anionic charge on the membrane facilitates the binding due to electrostatic interactions. Solid-state NMR results have suggested a parallel NCCN association of monomers in dimers and association of dimers to form octameric or decameric β-barrels. We find that this structure is not energetically plausible for binding to bilayers, because in this configuration the hydrophobic sides of two monomers point in opposite directions. In contrast, the antiparallel NCCN and especially the parallel NCNC octamers are stable and exhibit a favorable binding energy to the pore. The results of 100-ns simulations in explicit bilayers corroborate the higher stability of the parallel NCNC barrel compared with the parallel NCCN barrel. The ability to form pores in zwitterionic membranes provides a rationalization for the peptide's cytotoxicity. The discrepancies between our results and experiment are discussed, and new experiments are proposed to resolve them and to test the validity of the models.

A proline-hinge alters the characteristics of the amphipathic α-helical AMPs

HP (2-20) is a 19-aa, amphipathic, α-helical peptide with antimicrobial properties that was derived from the N-terminus of Helicobacter pylori ribosomal protein L1. We previously showed that increasing the net hydrophobicity of HP (2-20) by substituting Trp for Gln(17) and Asp(19) (Anal 3) increased the peptide's antimicrobial activity. In hydrophobic medium, Anal 3 forms an amphipathic structure consisting of an N-terminal random coil region (residues 2-5) and an extended helical region (residues 6-20). To investigate the structure-activity relationship of Anal 3, we substituted Pro for Glu(9) (Anal 3-Pro) and then examined the new peptide's three-dimensional structure, antimicrobial activity and mechanism of action. Anal 3-Pro had an α-helical structure in the presence of trifluoroethanol (TFE) and sodium dodecyl sulfate (SDS). NMR spectroscopic analysis of Anal 3-Pro's tertiary structure in SDS micelles confirmed that the kink potential introduced by Pro(10) was responsible for the helix distortion. We also found that Anal 3-Pro exhibited about 4 times greater antimicrobial activity than Anal 3. Fluorescence activated flow cytometry and confocal fluorescence microscopy showed that incorporating a Pro-hinge into Anal 3 markedly reduced its membrane permeability so that it accumulated in the cytoplasm without remaining in the cell membrane. To investigate the translocation mechanism, we assessed its ability to release of FITC-dextran. The result showed Anal 3-Pro created a pore <1.8 nm in diameter, which is similar to buforin II. Notably, scanning electron microscopic observation of Candida albicans revealed that Anal 3-Pro and buforin II exert similar effects on cell membranes, whereas magainin 2 exerts a different, more damaging, effect. In addition, Anal 3-Pro assumed a helix-hinge-helix structure in the presence of biological membranes and formed micropores in both bacterial and fungal membranes, through which it entered the cytoplasm and tightly bound to DNA. These results indicate that the bending region of Anal 3- Pro peptide is prerequisite for effective antibiotic activity and may facilitate easy penetration of the lipid bilayers of the cell membrane.

[Progress in cathelicidins antimicrobial peptides research]

Cathelicidins are a family of multi-functional antimicrobial peptides found in almost all types of vertebrates, where they play vital roles in the immune system. As they possess broad-spectrum antimicrobial properties, cathelicidins are not only strongly resistant to Gram-positive and Gram-negative bacteria, fungi, and viruses, but they are also active against many antibiotic-resistant clinical bacteria, adopting a special antimicrobial mechanism that is unlikely to lead to microbial resistance. Cathelicidins likewise possess simple structures, and low hemolytic and cytotoxic activities. Collectively, these features suggest potentially novel and exciting prospects for cathelicidins' application in medicine. Here, we review the structures, classification, activities, mechanisms, as well as prospective developments in the usage and application of cathelicidin antimicrobial peptides.

Multiscale models of the antimicrobial peptide protegrin-1 on gram-negative bacteria membranes

Antimicrobial peptides (AMPs) are naturally-occurring molecules that exhibit strong antibiotic properties against numerous infectious bacterial strains. Because of their unique mechanism of action, they have been touted as a potential source for novel antibiotic drugs. We present a summary of computational investigations in our lab aimed at understanding this unique mechanism of action, in particular the development of models that provide a quantitative connection between molecular-level biophysical phenomena and relevant biological effects. Our work is focused on protegrins, a potent class of AMPs that attack bacteria by associating with the bacterial membrane and forming transmembrane pores that facilitate the unrestricted transport of ions. Using fully atomistic molecular dynamics simulations, we have computed the thermodynamics of peptide-membrane association and insertion, as well as peptide aggregation. We also present a multi-scale analysis of the ion transport properties of protegrin pores, ranging from atomistic molecular dynamics simulations to mesoscale continuum models of single-pore electrodiffusion to models of transient ion transport from bacterial cells. Overall, this work provides a quantitative mechanistic description of the mechanism of action of protegrin antimicrobial peptides across multiple length and time scales.

Purification and structural characterization of a novel antibacterial peptide from Bellamya bengalensis: activity against ampicillin and chloramphenicol resistant Staphylococcus epidermidis

Increasing tendency of clinical bacterial strains resistant to conventional antibiotics has being a great challenge to the public's health. Antimicrobial peptides, a new class of antibiotics is known to have the activity against a wide range of bacteria resistant to conventional antibiotics. An antimicrobial peptide of 1676 Da was purified from Bellamya bengalensis, a fresh water snail, using ultrafiltration and reversed phase liquid chromatography. The effect of this peptide on Staphylococcus epidermidis resistant to ampicillin and chloramphenicol was investigated; the MIC and MBC values were 8 μg/ml and 16 μg/ml, respectively. Complete sequence of the peptide was determined by tandem mass spectrometry (MS/MS). Further, peptide net charge, hydrophobicity and molecular modeling were evaluated in silico for better understanding the probable mechanisms of action. The peptide showed the specificity to bacterial membranes. Hence, this reported peptide revealed a promising candidate to contribute in the development of therapeutic agent for Staphylococcal infections.

Antimicrobial peptides with cell-penetrating peptide properties and vice versa

Antimicrobial peptides (AMPs) are a group of peptides that are active against a diverse spectrum of microorganisms. Due to their mode of action, AMPs are a promising class of molecules that could overcome the problems of increasing resistance of bacteria to conventional antibiotics. Furthermore, AMPs are strongly membrane-active and some are able to translocate into cells without the necessity for permanent membrane permeabilization. This feature has brought them into focus for use as transport vectors in the context of drug delivery. Since the plasma membrane restricts transport of bioactive substances into cells, great research interest lies in the development of innovative ways to overcome this barrier and to increase bioavailability. In this context, peptide-based transport systems, such as cell-penetrating peptides (CPPs), have come into focus, and their efficiency has been demonstrated in many different applications. However, more recently, also some AMPs have been used as efficient vectors for intracellular translocation of various active molecules. This review summarizes recent efforts in this interesting field of drug delivery. Moreover, some examples of the application of CPPs as efficient antimicrobial substances will be discussed.

HIV-1 enhancing effect of prostatic acid phosphatase peptides is reduced in human seminal plasma

We recently reported that HIV-1 infection can be inhibited by innate antimicrobial components of human seminal plasma (SP). Conversely, naturally occurring peptidic fragments from the SP-derived prostatic acid phosphatase (PAP) have been reported to form amyloid fibrils called “SEVI” and enhance HIV-1 infection in vitro. In order to understand the biological consequence of this proviral effect, we extended these studies in the presence of human SP. PAP-derived peptides were agitated to form SEVI and incubated in the presence or absence of SP. While PAP-derived peptides and SEVI alone were proviral, the presence of 1% SP ablated their proviral activity in several different anti-HIV-1 assays. The anti-HIV-1 activity of SP was concentration dependent and was reduced following filtration. Supraphysiological concentrations of PAP peptides and SEVI incubated with diluted SP were degraded within hours, with SP exhibiting proteolytic activity at dilutions as high as 1∶200. Sub-physiological concentrations of two prominent proteases of SP, prostate-specific antigen (PSA) and matriptase, could degrade physiological and supraphysiological concentrations of PAP peptides and SEVI. While human SP is a complex biological fluid, containing both antiviral and proviral factors, our results suggest that PAP peptides and SEVI may be subject to naturally occurring proteolytic components capable of reducing their proviral activity.

Computational studies of protegrin antimicrobial peptides: a review

Antimicrobial peptides (AMPs) are small, naturally occurring peptides that exhibit strong antibacterial properties generally believed to be a result of selective bacterial membrane disruption. As a result, there has been significant interest in the development of therapeutic antibiotics based on AMPs; however, the poor understanding of the fundamental mechanism of action of these peptides has largely hampered such efforts. We present a summary of computational and theoretical investigations of protegrin, a particularly potent peptide that is both an excellent model for the mechanism of action of AMPs and a promising therapeutic candidate. Experimental investigations have shed light on many of the key steps in the action of protegrin: protegrin monomers are known to dimerize in various lipid environments; protegrin peptides interact strongly with lipid bilayer membranes, particularly anionic lipids; protegrins have been shown to form pores in lipid bilayers, which results in uncontrolled ion transport and may be a key factor in bacterial death. In this work, we present a comprehensive review of the computational and theoretical studies that have complemented and extended the information obtained from experimental work with protegrins, as well as a brief survey of the experimental biophysical studies that are most pertinent to such computational work. We show that a consistent, mechanistic description of the bactericidal mechanism of action of protegrins is emerging, and briefly outline areas where the current understanding is deficient. We hope that the research reviewed herein offers compelling evidence of the benefits of computational investigations of protegrins and other AMPs, as well as providing a useful guide to future work in this area.

Analysis of antimicrobial peptides from porcine neutrophils

Cationic host defence peptides are important components of innate immunity in pigs and other mammalians. Most of these peptides have a direct antimicrobial activity and they also have a broad spectrum of effects on the host immune system, which may be taken into account in the introduction of novel therapeutics. Our method permits simultaneous isolation of six antibacterial peptides, i.e. prophenin-1, prophenin-2, PR-39, and protegrins 1-3 from a porcine neutrophil crude extract and characterisation of them. Among the obtained peptides the greatest bactericidal activity expressed as MBC was seen in protegrins (10 μg/ml), whereas in the other studied peptides MBC was on the level of 20 μg/ml. Minimal inhibitory concentrations (MIC) reached 10 μg/ml for protegrins 1-3 and 20 μg/ml for prophenins, and PR-39. Within the bactericidal range all isolated peptides didn't show cytotoxicity on cell lines used in our experiment.

Antimicrobial protegrin-1 forms ion channels: molecular dynamic simulation, atomic force microscopy, and electrical conductance studies

Antimicrobial peptides (AMPs) are an emerging class of antibiotics for controlling health effects of antibiotic-resistant microbial strains. Protegrin-1 (PG-1) is a model antibiotic among beta-sheet AMPs. Antibiotic activity of AMPs involves cell membrane damage, yet their membrane interactions, their 3D membrane-associated structures and the mechanism underlying their ability to disrupt cell membrane are poorly understood. Using complementary approaches, including molecular dynamics simulations, atomic force microscopy (AFM) imaging, and planar lipid bilayer reconstitution, we provide computational and experimental evidence that PG-1, a beta-hairpin peptide, forms ion channels. Simulations indicate that PG-1 forms channel-like structures with loosely attached subunits when reconstituted in anionic lipid bilayers. AFM images show the presence of channel-like structures when PG-1 is reconstituted in dioleoylphosphatidylserine/palmitoyloleoyl phosphatidylethanolamine bilayers or added to preformed bilayers. Planar lipid bilayer electrical recordings show multiple single channel conductances that are consistent with the heterogeneous oligomeric channel structures seen in AFM images. PG-1 channel formation seems to be lipid-dependent: PG-1 does not easily show ion channel electrical activity in phosphatidylcholine membranes, but readily shows channel activity in membranes rich in phosphatidylethanolamine or phosphatidylserine. The combined results support a model wherein the beta-hairpin PG-1 peptide acts as an antibiotic by altering cell ionic homeostasis through ion channel formation in cell membranes.

C-terminal amidation of PMAP-23: translocation to the inner membrane of Gram-negative bacteria

PMAP-23 is a member of the cathelicidin family derived from pig myeloid cells and has potent antimicrobial activity. Amidation of the carboxyl terminus (C-terminus) of an antimicrobial peptide generally enhances its structural stability and antimicrobial activity or decreases its cytotoxicity. The aim of the present study was to investigate the effect of amidation on the mode of action in PMAP-23. Irrespective of amidation, PMAP-23 adopts a helix-hinge-helix structure in a membrane-mimetic environment. The antibacterial activities of PMAP-23C, which had a free C-terminus, and PMAP-23N, which had an amidated C-terminus, were similar against Gram-negative bacteria, reflecting a similar ability to neutralize lipopolysaccharide. However, PMAP-23N assumed a perpendicular orientation across the outer to the inner leaflet of the bacterial inner membrane, while PMAP-23C was orientated parallel to the lipid bilayer, as determined by following the blue shift in tryptophan fluorescence, as well as calcein release from liposomes and SYTOX Green uptake assays. These results suggest that N-terminal amidation of PMAP-23 provides structural stability and increases the peptide's cationic charge, facilitating translocation into the bacterial inner membrane.

Membrane aggregation and perturbation induced by antimicrobial peptide of S-thanatin

Thanatin, a 21-residue peptide, is an inducible insect peptide. In our previous study, we have identified a novel thanatin analog of S-thanatin, which exhibited a broad antimicrobial activity against bacteria and fungi with low hemolytic activity. This study was aimed to delineate the antimicrobial mechanism of S-thanatin and identify its interaction with bacterial membranes. In this study, membrane phospholipid was found to be the target for S-thanatin. In the presence of vesicles, S-thanatin interestingly led to the aggregation of anionic vesicles and sonicated bacteria. Adding S-thanatin to Escherichia coli suspension would result in the collapse of membrane and kill bacteria. The sensitivity assay of protoplast elucidated the importance of outer membrane (OM) for S-thanatin's antimicrobial activity. Compared with other antimicrobial peptide, S-thanatin produced chaotic membrane morphology and cell debris in electron microscopic appearance. These results supported our hypothesis that S-thanatin bound to negatively charged LPS and anionic lipid, impeded membrane respiration, exhausted the intracellular potential, and released periplasmic material, which led to cell death.

A novel antifungal analog peptide derived from protaetiamycine

Previously, the 9-mer analog peptides, 9Pbw2 and 9Pbw4, were designed based on a defensin-like peptide, protaetiamycine isolated from Protaetia brevitarsis. In this study, antifungal effects of the analog peptides were investigated. The antifungal susceptibility testing exhibited that 9Pbw4 contained more potent antifungal activities than 9Pbw2. A PI influx assay confirmed the effects of the analog peptides and demonstrated that the peptides exerted their activity by a membrane-active mechanism, in an energy-independent manner. As the noteworthy potency of 9Pbw4, the mechanism(s) of 9Pbw4 were further investigated. The membrane studies, using rhodamine-labeled giant unilamellar vesicle (GUV) and fluorescein isothiocyanate (FITC)-dextran loaded liposome, suggested that the membrane-active mechanism of 9Pbw4 could have originated from the poreforming action and the radii of pores was presumed to be anywhere from 1.8 nm to 3.3 nm. These results were confirmed by 3D-flow cytometric contour-plot analysis. The present study suggests a potential of 9Pbw4 as a novel antifungal peptide.

Poisson-Nernst-Planck models of nonequilibrium ion electrodiffusion through a protegrin transmembrane pore

Protegrin peptides are potent antimicrobial agents believed to act against a variety of pathogens by forming nonselective transmembrane pores in the bacterial cell membrane. We have employed 3D Poisson-Nernst-Planck (PNP) calculations to determine the steady-state ion conduction characteristics of such pores at applied voltages in the range of −100 to +100 mV in 0.1 M KCl bath solutions. We have tested a variety of pore structures extracted from molecular dynamics (MD) simulations based on an experimentally proposed octomeric pore structure. The computed single-channel conductance values were in the range of 290–680 pS. Better agreement with the experimental range of 40–360 pS was obtained using structures from the last 40 ns of the MD simulation, where conductance values range from 280 to 430 pS. We observed no significant variation of the conductance with applied voltage in any of the structures that we tested, suggesting that the voltage dependence observed experimentally is a result of voltage-dependent channel formation rather than an inherent feature of the open pore structure. We have found the pore to be highly selective for anions, with anionic to cationic current ratios (I_Cl−/I_K+) on the order of 10³. This is consistent with the highly cationic nature of the pore but surprisingly in disagreement with the experimental finding of only slight anionic selectivity. We have additionally tested the sensitivity of our PNP model to several parameters and found the ion diffusion coefficients to have a significant influence on conductance characteristics. The best agreement with experimental data was obtained using a diffusion coefficient for each ion set to 10% of the bulk literature value everywhere inside the channel, a scaling used by several other studies employing PNP calculations. Overall, this work presents a useful link between previous work focused on the structure of protegrin pores and experimental efforts aimed at investigating their conductance characteristics.

Protegrins are small peptides with strong antimicrobial properties, believed to kill bacteria primarily by forming nonselective pores in the bacterial membrane. This nonspecific and highly effective mechanism of action has created significant excitement about the use of protegrins as therapeutic antibiotics. However, a lack of understanding of the fundamental processes that lead to pore formation and bacterial death has proven to be a major bottleneck in the rational design of protegrin-based antibiotics. In the present work, we have carried out computational investigations of the diffusion of ions through a protegrin pore. We have thereby provided a connection between previous experimental and simulation work aimed at elucidating the structure of the protegrin pore and earlier experimental work investigating the ion transport characteristics of protegrin pores. The ion diffusion characteristics of protegrin pores are likely to be important in their ability to kill bacteria, as the uncontrolled flow of ions through a bacterial membrane will result in membrane depolarization and the loss of vital membrane functions. The present work thus represents an important first step in modeling and quantifying the timeline of events that lead to the killing of bacteria by protegrins. Furthermore, the computational tools that we have presented herein are easily extendible to similar systems, in particular other antimicrobial peptides.

Arginine dynamics in a membrane-bound cationic beta-hairpin peptide from solid-state NMR

The site-specific motion of Arg residues in a membrane-bound disulfide-linked antimicrobial peptide, protegrin-1 (PG-1), was investigated by using magic-angle-spinning solid-state NMR spectroscopy to better understand the membrane insertion and lipid interaction of this cationic membrane-disruptive peptide. The C-H and N-H dipolar couplings and 13C chemical shift anisotropies were measured in the anionic POPE/POPG membrane, and were found to be reduced from the rigid-limit values by varying extents; this indicates the presence of segmental motion. An Arg residue at the beta-turn region of the peptide showed much weaker spin interactions, which indicates larger amplitudes of motion than an Arg residue in the beta-strand region of the peptide. This is consistent with the exposure of the beta turn to the membrane surface and the immersion of the beta strand in the hydrophobic middle of the membrane, and supports the previously proposed oligomerization of the peptide into beta barrels in the anionic membrane. The 13C T2 and 1H T(1rho) relaxation times indicate that the beta-turn backbone undergoes large-amplitude intermediate-timescale motion in the fluid phase of the membrane; this causes significant line broadening and loss of spectral intensity. This study illustrates the strong correlation between the dynamics and structure of membrane proteins, and the capability of solid-state NMR spectroscopy to provide detailed information on site-specific dynamics in complex membrane-protein assemblies.

Influence of antimicrobial peptides on the formation of nonlamellar lipid mesophases

We have studied the influence of four antimicrobial peptides of different secondary and ternary structure--melittin (Mel), protegrin-1 (PG-1), peptidyl-glycylleucine-carboxyamide (PGLa), and gramicidin S (GS)--on the lamellar-to-nonlamellar transition of palmitoyloleoyl phosphatidylethanolamine (POPE) applying differential scanning calorimetry and small-angle X-ray diffraction. None of the peptides studied led to the formation of an inverted hexagonal phase observed for pure POPE at high temperatures. Instead either cubic or lamellar phases were stabilized to different degrees. GS was most effective in inducing a cubic phase, whereas Mel fully stabilized the lamellar phase. The behavior of POPE in the presence of PG-1 and PGLa was intermediate to GS and Mel. In addition to the known role of membrane elasticity we propose two mechanisms, which cause stabilization of the lamellar phase: electrostatic repulsion and lipid/peptide pore formation. Both mechanisms prevent transmembrane contact required to form either an inverted hexagonal phase or fusion pores, as precursors of the cubic phase.

Antimicrobial polypeptides are key anti-HIV-1 effector molecules of cervicovaginal host defense

Mucosal surfaces of the cervix and vagina are portals for heterosexual transmission of human immunodeficiency virus type 1 (HIV-1) and, therefore, play a fundamental role in the pathogenesis of primary infection. Cationic antimicrobial polypeptides including defensins are the principal effector molecules of mucosal innate immunity against microbes and viruses such as HIV. In cervicovaginal secretions, antimicrobial polypeptides constitute the majority of the intrinsic anti-HIV-1 activity, synergism between cationic polypeptides is complex, and full anti-HIV-1 activity involves the complete complement of cationic polypeptides. Periods in which cationic antimicrobial polypeptide expression is reduced are likely associated with increased susceptibility to HIV-1 infection. This review provides an overview of the role of cationic antimicrobial polypeptides in innate cervicovaginal anti-HIV-1 host defense, and discusses how hormones and bacterial infections can regulate their expression. Emphasis is placed on the theta-defensin (retrocyclin) class of anti-HIV-1 peptides and their potential for development as topical microbicides to prevent HIV-1 transmission.

Microbicidal properties and cytocidal selectivity of rhesus macaque theta defensins

Rhesus macaque theta-defensins (RTDs) are unique macrocyclic antimicrobial peptides. The three RTDs (RTD 1-3), isolated from macaque leukocytes, have broad-spectrum antimicrobial activities in vitro and share certain structural features with acyclic porcine protegrins, which are microbicidal peptides of the cathelicidin family. To understand the structural features that confer the respective cytocidal properties to theta-defensins and protegrins, we determined and compared the biological properties of RTD 1-3 and protegrin 1 (PG-1) in assays for antimicrobial activity, bacterial membrane permeabilization, and toxicity to human cells. RTD 1-3 and PG-1 had similar microbicidal potencies against Escherichia coli, Staphylococcus aureus, and Candida albicans in low-ionic-strength (10 mM) buffers at pH 7.4. The inclusion of physiologic sodium chloride partially inhibited the microbicidal activities of the RTDs, and the degree of inhibition depended on the buffer used in the assay. Similarly, the inclusion of 10% normal human serum partially antagonized the bactericidal activities of all four peptides. In contrast, the microbicidal activities of PG-1 and RTD 1-3 against E. coli were unaffected by physiologic concentrations of calcium chloride and magnesium chloride. Treatment of E. coli ML35 cells with RTD 1-3 or PG-1 rapidly rendered the bacteria permeable to omicron-nitrophenyl-beta-D-galactopyranoside, and this was accompanied by the rapid entry of the RTDs. Finally, although PG-1 was toxic to human fibroblasts and caused a marked lysis of erythrocytes, the RTDs were not cytotoxic or hemolytic. Thus, compared to PG-1, RTD 1-3 possess substantially greater cytocidal selectivity against microbes. Surprisingly, the low cytotoxicity of the RTDs did not depend on the peptides' cyclic conformation.

Mechanism of supported membrane disruption by antimicrobial peptide protegrin-1

While pore formation has been suggested as an important step in the membrane disruption process induced by antimicrobial peptides, membrane pore formation has never been directly visualized. We report on the dynamics of membrane disruption by antimicrobial peptide protegrin-1 (PG-1) on dimyristoyl-sn-glycero-phosphocholine-supported bilayer patches obtained via atomic force microscopy. The action of PG-1 is found to be concentration-dependent. At low PG-1 concentrations (1 < [PG-1] < 4 microg/mL), the peptide destabilizes the edge of the membrane to form fingerlike structures. At higher concentrations, PG-1 induces the formation of a sievelike nanoporous structure in the membrane. The highest degree of disruption is attained at concentrations >or=20 microg/mL, at which PG-1 disrupts the entire membrane, transforming it into stripelike structures with a well-defined and uniform stripe width. This first direct visualization of these membrane structural transformations helps elucidate the PG-1-induced membrane disruption mechanism.

Structure-activity relationship of HP (2-20) analog peptide: enhanced antimicrobial activity by N-terminal random coil region deletion

HP (2-20) (AKKVFKRLEKLFSKIQNDK) is a 19-aa antimicrobial peptide derived from N-terminus of Helicobacter pylori Ribosomal protein L1 (RpL1). In the previous study, several analogs with amino acid substitutions were designed to increase or decrease only the net hydrophobicity. In particular, substitutions of Gln(16) and Asp(18) with Trp (Anal 3) for hydrophobic amino acid caused a dramatic increase in antibiotic activity without a hemolytic effect. HP-A3 is a potent antimicrobial peptide that forms, in a hydrophobic medium, an amphipathic structure consisting of an N-terminal random coil region (residues 2-5) and extended C-terminal regular alpha-helical region (residues 6-20). To obtain the short and potent alpha-helical antimicrobial peptide, we synthesized a N-terminal random coil deleted HP-A3 (A3-NT) and examined their antimicrobial activity and mechanism of action. The resulting 15mer peptide showed increased antibacterial and antifungal activity to 2- and 4-fold, respectively, without hemolysis. Confocal fluorescence microscopy studies showed that A3-NT was accumulated in the plasma membrane. Flow cytometric analysis revealed that A3-NT acted in salt- and energy-independent manner. Furthermore, A3-NT causes significant morphological alterations of the bacterial surfaces as shown by scanning electron microscopy. Circular dichroism (CD) analysis revealed that A3-NT showed higher alpha-helical contents than the HP-A3 peptide in 50% TFE solution. Therefore, the cell-lytic efficiency of HP-A3, which depended on the alpha-helical content of peptide, correlated linearly with their antimicrobial potency.

Antimicrobial activity of omwaprin, a new member of the waprin family of snake venom proteins

We have isolated and characterized omwaprin, a 50-amino-acid cationic protein from the venom of inland taipan (Oxyuranus microlepidotus). It is a new member of the waprin family of snake venom proteins. A synthetic gene was designed and constructed for expressing the recombinant protein in Escherichia coli. Recombinant omwaprin was used for carrying out functional analyses. The protein is non-toxic to Swiss albino mice at doses of up to 10 mg/kg when administered intraperitoneally. However, it shows selective and dose-dependant antibacterial activity against Gram-positive bacteria. The minimum inhibitory doses were in the range 2-10 microg for selected species of bacteria in radial diffusion assays. The antibacterial activity is salt-tolerant up to 350 mM NaCl. However, omwaprin lost its antibacterial activity upon reduction and alkylation of its cysteine residues, or upon deletion of six N-terminal amino acid residues, four of which are positively charged. These observations indicate that the three-dimensional structure constrained by four disulfide bonds and the N-terminal residues are essential for its activity. The mechanism of action is via membrane disruption, as shown by scanning electron microscopy. Importantly, omwaprin lacks haemolytic activity on human erythrocytes. This demonstrates the specificity of omwaprin for bacterial membranes. Unlike other reported WAP (whey acidic protein) domain-containing antibacterial proteins, including elafin, EPPIN (epididymal proteinase inhibitor), SWAM1 and SWAM2 [single WAP (whey acidic protein) motif proteins 1 and 2] and SLPI (secretory leucocyte proteinase inhibitor), omwaprin shows species-specific activity on the Gram-positive bacteria tested.

Structure of the antimicrobial beta-hairpin peptide protegrin-1 in a DLPC lipid bilayer investigated by molecular dynamics simulation

All atom molecular dynamics simulations of the 18-residue beta-hairpin antimicrobial peptide protegrin-1 (PG-1, RGGRLCYCRRRFCVCVGR-NH(2)) in a fully hydrated dilauroylphosphatidylcholine (DLPC) lipid bilayer have been implemented. The goal of the reported work is to investigate the structure of the peptide in a membrane environment (previously solved only in solution [R.L. Fahrner, T. Dieckmann, S.S.L. Harwig, R.I. Lehrer, D. Eisenberg, J. Feigon, Solution structure of protegrin-1, a broad-spectrum antimicrobial peptide from porcine leukocytes. Chemistry and Biology, 3 (1996) 543-550]), and to delineate specific peptide-membrane interactions which are responsible for the peptide's membrane binding properties. A novel, previously unknown, "kick" shaped conformation of the peptide was detected, where a bend at the C-terminal beta-strand of the peptide caused the peptide backbone at residues 16-18 to extend perpendicular to the beta-hairpin plane. This bend was driven by a highly persistent hydrogen-bond between the polar peptide side-chain of TYR7 and the unshielded backbone carbonyl oxygen atom of GLY17. The H-bond formation relieves the unfavorable free energy of insertion of polar groups into the hydrophobic membrane core. PG-1 was anchored to the membrane by strong electrostatic binding of the protonated N-terminus of the peptide to the lipid head group phosphate anions. The orientation of the peptide in the membrane, and its influence on bilayer structural and dynamic properties are in excellent agreement with solid state NMR measurements [S. Yamaguchi, T. Hong, A. Waring, R.I. Lehrer, M. Hong, Solid-State NMR Investigations of Peptide-Lipid Interaction and Orientation of a b-Sheet Antimicrobial Peptide, Protegrin, Biochemistry, 41 (2002) 9852-9862]. Importantly, two simulations which started from different initial orientations of the peptide converged to the same final equilibrium orientation of the peptide relative to the bilayer. The kick-shaped conformation was observed only in one of the two simulations.

Roles of Salt and Conformation in the Biological and Physicochemical Behavior of Protegrin-1 and Designed Analogues: Correlation of Antimicrobial, Hemolytic, and Lipid Bilayer-Perturbing Activities

Protegrins are short (16-18 residues) cationic peptides from porcine leukocytes that display potent, broad-spectrum antimicrobial activity. Protegrin-1 (PG-1), one of five natural homologues, adopts a rigid beta-hairpin structure that is stabilized by two disulfide bonds. We have previously employed the principles of beta-hairpin design to develop PG-1 variants that lack disulfide bonds but nevertheless display potent antimicrobial activity [Lai, J. R., Huck, B. R., Weisblum, B., and Gellman, S. H. (2002) Biochemistry 41, 12835-12842.]. The activity of these disulfide-free variants, however, is attenuated in the presence of salt, and the activity of PG-1 itself is not. Salt-induced inactivation of host-defense peptides, such as human defensins, is thought to be important in some pathological situations (e.g., cystic fibrosis), and the variation in salt-sensitivity among our PG-1 analogues offers a model system with which to explore the origins of these salt effects. We find that the variations in antimicrobial activity among our peptides are correlated with the folding propensities of these molecules and with the extent to which the peptides induce leakage of contents from synthetic liposomes. Comparable correlations were observed between folding and hemolytic activity. The extent to which added salt reduces antimicrobial activity parallels salt effects on vesicle perturbation, which suggests that the biological effects of high salt concentrations arise from modulation of peptide-membrane interactions.

Membrane-dependent oligomeric structure and pore formation of a beta-hairpin antimicrobial peptide in lipid bilayers from solid-state NMR

We used solid-state NMR spectroscopy to investigate the oligomeric structure and insertion of protegrin-1 (PG-1), a beta-hairpin antimicrobial peptide, in lipid bilayers that mimic either the bacterial inner membrane [palmitoyloleoylphosphatidyl ethanolamine and palmitoyloleoylphosphatidylglycerol (POPE/POPG) bilayers] or the red blood cell membrane [neutral palmitoyloleoylphosphatidylcholine (POPC)/cholesterol bilayers]. (1)H spin diffusion from lipids to the peptide indicates that PG-1 contacts both the lipid acyl chains and the headgroups in the anionic membrane but resides far from the lipid chains in the POPC/cholesterol bilayer. (19)F spin diffusion data indicates that 75% of the beta-hairpins have homodimerized N strands and C strands in the anionic membrane. The resulting (NCCN)(n) multimer suggests a membrane-inserted beta-barrel enclosing a water pore. The lipids surrounding the beta-barrel have high orientational disorder and chain upturns, thus they may act as fillers for the pore. These results revise several features of the toroidal pore model, first proposed for magainin and subsequently applied to PG-1. In the POPC/cholesterol membrane, the N and C strands of PG-1 cluster into tetramers, suggesting the formation of beta-sheets on the membrane surface. Thus, the membrane composition plays a decisive role in defining the assembly and insertion of PG-1. The different oligomeric structures of PG-1 help to explain its greater toxicity for bacteria than for eukaryotic cells.