Determining the Effects of Membrane-Interacting Peptides on Membrane Integrity

Design, synthesis and structure-activity relationship (SAR) studies of an unusual class of non-cationic fatty amine-tripeptide conjugates as novel synthetic antimicrobial agents

Cationic ultrashort lipopeptides (USLPs) are promising antimicrobial candidates to combat multidrug-resistant bacteria. Using DICAMs, a newly synthesized family of tripeptides with net charges from -2 to +1 and a fatty amine conjugated to the C-terminus, we demonstrate that anionic and neutral zwitterionic USLPs can possess potent antimicrobial and membrane-disrupting activities against prevalent human pathogens such as Streptococcus pneumoniae and Streptococcus pyogenes. The strongest antimicrobials completely halt bacterial growth at low micromolar concentrations, reduce bacterial survival by several orders of magnitude, and may kill planktonic cells and biofilms. All of them comprise either an anionic or neutral zwitterionic peptide attached to a long fatty amine (16-18 carbon atoms) and show a preference for anionic lipid membranes enriched in phosphatidylglycerol (PG), which excludes electrostatic interactions as the main driving force for DICAM action. Hence, the hydrophobic contacts provided by the long aliphatic chains of their fatty amines are needed for DICAM's membrane insertion, while negative-charge shielding by salt counterions would reduce electrostatic repulsions. Additionally, we show that other components of the bacterial envelope, including the capsular polysaccharide, can influence the microbicidal activity of DICAMs. Several promising candidates with good-to-tolerable therapeutic ratios are identified as potential agents against S. pneumoniae and S. pyogenes. Structural characteristics that determine the preference for a specific pathogen or decrease DICAM toxicity have also been investigated.

Modulating lipid bilayer permeability and structure: Impact of hydrophobic chain length, C-3 hydroxyl group, and double bond in sphingosine

Sphingosine, an amphiphilic molecule, plays a pivotal role as the core structure of sphingolipids, essential constituents of cell membranes. Its unique capability to enhance the permeability of lipid membranes profoundly influences crucial life processes. The molecular structure of sphingosine dictates its mode of entry into lipid bilayers and governs its interactions with lipids, thereby determining membrane permeability. However, the incomplete elucidation of the relationship between the molecular structure of sphingosine and the permeability of lipid membranes persists due to challenges associated with synthesizing sphingosine molecules. A series of sphingosine-derived molecules, featuring diverse hydrophobic chain lengths and distinct headgroup structure, were meticulously designed and successfully synthesized. These molecules were employed to investigate the permeability of large unilamellar vesicles, functioning as model lipid bilayers. With a decrease in the hydrophobic chain length of sphingosine from C15 to C11, the transient leakage ratio of vesicle contents escalated from ∼ 13 % to ∼ 28 %. Although the presence of double bond did not exert a pronounced influence on transient leakage, it significantly affected the continuous leakage ratio. Conversely, modifying the chirality of the C-3 hydroxyl group gives the opposite result. Notably, methylation at the C-3 hydroxyl significantly elevates transient leakage while suppressing the continuous leakage ratio. Additionally, sphingosines that significantly affect vesicle permeability tend to have a more pronounced impact on cell viability. Throughout this leakage process, the charge state of sphingosine-derived molecule aggregates in the solution emerged as a pivotal factor influencing vesicle permeability. Fluorescence lifetime experiments further revealed discernible variations in the effect of sphingosine molecular structure on the mobility of hydrophobic regions within lipid bilayers. These observed distinctions emphasize the impact of molecular structure on intermolecular interactions, extending to the microscopic architecture of membranes, and underscore the significance of subtle alterations in molecular structure and their associated aggregation behaviors in governing membrane permeability.

Characterization of a New Immunosuppressive and Antimicrobial Peptide, DRS-DA2, Isolated from the Mexican Frog, Pachymedusa dacnicolor

Inflammatory and antimicrobial diseases constitute a major burden for society, and fighting them is a WHO strategic priority. Most of the treatments available to fight inflammatory diseases are anti-inflammatory drugs, such as corticosteroids or immunomodulators that lack cellular specificity and lead to numerous side effects. In addition to suppressing undesired inflammation and reducing disease progression, these drugs lessen the immune system protective functions. Furthermore, treating infectious diseases is more and more challenging due to the rise of microbial resistance to antimicrobial drugs. Thus, controlling the inflammatory process locally without compromising the ability to combat infections is an essential feature in the treatment of inflammatory diseases. We isolated three forms (DRS-DA2N, DRS-DA2NE, and DRS-DA2NEQ) of the same peptide, DRS-DA2, which belongs to the dermaseptin family, from the Mexican tree frog Pachymedusa dacnicolor. Interestingly, DRS-DA2N and DRS-DA2NEQ exhibit a dual activity by inducing the death of leukocytes as well as that of Gram-negative and Gram-positive bacteria, including multiresistant strains, without affecting other cells such as epithelial cells or erythrocytes. We showed that the death of both immune cells and bacteria is induced rapidly by DRS-DA2 and that the membrane is permeabilized, leading to the loss of membrane integrity. We also validated the capacity of DRS-DA2 to regulate the pool of inflammatory cells in vivo in a mouse model of noninfectious peritonitis. After the induction of peritonitis, a local injection of DRS-DA2N could decrease the number of inflammatory cells locally in the peritoneal cavity without inducing a systemic effect, as no changes in the number of inflammatory cells could be detected in blood or in the bone marrow. Collectively, these data suggest that this peptide could be a promising tool in the treatment of inflammatory diseases, such as inflammatory skin diseases, as it could reduce the number of inflammatory cells locally without suppressing the ability to combat infections.

The effect of membrane thickness on the membrane permeabilizing activity of the cyclic lipopeptide tolaasin II

Tolaasin II is an amphiphilic, membrane-active, cyclic lipopeptide produced by Pseudomonas tolaasii and is responsible for brown blotch disease in mushroom. To better understand the mode of action and membrane selectivity of tolaasin II and related lipopeptides, its permeabilizing effect on liposomes of different membrane thickness was characterized. An equi-activity analysis served to distinguish between the effects of membrane partitioning and the intrinsic activity of the membrane-bound peptide. It was found that thicker membranes require higher local peptide concentrations to become leaky. More specifically, the mole ratio of membrane-bound peptide per lipid needed to induce 50% leakage of calcein within 1 h, R_e ⁵⁰, increased monotonically with membrane thickness from 0.0016 for the 14:1 to 0.0070 for the 20:1 lipid-chains. Moreover, fast but limited leakage kinetics in the low-lipid regime were observed implying a mode of action based on membrane asymmetry stress in this time and concentration window. While the assembly of the peptide to oligomeric pores of defined length along the bilayer z-axis can in principle explain inhibition by increasing membrane thickness, it cannot account for the observed limited leakage. Therefore, reduced intrinsic membrane-permeabilizing activity with increasing membrane thickness is attributed here to the increased mechanical strength and order of thicker membranes.

The Membrane Activity of the Amphibian Temporin B Peptide Analog TB_KKG6K Sheds Light on the Mechanism That Kills Candida albicans

Temporin B (TB) is a 13-amino-acid-long, cationic peptide secreted by the granular glands of the European frog Rana temporaria. We recently showed that the modified TB peptide analog TB_KKG6K rapidly killed planktonic and sessile Candida albicans at low micromolar concentrations and was neither hemolytic nor cytotoxic to mammalian cells in vitro. The present study aimed to shed light into its mechanism of action, with a focus on its fungal cell membrane activity. We utilized different fluorescent dyes to prove that it rapidly induces membrane depolarization and permeabilization. Studies on model membrane systems revealed that the TB analog undergoes hydrophobic and electrostatic membrane interactions, showing a preference for anionic lipids, and identified phosphatidylinositol and cardiolipin as possible peptide targets. Fluorescence microscopy using fluorescein isothiocyanate-labeled TB_KKG6K in the presence of the lipophilic dye FM4-64 indicated that the peptide compromises membrane integrity and rapidly enters C. albicans cells in an energy-independent manner. Peptide-treated cells analyzed by cryo-based electron microscopy exhibited no signs of cell lysis; however, subcellular structures had disintegrated, suggesting that intracellular activity may form part of the killing mechanism of the peptide. Taken together, this study proved that TB_KKG6K compromises C. albicans membrane function, which explains the previously observed rapid, fungicidal mode of action and supports its great potential as a future anti-Candida therapeutic. IMPORTANCE Fungal infections with the opportunistic human pathogen C. albicans are associated with high mortality rates in immunocompromised patients. This is partly due to the yeast's ability to rapidly develop resistance toward currently available antifungals. Small, cationic, membrane-active peptides are promising compounds to fight against resistance development, as many of them effectuate rapid fungal cell death. This fast killing is believed to hamper the development of resistance, as the fungi do not have sufficient time to adapt to the antifungal compound. We previously reported that the synthetic variant of the amphibian TB peptide, TB_KKG6K, rapidly kills C. albicans. In the current study, the mechanism of action of the TB analog was investigated. We show that this TB analog is membrane-active and impairs cell membrane function, highlighting its potential to be developed as an attractive alternative anti-C. albicans therapeutic that may hinder the development of resistance.

A Multiplexed Cell-Free Assay to Screen for Antimicrobial Peptides in Double Emulsion Droplets

The global surge in bacterial resistance against traditional antibiotics triggered intensive research for novel compounds, with antimicrobial peptides (AMPs) identified as a promising candidate. Automated methods to systematically generate and screen AMPs according to their membrane preference, however, are still lacking. We introduce a novel microfluidic system for the simultaneous cell-free production and screening of AMPs for their membrane specificity. On our device, AMPs are cell-free produced within water-in-oil-in-water double emulsion droplets, generated at high frequency. Within each droplet, the peptides can interact with different classes of co-encapsulated liposomes, generating a membrane-specific fluorescent signal. The double emulsions can be incubated and observed in a hydrodynamic trapping array or analyzed via flow cytometry. Our approach provides a valuable tool for the discovery and development of membrane-active antimicrobials.

Applying flow cytometry to identify the modes of action of membrane-active peptides in a label-free and high-throughput fashion

Membrane-active peptides (MAPs) have several potential therapeutic uses, including as antimicrobial drugs. Many traditional methods used to evaluate the membrane interactions of MAPs have limited applicability. Low-throughput methods, such as microscopy, provide detailed information but often rely on fluorophore-labeled MAPs, and high-throughput assays, such as the calcein release assay, cannot assess the mechanism behind the disruption of vesicular-based lipid membranes. Here we present a flow cytometric assay that provides detailed information about the peptide-lipid membrane interactions on single artificial lipid vesicles while being high-throughput (1000-2000 vesicles/s) and based on label-free MAPs. We synthesized and investigated six MAPs with different modes of action to evaluate the versatility of the assay. The assay is based on the flow cytometric readouts from artificial lipid vesicles, including the fluorescence from membrane-anchored and core-encapsulated fluorophores, and the vesicle concentration. From these parameters, we were able to distinguish between MAPs that induce vesicle solubilization, permeation (pores/membrane distortion), and aggregation or fusion. Our flow cytometry findings have been verified by traditional methods, including the calcein release assay, dynamic light scattering, and fluorescence microscopy on giant unilamellar vesicles. We envision that the presented flow cytometric assay can be used for various types of peptide-lipid membrane studies, e.g. to identify new antibiotics. Moreover, the assay can easily be expanded to derive additional valuable information.

A Limitation of Using Dithionite Quenching to Determine the Topology of Membrane-inserted Proteins

Determining the topology of membrane-inserted proteins and peptides often relies upon indirect fluorescent measurements. One such technique uses NBD, an environmentally sensitive fluorophore that can be covalently linked to proteins. Relative to a hydrophilic environment, NBD in a hydrophobic environment shows an increase in emission intensity and a shift to shorter wavelengths. To gain further insight, NBD fluorescence can be chemically quenched using dithionite. As dithionite is an anion, it is only expected to penetrate the outer leaflet interfacial region and should be excluded from the hydrocarbon core, the inner leaflet, and the lumen of LUV. This assumption holds at neutral pH, where a large number of NBD/dithionite experiments are carried out. Here, we report control experiments in which LUV were directly labeled with NBD-PE to assess dithionite quenching in acidic conditions. Results showed that at acidic pH, dithionite moved more freely across the bilayer to quench the inner leaflet. For the buffer conditions used, dithionite exhibited a sharp change in behavior between pH 5.5 and 6.0. Therefore, in acidic conditions, dithionite could not differentiate in which leaflet the NBD resided.

Membrane-selective nanoscale pores in liposomes by a synthetically evolved peptide: implications for triggered release

Peptides that form nanoscale pores in lipid bilayers have potential applications in triggered release, but only if their selectivity for target synthetic membranes over bystander biomembranes can be optimized. Previously, we identified a novel family of α-helical pore-forming peptides called "macrolittins", which release macromolecular cargoes from phosphatidylcholine (PC) liposomes at concentrations as low as 1 peptide per 1000 lipids. In this work, we show that macrolittins have no measurable cytolytic activity against multiple human cell types even at high peptide concentration. This unprecedented selectivity for PC liposomes over cell plasma membranes is explained, in part, by the sensitivity of macrolittin activity to physical chemical properties of the bilayer hydrocarbon core. In the presence of cells, macrolittins release all vesicle-entrapped cargoes (proteins and small molecule drugs) which are then readily uptaken by cells. Triggered release occurs without any direct effect of the peptide on the cells, and without vesicle-vesicle or vesicle-cell interactions.

Efficacy of short-synthetic antifungal peptides on pathogenic Aspergillus flavus

The efficacies of three short synthetic antifungal peptides were tested for their inhibitory action on pathogenic fungi, Aspergillus flavus. The sequences of the short synthetic peptides are PPD1- FRLHF, 66-10-FRLKFH, 77-3- FRLKFHF, respectively. These test peptides inhibited fungal growth and showed a membranolytic activity. The fungal biomass and ergosterol levels were significantly low in peptides treated samples. Further, the fungal cell wall component chitin was also found to be lower in peptides treated samples. Scanning electron microscopic images also showed highly wrinkled fungal mycelia. Significant membrane permeabilisation as well as potassium ion leakage was also observed in fungal samples treated with peptides. To assess the membrane damage, the uptake of Sytox green dye was employed. At tested concentration, peptides induced fungal membrane damage as evidenced by the green fluorescence. Further, at tested concentration, these peptides induced an oxidative stress in A.flavus as evidenced by an increase in the ROS production, malondialdehyde levels, increase in the antioxidant enzymes - superoxide dismutase, catalase with concomitant decrease in the reduced glutathione content. Additionally, a growth dependent reduction in aflatoxin levels were also observed in peptides treated samples. Docking studies on the interaction of the peptides with a trans-membrane protein calcium ATPase of A. flavus showed that all the peptides were able to bind to the protein with high z rank score. The activity of the calcium ATPase was significantly decreased in peptides treated fungal samples, thereby validating the docking results. Among all the tested peptides, 77-3 peptide exhibited the maximal membrane damage property.

Functional and Structural Variation among Sticholysins, Pore-Forming Proteins from the Sea Anemone Stichodactyla helianthus

Venoms constitute complex mixtures of many different molecules arising from evolution in processes driven by continuous prey-predator interactions. One of the most common compounds in these venomous cocktails are pore-forming proteins, a family of toxins whose activity relies on the disruption of the plasmatic membranes by forming pores. The venom of sea anemones, belonging to the oldest lineage of venomous animals, contains a large amount of a characteristic group of pore-forming proteins known as actinoporins. They bind specifically to sphingomyelin-containing membranes and suffer a conformational metamorphosis that drives them to make pores. This event usually leads cells to death by osmotic shock. Sticholysins are the actinoporins produced by Stichodactyla helianthus. Three different isotoxins are known: Sticholysins I, II, and III. They share very similar amino acid sequence and three-dimensional structure but display different behavior in terms of lytic activity and ability to interact with cholesterol, an important lipid component of vertebrate membranes. In addition, sticholysins can act in synergy when exerting their toxin action. The subtle, but important, molecular nuances that explain their different behavior are described and discussed throughout the text. Improving our knowledge about sticholysins behavior is important for eventually developing them into biotechnological tools.

Membrane activity of two short Trp-rich amphipathic peptides

Short linear antimicrobial peptides are attractive templates for developing new antibiotics. Here, it is described a study of the interaction between two short Trp-rich peptides, horine and verine-L, and model membranes. Isothermal titration calorimetry studies showed that the affinity of these peptides towards large unilamellar vesicles (LUV) having a lipid composition mimicking the lipid composition of S. aureus membranes is ca. 30-fold higher than that towards E. coli mimetics. The former interaction is driven by enthalpy and entropy, while the latter case is driven by entropy, suggesting differences in the forces that play a role in the binding to the two types of model membranes. Upon membrane binding the peptides acquired different conformations according to circular dichroism (CD) studies; however, in both cases CD studies indicated stacked W-residues. Peptide-induced membrane permeabilization, lipid flip-flop, molecular packing at the membrane-water interface, and lateral lipid segregation were observed in all cases. However, the extent of these peptide-induced changes on membrane properties was always higher in S. aureus than E. coli mimetics. Both peptides seem to act via a similar mechanism of membrane permeabilization of S. aureus membrane mimetics, while their mechanisms seem to differ in the case of E. coli. This may be the result of differences in both the peptides´ structure and the membrane lipid composition between both types of bacteria.

The Mechanism of Membrane Permeabilization by Peptides: Still an Enigma

Peptide-induced permeabilization of lipid vesicles has been measured for decades and has provided many insights into the sequence-structure-function relationships of membrane-active peptides. However, researchers in the field have noted that many experiments show transient permeabilization, in which a burst of leakage occurs immediately after peptide addition, followed by a slowdown or cessation of leakage before all contents have been released. This widely observed, but rarely studied, phenomenon is not explained by standard equilibrium pore models that are commonly invoked in both experimental and computational studies. Here we discuss observations of transient permeabilization, and we outline a pathway towards understanding this enigmatic phenomenon.

Mechanistic Landscape of Membrane-Permeabilizing Peptides

Membrane permeabilizing peptides (MPPs) are as ubiquitous as the lipid bilayer membranes they act upon. Produced by all forms of life, most membrane permeabilizing peptides are used offensively or defensively against the membranes of other organisms. Just as nature has found many uses for them, translational scientists have worked for decades to design or optimize membrane permeabilizing peptides for applications in the laboratory and in the clinic ranging from antibacterial and antiviral therapy and prophylaxis to anticancer therapeutics and drug delivery. Here, we review the field of membrane permeabilizing peptides. We discuss the diversity of their sources and structures, the systems and methods used to measure their activities, and the behaviors that are observed. We discuss the fact that "mechanism" is not a discrete or a static entity for an MPP but rather the result of a heterogeneous and dynamic ensemble of structural states that vary in response to many different experimental conditions. This has led to an almost complete lack of discrete three-dimensional active structures among the thousands of known MPPs and a lack of useful or predictive sequence-structure-function relationship rules. Ultimately, we discuss how it may be more useful to think of membrane permeabilizing peptides mechanisms as broad regions of a mechanistic landscape rather than discrete molecular processes.

Identification of Peptides in Flowers of Sambucus nigra with Antimicrobial Activity against Aquaculture Pathogens

The elder (Sambucus spp.) tree has a number of uses in traditional medicine. Previous studies have demonstrated the antimicrobial properties of elderberry liquid extract against human pathogenic bacteria and also influenza viruses. These properties have been mainly attributed to phenolic compounds. However, other plant defense molecules, such as antimicrobial peptides (AMPs), may be present. Here, we studied peptide extracts from flowers of Sambucus nigra L. The mass spectrometry analyses determined peptides of 3 to 3.6 kDa, among them, cysteine-rich peptides were identified with antimicrobial activity against various Gram-negative bacteria, including recurrent pathogens of Chilean aquaculture. In addition, membrane blebbing on the bacterial surface after exposure to the cyclotide was visualized by SEM microscopy and SYTOX Green permeabilization assay showed the ability to disrupt the bacterial membrane. We postulate that these peptides exert their action by destroying the bacterial membrane.

Spontaneous Membrane Translocating Peptides: The Role of Leucine-Arginine Consensus Motifs

We previously used an orthogonal high-throughput screen to select peptides that spontaneously cross synthetic lipid bilayers without bilayer disruption. Many of the 12-residue spontaneous membrane translocating peptides (SMTPs) selected from the library contained a 5-residue consensus motif, LRLLR in positions 5-9. We hypothesized that the conserved motif could be a necessary and sufficient minimal motif for translocation. To test this and to explore the mechanism of spontaneous membrane translocation, we synthesized seven arginine placement variants of LRLLRWC and compared their membrane partitioning, translocation, and perturbation to one of the parent SMTPs, called "TP2". Several motif variant peptides translocate into synthetic vesicles with rates that are similar to TP2. However, the peptide containing the selected motif, LRLLRWC, was not the fastest; sequence context is also important for translocation efficiency. Although none of these peptides permeabilize bilayers, the motif peptides translocate faster at higher peptide to lipid ratios, suggesting that bilayer perturbation and/or cooperative interactions are important for their translocation. On the other hand, TP2 translocates slower as its concentration is increased, suggesting that TP2 translocates as a monomer and is inhibited by lateral interactions in the membrane. TP2 and the LRLLR motif peptide induce lipid translocation, suggesting that lipids chaperone them across the bilayer. The other motif peptides do not induce lipid flip-flop, suggesting an alternate mechanism. Concatenated motifs translocate slower than the motifs alone. Variants of TP2 with shorter and longer arginine side-chain analogs translocate slower than TP2. In summary, these results suggest that multiple patterns of leucine and arginine can support spontaneous membrane translocation, and that sequence context is important for the contribution of the motifs. Because motifs do not make simple, additive contributions to spontaneous translocation, rational engineering of novel SMTPs will remain difficult, providing even more reason to pursue SMTP discovery with synthetic molecular evolution.

Ebola Virus Delta Peptide Is a Viroporin

The Ebola virus (EBOV) genome encodes a partly conserved 40-residue nonstructural polypeptide, called the delta peptide, that is produced in abundance during Ebola virus disease (EVD). The function of the delta peptide is unknown, but sequence analysis has suggested that delta peptide could be a viroporin, belonging to a diverse family of membrane-permeabilizing small polypeptides involved in replication and pathogenesis of numerous viruses. Full-length and conserved C-terminal delta peptide fragments permeabilize the plasma membranes of nucleated cells of rodent, dog, monkey, and human origin; increase ion permeability across confluent cell monolayers; and permeabilize synthetic lipid bilayers. Permeabilization activity is completely dependent on the disulfide bond between the two conserved cysteines. The conserved C-terminal portion of the peptide is biochemically stable in human serum, and most serum-stable fragments have full activity. Taken together, the evidence strongly suggests that Ebola virus delta peptide is a viroporin and that it may be a novel, targetable aspect of Ebola virus disease pathology.IMPORTANCE During the unparalleled West African outbreak of Ebola virus disease (EVD) that began in late 2013, the lack of effective countermeasures resulted in chains of serial infection and a high mortality rate among infected patients. A better understanding of disease pathology is desperately needed to develop better countermeasures. We show here that the Ebola virus delta peptide, a conserved nonstructural protein produced in large quantities by infected cells, has the characteristics of a viroporin. This information suggests a critical role for the delta peptide in Ebola virus disease pathology and as a possible target for novel countermeasures.

Macromolecule Biosynthesis Assay and Fluorescence Spectroscopy Methods to Explore Antimicrobial Peptide Mode(s) of Action

Antimicrobial peptides (AMPs) are viable alternatives to the currently available antimicrobials, and numerous studies have investigated their possible use as therapeutic agents for specific clinical applications. AMPs are a diverse class of antimicrobials that often act upon the bacterial cell membrane but may exhibit additional modes of action. Identification of the multiple modes of action requires a comprehensive study at subinhibitory concentrations and careful data analysis since additional modes of action can be eclipsed by AMP action on the cell membrane.Techniques that measure the biosynthesis rate of macromolecules (e.g., DNA, RNA, protein, and cell wall) and the cytoplasmic membrane proton motive force (PMF) energy can help to unravel the diverse modes of action of AMPs. Here, we present an overview of macromolecule biosynthesis rate measurement and fluorescence spectroscopy methods to identify AMP mode(s) of action. Detailed protocols designed to measure inhibition of DNA, RNA, protein, and cell wall synthesis or membrane de-energization are presented and discussed for optimal application of these two techniques as well as to enable accurate interpretation of the experimental findings.

Applying Fluorescence Correlation Spectroscopy to Investigate Peptide-Induced Membrane Disruption

There is considerable interest in understanding the interactions of antimicrobial peptides with phospholipid membranes. Fluorescence correlation spectroscopy (FCS) is a powerful experimental technique that can be used to gain insight into these interactions. Specifically, FCS can be used to quantify leakage of fluorescent molecules of different sizes from large unilamellar lipid vesicles, thereby providing a tool for estimating the size of peptide-induced membrane disruptions. If fluorescently labeled lipids are incorporated into the membranes of the vesicles, FCS can also be used to obtain information about whether leakage occurs due to localized membrane perturbations or global membrane destabilization. Here, we outline a detailed step-by-step protocol on how to optimally implement an FCS-based leakage assay. To make the protocol easily accessible to other researchers, it has been supplemented with a number of practical tips and tricks.

Conformational Fine-Tuning of Pore-Forming Peptide Potency and Selectivity

To better understand the sequence-structure-function relationships that control the activity and selectivity of membrane-permeabilizing peptides, we screened a peptide library, based on the archetypal pore-former melittin, for loss-of-function variants. This was accomplished by assaying library members for failure to cause leakage of entrapped contents from synthetic lipid vesicles at a peptide-to-lipid ratio of 1:20, 10-fold higher than the concentration at which melittin efficiently permeabilizes the same vesicles. Surprisingly, about one-third of the library members are inactive under these conditions. In the negative peptides, two changes of hydrophobic residues to glycine were especially abundant. We show that loss-of-function activity can be completely recapitulated by a single-residue change of the leucine at position 16 to glycine. Unlike the potently cytolytic melittin, the loss-of-function peptides, including the single-site variant, are essentially inactive against phosphatidylcholine vesicles and multiple types of eukaryotic cells. Loss of function is shown to result from a shift in the binding-folding equilibrium away from the active, bound, α-helical state toward the inactive, unbound, random-coil state. Accordingly, the addition of anionic lipids to synthetic lipid vesicles restored binding, α-helical secondary structure, and potent activity of the "negative" peptides. While nontoxic to mammalian cells, the single-site variant has potent bactericidal activity, consistent with the anionic nature of bacterial membranes. The results show that conformational fine-tuning of helical pore-forming peptides is a powerful way to modulate their activity and selectivity.