Influence of tryptophan residues on melittin's hemolytic activity

Antimicrobial Peptides and Their Mimetics: Promising Candidates of Next-Generation Therapeutic Agents Combating Multidrug-Resistant Bacteria

The increasing morbidity and mortality caused by multidrug-resistant bacteria alerts human beings to the fact that conventional antibiotics are no longer reliable and effective alternatives are imperatively needed. Owing to wide range of sources, diverse structures, and unique mode of action, antimicrobial peptides have been highly anticipated and extensively studied in recent years. Besides, the integration of artificial intelligence helps researchers gain access to the vast unexplored chemical space, which opens more opportunities for the optimization and design of novel structures. Moreover, Due to advances in chemistry and synthetic biology, researchers have also begun to focus on the potential of chemical mimetics of antimicrobial peptides. In this review, a comprehensive discussion about natural and synthesized antimicrobial peptides as well as their chemical mimetics is made, so as to provide a comprehensive summary of this field and inspire follow-up research.

BsR1, a broad-spectrum antibacterial peptide with potential for plant protection

IMPORTANCE

This study addresses the critical need for new antibacterial drugs in the face of bacterial multidrug resistance resulting from antibiotic overuse. It highlights the significance of antimicrobial peptides as essential components of innate immunity in animals and plants, which have been proven effective against multidrug-resistant bacteria and are difficult to develop resistance against. This study successfully synthesizes a broad-spectrum antibacterial peptide, BsR1, with strong inhibitory activities against various Gram-positive and Gram-negative bacteria. BsR1 demonstrates favorable stability and a mode of action that damages bacterial cell membranes, leading to cell death. It also exhibits biological safety and shows potential in enhancing disease resistance in rice. This research offers a novel approach and potential medication for antibacterial drug development, presenting a valuable tool in combating pathogenic microorganisms, particularly in plants.

Recent advances in melittin-based nanoparticles for antitumor treatment: from mechanisms to targeted delivery strategies

As a naturally occurring cytolytic peptide, melittin (MLT) not only exhibits a potent direct tumor cell-killing effect but also possesses various immunomodulatory functions. MLT shows minimal chances for developing resistance and has been recognized as a promising broad-spectrum antitumor drug because of this unique dual mechanism of action. However, MLT still displays obvious toxic side effects during treatment, such as nonspecific cytolytic activity, hemolytic toxicity, coagulation disorders, and allergic reactions, seriously hampering its broad clinical applications. With thorough research on antitumor mechanisms and the rapid development of nanotechnology, significant effort has been devoted to shielding against toxicity and achieving tumor-directed drug delivery to improve the therapeutic efficacy of MLT. Herein, we mainly summarize the potential antitumor mechanisms of MLT and recent progress in the targeted delivery strategies for tumor therapy, such as passive targeting, active targeting and stimulus-responsive targeting. Additionally, we also highlight the prospects and challenges of realizing the full potential of MLT in the field of tumor therapy. By exploring the antitumor molecular mechanisms and delivery strategies of MLT, this comprehensive review may inspire new ideas for tumor multimechanism synergistic therapy.

Challenges and advances in antimicrobial peptide development

Microbial resistance is a major concern for public health worldwide, mainly because of the inappropriate use of antimicrobials. In this scenario, antimicrobial peptides (AMPs) have emerged as a potential therapeutic alternative means by which to control infectious diseases, because of their broad spectrum of action. However, some challenges can make their clinical application problematic, including metabolic instability and toxicity. Here, we provide a clear description of AMPs as promising molecules for the development of unusual antimicrobial drugs. We also describe current strategies used to overcome the main difficulties related to AMP clinical application, including different peptide designs and nanoformulation.

Peptide-Based Nanoparticles for Systemic Extrahepatic Delivery of Therapeutic Nucleotides

Peptide-based nanoparticles (PBN) for nucleotide complexation and targeting of extrahepatic diseases are gaining recognition as potent pharmaceutical vehicles for fine-tuned control of protein production (up- and/or down-regulation) and for gene delivery. Herein, we review the principles and mechanisms underpinning self-assembled formation of PBN, cellular uptake, endosomal release, and delivery to extrahepatic disease sites after systemic administration. Selected examples of PBN that have demonstrated recent proof of concept in disease models in vivo are summarized to offer the reader a comparative view of the field and the possibilities for clinical application.

Inhibition of Melittin Activity Using a Small Molecule with an Indole Ring

We investigated d-amino acids as potential inhibitors targeting l-peptide toxins. Among the l- and d-amino acids tested, we found that d-tryptophan (d-Trp) acted as an inhibitor of melittin-induced hemolysis. We then evaluated various Trp derivatives and found that 5-chlorotryptamine (5CT) had the largest inhibitory effect on melittin. The indole ring, amino group, and steric hindrance of an inhibitor played important roles in the inhibition of melittin activity. Despite the small size and simple molecular structure of 5CT, its IC₅₀ was approximately 13 μg/mL. Fluorescence quenching, circular dichroism measurements, and size-exclusion chromatography revealed that 5CT interacted with Trp19 in melittin and affected the formation of the melittin tetramer involved in hemolysis. Molecular dynamics simulation of melittin also indicated that the interaction of 5CT with Trp19 in melittin affected the formation of the tetramer.

Anionically Functionalized Glycogen Encapsulates Melittin by Multivalent Interaction

We developed acid-functionalized glycogen conjugates as supramolecular carriers for efficient encapsulation and inhibition of a model cationic peptide melittin─the main component of honeybee venom. For this purpose, we synthesized and characterized a set of glycogens, functionalized to various degrees by several different acid groups. These conjugates encapsulate melittin up to a certain threshold amount, beyond which they precipitate. Computer simulations showed that sufficiently functionalized conjugates electrostatically attract melittin, resulting in its efficient encapsulation in a broad pH range around the physiological pH. Hemolytic assays confirmed in vitro that the effective inhibition of melittin's hemolytic activity occurs for highly functionalized samples, whereas no inhibition is observed when using low-functionalized conjugates. It can be concluded that functional glycogens are promising carriers for cationic molecular cargos or antidotes against animal venoms under conditions, in which suitable properties such as biodegradability and biocompatibility are crucial.

Melittin Tryptophan Substitution with a Fluorescent Amino Acid Reveals the Structural Basis of Selective Antitumor Effect and Subcellular Localization in Tumor Cells

Melittin is a membrane-active peptide with strong anticancer activity against various cancers. Despite decades of research, the role of the singular Trp in the anticancer activity and selectivity of melittin remains poorly understood. Here, we propose a theranostic solution based on the substitution of Trp19 with a noncanonical fluorescent amino acid (DapAMCA). The introduction of DapAMCA residue in melittin stabilized the helical structure of the peptide, as evaluated by circular dichroism spectra and molecular dynamics simulations. In vitro hemolytic and anticancer activity assays revealed that introducing DapAMCA residue in melittin changed its mode of action with the cell membrane, resulting in reduced hemolytic toxicity and an improved the selectivity index (SI), with up to a five-fold increase compared to melittin. In vitro fluorescence imaging of DapAMCA-labeled melittin (MELFL) in cancer cells demonstrated high membrane-penetrating activity, with strong nuclear and nucleolar localization ability. These findings provide implications for novel anticancer therapies based on Trp-substituted designs and nuclear/nucleolar targeted therapy.

Traditional and Computational Screening of Non-Toxic Peptides and Approaches to Improving Selectivity

Peptides have positively impacted the pharmaceutical industry as drugs, biomarkers, or diagnostic tools of high therapeutic value. However, only a handful have progressed to the market. Toxicity is one of the main obstacles to translating peptides into clinics. Hemolysis or hemotoxicity, the principal source of toxicity, is a natural or disease-induced event leading to the death of vital red blood cells. Initial screenings for toxicity have been widely evaluated using erythrocytes as the gold standard. More recently, many online databases filled with peptide sequences and their biological meta-data have paved the way toward hemolysis prediction using user-friendly, fast-access machine learning-driven programs. This review details the growing contributions of in silico approaches developed in the last decade for the large-scale prediction of erythrocyte lysis induced by peptides. After an overview of the pharmaceutical landscape of peptide therapeutics, we highlighted the relevance of early hemolysis studies in drug development. We emphasized the computational models and algorithms used to this end in light of historical and recent findings in this promising field. We benchmarked seven predictors using peptides from different data sets, having 7-35 amino acids in length. According to our predictions, the models have scored an accuracy over 50.42% and a minimal Matthew's correlation coefficient over 0.11. The maximum values for these statistical parameters achieved 100.0% and 1.00, respectively. Finally, strategies for optimizing peptide selectivity were described, as well as prospects for future investigations. The development of in silico predictive approaches to peptide toxicity has just started, but their important contributions clearly demonstrate their potential for peptide science and computer-aided drug design. Methodology refinement and increasing use will motivate the timely and accurate in silico identification of selective, non-toxic peptide therapeutics.

Analogues of a Cyclic Antimicrobial Peptide with a Flexible Linker Show Promising Activity against Pseudomonas aeruginosa and Staphylococcus aureus

The emergence of multi-drug resistant bacteria is becoming a major health concern. New strategies to combat especially Gram-negative pathogens are urgently needed. Antimicrobial peptides (AMPs) found in all multicellular organisms act as a first line of defense in immunity. In recent years, AMPs have attracted increasing attention as potential antibiotics. Naturally occurring antimicrobial cyclic lipopeptides include colistin and daptomycin, both of which contain a flexible linker. We previously reported a cyclic AMP BSI-9 cyclo(Lys-Nal-Lys-Lys-Bip-O₂Oc-Nal-Lys-Asn) containing a flexible linker, with a broad spectrum of activity against bacterial strains and low hemolytic activity. In this study, improvement of the antimicrobial activity of BSI-9, against the European Committee on Antimicrobial Susceptibility Testing (EUCAST) strains of S. aureus, E. coli, A. baumannii, and P. aeruginosa was examined. This led to synthesis of eighteen peptide analogues of BSI-9, produced in four individual stages, with a different focus in each stage; cyclization point, hydrophobicity, cationic side-chain length, and combinations of the last two. Specifically the modified compound 11, exhibited improved activity against Staphylococcus aureus and Pseudomonas aeruginosa with MIC of 4 µg/mL and 8 µg/mL, respectively, compared to the original BSI-9, which had an MIC of 16–32 µg/mL.

Strategies in Translating the Therapeutic Potentials of Host Defense Peptides

The golden era of antibiotics, heralded by the discovery of penicillin, has long been challenged by the emergence of antimicrobial resistance (AMR). Host defense peptides (HDPs), previously known as antimicrobial peptides, are emerging as a group of promising antimicrobial candidates for combatting AMR due to their rapid and unique antimicrobial action. Decades of research have advanced our understanding of the relationship between the physicochemical properties of HDPs and their underlying antimicrobial and non-antimicrobial functions, including immunomodulatory, anti-biofilm, and wound healing properties. However, the mission of translating novel HDP-derived molecules from bench to bedside has yet to be fully accomplished, primarily attributed to their intricate structure-activity relationship, toxicity, instability in host and microbial environment, lack of correlation between in vitro and in vivo efficacies, and dwindling interest from large pharmaceutical companies. Based on our previous experience and the expanding knowledge gleaned from the literature, this review aims to summarize the novel strategies that have been employed to enhance the antimicrobial efficacy, proteolytic stability, and cell selectivity, which are all crucial factors for bench-to-bedside translation of HDP-based treatment. Strategies such as residues substitution with natural and/or unnatural amino acids, hybridization, L-to-D heterochiral isomerization, C- and N-terminal modification, cyclization, incorporation with nanoparticles, and "smart design" using artificial intelligence technology, will be discussed. We also provide an overview of HDP-based treatment that are currently in the development pipeline.

Melittin-Containing Hybrid Peptide Hydrogels for Enhanced Photothermal Therapy of Glioblastoma

The design of biocompatible and efficacious anticancer biomaterials to achieve relatively low tumor recurrence rates is the main pursuit of cancer photothermal therapy (PTT). RADA16-I is a synthetic amphiphilic peptide with the sequence RADARADARADARADA that can self-assemble into a peptide nanofiber hydrogel. In this study, we synthesized a novel melittin-RADA₃₂-indocyanine green (ICG) hydrogel ("MRI hydrogel"), which contains melittin in the peptide hydrogel backbone and ICG in the hydrogel matrix, for enhanced PTT of glioblastomas. The MRI hydrogel exhibited physiologic characteristics similar to those of the RADA₁₆ hydrogel, while displaying concentration-dependent cytotoxicity to C6 glioma cells and photothermal effects. The in vivo biodistribution of the MRI hydrogel was visualized by near-infrared fluorescence and photoacoustic imaging. More importantly, in vivo PTT provided by the MRI hydrogel significantly reduced the tumor size and the tumor recurrence rate compared with the RADR₁₆-ICG hydrogel and other controls, suggesting a synergistic effect of MRI hydrogel-carried melittin and ICG-based PTT treatment. Thus, MRI provides an alternative tool for the safe and efficient PTT treatment of tumors.

Heterologous expression of abaecin peptide from Apis mellifera in Pichia pastoris

Background

Antimicrobial peptides (AMPs) are the first line of host immune defense against pathogens. Among AMPs from the honeybee Apis mellifera, abaecin is a major broad-spectrum antibacterial proline-enriched cationic peptide.

Results

For heterologous expression of abaecin in Pichia pastoris, we designed an ORF with HisTag, and the codon usage was optimized. The gene was chemically synthetized and cloned in the pUC57 vector. The new ORF was sub-cloned in the pPIC9 expression vector and transformed into P. pastoris. After selection of positive clones, the expression was induced by methanol. The supernatant was analyzed at different times to determine the optimal time for the recombinant peptide expression. As a proof-of-concept, Escherichia coli was co-incubated with the recombinant peptide to verify its antimicrobial potential.

Discussion

Briefly, the recombinant Abaecin (rAbaecin) has efficiently decreased E. coli growth (P < 0.05) through an in vitro assay, and may be considered as a novel therapeutic agent that may complement other conventional antibiotic therapies.

Hemolytic Activity of Antimicrobial Peptides

For antimicrobial peptides to be interesting for systemic applications, they must show low toxicity against erythrocytes. In this chapter, we describe a protocol for measuring the ability of AMPs to lyse human red blood cells, using melittin as positive control.

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.

Mechanistic and functional aspects of the interaction of AR-23 with mammalian cell membrane and improvement of branched polyethylenimine-mediated gene transfection

BACKGROUND

Previous studies have suggested that reducing the positive charge of melittin could increase endosomal release activity and improve branched polyethylenimine (BPEI)-mediated transfection. AR-23 is a melittin-related peptide from Rana tagoi, which shows 81% sequence identity with melittin but has less positively-charged residues than melittin. The present study aimed to investigate the mechanistic and functional aspects of the interaction of AR-23 with mammalian cells and thus improve BPEI-mediated gene transfection.

METHODS

AR23 and two AR-23 analogs (AR-20 without positively-charged residues and AR-26 with the same positively-charged residues as melittin) were analyzed. Circular dichroism (CD) spectrometry was used to analyze the secondary structures of the peptides. Peptide-induced depolarization of cell membrane, the membrane-lytic activity of the peptides, and their potency with respect to enhancing the cellular uptake of calcein were evaluated. The physicochemical characters of complexes were measured and the effect of the peptides on BPEI-mediated transfection was determined.

RESULTS

The CD spectra results indicated that a positive charge in AR-23 played a crucial role in maintaining the α-helical conformation, whereas an extra positive charge could not increase α-helical formation. AR-23 displayed a similar depolarization ability to melittin. However, AR-23 showed a lower membrane lytic activity under physiological conditions and a higher lytic activity at endosomal pH than melittin and AR-26, which possess more positive charges. Compared to melittin and AR-26, AR-23, with a higher endosomal escaping activity, resulted in a higher enhancement of BPEI-mediated gene transfection, as well as the maintainance of a lower cytotoxicity.

CONCLUSIONS

We suggest that AR-23 may be considered as a potential enhancer for improving the transfection efficiency of cationic polymers.

Clinical efficacy of a specifically targeted antimicrobial peptide mouth rinse: targeted elimination of Streptococcus mutans and prevention of demineralization

BACKGROUND/AIMS

Streptococcus mutans, the major etiological agent of dental caries, has a measurable impact on domestic and global health care costs. Though persistent in the oral cavity despite conventional oral hygiene, S. mutans can be excluded from intact oral biofilms through competitive exclusion by other microorganisms. This suggests that therapies capable of selectively eliminating S. mutans while limiting the damage to the normal oral flora might be effective long-term interventions to fight cariogenesis. To meet this challenge, we designed C16G2, a novel synthetic specifically targeted antimicrobial peptide with specificity for S. mutans. C16G2 consists of a S. mutans-selective 'targeting region' comprised of a fragment from S. mutans competence stimulation peptide (CSP) conjoined to a 'killing region' consisting of a broad-spectrum antimicrobial peptide (G2). In vitro studies have indicated that C16G2 has robust efficacy and selectivity for S. mutans, and not other oral bacteria, and affects targeted bacteria within seconds of contact.

METHODS

In the present study, we evaluated C16G2 for clinical utility in vitro, followed by a pilot efficacy study to examine the impact of a 0.04% (w/v) C16G2 rinse in an intra-oral remineralization/demineralization model.

RESULTS AND CONCLUSIONS

C16G2 rinse usage was associated with reductions in plaque and salivary S. mutans, lactic acid production, and enamel demineralization. The impact on total plaque bacteria was minimal. These results suggest that C16G2 is effective against S. mutans in vivo and should be evaluated further in the clinic.

The role of electrostatic interactions in the membrane binding of melittin

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

Lipid membrane editing with peptide cargo linkers in cells and synthetic nanostructures

Current strategies for deploying synthetic nanocarriers involve the creation of agents that incorporate targeting ligands, imaging agents, and/or therapeutic drugs into particles as an integral part of the formulation process. Here we report the development of an amphipathic peptide linker that enables postformulation editing of payloads without the need for reformulation to achieve multiplexing capability for lipidic nanocarriers. To exemplify the flexibility of this peptide linker strategy, 3 applications were demonstrated: converting nontargeted nanoparticles into targeting vehicles; adding cargo to preformulated targeted nanoparticles for in vivo site-specific delivery; and labeling living cells for in vivo tracking. This strategy is expected to enhance the clinical application of molecular imaging and/or targeted therapeutic agents by offering extended flexibility for multiplexing targeting ligands and/or drug payloads that can be selected after base nanocarrier formulation.

Engineered Saccharomyces cerevisiae strain for improved xylose utilization with a three-plasmid SUMO yeast expression system

A three-plasmid yeast expression system utilizing the portable small ubiquitin-like modifier (SUMO) vector set combined with the efficient endogenous yeast protease Ulp1 was developed for production of large amounts of soluble functional protein in Saccharomyces cerevisiae. Each vector has a different selectable marker (URA, TRP, or LEU), and the system provides high expression levels of three different proteins simultaneously. This system was integrated into the protocols on a fully automated plasmid-based robotic platform to screen engineered strains of S. cerevisiae for improved growth on xylose. First, a novel PCR assembly strategy was used to clone a xylose isomerase (XI) gene into the URA-selectable SUMO vector and the plasmid was placed into the S. cerevisiae INVSc1 strain to give the strain designated INVSc1-XI. Second, amino acid scanning mutagenesis was used to generate a library of mutagenized genes encoding the bioinsecticidal peptide lycotoxin-1 (Lyt-1) and the library was cloned into the TRP-selectable SUMO vector and placed into INVSc1-XI to give the strain designated INVSc1-XI-Lyt-1. Third, the Yersinia pestis xylulokinase gene was cloned into the LEU-selectable SUMO vector and placed into the INVSc1-XI-Lyt-1 yeast. Yeast strains expressing XI and xylulokinase with or without Lyt-1 showed improved growth on xylose compared to INVSc1-XI yeast.

Probing melittin helix-coil equilibria in solutions and vesicles

Melittin is a toxic, amphipathic peptide that rearranges from a random coil in solution to a helical structure upon binding to cell membranes or lipid vesicles. We have found that mutation of the valine at position five of the peptide to a phenylalanine or 3-nitrotyrosine induces aggregation and helix formation at low concentrations (20-80 microM). Donor-acceptor distances obtained from analyses of fluorescence energy transfer kinetics experiments with the 3-nitrotyrosine mutant indicate that both coil and helix structures are present in 2 and 20 microM aqueous solutions. The helical peptide population increases upon addition of phospholipid vesicles or in high ionic strength solutions.

Melittin: a membrane-active peptide with diverse functions

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

Conformational sampling of peptides in cellular environments

Biological systems provide a complex environment that can be understood in terms of its dielectric properties. High concentrations of macromolecules and cosolvents effectively reduce the dielectric constant of cellular environments, thereby affecting the conformational sampling of biomolecules. To examine this effect in more detail, the conformational preference of alanine dipeptide, poly-alanine, and melittin in different dielectric environments is studied with computer simulations based on recently developed generalized Born methodology. Results from these simulations suggest that extended conformations are favored over alpha-helical conformations at the dipeptide level at and below dielectric constants of 5-10. Furthermore, lower-dielectric environments begin to significantly stabilize helical structures in poly-alanine at epsilon = 20. In the more complex peptide melittin, different dielectric environments shift the equilibrium between two main conformations: a nearly fully extended helix that is most stable in low dielectrics and a compact, V-shaped conformation consisting of two helices that is preferred in higher dielectric environments. An additional conformation is only found to be significantly populated at intermediate dielectric constants. Good agreement with previous studies of different peptides in specific, less-polar solvent environments, suggest that helix stabilization and shifts in conformational preferences in such environments are primarily due to a reduced dielectric environment rather than specific molecular details. The findings presented here make predictions of how peptide sampling may be altered in dense cellular environments with reduced dielectric response.

Interactions between the plasma membrane and the antimicrobial peptide HP (2-20) and its analogues derived from Helicobacter pylori

HP (2-20), a 19-residue peptide derived from the N-terminus of Helicobacter pylori ribosomal protein L1, has antimicrobial activity but is not cytotoxic to human erythrocytes. We synthesized several peptide analogues to investigate the effects of substitutions on structure and antimicrobial activity. Replacement of Gln16 and Asp18 with tryptophan [anal-3 (analogue-3)] caused a dramatic increase in lytic activities against bacteria and fungi. By contrast, a decrease in amphiphilicity caused by replacement of Phe5 or Leu11 with serine was accompanied by a reduction in antimicrobial activity. Analysis of the tertiary structures of the peptides in SDS micelles by NMR spectroscopy revealed that they have a well-defined a-helical structure. Among the analogues, anal-3 has the longest a-helix, from Val4 to Trp18. The enhanced hydrophobicity and increased a-helicity results in enhanced antimicrobial activity in anal-3 without an increase in haemolytic activity. Fluorescence experiments proved that the bacterial-cell selectivity of the anal-3 peptide is due to its high binding affinity for negatively charged phospholipids in bacterial cells. Results showing the effect of spin-labels on the NMR spectra indicated that the side chains in the hydrophobic phase of the amphiphilic a-helix are buried on the surface of the micelle and the tryptophan indole ring is anchored in the membrane surface. Because anal-3 shows high selectivity towards bacterial and fungal cells, it may provide an avenue for the development of new antibiotics.

Antibiotic activity and structural analysis of the scorpion-derived antimicrobial peptide IsCT and its analogs

IsCT is a non-cell-selective antimicrobial peptide isolated from the scorpion Opisthacanthus madagascariensis that has potent cytolytic activity against both mammalian and bacterial cells. To investigate the structure-activity relationships of IsCT and to design novel peptide antibiotics with bacterial cell selectivity, we synthesized several analogs of IsCT and determined their three-dimensional structures in solution by 2D-NMR spectroscopy. IsCT has a linear alpha-helical structure from Gly3 to Phe13, and [K7]-IsCT has a linear alpha-helical structure from Leu2 to Phe13. [K7, P8, K11]-IsCT, which has a bend in its middle region, exhibited the highest antibacterial activity without hemolytic activity, suggesting that its proline-induced bend is an important determinant of this selectivity. Tryptophan fluorescence showed that the high selectivity of [K7, P8, K11]-IsCT toward bacterial cells is closely correlated with its highly selective interaction with negatively charged phospholipids. Its potent activity against antibiotic-resistant bacteria suggests that [K7, P8, K11]-IsCT may serve as a promising lead candidate in the development of new peptide antibiotics.

Effects of l- or d-Pro incorporation into hydrophobic or hydrophilic helix face of amphipathic α-helical model peptide on structure and cell selectivity

A synthetic amphipathic alpha-helical model peptide, KLW, displays non-cell selective cytotoxicity. To investigate the effects of L- or D-Pro kink incorporation into hydrophobic or hydrophilic helix face of KLW on structure, cell selectivity, and membrane-binding affinity, we designed a series of four peptides, in which Leu(9) and Lys(11) in the hydrophobic and hydrophilic helix face of KLW, respectively, are substituted with L- or D-Pro. A L- or D-Pro substitution (KLW-L9P or KLW-L9p) of Leu(9) at the hydrophobic helix face of KLW induced a more significant reduction in hemolytic activity with improved antibacterial activity than that (KLW-K11P or KLW-K11p) of Lys(11) in the hydrophilic helix face. In addition, D-Pro-containing peptides (KLW-L9p and KLW-K11p) displayed less hemolytic activity than L-Pro-containing peptides (KLW-L9P and KLW-K11P). Tryptophan fluorescence studies revealed that bacterial cell selectivity of KLW-L9P, KLW-L9p, and KLW-K11p is closely related to selective interactions with negatively charged phospholipids. CD analysis revealed that L- or D-Pro incorporation into KLW reduces the alpha-helicity of the peptide and D-Pro incorporation induces more significant disruption in alpha-helical structure than L-Pro incorporation. Our results collectively suggest that D-Pro incorporation into the hydrophobic helix face of non-cell selective amphipathic alpha-helical peptides may be useful for the design of novel antimicrobial peptides possessing high bacterial cell selectivity without hemolytic activity.

Sequence requirements for the activity of membrane-active peptides

Synthetic peptides were constructed with the sequence of the first 20 residues of melittin and terminating with a range of different amino acid amides. These were found to have haemolytic and cytolytic activity similar to that of melittin, provided that certain charge constraints were observed. The nature of the 21st residue was not critical except when the residue introduced a negative charge. The presence of at least two positive charges in the molecule was found to be essential for activity. One of these charges could be the amino-terminal amine. Peptides could be inactivated by the addition of a non-acidic presequence which was acetylated at the N-terminus. Introducing a protease cleavable sequence into an N-terminal extension of the peptides produced analogues with low haemolytic activity that could be activated by proteolytic action. A peptide with extra positive charges introduced on the hydrophilic face of the helix possessed a haemolytic activity that was greater than that of melittin.

Electrostatic and hydrophobic forces tether the proximal region of the angiotensin II receptor (AT1A) carboxyl terminus to anionic lipids

The carboxyl terminus of the type-1 angiotensin II receptor (AT(1A)) is a focal point for receptor activation and deactivation. Synthetic peptides corresponding to the membrane-proximal, first 20 amino acids of the carboxyl terminus adopt an alpha-helical conformation in organic solvents, suggesting that the secondary structure of this region may be sensitive to hydrophobic environments. Using surface plasmon resonance, immobilized lipid chromatography, and circular dichroism, we examined whether this positively charged, amphipathic alpha-helical region of the AT(1A) receptor can interact with lipid components in the cell membrane and thereby modulate local receptor attachment and structure. A synthetic peptide corresponding to the proximal region of the AT(1A) receptor carboxyl terminus (Leu(305) to Lys(325)) was shown by surface plasmon resonance to bind with high affinity to the negatively charged lipid, dimyristoyl L-alpha-phosphatidyl-DL-glycerol (DMPG), but poorly to the zwitterionic lipid, dimyristoyl L-alpha-phosphatidylcholine (DMPC). In contrast, a peptide analogue possessing substitutions at four lysine residues (corresponding to Lys(307,308,310,311)) displayed poor association with either lipid, indicating a crucial anionic component to the interaction. Circular dichroism analysis revealed that both the wild-type and substituted peptides possessed alpha-helical propensity in methanol and trifluoroethanol, while the wild-type peptide also adopted partially inserted helical structure in DMPG and DMPC liposomes. In contrast, the substituted peptide exhibited spectra that suggested the presence of beta-sheet and alpha-helical structure in both liposomes. Immobilized lipid chromatography was used to characterize the hydrophobic component of the membrane interaction, and the results demonstrated that hydrophobic and electrostatic interactions mediated the binding of the wild-type peptide but that the substituted peptide bound to the model membranes mainly via hydrophobic forces. We propose that, in intact AT(1A) receptors, the proximal carboxyl terminus associates with the cytoplasmic face of the cell membrane via a high-affinity, anionic phospholipid-specific tethering that serves to increase the amphipathic helicity of this region. Such associations may be important for receptor function and common for G protein-coupled receptors.

Biochemical characterisation and genetic analysis of aureocin A53, a new, atypical bacteriocin from Staphylococcus aureus

Aureocin A53 is produced by Staphylococcus aureus A53. It is encoded on a 10.4 kb plasmid, pRJ9, and is active against Listeria monocytogenes. Aureocin A53 is a highly cationic 51-residue peptide containing ten lysine and five tryptophan residues. Aureocin A53 was purified to homogeneity by hydrophobic-interaction, cation-exchange, and reverse-phase chromatography. MALDI-TOF mass spectrometry yielded a molecular mass of 6012.5 Da, which was 28 Da higher than predicted from the structural gene sequence of the bacteriocin. The mass increment resulted from an N-formylmethionine residue, indicating that the aureocin A53 is synthesised and secreted without a typical bacteriocin leader sequence or sec-dependent signal peptide. The structural identity of aureocin A53 was verified by Edman sequencing after de-blocking with cyanogen bromide and extensive mass spectrometry analysis of enzymatically and laser-generated fragments. The complete sequence of pRJ9 was determined and none of the open reading frames identified in the vicinity of the structural gene aucA showed similarity to genes that are typically found in bacteriocin gene clusters. Thus, neither a dedicated protease or transporter, nor modifying enzymes and regulatory elements seemed to be involved in the production of aureocin A53. Further unique features that distinguish aureocin A53 from other peptide bacteriocins include remarkable protease stability and a defined, rigid structure in aqueous solution.

Structure and activity of D-Pro14 melittin

D-Pro14 melittin was synthesized to investigate the effect of increasing the angle of the bend in the hinge region between the helical segments of the molecule. Structural analysis by nuclear magnetic resonance indicated that, in methanol, the molecule consisted of two helices separated at Pro14, as in melittin. However, the two helices in D-Pro14 melittin were laterally displaced relative to each other by approximately 7 A, and in addition, there was a small rotation of the carboxyl-terminal helix relative to the amino-terminal helix around the long axis of the molecule. The peptide had less than 5% of the cytolytic activity of melittin. Modification of Arg22 with the 2,2,5,7,8-pentamethyl-chroman-6-sulphonyl (pmc) group restored hemolytic activity to close to that of unmodified melittin. Replacement of Arg22 with Phe was less effective in restoring hemolytic activity. Electron-paramagnetic resonance studies suggest that there is a positive correlation between hemolytic activity of the peptides and interaction with phospholipid bilayers.

Role of melittin-like region within phospholipase A(2)-activating protein in biological function

Phospholipase A(2)-activating protein (PLAA) has been implicated in the production of prostaglandins (e.g. PGE(2)) via activation of phospholipases in various stimulated cell types. Human PLAA, with 738 amino acid (aa) residues, contains a region of 38% homology (aa 503-538) with the 26-aa long melittin peptide, a major component of bee venom and a reported regulator of phospholipase A(2) and phospholipase D activity. To learn more about the role of PLAA in the production of eicosanoids and other inflammatory mediators, we synthesized a murine PLAA peptide (36-aa long) having homology to melittin, as well as to human and rat PLAA. The PLAA peptide and melittin increased the expression of genes encoding the proinflammatory cytokine tumor necrosis factor alpha (TNFalpha) and cyclooxygenase-2 (COX-2), which is involved in PGE(2) production. We determined that the C-terminal region of the PLAA peptide (aa 515-538) was essential, since truncation of the C-terminal end of the PLAA peptide significantly reduced expression of genes encoding TNFalpha and COX-2 in macrophages. We concluded that PLAA could be important in the regulation of the inflammatory response because of its stimulatory effects on eicosanoid and cytokine synthesis. Consequently, control of plaa gene expression could be a target for the development of new drugs to control the inflammatory response.

Salt resistance and synergistic effect with vancomycin of alpha-helical antimicrobial peptide P18

P18 (KWKLFKKIPKFLHLAKKF-NH(2)) is an alpha-helical antimicrobial peptide designed from a cecropin A-magainin 2 hybrid. In this study, P18 was found to show strong antimicrobial activity against several antibiotic-resistant bacterial and fungal strains. Both the salt resistance on antimicrobial activity and the synergistic effect with clinically used antibiotic agents are critical factors in developing effective peptide antibiotic drugs. For this reason, we investigated the salt resistance of P18 to antagonism by NaCl, CaCl(2), and MgCl(2) on antimicrobial activity and the synergistic effect of P18 with vancomycin against vancomycin-resistant Enterococcus faecium (VREF). Compared to magainin 2, P18 showed strong resistance on antimicrobial activity against bacterial strains and C. albicans under high NaCl concentrations of 100-200 mM. In addition, P18 displayed much greater salt resistance on antibacterial activity against Gram-negative bacteria at the physiological or elevated concentrations of CaCl(2) and MgCl(2) than magainin 2. Furthermore, the combination study revealed that P18 has a relatively effective synergistic effect with vancomycin against VREF. Thus, these results support that P18 may prove to be a salt-resistant antibiotic peptide potentially useful in the treatment of cystic fibrosis patients as well as a valuable adjuvant for antimicrobial chemotherapy.

Measurement of the affinity of melittin for zwitterionic and anionic membranes using immobilized lipid biosensors

The binding of melittin to zwitterionic dimyristyphosphatidylcholine (DMPC) and anionic dimyristylphosphatidylglycerol (DMPG) was analysed using two different immobilized model membrane systems. The first system used surface plasmon resonance (SPR), which monitors the real-time binding of peptides to an immobilized hybrid bilayer. SPR experiments reflected a stronger binding of melittin for DMPG than for DMPC, while kinetic analysis suggested the existence of at least two distinct binding steps. The second lipid biosensor system involved an immobilized phospholipid monolayer covalently attached to a microporous silica surface. The binding of melittin to the immobilized monolayer was then monitored using dynamic elution chromatography with varied methanol concentrations to analyse the binding of melittin to DMPC and DMPG. The nonlinear binding behaviour observed for melittin with the phosphatidylcholine (PC) and phosphatidylglycerol (PG) monolayers compared with the linear retention plots and Gaussian peak shapes observed for the control molecule demonstrated that melittin undergoes significant conformational and orientational changes upon binding to the immobilized PC and PG ligands. The dependence of log k' on per cent methanol also demonstrated a bimodal interaction whereby hydrophobic forces predominated at higher temperatures and methanol concentrations, while other forces, presumably electrostatic in nature, also made a contribution to the affinity of the peptides for the lipid monolayer, particularly at lower temperatures. The complementary use of these two lipid biosensors thus allows the role of hydrophobic and electrostatic forces in peptide-membrane interactions to be studied.

Prostaglandin levels in stimulated macrophages are controlled by phospholipase A2-activating protein and by activation of phospholipase C and D

Prostaglandins (PG), which are responsible for a large array of biological functions in eukaryotic cells, are produced from arachidonic acid by phospholipases and cyclooxygenase enzymes COX-1 and COX-2. We demonstrated that PG levels in cells were partly controlled by a regulatory protein, phospholipase A2 (PLA2)-activating protein (PLAA). Treatment of murine macrophages with lipopolysaccharide, interleukin-1beta, and tumor necrosis factor-alpha increased PLAA levels at early time points (2-30 min), which correlated with an up-regulation in cytosolic PLA2 and PGE2 levels. Both COX-2 and secretory PLA2 were also increased in lipopolysaccharide-stimulated macrophages, however, at later time points of 4-24 h. The role of PLAA in eicosanoid formation in macrophages was confirmed by the use of an antisense plaa oligonucleotide. Within amino acid residues 503-538, PLAA exhibited homology with melittin, and increased PGE(2) production was noted in macrophages stimulated with melittin. In addition to PLA2, we demonstrated that activation of phospholipase C and D significantly controlled PGE2 production. Finally, increased antigen levels of PLAA, COX-2, and phospholipases were demonstrated in biopsy specimens from patients with varying amounts of intestinal mucosal inflammation, which corresponded to increased levels of phospholipase activity. These results could provide a basis for the development of new therapeutic tools to control inflammation.

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

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

Amphipathic, alpha-helical antimicrobial peptides

Gene-encoded antimicrobial peptides are an important component of host defense in animals ranging from insects to mammals. They do not target specific molecular receptors on the microbial surface, but rather assume amphipathic structures that allow them to interact directly with microbial membranes, which they can rapidly permeabilize. They are thus perceived to be one promising solution to the growing problem of microbial resistance to conventional antibiotics. A particularly abundant and widespread class of antimicrobial peptides are those with amphipathic, alpha-helical domains. Due to their relatively small size and synthetic accessibility, these peptides have been extensively studied and have generated a substantial amount of structure-activity relationship (SAR) data. In this review, alpha-helical antimicrobial peptides are considered from the point of view of six interrelated structural and physicochemical parameters that modulate their activity and specificity: sequence, size, structuring, charge, amphipathicity, and hydrophobicity. It begins by providing an overview of how these vary in peptides from different natural sources. It then analyzes how they relate to the currently accepted model for the mode of action of alpha-helical peptides, and discusses what the numerous SAR studies that have been carried out on these compounds and their analogues can tell us. A comparative analysis of the many alpha-helical, antimicrobial peptide sequences that are now available then provides further information on how these parameters are distributed and interrelated. Finally, the systematic variation of parameters in short model peptides is used to throw light on their role in antimicrobial potency and specificity. The review concludes with some considerations on the potentials and limitations for the development of alpha-helical, antimicrobial peptides as antiinfective agents.

Studies on the conformational properties of CP-10(42-55), the hinge region of CP-10, using circular dichroism and RP-HPLC

The conformational properties of CP-10(42-55), a peptide corresponding to the hinge region of CP-10, were investigated using circular dichroism spectroscopy and reverse-phase high-performance liquid chromatography (RP-HPLC). The circular dichroism studies indicated that CP-10(42-55) formed considerable secondary structure in the presence of hydrophobic solution environments including 50% acetonitrile, 50% trifluoroethanol and 200 mM sodium dodecyl sulfate, which comprised a mixture of alpha-helix and beta-sheet. The effect of temperature on the conformation of CP-10(42-55) was investigated between 5 and 40 degrees C, with very small changes in the spectra being observed. RP-HPLC was then used to investigate the effect of temperature on the conformation of CP-10(42-55) in the presence of a hydrophobic surface. Using a C18-adsorbent, CP-10(42-55) exhibited a conformational transition at 25 degrees C, which was associated with an increase in the chromatographic contact area and the binding affinity of the peptide for the stationary phase. In addition, near-planar bandbroadening behaviour indicated that conformational species interconverted with rapid rate constants compared with the chromatographic time scale. These results indicated that the conformational change at 25 degrees C in the RP-HPLC system most likely corresponds to the unfolding of an alpha-helical and/or beta-sheet structure to an extended coil structure. Therefore, the strong chemotactic properties of this peptide may be attributed to its ability to form considerable secondary structure in the presence of a hydrophobic environment.

The role of tryptophan residues in the hemolytic activity of stonustoxin,a lethal factor from stonefish (Synanceja horrida) venom

Stonustoxin (SNTX) is a pore-forming cytolytic lethal factor, isolated from the venom of the stonefish Synanceja horrida, that has potent hemolytic activity. The role of tryptophan residues in the hemolytic activity of SNTX was investigated. Oxidation of tryptophan residues of SNTX with N-bromosuccinimide (NBS) resulted in loss of hemolytic activity. Binding of 8-anilino-1-naphthalenesulphonate (ANS) to SNTX resulted in occlusion of tryptophan residues that resulted in loss of hemolytic activity. Circular dichroism and fluorescence studies indicated that ANS binding resulted in a conformational change of SNTX, in particular, a relocation of surface tryptophan residues to the hydrophobic interior. NBS-modification resulted in oxidised surface tryptophan residues that did not relocate to the hydrophobic interior. These results suggest that native surface tryptophan residues play a pivotal role in the hemolytic activity of STNX, possibly by being an essential component of a hydrophobic surface necessary for pore-formation. This study is the first report on the essentiality of tryptophan residues in the activity of a lytic and lethal factor from a fish venom.

Tyr-->Trp-substituted peptide 115-129 of a Lys49 phospholipase A(2) expresses enhanced membrane-damaging activities and reproduces its in vivo myotoxic effect

Myotoxin II is a group II Lys49 phospholipase A(2) (PLA(2)) isolated from the venom of the snake Bothrops asper. Previous studies on a synthetic peptide derived from its heparin-binding, cationic/hydrophobic sequence 115-129 demonstrated a direct functional role of this particular region in the in vitro cytolytic and bactericidal actions of the protein. Nevertheless, no significant myonecrosis has been observed after local intramuscular injection of peptide 115-129 (p115-129) in mice. Since the membrane-damaging action of p115-129 was proposed to depend on its amphiphilic character, the present study examined the effects of substituting its cluster of three tyrosine residues by tryptophan residues, on its toxic/pharmacological activities in vitro and in vivo. This substitution resulted in a drastic enhancement of the membrane-damaging activities of the peptide (p115-W3), together with the clear expression of myotoxic activity in vivo. Both the heparin-binding and antigenic characteristics of p115-129 were essentially conserved in p115-W3, suggesting that the modification did not lead to radical structural alterations. In addition to myotoxicity, cytotoxicity, and bactericidal action, p115-W3 exerted edema-forming activity in the mouse footpad assay. Thus, the synthetic 13-mer p115-W3 reproduced all the known toxic effects of myotoxin II. In spite of its potent membrane-damaging actions, p115-W3 did not acquire direct hemolytic activity upon mouse erythrocytes, an effect which is not present in myotoxin II, but that has been ascribed to the presence of tryptophan in other cationic, membrane-damaging peptides such as mellitin from bee venom. The myotoxic activity of p115-W3 herein described constitutes the first example of a short, PLA(2)-based linear synthetic peptide with the ability to reproduce this effect of a parent protein in vivo. This finding is in clear support of the proposed relevance of the C-terminal region 115-129 in all the membrane-damaging mechanisms exerted by myotoxin II, including the myotoxic mechanism.

NMR structural characterization of cecropin A(1-8) - magainin 2(1-12) and cecropin A (1-8) - melittin (1-12) hybrid peptides

In order to elucidate the structure-antibiotic activity relationships of the peptides, the three-dimensional structures of two hybrid peptides, CA(1-8) - MA(1-12) and CA(1-8) - ME(1-12) in trifluoroethanol-containing aqueous solution were investigated by NMR spectroscopy. Both CA(1-8) - MA(1-12) and CA(1-8) - ME(1-12) have strong antibacterial activity but only CA(1-8) - ME(1-12) has hemolytic activity against human erythrocytes. CA(1-8) - MA(1-12) has a hydrophobic 310-helix of only two turns combined with one short helix in the N-terminus with a flexible hinge section in between. CA(1-8) - MA(1-12) has a severely bent structure in the middle of the peptide. These structural features as well as the low hydrophobicity of CA(1-8) - MA(1-12) seem to be crucial for the selective lysis against the membrane of prokaryotic cells. CA(1-8) - ME(1-12) has an alpha-helical structure of about three turns in the melittin domain and a flexible structure with one turn in the cecropin domain connected with a flexible hinge section in between, and these might be the structural features required for membrane disruption against prokaryotic and eukaryotic cells. The central hinge region (Gly9-Ile10-Gly11) in an amphipathic antibacterial peptide is considered to play an important role in providing the conformational flexibility required for ion channel formation of the C-terminal hydrophobic alpha-helix on cell membrane.

A conserved tryptophan-rich motif in the membrane-proximal region of the human immunodeficiency virus type 1 gp41 ectodomain is important for Env-mediated fusion and virus infectivity

Mutations were introduced into the ectodomain of the human immunodeficiency virus type 1 (HIV-1) transmembrane envelope glycoprotein, gp41, within a region immediately adjacent to the membrane-spanning domain. This region, which is predicted to form an alpha-helix, contains highly conserved hydrophobic residues and is unusually rich in tryptophan residues. In addition, this domain overlaps the epitope of a neutralizing monoclonal antibody, 2F5, as well as the sequence corresponding to a peptide, DP-178, shown to potently neutralize virus. Site-directed mutagenesis was used to create deletions, substitutions, and insertions centered around a stretch of 17 hydrophobic and uncharged amino acids (residues 666 to 682 of the HXB2 strain of HIV-1) in order to determine the role of this region in the maturation and function of the envelope glycoprotein. Deletion of the entire stretch of 17 amino acids abrogated the ability of the envelope glycoprotein to mediate both cell-cell fusion and virus entry without affecting the normal maturation, transport, or CD4-binding ability of the protein. This phenotype was also demonstrated by substituting alanine residues for three of the five tryptophan residues within this sequence. Smaller deletions, as well as multiple amino acid substitutions, were also found to inhibit but not block cell-cell fusion. These results demonstrate the crucial role of a tryptophan-rich motif in gp41 during a post-CD4-binding step of glycoprotein-mediated fusion. The basis for the invariant nature of the tryptophans, however, appears to be at the level of glycoprotein incorporation into virions. Even the substitution of phenylalanine for a single tryptophan residue was sufficient to reduce Env incorporation and drop the efficiency of virus entry approximately 10-fold, despite the fact that the same mutation had no significant effect on syncytium formation.