Cholesterol as a target for toxins

Potential of Pm11 antimicrobial peptide against bovine mastitis pathogens

OBJECTIVE

To investigate an alternative treatment for bovine mastitis by using Pm11 antimicrobial peptide.

SAMPLE

5 bovine mastitis pathogens that were previously isolated from cows affected by either clinical or subclinical mastitis.

PROCEDURES

The current study introduces Pm11 antimicrobial peptide as an alternative treatment for bovine mastitis. The antibacterial activity of Pm11 was tested against Escherichia coli strain SCM1249, Klebsiella spp strain SCM1282, Staphylococcus aureus strain CM967, Streptococcus agalactiae strain SCM1084, and Streptococcus uberis strain SCM1310 using minimum bactericidal concentrations (MBCs) and time-kill kinetics. The pathogens' morphological changes were demonstrated using a scanning electron microscope (SEM). The cytotoxicity of Pm11 was assessed using the minimum hemolytic concentration assay.

RESULTS

MBCs ranged from 2.5 to 10 μM and IC50 ranged from 0.32 to 2.07 μM. Time-kill kinetics at MBC demonstrated that Pm11 reduced viable cell counts of S agalactiae strain SCM1084 and S uberis strain SCM1310 from 105 to 0 CFU/mL within 1 h. E coli strain SCM1249 and S aureus strain CM967 were reduced from 105 to 0 CFU/mL within 4 h. The average Pm11-induced hemolytic activity was < 10% for all Pm11 concentrations tested except at the maximum concentration tested (160 μM: 10.19 ± 2.29%). Based on SEM, Pm11 induced morphological and cellular changes in S aureus and E coli.

CLINICAL RELEVANCE

Pm11 antimicrobial peptide demonstrated in vitro antibacterial activity against the common bovine mastitis pathogens E coli, S aureus, S agalactiae, and S uberis, except Klebsiella spp, and should be further investigated in vivo.

Characterization of anti-microbial peptides and proteins from maggots of Calliphoridae and Sarcophagidae fly species (Diptera)

Removal of infected wounds using maggots has been known for centuries. Early research has shown that the maggot exosecretion, whole body, and fecal waste products of Calliphoridae and Sarcophagidae species contain a variety of alkaline peptides capable of inhibiting bacterial growth. Since the wide application of antibiotics such as penicillin, a number of bacterial infections have become insensitive to antibiotic treatment. In many of these instances, maggot therapy has been successfully applied for the treatment of chronic wounds. To identify and compare the expression patterns of anti-microbial peptides (AMPs) from some dipteran species, transcriptome analyses were conducted for the maggots of 11 Calliphoridae and Sarcophagidae species. Species of the subfamily Calliphorinae showed relatively higher expression levels of AMPs and anti-microbial proteins compared with those of Luciliinae and Sarcophagidae species. Furthermore, among all of the dipteran species examined, Lucilia illustris exhibited the highest transcription levels of AMPs. Cecropin A2 and defensin, whose expression levels were the highest among the anti-microbial peptides, were synthesized to test their biological activity. The synthesized peptides showed anti-microbial activities without hemolytic activities. In particular, cecropin A2 of L. illustris exhibited the highest anti-microbial activity against all of the bacteria and fungi examined, thereby possessing the potential to be developed as a new alternative to antibiotics. This comparative transcriptomic study may provide new insights into anti-microbial compositions of some dipteran species.

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.

Control of Aspergillus flavus growth and aflatoxin production in transgenic maize kernels expressing a tachyplesin-derived synthetic peptide, AGM182

Aspergillus flavus is an opportunistic, saprophytic fungus that infects maize and other fatty acid-rich food and feed crops and produces toxic and carcinogenic secondary metabolites known as aflatoxins. Contamination of maize with aflatoxin poses a serious threat to human health in addition to reducing the crop value leading to a substantial economic loss. Here we report designing a tachyplesin1-derived synthetic peptide AGM182 and testing its antifungal activity both in vitro and in planta. In vitro studies showed a five-fold increase in antifungal activity of AGM182 (vs. tachyplesin1) against A. flavus. Transgenic maize plants expressing AGM182 under maize Ubiquitin-1 promoter were produced through Agrobacterium-mediated transformation. PCR products confirmed integration of the AGM182 gene, while RT-PCR of maize RNA confirmed the presence of AGM182 transcripts. Maize kernel screening assay using a highly aflatoxigenic A. flavus strain (AF70) showed up to 72% reduction in fungal growth in the transgenic AGM182 seeds compared to isogenic negative control seeds. Reduced fungal growth in the AGM182 transgenic seeds resulted in a significant reduction in aflatoxin levels (76-98%). The results presented here show the power of computational and synthetic biology to rationally design and synthesize an antimicrobial peptide against A. flavus that is effective in reducing fungal growth and aflatoxin contamination in an economically important food and feed crop such as maize.

Channel Formation and Membrane Deformation via Sterol-Aided Polymorphism of Amphidinol 3

Amphidinol 3 (AM3) is an anti-fungal polyene extracted from a marine dinoflagellate. Here, we examined the ion channel activity and membrane-embedded structure of AM3 using a lipid bilayer method and atomic force microscopy (AFM). AM3 exhibited large-conductance (~1 nS) and non-selective single-channel activity only when sterols were present in the membrane leaflet of the AM3-added side. The variable conductance suggests the formation of a multimeric barrel-stave pore. At high AM3 concentrations, giant-conductance “jumbo” channels (~40 nS) emerged. AFM revealed a thicker raft-like membrane phase with the appearance of a wrinkled surface, in which phase pores (diameter: ~10 nm) were observed. The flip-flop of ergosterol occurred only after the appearance of the jumbo channel, indicating that the jumbo channel induced a continuity between the outer and inner leaflets of the membrane: a feature characteristic of toroidal-like pores. Thus, AM3 forms different types of sterol-aided polymorphic channels in a concentration dependent manner.

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.

An in vitro test system for evaluation of SCAP–SREBP pathway inhibitory activities of Traditional Chinese Medicines

In vitro assay system demonstrates that Rhizoma Alismatis and Semen Cassiae show beneficial effects on inhibition of SCAP–SREBP pathway activities.

Effect of antimicrobial peptide on the dynamics of phosphocholine membrane: role of cholesterol and physical state of bilayer

Antimicrobial peptides are universal in all forms of life and are well known for their strong interaction with the cell membrane. This makes them a popular target for investigation of peptide-lipid interactions. Here we report the effect of melittin, an important antimicrobial peptide, on the dynamics of membranes based on 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) lipid in both the solid gel and fluid phases. To probe the phase transition, elastic neutron intensity temperature scans have been carried out on DMPC-based unilamellar vesicles (ULV) with and without melittin. We have found that addition of a small amount (0.2 mol%) melittin eliminates the steep fall in the elastic intensity at 296 K associated with the solid gel to fluid phase transition, which is observed for pure DMPC vesicles. Quasielastic neutron scattering (QENS) experiments have been carried out on DMPC ULV in the solid gel and fluid phases with and without 0.2 mol% melittin. The data analysis invariably shows the presence of lateral and internal motions of the DMPC molecule. We found that melittin does have a profound effect on the dynamics of lipid molecules, especially on the lateral motion, and affects it in a different way, depending on the phase of the bilayers. In the solid gel phase, it acts as a plasticizer, enhancing the lateral motion of DMPC. However, in the fluid phase it acts as a stiffening agent, restricting the lateral motion of the lipid molecules. These observations are consistent with the mean squared displacements extracted from the elastic intensity temperature scans. Their importance lies in the fact that many membrane processes, including signaling and energy transduction pathways, are controlled to a great extent by the lateral diffusion of lipids in the membrane. To investigate the effect of melittin on vesicles supplemented with cholesterol, QENS experiments have also been carried out on DMPC ULV with cholesterol in the presence and absence of 0.2 mol% melittin. Remarkably, the effects of melittin on the membrane dynamics disappear in the presence of 20 mol% cholesterol. Our measurements indicate that the destabilizing effect of the peptide melittin on membranes can be mitigated by the presence of cholesterol. This study might provide new insights into the mechanism of action of antimicrobial peptides and their selective toxicity towards foreign microorganisms.

The interaction of aurein 2.5 with fungal membranes

Aurein 2.5 (GLFDIVKKVVGAFGSL-NH2) is an antimicrobial peptide, which was seen to have activity against Stachybotris chartarum, Penicillium roseopurpureum and Aspergillus flavus with minimum fungicidal concentrations in the range 250-500 μM. S. chartarum showed enhanced susceptibility to lysis as compared to P. roseopurpureum and A. flavus, (44, 26 and 28 % respectively). Monolayers formed from lipid membrane extracts derived from S. chartarum, P. roseopurpureum and A. flavus showed maximal surface pressure changes of 13.5, 10.3 and 10.2 mN m(-1) respectively. However, aurein 2.5 adopted similar levels of α-helical structure (circa 45 %) in the presence of vesicles formed from membrane lipid extracts derived from all three fungi. These data imply that differential activity is not due to targeting and membrane association but linked to the ability of the bound peptide to lyse the cells. At sterol levels mimetic of eukaryotic systems, high levels of α-helical structure (circa 50 %) were also observed and hence similar binding. However, enhanced sterol levels (>0.6) led to a reduction in monolayer membrane interaction, suggesting that the sterols influence efficacy. Consistent with this suggestion, thermodynamic analysis showed that the peptide was able to destabilise model fungal monolayers, as indicated by negative values of ∆Gmix.

On the selectivity and efficacy of defense peptides with respect to cancer cells

Here, we review potential determinants of the anticancer efficacy of innate immune peptides (ACPs) for cancer cells. These determinants include membrane-based factors, such as receptors, phosphatidylserine, sialic acid residues, and sulfated glycans, and peptide-based factors, such as residue composition, sequence length, net charge, hydrophobic arc size, hydrophobicity, and amphiphilicity. Each of these factors may contribute to the anticancer action of ACPs, but no single factor(s) makes an overriding contribution to their overall selectivity and toxicity. Differences between the anticancer actions of ACPs seem to relate to different levels of interplay between these peptide and membrane-based factors.

Prosurvival effect of DHCR24/Seladin-1 in acute and chronic responses to oxidative stress

DHCR24/seladin-1, a crucial enzyme in sterol synthesis, is of lower abundance in brain areas affected by Alzheimer's disease. While high levels of DHCR24/seladin-1 exert antiapoptotic function by conferring resistance against oxidative stress, the molecular mechanism for this protective effect is not fully understood. Here we show that DHCR24/seladin-1 expression is up-regulated in an acute response and down-regulated in a chronic response to oxidative stress. High levels of DHCR24/seladin-1 were associated with elevated cholesterol concentrations and a general increase in cholesterol biosynthesis upon oxidative stress exposure in neuroblastoma SH-SY5Y cells. DHCR24/seladin-1 overexpression conferred resistance to oxidative stress in a cholesterol-dependent manner. Mutating the reductase activity within DHCR24/seladin-1 abolished this protective effect. Conversely, DHCR24/seladin-1 levels diminished upon chronic exposure to oxidative stress. Low levels of DHCR24/seladin-1 were associated with reduced p53 levels, independent of DHCR24 activity and cholesterol concentrations. Additionally, ablation of DHCR24/seladin-1 prevented apoptosis of primary neurons in a p53-dependent manner and reduced the response of critical p53 targets due to deficient stabilization of p53 and therefore elevated p53 ubiquitination and degradation. Our findings reveal a dual capacity of DHCR24/seladin-1, which appears to be involved in two mechanistically independent prosurvival effects, exerting an acute response and a chronic response to oxidative stress.

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.

Interaction of melittin with membrane cholesterol: a fluorescence approach

We have monitored the organization and dynamics of the hemolytic peptide melittin in membranes containing cholesterol by utilizing the intrinsic fluorescence properties of its functionally important sole tryptophan residue and circular dichroism spectroscopy. The significance of this study is based on the fact that the natural target for melittin is the erythrocyte membrane, which contains high amounts of cholesterol. Our results show that the presence of cholesterol inhibits melittin-induced leakage of lipid vesicles and the extent of inhibition appears to be dependent on the concentration of membrane cholesterol. The presence of cholesterol is also shown to reduce binding of melittin to membranes. Our results show that fluorescence parameters such as intensity, emission maximum, and lifetime of membrane-bound melittin indicate a change in polarity in the immediate vicinity of the tryptophan residue probably due to increased water penetration in presence of cholesterol. This is supported by results from fluorescence quenching experiments using acrylamide as the quencher. Membrane penetration depth analysis by the parallax method shows that the melittin tryptophan is localized at a relatively shallow depth in membranes containing cholesterol. Analysis of energy transfer results using melittin tryptophan (donor) and dehydroergosterol (acceptor) indicates that dehydroergosterol is not randomly distributed and is preferentially localized around the tryptophan residue of membrane-bound melittin, even at the low concentrations used. Taken together, our results are relevant in understanding the interaction of melittin with membranes in general, and with cholesterol-containing membranes in particular, with possible relevance to its interaction with the erythrocyte membrane.

Reduced sensitivity of Niemann-Pick C1-deficient cells to theta-toxin (perfringolysin O): sequestration of toxin to raft-enriched membrane vesicles

Theta-toxin (perfringolysin O) binds to cell surface cholesterol and forms oligomeric pores that cause membrane damage. Both in cytotoxicity and cell survival assays, a mutant Chinese hamster ovary cell line NPC1(-) that lacked Niemann-Pick C1 showed reduced sensitivity to theta-toxin, compared with wild-type (wt) cells. BCtheta is a derivative of theta-toxin that retains cholesterol-binding activity but lacks cytotoxicity. Confocal and electron microscopy revealed the presence of multiple vesicles which bound BCtheta, both on the cell surface and in the extracellular space of these cells. BCtheta binding to raft microdomains was verified by its resistance to 1% Triton X-100 at 4 degrees C and recovery of bound BCtheta in floating low-density fractions on sucrose density gradient fractionation. BCtheta-labeled vesicles were abolished when NPC1(-) cells were depleted of lipoproteins and also when treated with a Rho-associated kinase inhibitor Y-27632. In addition, similar vesicles were observed in wt cells treated with progesterone. In parallel with these results, theta-toxin sensitivity of NPC1(-) cells was increased when cells were depleted of lipoproteins or treated with Y-27632, whereas that of wt cells was decreased by progesterone. Our findings suggest that sequestration of toxin to raft-enriched cell surface vesicles may underlie reduced sensitivity of NPC1-deficient cells to theta-toxin.

Deficiency in ethanolamine plasmalogen leads to altered cholesterol transport

Plasmalogens are a major sub-class of ethanolamine and choline phospholipids in which the sn-1 position has a long chain fatty alcohol attached through a vinyl ether bond. These phospholipids are proposed to play a role in membrane fusion-mediated events. In this study, we investigated the role of the ethanolamine plasmalogen plasmenylethanolamine (PlsEtn) in intracellular cholesterol transport in Chinese hamster ovary cell mutants NRel-4 and NZel-1, which have single gene defects in PlsEtn biosynthesis. We found that PlsEtn was essential for specific cholesterol transport pathways, those from the cell surface or endocytic compartments to acyl-CoA/cholesterol acyltransferase in the endoplasmic reticulum. The movement of cholesterol from the endoplasmic reticulum or endocytic compartments to the cell surface was normal in PlsEtn-deficient cells. Also, vesicle trafficking was normal in PlsEtn-deficient cells, as measured by fluid phase endocytosis and exocytosis, as was the movement of newly-synthesized proteins to the cell surface. The mutant cholesterol transport phenotype was due to the lack of PlsEtn, since it was corrected when NRel-4 cells were transfected with a cDNA encoding the missing enzyme or supplied with a metabolic intermediate that enters the PlsEtn biosynthetic pathway downstream of the defect. Future work must determine the precise role that plasmalogens have on cholesterol transport to the endoplasmic reticulum.

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.

Two structural transitions in membrane pore formation by pneumolysin, the pore-forming toxin of Streptococcus pneumoniae

The human pathogen Streptococcus pneumoniae produces soluble pneumolysin monomers that bind host cell membranes to form ring-shaped, oligomeric pores. We have determined three-dimensional structures of a helical oligomer of pneumolysin and of a membrane-bound ring form by cryo-electron microscopy. Fitting the four domains from the crystal structure of the closely related perfringolysin reveals major domain rotations during pore assembly. Oligomerization results in the expulsion of domain 3 from its original position in the monomer. However, domain 3 reassociates with the other domains in the membrane pore form. The base of domain 4 contacts the bilayer, possibly along with an extension of domain 3. These results reveal a two-stage mechanism for pore formation by the cholesterol-binding toxins.

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.

Evidence for a cholesterol transport pathway from lysosomes to endoplasmic reticulum that is independent of the plasma membrane

We have studied the movement of low density lipoprotein (LDL)-derived cholesterol in cultured Chinese hamster ovary cells. Our hypothesis is that when LDL cholesterol is effluxed from lysosomes, the bulk of LDL cholesterol is mobilized to the plasma membrane, while another pathway delivers LDL cholesterol from lysosomes to acyl-CoA/cholesterol acyltransferase (ACAT) in the endoplasmic reticulum. Three lines of evidence support this model. First, LDL cholesterol transport to ACAT can be blocked without inhibiting the movement of cholesterol from lysosomes to plasma membrane or from plasma membrane to endoplasmic reticulum. Second, LDL cholesterol transport to ACAT is normal in a Chinese hamster ovary mutant with defective plasma membrane-to-ACAT movement. Third, LDL cholesterol is not diluted by the plasma membrane cholesterol pool before reaching ACAT. Our evidence supports a vesicular model of cholesterol transport from lysosomes to the endoplasmic reticulum that is independent of the plasma membrane.

A conserved tryptophan in pneumolysin is a determinant of the characteristics of channels formed by pneumolysin in cells and planar lipid bilayers

Pneumolysin is one of the family of thiol-activatable, cytolytic toxins. Within these toxins the amino acid sequence Trp-Glu-Trp-Trp is conserved. Mutations made in this region of pneumolysin, residues 433-436 inclusive, did not affect cell binding or the formation of toxin oligomers in the target cell membrane. However, the mutations did affect haemolysis, leakage of low-molecular-mass metabolites from Lettre cells and the induction of conductance channels across planar lipid bilayers. Of eight modified pneumolysins examined, Trp-433-->Phe showed the smallest amount of haemolysis or leakage (less than 5% of wild type). Pneumolysin-induced leakage from Lettre cells was sensitive to inhibition by bivalent cations but the extent of inhibition varied depending on the modification. Leakage by the mutant Trp-433-->Phe was least sensitive to cation inhibition. The ion-conducting channels formed across planar lipid bilayers exhibit small (less than 30 pS), medium (30 pS-1 nS) and large (more than 1 nS) conductance steps. Small- and medium-sized channels were preferentially closed by bivalent cations. In contrast with wild-type toxin, which formed predominantly small channels, the modified toxin Trp-433-->Phe formed large channels that were insensitive to cation-induced closure. Polysaccharides of molecular mass more than 15 kDa inhibited haemolysis by wild-type toxin, but polysaccharide of up to 40 kDa did not prevent haemolysis by Trp-433-->Phe. Electron microscopy revealed that Trp-433-->Phe formed oligomeric arc and ring structures with dimensions identical with those of wild-type toxin, and that the ratio of arcs to rings formed was the same for wild-type toxin and the Trp-433-->Phe variant. We conclude that the change Trp-433-->Phe affects channel formation at a point subsequent to binding to the cell membrane and the formation of oligomers, and that the size of arc and ring structures revealed by electron microscopy does not reflect the functional state of the channels.

The effect of sterols on the sensitivity of membranes to the channel-forming antifungal antibiotic, syringomycin E

The ability of three sterols of different structure to influence the interaction of syringomycin E (an antifungal antibiotic that forms voltage dependent channels in planar lipid bilayers) with a planar lipid bilayer was evaluated. The rate of increase of bilayer conductance induced by syringomycin E was about 1000-times less in bilayers containing 50 mol% of cholesterol compared to bilayers without sterols. The effect of ergosterol (the primary sterol of fungal cells) on the sensitivity of bilayers to syringomycin E was much weaker than that of cholesterol, while stigmasterol (one of the main sterols of plant cells) did not significantly influence the ability of syringomycin E to induce a conductance increase in the bilayer. None of the sterols altered the single channel conductance properties of syringomycin E. These observations suggest that cholesterol affects the sensitivity of target membranes to syringomycin E by enlarging the energy barrier for channel formation rather than participating in channel formation itself.

The role of amphipathicity in the folding, self-association and biological activity of multiple subunit small proteins

The effect that altering amphipathicity has on the folding process and self association of melittin, a small model protein, has been investigated using single amino acid substitutions of lysine 7, a residue distant from the contact residues involved in the hydrophobic core of tetrameric melittin. While substitutions of such a residue were not expected to interfere with the packing process, the largest alterations in the potential overall amphipathicity of melittin were found to prevent the folding into an alpha-helical conformation to occur and, in turn, to prevent the self association. Amphipathic alpha-helices were found to be a key determining feature in the early folding process of the self association of peptides and protein segments. Those substitutions, which prevented the inducible amphipathic folding ability, were also found to result in a loss in hemolytic and antimicrobial activity. These results, combined with studies of the binding to artificial liposomes and to polysialic acids, indicate that the losses in activity were due to an initial inability to be induced into an amphipathic alpha-helix and to self associate. Ultimately, melittin's self association is proposed to be required to penetrate the carbohydrate barrier present in biological membranes.

Characterization of the solution properties and conformation of pneumolysin, the membrane-damaging toxin of Streptococcus pneumoniae

Pneumolysin is a membrane-damaging toxin produced by Streptococcus pneumoniae. In order to understand fully the mode of action of this toxin, it is necessary to have an appreciation of the size, self-association behaviour and solution conformation of pneumolysin. A combination of analytical ultracentrifugation methodologies has shown that pneumolysin lacks self-association behaviour in solution and has provided a weight-average M(r) (M omega) of 52,000 +/- 2000, which was in agreement with that derived from the amino acid sequence. By determining a sedimentation coefficient (S20,w0) of 3.35 +/- 0.10 S, it was possible to suggest a model for the gross solution conformation of pneumolysin monomers. Spectroscopic methods provide additional secondary and tertiary structure information.

Influence of tryptophan residues on melittin's hemolytic activity

Earlier studies of melittin have shown that the Trp residue at position 19 is significantly involved in its hemolytic activity. Tryptophan residues have also been reported to play a specific and important role in a number of other biological interactions. In the present study, we investigated what effect the introduction of a second Trp residue would have on melittin's hemolytic activity. This was accomplished through the synthesis and analysis of a complete set of 25 single-position, synthetic Trp substitution analogs. Significant increases in activity were observed upon substituting Trp at a single residue at either extreme of melittin's two alpha-helices, or in its 'hinge' region. Decreases in activity were found upon replacing any of melittin's Leu residues with Trp. The changes in activity of all of the analogs relative to melittin were found to be correlated to their behavior during RP-HPLC, as was their variation in percent helicity in the presence of liposomes.

Dual specificity of sterol-mediated glycoalkaloid induced membrane disruption

In this study the effects of the glycoalkaloids alpha-solanine, alpha-chaconine, alpha-tomatine and the aglycone solanidine on model membranes composed of PC in the absence and presence of sterols have been analysed via permeability measurements and different biophysical methods. The main result is that glycoalkaloids are able to interact strongly with sterol containing membranes thereby causing membrane disruption in a way which is specific for the type of glycoalkaloid and sterol. For this dual specificity both the sugar moiety of the glycoalkaloid and the side-chain of the sterol on position 24 turned out to be of major importance for the membrane disrupting activity. The order of potency of the glycoalkaloids was alpha-tomatine > alpha-chaconine > alpha-solanine. The plant sterols beta-sitosterol and fucosterol showed higher affinity for glycoalkaloids as compared to cholesterol and ergosterol. The mode of action of the glycoalkaloids is proposed to consist of three main steps: (1) Insertion of the aglycone part in the bilayer. (2) Complex formation of the glycoalkaloid with the sterols present. (3) Rearrangement of the membrane caused by the formation of a network of sterol-glycoalkaloid complexes resulting in a transient disruption of the bilayer during which leakage occurs.