pH-dependent bilayer destabilization and fusion of phospholipidic large unilamellar vesicles induced by diphtheria toxin and its fragments A and B

Characterization of diphtheria toxin's catalytic domain interaction with lipid membranes

In response to a low environmental pH and with the help of the B fragment (DTB) the catalytic domain of diphtheria toxin (DTA) crosses the endosomal membrane to inhibit protein synthesis. In this study, we investigated the interaction of DTA with lipid membranes by biochemical and biophysical approaches. Data obtained from proteinase K and trypsin digestion experiments of membrane-inserted DTA suggested that residues 134-157 may adopt a transmembrane orientation and residues 77-100 could be membrane-associated, adopting either a surface or a transmembrane orientation. Fourier transform infrared spectroscopy analysis (FTIR) was used to characterize the secondary and tertiary structure of DTA along its pathway, from the native secreted form at pH 7.2 to the refolded structure at neutral pH after interaction with and desorption from a lipid membrane. We found that the association of DTA with lipid membranes at low pH was characterized by an increase of beta-sheet structures and that the refolded structure at neutral pH after interaction with the membrane was identical to the native structure at the same pH. We also investigated the desorption of DTA from the membrane at neutral pH as a function of temperature. Although a complete desorption was observed at 37 degrees C, no desorption took place at 4 degrees C. A model of translocation involving the possibility that DTA might insert one or several transient transmembrane domains during translocation is discussed.

Structural changes of the prion protein in lipid membranes leading to aggregation and fibrillization

Prion diseases are associated with a major refolding event of the normal cellular prion protein, PrP(C), where the predominantly alpha-helical and random coil structure of PrP(C) is converted into a beta-sheet-rich aggregated form, PrP(Sc). Under normal physiological conditions PrP(C) is attached to the outer leaflet of the plasma membrane via a GPI anchor, and it is plausible that an interaction between PrP and lipid membranes could be involved in the conversion of PrP(C) into PrP(Sc). Recombinant PrP can be refolded into an alpha-helical structure, designated alpha-PrP isoform, or into beta-sheet-rich states, designated beta-PrP isoform. The current study investigates the binding of beta-PrP to model lipid membranes and compares the structural changes in alpha- and beta-PrP induced upon membrane binding. beta-PrP binds to negatively charged POPG membranes and to raft membranes composed of DPPC, cholesterol, and sphingomyelin. Binding of beta-PrP to raft membranes results in substantial unfolding of beta-PrP. This membrane-associated largely unfolded state of PrP is slowly converted into fibrils. In contrast, beta-PrP and alpha-PrP gain structure with POPG membranes, which instead leads to amorphous aggregates. Furthermore, binding of beta-PrP to POPG has a disruptive effect on the integrity of the lipid bilayer, leading to total release of vesicle contents, whereas raft vesicles are not destabilized upon binding of beta-PrP.

Membrane destabilization induced by beta-amyloid peptide 29-42: importance of the amino-terminus

Increasing evidence implicates interactions between Abeta-peptides and membrane lipids in Alzheimer's disease. To gain insight into the potential role of the free amino group of the N-terminus of Abeta29-42 fragment in these processes, we have investigated the ability of Abeta29-42 unprotected and Abeta29-42 N-protected to interact with negatively-charged liposomes and have calculated the interaction with membrane lipids by conformational analysis. Using vesicles mimicking the composition of neuronal membranes, we show that both peptides have a similar capacity to induce membrane fusion and permeabilization. The fusogenic effect is related to the appearance of non-bilayer structures where isotropic motions occur as shown by 31P and 2H NMR studies. The molecular modeling calculations confirm the experimental observations and suggest that lipid destabilization could be due to the ability of both peptides to adopt metastable positions in the presence of lipids. In conclusion, the presence of a free or protected (acetylated) amino group in the N-terminus of Abeta29-42 is therefore probably not crucial for destabilizing properties of the C-terminal fragment of Abeta peptides.

Binding of prion protein to lipid membranes and implications for prion conversion

The binding of the Syrian hamster prion protein, SHaPrP(90-231), to model lipid membranes was investigated by tryptophan fluorescence. Membranes composed of negatively charged or zwitterionic lipids, and raft-like membranes containing dipalmitoylphosphatidylcholine(1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), cholesterol and sphingomyelin, were investigated. It was found that SHaPrP(90-231) binds to negatively charged lipid membranes and raft-like membranes. Binding of PrP to negatively charged lipid membranes involves both electrostatic and hydrophobic lipid-protein interactions and results in partial insertion of PrP into the lipid bilayer. This membrane-inserted conformation of PrP is richer in beta-sheet structure and has a disruptive effect on the integrity of the lipid bilayer, leading to total release of vesicle contents. In contrast, the binding of PrP to raft-like membranes is driven by hydrophobic lipid-protein interactions and induces the formation of alpha-helical structure. This conformation of PrP with a high content of alpha-helix is formed only at pH 7 and does not destabilize the lipid bilayer. Our findings support the view that an interaction of PrP with lipid membranes could play a role in PrP conversion.

Experimental and conformational analyses of interactions between butenafine and lipids

Butenafine (N-4-tert-butylbenzyl-N-methyl-1-naphtalenemethylamine hydrochloride) is an antifungal agent of the benzylamine class that has excellent therapeutic efficacy and a remarkably long duration of action when applied topically to treat various mycoses. Given the lipophilic nature of the molecule, efficacy may be related to an interaction with cell membrane phospholipids and permeabilization of the fungal cell wall. Similarly, high lipophilicity could account for the long duration of action, since fixation to lipids in cutaneous tissues might allow them to act as local depots for slow release of the drug. We have therefore used computer-assisted conformational analysis to investigate the interaction of butenafine with lipids and extended these observations with experimental studies in vitro using liposomes. Conformational analysis of mixed monolayers of phospholipids with the neutral and protonated forms of butenafine highlighted a possible interaction with both the hydrophilic and hydrophobic domains of membrane phospholipids. Studies using liposomes demonstrated that butenafine increases membrane fluidity [assessed by fluorescence polarization of 1-(4-trimethylammonium-phenyl)-6-phenyl-1,3,5-hexatriene and 1,6-diphenylhexatriene] and membrane permeability (studied by release of calcein from liposomes). The results show, therefore, that butenafine readily interacts with lipids and is incorporated into membrane phospholipids. These findings may help explain the excellent antifungal efficacy and long duration of action of this drug when it is used as a topical antifungal agent in humans.

Characterization of the interaction of IpaB and IpaD, proteins required for entry of Shigella flexneri into epithelial cells, with a lipid membrane

Entry of Shigella flexneri into epithelial cells and lysis of the phagosome involve the IpaB, IpaC, and IpaD proteins, which are secreted by type III secretion machinery. We report here the purification of IpaB and IpaD and the characterization of their lipid-binding properties as a function of pH. The interaction of IpaB with the membrane was quite independent of the pH whereas that of IpaD took place only at low pH. To support the data obtained with the purified proteins, we designed a system in which protein secretion by live bacteria was induced in the presence of liposomes, thereby allowing interaction of proteins with lipids directly after secretion and bypassing any purification step. In these conditions, both IpaB and IpaC, as well as minor amounts of IpaA and IpgD, were associated with the membrane and the ratio of IpaB to IpaC was modulated by the pH. The relevance of these results with respect to the dual roles of IpaB, IpaC and IpaD in induction of membrane ruffles and lysis of the endosomal membrane is discussed.

The pH-dependent interaction of cinnamomin with lipid membranes investigated by fluorescence methods

Cinnamomin, a new type II ribosome-inactivating protein (RIP), was found to be able to induce the release of calcein loaded in lecithin small unilamellar vesicles and the fusion or aggregation of the lecithin liposomes. Such induction could be promoted severalfold by a pH 5.0 environment, a condition similar to that in endocytic vesicles. Lowering the pH from 7.5 to 5.0 evoked conformational changes of cinnamomin and unmasked its hydrophobic areas, including the exposure of 1-anilino-8-naphthalenesulfonate (1,8-ANS) binding sites of the molecule. Some tryptophan residues with affinity to acrylamide were demonstrated to participate in the lipid-protein interaction. The pH dependent fusogenicity of type II RIP might suggest its in vivo function as a fusogen to exert its cytotoxicity.

Structure and interaction of VacA of Helicobacter pylori with a lipid membrane

In its mature form, the VacA toxin of Helicobacter pylori is a 95-kDa protein which is released from the bacteria as a low-activity complex. This complex can be activated by low-pH treatment that parallels the activity of the toxin on target cells. VacA has been previously shown to insert itself into lipid membranes and to induce anion-selective channels in planar lipid bilayers. Binding of VacA to lipid vesicles and its ability to induce calcein release from these vesicles were systematically compared as a function of pH. These two phenomena show a different pH-dependence, suggesting that the association with the lipid membrane may be a two-step mechanism. The secondary and tertiary structure of VacA as a function of pH and the presence of lipid vesicles were investigated by Fourier-transform infrared spectroscopy. The secondary structure of VacA is identical whatever the pH and the presence of a lipid membrane, but the tertiary structure in the presence of a lipid membrane is dependent on pH, as evidenced by H/D exchange.

Transcellular and lipophilic complex-enhanced intestinal absorption of human growth hormone

PURPOSE

To evaluate the transcellular mechanism of novel enhancers absorption enhancement of human growth hormone (hGH), by examining the involvement of a P-glycoprotein-like efflux system, changes in membrane fluidity, and membrane damage.

METHODS

Caco-2 cell monolayers were grown on Snapwell filter supports and placed in a side-by-side diffusion apparatus. Transport in both the apical to basolateral (AP to BL) and basolateral to apical (BL to AP) direction was measured at different temperatures and in the presence of potential inhibitors. Fluorescence anisotropy measurement was used to measure membrane fluidity. The fluorescence anisotropy of DPH- and TMA-DPH-labeled cell suspensions was measured at room temperature. LDH (a measure of cytosolic lactate dehydrogenase) leakage assay was used to evaluate cytotoxicity.

RESULTS

The bi-directional transepithelial fluxes of hGH in the presence of these novel enhancers across Caco-2 cells showed marked asymmetry. Average permeability coefficient values obtained in the apical to basolateral (AP to BL) direction were lower than those of the reverse (BL to AP) direction. On the other hand, the fluxes for hGH alone were symmetric. When P-gp-like efflux inhibitors were included in the transport medium, the permeability coefficient value of BL to AP direction was significantly decreased while the transport was increased in the reverse direction in the presence of novel enhancers. In addition, lowering the temperature to 25 degrees C completely eliminated the asymmetry of hGH transport in the presence of novel enhancers. It was also shown by fluorescence anisotropy that these novel enhancers alone only slightly increased membrane fluidity. On the other hand, upon addition of hGH to the novel enhancers, the cell membrane showed a dramatic change as compared to treatment with novel enhancers alone. The results from the LDH assay showed that the novel enhancers and/or hGH did not cause cell damage, at least up to 1 hour, and the damage seen at the 2 hour point is also much lower than other known enhancers.

CONCLUSIONS

This study shows that human growth hormone alone cannot be transported across Caco-2 cells, except in small quantities, by passive diffusion, but in the presence of novel enhancers, human growth hormone permeation is substantial. In addition, the asymmetry of transport of the complexed hGH appears to be due to a P-gp-like efflux system. Assuming that the present substrate specificity of the P-gp-like efflux system shows the same preference for hydrophobic molecules as p-gp, the present work also indirectly shows that human growth hormone has become more lipophilic in the presence of these novel enhancers. Furthermore, membrane fluidity data also supports the premise that these novel enhancers interact and stabilize hGH, to make them more hydrophobic and easier to be transported through cell membranes.

Application of membrane-active peptides for drug and gene delivery across cellular membranes

Naturally occurring peptides and protein domains with amphipathic sequences play a dominant role in physiological, lipid membrane-reorganizing processes like fusion, disruption, or pore formation. More recently this capacity to modulate membrane integrity has been exploited for drug delivery into cells. Incorporation of synthetic membrane-active peptides into delivery systems has been found to enhance intracellular delivery of drugs including oligonucleotides, peptides, or plasmid DNA. In the majority of applications, the amphipathic peptides are designed to act after uptake by endocytosis, releasing the delivered agent from intracellular vesicles to the cytoplasm. Alternatively, peptides might mediate direct drug transfer across the plasma membrane. Although encouraging results have been obtained with the use of synthetic peptides to enhance cellular delivery of various compounds, the naturally evolved mechanisms observed in the entry of viruses or protein toxins are still far more efficient. For the development of improved synthetic peptides and carrier systems a better understanding of the molecular details of membrane-destabilization and reorganization will be essential.

The acid activation of Helicobacter pylori toxin VacA: structural and membrane binding studies

The cell vacuolating activity of the protein toxin VacA, released by Helicobacter pylori, is strongly increased in vitro by exposure to acidic pH followed by neutralization. This short acid exposure does not increase significantly the binding of VacA to cell or to lipid membranes. However, membrane photolabeling with photoactivatable radioactive phospholipids and ANS binding studies show that VacA transiently exposed to pH equal or lower than 5 changes conformation and exposes on its surface hydrophobic segments. Both the 32 and the 58 kDa subunits of the toxin insert in the lipid bilayer and interact with the fatty acid chains of phospholipids. Membrane binding and penetration are enhanced by incubating target cells or liposomes with the toxin at mild acidic pH values, similar to those present around H. pylori on the stomach mucosa. These findings are discussed with respect to the critical step in cell intoxication consisting in the translocation of the active toxin domain into the cell cytosol. We suggest that membrane translocation takes place at the plasma membrane level.

Enhanced efficiency of a targeted fusogenic peptide

Membrane targeting was investigated as a potential strategy to increase the fusogenic activity of an isolated fusion peptide. This was achieved by coupling the fusogenic carboxy-terminal part of the beta-amyloid peptide (Abeta, amino acids 29-40), involved in Alzheimer's disease, to a positively charged peptide (PIP2-binding peptide, PBP) interacting specifically with a naturally occurring negatively charged phospholipid, phosphatidylinositol 4, 5-bisphosphate (PIP2). Peptide-induced vesicle fusion was spectroscopically evidenced by: (i) mixing of membrane lipids, (ii) mixing of aqueous vesicular contents, and (iii) an irreversible increase in vesicle size, at concentrations five to six times lower than the Abeta(29-40) peptide. In contrast, at these concentrations the PBP-Abeta(29-40) peptide did not display any significant activity on neutral vesicles, indicating that negatively charged phospholipids included as targets in the membranes, are required to compensate for the lower hydrophobicity of this peptide. When the alpha-helical structure of the chimeric peptide was induced by dissolving it in trifluoroethanol, an increase of the fusogenic potential of the peptide was observed, supporting the hypothesis that the alpha-helical conformation of the peptide is crucial to trigger the lipid-peptide interaction. The specificity of the interaction between PIP2 and the PBP moiety, was shown by the less efficient targeting of the chimeric peptide to membranes charged with phosphatidylserine. These data thus demonstrate that the specific properties of both the Abeta(29-40) and the PBP peptide are conserved in the chimeric peptide, and that a synergetic effect is reached through chemical linkage of these two fragments.

The 118-135 peptide of the human prion protein forms amyloid fibrils and induces liposome fusion

The prion protein (PrPC) is a glycoprotein of unknown function normally found at the surface of neurons and of glial cells. It is involved in diseases such as bovine spongiform encephalopathy, and Creutzfeldt-Jakob disease in the human, where PrPC is converted into an altered form (termed PrPSc). PrPSc is highly resistant towards proteolytic degradation and accumulates in the central nervous system of affected individuals. By analogy with the pathological events occuring during the development of Alzheimer's disease, controverses still exist regarding the relationship between amyloidogenesis, prion aggregation and neuronal loss. To unravel the mechanism of PrP neurotoxicity and understand the interaction of PrP with cellular membranes, a series of natural and variant peptides spanning residues 118 to 135 of PrP was synthesized. The potential of these peptides to induce fusion of unilamellar lipid vesicles was investigated. According to computer modeling calculations, the 120 to 133 domain of PrP is predicted to be a tilted lipid-associating peptide, and to insert in a oblique way into a lipid bilayer through its N-terminal end. In addition to amyloidogenic properties exhibited in vitro by these peptides, peptide-induced vesicle fusion was demonstrated by several techniques, including lipid- and core-mixing assays. Elongation of the 120 to 133 peptide towards the N- and C-terminal ends of the PrP sequence showed that the 118 to 135 PrP peptide has maximal fusogenic properties, while the variant peptides had no effect. Due to their high hydrophobicity, all peptides tested were able to interact with liposomes to induce leakage of encapsulated calcein. We demonstrate also that the propensity of the peptides to fold as an alpha-helix increases their fusogenic activity, thus accounting for the maximal fusogenic activity of the most stable helix at residues 118 to 135. These data suggest that, by analogy with the C-terminal domain of the beta-amyloid peptide, the fusogenic properties exhibited by the prion peptides might contribute to the neurotoxicity of these peptides by destabilizing cellular membranes.

Interaction with a lipid membrane: a key step in bacterial toxins virulence

Bacterial toxins are secreted as soluble proteins. However, they have to interact with a cell lipid membrane either to permeabilize the cells (pore forming toxins) or to enter into the cytosol to express their enzymatic activity (translocation toxins). The aim of this review is to suggest that the strategies developed by toxins to insert in a lipid membrane is mediated by their structure. Two categories, which contains both pore forming and translocation toxins, are emerging: alpha helical proteins containing hydrophobic domains and beta sheets proteins in which no hydrophobicity can be clearly detected. The first category would rather interact with the membrane through multi-spanning helical domains whereas the second category would form a beta barrel in the membrane.

Fusogenic properties of the C-terminal domain of the Alzheimer beta-amyloid peptide

A series of natural peptides and mutants, derived from the Alzheimer beta-amyloid peptide, was synthesized, and the potential of these peptides to induce fusion of unilamellar lipid vesicles was investigated. These peptide domains were identified by computer modeling and correspond to respectively the C-terminal (e.g. residues 29-40 and 29-42) and a central domain (13-28) of the beta-amyloid peptide. The C-terminal peptides are predicted to insert in an oblique way into a lipid membrane through their N-terminal end, while the mutants are either parallel or perpendicular to the lipid bilayer. Peptide-induced vesicle fusion was demonstrated by several techniques, including lipid-mixing and core-mixing assays using pyrene-labeled vesicles. The effect of peptide elongation toward the N-terminal end of the entire beta-amyloid peptide was also investigated. Peptides corresponding to residues 22-42 and 12-42 were tested using the same techniques. Both the 29-40 and 29-42 beta-amyloid peptides were able to induce fusion of unilamellar lipid vesicles and calcein leakage, and the amyloid 29-42 peptide was the most potent fusogenic peptide. Neither the two mutants or the 13-28 beta-amyloid peptide had any fusogenic activity. Circular dichroism measurements showed an increase of the alpha-helical content of the two C-terminal peptides at increasing concentrations of trifluoroethanol, which was accompanied by an increase of the fusogenic potential of the peptides. Our data suggest that the alpha-helical content and the angle of insertion of the peptide into a lipid bilayer are critical for the fusogenic activity of the C-terminal domain of the amyloid peptide. The differences observed between the fusogenic capacity of the amyloid 29-40 and 29-42 peptides might result from differences in the degree of penetration of the peptides into the membrane and the resulting membrane destabilization. The longer peptides, residues 22-42 and 12-42, had decreased, but significant, fusogenic properties associated with perturbation of the membrane permeability. These data suggest that the fusogenic properties of the C-terminal domain of the beta-amyloid peptide might contribute to the cytotoxicity of the peptide by destabilizing the cell membrane.

Interaction of the lantibiotic nisin with membranes revealed by fluorescence quenching of an introduced tryptophan

Nisin is a lantibiotic produced by strains of Lactococcus lactis subsp. lactis. The target for nisin action is the cytoplasmic membrane of gram-positive bacteria. To aid understanding of its mode of action, the interaction of nisin with vesicles of differing phospholipid composition were investigated by fluorescence techniques, using a variant of nisin in which the isoleucine at position 30 was replaced by a tryptophan residue. Activity of the site-directed variant containing tryptophan was established to be similar to that of the wild-type peptide. Fluorescence experiments showed a blue shift of the emission wavelength maximum in the presence of lipid vesicles, indicating that the tryptophan residue enters a more hydrophobic environment. Quenching experiments with aqueous and membrane-restricted quenchers (iodide and spin-labelled lipids, respectively) both confirmed a non-aqueous environment for the Trp30 residue, and implied that the residue resides between 0.36 nm and 0.52 nm from the centre of the membrane, depending on the lipid identity. The results clearly demonstrate that nisin interacts strongly with the hydrophobic phase of lipid vesicles. This interaction is stronger in the presence of negatively charged lipids suggesting their importance in the functional interaction of nisin with membranes.

Lipid interaction of the 37-kDa and 58-kDa fragments of the Helicobacter pylori cytotoxin

Helicobacter pylori cytotoxin vacA (95 kDa) causes a vacuolar degeneration of epithelial cells. There is evidence that this protein toxin acts inside cells, and hence has to cross a cell membrane. This cytotoxin is frequently obtained as two fragments of 58 kDa (p58) and 37 kDa (p37) and it is available only in minute amounts. Here, its membrane interaction was studied with the two fragments, produced in Escherichia coli. Light scattering and energy transfer experiments show that p37 and p58 cause aggregation and fusion of small unilamellar lipid vesicles; only a reversible aggregation is induced at neutral pH, whereas at acid pH fusion also takes place. p58, but not p37, causes potassium efflux from liposomes and this occurs only at acid pH. Hydrophobic photolabelling with photoactivatable phosphatidylcholines inserted into liposomes shows that both fragments are labelled at neutral pH. The amount of labelling of the two fragments is much higher at acid pH, consistent with a further penetration into the hydrophobic core of the lipid bilayer. Tryptophan fluorescence measurements indicate that the two fragments undergo a pH-driven conformational change. These data are consistent with cytotoxin entry in the cell cytosol via an intracellular acidic compartment.

Secondary structure and membrane interaction of PR-39, a Pro+Arg-rich antibacterial peptide

PR-39 is a 4719-Da peptide isolated from pig intestine and belonging to the recently discovered family of Pro+Arg-rich antibacterial peptides. PR-39 does not lyse Escherichia coli, instead the lethal action is probably linked to the termination of DNA and protein synthesis [Boman, H. G., Agerberth, B. & Boman, A. (1993) Infect. Immun. 61, 2978-2984]. Circular dichroism and Fourier-transform infrared spectroscopy have been used to investigate the secondary structure of PR-39 in the absence or presence of lipids. According to the circular dichroic data, this structure is not altered upon incubation of PR-39 with negatively charged vesicles, although the infrared spectra suggest that the hydrogen bond pattern is modified upon the peptide-lipid interaction. This is detected by a shift in the maximum wavelength of absorption of PR-39 from 1636 cm-1 in the absence of lipids to 1645 cm-1 in the presence of lipids. We have further addressed the question of the possible mechanism of interaction of PR-39 with model membranes (liposomes and planar lipid bilayers) whose lipid compositions mimick that of the E. coli inner membrane. PR-39 induced a calcein release from large unilamellar vesicles, which is dependent upon the peptide concentration and upon the presence of negatively charged lipid (glycerophosphoglycerol) in the membrane. The binding study of PR-39 to dioleoylglycerophosphoglycerol vesicles suggests that nearly 100% of the added peptide is membrane-bound. Addition of PR-39 to a planar lipid bilayer induced a linear increase in the current but no channel formation was observed since no discrete steps of conductance occurred.

Pardaxin: channel formation by a shark repellant peptide from fish

The results of the various studies describing the mechanism involved in pore formation by pardaxin and some of its analogues, support a 'barrel-stave' model (Ehrenstein amd Lecar, 1977). In this model pardaxin exerts its activity via three successive steps: (i) a fast binding step (as reflected by the rapid increase of NBD fluorescence in the presence of vesicles); (ii) insertion of peptides into the lipid bilayer; and (iii) the monomers aggregate into a barrel-like formation in which a central aqueous pore surrounded by proteins is formed. This pore increases in diameter through the progressive recruitment of additional monomers. Both the fluorescence energy transfer (FET) studies and the observation of a significant difference in the increase of NBD fluorescence, depending on which terminal was labelled by the fluorophore, support a model by which aggregates are formed in an ordered parallel manner, where the C-terminus is more exposed to the aqueous phase.

Topology of diphtheria toxin B fragment inserted in lipid vesicles

Diphtheria toxin (DT) is a bacterial protein that crosses the membrane of endosomes of target cells in response to the low endosomal pH. In this paper, we have inserted diphtheria toxin in asolectin vesicles at pH 5.0 and treated the reconstituted system with pronase. The peptides that were protected from digestion were separated by gel electrophoresis, transferred to a membrane and their N-terminal sequences were determined. All peptides belong to the B fragment of DT and cover residues 194-223, 265-375 and 429-528. The secondary structures of the peptides inserted in the membrane, determined by Fourier-transformed infrared spectroscopy, were shown to be mostly alpha-helices and beta-sheets (44% and 53%, respectively). On the basis of these data and the recently published X-ray structure of DT, we are proposing a topology for the DTB fragment in the membrane.

Alterations in membrane permeability induced by aminoglycoside antibiotics: studies on liposomes and cultured cells

Aminoglycoside antibiotics bind to negatively-charged membranes in vitro as well as in vivo. We have examined if this binding could be associated with a change in the properties of membrane permeability. We have used a series of aminoglycoside derivatives and two independent test systems, namely (i) the release of calcein and of Mn2+ from phosphatidylinositol-containing large unilamellar vesicles, and (ii) the influx of Ca2+ into cultured macrophages. We found that certain aminoglycosides (e.g., streptomycin, isepamicin) markedly increase the membrane permeability whereas others (e.g., gentamicin) barely or do not influence it. This increase, when it occurs, is slower or less extensive than observed with pore-forming agents (mellitin, nystatin) or a Ca(2+)-ionophore (ionomycin). It is not observed with an agent [bis(beta-diethylaminoethylether)hexestrol] known to cause membrane fusion, and is not associated with any detectable change in membrane fluidity. In computer-aided conformational analysis of mixed monolayers between phosphatidylinositol and the aminoglycosides studied, it was found that those derivatives inducing an increase in membrane permeability in our experiments adopted an orientation rather perpendicular to the interface, whereas those with no or only a moderate effect were placed in a parallel orientation to this interface. The perpendicular orientation might cause a local condition of disorder which could explain the effects observed.

Ion channel and membrane translocation of diphtheria toxin

Diphtheria toxin is the best studied member of a family of bacterial protein toxins which act inside cells. To reach their cytoplasmic targets, these toxins, which include tetanus and botulinum neurotoxins and anthrax toxin, have to cross the hydrophobic membrane barrier. All of them have been shown to form ion channels across planar lipid bilayer and, in the case of diphtheria toxin, also in the plasma membrane of cells. A relation between the ion channel and the process of membrane translocation has been suggested and two different models have been put forward to account for these phenomena. The two models are discussed on the basis of the available experimental evidence and in terms of the focal points of difference, amenable to further experimental investigations.

Triggered release of hydrophilic agents from plasmalogen liposomes using visible light or acid

Triggered release from liposomes composed of semi-synthetic 1-alk-1'-enyl-2-acyl-sn-glycero-3-phosphocholine (plasmalogen) lipids has been demonstrated using either aerobic visible illumination or low pH to induce leakage. The photodynamic release system consists of three functional components: (1) small (less than 1000 A) unilamellar plasmalogen vesicles (SUVs) containing encapsulated glucose, (2) oxygen and (3) zinc phthalocyanine (ZnPc) incorporated within the hydrophobic region of the SUV membrane. Irradiation (lambda greater than 640 nm) at 37 degrees C of air-saturated 1-alk-1'-enyl-2-palmitoyl-sn-glycero-3-phosphocholine (PlasPPC)/1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) (8:1, mol/mol) liposomes at physiologically relevant temperatures results in glucose release rates that are twice those of the corresponding dark control. Photolysis of argon-saturated PlasPPC/DPPC liposomes or of identical vesicles lacking either ZnPc or the plasmalogen vinyl ether bond exhibit glucose release curves which are indistinguishable from the dark control. Irradiation under identical conditions, but in the presence of 100 mM sodium azide, also results in no increased rate of glucose release above that of the dark control. TLC analysis indicates that oxidized lipid species are produced only in air-saturated, irradiated plasmalogen liposomes. The acid lability of the plasmalogen vinyl ether linkage has also been used to trigger release of entrapped calcein. At pH 4.2, the release rate at 37 degrees C is increased 4-fold over rates observed at pH 8. TLC analysis indicates formation of a lysoplasmalogen product. Taken together, these results indicate that both photodynamic and acid triggering can be used to increase plasmalogen liposome permeability and suggest that these liposomes are potentially useful for drug delivery applications.

Lipid interaction of Pseudomonas aeruginosa exotoxin A. Acid-triggered permeabilization and aggregation of lipid vesicles

We have investigated the interaction of Pseudomonas exotoxin A with small unilamellar vesicles comprised of different phospholipids as a function of pH, toxin, and lipid concentration. We have found that this toxin induces vesicle permeabilization, as measured by the release of a fluorescent dye. Permeabilization is due to the formation of ion-conductive channels which we have directly observed in planar lipid bilayers. The toxin also produces vesicle aggregation, as indicated by an increase of the turbidity. Aggregation and permeabilization have completely different time course and extent upon toxin dose and lipid composition, thus suggesting that they are two independent events. Both time constants decrease by lowering the pH of the bulk phase or by introducing a negative lipid into the vesicles. Our results indicate that at least three steps are involved in the interaction of Pseudomonas exotoxin A with lipid vesicles. After protonation of one charged group the toxin becomes competent to bind to the surface of the vesicles. Binding is probably initiated by an electrostatic interaction because it is absolutely dependent on the presence of acidic phospholipids. Binding is a prerequisite for the subsequent insertion of the toxin into the lipid bilayer, with a special preference for phosphatidylglycerol-containing membranes, to form ionic channels. At high toxin and vesicle concentrations, bound toxin may also induce aggregation of the vesicles, particularly when phosphatidic acid is present in the lipid mixture. A quenching of the intrinsic tryptophan fluorescence of the protein, which is induced by lowering the pH of the solution, becomes more drastic in the presence of lipid vesicles. However, this further quenching takes so long that it cannot be a prerequisite to either vesicle permeabilization or aggregation. Pseudomonas exotoxin A shares many of these properties with other bacterial toxins like diphtheria and tetanus toxin.

Anthrax toxin protective antigen: low-pH-induced hydrophobicity and channel formation in liposomes

To probe the role of the protective antigen (PA) component of anthrax toxin in toxin entry into animals cells, we examined the membrane channel-forming properties and hydrophobicity of intact and trypsin-cleaved forms of the protein at various pH values. At neutral pH neither form caused release of entrapped K+ from unilamellar lipid vesicles. At pH values below 6.0, however, K+ was rapidly released upon addition of either the nicked PA (PAN) or the 63 kDa tryptic fragment of PA (PA63), which has been implicated in the toxin entry process. Under the same conditions intact PA exhibited only weak channel-forming activity, and PA20, the complementary tryptic fragment, showed no such activity. Both PA and PA63 exhibited enhanced hydrophobicity at acidic pH values, but the enhancement was greater and the pH threshold higher with PA63. Our findings indicate that proteolytic removal of PA20 from intact PA enables the residual protein, PA63, to adopt a conformation at mildly acidic pH values that permits it to insert readily and form channels in membranes. Thus acidic conditions within endocytic vesicles may trigger membrane insertion of PA63, which in turn promotes translocation of ligated effector moieties, edema factor or lethal factor, across the vesicle membrane into the cytosol.

Folding changes in membrane-inserted diphtheria toxin that may play important roles in its translocation

Diphtheria toxin membrane penetration is triggered by the low pH within the endosome lumen. Subsequent exposure to the neutral pH of the cytoplasm is believed to aid in translocation of the catalytic A domain of the toxin into the cytoplasm. To understand the effects of low pH and subsequent exposure to neutral pH on translocation, we studied toxin conformation in solution and in toxin inserted in model membranes. Two conformations were found at low pH. One form, L', predominates below 25-30 degrees C, and the other, L", predominates above 25-30 degrees C and is formed from the L' state by an unfolding event. Both forms are hydrophobic and penetrate deeply into membranes. After pH neutralization, the L' and L'' conformations give rise to two new conformations, R' and R'', respectively. The R' and R" conformations differ from each other in that in the R' state the A domain remains folded, whereas in the R" state the A domain is unfolded. This is confirmed by the finding that only the R' state possesses the capacity to bind and hydrolyze NAD+. It is also supported by the finding that the R'' state can also be formed by thermal unfolding of the R' state. The R conformations differ from the low-pH L conformations in that although they remain largely membrane-inserted, it appears that a large portion of the toxin is no longer in contact with the hydrophobic core of the bilayer.(ABSTRACT TRUNCATED AT 250 WORDS)

Lipid interaction of diphtheria toxin and mutants. A study with phospholipid and protein monolayers

To study the structural change of diphtheria toxin (DT) induced by low pH and its influence on the interaction with membrane lipids, protein and lipid monolayers were formed and characterized. DT at neutral and acidic pH forms stable monolayers, whose surface-pressure-increase curves allow an estimation of the apparent molecular area of 29.5 nm2/molecule at pH 7.4 (corresponding to a radius of 3.06 nm) and 34.5 nm2/molecule at pH 5.0 (corresponding to a radius of 3.32 nm). DT at pH 7.4 does not insert into phospholipid monolayers, while at pH 5.0 it penetrates into the lipid layer with a portion of apparent molecular area of 21.0 nm2/molecule (corresponding to a radius of 2.6 nm). The low-pH driven lipid interaction of the toxin is favoured by the presence of acidic phospholipids, without an apparent requirement for a particular class of negative lipids. The DT mutants crm 45 and crm 197 are capable of hydrophobic interaction already at neutral pH and cause an increase of surface pressure with a further increase upon acidification.

Cyclic AMP accumulation in HeLa cells induced by cholera toxin. Involvement of the ceramide moiety of GM1 ganglioside

The influence of ceramide composition on the rate of GM1 association to HeLa cells has been investigated by incubating the cells in the presence of either native ganglioside or molecular species carrying highly homogeneous long chain base moieties, fractionated from native GM1. The GM1 ganglioside species carrying the unsaturated C18 long chain base moiety proved to have the fastest rate of association, whereas the saturated species carrying 20 carbon atoms had the slowest rate. After having increased the GM1 cell content (65-fold) by incubation with the various ganglioside species, the cells were incubated with cholera toxin and the time course of cyclic AMP accumulation was monitored. Remarkable differences among cells enriched with the various molecular species were found in the duration of the lag time preceding the accumulation of cyclic AMP, the shortest being displayed by the unsaturated C18 species. Moreover, the amount of cyclic AMP accumulated after a given time of incubation with cholera toxin was significantly higher when the C18:1-GM1 species was present than with native GM1. Fluorescence anisotropy experiments, carried out using the probe 1,3-diphenylhexatriene, show that the GM1 ganglioside ceramide moiety was also modifying the cell membrane fluidity of the host.

Histidine-21 is involved in diphtheria toxin NAD+ binding

Histidine-21 is the sole histidine present in the A chain of diphtheria toxin and recent evidence suggests that it is involved in NAD+ binding. Fluorimetric assays of NAD+ binding and diethylpyrocarbonate modification performed at different pH values provide further insights on the role of this residue and indicate that its pKa value is 6.3. Conformational changes of subunit A of diphtheria toxin have been detected by analysis of tryptophan fluorescence in the pH 2.5-4 and pH 9-10.5 ranges. This indicates that histidine-21 is unlikely to be involved in the low pH-driven conformational change of diphtheria toxin.