Spin-label electron paramagnetic resonance studies on the interaction of avidin with dimyristoyl-phosphatidylglycerol membranes

Role of membranotropic sequences from herpes simplex virus type I glycoproteins B and H in the fusion process

The entry of enveloped viruses involves attachment followed by close apposition of the viral and plasma membranes. Then, either on the cell surface or in an endocytotic vesicle, the two membranes fuse by an energetically unfavourable process requiring the destabilisation of membrane microenvironment in order to release the viral nucleocapsid into the cytoplasm. The core fusion machinery, conserved throughout the herpesvirus family, involves glycoprotein B (gB) and the non-covalently associated complex of glycoproteins H and L (gH/gL). Both gB and gH possess several hydrophobic domains necessary for efficient induction of fusion, and synthetic peptides corresponding to these regions are able to associate to membranes and induce fusion of artificial liposomes. Here, we describe the first application of surface plasmon resonance (SPR) to the study of the interaction of viral membranotropic peptides with model membranes in order to enhance our molecular understanding of the mechanism of membrane fusion. SPR spectroscopy data are supported by tryptophan fluorescence, circular dichroism and electron spin resonance spectroscopy (ESR). We selected peptides from gB and gH and also analysed the behaviour of HIV gp41 fusion peptide and the cationic antimicrobial peptide melittin. The combined results of SPR and ESR showed a marked difference between the mode of action of the HSV peptides and the HIV fusion peptide compared to melittin, suggesting that viral-derived membrane interacting peptides all act via a similar mechanism, which is substantially different from that of the non-cell selective lytic peptide melittin.

Structural characterization of the transmembrane proximal region of the hepatitis C virus E1 glycoprotein

A detailed knowledge of the mechanism of virus entry represents one of the most promising approaches to develop new therapeutic strategies. However, viral fusion is a very complex process involving fusion glycoproteins present on the viral envelope. In the two hepatitis C virus envelope proteins, E1 and E2, several membranotropic regions with a potential role in the fusion process have been identified. Among these, we have selected the 314-342 E1 region. Circular Dichroism data indicate that the peptide exhibits a clear propensity to adopt a helical folding in different membrane mimicking media, such as mixtures of water with fluorinated alcohols and phospholipids, with a slight preference for negative charged bilayers. The 3D structure of E1(314-342) peptide, calculated by 2D-NMR in a low-polarity environment, consists of two helical stretches encompassing residues 319-323 and 329-338 respectively. The peptide, presenting a largely apolar character, interacts with liposomes, as indicated by fluorescence and electron spin resonance spectra. The strength of the interaction and the deepness of peptide insertion in the phospholipid membrane are modulated by the bilayer composition, the interaction with anionic phospholipids being among the strongest ever observed. The presence of cholesterol also affects the peptide-bilayer interaction, favoring the peptide positioning close to the bilayer surface. Overall, the experimental data support the idea that this region of E1 might be involved in membrane destabilization and viral fusion; therefore it may represent a good target to develop anti-viral molecules.

Interaction of short modified peptides deriving from glycoprotein gp36 of feline immunodeficiency virus with phospholipid membranes

A tryptophan-rich octapeptide, C8 (Ac-Trp-Glu-Asp-Trp-Val-Gly-Trp-Ile-NH(2)), modelled on the membrane-proximal external region of the feline immunodeficiency virus (FIV) gp36 glycoprotein ectodomain, exhibits potent antiviral activity against FIV. A mechanism has been proposed by which the peptide, being positioned on the surface of the cell membrane, inhibits its fusion with the virus. In the present work, peptide-lipid interactions of C8 with dimyristoyl phosphatidylcholine liposomes are investigated using electron spin resonance spectroscopy of spin-labelled lipids. Three other peptides, obtained from modifications of C8, have also been investigated, in an attempt to clarify the essential molecular features of the interactions involving the tryptophan residues. The results show that C8 adsorbs strongly on the bilayer surface. Membrane binding requires not only the presence of the Trp residues in the sequence, but also their common orientation on one side of the peptide that is engendered by the WX(2) WX(2) W motif. Membrane interaction correlates closely with peptide antiviral activity, indicating that the membrane is essential in stabilizing the peptide conformation that will be able to inhibit viral infection.

Influence of structural factors on the enhanced activity of moxifloxacin: a fluorescence and EPR spectroscopic study

Partition coefficients of moxifloxacin in liposomes of dimyristoyl-L-alpha-phosphatidylcholine or dimyristoyl-L-alpha-phosphatidylglycerol and water were determined by spectrophotometry and fluorimetry. The K (p) values obtained were larger than those reported for most of the other fluoroquinolones, a consequence of the structural changes observed in the molecule of moxifloxacin, which in turn change its acid/base properties. Introduction of a methoxy group at position 8 and a diazabicyclonyl ring at position 7 in the basic fluoroquinolone structure alters the charge distribution at the physiological pH of 7.4, and these changes seem to be responsible for its improved antibacterial potency and broader spectrum of activity. Location studies have also been performed using fluorescence and electron paramagnetic resonance (EPR) spectroscopies. The results show that moxifloxacin must be located near the phospholipid headgroups, similar to other fluoroquinolones, but contributions from a hydrophobic component were also detected. These results suggest that the enhanced activity of this drug may be related to a more facilitated entrance into the bacterial cell, perhaps including a mediator step involving electrostatic interaction with a hydrophobic component; this step then controls the extent or orientation of insertion and improves the electrostatic interaction.

Electron paramagnetic resonance studies of magnetically aligned phospholipid bilayers utilizing a phospholipid spin label: The effect of cholesterol

X-band EPR spectroscopy has been employed to study the dynamic properties of magnetically aligned phospholipid bilayers (bicelles) utilizing a variety of phosphocholine spin labels (n-PCSL) as a function of cholesterol content. The utilization of both perpendicular and parallel aligned bicelles in EPR spectroscopy provides a more detailed structural and orientational picture of the phospholipid bilayers. The magnetically aligned EPR spectra of the bicelles and the hyperfine splitting values reveal that the addition of cholesterol increases the phase transition temperature and alignment temperature of the DMPC/DHPC bicelles. The corresponding molecular order parameter, Smol, of the DMPC/DHPC bicelles increased upon addition of cholesterol. Cholesterol also decreased the rotational motion and increased the degree of anisotropy in the interior region of the bicelles. This report reveals that the dynamic properties of DMPC/DHPC bicelles agree well with other model membrane systems and that the magnetically aligned bicelles are an excellent model membrane system.

Interaction of tetraphenyl-porphyrin derivatives with DPPC-liposomes: an EPR study

The effect of the symmetry and polarity of the porphyrin molecules on their membrane localization and interaction with membrane lipids were investigated by electron paramagnetic resonance (EPR). For this purpose, two glycoconjugated tetraphenyl porphyrin derivatives were selected, respectively, symmetrically and asymmetrically substituted. Small unilamellar liposomes composed of dipalmitoylphosphatidylcholine (DPPC) and spin labeled stearic acids were prepared. The spin probe was located at the 5th or 7th or 12th or 16th position of the hydrocarbon chain in order to monitor various regions of the lipid bilayer. EPR spectra of porphyrin-free and porphyrin-bound liposomes were recorded at various temperatures below and above the phase transition temperature of DPPC. The effect on membrane fluidity proved to be stronger with the asymmetrical porphyrin derivative than with the symmetrical one. The rigidity increased when the spin label was near lipid head groups. The difference observed between control and porphyrin-treated samples when measured below the main lipid transition temperature disappeared at higher temperature. When the spin label was near the end of the hydrophobic tails, the symmetrical porphyrin derivative caused increase in fluidity, while the asymmetrical one slightly decreased it. To explain this phenomenon we propose that the asymmetrical derivative exerts a stronger ordering effect caused by its fluorophenyl group located at the level of the lipid heads, which is attenuated to the hydrophobic tails. The perturbing effect of the symmetric derivative could not lead to similar extent of ordering at the head groups and looses the hydrocarbon chains deeper in the membrane.

Molecular dynamics of the cyclic lipodepsipeptides' action on model membranes: effects of syringopeptin22A, syringomycin E, and syringotoxin studied by EPR technique

Interaction of pore-forming toxins, syringopeptin22A (SP22A), syringomycin E (SRE) and syringotoxin (ST), with model membranes were investigated. Liposomes were prepared from saturated phospholipids (DPPC or DMPC) or from binary mixtures of DPPC with varying amount of DOPC or cholesterol. The effects of the three toxins on the molecular order and dynamics of the lipids were studied using electron paramagnetic resonance (EPR) techniques. SP22A was the most-, SRE less-, and ST the least effective to increase the ordering and to decrease the rotational correlation time of the lipid molecules. The effects were more pronounced: (a) on small unilamellar vesicles (SUVs) than on multilamellar vesicles (MUVs); (b) on pure DPPC than on DPPC-cholesterol or DPPC-DOPC mixtures. Fluidity changes, determined from EPR spectra at different concentrations of the toxin, suggested the shell structure of the lipid molecules in pore formation. EPR spectra observed at different depth of the hydrocarbon chain of the lipid molecules implied an active role of the lipid molecules in the architecture of the pores created in the presence of the three toxins. Temperature dependence of the fluidity of the SUVs treated with toxins has shown an abrupt and irreversible change in the molecular dynamics of the lipid molecules at a temperature close to the pretransition, depending on the toxin species and the lipid composition. Coalescence and aggregation of the SUVs were proposed as the origin of this irreversible change.

Ordered and disordered phases coexist in plasma membrane vesicles of RBL-2H3 mast cells. An ESR study

Four chain spin labels and a spin-labeled cholestane were used to study the dynamic structure of plasma membrane vesicles (PMV) prepared from RBL-2H3 mast cells at temperatures ranging from 22 degrees C to 45 degrees C. Analysis shows that the spectra from most labels consist of two components. The abundant spectral components exhibit substantial ordering that is intermediate between that of a liquid-ordered (Lo) phase, and that of a liquid-crystalline (Lc) phase as represented by model membranes. Also, rotational diffusion rates of the spin labels are comparable to those in the Lo phase. In contrast, the ordering for the less abundant components is much lower. These results indicate that a Lo-like region or phase (the abundant component) and an Lc-like region or phase (the less abundant component) coexist in the PMV. In contrast, membranes reconstituted from extracted lipids exhibit the more ordered phase only. This suggests that membrane-associated proteins are important for the coexistence of Lo-like and Lc-like regions in the plasma membrane. In addition, binding of the myristoylated protein, ARF6 to PMV, leads to a new spectral component for a headgroup lipid spin label that indicates the formation of plasma membrane defects by this low molecular weight GTPase.

Alteration in the Temperature-Dependent Content Release Property of Thermosensitive Liposomes in Plasma

The effect of plasma components on the temperature-dependent content release property of thermosensitive liposomes has been described. Temperature-sensitive liposomes containing mitomycin C (MMC) were prepared from dipalmitoylphosphatidylcholine (DPPC liposomes) and a 7 : 3 mixture of DPPC and dipalmitoylophosphatidylglycerol (DPPC/DPPG liposomes). We defined in this study the difference in the content release between 38 degrees C and 44 degrees C as an index of the temperature-dependent content release efficiency (Delta% release). In the absence of rat plasma, the Delta% release of the DPPC liposomes and the DPPC/DPPG liposomes was 83% and 71%, respectively. However, when the release study was conducted with rat plasma, the Delta% release increased to about 96% for both liposomes. In addition, while the DPPC liposomes were destabilized by rat plasma below the gel-to-liquid crystalline phase transition temperature (T(m)), MMC leakage from the DPPC/DPPG liposomes below T(m) was suppressed by rat plasma. Moreover, the plasma protein binding onto lipid bilayer was concomitant with the gel-to-liquid crystalline phase transition and then enhanced the temperature-dependent release from the DPPC/DPPG liposomes. The possible mechanism of interaction between liposomes and plasma proteins, especially serum albumin, was discussed based on differential scanning calorimetry and protein binding experiments.