Membrane fusion induced by the HIV type 1 fusion peptide: modulation by factors affecting glycoprotein 41 activity and potential anti-HIV compounds

ATR-FTIR studies in pore forming and membrane induced fusion peptides

Infrared (IR) spectroscopy has been shown to be very reliable for the characterization, identification and quantification of structural data. Particularly, the Attenuated Total Reflectance (ATR) technique which became one of the best choices to study the structure and organization of membrane proteins and membrane-bound peptides in biologically relevant membranes. An important advantage of IR spectroscopy is its ability to analyze material under a very wide range of conditions including solids, liquids and gases. This method allows elucidation of component secondary structure elements of a peptide or protein in a global manner, and by using site specific isotope labeling allows determination of specific regions. A few advantages in using ATR-FTIR spectroscopy include; a relatively simple technique, allow the determination of peptide orientation in the membrane, allow the determination of secondary structures of very small peptides, and importantly, the method is sensitive to isotopic labeling on the scale of single amino acids. Many studies were reported on the use of ATR-FTIR spectroscopy in order to study the structure and orientation of membrane bound hydrophobic peptides and proteins. The list includes native and de-novo designed peptides, as well as those derived from trans-membrane domains of various receptors (TMDs). The present review will focus on several examples that demonstrate the potential and the simplicity in using the ATR-FTIR approach to determine secondary structures of proteins and peptides when bound, inserted, and oligomerized within membranes. The list includes (i) a channel forming protein/peptide: the Ca(2+) channel phospholamban, (ii) a cell penetrating peptide, (iii) changes in the structure of a transmembrane domain located within ordered and non-ordered domains, and (iv) isotope edited FTIR to directly assign structure to the membrane associated fusion peptide in context of a Key gp41 Structural Motif. Importantly, a unique advantage of infrared spectroscopy is that it allows a simultaneous study of the structure of lipids and proteins in intact biological membranes without an introduction of foreign perturbing probes. Because of the long IR wavelength, light scattering problems are virtually non-existent. This allows the investigation of highly aggregated materials or large membrane fragments. This article is part of a Special Issue entitled: FTIR in membrane proteins and peptide studies.

Roles for biological membranes in regulating human immunodeficiency virus replication and progress in the development of HIV therapeutics that target lipid metabolism

Infection by the human immunodeficiency virus (HIV) involves a number of important interactions with lipid components in host membranes that regulate binding, fusion, internalization, and viral assembly. Available data suggests that HIV actively modifies the sphingolipid content of cellular membranes to create focal environments that are favorable for infection. In this review, we summarize the roles that membrane lipids play in HIV infection and discuss the current status of therapeutics that attempt to modify biological membranes to inhibit HIV.

The membrane-proximal external region of the human immunodeficiency virus type 1 envelope: dominant site of antibody neutralization and target for vaccine design

Enormous efforts have been made to produce a protective vaccine against human immunodeficiency virus type 1; there has been little success. However, the identification of broadly neutralizing antibodies against epitopes on the highly conserved membrane-proximal external region (MPER) of the gp41 envelope protein has delineated this region as an attractive vaccine target. Furthermore, emerging structural information on the MPER has provided vaccine designers with new insights for building relevant immunogens. This review describes the current state of the field regarding (i) the structure and function of the gp41 MPER; (ii) the structure and binding mechanisms of the broadly neutralizing antibodies 2F5, 4E10, and Z13; and (iii) the development of an MPER-targeting vaccine. In addition, emerging approaches to vaccine design are presented.

Novel paradigms for drug discovery: computational multitarget screening

An established paradigm in current drug development is (i) to identify a single protein target whose inhibition is likely to result in the successful treatment of a disease of interest; (ii) to assay experimentally large libraries of small-molecule compounds in vitro and in vivo to identify promising inhibitors in model systems; and (iii) to determine whether the findings are extensible to humans. This complex process, which is largely based on trial and error, is risk-, time- and cost-intensive. Computational (virtual) screening of drug-like compounds simultaneously against the atomic structures of multiple protein targets, taking into account protein-inhibitor dynamics, might help to identify lead inhibitors more efficiently, particularly for complex drug-resistant diseases. Here we discuss the potential benefits of this approach, using HIV-1 and Plasmodium falciparum infections as examples. We propose a virtual drug discovery 'pipeline' that will not only identify lead inhibitors efficiently, but also help minimize side-effects and toxicity, thereby increasing the likelihood of successful therapies.

Interfacial pre-transmembrane domains in viral proteins promoting membrane fusion and fission

Membrane fusion and fission underlie two limiting steps of enveloped virus replication cycle: access to the interior of the host-cell (entry) and dissemination of viral progeny after replication (budding), respectively. These dynamic processes proceed mediated by specialized proteins that disrupt and bend the lipid bilayer organization transiently and locally. We introduced Wimley–White membrane-water partitioning free energies of the amino acids as an algorithm for predicting functional domains that may transmit protein conformational energy into membranes. It was found that many viral products possess unusually extended, aromatic-rich pre-transmembrane stretches predicted to stably reside at the membrane interface. Here, we review structure–function studies, as well as data reported on the interaction of representative peptides with model membranes, all of which sustain a functional role for these domains in viral fusion and fission. Since pre-transmembrane sequences also constitute antigenic determinants in a membrane-bound state, we also describe some recent results on their recognition and blocking at membrane interface by neutralizing antibodies.

3D-Structure of the interior fusion peptide of HGV/GBV-C by 1H NMR, CD and molecular dynamics studies

In this work, we present a structural characterization of the putative fusion peptide E2(279-298) corresponding to the E2 envelope protein of the HGV/GBV-C virus by (1)H NMR, CD and MD studies performed in H(2)O/TFE and in lipid model membranes. The peptide is largely unstructured in water, whereas in H(2)O/TFE and in model membranes it adopts an helical structure (approximately 65-70%). The partitioning free energy DeltaG ranges from -6 to -7.5 kcal mol(-1). OCD measurements on peptide-containing hydrated and oriented lipid multilayers showed that the peptide adopts a predominantly surface orientation. The (1)H NMR data (observed NOEs, deuterium exchange rates, Halpha chemical shift index and vicinal coupling constants) and the molecular dynamics calculations support the conclusions that the peptide adopts a stable helix in the C-terminal 9-18 residues slightly inserted into the lipid bilayer and a major mobility in the amino terminus of the sequence (1-8 residues).

Membrane perturbing actions of HIV type 1 glycoprotein 41 domains are inhibited by helical C-peptides

To study the membrane actions of various domains of HIV-1 glycoprotein 41,000 (gp41), synthetic peptides were prepared corresponding to the N-terminal fusion region (FP; gp41 residues 519-541), the nearby N-leucine zipper domain (N-peptides; DP-107; gp41 residues 560-597), the C-leucine zipper domain (C-peptides; DP-178; gp41 residues 645-680), and the viral envelope adjacent domain that partially overlaps DP-178 (Pre-TM; gp41 residues 671-690). With erythrocytes, FP, DP-107, and Pre-TM induced hemolysis and cell aggregation; the order for hemolytic activity was Pre-TM > FP > DP-107, but each was equally effective in aggregating cells at the highest peptide concentrations tested. DP-178 produced neither hemolysis nor aggregation, but efficiently reduced FP-, DP-107-, and Pre-TM-induced membrane actions. Fourier transform infrared spectroscopy indicated that the membrane perturbations of Pre-TM, as well as the ability of DP-178 to block membrane activities of other gp41 domains, are dependent on Pre-TM and DP-178 each maintaining helical conformations and tryptophans at residues 673, 677, and 679. These results suggest that the corresponding N-terminal fusion, N-leucine zipper, and viral membrane-adjacent regions of HIV-1 gp41 may similarly promote key membrane perturbations underlying the merging of the viral envelope with the cell surface. Further, the antiviral mechanism of exogenous DP-178 (clinically approved enfuvirtide) may be partially explained by its coordinate inhibition of the fusogenic actions of the FP, DP-107, and Pre-TM regions of gp41.

The N-terminal 12 Residue Long Peptide of HIV gp41 is the Minimal Peptide Sufficient to Induce Significant T-cell-like Membrane Destabilization in Vitro

Here, we predicted the minimal N-terminal fragment of gp41 required to induce significant membrane destabilization using IMPALA. This algorithm is dedicated to predict peptide interaction with a membrane. We based our prediction of the minimal fusion peptide on the tilted peptide theory. This theory proposes that some protein fragments having a peculiar distribution of hydrophobicity adopt a tilted orientation at a hydrophobic/hydrophilic interface. As a result of this orientation, tilted peptides should disrupt the interface. We analysed in silico the membrane-interacting properties of gp41 N-terminal peptides of different length derived from the isolate BRU and from an alignment of 710 HIV strains available on the Los Alamos National Laboratory. Molecular modelling results indicated that the 12 residue long peptide should be the minimal fusion peptide. We then assayed lipid-mixing and leakage of T-cell-like liposomes with N-terminal peptides of different length as first challenge of our predictions. Experimental results confirmed that the 12 residue long peptide is necessary and sufficient to induce membrane destabilization to the same extent as the 23 residue long fusion peptide. In silico analysis of some fusion-incompetent mutants presented in the literature further revealed that they cannot insert into a modelled membrane correctly tilted. According to this work, the tilted peptide model appears to explain at least partly the membrane destabilization properties of HIV fusion peptide.

Hexapeptides that interfere with HIV-1 fusion peptide activity in liposomes block GP41-mediated membrane fusion

Upon receptor-mediated activation, the gp41 hydrophobic, conserved fusion peptide inserts into the target membrane and promotes the kind of perturbations required for the progression of the HIV-cell fusion reaction. Using a synthetic combinatorial library we have identified all d-amino acid hexapeptide sequences that inhibited the fusion peptide capacity of perturbing model membranes. Two hexapeptides that effectively inhibited the fusion peptide in these systems were subsequently shown to inhibit cell-cell fusion promoted by gp41 expressed at cell surfaces. These observations might be of importance for understanding the mechanisms underlying fusion peptide activity and suggest new strategies for screening compounds that target these viral sequences.

The hydrophobic internal region of bovine prion protein shares structural and functional properties with HIV type 1 fusion peptide

The conserved fusion peptide at the N-terminus of HIV-1 envelope glycoprotein gp41 is involved in the virus-cell fusion reaction and in the cytopathic effects promoted by expression in single cells. The conserved bovine prion protein 121KHVAGAAAAGAVVGGLGGYMLGSAMSR147 transmembrane region (BPrP(tm)) contains a sequence rich in Gly residues [i.e., 130GAVVGGLGGYMLGSAMSR147 (BPrP(mi))] that shows homology with HIV-1 fusion peptide. As in the case of the latter peptide, analysis of the BPrP(mi) interfacial hydrophobicity confirms that this stretch bears an intrinsic tendency to partitioning from the aqueous phase into membranes. Experimental analyses of BPrP(mi)-lipid interactions suggest several similarities between this sequence and HIV-1 fusion peptide. Infrared spectroscopy reveals a conformational plasticity of the membrane-associated prion sequence comparable to that displayed by the viral sequence. The adoption of a mainly alpha-helical structure correlates with the formation of lytic pores. This helical structure can be converted into a beta-sheet conformation by addition of calcium, a process that is accompanied by vesicle membrane fusion. In contrast, transmembrane BPrP(tm) associates with membranes in a nonlytic, mainly helical structure but also containing some random coil. Upon addition of calcium, the random coils disappear while the helical conformation remains. In the absence of membranes both prion and HIV-1 sequences form amyloid-type fibers. It is proposed that during the pathological processes induced by secreted PrPSc and its proteolytic fragments, conformational polymorphism displayed by membrane-inserted BPrP(mi) may play a role at perturbing the general architecture of the membrane lipid bilayer and inducing protein-protein aggregation at membrane surfaces. These findings suggest the existence of common mechanisms underlying cytotoxicity by PrP and HIV-1 gp41.

Are fusion peptides a good model to study viral cell fusion?

Fusion peptides are hydrophobic and conserved sequences located within glycoprotein ectodomains that protrude from the virion surface. Direct participation of fusion peptides in the viral membrane fusion phenomenon has been inferred from genetic analyses showing that even a single residue substitution or a deletion within these sequences may completely block the process. However, the specific fusion peptide activities associated to the multi-step fusion mechanism are not well defined. Based on the assumption that fusion peptides are transferred into target membranes, biophysical methodologies have been applied to study integration into model membranes of synthetic fragments representing functional and non-functional sequences. From these studies, it is inferred that, following insertion, functional sequences generate target membrane perturbations and adopt specific structural arrangements within. Further characterization of these artificial systems may help in understanding the molecular processes that bring initial bilayer destabilizations to the eventual opening of a fusion pore.

Conformational mapping of the N-terminal peptide of HIV-1 gp41 in membrane environments using (13)C-enhanced Fourier transform infrared spectroscopy

The N-terminal domain of HIV-1 glycoprotein 41000 (FP; residues 1--23; AVGIGALFLGFLGAAGSTMGARSCONH(2)) participates in fusion processes underlying virus--cell infection. Here, we use physical techniques to study the secondary conformation of synthetic FP in aqueous, structure-promoting, lipid and biomembrane environments. Circular dichroism and conventional, (12)C-Fourier transform infrared (FTIR) spectroscopy indicated the following alpha-helical levels for FP in 1-palmitoyl-2-oleoylphosphatidylglycerol (POPG) liposomes-hexafluoroisopropanol (HFIP)>trifluoroethanol (TFE)>phosphate-buffered saline (PBS). (12)C-FTIR spectra also showed disordered FP structures in these environments, along with substantial beta-structures for FP in TFE or PBS. In further experiments designed to map secondary conformations to specific residues, isotope-enhanced FTIR spectroscopy was performed using a suite of FP peptides labeled with (13)C-carbonyl at multiple sites. Combining these (13)C-enhanced FTIR results with molecular simulations indicated the following model for FP in HFIP: alpha-helix (residues 3-16) and random and beta-structures (residues 1-2 and residues 17-23). Additional (13)C-FTIR analysis indicated a similar conformation for FP in POPG at low peptide loading, except that the alpha-helix extends over residues 1-16. At low peptide loading in either human erythrocyte ghosts or lipid extracts from ghosts, (13)C-FTIR spectroscopy showed alpha-helical conformations for the central core of FP (residues 5-15); on the other hand, at high peptide loading in ghosts or lipid extracts, the central core of FP assumed an antiparallel beta-structure. FP at low loading in ghosts probably inserts deeply as an alpha-helix into the hydrophobic membrane bilayer, while at higher loading FP primarily associates with ghosts as an aqueous-accessible, beta-sheet. In future studies, (13)C-FTIR spectroscopy may yield residue-specific conformations for other membrane-bound proteins or peptides, which have been difficult to analyze with more standard methodologies.

Stepwise deletion of the HIV type 1 glycoprotein 41 N terminus leads to an increasing export of microvesicles containing uncleaved Env glycoprotein

Deletion of two or more amino acid residues from the N terminus of HIV-1 gp41 leads to an increasing loss of cleavability of the envelope (Env) precursor on introduction of an env-expressing vector into HeLa-T4+ cells. In protein analysis, this is paralleled by the appearance of a second form of uncleaved Env precursor that is terminally sialylated. Cell-derived microvesicles that preferentially incorporate this form of Env precursor were found in the culture medium. The same applies to a mutant with a nonfunctional cleavage site, indicating that a pathway by which uncleaved Env glycoprotein leaves the cell exists. The amount of exported glycoprotein is augmented as compared with wild-type Env. Transfection with a wild-type Env-expressing vector leads to the presence of extracellular microvesicles that contain only the transmembrane domain of HIV-1 Env. Microvesicles derived from wild-type Env and mutant Env contain sialylated glycoproteins that are resistant to exo- and endoglycosidase treatment unless the particles have been previously lysed by detergent. This raises the possibility that the C-terminal domains of the glycoproteins are exposed on the surface of the exported microvesicles.

Lipid rafts and HIV pathogenesis: host membrane cholesterol is required for infection by HIV type 1

In a previous study we showed that budding of HIV-1 particles occurs at highly specialized membrane microdomains known as lipid rafts. These microdomains are characterized by a distinct lipid composition that includes high concentrations of cholesterol, sphingolipids, and glycolipids. Since cholesterol is known to play a key role in the entry of some other viruses, our observation of HIV budding from lipid rafts led us to investigate the role in HIV-1 entry of cholesterol and lipid rafts in the plasma membrane of susceptible cells. We have used 2-OH-propyl-beta-cyclodextrin (beta-cyclodextrin) to deplete cellular cholesterol and disperse lipid rafts. Our results show that removal of cellular cholesterol rendered primary cells and cell lines highly resistant to HIV-1-mediated syncytium formation and to infection by both CXCR4- and CCR5-specific viruses. beta-Cyclodextrin treatment of cells partially reduced HIV-1 binding, while rendering chemokine receptors highly sensitive to antibody-mediated internalization. There was no effect on CD4 expression. All of the above-described effects were readily reversed by incubating cholesterol-depleted cells with low concentrations of cholesterol-loaded beta-cyclodextrin to restore cholesterol levels. Cholesterol depletion made cells resistant to SDF-1-induced binding to ICAM-1 through LFA-1. Since LFA-1 contributes significantly to cell binding by HIV-1, this latter effect may have contributed to the observed reduction in HIV-1 binding to cells after treatment with beta-cyclodextrin. Our results indicate that cholesterol may be critical to the HIV-1 coreceptor function of chemokine receptors and is required for infection of cells by HIV-1.

Membrane-perturbing domains of HIV type 1 glycoprotein 41

Structural and functional studies were performed to assess the membrane actions of peptides based on HIV-1 glycoprotein 41,000 (gp41). Previous site-directed mutagenesis of gp41 has shown that amino acid changes in either the N-terminal fusion or N-leucine zipper region depressed viral infection and syncytium formation, while modifications in the C-leucine zipper domain both increased and decreased HIV fusion. Here, synthetic peptides were prepared corresponding to the N-terminal fusion region (FP-I; gp41 residues 519-541), the nearby N-leucine zipper domain (DP-107; gp41 residues 560-597), and the C-leucine zipper domain (DP-178; gp41 residues 645-680). With erythrocytes, FP-I or DP-107 induced dose-dependent hemolysis and promoted cell aggregation; FP-I was more hemolytic than DP-107, but each was equally effective in aggregating cells. DP-178 produced neither hemolysis nor aggregation, but blocked either FP-I- or DP-107-induced hemolysis and aggregation. Combined with previous nuclear magnetic resonance and Fourier transform infrared spectroscopic results, circular dichroism (CD) spectroscopy showed that the alpha-helicity for these peptides in solution decreased in the order: DP-107 >> DP-178 > FP-I. CD analysis also indicated binding of DP-178 to either DP-107 or FP-I. Consequently, DP-178 may inhibit the membrane actions mediated by either FP-I or DP-107 through direct peptide interactions in solution. These peptide results suggest that the corresponding N-terminal fusion and N-leucine zipper regions participate in HIV infection, by promoting membrane perturbations underlying the merging of the viral envelope with the cell surface. Further, the C-leucine zipper domain in "prefusion" HIV may inhibit these membrane activities by interacting with the N-terminal fusion and N-leucine zipper domains in unactivated gp41. Last, exogenous DP-178 may bind to the N-terminal and N-leucine zipper domains of gp41 that become exposed on HIV stimulation, thereby preventing the fusogenic actions of these gp41 regions leading to infection.

Paramyxovirus F1 protein has two fusion peptides: implications for the mechanism of membrane fusion

Viral fusion proteins contain a highly hydrophobic segment, named the fusion peptide, which is thought to be responsible for the merging of the cellular and viral membranes. Paramyxoviruses are believed to contain a single fusion peptide at the N terminus of the F1 protein. However, here we identified an additional internal segment in the Sendai virus F1 protein (amino acids 214-226) highly homologous to the fusion peptides of HIV-1 and RSV. A synthetic peptide, which includes this region, was found to induce membrane fusion of large unilamellar vesicles, at concentrations where the known N-terminal fusion peptide is not effective. A scrambled peptide as well as several peptides from other regions of the F1 protein, which strongly bind to membranes, are not fusogenic. The functional and structural characterization of this active segment suggest that the F1 protein has an additional internal fusion peptide that could participate in the actual fusion event. The presence of homologous regions in other members of the same family suggests that the concerted action of two fusion peptides, one N-terminal and the other internal, is a general feature of paramyxoviruses.

Interactions of peptides with liposomes: pore formation and fusion

Leakage from liposomes induced by several peptides is reviewed and a pore model is described. According to this model peptide molecules become incorporated into the vesicle bilayer and aggregate reversibly or irreversibly within the surface. When a peptide aggregate reaches a critical size, peptide translocation can occur and a pore is formed. With the peptide GALA the pores are stable and persist for at least 10 minutes. The model predicts that for a given lipid/peptide ratio, the extent of leakage should decrease as the vesicle diameter decreases, and for a given amount of peptide bound per vesicle less leakage would be observed at higher temperatures due to the increase in reversibility of surface aggregates of the peptide. Effect of membrane composition on pore formation is reviewed. When cholesterol was included in the liposomes the efficiency of inducation of leakage by the peptide GALA was reduced due to reduced binding and increased reversibility of surface aggregation of the peptide. Phospholipids which contain less ordered acyl-chains and have a slightly wedge-like shape, can better accommodate peptide surface aggregates, and consequently insertion and translocation of the peptide may be less favored. Demonstrations of antagonism between pore formation and fusion are presented. The choice of factors which promote vesicle aggregation, e.g., larger peptides, increased vesicle and peptide concentration results in enhanced vesicle fusion at the expense of formation of intravesicular pores. FTIR studies with HIV-1 fusion peptides indicate that in systems where extensive vesicle fusion occurred the beta conformation of the peptides was predominant, whereas the alpha conformation was exhibited in cases where leakage was the main outcome. Antagonism between leakage and fusion was exhibited by 1-palmitoyl-2-oleoylphosphatidylglycerol vesicles, where the order of addition of peptide (HIV(arg)) or Ca(2+)dictated whether pore formation or vesicle fusion would occur. The current study emphasizes that the addition of Ca(2+), which promotes vesicle aggregation can also reduce peptide translocation in isolated vesicles.

Biochemical consequences of a mutation that controls the cholesterol dependence of Semliki Forest virus fusion

The enveloped alphavirus Semliki Forest virus (SFV) infects cells via a low-pH-triggered membrane fusion reaction that requires cholesterol and sphingolipid in the target membrane. Cholesterol-depleted insect cells are highly resistant to alphavirus infection and were used to select srf-3, an SFV mutant that is approximately 100-fold less cholesterol dependent for infection due to a single amino acid change in the E1 spike subunit, proline 226 to serine. Sensitive lipid-mixing assays here demonstrated that the in vitro fusion of srf-3 and wild-type (wt) virus with cholesterol-containing liposomes had comparable kinetics, activation energies, and sphingolipid dependence. In contrast, srf-3 fusion with sterol-free liposomes was significantly more efficient than that of wt virus. Thus, the srf-3 mutation does not affect its general fusion properties with purified lipid bilayers but causes a marked and specific reduction in cholesterol dependence. Upon exposure to low pH, the E1 spike subunit undergoes distinct conformational changes, resulting in the exposure of an acid conformation-specific epitope and formation of an E1 homotrimer. These conformational changes were strongly cholesterol and sphingolipid dependent for wt SFV and strikingly less cholesterol dependent for srf-3. Our results thus demonstrate the functional importance of fusogenic E1 conformational changes in the control of SFV cholesterol dependence.

Membrane fusion induced by a short fusogenic peptide is assessed by its insertion and orientation into target bilayers

To clarify the molecular mechanism by which an amphipathic negatively charged peptide consisting of 11 residues (WAE) induces fusion, and the relevance of these features for fusion, its mode of insertion and orientation into target bilayers were investigated. Using attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) in combination with techniques based on tryptophan fluorescence, the peptide was found to form an alpha-helix, shallowly inserted into the membrane to which it is anchored. Interestingly, in the presence of target membranes, WAE inserts into the target bilayer as an alpha-helix oriented almost parallel to the lipid acyl chains. The accessibility of the peptide to either acrylamide (as an aqueous quencher of Trp fluorescence) or deuterium oxide (on the course of an FTIR deuteration kinetics) was lower in the presence than in the absence of target membranes, confirming that under those conditions, the peptide was shielded from the aqueous environment. Since fusion experiments have shown a temperature dependence, the effect of this later parameter on the structure and mode of insertion of the peptide was also analyzed. In the presence of target membrane, but not in their absence, the amount of alpha-helical structure increased with temperature, reflecting a similar temperature-dependent increase in the rate and extent of WAE-induced fusion. Also, the extent of penetration of the helix into the target membrane was greater at 37 degrees C than at lower temperatures. This temperature-dependent distinction was revealed by a decreased accessibility of the peptide to deuterium oxide and acrylamide at 37 degrees C as compared to that at lower temperatures. These data underscore the role of peptide structure, peptide penetration, and orientation in the mechanism of protein-induced membrane fusion.

Phosphatidylinositol-dependent membrane fusion induced by a putative fusogenic sequence of Ebola virus

The membrane-interacting abilities of three sequences representing the putative fusogenic subdomain of the Ebola virus transmembrane protein have been investigated. In the presence of calcium, the sequence EBO(GE) (GAAIGLAWIPYFGPAAE) efficiently fused unilamellar vesicles composed of phosphatidylcholine, phosphatidylethanolamine, cholesterol, and phosphatidylinositol (molar ratio, 2:1:1:0.5), a mixture that roughly resembles the lipid composition of the hepatocyte plasma membrane. Analysis of the lipid dependence of the process demonstrated that the fusion activity of EBO(GE) was promoted by phosphatidylinositol but not by other acidic phospholipids. In comparison, EBO(EA) (EGAAIGLAWIPYFGPAA) and EBO(EE) (EGAAIGLAWIPYFGPAAE) sequences, which are similar to EBO(GE) except that they bear the negatively charged glutamate residue at the N terminus and at both the N and C termini, respectively, induced fusion to a lesser extent. As revealed by binding experiments, the glutamate residue at the N terminus severely impaired peptide-vesicle interaction. In addition, the fusion-competent EBO(GE) sequence did not associate significantly with vesicles lacking phosphatidylinositol. Tryptophan fluorescence quenching by vesicles containing brominated phospholipids indicated that the EBO(GE) peptide penetrated to the acyl chain level only when the membranes contained phosphatidylinositol. We conclude that binding and further penetration of the Ebola virus putative fusion peptide into membranes might be governed by the nature of the N-terminal residue and by the presence of phosphatidylinositol in the target membrane. Moreover, since insertion of such a peptide leads to membrane destabilization and fusion, the present data would be compatible with the involvement of this sequence in Ebola virus fusion.