Fusion of Semliki Forest virus with cholesterol-containing liposomes at low pH: a specific requirement for sphingolipids

Membrane Sphingomyelin in Host Cells Is Essential for Nucleocapsid Penetration into the Cytoplasm after Hemifusion during Rubella Virus Entry

The lipid composition of the host cell membrane is one of the key determinants of the entry of enveloped viruses into cells. To elucidate the detailed mechanisms behind the cell entry of rubella virus (RuV), one of the enveloped viruses, we searched for host factors involved in such entry by using CRISPR/Cas9 genome-wide knockout screening, and we found sphingomyelin synthase 1 (SMS1), encoded by the SGMS1 gene, as a candidate. RuV growth was strictly suppressed in SGMS1-knockout cells and was completely recovered by the overexpression of enzymatically active SMS1 and partially recovered by that of SMS2, another member of the SMS family, but not by that of enzymatically inactive SMS1. An entry assay using pseudotyped vesicular stomatitis virus possessing RuV envelope proteins revealed that sphingomyelin generated by SMSs is crucial for at least RuV entry. In SGMS1-knockout cells, lipid mixing between the RuV envelope membrane and the membrane of host cells occurred, but entry of the RuV genome from the viral particles into the cytoplasm was strongly inhibited. This indicates that sphingomyelin produced by SMSs is essential for the formation of membrane pores after hemifusion occurs during RuV entry. IMPORTANCE Infection with rubella virus during pregnancy causes congenital rubella syndrome in infants. Despite its importance in public health, the detailed mechanisms of rubella virus cell entry have only recently become somewhat clearer. The E1 protein of rubella virus is classified as a class II fusion protein based on its structural similarity, but it has the unique feature that its activity is dependent on calcium ion binding in the fusion loops. In this study, we found another unique feature, as cellular sphingomyelin plays a critical role in the penetration of the nucleocapsid into the cytoplasm after hemifusion by rubella virus. This provides important insight into the entry mechanism of rubella virus. This study also presents a model of hemifusion arrest during cell entry by an intact virus, providing a useful tool for analyzing membrane fusion, a biologically important phenomenon.

Role of Protein-Lipid Interactions in Viral Entry

The viral entry consists of several sequential events that ensure the attachment of the virus to the host cell and the introduction of its genetic material for the continuation of the replication cycle. Both cellular and viral lipids have gained a wider focus in recent years in the field of viral entry, as they are found to play key roles in different steps of the process. The specific role is summarized that lipids and lipid membrane nanostructures play in viral attachment, fusion, and immune evasion and how they can be targeted with antiviral therapies. Finally, some of the limitations of techniques commonly used for protein-lipid interactions studies are discussed, and new emerging tools are reviewed that can be applied to this field.

Cholesterol: A key player in membrane fusion that modulates the efficacy of fusion inhibitor peptides

The interaction of cholesterol with the neighboring lipids modulates several physical properties of the membrane. Mostly, it affects membrane fluidity, membrane permeability, lateral diffusion of lipids, bilayer thickness, and water penetration into the lipid bilayer. Due to the smaller head group to hydrophobic cross-sectional area of the tail, cholesterol induces intrinsic negative curvature to the membrane. The interaction of cholesterol with sphingolipids forms lipid rafts; generates phase separation in the membrane. The cholesterol-dependent modifications of membrane physical properties modulate viral infections by affecting the fusion between viral and host cell membranes. Cholesterol demonstrates a strong impact on the structure, depth of penetration, conformation, and organization of fusion peptides in membrane milieu. Further, cholesterol has been implicated to modify the fusion inhibitory efficiency of peptide-based membrane fusion inhibitors.

Ceramide and Related Molecules in Viral Infections

Ceramide is a lipid messenger at the heart of sphingolipid metabolism. In concert with its metabolizing enzymes, particularly sphingomyelinases, it has key roles in regulating the physical properties of biological membranes, including the formation of membrane microdomains. Thus, ceramide and its related molecules have been attributed significant roles in nearly all steps of the viral life cycle: they may serve directly as receptors or co-receptors for viral entry, form microdomains that cluster entry receptors and/or enable them to adopt the required conformation or regulate their cell surface expression. Sphingolipids can regulate all forms of viral uptake, often through sphingomyelinase activation, and mediate endosomal escape and intracellular trafficking. Ceramide can be key for the formation of viral replication sites. Sphingomyelinases often mediate the release of new virions from infected cells. Moreover, sphingolipids can contribute to viral-induced apoptosis and morbidity in viral diseases, as well as virus immune evasion. Alpha-galactosylceramide, in particular, also plays a significant role in immune modulation in response to viral infections. This review will discuss the roles of ceramide and its related molecules in the different steps of the viral life cycle. We will also discuss how novel strategies could exploit these for therapeutic benefit.

Both Sphingomyelin and Cholesterol in the Host Cell Membrane Are Essential for Rubella Virus Entry

Rubella virus (RuV) causes a systemic infection, and transplacental fetal infection causes congenital rubella syndrome. In this study, we showed that treatment of cells with sphingomyelinase inhibited RuV infection. Assays using inhibitors of serine palmitoyl transferase and ceramide transport protein demonstrated the contribution of sphingomyelin (SM) to RuV infection. Compelling evidence for direct binding of RuV to lipid membranes at neutral pH was obtained using liposome coflotation assays. The absence of either SM or cholesterol (Chol) abrogated the RuV-liposome interaction. SM and Chol (SM/Chol) were also critical for RuV binding to erythrocytes and lymphoid cells. Removal of Ca²⁺ from the assay buffer or mutation of RuV envelope E1 protein Ca²⁺-binding sites abrogated RuV binding to liposomes, erythrocytes, and lymphoid cells. However, RuV bound to various nonlymphoid adherent cell lines independently of extracellular Ca²⁺ or SM/Chol. Even in these adherent cell lines, both the E1 protein Ca²⁺-binding sites and cellular SM/Chol were essential for the early stage of RuV infection, possibly affecting envelope-membrane fusion in acidic compartments. Myelin oligodendrocyte glycoprotein (MOG) has recently been identified as a cellular receptor for RuV. However, RuV bound to MOG-negative cells in a Ca²⁺-independent manner. Collectively, our data demonstrate that RuV has two distinct binding mechanisms: one is Ca²⁺ dependent and the other is Ca²⁺ independent. Ca²⁺-dependent binding observed in lymphoid cells occurs by the direct interaction between E1 protein fusion loops and SM/Chol-enriched membranes. Clarification of the mechanism of Ca²⁺-independent RuV binding is an important next step in understanding the pathology of RuV infection.IMPORTANCE Rubella has a significant impact on public health as infection during early pregnancy can result in babies being born with congenital rubella syndrome. Even though effective rubella vaccines are available, rubella outbreaks still occur in many countries. We studied the entry mechanism of rubella virus (RuV) and found that RuV binds directly to the host plasma membrane in the presence of Ca²⁺ at neutral pH. This Ca²⁺-dependent binding is specifically directed to membranes enriched in sphingomyelin and cholesterol and is critical for RuV infection. Importantly, RuV also binds to many cell lines in a Ca²⁺-independent manner. An unidentified RuV receptor(s) is involved in this Ca²⁺-independent binding. We believe that the data presented here may aid the development of the first anti-RuV drug.

Fusion of a New World Alphavirus with Membrane Microdomains Involving Partially Reversible Conformational Changes in the Viral Spike Proteins

Alphaviruses are enveloped arboviruses mainly proposed to infect host cells by receptor-mediated endocytosis followed by fusion between the viral envelope and the endosomal membrane. The fusion reaction is triggered by low pH and requires the presence of both cholesterol and sphingolipids in the target membrane, suggesting the involvement of lipid rafts in the cell entry mechanism. In this study, we show for the first time the interaction of an enveloped virus with membrane microdomains isolated from living cells. Using Mayaro virus (MAYV), a New World alphavirus, we verified that virus fusion to these domains occurred to a significant extent upon acidification, although its kinetics was quite slow when compared to that of fusion with artificial liposomes demonstrated in a previous work. Surprisingly, when virus was previously exposed to acidic pH, a condition previously shown to inhibit alphavirus binding and fusion to target membranes as well as infectivity, and then reneutralized, its ability to fuse with membrane microdomains at low pH was retained. Interestingly, this observation correlated with a partial reversion of low pH-induced conformational changes in viral proteins and retention of virus infectivity upon reneutralization. Our results suggest that MAYV entry into host cells could alternatively involve internalization via lipid rafts and that the conformational changes triggered by low pH in the viral spike proteins during the entry process are partially reversible.

Replication of alphaviruses: a review on the entry process of alphaviruses into cells

Alphaviruses are small, enveloped viruses, ~70 nm in diameter, containing a single-stranded, positive-sense, RNA genome. Viruses belonging to this genus are predominantly arthropod-borne viruses, known to cause disease in humans. Their potential threat to human health was most recently exemplified by the 2005 Chikungunya virus outbreak in La Reunion, highlighting the necessity to understand events in the life-cycle of these medically important human pathogens. The replication and propagation of viruses is dependent on entry into permissive cells. Viral entry is initiated by attachment of virions to cells, leading to internalization, and uncoating to release genetic material for replication and propagation. Studies on alphaviruses have revealed entry via a receptor-mediated, endocytic pathway. In this paper, the different stages of alphavirus entry are examined, with examples from Semliki Forest virus, Sindbis virus, Chikungunya virus, and Venezuelan equine encephalitis virus described.

Alphavirus Entry and Membrane Fusion

The study of enveloped animal viruses has greatly advanced our understanding of the general properties of membrane fusion and of the specific pathways that viruses use to infect the host cell. The membrane fusion proteins of the alphaviruses and flaviviruses have many similarities in structure and function. As reviewed here, alphaviruses use receptor-mediated endocytic uptake and low pH-triggered membrane fusion to deliver their RNA genomes into the cytoplasm. Recent advances in understanding the biochemistry and structure of the alphavirus membrane fusion protein provide a clearer picture of this fusion reaction, including the protein’s conformational changes during fusion and the identification of key domains. These insights into the alphavirus fusion mechanism suggest new areas for experimental investigation and potential inhibitor strategies for anti-viral therapy.

Class I and class II viral fusion protein structures reveal similar principles in membrane fusion (Review)

Recent crystal structures of Flavivirus and Alphavirus fusion proteins (class II) confirm two major principles of protein machineries that mediate the merger of two opposing lipid bilayers. First, the fusion protein can bridge both membranes tethered by two membrane anchors. Second, refolding or domain rearrangement steps lead to the positioning of both anchors into close proximity at the same end of an elongated structure. Although these two steps are in principle sufficient to pull two opposing membranes together and initiate membrane fusion, accumulating evidence suggests that the process requires the concerted action of a number of fusion proteins at and outside the contact sites. This review will focus on the structures of viral class I and class II fusion proteins and their similarities in facilitating membrane fusion.

Antiepileptic ceramides from the Red Sea sponge Negombata corticata

A new antiepileptic ceramide mixture 1 was isolated from the Red Sea sponge Negombata corticata. The structures of the metabolites were determined by extensive spectroscopic analysis. The anticonvulsant activity of 1 was measured in vivo using the pentylenetetrazole-induced seizure model. This finding has important implications for biological studies with this class of compounds.

Cubic phases in phosphatidylcholine-cholesterol mixtures: cholesterol as membrane "fusogen"

X-ray diffraction reveals that mixtures of some unsaturated phosphatidylcholines (PCs) with cholesterol (Chol) readily form inverted bicontinuous cubic phases that are stable under physiological conditions. This effect was studied in most detail for dioleoyl PC/Chol mixtures with molar ratios of 1:1 and 3:7. Facile formation of Im3m and Pn3m phases with lattice constants of 30-50 nm and 25-30 nm, respectively, took place in phosphate-buffered saline, in sucrose solution, and in water near the temperature of the Lalpha-HII transition of the mixtures, as well as during cooling of the HII phase. Once formed, the cubic phases displayed an ability to supercool and replace the initial Lalpha phase over a broad range of physiological temperatures. Conversion into stable cubic phases was also observed for mixtures of Chol with dilinoleoyl PC but not for mixtures with palmitoyl-linoleoyl PC or palmitoyl-oleoyl PC, for which only transient cubic traces were recorded at elevated temperatures. A saturated, branched-chain PC, diphytanoyl PC, also displayed a cubic phase in mixture with Chol. Unlike the PEs, the membrane PCs are intrinsically nonfusogenic lipids: in excess water they only form lamellar phases and not any of the inverted phases on their own. Thus, the finding that Chol induces cubic phases in mixtures with unsaturated PCs may have important implications for its role in fusion. In ternary mixtures, saturated PCs and sphingomyelin are known to separate into liquid-ordered domains along with Chol. Our results thus suggest that unsaturated PCs, which are excluded from these domains, could form fusogenic domains with Chol. Such a dual role of Chol may explain the seemingly paradoxical ability of cell membranes to simultaneously form rigid, low-curvature raft-like patches while still being able to undergo facile membrane fusion.

Class II virus membrane fusion proteins

Enveloped animal viruses fuse their membrane with a host cell membrane, thus delivering the virus genetic material into the cytoplasm and initiating infection. This critical membrane fusion reaction is mediated by a virus transmembrane protein known as the fusion protein, which inserts its hydrophobic fusion peptide into the cell membrane and refolds to drive the fusion reaction. This review describes recent advances in our understanding of the structure and function of the class II fusion proteins of the alphaviruses and flaviviruses. Inhibition of the fusion protein refolding reaction confirms its importance in fusion and suggests new antiviral strategies for these medically important viruses.

How principles of domain formation in model membranes may explain ambiguities concerning lipid raft formation in cells

Sphingolipid and cholesterol-rich liquid ordered lipid domains (lipid rafts) have been studied in both eukaryotic cells and model membranes. However, while the coexistence of ordered and disordered liquid phases can now be easily demonstrated in model membranes, the situation in cell membranes remains ambiguous. Unlike the usual situation in model membranes, under most conditions, cell membranes rich in sphingolipid and cholesterol may have a "granular" organization in which the size of ordered and/or disordered domains is extremely small and domains may be of borderline stability. This review attempts to explain the origin of the divergence between of our understanding of rafts in model membranes and in cells, and how the physical properties of model membranes can help explain many of the ambiguities concerning raft formation and properties in cells. How physical principles of ordered domain formation relate to limitations of detergent insolubility and cholesterol depletion methods used to infer the presence of rafts in cells is also discussed. Possible modifications of these techniques that may increase their reliability are considered. It will be necessary to study model membrane systems more closely approximating cell membranes in order gain a complete understanding of raft properties in cells. Very high concentrations of membrane cholesterol and proteins may explain key physical characteristics of domains in cellular membranes, and are the two of the most obvious factors requiring additional study.

Reversible acid-induced inactivation of the membrane fusion protein of Semliki Forest virus

Previously, it has been shown that the exposure of Semliki Forest virus (SFV) to a mildly acidic environment induces a rapid and complete loss of the ability of the virus to bind and fuse to target membranes added subsequently. In the present study, incubation of SFV at low pH followed by a specific reneutralization step resulted in a partial reversion of this loss of viral fusion capacity, as assessed in a liposomal model system. Also, the ability of the viral E1 fusion protein to undergo liposome-stimulated trimerization was restored. Furthermore, acid-treated and neutralized SFV largely retained infectivity. Exposure of SFV to low pH induced dissociation of the E1/E2 heterodimer, which was not reversed upon neutralization. It is concluded that the SFV E1 fusion protein, after acid-induced dissociation from E2, rapidly adopts an intermediate, nontrimeric conformation in which it is no longer able to interact with target membrane lipids. Neutralization restores the ability of E1 to interact with membranes. This interaction, however, remains strictly dependent on low pH.

Neuronal cell death in alphavirus encephalomyelitis

Alphaviruses are mosquito-borne, enveloped, plus-strand RNA viruses that cause a spectrum of diseases in humans that include fever, rash, arthritis, meningitis, and encephalomyelitis. Sindbis virus (SINV) is the prototype alphavirus, causes encephalomyelitis in mice, and provides a model system for studying the pathogenesis of alphavirus-induced neurological disease. Major target cells for SINV infection in the central nervous system (CNS) are neurons, and both host and viral factors determine the fate of infected neurons. Young animals are most susceptible to fatal disease. This correlates with the ability of SINV to induce apoptosis in immature neurons. In vitro, apoptotic death of neuroblastoma cells can be induced by fusion of the virus envelope with the endosomal membrane and does not require infectious virus. This fusion process activates acid sphingomyelinase that cleaves sphingomyelin to release ceramide, an initiator of apoptosis. Within an hour, poly(ADP-ribose) polymerase is activated, and this is followed by release of cytochrome c and activation of effector caspases. SINV-induced cell death can be delayed or prevented by treatment with antioxidants or caspase inhibitors and by intracellular expression of Bcl-2, Beclin-1, or protease inhibitors. Older animals survive infection unless infected with a neurovirulent strain of SINV. In these mice, anterior horn motor neurons die by a primarily necrotic process that is influenced by excitotoxic amino acids and inflammation, whereas hippocampal neurons can be either apoptotic or necrotic. Death also occurs in uninfected neurons in the vicinity of infected neurons and can be delayed or prevented by treatment with glutamate receptor antagonists.

The 2003 ASBMB-Avanti Award in Lipids Address: Applications of novel synthetic lipids to biological problems

This paper is an overview of the 2003 Avanti Award in Lipids address that was presented by Robert Bittman at the American Society for Biochemistry and Molecular Biology (ASBMB) Annual Meeting held in San Diego, CA in conjunction with meetings of five other FASEB Societies, April 15, 2003. The theme of the lecture is: "How can the chemical synthesis of unnatural lipids provide insights into problems ranging from cell biology to biophysics?" The following examples are presented: (1) novel ceramide analogs as experimental anticancer agents, (2) photoactivatable sphingosine 1-phosphate analogs as probes of protein targets of this bioactive lipid, (3) a 13C-enriched cerebroside as a quantitative probe of glycosphingolipid (GSL) transbilayer distribution in bilayers with and without sphingomyelin, (4) cis and trans unsaturated sphingomyelin analogs as modulators of the existence of cholesterol-enriched microdomains (rafts) that may facilitate fusion of alphaviruses with target membranes, (5) ceramide as an indirect enhancer of the permeabilization of membranes induced by cholesterol-specific cytolysins, (6) fluorescent GSL analogs of widely disparate structure as probes of the molecular features responsible for the selective internalization of GSLs in caveolae of living mammalian cells, (7) enantiomeric lysophosphatidic acid (LPA) analogs as probes of receptor subtypes that mediate LPA signaling, and (8) phosphonocholine analogs of the antitumor ether lipid ET-18-OCH3 as tools for discerning the primary targets that are critical for cytotoxic activity in tumor cells.

Characterization of a membrane-associated trimeric low-pH-induced Form of the class II viral fusion protein E from tick-borne encephalitis virus and its crystallization

The interaction of a dimeric membrane anchor-free form of the envelope protein E (sE dimer) from tick-borne encephalitis virus with liposomes at acidic pH levels leads to its conversion into membrane-inserted sE trimers. Electron microscopy shows that these trimers have their long dimensions along the threefold molecular axis, which is oriented perpendicularly to the plane of the membrane, where the protein inserts via the internal fusion peptide. Liposomes containing sE at their surface display paracrystalline arrays of protein in a closely packing arrangement in which each trimer is surrounded by six others, suggesting cooperativity in the insertion process. sE trimers, solubilized with nonionic detergents, yielded three-dimensional crystals suitable for X-ray diffraction analysis.

Visualization of the target-membrane-inserted fusion protein of Semliki Forest virus by combined electron microscopy and crystallography

Semliki Forest virus enters cells by receptor-mediated endocytosis. The acidic environment of the endosome triggers a membrane fusion reaction that is mediated by the E1 glycoprotein. During fusion, E1 rearranges from an E1/E2 heterodimer to a highly stable, membrane-inserted E1 homotrimer (E1HT). In this study, we analyzed E1HT by a combination of electron cryomicroscopy, electron crystallography of negatively stained 2D crystals, and fitting of the available X-ray structure of the monomeric E1 ectodomain into the resulting 3D reconstruction. The visualized E1HT reveals that the ectodomain has reoriented vertically and inserted the distal tip of domain II into the lipid bilayer. Our data allow the visualization of a viral fusion protein inserted in its target membrane and demonstrate that insertion is a cooperative process, resulting in rings composed of five to six homotrimers.

Novel mutations that control the sphingolipid and cholesterol dependence of the Semliki Forest virus fusion protein

The enveloped alphavirus Semliki Forest virus (SFV) infects cells via a membrane fusion reaction mediated by the E1 membrane protein. Efficient SFV-membrane fusion requires the presence of cholesterol and sphingolipid in the target membrane. Here we report on two mutants, srf-4 and srf-5, selected for growth in cholesterol-depleted cells. Like the previously isolated srf-3 mutant (E1 proline 226 to serine), the phenotypes of the srf-4 and srf-5 mutants were conferred by single-amino-acid changes in the E1 protein: leucine 44 to phenylalanine and valine 178 to alanine, respectively. Like srf-3, srf-4 and srf-5 show striking increases in the cholesterol independence of growth, infection, membrane fusion, and exit. Unexpectedly, and unlike srf-3, srf-4 and srf-5 showed highly efficient fusion with sphingolipid-free membranes in both lipid- and content-mixing assays. Both srf-4 and srf-5 formed E1 homotrimers of decreased stability compared to the homotrimers of the wild type and the srf-3 mutant. All three srf mutations lie in the same domain of E1, but the srf-4 and srf-5 mutations are spatially separated from srf-3. When expressed together, the three mutations could interact to produce increased sterol independence and to cause temperature-sensitive E1 transport. Thus, the srf-4 and srf-5 mutations identify novel regions of E1 that are distinct from the fusion peptide and srf-3 region and modulate the requirements for both sphingolipid and cholesterol in virus-membrane fusion.

Sphingolipid and cholesterol dependence of alphavirus membrane fusion. Lack of correlation with lipid raft formation in target liposomes

Semliki Forest virus (SFV) and Sindbis virus (SIN) are enveloped viruses that infect their host cells by receptor-mediated endocytosis and subsequent fusion from within acidic endosomes. Fusion of the viral envelope requires the presence of both cholesterol and sphingolipids in the target membrane. This is suggestive of a possible involvement of sphingolipid-cholesterol microdomains, or "lipid rafts," in the membrane fusion and cell entry process of the virus. In this study, large unilamellar vesicles (LUVs) were prepared from synthetic sphingolipids and sterols that vary with respect to their capacity to promote microdomain formation, as assessed by gradient flotation analysis in the presence of Triton X-100. SFV and SIN fused with LUVs irrespective of the presence or absence of Triton X-100-insoluble microdomains. These results suggest that SFV and SIN do not require the presence of lipid rafts for fusion with target membranes. Furthermore, it is not necessary for sphingolipids to reside in a detergent-insoluble complex with cholesterol to promote SFV or SIN fusion.

Pyrene-labeled lipids as tools in membrane biophysics and cell biology

Pyrene is one of the most frequently used lipid-linked fluorophores. Its most characteristic features are a long excited state lifetime and (local) concentration-dependent formation of excimers. Pyrene is also hydrophobic and thus does not significantly distort the conformation of the labeled lipid molecule. These characteristics make pyrene lipids well-suited for studies on a variety of biophysical phenomena like lateral diffusion, inter- or transbilayer movement of lipids and lateral organization of membranes. Pyrene lipids have also been widely employed to determine protein binding to membranes, lipid conformation and the activity of lipolytic enzymes. In cell biology, pyrene lipids are promising tools for studies on lipid trafficking and metabolism, as well as for microscopic mapping of membrane properties. The main disadvantage of pyrene lipids is the relatively large size of the fluorophore. Another disadvantage is that they require UV-excitation, which is not feasible with all microscopes.

Synthesis of new trans double-bond sphingolipid analogues: Delta(4,6) and Delta(6) ceramides

Unsaturation was introduced at Delta(4,6) and Delta(6) of the sphingoid chain of naturally occurring ceramide 1 via a beta-keto sulfoxide (12) and sulfone (18) derived from N-Boc-L-serine methyl ester acetonide (9), affording two novel ceramide analogues, (2S,3R)-2-octanoylamidooctadeca-(4E,6E)-diene-1,3-diol (2) and (2S,3R)-2-octanoylamidooctadec-(6E)-ene-1,3-diol (3). After C-alkylation of 12 with (E)-1-bromo-2-tetradecene (8), a trans double bond was installed by elimination of PhS(O)H, providing conjugated dienone oxazolidine 13. Reaction of 18 with 8, followed by desulfonation (Al(Hg)), afforded keto-oxazolidine 20, which bears a (E)-Delta(6) double bond. The syntheses of analogues 2 and 3 from ketones 13 and 20, respectively, were completed by the following sequence of reactions: diastereoselective reduction (NaBH(4)/CeCl(3) or DIBAL-H), hydrolysis of the oxazolidine ring, liberation of the amino group, and installation of the N-amide group.

The fusion peptide of Semliki Forest virus associates with sterol-rich membrane domains

Semliki Forest virus (SFV) is an enveloped alphavirus whose membrane fusion is triggered by low pH and promoted by cholesterol and sphingolipid in the target membrane. Fusion is mediated by E1, a viral membrane protein containing the putative fusion peptide. Virus mutant studies indicate that SFV's cholesterol dependence is controlled by regions of E1 outside of the fusion peptide. Both E1 and E1*, a soluble ectodomain form of E1, interact with membranes in a reaction dependent on low pH, cholesterol, and sphingolipid and form highly stable homotrimers. Here we have used detergent extraction and gradient floatation experiments to demonstrate that E1* associated selectively with detergent-resistant membrane domains (DRMs or rafts). In contrast, reconstituted full-length E1 protein or influenza virus fusion peptide was not associated with DRMs. Methyl beta-cyclodextrin quantitatively extracted both cholesterol and E1* from membranes in the absence of detergent, suggesting a strong association of E1* with sterol. Monoclonal antibody studies demonstrated that raft association was mediated by the proposed E1 fusion peptide. Thus, although other regions of E1 are implicated in the control of virus cholesterol dependence, once the SFV fusion peptide inserts in the target membrane it has a high affinity for membrane domains enriched in cholesterol and sphingolipid.

PE2 cleavage mutants of Sindbis virus: correlation between viral infectivity and pH-dependent membrane fusion activation of the spike heterodimer

The spike glycoprotein E2 of Sindbis virus (SIN) is synthesized in the infected cell as a PE2 precursor protein, which matures through cleavage by a cellular furin-like protease. Previous work has shown that SIN mutants impaired in PE2 cleavage are noninfectious on BHK-21 cells, the block in infection being localized at a step after virus-receptor interaction but prior to RNA replication. Here, we studied the membrane fusion properties of SIN PE2 cleavage mutants and observed that these viruses are impaired in their ability to form an E1 homotrimer and to fuse with liposomes at a mildly acidic pH. The block in spike rearrangement and fusion could be overridden by exposure of the mutant viruses to very low pH (<4.5). Cleavage mutants with second-site resuscitating mutations in PE2 were highly infectious for BHK-21 cells. The ability of these viruses to form E1 homotrimers and to fuse at a mildly acidic pH was completely restored despite a sustained lack of PE2 cleavage.

In vivo generation and characterization of a soluble form of the Semliki forest virus fusion protein

During infection of host cells, a number of enveloped animal viruses are known to produce soluble forms of viral membrane glycoproteins lacking the transmembrane domain. The roles of such soluble glycoproteins in viral life cycles are incompletely understood, but in several cases they are believed to modulate host immune response and viral pathogenesis. Semliki Forest virus (SFV) is an enveloped alphavirus that infects cells through low-pH-dependent fusion and buds from the plasma membrane. Fusion is mediated by the E1 subunit of the SFV spike protein. Previous studies described the in vivo generation of E1s, a truncated soluble form of E1, under conditions in which budding is inhibited in mammalian host cells. We have here examined the properties of E1s generation and the biological activity of E1s. E1s cleavage required spike protein transport out of the endoplasmic reticulum and was independent of virus infection. Cell surface E1 efficiently acted as a precursor for E1s. E1s generation was strongly pH dependent in BHK cells, with optimal cleavage at a pH of < or =7.0, conditions that inhibited the budding of SFV but not the budding of the rhabdovirus vesicular stomatitis virus. The pH dependence of E1s production and SFV budding was unaffected by the stability of the spike protein dimer but was a function of the host cell. Similar to the intact virus and in vitro-generated E1 ectodomain, treatment of E1s at low pH in the presence of target membranes triggered specific acid-dependent conformational changes. Thus, under a variety of conditions, SFV-infected cells can produce a soluble form of E1 that is biologically active.

Coupling of cholesterol and cone-shaped lipids in bilayers augments membrane permeabilization by the cholesterol-specific toxins streptolysin O and Vibrio cholerae cytolysin

Vibrio cholerae cytolysin (VCC) forms oligomeric pores in lipid bilayers containing cholesterol. Membrane permeabilization is inefficient if the sterol is embedded within bilayers prepared from phosphatidylcholine only but is greatly enhanced if the target membrane also contains ceramide. Although the enhancement of VCC action is stereospecific with respect to cholesterol, we show here that no such specificity applies to the two stereocenters in ceramide; all four stereoisomers of ceramide enhanced VCC activity in cholesterol-containing bilayers. A wide variety of ceramide analogs were as effective as D-erythro-ceramide, as was diacylglycerol, suggesting that the effect of ceramide exemplifies a general trend of lipids with a small headgroup to augment the activity of VCC. Incorporation of these cone-shaped lipids into cholesterol-containing bilayers also gave similar effects with streptolysin O, another cholesterol-specific but structurally unrelated cytolysin. In contrast, the activity of staphylococcal alpha-hemolysin, which does not share with the other toxins the requirement for cholesterol, was far less affected by the presence of lipids with a conical shape. The collective data indicate that sphingolipids and glycerolipids do not interact with the cytolysins specifically. Instead, lipids that have a conical molecular shape appear to effect a change in the energetic state of membrane cholesterol that in turn augments the interaction of the sterol with the cholesterol-specific cytolysins.

Exposure to low pH is not required for penetration of mosquito cells by Sindbis virus

It is widely held that the penetration of cells by alphaviruses is dependent on exposure to the acid environment of an endosome. The alphavirus Sindbis virus replicates in both vertebrate and invertebrate cell cultures. We have found that exposure to an acid environment may not be required for infection of cells of the insect host. In this work, we investigated the effects of two agents (NH(4)Cl and chloroquine), which raise the pH of intracellular compartments (lysosomotropic weak bases) on the infection and replication of Sindbis virus in cells of the insect host Aedes albopictus. The results show that both of these agents increase the pH of endosomes, as indicated by protection against diphtheria toxin intoxication. NH(4)Cl blocked the production of infectious virus and blocked virus RNA synthesis when added prior to infection. Chloroquine, in contrast to its effect on vertebrate cells, had no inhibitory effect on infectious virus production in mosquito cells even when added prior to infection. Treatment with NH(4)Cl did not prevent the penetration of virus RNA into the cell cytoplasm or translation of the RNA to produce a precursor to virus nonstructural proteins. These data suggest that while these two drugs raise the pH of endosomes, they do not block insect cell penetration. These data support previous results published by our laboratory suggesting that exposure to an acid environment within the cell may not be an obligatory step in the process of infection of cells by alphaviruses.

Differential roles of two conserved glycine residues in the fusion peptide of Semliki Forest virus

Semliki Forest Virus (SFV) is an enveloped alphavirus that infects cells by a low-pH-dependent membrane fusion reaction. SFV fusion is catalyzed by the spike protein E1 subunit, which contains a putative fusion peptide between residues 79 and 97. Prior mutagenesis studies demonstrated that an E1 G91D mutation blocks both virus-membrane fusion and the formation of a highly stable E1 trimer believed to be a critical fusion intermediate. We have here demonstrated that the G91D mutant was also inactive in hemifusion, suggesting that the E1 homotrimer is important in the initial stages of lipid mixing. Revertant analysis of a G91 deletion mutant indicated that G91 was crucial for the viability of SFV. In contrast, a G83D mutation produced infectious virus with both efficient fusion and homotrimer formation. Thus, the G83 position, although highly conserved among alphaviruses, was functional if replaced with a charged amino acid.

Synthesis of ceramide analogues having the C(4)-C(5) bond of the long-chain base as part of an aromatic or heteroaromatic system

Two efficient and stereoselective methods are described for the preparation of aryl and heteroaryl ceramide analogues 2 and 3. The first route involves the addition of an aryllithium or a heteroaryllithium reagent (7a or 25a, respectively) to the L-serine-derived aldehyde 4, followed by hydrolysis of the oxazolidine, liberation of the amino group, and N-acylation. The second route, which was used to prepare arylceramide analogue 2 in eight steps and 28% overall yield starting with 3-bromobenzaldehyde, utilizes a Heck reaction to afford (E)-alpha,beta-unsaturated ester 16, then osmium-catalyzed asymmetric dihydroxylation for the introduction of the desired chirality at C-2 and C-3. Regioselective alpha-azidation of alpha-O-nosyl-beta-hydroxyester 18 with sodium azide, followed by LiAlH(4) reduction of the azido and ester groups and N-acylation, complete the synthesis of arylceramide analogue 2.

Formation and characterization of the trimeric form of the fusion protein of Semliki Forest Virus

Enveloped animal viruses infect cells via fusion of the viral membrane with a host cell membrane. Fusion is mediated by a viral envelope glycoprotein, which for a number of enveloped animal viruses rearranges itself during fusion to form a trimeric alpha-helical coiled-coil structure. This conformational change from the metastable, nonfusogenic form of the spike protein to the highly stable form involved in fusion can be induced by physiological activators of virus fusion and also by a variety of destabilizing conditions. The E1 spike protein subunit of Semliki Forest virus (SFV) triggers membrane fusion upon exposure to mildly acidic pH and forms a homotrimer that appears necessary for fusion. We have here demonstrated that formation of the E1 homotrimer was efficiently triggered under low-pH conditions but not by perturbants such as heat or urea, despite their induction of generalized conformational changes in the E1 and E2 subunits and partial exposure of an acid-specific E1 epitope. We used a sensitive fluorescence assay to show that neither heat nor urea treatment triggered SFV-liposome fusion at neutral pH, although either treatment inactivated subsequent low-pH-triggered fusion activity. Once formed, the low-pH-induced E1 homotrimer was very stable and was only dissociated under harsh conditions such as heating in sodium dodecyl sulfate. Taken together, these data, as well as protein structure predictions, suggest a model in which the less stable native E1 subunit specifically responds to low pH to form the more stable E1 homotrimer via conformational changes different from those of the coiled-coil type of fusion proteins.

Membrane fusion activity of tick-borne encephalitis virus and recombinant subviral particles in a liposomal model system

We present a kinetic analysis of the membrane fusion activity of tick-borne encephalitis (TBE) virus and TBE-derived recombinant subviral particles (RSPs) in a liposomal model system. Fusion was monitored using a fluorescence assay involving pyrene-labeled phospholipids. Fusion was strictly dependent on low pH, with the optimum being at pH 5.3-5.5 and the threshold at pH 6.8. Fusion did not require a protein or carbohydrate receptor in the target liposomes. Preexposure to low pH of the virus alone resulted in inactivation of its fusion activity. At the optimum pH for fusion and 37 degrees C, the rate and extent of fusion were very high, with more than 50% of the virus fusing within 2 s and the final extent of fusion being 70%. Lowering of the temperature did not result in a significant decrease in the rate and extent of fusion, suggesting that TBE virus fusion is a facile process with a low activation energy, possibly due to the flat orientation of the E glycoprotein on the viral surface facilitating the establishment of direct intermembrane contact. The fusion characteristics of TBE virus and RSPs were similar, indicating that RSPs provide a reliable and convenient model for further study of the membrane fusion properties of TBE virus.

An epitope of the Semliki Forest virus fusion protein exposed during virus-membrane fusion

Semliki Forest virus (SFV) is an enveloped alphavirus that infects cells via a membrane fusion reaction triggered by acidic pH in the endocytic pathway. Fusion is mediated by the spike protein E1 subunit, an integral membrane protein that contains the viral fusion peptide and forms a stable homotrimer during fusion. We have characterized four monoclonal antibodies (MAbs) specific for the acid conformation of E1. These MAbs did not inhibit fusion, suggesting that they bind to an E1 region different from the fusion peptide. Competition analyses demonstrated that all four MAbs bound to spatially related sites on acid-treated virions or isolated spike proteins. To map the binding site, we selected for virus mutants resistant to one of the MAbs, E1a-1. One virus isolate, SFV 4-2, showed reduced binding of three acid-specific MAbs including E1a-1, while its binding of one acid-specific MAb as well as non-acid-specific MAbs to E1 and E2 was unchanged. The SFV 4-2 mutant was fully infectious, formed the E1 homotrimer, and had the wild-type pH dependence of infection. Sequence analysis demonstrated that the relevant mutation in SFV 4-2 was a change of E1 glycine 157 to arginine (G157R). Decreased binding of MAb E1a-1 was observed under a wide range of assay conditions, strongly suggesting that the E1 G157R mutation directly affects the MAb binding site. These data thus localize an E1 region that is normally hidden in the neutral pH structure and becomes exposed as part of the reorganization of the spike protein to its fusion-active conformation.

Low-pH-dependent fusion of Sindbis virus with receptor-free cholesterol- and sphingolipid-containing liposomes

There is controversy as to whether the cell entry mechanism of Sindbis virus (SIN) involves direct fusion of the viral envelope with the plasma membrane at neutral pH or uptake by receptor-mediated endocytosis and subsequent low-pH-induced fusion from within acidic endosomes. Here, we studied the membrane fusion activity of SIN in a liposomal model system. Fusion was followed fluorometrically by monitoring the dilution of pyrene-labeled lipids from biosynthetically labeled virus into unlabeled liposomes or from labeled liposomes into unlabeled virus. Fusion was also assessed on the basis of degradation of the viral core protein by trypsin encapsulated in the liposomes. SIN fused efficiently with receptor-free liposomes, consisting of phospholipids and cholesterol, indicating that receptor interaction is not a mechanistic requirement for fusion of the virus. Fusion was optimal at pH 5.0, with a threshold at pH 6.0, and undetectable at neutral pH, supporting a cell entry mechanism of SIN involving fusion from within acidic endosomes. Under optimal conditions, 60 to 85% of the virus fused, depending on the assay used, corresponding to all of the virus bound to the liposomes as assessed in a direct binding assay. Preincubation of the virus alone at pH 5.0 resulted in a rapid loss of fusion capacity. Fusion of SIN required the presence of both cholesterol and sphingolipid in the target liposomes, cholesterol being primarily involved in low-pH-induced virus-liposome binding and the sphingolipid catalyzing the fusion process itself. Under low-pH conditions, the E2/E1 heterodimeric envelope glycoprotein of the virus dissociated, with formation of a trypsin-resistant E1 homotrimer, which kinetically preceded the fusion reaction, thus suggesting that the E1 trimer represents the fusion-active conformation of the viral spike.

The cholesterol requirement for sindbis virus entry and exit and characterization of a spike protein region involved in cholesterol dependence

Semliki Forest virus (SFV) and Sindbis virus (SIN) are enveloped alphaviruses that enter cells via low-pH-triggered fusion in the endocytic pathway and exit by budding from the plasma membrane. Previous studies with cholesterol-depleted insect cells have shown that SFV requires cholesterol in the cell membrane for both virus fusion and efficient exit of progeny virus. An SFV mutant, srf-3, shows efficient fusion and exit in the absence of cholesterol due to a single point mutation in the E1 spike subunit, proline 226 to serine. We have here characterized the role of cholesterol in the entry and exit of SIN, an alphavirus quite distantly related to SFV. Growth, primary infection, fusion, and exit of SIN were all dramatically inhibited in cholesterol-depleted cells compared to control cells. Based on sequence differences within the E1 226 region between SFV, srf-3, and SIN, we constructed six SIN mutants with alterations within this region and characterized their cholesterol dependence. A SIN mutant, SGM, that had the srf-3 amino acid sequence from E1 position 224 to 235 showed increases of approximately 100-fold in infection and approximately 250-fold in fusion with cholesterol-depleted cells compared with infection and fusion of wild-type SIN. Pulse-chase analysis demonstrated that SGM exit from cholesterol-depleted cells was markedly more efficient than that of wild-type SIN. Thus, similar to SFV, SIN was cholesterol dependent for both virus entry and exit, and the cholesterol dependence of both steps could be modulated by sequences within the E1 226 region.

Evidence for the extended phospholipid conformation in membrane fusion and hemifusion

Molecular-level mechanisms of fusion and hemifusion of large unilamellar dioleoyl phosphatidic acid/phosphocholine (DOPA/DOPC, 1:1 molar ratio) vesicles induced by millimolar Ca2+ and Mg2+, respectively, were investigated using fluorescence spectroscopy. In keeping with reduction of membrane free volume Vf, both divalent cations increased the emission polarization for 1,6-diphenyl-1,3, 5-hexatriene (DPH). An important finding was a decrease in excimer/monomer emission intensity ratio (Ie/Im) for the intramolecular excimer-forming probe 1, 2-bis[(pyren-1-)yl]decanoyl-sn-glycero-3-phosphocholine (bis-PDPC) in the course of fusion and hemifusion. Comparison with another intramolecular excimer-forming probe, namely, 1-[(pyren-1)-yl]decanoyl-2-[(pyren-1)-yl]tetradecanoyl-sn-gl ycero-3-p hosphocholine (PDPTPC), allowed us to exclude changes in acyl chain alignment to be causing the decrement in Ie/Im. As a decrease in Vf should increase Ie/Im for bis-PDPC and because contact site between adhering liposomes was required we conclude the most feasible explanation to be the adoption of the extended conformation (P.K.J., Chem. Phys. Lipids 63:251-258) by bis-PDPC. In this conformation the two acyl chains are splaying so as to become embedded in the opposing leaflets of the two adhered bilayers, with the headgroup remaining between the adjacent surfaces. Our data provide evidence for a novel mechanism of fusion of the lipid bilayers.

Membrane fusion activity of Semliki Forest virus in a liposomal model system: specific inhibition by Zn2+ ions

Semliki Forest virus (SFV) has been shown previously to fuse efficiently with cholesterol- and sphingolipid-containing liposomal model membranes in a low-pH-dependent manner. Several steps can be distinguished in this process, including low-pH-induced irreversible binding of the virus to the liposomes, facilitated by target membrane cholesterol, and subsequent fusion of the viral membrane with the liposomal bilayer, specifically catalyzed by target membrane sphingolipid. Binding and fusion are mediated by the heterodimeric viral envelope glycoprotein E2/E1. At low pH the heterodimer dissociates, and the E1 monomers convert to a homotrimeric structure, the presumed fusion-active conformation of the viral spike. In this paper, we demonstrate that SFV-liposome fusion is specifically inhibited by Zn2+ ions. The inhibition is at the level of the fusion reaction itself, since virus-liposome binding was found to be unaffected. Zn2+ did not inhibit E2/E1 dissociation, but severely inhibited exposure of an acid-specific epitope on E1, E1 homotrimer formation, and acquisition of trypsin-resistance. It is concluded that virus--liposome binding solely requires low-pH-induced E2/E1 heterodimer dissociation, while fusion depends on further rearrangements in the E1 spike protein. As these rearrangements occur subsequent to the binding step, their precise course, including the formation of a fusion complex, may be influenced by interaction of E1 with target membrane lipids.

Inhibition of influenza virus fusion by polyanionic proteins

Anionic charge-modified human serum albumin (HSA) has previously been shown to exert potent in vitro activity against human immunodeficiency virus type 1 (HIV-1). In these studies, introduction of the additional negative charges was performed by derivatizing the epsilon-amino groups of lysine residues with succinic (Suc-HSA) or cis-aconitic anhydride (Aco-HSA), by which primary amino groups are replaced with carboxylic acids. The anti-HIV-1 activity was related to inhibition of gp41-mediated membrane fusion. Here, we investigated the activity of aconitylated and succinylated proteins on influenza virus membrane fusion, which is mediated by the viral membrane glycoprotein hemagglutinin (HA). Aco-HSA and Suc-HSA markedly inhibited the rates and extents of fusion of fluorescently labeled virosomes bearing influenza HA, with target membranes derived from erythrocytes. The inhibitory activity was dependent on the overall negative-charge density; HSA modified with 36 or less extra negative charges failed to inhibit fusion. The inhibition of fusion showed a certain degree of specificity for the protein carrying the negative charges: polyanionic HSA and beta-lactoglobulin A derivatives had fusion-inhibitory activity, whereas succinylated BSA, lactalbumin, lactoferrin, lysozyme, and transferrin were inactive. Aco60-HSA and Aco-beta-lactoglobulin A inhibited influenza virus membrane fusion in a concentration-dependent manner, IC50 values being about 4 and 10 microg/mL, respectively. HA-mediated membrane fusion is pH dependent. Aco60-HSA did not induce a shift in the pH threshold or in the pH optimum. Fusion with liposomes of another low pH-dependent virus, Semliki Forest virus, was not specifically affected by any of the compounds reported here. In view of some structural and functional similarities between influenza HA and the HIV-1 gp120/gp41 complex, it is tempting to postulate that the current results might have some implications for the anti-HIV-1 mechanism of polyanionic proteins.

Solubilization and reconstitution of vesicular stomatitis virus envelope using octylglucoside

Reconstituted vesicular stomatitis virus envelopes or virosomes are formed by detergent removal from solubilized intact virus. We have monitored the solubilization process of the intact vesicular stomatitis virus by the nonionic surfactant octylglucoside at various initial virus concentrations by employing turbidity measurements. This allowed us to determine the phase boundaries between the membrane and the mixed micelles domains. We have also characterized the lipid and protein content of the solubilized material and of the reconstituted envelope. Both G and M proteins and all of the lipids of the envelope were extracted by octylglucoside and recovered in the reconstituted envelope. Fusion activity of the virosomes tested either on Vero cells or on liposomes showed kinetics and pH dependence similar to those of the intact virus.

Cholesterol-induced interfacial area condensations of galactosylceramides and sphingomyelins with identical acyl chains

The interfacial interactions occurring between cholesterol and either galactosylceramides (GalCers) or sphingomyelins (SMs) with identical acyl chains have been investigated using Langmuir film balance techniques. Included among the synthesized GalCers and SMs were species containing palmitoyl (16:0), stearoyl (18:0), oleoyl [18:1 delta 9(c)], nervonoyl [24:1 delta 15(c)], or linoleoyl [18:2 delta 9,12(c)] acyl residues. The cholesterol-induced condensations in the average molecular areas of the monolayers were determined by classic mean molecular area vs composition plots as well as by expressing the changes in terms of sphingolipid cross-sectional area reduction over the surface pressure range from 1 to 40 mN/m (at 1 mN/m intervals). The results show that, at surface pressures approximating bilayer conditions (30 mN/m), acyl heterogeneity in naturally occurring SMs (bovine of egg SM) enhanced the area condensation induced by cholesterol compared with their predominant molecular species (e.g. 18:0 SM in bovine SM; 16:0 SM in egg SM). Nonetheless, cholesterol always had a greater condensing effect on SM compared to GalCer when these sphingolipids were acyl chain matched and in similar phase states (prior to mixing with cholesterol). Also, the cholesterol-induced area changes for a given sphingolipid type (e.g. SM or GalCer) were similar whether the acyl chains were saturated, cis-delta 9-monounsaturated, or cis-delta 9,12-diunsaturated if the sphingolipids were in similar phase states (prior to mixing with cholesterol) and compared at equivalent surface pressures. These results indicate that, under conditions were hydrocarbon structure is matched, the sphingolipid head group plays a dominant role in determining the extent to which cholesterol reduces sphingolipid cross-sectional area. Despite the larger cholesterol-induced area condensations observed in SMs compared to those in GalCers, the molecular-packing densities showed that equimolar GalCer-cholesterol films were generally packed as tight as or slightly tighter than those of the SM-cholesterol films. The results are discussed in terms of a molecular model for sphingolipid-cholesterol interactions. Our findings also do only raise questions as to whether cholesterol-induced condensation data provide a reliable measure of the affinity, i.e. interaction strength, between cholesterol and different lipids but also provide insight regarding the stability of sterol/sphingolipid 1-1 rich microdomains thought to exist in caveolae and other cell membrane regions.

Sphingolipids activate membrane fusion of Semliki Forest virus in a stereospecific manner

The alphavirus Semliki Forest virus (SFV) enters cells through receptor-mediated endocytosis. Subsequently, triggered by the acid pH in endosomes, the viral envelope fuses with the endosomal membrane. Membrane fusion of SFV has been shown previously to be dependent on the presence of cholesterol in the target membrane. Recently, we have demonstrated that fusion of SFV also requires sphingolipids [Nieva, J. L., Bron, R., Corver, J., & Wilschut, J. (1994) EMBO J. 13, 2797-2804]. In the present paper, we show that the activation of low-pH-dependent fusion of SFV by sphingolipids is a stereospecific process. Pyrene-labeled SFV fused rapidly and extensively with liposomes consisting of a mixture of phosphatidylcholine, phosphatidylethanolamine, and cholesterol, supplemented with low concentrations of D-erythro-ceramide, representing the naturally occurring sphingolipid stereoisomer. Fusion was assessed by a decrease in the pyrene excimer fluorescence. L-erythro-, D-threo-, and L-threo-ceramide did not support fusion of the virus. Similar results were obtained with the corresponding sphingomyelin stereoisomers. The stereospecificity of SFV fusion activation was confirmed by using an assay based on degradation of the viral capsid protein by trypsin encapsulated in the target liposomes. Fusion mediated by D-erythro-ceramide was not affected by the additional presence in the target liposomes of ceramide stereoisomers incapable of fusion activation. Binding of the virus to the liposomes, as assessed by flotation on sucrose density gradients, was not dependent on the presence of fusion-competent or fusion-incompetent sphingolipids in the liposomes. The results of this study support the notion that a stereospecific interaction of the viral fusion protein with D-erythro sphingolipids in the target membrane represents an essential step in the activation of the fusion capacity of SFV.