Inner but not outer membrane leaflets control the transition from glycosylphosphatidylinositol-anchored influenza hemagglutinin-induced hemifusion to full fusion

The late endosome-resident lipid bis(monoacylglycero)phosphate is a cofactor for Lassa virus fusion

Arenavirus entry into host cells occurs through a low pH-dependent fusion with late endosomes that is mediated by the viral glycoprotein complex (GPC). The mechanisms of GPC-mediated membrane fusion and of virus targeting to late endosomes are not well understood. To gain insights into arenavirus fusion, we examined cell-cell fusion induced by the Old World Lassa virus (LASV) GPC complex. LASV GPC-mediated cell fusion is more efficient and occurs at higher pH with target cells expressing human LAMP1 compared to cells lacking this cognate receptor. However, human LAMP1 is not absolutely required for cell-cell fusion or LASV entry. We found that GPC-induced fusion progresses through the same lipid intermediates as fusion mediated by other viral glycoproteins–a lipid curvature-sensitive intermediate upstream of hemifusion and a hemifusion intermediate downstream of acid-dependent steps that can be arrested in the cold. Importantly, GPC-mediated fusion and LASV pseudovirus entry are specifically augmented by an anionic lipid, bis(monoacylglycero)phosphate (BMP), which is highly enriched in late endosomes. This lipid also specifically promotes cell fusion mediated by Junin virus GPC, an unrelated New World arenavirus. We show that BMP promotes late steps of LASV fusion downstream of hemifusion–the formation and enlargement of fusion pores. The BMP-dependence of post-hemifusion stages of arenavirus fusion suggests that these viruses evolved to use this lipid as a cofactor to selectively fuse with late endosomes.

Pathogenic arenaviruses pose a serious health threat. The viral envelope glycoprotein GPC mediates attachment to host cells and drives virus entry via endocytosis and low pH-dependent fusion within late endosomes. Understanding the host factors and processes that are essential for arenavirus fusion may identify novel therapeutic targets. To delineate the mechanism of arenavirus entry, we examined cell-cell fusion induced by the Old World Lassa virus GPC proteins at low pH. Lassa GPC-mediated fusion was augmented by the human LAMP1 receptor and progressed through lipid curvature-sensitive intermediates, such as hemifusion (merger of contacting leaflets of viral and cell membrane without the formation of a fusion pore). We found that most GPC-mediated fusion events were off-path hemifusion structures and that the transition from hemifusion to full fusion and fusion pore enlargement were specifically promoted by an anionic lipid, bis(monoacylglycero)phosphate, which is highly enriched in late endosomes. This lipid also specifically promotes fusion of unrelated New World Junin arenavirus. Our results imply that arenaviruses evolved to use bis(monoacylglycero)phosphate to enter cells from late endosomes.

Myomerger promotes fusion pore by elastic coupling between proximal membrane leaflets and hemifusion diaphragm

Myomerger is a muscle-specific membrane protein involved in formation of multinucleated muscle cells by mediating the transition from the early hemifusion stage to complete fusion. Here, we considered the physical mechanism of the Myomerger action based on the hypothesis that Myomerger shifts the spontaneous curvature of the outer membrane leaflets to more positive values. We predicted, theoretically, that Myomerger generates the outer leaflet elastic stresses, which propagate into the hemifusion diaphragm and accelerate the fusion pore formation. We showed that Myomerger ectodomain indeed generates positive spontaneous curvature of lipid monolayers. We substantiated the mechanism by experiments on myoblast fusion and influenza hemagglutinin-mediated cell fusion. In both processes, the effects of Myomerger ectodomain were strikingly similar to those of lysophosphatidylcholine known to generate a positive spontaneous curvature of lipid monolayers. The control of post-hemifusion stages by shifting the spontaneous curvature of proximal membrane monolayers may be utilized in diverse fusion processes.

The regulatory role of Myomaker and Myomixer-Myomerger-Minion in muscle development and regeneration

Skeletal muscle plays essential roles in motor function, energy, and glucose metabolism. Skeletal muscle formation occurs through a process called myogenesis, in which a crucial step is the fusion of mononucleated myoblasts to form multinucleated myofibers. The myoblast/myocyte fusion is triggered and coordinated in a muscle-specific way that is essential for muscle development and post-natal muscle regeneration. Many molecules and proteins have been found and demonstrated to have the capacity to regulate the fusion of myoblast/myocytes. Interestingly, two newly discovered muscle-specific membrane proteins, Myomaker and Myomixer (also called Myomerger and Minion), have been identified as fusogenic regulators in vertebrates. Both Myomaker and Myomixer-Myomerger-Minion have the capacity to directly control the myogenic fusion process. Here, we review and discuss the latest studies related to these two proteins, including the discovery, structure, expression pattern, functions, and regulation of Myomaker and Myomixer-Myomerger-Minion. We also emphasize and discuss the interaction between Myomaker and Myomixer-Myomerger-Minion, as well as their cooperative regulatory roles in cell-cell fusion. Moreover, we highlight the areas for exploration of Myomaker and Myomixer-Myomerger-Minion in future studies and consider their potential application to control cell fusion for cell-therapy purposes.

Lipid Mixing Assay for Murine Myoblast Fusion and Other Slow Cell-cell Fusion Processes

Lipid mixing (redistribution of lipid probes between fusing membranes) has been widely used to study early stages of relatively fast viral and intracellular fusion processes that take seconds to minutes. Lipid mixing assays are especially important for identification of hemifusion intermediates operationally defined as lipid mixing without content mixing. Due to unsynchronized character and the slow rate of the differentiation processes that prime the cells for cell-cell fusion processes in myogenesis, osteoclastogenesis and placentogenesis, these fusions take days. Application of lipid mixing assays to detect early fusion intermediates in these very slow fusion processes must consider the continuous turnover of plasma membrane components and potential fusion-unrelated exchange of the lipid probes between the membranes. Here we describe the application of lipid mixing assay in our work on myoblast fusion stage in development and regeneration of skeletal muscle cells. Our approach utilizes conventional in vitro model of myogenic differentiation and fusion based on murine C2C12 cells. When we observe the appearance of first multinucleated cells, we lift the cells and label them with either fluorescent lipid DiI as a membrane probe or CellTracker^TM Green as a content probe. Redistribution of the probes between the cells is scored by fluorescence microscopy. Hemifused cells are identified as mononucleated cells labeled with both content- and membrane probes. The interpretation must be supported by a system of negative controls with fusion-incompetent cells to account for and minimize contributions of fusion-unrelated exchange of the lipid probes. This approach with minor modifications has been used for investigating fusion of primary murine myoblasts, osteoclast precursors and fusion mediated by a gamete fusogen HAP2, and likely can be adopted for other slow cell-cell fusion processes.

Interferon-induced transmembrane protein 3 blocks fusion of sensitive but not resistant viruses by partitioning into virus-carrying endosomes

Late endosome-resident interferon-induced transmembrane protein 3 (IFITM3) inhibits fusion of diverse viruses, including Influenza A virus (IAV), by a poorly understood mechanism. Despite the broad antiviral activity of IFITM3, viruses like Lassa virus (LASV), are fully resistant to its inhibitory effects. It is currently unclear whether resistance arises from a highly efficient fusion machinery that is capable of overcoming IFITM3 restriction or the ability to enter from cellular sites devoid of this factor. Here, we constructed and validated a functional IFITM3 tagged with EGFP or other fluorescent proteins. This breakthrough allowed live cell imaging of virus co-trafficking and fusion with endosomal compartments in cells expressing fluorescent IFITM3. Three-color single virus and endosome tracking revealed that sensitive (IAV), but not resistant (LASV), viruses become trapped within IFITM3-positive endosomes where they underwent hemifusion but failed to release their content into the cytoplasm. IAV fusion with IFITM3-containing compartments could be rescued by amphotericin B treatment, which has been previously shown to antagonize the antiviral activity of this protein. By comparison, virtually all LASV particles trafficked and fused with endosomes lacking detectable levels of fluorescent IFITM3, implying that this virus escapes restriction by utilizing endocytic pathways that are distinct from the IAV entry pathways. The importance of virus uptake and transport pathways is further reinforced by the observation that LASV glycoprotein-mediated cell-cell fusion is inhibited by IFITM3 and other members of the IFITM family expressed in target cells. Together, our results strongly support a model according to which IFITM3 accumulation at the sites of virus fusion is a prerequisite for its antiviral activity and that this protein traps viral fusion at a hemifusion stage by preventing the formation of fusion pores. We conclude that the ability to utilize alternative endocytic pathways for entry confers IFITM3-resistance to otherwise sensitive viruses.

Myomaker and Myomerger Work Independently to Control Distinct Steps of Membrane Remodeling during Myoblast Fusion

Classic mechanisms for membrane fusion involve transmembrane proteins that assemble into complexes and dynamically alter their conformation to bend membranes, leading to mixing of membrane lipids (hemifusion) and fusion pore formation. Myomaker and Myomerger govern myoblast fusion and muscle formation but are structurally divergent from traditional fusogenic proteins. Here, we show that Myomaker and Myomerger independently mediate distinct steps in the fusion pathway, where Myomaker is involved in membrane hemifusion and Myomerger is necessary for fusion pore formation. Mechanistically, we demonstrate that Myomerger is required on the cell surface where its ectodomains stress membranes. Moreover, we show that Myomerger drives fusion completion in a heterologous system independent of Myomaker and that a Myomaker-Myomerger physical interaction is not required for function. Collectively, our data identify a stepwise cell fusion mechanism in myoblasts where different proteins are delegated to perform unique membrane functions essential for membrane coalescence.

Lipid-dependence of target membrane stability during influenza viral fusion

Although influenza kills about a half million people each year, even after excluding pandemics, there is only one set of antiviral drugs: neuraminidase inhibitors. By using a new approach utilizing giant unilamellar vesicles and infectious X-31 influenza virus, and testing for the newly identified pore intermediate of membrane fusion, we observed ∼30-87% poration, depending upon lipid composition. Testing the hypothesis that spontaneous curvature (SC) of the lipid monolayer controls membrane poration, our Poisson model and Boltzmann energetic considerations suggest a transition from a leaky to a non-leaky fusion pathway depending on the SC of the target membrane. When the target membrane SC is below approximately -0.20 nm^-1 fusion between influenza virus and target membrane is predominantly non-leaky while above that fusion is predominantly leaky, suggesting that influenza hemagglutinin (HA)-catalyzed topological conversion of target membranes during fusion is associated with a loss of membrane integrity.

Insertion of Dengue E into lipid bilayers studied by neutron reflectivity and molecular dynamics simulations

The envelope (E) protein of Dengue virus rearranges to a trimeric hairpin to mediate fusion of the viral and target membranes, which is essential for infectivity. Insertion of E into the target membrane serves to anchor E and possibly also to disrupt local order within the membrane. Both aspects are likely to be affected by the depth of insertion, orientation of the trimer with respect to the membrane normal, and the interactions that form between trimer and membrane. In the present work, we resolved the depth of insertion, the tilt angle, and the fundamental interactions for the soluble portion of Dengue E trimers (sE) associated with planar lipid bilayer membranes of various combinations of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-rac-glycerol (POPG), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE), and cholesterol (CHOL) by neutron reflectivity (NR) and by molecular dynamics (MD) simulations. The results show that the tip of E containing the fusion loop (FL) is located at the interface of the headgroups and acyl chains of the outer leaflet of the lipid bilayers, in good agreement with prior predictions. The results also indicate that E tilts with respect to the membrane normal upon insertion, promoted by either the anionic lipid POPG or CHOL. The simulations show that tilting of the protein correlates with hydrogen bond formation between lysines and arginines located on the sides of the trimer close to the tip (K246, K247, and R73) and nearby lipid headgroups. These hydrogen bonds provide a major contribution to the membrane anchoring and may help to destabilize the target membrane.

SERINC5 protein inhibits HIV-1 fusion pore formation by promoting functional inactivation of envelope glycoproteins

The host proteins, SERINC3 and SERINC5, have been recently shown to incorporate into HIV-1 particles and compromise their ability to fuse with target cells, an effect that is antagonized by the viral Nef protein. Envelope (Env) glycoproteins from different HIV-1 isolates exhibit a broad range of sensitivity to SERINC-mediated restriction, and the mechanism by which SERINCs interfere with HIV-1 fusion remains unclear. Here, we show that incorporation of SERINC5 into virions in the absence of Nef inhibits the formation of small fusion pores between viruses and cells. Strikingly, we found that SERINC5 promotes spontaneous functional inactivation of sensitive but not resistant Env glycoproteins. Although SERINC5-Env interaction was not detected by co-immunoprecipitation, incorporation of this protein enhanced the exposure of the conserved gp41 domains and sensitized the virus to neutralizing antibodies and gp41-derived inhibitory peptides. These results imply that SERINC5 restricts HIV-1 fusion at a step prior to small pore formation by selectively inactivating sensitive Env glycoproteins, likely through altering their conformation. The increased HIV-1 sensitivity to anti-gp41 antibodies and peptides suggests that SER5 also delays refolding of the remaining fusion-competent Env trimers.

Interferon-inducible LY6E Protein Promotes HIV-1 Infection

LY6E is a glycosylphosphatidylinositol-anchored, IFN-inducible protein that regulates T lymphocytes proliferation, differentiation, and development. Single-nucleotide polymorphism rs2572886 in the LY6 family protein locus has been shown to associate with accelerated progression to AIDS. In this study, we show that LY6E promotes HIV, type 1 (HIV-1) infection by enhancing viral entry and gene expression. Knockdown of LY6E in human peripheral blood mononuclear, SupT1, and THP-1 cells diminishes HIV-1 replication. Virion-cell and cell-cell fusion experiments revealed that LY6E promotes membrane fusion of the viral entry step. Interestingly, we find that LTR-driven HIV-1 gene expression is also enhanced by LY6E, suggesting additional roles of LY6E in HIV-1 replication. HIV-1 infection induces LY6E expression in human peripheral blood mononuclear cells, concomitant with increased production of type I IFN and some classical IFN-stimulated genes. Altogether, our results demonstrate that IFN-inducible LY6E promotes HIV-1 entry and replication and highlight a positive regulatory role of IFN-induced proteins in HIV-1 infection. Our work emphasizes the complexity of IFN-mediated signaling in HIV-host interaction and AIDS pathogenesis.

Steric hindrance of SNARE transmembrane domain organization impairs the hemifusion-to-fusion transition

SNAREs fuse membranes in several steps. Trans-SNARE complexes juxtapose membranes, induce hemifused stalk structures, and open the fusion pore. A recent penetration model of fusion proposed that SNAREs force the hydrophilic C-termini of their transmembrane domains through the hydrophobic core of the membrane(s). In contrast, the indentation model suggests that the C-termini open the pore by locally compressing and deforming the stalk. Here we test these models in the context of yeast vacuole fusion. Addition of small hydrophilic tags renders bilayer penetration by the C-termini energetically unlikely. It preserves fusion activity, however, arguing against the penetration model. Addition of large protein tags to the C-termini permits SNARE activation, trans-SNARE pairing, and hemifusion but abolishes pore opening. Fusion proceeds if the tags are detached from the membrane by a hydrophilic spacer or if only one side of the trans-SNARE complex carries a protein tag. Thus, both sides of a trans-SNARE complex can drive pore opening. Our results are consistent with an indentation model in which multiple SNARE C-termini cooperate in opening the fusion pore by locally deforming the inner leaflets.

Myristoylation of the Arenavirus Envelope Glycoprotein Stable Signal Peptide Is Critical for Membrane Fusion but Dispensable for Virion Morphogenesis

Arenaviruses are responsible for severe and often fatal hemorrhagic disease. In the absence of effective antiviral therapies and vaccines, these viruses pose serious threats to public health and biodefense. Arenaviruses enter the host cell by fusion of the viral and endosomal membranes, a process mediated by the virus envelope glycoprotein GPC. Unlike other class I viral fusion proteins, GPC retains its stable signal peptide (SSP) as an essential third subunit in the mature complex. SSP spans the membrane twice and is myristoylated at its cytoplasmic N terminus. Mutations that abolish SSP myristoylation have been shown to reduce pH-induced cell-cell fusion activity of ectopically expressed GPC to ∼20% of wild-type levels. In order to examine the role of SSP myristoylation in the context of the intact virus, we used reverse genetics to generate Junín viruses (Candid #1 isolate) in which the critical glycine-2 residue in SSP was either replaced by alanine (G2A) or deleted (ΔG2). These mutant viruses produced smaller foci of infection in Vero cells and showed an ∼5-fold reduction in specific infectivity, commensurate with the defect in cell-cell fusion. However, virus assembly and GPC incorporation into budded virions were unaffected. Our findings suggest that the myristate moiety is cryptically disposed in the prefusion GPC complex and may function late in the fusion process to promote merging of the viral and cellular membranes.

IMPORTANCE

Hemorrhagic fever arenaviruses pose significant threats to public health and biodefense. Arenavirus entry into the host cell is promoted by the virus envelope glycoprotein GPC. Unlike other viral envelope glycoproteins, GPC contains a myristoylated stable signal peptide (SSP) as an essential third subunit. Myristoylation has been shown to be important for the membrane fusion activity of recombinantly expressed GPC. Here, we use reverse genetics to study the role of SSP myristoylation in the context of the intact virion. We find that nonmyristoylated GPC mutants of the Candid #1 strain of Junín virus display a commensurate deficiency in their infectivity, albeit without additional defects in virion assembly and budding. These results suggest that SSP myristoylation may function late in the fusion process to facilitate merging of the viral and cellular membranes. Antiviral agents that target this novel aspect of GPC membrane fusion may be useful in the treatment of arenavirus hemorrhagic fevers.

Induction of Cell-Cell Fusion by Ebola Virus Glycoprotein: Low pH Is Not a Trigger

Ebola virus (EBOV) is a highly pathogenic filovirus that causes hemorrhagic fever in humans and animals. Currently, how EBOV fuses its envelope membrane within an endosomal membrane to cause infection is poorly understood. We successfully measure cell-cell fusion mediated by the EBOV fusion protein, GP, assayed by the transfer of both cytoplasmic and membrane dyes. A small molecule fusion inhibitor, a neutralizing antibody, as well as mutations in EBOV GP known to reduce viral infection, all greatly reduce fusion. By monitoring redistribution of small aqueous dyes between cells and by electrical capacitance measurements, we discovered that EBOV GP-mediated fusion pores do not readily enlarge—a marked difference from the behavior of other viral fusion proteins. EBOV GP must be cleaved by late endosome-resident cathepsins B or L in order to become fusion-competent. Cleavage of cell surface-expressed GP appears to occur in endosomes, as evidenced by the fusion block imposed by cathepsin inhibitors, agents that raise endosomal pH, or an inhibitor of anterograde trafficking. Treating effector cells with a recombinant soluble cathepsin B or thermolysin, which cleaves GP into an active form, increases the extent of fusion, suggesting that a fraction of surface-expressed GP is not cleaved. Whereas the rate of fusion is increased by a brief exposure to acidic pH, fusion does occur at neutral pH. Importantly, the extent of fusion is independent of external pH in experiments in which cathepsin activity is blocked and EBOV GP is cleaved by thermolysin. These results imply that low pH promotes fusion through the well-known pH-dependent activity of cathepsins; fusion induced by cleaved EBOV GP is a process that is fundamentally independent of pH. The cell-cell fusion system has revealed some previously unappreciated features of EBOV entry, which could not be readily elucidated in the context of endosomal entry.

The devastation and transmissibility of Ebola virus (EBOV) are well known. However, the manner in which EBOV enters host cells through endosomal membrane remains elusive. Here, we have developed a convenient experimental system to mimic EBOV fusion in endosomes: cells expressing the fusion protein of EBOV, GP, on their surface are fused to target cells. This system exhibits the known key properties of EBOV fusion. We show that the pH-dependence of EBOV fusion is caused by the pH-dependence of cathepsins, proteases known to cleave EBOV GP into a fusion-competent form. We demonstrate that the fusion activity of this cleaved form is independent of pH. We further show that the enlargement of the fusion pore created by EBOV GP is unusually slow in reaching sizes necessary to pass EBOV’s genome—this is atypical of virally created fusion pores. This cell-cell fusion system should provide a useful platform for developing drugs against EBOV infection.

Viral fusion protein transmembrane domain adopts β-strand structure to facilitate membrane topological changes for virus-cell fusion

The C-terminal transmembrane domain (TMD) of viral fusion proteins such as HIV gp41 and influenza hemagglutinin (HA) is traditionally viewed as a passive α-helical anchor of the protein to the virus envelope during its merger with the cell membrane. The conformation, dynamics, and lipid interaction of these fusion protein TMDs have so far eluded high-resolution structure characterization because of their highly hydrophobic nature. Using magic-angle-spinning solid-state NMR spectroscopy, we show that the TMD of the parainfluenza virus 5 (PIV5) fusion protein adopts lipid-dependent conformations and interactions with the membrane and water. In phosphatidylcholine (PC) and phosphatidylglycerol (PG) membranes, the TMD is predominantly α-helical, but in phosphatidylethanolamine (PE) membranes, the TMD changes significantly to the β-strand conformation. Measured order parameters indicate that the strand segments are immobilized and thus oligomerized. (31)P NMR spectra and small-angle X-ray scattering (SAXS) data show that this β-strand-rich conformation converts the PE membrane to a bicontinuous cubic phase, which is rich in negative Gaussian curvature that is characteristic of hemifusion intermediates and fusion pores. (1)H-(31)P 2D correlation spectra and (2)H spectra show that the PE membrane with or without the TMD is much less hydrated than PC and PG membranes, suggesting that the TMD works with the natural dehydration tendency of PE to facilitate membrane merger. These results suggest a new viral-fusion model in which the TMD actively promotes membrane topological changes during fusion using the β-strand as the fusogenic conformation.

Trimeric transmembrane domain interactions in paramyxovirus fusion proteins: roles in protein folding, stability, and function

Paramyxovirus fusion (F) proteins promote membrane fusion between the viral envelope and host cell membranes, a critical early step in viral infection. Although mutational analyses have indicated that transmembrane (TM) domain residues can affect folding or function of viral fusion proteins, direct analysis of TM-TM interactions has proved challenging. To directly assess TM interactions, the oligomeric state of purified chimeric proteins containing the Staphylococcal nuclease (SN) protein linked to the TM segments from three paramyxovirus F proteins was analyzed by sedimentation equilibrium analysis in detergent and buffer conditions that allowed density matching. A monomer-trimer equilibrium best fit was found for all three SN-TM constructs tested, and similar fits were obtained with peptides corresponding to just the TM region of two different paramyxovirus F proteins. These findings demonstrate for the first time that class I viral fusion protein TM domains can self-associate as trimeric complexes in the absence of the rest of the protein. Glycine residues have been implicated in TM helix interactions, so the effect of mutations at Hendra F Gly-508 was assessed in the context of the whole F protein. Mutations G508I or G508L resulted in decreased cell surface expression of the fusogenic form, consistent with decreased stability of the prefusion form of the protein. Sedimentation equilibrium analysis of TM domains containing these mutations gave higher relative association constants, suggesting altered TM-TM interactions. Overall, these results suggest that trimeric TM interactions are important driving forces for protein folding, stability and membrane fusion promotion.

The lipid composition and physical properties of the yeast vacuole affect the hemifusion-fusion transition

Yeast vacuole fusion requires the formation of SNARE bundles between membranes. Although the function of vacuolar SNAREs is controlled in part by regulatory lipids, the exact role of the membrane in regulating fusion remains unclear. Because SNAREs are membrane-anchored and transmit the force required for fusion to the bilayer, we hypothesized that the lipid composition and curvature of the membrane aid in controlling fusion. Here, we examined the effect of altering membrane fluidity and curvature on the functionality of fusion-incompetent SNARE mutants that are thought to generate insufficient force to trigger the hemifusion-fusion transition. The hemifusion-fusion transition was inhibited by disrupting the 3Q:1R stoichiometry of SNARE bundles with the mutant SNARE Vam7p(Q283R) . Similarly, replacing the transmembrane domain of the syntaxin homolog Vam3p with a lipid anchor allowed hemifusion, but not content mixing. Hemifusion-stalled reactions containing either of the SNARE mutants were stimulated to fuse with chlorpromazine, an amphipathic molecule that alters membrane fluidity and curvature. The activity of mutant SNAREs was also rescued by the overexpression of SNAREs, thus multiplying the force transferred to the membrane. Thus, we conclude that either increasing membrane fluidity, or multiplying SNARE-generated energy restored the fusogenicity of mutant SNAREs that are stalled at hemifusion. We also found that regulatory lipids differentially modulated the complex formation of wild-type SNAREs. Together, these data indicate that the physical properties and the lipid composition of the membrane affect the function of SNAREs in promoting the hemifusion-fusion transition.

IFITM proteins restrict viral membrane hemifusion

The interferon-inducible transmembrane (IFITM) protein family represents a new class of cellular restriction factors that block early stages of viral replication; the underlying mechanism is currently not known. Here we provide evidence that IFITM proteins restrict membrane fusion induced by representatives of all three classes of viral membrane fusion proteins. IFITM1 profoundly suppressed syncytia formation and cell-cell fusion induced by almost all viral fusion proteins examined; IFITM2 and IFITM3 also strongly inhibited their fusion, with efficiency somewhat dependent on cell types. Furthermore, treatment of cells with IFN also markedly inhibited viral membrane fusion and entry. By using the Jaagsiekte sheep retrovirus envelope and influenza A virus hemagglutinin as models for study, we showed that IFITM-mediated restriction on membrane fusion is not at the steps of receptor- and/or low pH-mediated triggering; instead, the creation of hemifusion was essentially blocked by IFITMs. Chlorpromazine (CPZ), a chemical known to promote the transition from hemifusion to full fusion, was unable to rescue the IFITM-mediated restriction on fusion. In contrast, oleic acid (OA), a lipid analog that generates negative spontaneous curvature and thereby promotes hemifusion, virtually overcame the restriction. To explore the possible effect of IFITM proteins on membrane molecular order and fluidity, we performed fluorescence labeling with Laurdan, in conjunction with two-photon laser scanning and fluorescence-lifetime imaging microscopy (FLIM). We observed that the generalized polarizations (GPs) and fluorescence lifetimes of cell membranes expressing IFITM proteins were greatly enhanced, indicating higher molecularly ordered and less fluidized membranes. Collectively, our data demonstrated that IFITM proteins suppress viral membrane fusion before the creation of hemifusion, and suggested that they may do so by reducing membrane fluidity and conferring a positive spontaneous curvature in the outer leaflets of cell membranes. Our study provides novel insight into the understanding of how IFITM protein family restricts viral membrane fusion and infection.

Many pathogenic viruses contain an envelope that must fuse with the cell membrane in order to gain entry and initiate infection. This process is mediated by one or more glycoproteins present on the surface of the virions, known as viral fusion proteins. Recently, a family of interferon-inducible transmembrane (IFITM) protein has been shown to block viral infection, including those of highly pathogenic viruses. Here we provide evidence that these IFITM proteins potently suppress membrane fusion induced by representatives of all three classes of viral fusion proteins. Interestingly, we found that the block is not at the steps of receptor binding or low pH that triggers conformational changes of viral fusion proteins required for membrane fusion. Rather, we discovered that the creation of hemifusion, an intermediate in which the outer membranes of the two lipid bilayers have merged but the inner membranes still remain intact is blocked by IFITM proteins. We further demonstrated that overexpression of IFITM proteins rigidify the cell membrane, thereby reducing membrane fluidity and fusion potential. Our study provides novel insight into the understanding of how IFITM proteins restrict viral entry and infection.

HIV entry and envelope glycoprotein-mediated fusion

HIV entry involves binding of the trimeric viral envelope glycoprotein (Env) gp120/gp41 to cell surface receptors, which triggers conformational changes in Env that drive the membrane fusion reaction. The conformational landscape that the lipids and Env navigate en route to fusion has been examined by biophysical measurements on the microscale, whereas electron tomography, x-rays, and NMR have provided insights into the process on the nanoscale and atomic scale. However, the coupling between the lipid and protein pathways that give rise to fusion has not been resolved. Here, we discuss the known and unknown about the overall HIV Env-mediated fusion process.

Membrane fusion and cell entry of XMRV are pH-independent and modulated by the envelope glycoprotein's cytoplasmic tail

Xenotropic murine leukemia virus-related virus (XMRV) is a gammaretrovirus that was originally identified from human prostate cancer patients and subsequently linked to chronic fatigue syndrome. Recent studies showed that XMRV is a recombinant mouse retrovirus; hence, its association with human diseases has become questionable. Here, we demonstrated that XMRV envelope (Env)-mediated pseudoviral infection is not blocked by lysosomotropic agents and cellular protease inhibitors, suggesting that XMRV entry is not pH-dependent. The full length XMRV Env was unable to induce syncytia formation and cell-cell fusion, even in cells overexpressing the viral receptor, XPR1. However, truncation of the C-terminal 21 or 33 amino acid residues in the cytoplasmic tail (CT) of XMRV Env induced substantial membrane fusion, not only in the permissive 293 cells but also in the nonpermissive CHO cells that lack a functional XPR1 receptor. The increased fusion activities of these truncations correlated with their enhanced SU shedding into culture media, suggesting conformational changes in the ectodomain of XMRV Env. Noticeably, further truncation of the CT of XMRV Env proximal to the membrane-spanning domain severely impaired the Env fusogenicity, as well as dramatically decreased the Env incorporations into MoMLV oncoretroviral and HIV-1 lentiviral vectors resulting in greatly reduced viral transductions. Collectively, our studies reveal that XMRV entry does not require a low pH or low pH-dependent host proteases, and that the cytoplasmic tail of XMRV Env critically modulates membrane fusion and cell entry. Our data also imply that additional cellular factors besides XPR1 are likely to be involved in XMRV entry.

Beyond anchoring: the expanding role of the hendra virus fusion protein transmembrane domain in protein folding, stability, and function

While work with viral fusion proteins has demonstrated that the transmembrane domain (TMD) can affect protein folding, stability, and membrane fusion promotion, the mechanism(s) remains poorly understood. TMDs could play a role in fusion promotion through direct TMD-TMD interactions, and we have recently shown that isolated TMDs from three paramyxovirus fusion (F) proteins interact as trimers using sedimentation equilibrium (SE) analysis (E. C. Smith, et al., submitted for publication). Immediately N-terminal to the TMD is heptad repeat B (HRB), which plays critical roles in fusion. Interestingly, addition of HRB decreased the stability of the trimeric TMD-TMD interactions. This result, combined with previous findings that HRB forms a trimeric coiled coil in the prefusion form of the whole protein though HRB peptides fail to stably associate in isolation, suggests that the trimeric TMD-TMD interactions work in concert with elements in the F ectodomain head to stabilize a weak HRB interaction. Thus, changes in TMD-TMD interactions could be important in regulating F triggering and refolding. Alanine insertions between the TMD and HRB demonstrated that spacing between these two regions is important for protein stability while not affecting TMD-TMD interactions. Additional mutagenesis of the C-terminal end of the TMD suggests that β-branched residues within the TMD play a role in membrane fusion, potentially through modulation of TMD-TMD interactions. Our results support a model whereby the C-terminal end of the Hendra virus F TMD is an important regulator of TMD-TMD interactions and show that these interactions help hold HRB in place prior to the triggering of membrane fusion.

Imaging single retrovirus entry through alternative receptor isoforms and intermediates of virus-endosome fusion

A large group of viruses rely on low pH to activate their fusion proteins that merge the viral envelope with an endosomal membrane, releasing the viral nucleocapsid. A critical barrier to understanding these events has been the lack of approaches to study virus-cell membrane fusion within acidic endosomes, the natural sites of virus nucleocapsid capsid entry into the cytosol. Here we have investigated these events using the highly tractable subgroup A avian sarcoma and leukosis virus envelope glycoprotein (EnvA)-TVA receptor system. Through labeling EnvA pseudotyped viruses with a pH-sensitive fluorescent marker, we imaged their entry into mildly acidic compartments. We found that cells expressing the transmembrane receptor (TVA950) internalized the virus much faster than those expressing the GPI-anchored receptor isoform (TVA800). Surprisingly, TVA800 did not accelerate virus uptake compared to cells lacking the receptor. Subsequent steps of virus entry were visualized by incorporating a small viral content marker that was released into the cytosol as a result of fusion. EnvA-dependent fusion with TVA800-expressing cells occurred shortly after endocytosis and delivery into acidic endosomes, whereas fusion of viruses internalized through TVA950 was delayed. In the latter case, a relatively stable hemifusion-like intermediate preceded the fusion pore opening. The apparent size and stability of nascent fusion pores depended on the TVA isoforms and their expression levels, with TVA950 supporting more robust pores and a higher efficiency of infection compared to TVA800. These results demonstrate that surface receptor density and the intracellular trafficking pathway used are important determinants of efficient EnvA-mediated membrane fusion, and suggest that early fusion intermediates play a critical role in establishing low pH-dependent virus entry from within acidic endosomes.

Differential infectious entry of human influenza A/NWS/33 virus (H1N1) in mammalian kidney cells

In this report we focused our interest on the early events of the replication cycle of NWS/33 human influenza A (NWS) virus in MDCK (canine), LLC-MK2 (simian), and NSK (swine) kidney cells, with different susceptibility upon infection. We have previously demonstrated that actin organization induces restriction to viral replication during the early stages of NWS virus infection in simian kidney cells. To explore how cell endocytic mechanisms are hijacked by NWS virus and may modulate the outcome of viral infection, the effect of drugs affecting selectively the entry via clathrin-coated pits, caveolar/raft-dependent endocytosis and macropinocytosis was analyzed. Results point to critical differences in terms of internalization pathways exploited by NWS virus to enter the examined cell models. Moreover, we show that some ways of entry do not allow an effective virus internalization, depending on the cell type. Understanding how specific cell functions/components may regulate early phases of viral replication allows us to deepen our knowledge on influenza virus infection and provides new insights for anti-viral researches.

Dengue virus ensures its fusion in late endosomes using compartment-specific lipids

Many enveloped viruses invade cells via endocytosis and use different environmental factors as triggers for virus-endosome fusion that delivers viral genome into cytosol. Intriguingly, dengue virus (DEN), the most prevalent mosquito-borne virus that infects up to 100 million people each year, fuses only in late endosomes, while activation of DEN protein fusogen glycoprotein E is triggered already at pH characteristic for early endosomes. Are there any cofactors that time DEN fusion to virion entry into late endosomes? Here we show that DEN utilizes bis(monoacylglycero)phosphate, a lipid specific to late endosomes, as a co-factor for its endosomal acidification-dependent fusion machinery. Effective virus fusion to plasma- and intracellular- membranes, as well as to protein-free liposomes, requires the target membrane to contain anionic lipids such as bis(monoacylglycero)phosphate and phosphatidylserine. Anionic lipids act downstream of low-pH-dependent fusion stages and promote the advance from the earliest hemifusion intermediates to the fusion pore opening. To reach anionic lipid-enriched late endosomes, DEN travels through acidified early endosomes, but we found that low pH-dependent loss of fusogenic properties of DEN is relatively slow in the presence of anionic lipid-free target membranes. We propose that anionic lipid-dependence of DEN fusion machinery protects it against premature irreversible restructuring and inactivation and ensures viral fusion in late endosomes, where the virus encounters anionic lipids for the first time during entry. Currently there are neither vaccines nor effective therapies for DEN, and the essential role of the newly identified DEN-bis(monoacylglycero)phosphate interactions in viral genome escape from the endosome suggests a novel target for drug design.

Dengue virus infection is a growing public health problem with up to 100 million cases annually, and neither vaccines nor effective therapies are available. To search for the ways of preventing and treating dengue infections we need to better understand their molecular mechanisms. As with many other viruses, dengue virus enters cells by fusion between the viral membrane and the membrane of intracellular vesicles (endosomes). In this work we explored the fusion stage of dengue virus entry in different experimental systems ranging from virus fusion to artificial lipid membranes to fusion inside the cells. While earlier work on dengue virus entry has focused on viral protein that mediates fusion, we found that effective action of this protein requires specific lipid composition of the membrane the virus fuses to. In effect, this lipid dependence allows virus to control intracellular location of the fusion event and, thus, the place of its RNA release by exploiting cell-controlled differences between lipid compositions of different organelles the virus travels through. The essential role of the interactions between dengue virus and its lipid cofactors during viral entry suggests that these interactions may be targeted in drug design.

Role of Abl kinase and the Wave2 signaling complex in HIV-1 entry at a post-hemifusion step

Entry of human immunodeficiency virus type 1 (HIV-1) commences with binding of the envelope glycoprotein (Env) to the receptor CD4, and one of two coreceptors, CXCR4 or CCR5. Env-mediated signaling through coreceptor results in Galphaq-mediated Rac activation and actin cytoskeleton rearrangements necessary for fusion. Guanine nucleotide exchange factors (GEFs) activate Rac and regulate its downstream protein effectors. In this study we show that Env-induced Rac activation is mediated by the Rac GEF Tiam-1, which associates with the adaptor protein IRSp53 to link Rac to the Wave2 complex. Rac and the tyrosine kinase Abl then activate the Wave2 complex and promote Arp2/3-dependent actin polymerization. Env-mediated cell-cell fusion, virus-cell fusion and HIV-1 infection are dependent on Tiam-1, Abl, IRSp53, Wave2, and Arp3 as shown by attenuation of fusion and infection in cells expressing siRNA targeted to these signaling components. HIV-1 Env-dependent cell-cell fusion, virus-cell fusion and infection were also inhibited by Abl kinase inhibitors, imatinib, nilotinib, and dasatinib. Treatment of cells with Abl kinase inhibitors did not affect cell viability or surface expression of CD4 and CCR5. Similar results with inhibitors and siRNAs were obtained when Env-dependent cell-cell fusion, virus-cell fusion or infection was measured, and when cell lines or primary cells were the target. Using membrane curving agents and fluorescence microscopy, we showed that inhibition of Abl kinase activity arrests fusion at the hemifusion (lipid mixing) step, suggesting a role for Abl-mediated actin remodeling in pore formation and expansion. These results suggest a potential utility of Abl kinase inhibitors to treat HIV-1 infected patients.

Negative potentials across biological membranes promote fusion by class II and class III viral proteins

Fusion of virions pseudotyped with a class II, SFV E1 or VEEV E, or a class III protein, VSV G is promoted by negative potentials and hindered by positive potentials across the target cell. Hemifusion is independent of polarity. Reversion of hemifused membranes into two distinct ones is responsible for voltage-dependence and inhibition of fusion.

Voltage was investigated as a factor in the fusion of virions. Virions, pseudotyped with a class II, SFV E1 or VEEV E, or a class III protein, VSV G, were prepared with GFP within the core and a fluorescent lipid. This allowed both hemifusion and fusion to be monitored. Voltage clamping the target cell showed that fusion is promoted by a negative potential and hindered by a positive potential. Hemifusion occurred independent of polarity. Lipid dye movement, in the absence of content mixing, ceased before complete transfer for positive potentials, indicating that reversion of hemifused membranes into two distinct membranes is responsible for voltage dependence and inhibition of fusion. Content mixing quickly followed lipid dye transfer for a negative potential, providing a direct demonstration that hemifusion induced by class II and class III viral proteins is a functional intermediate of fusion. In the hemifused state, virions that fused exhibited slower lipid transfer than did nonfusing virions. All viruses with class II or III fusion proteins may utilize voltage to achieve infection.

Inhibition of RNA recruitment and replication of an RNA virus by acridine derivatives with known anti-prion activities

Background

Small molecule inhibitors of RNA virus replication are potent antiviral drugs and useful to dissect selected steps in the replication process. To identify antiviral compounds against Tomato bushy stunt virus (TBSV), a model positive stranded RNA virus, we tested acridine derivatives, such as chlorpromazine (CPZ) and quinacrine (QC), which are active against prion-based diseases.

Methodology/Principal Findings

Here, we report that CPZ and QC compounds inhibited TBSV RNA accumulation in plants and in protoplasts. In vitro assays revealed that the inhibitory effects of these compounds were manifested at different steps of TBSV replication. QC was shown to have an effect on multiple steps, including: (i) inhibition of the selective binding of the p33 replication protein to the viral RNA template, which is required for recruitment of viral RNA for replication; (ii) reduction of minus-strand synthesis by the tombusvirus replicase; and (iii) inhibition of translation of the uncapped TBSV genomic RNA. In contrast, CPZ was shown to inhibit the in vitro assembly of the TBSV replicase, likely due to binding of CPZ to intracellular membranes, which are important for RNA virus replication.

Conclusion/Significance

Since we found that CPZ was also an effective inhibitor of other plant viruses, including Tobacco mosaic virus and Turnip crinkle virus, it seems likely that CPZ has a broad range of antiviral activity. Thus, these inhibitors constitute effective tools to study similarities in replication strategies of various RNA viruses.

Cholesterol, regulated exocytosis and the physiological fusion machine

Exocytosis is a highly conserved and essential process. Although numerous proteins are involved throughout the exocytotic process, the defining membrane fusion step appears to occur through a lipid-dominated mechanism. Here we review and integrate the current literature on protein and lipid roles in exocytosis, with emphasis on the multiple roles of cholesterol in exocytosis and membrane fusion, in an effort to promote a more molecular systems-level view of the as yet poorly understood process of Ca2+-triggered membrane mergers.

Complex lipid requirements for SNARE- and SNARE chaperone-dependent membrane fusion

Membrane fusion without lysis has been reconstituted with purified yeast vacuolar SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors), the SNARE chaperones Sec17p/Sec18p and the multifunctional HOPS complex, which includes a subunit of the SNARE-interactive Sec1-Munc18 family, and vacuolar lipids: phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylinositol (PI), phosphatidylserine (PS), phosphatidic acid (PA), cardiolipin (CL), ergosterol (ERG), diacylglycerol (DAG), and phosphatidylinositol 3-phosphate (PI3P). We now report that many of these lipids are required for rapid and efficient fusion of the reconstituted SNARE proteoliposomes in the presence of SNARE chaperones. Omission of either PE, PA, or PI3P from the complete set of lipids strongly reduces fusion, and PC, PE, PA, and PI3P constitute a minimal set of lipids for fusion. PA could neither be replaced by other lipids with small headgroups such as DAG or ERG nor by the acidic lipids PS or PI. PA is needed for full association of HOPS and Sec18p with proteoliposomes having a minimal set of lipids. Strikingly, PA and PE are as essential for SNARE complex assembly as for fusion, suggesting that these lipids facilitate functional interactions among SNAREs and SNARE chaperones.

The six-helix bundle of human immunodeficiency virus Env controls pore formation and enlargement and is initiated at residues proximal to the hairpin turn

Residues that create the grooves of the human immunodeficiency virus type 1 (HIV-1) Env triple-stranded coiled coil (HR1) and the residues that pack into the grooves (HR2) to complete the formation of the six-helix bundle (6HB) were mutated. The extent and kinetics of fusion as well as pore enlargement were measured for each mutant. Mutations near the hairpin turns of each monomer of the 6HB were more important than those far from the turn, for both HR1 and HR2. This result is consistent with the idea that binding of HR2 to the HR1 grooves is initiated near the hairpin turn of each monomer. Mutations at the distal portions also reduced fusion, albeit to a smaller extent. An intermediate of fusion (temperature-arrested state [TAS]) was formed, and the consequences of mutation were compared; a mutant that exhibited less fusion also showed slower kinetics from TAS. This suggests that formation of the bundle is a rate-limiting step downstream of the intermediate state. The rate of enlargement of a fusion pore also correlated with the extent and kinetics of fusion. The rate of pore enlargement was severely reduced by mutation. This supports our prior conclusion that formation of the 6HB occurs after pore creation and strongly suggests that the free energy released by bundle formation is directly used to promote pore growth.

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.

Physical transfer of membrane and cytoplasmic components as a general mechanism of cell-cell communication

Recent evidence from different research areas has revealed a novel mechanism of cell-cell communication by spontaneous intercellular transfer of cellular components (ICT). Here we studied this phenomenon by co-culturing different cells that contain distinct levels of proteins or markers for the plasma membrane or cytoplasm. We found that a variety of transmembrane proteins are transferable between multiple cell types. Membrane lipids also show a high efficiency of intercellular transfer. Size-dependent cytoplasmic transfer allows exchange of cytoplasmic macromolecules up to 40 kDa between somatic cells, and up to 2000 kDa between uncommitted human precursor cells and human umbilical vein endothelial cells. Protein transfer, lipid transfer and cytoplasmic component transfer can occur simultaneously and all require direct cell-cell contact. Analyses of the properties of ICT, together with a close examination of cell-cell interactions, suggest that the spontaneous ICT of different cellular components might have a common underlying process: transient local membrane fusions formed when neighboring cells undergo close cell-cell contact.

The Autographa californica multicapsid nucleopolyhedrovirus GP64 protein: analysis of transmembrane domain length and sequence requirements

GP64, the major envelope glycoprotein of the Autographa californica multicapsid nucleopolyhedrovirus budded virion, is important for host cell receptor binding and mediates low-pH-triggered membrane fusion during entry by endocytosis. Previous transmembrane (TM) domain replacement studies showed that the TM domain serves a critical role in GP64 function. To extend the prior studies and examine specific sequence requirements of the TM domain, we generated a variety of GP64 TM domain mutations. The mutations included 4- to 8-amino-acid deletions, as well as single and multiple point mutations. While most TM domain deletion constructs remained fusion competent, those containing deletions of eight amino acids from the C terminus did not mediate detectable fusion. The addition of a hydrophobic amino acid (A, L, or V) to the C terminus of construct C8 (a construct that contains a TM domain deletion of eight amino acids from the C terminus) restored fusion activity. These data suggest that the membrane fusion function of GP64 is dependent on a critical length of the hydrophobic TM domain. All GP64 proteins with a truncated TM domain mediated detectable virion budding with dramatically lower levels of efficiency than wild-type GP64. The effects of deletions of various lengths and positions in the TM domain were also examined for their effects on viral infectivity. Further analysis of the TM domain by single amino acid substitutions and 3-alanine scanning mutations identified important but not essential amino acid positions. These studies showed that amino acids at positions 485 to 487 and 503 to 505 are important for cell surface expression of GP64, while amino acids at positions 483 to 484 and 494 to 496 are important for virus budding. Overall, our results show that specific features and amino acid sequences, particularly the length of the hydrophobic TM domain, play critical roles in membrane anchoring, membrane fusion, virus budding, and infectivity.

Functional analysis of the transmembrane domain in paramyxovirus F protein-mediated membrane fusion

To enter cells, enveloped viruses use fusion-mediating glycoproteins to facilitate the merger of the viral and host cell membranes. These glycoproteins undergo large-scale irreversible refolding during membrane fusion. The paramyxovirus parainfluenza virus 5 mediates membrane merger through its fusion protein (F). The transmembrane (TM) domains of viral fusion proteins are typically required for fusion. The TM domain of F is particularly interesting in that it is potentially unusually long; multiple calculations suggest a TM helix length between 25 and 48 residues. Oxidative cross-linking of single-cysteine substitutions indicates the F TM trimer forms a helical bundle within the membrane. To assess the functional role of the paramyxovirus parainfluenza virus 5 F protein TM domain, alanine scanning mutagenesis was performed. Two residues located in the outer leaflet of the bilayer are critical for fusion. Multiple amino acid substitutions at these positions indicate the physical properties of the side chain play a critical role in supporting or blocking fusion. Analysis of intermediate steps in F protein refolding indicated that the mutants were not trapped at the open stalk intermediate or the prehairpin intermediate. Incorporation of a known F protein destabilizing mutation that causes a hyperfusogenic phenotype restored fusion activity to the mutants. Further, altering the curvature of the lipid bilayer by addition of oleic acid promoted fusion of the F protein mutants. In aggregate, these data indicate that the TM domain plays a functional role in fusion beyond merely anchoring the protein in the viral envelope and that it can affect the structures and steady-state concentrations of the various conformational intermediates en route to the final postfusion state. We suggest that the unusual length of this TM helix might allow it to serve as a template for formation of or specifically stabilize the lipid stalk intermediate in fusion.

Reovirus FAST protein transmembrane domains function in a modular, primary sequence-independent manner to mediate cell-cell membrane fusion

The FAST proteins are a unique family of virus-encoded cell-cell membrane fusion proteins. In the absence of a cleavable N-terminal signal peptide, a single-pass transmembrane domain (TMD) functions as a reverse signal-anchor to direct the FAST proteins into the plasma membrane in an N(exo)/C(cyt) topology. There is little information available on the role of the FAST protein TMD in the cell-cell membrane fusion reaction. We show that in the absence of conservation in the length or primary amino acid sequence, the p14 TMD can be functionally exchanged with the TMDs of the p10 and p15 FAST proteins. This is not the case for chimeric p14 proteins containing the TMDs of two different enveloped viral fusion proteins or a cellular membrane protein; such chimeric proteins were defective for both pore formation and syncytiogenesis. TMD structural features that are conserved within members of the FAST protein family presumably play direct roles in the fusion reaction. Molecular modeling suggests that the funnel-shaped architecture of the FAST protein TMDs may represent such a conserved structural and functional motif. Interestingly, although heterologous TMDs exert diverse influences on the trafficking of the p14 FAST protein, these TMDs are capable of functioning as reverse signal-anchor sequences to direct p14 into lipid rafts in the correct membrane topology. The FAST protein TMDs are therefore not primary determinants of type III protein topology, but they do play a direct, sequence-independent role in the membrane fusion reaction.

Low-pH triggering of human metapneumovirus fusion: essential residues and importance in entry

Human metapneumovirus (HMPV) is a significant respiratory pathogen classified in the Pneumovirinae subfamily of the paramyxovirus family. Recently, we demonstrated that HMPV F protein-promoted cell-cell fusion is stimulated by exposure to low pH, in contrast to what is observed for other paramyxovirus F proteins. In the present study, we examined the potential role of histidine protonation in HMPV F fusion and investigated the role of low pH in HMPV viral entry. Mutagenesis of the three ectodomain histidine residues of the HMPV F protein demonstrated that the mutation of a histidine in the heptad repeat B linker domain (H435) ablated fusion activity without altering cell surface expression or proteolytic processing significantly. Modeling of the HMPV F protein revealed several basic residues surrounding this histidine residue, and the mutation of these residues also reduced fusion activity. These results suggest that electrostatic repulsion in the heptad repeat B linker region may contribute to the triggering of HMPV F. In addition, we examined the effect of inhibitors of endosomal acidification or endocytosis on the entry of a recombinant green fluorescent protein-expressing HMPV. Interestingly, chemicals that raise the pH of endocytic vesicles resulted in a 30 to 50% decrease in HMPV infection, while the inhibitors of endocytosis reduced infection by as much as 90%. These data suggest that HMPV utilizes an endocytic entry mechanism, in contrast to what has been hypothesized for most paramyxoviruses. In addition, our results indicate that HMPV uses the low pH of the endocytic pathway to enhance infectivity, though the role of low pH likely differs from classically described mechanisms.

Membrane fusion

Subcellular compartmentalization, cell growth, hormone secretion and neurotransmission require rapid, targeted, and regulated membrane fusion. Fusion entails extensive lipid rearrangements by two apposed (docked) membrane vesicles, joining their membrane proteins and lipids and mixing their luminal contents without lysis. Fusion of membranes in the secretory pathway involves Rab GTPases; their bound ‘effector’ proteins, which mediate downstream steps; SNARE proteins, which can ‘snare’ each other, in cis (bound to one membrane) or in trans (anchored to apposed membranes); and SNARE-associated proteins (SM proteins; NSF or Sec18p; SNAP or Sec17p; and others) cooperating with specific lipids to catalyze fusion. In contrast, mitochondrial and cell-cell fusion events are regulated by and use distinct catalysts.

The block to membrane fusion differs with the site of ligand insertion in modified retroviral envelope proteins

Efforts to achieve cell type-specific transduction of retroviral vectors for gene therapy have centred on modification of the envelope protein (Env). Typically, addition of a ligand to Env gives binding to the new or target receptor, but little or no infection, and affects the subunit association of the modified Env. We previously discovered two point mutations that increase targeted infection by over 1000-fold when added to an Env modified by N-terminal insertion of the receptor-binding domain from amphotropic murine leukemia virus Env. Here, we asked whether these mutations would similarly increase transduction by Env modified with a clinically relevant ligand, human interleukin-13 (IL-13L). Addition of the point mutations stabilized the weak subunit association observed in some IL-13L-modified Env proteins, but infection via the target IL-13 receptor still did not occur. Fluorescence-based cell-cell fusion assays and studies with a membrane-curving agent revealed that defects in membrane fusion differed with the site of ligand insertion. When IL-13 was inserted into the N terminus of Env, membrane fusion was blocked prior to membrane-lipid mixing, regardless of whether flanking flexible linkers were added. Unexpectedly, insertion of IL-13 in the proline-rich region showed evidence of initiation of fusion and fusion-peptide exposure, but fusion was blocked at a subsequent step prior to fusion-pore formation. Thus, the site of ligand insertion influenced initiation of membrane fusion and its progression. These observations suggest that a novel site for ligand insertion must be identified before clinically useful targeted transduction will be achieved.

Hydrophobic inactivation of influenza viruses confers preservation of viral structure with enhanced immunogenicity

The use of inactivated influenza virus for the development of vaccines with broad heterosubtypic protection requires selective inactivation techniques that eliminate viral infectivity while preserving structural integrity. Here we tested if a hydrophobic inactivation approach reported for retroviruses could be applied to the influenza virus. By this approach, the transmembrane domains of viral envelope proteins are selectively targeted by the hydrophobic photoactivatable compound 1,5-iodonaphthyl-azide (INA). This probe partitions into the lipid bilayer of the viral envelope and upon far UV irradiation reacts selectively with membrane-embedded domains of proteins and lipids while the protein domains that localize outside the bilayer remain unaffected. INA treatment of influenza virus blocked infection in a dose-dependent manner without disrupting the virion or affecting neuraminidase activity. Moreover, the virus maintained the full activity in inducing pH-dependent lipid mixing, but pH-dependent redistribution of viral envelope proteins into the target cell membrane was completely blocked. These results indicate that INA selectively blocks fusion of the virus with the target cell membrane at the pore formation and expansion step. Using a murine model of influenza virus infection, INA-inactivated influenza virus induced potent anti-influenza virus serum antibody and T-cell responses, similar to live virus immunization, and protected against heterosubtypic challenge. INA treatment of influenza A virus produced a virus that is noninfectious, intact, and fully maintains the functional activity associated with the ectodomains of its two major envelope proteins, neuraminidase and hemagglutinin. When used as a vaccine given intranasally (i.n.), INA-inactivated influenza virus induced immune responses similar to live virus infection.

Fusion induced by a class II viral fusion protein, semliki forest virus E1, is dependent on the voltage of the target cell

Cells expressing the low pH-triggered class II viral fusion protein E1 of Semliki Forest virus (SFV) were fused to target cells. Fusion was monitored by electrical capacitance and aqueous dye measurements. Electrical voltage-clamp measurements showed that SFV E1-induced cell-cell fusion occurred quickly after acidification for a trans-negative potential across the target membrane (i.e., negative potential inside the target cell) but that a trans-positive potential eliminated all fusion. Use of an ionophore to control potentials for a large population of cells confirmed the dependence of fusion on voltage polarity. In contrast, fusion induced by the class I fusion proteins of human immunodeficiency virus, avian sarcoma leukosis virus, and influenza virus was independent of the voltage polarity across the target cell. Initial pore size and pore growth were also independent of voltage polarity for the class I proteins. An intermediate of SFV E1-induced fusion was created by transient acidification at low temperature. Membranes were hemifused at this intermediate state, and raising the temperature at neutral pH allowed full fusion to occur. Capacitance measurements showed that maintaining a trans-positive potential definitely blocked fusion at steps following the creation of the hemifusion intermediate and may have inhibited fusion at prior steps. It is proposed that the trans-negative voltage across the endosomal membrane facilitates fusion after low-pH-induced conformational changes of SFV E1 have occurred.

Assays of vacuole fusion resolve the stages of docking, lipid mixing, and content mixing

Membrane fusion entails organelle docking and subsequent mixing of membrane bilayers and luminal compartments. We now present an in vitro assay of fusion, using yeast vacuoles bearing domains of either Fos or Jun fused to complementary halves of beta-lactamase. Upon fusion, these proteins associate to yield beta-lactamase activity. This assay complements the standard fusion assay (activation of pro-Pho8p in protease-deficient vacuoles by proteases from pho8Delta vacuoles). Both the beta-lactamase and pro-Pho8p activation assays of fusion show the same long kinetic delay between SNARE pairing and luminal compartment mixing. Lipid-mixing occurs rapidly after SNARE pairing but well before aqueous compartment mixing. These results support a model in which SNARE pairing leads to rapid hemifusion, followed by slow further lipid rearrangement and aqueous compartment mixing.

Polybasic KKR motif in the cytoplasmic tail of Nipah virus fusion protein modulates membrane fusion by inside-out signaling

The cytoplasmic tails of the envelope proteins from multiple viruses are known to contain determinants that affect their fusogenic capacities. Here we report that specific residues in the cytoplasmic tail of the Nipah virus fusion protein (NiV-F) modulate its fusogenic activity. Truncation of the cytoplasmic tail of NiV-F greatly inhibited cell-cell fusion. Deletion and alanine scan analysis identified a tribasic KKR motif in the membrane-adjacent region as important for modulating cell-cell fusion. The K1A mutation increased fusion 5.5-fold, while the K2A and R3A mutations decreased fusion 3- to 5-fold. These results were corroborated in a reverse-pseudotyped viral entry assay, where receptor-pseudotyped reporter virus was used to infect cells expressing wild-type or mutant NiV envelope glycoproteins. Differential monoclonal antibody binding data indicated that hyper- or hypofusogenic mutations in the KKR motif affected the ectodomain conformation of NiV-F, which in turn resulted in faster or slower six-helix bundle formation, respectively. However, we also present evidence that the hypofusogenic phenotypes of the K2A and R3A mutants were effected via distinct mechanisms. Interestingly, the K2A mutant was also markedly excluded from lipid rafts, where approximately 20% of wild-type F and the other mutants can be found. Finally, we found a strong negative correlation between the relative fusogenic capacities of these cytoplasmic-tail mutants and the avidities of NiV-F and NiV-G interactions (P = 0.007, r(2) = 0.82). In toto, our data suggest that inside-out signaling by specific residues in the cytoplasmic tail of NiV-F can modulate its fusogenicity by multiple distinct mechanisms.

Membranes of the world unite!

Despite diverse origins, cellular fusion mechanisms converge at a pathway of phospholipid bilayer fusion. In this mini-review, we discuss how proteins can mediate each of the three major stages in the fusion pathway: contact, hemifusion, and the opening of an expanding fusion pore.

Heterogeneity of early intermediates in cell-liposome fusion mediated by influenza hemagglutinin

To explore early intermediates in membrane fusion mediated by influenza virus hemagglutinin (HA) and their dependence on the composition of the target membrane, we studied lipid mixing between HA-expressing cells and liposomes containing phosphatidylcholine (PC) with different hydrocarbon chains. For all tested compositions, our results indicate the existence of at least two types of intermediates, which differ in their lifetimes. The composition of the target membrane affects the stability of fusion intermediates at a stage before lipid mixing. For less fusogenic distearoyl PC-containing liposomes at 4 degrees C, some of the intermediates inactivate, and no intermediates advance to lipid mixing. Fusion intermediates that formed for the more fusogenic dioleoyl PC-containing liposomes did not inactivate and even yielded partial lipid mixing at 4 degrees C. Thus, a more fusogenic target membrane effectively blocks nonproductive release of the conformational energy of HA. Even for the same liposome composition, HA forms two types of fusion intermediates, dissimilar in their stability and propensity to fuse. This diversity of fusion intermediates emphasizes the importance of local membrane composition and local protein concentration in fusion of heterogeneous biological membranes.

Membrane transformation during malaria parasite release from human red blood cells

Three opposing pathways are proposed for the release of malaria parasites from infected erythrocytes: coordinated rupture of the two membranes surrounding mature parasites; fusion of erythrocyte and parasitophorus vacuolar membranes (PVM); and liberation of parasites enclosed within the vacuole from the erythrocyte followed by PVM disintegration. Rupture by cell swelling should yield erythrocyte ghosts; membrane fusion is inhibited by inner-leaflet amphiphiles of positive intrinsic curvature, which contrariwise promote membrane rupture; and without protease inhibitors, parasites would leave erythrocytes packed within the vacuole. Therefore, we visualized erythrocytes releasing P. falciparum using fluorescent microscopy of differentially labeled membranes. Release did not yield erythrocyte ghosts, positive-curvature amphiphiles did not inhibit release but promoted it, and release of packed merozoites was shown to be an artifact. Instead, two sequential morphological stages preceded a convulsive rupture of membranes and rapid radial discharge of separated merozoites, leaving segregated internal membrane fragments and plasma membrane vesicles or blebs at the sites of parasite egress. These results, together with the modulation of release by osmotic stress, suggest a pathway of parasite release that features a biochemically altered erythrocyte membrane that folds after pressure-driven rupture of membranes.