The initial fusion pore induced by baculovirus GP64 is large and forms quickly

Drosophila Myoblast Fusion: Invasion and Resistance for the Ultimate Union

Cell-cell fusion is indispensable for creating life and building syncytial tissues and organs. Ever since the discovery of cell-cell fusion, how cells join together to form zygotes and multinucleated syncytia has remained a fundamental question in cell and developmental biology. In the past two decades, Drosophila myoblast fusion has been used as a powerful genetic model to unravel mechanisms underlying cell-cell fusion in vivo. Many evolutionarily conserved fusion-promoting factors have been identified and so has a surprising and conserved cellular mechanism. In this review, we revisit key findings in Drosophila myoblast fusion and highlight the critical roles of cellular invasion and resistance in driving cell membrane fusion.

Baculovirus Entry and Egress from Insect Cells

Baculoviruses are large DNA viruses of insects that are highly pathogenic in many hosts. In the infection cycle, baculoviruses produce two types of virions. These virion phenotypes are physically and functionally distinct, and each serves a critical role in the biology of the virus. One phenotype, the occlusion-derived virus (ODV), is occluded within a crystallized protein that facilitates oral infection of the host. A large complex of at least nine ODV envelope proteins called per os infectivity factors are critically important for ODV infection of insect midgut epithelial cells. Viral egress from midgut cells is by budding to produce a second virus phenotype, the budded virus (BV). BV binds, enters, and replicates in most other tissues of the host insect. Cell recognition and entry by BV are mediated by a single major envelope glycoprotein: GP64 in some baculoviruses and F in others. Entry and egress by the two virion phenotypes occur by dramatically different mechanisms and reflect a life cycle in which ODV is specifically adapted for oral infection while BV mediates dissemination of the infection within the animal.

Nanodisc-cell fusion: control of fusion pore nucleation and lifetimes by SNARE protein transmembrane domains

The initial, nanometer-sized connection between the plasma membrane and a hormone- or neurotransmitter-filled vesicle –the fusion pore– can flicker open and closed repeatedly before dilating or resealing irreversibly. Pore dynamics determine release and vesicle recycling kinetics, but pore properties are poorly known because biochemically defined single-pore assays are lacking. We isolated single flickering pores connecting v-SNARE-reconstituted nanodiscs to cells ectopically expressing cognate, “flipped” t-SNAREs. Conductance through single, voltage-clamped fusion pores directly reported sub-millisecond pore dynamics. Pore currents fluctuated, transiently returned to baseline multiple times, and disappeared ~6 s after initial opening, as if the fusion pore fluctuated in size, flickered, and resealed. We found that interactions between v- and t-SNARE transmembrane domains (TMDs) promote, but are not essential for pore nucleation. Surprisingly, TMD modifications designed to disrupt v- and t-SNARE TMD zippering prolonged pore lifetimes dramatically. We propose that the post-fusion geometry of the proteins contribute to pore stability.

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.

Molecular dynamics simulation analysis of membrane defects and pore propensity of hemifusion diaphragms

Membrane fusion often exhibits slow dynamics in electrophysiological experiments, involving prespike foot and fusion pore-flickering, but the structural basis of such phenomena remains unclear. Hemifusion intermediates have been implicated in the early phase of membrane fusion. To elucidate the dynamics of formation of membrane defects and pores within the hemifusion diaphragm (HD), atomistic and coarse-grained models of hemifusion intermediates were constructed using dipalmitoylphosphatidylcholine or dioleoylphosphatidylcholine membranes. The work necessary to displace a lipid molecule to the hydrophobic core of the bilayer was measured. For a lipid within the HD with radius of 4 nm, the work was ∼80 kJ/mol, similar to that in a planar bilayer. The work was much less (∼40 kJ/mol) when the HD was surrounded by a steep stalk, i.e., stalk wings forming a large angle at the junction of three bilayers. In the latter case, the lipid displacement engendered formation of a pore contacting the HD rim. The work was similarly small (40 kJ/mol) for a small HD of 1.5 nm radius, where a pore formed and grew rapidly, quickly generating a toroidal structure (<40 ns). Combining the steep stalk and the small HD decreased the work further, although quantitative analysis was difficult because the latter system was not in a stable equilibrium state. Results suggest that fine tuning of fusion dynamics requires strict control of the HD size and the angle between the expanded stalk and HD. In additional free simulations, the steep stalk facilitated widening of a preformed pore contacting the HD rim.

Functional analysis of the Autographa californica multiple nucleopolyhedrovirus GP64 terminal fusion loops and interactions with membranes

The Autographa californica multiple nucleopolyhedrovirus (AcMNPV) glycoprotein GP64 is the major envelope protein of the budded virus (BV). GP64 is a class III fusion protein that mediates BV attachment to the cell surface and low-pH-triggered membrane fusion between the BV envelope and the endosome membrane during entry. Class III fusion proteins contain terminal looped structures that are believed to interact with membranes. To examine the functions of 3 loops found at the apex of the GP64 postfusion structure, we generated 2-alanine substitutions that scanned the two so-called fusion loops (loop 1 and loop 2) plus an adjacent loop structure (loop 3) that is closely attached to loop 2 and is also found at the apex of the GP64 postfusion structure. We identified essential residues from Y75 to T86 (loop 1) and N149 to H156 (loop 2) that are required for fusion activity, but no essential residues in loop 3. Further analysis revealed that critical fusion loop residues fall within two groups that are associated with either membrane merger (hemifusion) or fusion pore expansion. We next examined the interactions of soluble GP64 proteins and BV with membranes composed of various phospholipids. BV interacted directly with small unilamellar vesicles (SUVs) comprised of phospholipids phosphatidylcholine and phosphatidic acid (PC/PA) or phosphatidylcholine and phosphatidylserine (PC/PS) under neutral and acidic pH. We also examined the interactions of soluble GP64 constructs containing substitutions of the most hydrophobic residues within each of the two fusion loops. We found that a 2-residue substitution in either single loop (loop 1 [positions 81 and 82] or loop 2 [positions 153 and 154]) was not sufficient to substantially reduce the GP64-liposome interaction, but the same substitutions in both fusion loops severely reduced the GP64-liposome association at neutral pH. These results suggest that critical hydrophobic residues in both fusion loops may be involved in the interaction of GP64 with host cellular membranes and direct GP64-membrane interactions may represent a receptor-binding step prior to a low-pH-triggered conformational change.

Intracellular curvature-generating proteins in cell-to-cell fusion

Cell-to-cell fusion plays an important role in normal physiology and in different pathological conditions. Early fusion stages mediated by specialized proteins and yielding fusion pores are followed by a pore expansion stage that is dependent on cell metabolism and yet unidentified machinery. Because of a similarity of membrane bending in the fusion pore rim and in highly curved intracellular membrane compartments, in the present study we explored whether changes in the activity of the proteins that generate these compartments affect cell fusion initiated by protein fusogens of influenza virus and baculovirus. We raised the intracellular concentration of curvature-generating proteins in cells by either expressing or microinjecting the ENTH (epsin N-terminal homology) domain of epsin or by expressing the GRAF1 (GTPase regulator associated with focal adhesion kinase 1) BAR (Bin/amphiphysin/Rvs) domain or the FCHo2 (FCH domain-only protein 2) F-BAR domain. Each of these treatments promoted syncytium formation. Cell fusion extents were also influenced by treatments targeting the function of another curvature-generating protein, dynamin. Cell-membrane-permeant inhibitors of dynamin GTPase blocked expansion of fusion pores and dominant-negative mutants of dynamin influenced the syncytium formation extents. We also report that syncytium formation is inhibited by reagents lowering the content and accessibility of PtdIns(4,5)P₂, an important regulator of intracellular membrane remodelling. Our findings indicate that fusion pore expansion at late stages of cell-to-cell fusion is mediated, directly or indirectly, by intracellular membrane-shaping proteins.

Autographa californica multiple nucleopolyhedrovirus GP64 protein: roles of histidine residues in triggering membrane fusion and fusion pore expansion

The Autographa californica multiple nucleopolyhedrovirus (AcMNPV) GP64 protein mediates membrane fusion during entry. Fusion results from a low-pH-triggered conformational change in GP64 and subsequent interactions with the membrane bilayers. The low-pH sensor and trigger of the conformational change are not known, but histidine residues are implicated because the pK(a) of histidine is near the threshold for triggering fusion by GP64. We used alanine substitutions to examine the roles of all individual and selected clusters of GP64 histidine residues in triggering and mediating fusion by GP64. Three histidine residues (H152, H155, and H156), located in fusion loop 2, were identified as important for membrane fusion. These three histidine residues were important for efficient pore expansion but were not required for the pH-triggered conformational change. In contrast, a cluster of three histidine residues (H245, H304, and H430) located near the base of the central coiled coil was identified as a putative sensor for low pH. Three alanine substitutions in cluster H245/H304/H430 resulted in dramatically reduced membrane fusion and the apparent loss of the prefusion conformation at neutral pH. Thus, the H245/H304/H430 cluster of histidines may function or participate as a pH sensor by stabilizing the prefusion structure of GP64.

An invasive podosome-like structure promotes fusion pore formation during myoblast fusion

Recent studies in Drosophila have implicated actin cytoskeletal remodeling in myoblast fusion, but the cellular mechanisms underlying this process remain poorly understood. Here we show that actin polymerization occurs in an asymmetric and cell type-specific manner between a muscle founder cell and a fusion-competent myoblast (FCM). In the FCM, a dense F-actin-enriched focus forms at the site of fusion, whereas a thin sheath of F-actin is induced along the apposing founder cell membrane. The FCM-specific actin focus invades the apposing founder cell with multiple finger-like protrusions, leading to the formation of a single-channel macro fusion pore between the two muscle cells. Two actin nucleation-promoting factors of the Arp2/3 complex, WASP and Scar, are required for the formation of the F-actin foci, whereas WASP but not Scar promotes efficient foci invasion. Our studies uncover a novel invasive podosome-like structure (PLS) in a developing tissue and reveal a previously unrecognized function of PLSs in facilitating cell membrane juxtaposition and fusion.

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.

Visualizing fusion of pseudotyped HIV-1 particles in real time by live cell microscopy

Background

Most retroviruses enter their host cells by fusing the viral envelope with the plasma membrane. Although the protein machinery promoting fusion has been characterized extensively, the dynamics of the process are largely unknown.

Results

We generated human immunodeficiency virus-1 (HIV-1) particles pseudotyped with the envelope (Env) protein of ecotropic murine leukemia virus eMLV to study retrovirus entry at the plasma membrane using live-cell microscopy. This Env protein mediates highly efficient pH independent fusion at the cell surface and can be functionally tagged with a fluorescent protein. To detect fusion events, double labeled particles carrying one fluorophor in Env and the other in the matrix (MA) domain of Gag were generated and characterized. Fusion events were defined as loss of Env signal after virus-cell contact. Single particle tracking of >20,000 individual traces in two color channels recorded 28 events of color separation, where particles lost the Env protein, with the MA layer remaining stable at least for a short period. Fourty-five events were detected where both colors were lost simultaneously. Importantly, the first type of event was never observed when particles were pseudotyped with a non-fusogenic Env.

Conclusion

These results reveal rapid retroviral fusion at the plasma membrane and permit studies of the immediate post-fusion events.

The pre-transmembrane domain of the Autographa californica multicapsid nucleopolyhedrovirus GP64 protein is critical for membrane fusion and virus infectivity

The envelope glycoprotein, GP64, of the baculovirus Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) is a class III viral fusion protein that mediates pH-triggered membrane fusion during virus entry. Viral fusion glycoproteins from many viruses contain a short region in the ectodomain and near the transmembrane domain, referred to as the pre-transmembrane (PTM) domain. In some cases, the PTM domain is rich in aromatic amino acids and plays an important role in membrane fusion. Although the 23-amino-acid (aa) PTM domain of AcMNPV GP64 lacks aromatic amino acids, we asked whether this region might also play a significant role in membrane fusion. We generated alanine scanning and single and multiple amino acid substitutions in the GP64 PTM domain. We specifically focused on amino acid positions conserved between baculovirus GP64 and thogotovirus GP75 proteins, as well as hydrophobic and charged amino acids. For each PTM-modified construct, we examined trimerization, cell surface localization, and membrane fusion activity. Membrane merger and pore formation were also examined. We identified eight aa positions that are important for membrane fusion activity. Critical positions were not clustered in the linear sequence but were distributed throughout the PTM domain. While charged residues were not critical or essential, three hydrophobic amino acids (L465, L476, and L480) played an important role in membrane fusion activity and appear to be involved in formation of the fusion pore. We also asked whether selected GP64 constructs were capable of rescuing a gp64null AcMNPV virus. These studies suggested that several conserved residues (T463, G460, G462, and G474) were not required for membrane fusion but were important for budding and viral infectivity.

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.

Fusion-pore expansion during syncytium formation is restricted by an actin network

Cell-cell fusion in animal development and in pathophysiology involves expansion of nascent fusion pores formed by protein fusogens to yield an open lumen of cell-size diameter. Here we explored the enlargement of micron-scale pores in syncytium formation, which was initiated by a well-characterized fusogen baculovirus gp64. Radial expansion of a single or, more often, of multiple fusion pores proceeds without loss of membrane material in the tight contact zone. Pore growth requires cell metabolism and is accompanied by a local disassembly of the actin cortex under the pores. Effects of actin-modifying agents indicate that the actin cortex slows down pore expansion. We propose that the growth of the strongly bent fusion-pore rim is restricted by a dynamic resistance of the actin network and driven by membrane-bending proteins that are involved in the generation of highly curved intracellular membrane compartments.

The postfusion structure of baculovirus gp64 supports a unified view of viral fusion machines

Viral fusion proteins mediate the merger of host and viral membranes during cell entry for all enveloped viruses. Baculovirus glycoprotein gp64 (gp64) is unusual in promoting entry into both insect and mammalian cells and is distinct from established class I and class II fusion proteins. We report the crystal structure of its postfusion form, which explains a number of gp64's biological properties including its cellular promiscuity, identifies the fusion peptides and shows it to be the third representative of a new class (III) of fusion proteins with unexpected structural homology with vesicular stomatitis virus G and herpes simplex virus type 1 gB proteins. We show that domains of class III proteins have counterparts in both class I and II proteins, suggesting that all these viral fusion machines are structurally more related than previously thought.

Viral membrane fusion

Infection by viruses having lipid-bilayer envelopes proceeds through fusion of the viral membrane with a membrane of the target cell. Viral 'fusion proteins' facilitate this process. They vary greatly in structure, but all seem to have a common mechanism of action, in which a ligand-triggered, large-scale conformational change in the fusion protein is coupled to apposition and merger of the two bilayers. We describe three examples—the influenza virus hemagglutinin, the flavivirus E protein and the vesicular stomatitis virus G protein—in some detail, to illustrate the ways in which different structures have evolved to implement this common mechanism. Fusion inhibitors can be effective antiviral agents.

A functional F analogue of Autographa californica nucleopolyhedrovirus GP64 from the Agrotis segetum granulovirus

The envelope fusion protein F of Plutella xylostella granulovirus is a computational analogue of the GP64 envelope fusion protein of Autographa californica nucleopolyhedrovirus (AcMNPV). Granulovirus (GV) F proteins were thought to be unable to functionally replace GP64 in the AcMNPV pseudotyping system. In the present study the F protein of Agrotis segetum GV (AgseGV) was identified experimentally as the first functional GP64 analogue from GVs. AgseF can rescue virion propagation and infectivity of gp64-null AcMNPV. The AgseF-pseudotyped AcMNPV also induced syncytium formation as a consequence of low-pH-induced membrane fusion.

Initial size and dynamics of viral fusion pores are a function of the fusion protein mediating membrane fusion

BACKGROUND INFORMATION

Protein-mediated merger of biological membranes, membrane fusion, is an important process. To investigate the role of fusogenic proteins in the initial size and dynamics of the fusion pore (a narrow aqueous pathway, which widens to finalize membrane fusion), two different fusion proteins expressed in the same cell line were investigated: the major glycoprotein of baculovirus Autographa californica (GP64) and the HA (haemagglutinin) of influenza X31.

RESULTS

The host Sf9 cells expressing these viral proteins, irrespective of protein species, fused to human RBCs (red blood cells) upon acidification of the medium. A high-time-resolution electrophysiological study of fusion pore conductance revealed fundamental differences in (i) the initial pore conductance; pores created by HA were smaller than those created by GP64; (ii) the ability of pores to flicker; only HA-mediated pores flickered; and (iii) the time required for pore formation; HA-mediated pores took much longer to form after acidification.

CONCLUSION

HA and GP64 have divergent electrophysiological phenotypes even when they fuse identical membranes, and fusion proteins play a crucial role in determining initial fusion pore characteristics. The structure of the initial fusion pore detected by electrical conductance measurements is sensitive to the nature of the fusion protein.

Cholesterol promotes hemifusion and pore widening in membrane fusion induced by influenza hemagglutinin

Cholesterol-specific interactions that affect membrane fusion were tested for using insect cells; cells that have naturally low cholesterol levels (< 4 mol %). Sf9 cells were engineered (HAS cells) to express the hemagglutinin (HA) of the influenza virus X-31 strain. Enrichment of HAS cells with cholesterol reduced the delay between triggering and lipid dye transfer between HAS cells and human red blood cells (RBC), indicating that cholesterol facilitates membrane lipid mixing prior to fusion pore opening. Increased cholesterol also increased aqueous content transfer between HAS cells and RBC over a broad range of HA expression levels, suggesting that cholesterol also favors fusion pore expansion. This interpretation was tested using both trans-cell dye diffusion and fusion pore conductivity measurements in cholesterol-enriched cells. The results of this study support the hypothesis that host cell cholesterol acts at two stages in membrane fusion: (1) early, prior to fusion pore opening, and (2) late, during fusion pore expansion.

Identification of a GP64 subdomain involved in receptor binding by budded virions of the baculovirus Autographica californica multicapsid nucleopolyhedrovirus

Enveloped virus entry into host cells is typically initiated by an interaction between a viral envelope glycoprotein and a host cell receptor. For budded virions of the baculovirus Autographa californica multicapsid nucleopolyhedrovirus, the envelope glycoprotein GP64 is involved in host cell receptor binding, and GP64 is sufficient to mediate low-pH-triggered membrane fusion. To better define the role of GP64 in receptor binding, we generated and characterized a panel of antisera against subdomains of GP64. Eight subdomain-specific antisera were generated, and their reactivities with GP64 proteins and neutralization of virus infectivity and binding were examined. Antibodies directed against the N-terminal region of GP64 (amino acids 21 to 159) showed strong neutralization of infectivity and effectively inhibited binding of (35)S-labeled budded virions to Sf9 cells. In addition, we generated virions displaying truncated GP64 constructs. A construct displaying the N-terminal 274 amino acids (residues 21 to 294) of the ectodomain was sufficient to mediate virion binding. Additional studies of antisera directed against small subdomains revealed that an antiserum against a 40-amino-acid region (residues 121 to 160) neutralized virus infectivity. Site-directed mutagenesis was subsequently used for functional analysis of that region. Recombinant viruses expressing GP64 proteins with single amino acid substitutions within amino acids 120 to 124 and 142 to 148 replicated to high titers, suggesting that those amino acids were not critical for receptor binding or other important GP64 functions. In contrast, GP64 proteins with single amino acid substitutions of residues 153 and 156 were unable to substitute for wild-type GP64 and did not rescue a gp64 knockout virus. Further analysis showed that these substitutions substantially reduced binding of recombinant virus to Sf9 cells. Thus, the amino acid region from positions 21 to 159 was identified as a putative receptor binding domain, and amino acids 153 and 156 appear to be important for receptor binding.

Functional analysis of the transmembrane (TM) domain of the Autographa californica multicapsid nucleopolyhedrovirus GP64 protein: substitution of heterologous TM domains

GP64, the major envelope glycoprotein of the Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) budded virion, is important for host cell receptor binding and mediates low-pH-triggered membrane fusion during entry by endocytosis. In the current study, we examined the functional role of the AcMNPV GP64 transmembrane (TM) domain by replacing the 23-amino-acid GP64 TM domain with corresponding TM domain sequences from a range of viral and cellular type I membrane proteins, including Orgyia pseudotsugata MNPV (OpMNPV) GP64 and F, thogotovirus GP75, Lymantria dispar MNPV (LdMNPV) F, human immunodeficiency virus type 1 (HIV-1) GP41, human CD4 and glycophorin A (GpA), and influenza virus hemagglutinin (HA), and with a glycosylphosphatidylinositol (GPI) anchor addition sequence. In transient expression experiments with Sf9 cells, chimeric GP64 proteins containing either a GPI anchor or TM domains from LdMNPV F or HIV-1 GP41 failed to localize to the cell surface and thus appear to be incompatible with either GP64 structure or cell transport. All of the mutant constructs detected at the cell surface mediated hemifusion (outer leaflet merger) upon low-pH treatment, but only those containing TM domains from CD4, GpA, OpMNPV GP64, and thogotovirus GP75 mediated pore formation and complete membrane fusion activity. This supports a model in which partial fusion (hemifusion) proceeds by a mechanism that is independent of the TM domain and the TM domain participates in the enlargement or expansion of fusion pores after hemifusion. GP64 proteins containing heterologous TM domains mediated virion budding with dramatically differing levels of efficiency. In addition, chimeric GP64 proteins containing TM domains from CD4, GpA, HA, and OpMNPV F were incorporated into budded virions but were unable to rescue the infectivity of a gp64 null virus, whereas those with TM domains from OpMNPV GP64 and thogotovirus GP75 rescued infectivity. These results show that in addition to its basic role in membrane anchoring, the GP64 TM domain is critically important for GP64 trafficking, membrane fusion, virion budding, and virus infectivity. These critical functions were replaced only by TM domains from related viral membrane proteins.

Neutralizing antibodies decrease the envelope fluidity of HIV-1

For successful penetration of HIV-1, the formation of a fusion pore may be required in order to accumulate critical numbers of fusion-activated gp41 with the help of fluidization of the plasma membrane and viral envelope. An increase in temperature to 40 degrees C after viral adsorption at 25 degrees C enhanced the infectivity by 1.4-fold. The enhanced infectivity was inhibited by an anti-CXCR4 peptide, T140, and anti-V3 monoclonal antibodies (0.5beta and 694/98-D) by post-attachment neutralization, but not by non-neutralizing antibodies (670-30D and 246-D) specific for the C5 of gp120 and cluster I of gp41, respectively. Anti-HLA-II and an anti-HTLV-I gp46 antibody, LAT27, neutralized the molecule-carrying HIV-1(C-2(MT-2)). The anti-V3 antibodies suppressed the fluidity of the HIV-1(C-2) envelope, whereas the non-neutralizing antibodies did not. The anti-HLA-II antibody decreased the envelope fluidity of HIV-1(C-2(MT-2)), but not that of HIV-1(C-2). Therefore, fluidity suppression by these antibodies represents an important neutralization mechanism, in addition to inhibition of viral attachment.

Process optimization of large-scale production of recombinant adeno-associated vectors using dielectric spectroscopy

A well-characterized manufacturing process for the large-scale production of recombinant adeno-associated vectors (rAAV) for gene therapy applications is required to meet current and future demands for pre-clinical and clinical studies and potential commercialization. Economic considerations argue in favor of suspension culture-based production. Currently, the only feasible method for large-scale rAAV production utilizes baculovirus expression vectors and insect cells in suspension cultures. To maximize yields and achieve reproducibility between batches, online monitoring of various metabolic and physical parameters is useful for characterizing early stages of baculovirus-infected insect cells. In this study, rAAVs were produced at 40-l scale yielding ~1 x 10(15) particles. During the process, dielectric spectroscopy was performed by real time scanning in radio frequencies between 300 kHz and 10 MHz. The corresponding permittivity values were correlated with the rAAV production. Both infected and uninfected reached a maximum value; however, only infected cell cultures permittivity profile reached a second maximum value. This effect was correlated with the optimal harvest time for rAAV production. Analysis of rAAV indicated the harvesting time around 48 h post-infection (hpi), and 72 hpi produced similar quantities of biologically active rAAV. Thus, if operated continuously, the 24-h reduction in the production process of rAAV gives sufficient time for additional 18 runs a year corresponding to an extra production of ~2 x 10(16) particles. As part of large-scale optimization studies, this new finding will facilitate the bioprocessing scale-up of rAAV and other bioproducts.

Electron tomography of the contact between T cells and SIV/HIV-1: implications for viral entry

The envelope glycoproteins of primate lentiviruses, including human and simian immunodeficiency viruses (HIV and SIV), are heterodimers of a transmembrane glycoprotein (usually gp41), and a surface glycoprotein (gp120), which binds CD4 on target cells to initiate viral entry. We have used electron tomography to determine the three-dimensional architectures of purified SIV virions in isolation and in contact with CD4+ target cells. The trimeric viral envelope glycoprotein surface spikes are heterogeneous in appearance and typically ∼120 Å long and ∼120 Å wide at the distal end. Docking of SIV or HIV-1 on the T cell surface occurs via a neck-shaped contact region that is ∼400 Å wide and consistently consists of a closely spaced cluster of five to seven rod-shaped features, each ∼100 Å long and ∼100 Å wide. This distinctive structure is not observed when viruses are incubated with T lymphocytes in the presence of anti-CD4 antibodies, the CCR5 antagonist TAK779, or the peptide entry inhibitor SIVmac251 C34. For virions bound to cells, few trimers were observed away from this cluster at the virion–cell interface, even in cases where virus preparations showing as many as 70 envelope glycoprotein trimers per virus particle were used. This contact zone, which we term the “entry claw”, provides a spatial context to understand the molecular mechanisms of viral entry. Determination of the molecular composition and structure of the entry claw may facilitate the identification of improved drugs for the inhibition of HIV-1 entry.

Retroviruses such as simian immunodeficiency virus and HIV-1 enter target cells by exploiting the interaction between their surface glycoproteins and cell surface receptors. Knowledge of the structures of these glycoproteins and of the molecular details of their interaction with cell surface receptors is of fundamental interest in understanding viral entry mechanisms. Electron tomo-graphy is a powerful approach to determining the three-dimensional structures of large and heterogeneous sub-cellular assemblies such as virus–cell contact regions that cannot easily be analyzed by high-resolution structural methods such as X-ray crystallography. Here, we have used electron tomographic approaches to show that SIV and HIV-1 viruses make contact with T cells via a unique structure that we term the viral “entry claw”, which is typically composed of about six clustered rods of density that span the contact region. Investigation of the structure of the entry claw and the factors that promote its formation could lead to new insights into the design of more effective drugs to inhibit HIV entry.

A broad antiviral neutral glycolipid, fattiviracin FV-8, is a membrane fluidity modulator

To screen for an effective antiviral compound which acts as a membrane fluidity modulator, dichotomous effects on human immunodeficiency virus type 1 (HIV-1) infection due to different treatments of several glycolipids and lipids were examined. Continuous treatment of infected cells with 40 microg ml(-1) fattiviracin FV-8, a neutral glycolipid isolated from Streptomycetes, inhibited HIV-1 infection by 96%, whereas pretreatment with 400 microg ml(-1) enhanced infectivity 4.7-fold. The glycolipid showed similar effects as glycyrrhizin; it inhibited infection by broad enveloped viruses, blocked cell-cell fusion, reduced the infectivity of treated virions and enhanced susceptibility to viral infection and cell-cell fusion of cells pretreated with high doses of the compound. Suppression and enhancement was correlated with decreased and increased fluidity of plasma membrane of the fattiviracin FV-8-treated cells. Restricted movement of membrane molecules might impede the formation of a wide fusion pore, and therefore be critical to the entry of viruses. Thus, this can be applied as a new strategy to inhibit viral infections.

Genetic control of fusion pore expansion in the epidermis of Caenorhabditis elegans

Developmental cell fusion is found in germlines, muscles, bones, placentae, and stem cells. In Caenorhabditis elegans 300 somatic cells fuse during development. Although there is extensive information on the early intermediates of viral-induced and intracellular membrane fusion, little is known about late stages in membrane fusion. To dissect the pathway of cell fusion in C. elegans embryos, we use genetic and kinetic analyses using live-confocal and electron microscopy. We simultaneously monitor the rates of multiple cell fusions in developing embryos and find kinetically distinct stages of initiation and completion of membrane fusion in the epidermis. The stages of cell fusion are differentially blocked or retarded in eff-1 and idf-1 mutants. We generate kinetic cell fusion maps for embryos grown at different temperatures. Different sides of the same cell differ in their fusogenicity: the left and right membrane domains are fusion-incompetent, whereas the anterior and posterior membrane domains fuse with autonomous kinetics in embryos. All but one cell pair can initiate the formation of the largest syncytium. The first cell fusion does not trigger a wave of orderly fusions in either direction. Ultrastructural studies show that epidermal syncytiogenesis require eff-1 activities to initiate and expand membrane merger.

The C. elegans developmental fusogen EFF-1 mediates homotypic fusion in heterologous cells and in vivo

During cell-cell fusion, two cells' plasma membranes merge, allowing the cytoplasms to mix and form a syncytium. Little is known about the mechanisms of cell fusion. Here, we asked whether eff-1, shown previously to be essential for fusion in Caenorhabditis elegans, acts directly in the fusion machinery. We show that expression of EFF-1 transmembrane protein drives fusion of heterologous cells into multinucleate syncytia. We obtained evidence that EFF-1-mediated fusion involves a hemifusion intermediate characterized by membrane mixing without cytoplasm mixing. Furthermore, syncytiogenesis requires EFF-1 in both fusing cells. To test whether this mechanism also applies in vivo, we conducted genetic mosaic analysis of C. elegans and found that homotypic epidermal fusion requires EFF-1 in both cells. Thus, although EFF-1-mediated fusion shares characteristics with viral and intracellular fusion, including an apparent hemifusion step, it differs from these reactions in the homotypic organization of the fusion machinery.

The broad anti-viral agent glycyrrhizin directly modulates the fluidity of plasma membrane and HIV-1 envelope

Cell entry of enveloped viruses requires a wide-fusion-pore mechanism, involving clustering of fusion-activated proteins and fluidization of the plasma membrane and viral envelope. In the present study, GL (glycyrrhizin) is reported to lower membrane fluidity, thus suppressing infection by HIV, influenza A virus and vesicular stomatitis virus, but not by poliovirus. GL-treated HIV-1 particles showed reduced infectivity. GL also inhibited cell-to-cell fusion induced by HIV-1 and HTLV-I (human T-cell leukaemia virus type I). However, when cells treated with 1 mg/ml GL were placed in GL-free medium, they showed increased susceptibility to HIV-1 infection and HTLV-I fusion due to enhancement of membrane fluidity. The membrane dependence of GL and GL removal experiments suggest that GL does affect the cell entry of viruses. HIVs with more gp120 were less dependent on temperature and less sensitive to GL treatment than those with less gp120, indicating that the existence of more gp120 molecules resulted in a higher probability of forming a cluster of fusion-activated proteins.

Persistent gene expression in mouse nasal epithelia following feline immunodeficiency virus-based vector gene transfer

Gene transfer development for treatment or prevention of cystic fibrosis lung disease has been limited by the inability of vectors to efficiently and persistently transduce airway epithelia. Influenza A is an enveloped virus with natural lung tropism; however, pseudotyping feline immunodeficiency virus (FIV)-based lentiviral vector with the hemagglutinin envelope protein proved unsuccessful. Conversely, pseudotyping FIV with the envelope protein from influenza D (Thogoto virus GP75) resulted in titers of 10(6) transducing units (TU)/ml and conferred apical entry into well-differentiated human airway epithelial cells. Baculovirus GP64 envelope glycoproteins share sequence identity with influenza D GP75 envelope glycoproteins. Pseudotyping FIV with GP64 from three species of baculovirus resulted in titers of 10(7) to 10(9) TU/ml. Of note, GP64 from Autographa californica multicapsid nucleopolyhedrovirus resulted in high-titer FIV preparations (approximately 10(9) TU/ml) and conferred apical entry into polarized primary cultures of human airway epithelia. Using a luciferase reporter gene and bioluminescence imaging, we observed persistent gene expression from in vivo gene transfer in the mouse nose with A. californica GP64-pseudotyped FIV (AcGP64-FIV). Longitudinal bioluminescence analysis documented persistent expression in nasal epithelia for approximately 1 year without significant decline. According to histological analysis using a LacZ reporter gene, olfactory and respiratory epithelial cells were transduced. In addition, methylcellulose-formulated AcGP64-FIV transduced mouse nasal epithelia with much greater efficiency than similarly formulated vesicular stomatitis virus glycoprotein-pseudotyped FIV. These data suggest that AcGP64-FIV efficiently transduces and persistently expresses a transgene in nasal epithelia in the absence of agents that disrupt the cellular tight junction integrity.

Influence of membrane fluidity on human immunodeficiency virus type 1 entry

For penetration of human immunodeficiency virus type 1 (HIV-1), formation of fusion-pores might be required for accumulating critical numbers of fusion-activated gp41, followed by multiple-site binding of gp120 with receptors, with the help of fluidization of the plasma membrane and viral envelope. Correlation between HIV-1 infectivity and fluidity was observed by treatment of fluidity-modulators, indicating that infectivity was dependent on fluidity. A 5% decrease in fluidity suppressed the HIV-1 infectivity by 56%. Contrarily, a 5% increase in fluidity augmented the infectivity by 2.4-fold. An increased temperature of 40 degrees C or treatment of 0.2% xylocaine after viral adsorption at room temperature enhanced the infectivity by 2.6- and 1.5-fold, respectively. These were inhibited by anti-CXCR4 peptide, implying that multiple-site binding was accelerated at 40 degrees C or by xylocaine. Thus, fluidity of both the plasma membrane and viral envelope was required to form the fusion-pore and to complete the entry of HIV-1.

Class II fusion protein of alphaviruses drives membrane fusion through the same pathway as class I proteins

Viral fusion proteins of classes I and II differ radically in their initial structures but refold toward similar conformations upon activation. Do fusion pathways mediated by alphavirus E1 and influenza virus hemagglutinin (HA) that exemplify classes II and I differ to reflect the difference in their initial conformations, or concur to reflect the similarity in the final conformations? Here, we dissected the pathway of low pH–triggered E1-mediated cell–cell fusion by reducing the numbers of activated E1 proteins and by blocking different fusion stages with specific inhibitors. The discovered progression from transient hemifusion to small, and then expanding, fusion pores upon an increase in the number of activated fusion proteins parallels that established for HA-mediated fusion. We conclude that proteins as different as E1 and HA drive fusion through strikingly similar membrane intermediates, with the most energy-intensive stages following rather than preceding hemifusion. We propose that fusion reactions catalyzed by all proteins of both classes follow a similar pathway.

Evidence that rabies virus forms different kinds of fusion machines with different pH thresholds for fusion

Fusion of rabies virus with membranes is triggered at a low pH and is mediated by a viral glycoprotein (G). Fusion of rabies virus with liposomes was monitored by using a lipid mixing assay based on fluorescence resonance energy transfer. Fusion was detected below pH 6.4, and its extent increased with H(+) concentrations to be maximal around pH 6.15. The origin of the partial fusion activity of rabies virus under suboptimal pH conditions (i.e., between pH 6.15 and 6.4) was investigated. We demonstrate unambiguously that fusion at a suboptimal pH is distinct from the phenomenon of low-pH-induced inactivation and that it is not due to heterogeneity of the virus population. We also show that viruses that do not fuse under suboptimal pH conditions are indeed bound to the target liposomes and that the fusion complexes they have formed are blocked at an early stage of the fusion pathway. Our conclusion is that along the fusion reaction, different kinds of fusion machines with different pH thresholds for fusion can be formed. Possible explanations of this difference of pH sensitivity are discussed.

A study of low pH-induced refolding of Env of avian sarcoma and leukosis virus into a six-helix bundle

The fusion protein of avian sarcoma and leukosis virus is likely to fold into a six-helix bundle as part of its final configuration. A peptide, R99, inhibits fusion, probably by binding into the grooves of the triple-stranded coiled coil that becomes the central core of the six-helix bundle. The stages at which the envelope protein (Env) of avian sarcoma and leukosis virus subgroup A folds into a bundle during low pH-induced fusion were determined. Effector cells expressing Env were bound to target cells expressing the cognate receptor Tva, and intermediates of fusion were created. R99 was added and the extent of fusion inhibition was used to distinguish between a prebundle state with exposed grooves and a state in which the grooves were no longer exposed. The native conformation of Env was not sensitive to R99. But adding a soluble form of Tva to effector cells conferred sensitivity. Acidic pH applied at low temperature created an intermediate state of local hemifusion. Surprisingly, R99 caused these locally hemifused membranes to separate. This indicates that the grooves of Env were still exposed, that prebundle configurations of Env stabilized hemifused states, and that binding of R99 altered the conformation of Env. In the presence of an inhibitory lipid that blocks fusion before hemifusion, applying low pH at 37 degrees C created an intermediate in which R99 was without effect. This suggests that the six-helix bundle can form before hemifusion and that subsequent conformational changes, such as formation of the trimeric hairpin, are responsible for pore formation and/or growth.

The baculovirus GP64 protein mediates highly stable infectivity of a human respiratory syncytial virus lacking its homologous transmembrane glycoproteins

Baculovirus GP64 is a low-pH-dependent membrane fusion protein required for virus entry and cell-to-cell transmission. Recently, GP64 has generated interest for practical applications in mammalian systems. Here we examined the membrane fusion function of GP64 from Autographa californica multiple nucleopolyhedrovirus (AcMNPV) expressed in mammalian cells, as well as its capacity to functionally complement a mammalian virus, human respiratory syncytial virus (HRSV). Both authentic GP64 and GP(64/F), a chimeric protein in which the GP64 cytoplasmic tail domain was replaced with the 12 C-terminal amino acids of the HRSV fusion (F) protein, induced low-pH-dependent cell-cell fusion when expressed transiently in HEp-2 (human) cells. Levels of surface expression and syncytium formation were substantially higher at 33 degrees C than at 37 degrees C. The open reading frames (ORFs) encoding GP64 or GP(64/F), along with two marker ORFs encoding green fluorescent protein (GFP) and beta-glucuronidase (GUS), were used to replace all three homologous transmembrane glycoprotein ORFs (small hydrophobic SH, attachment G, and F) in a cDNA of HRSV. Infectious viruses were recovered that lacked the HRSV SH, G, and F proteins and expressed instead the GP64 or GP(64/F) protein and the two marker proteins GFP and GUS. The properties of these viruses, designated RSDeltaSH,G,F/GP64 or RSDeltaSH,G,F/GP(64/F), respectively, were compared to a previously described HRSV expressing GFP in place of SH but still containing the wild-type HRSV G and F proteins (RSDeltaSH [A. G. Oomens, A. G. Megaw, and G. W. Wertz, J. Virol., 77:3785-3798, 2003]). By immunoelectron microscopy, the GP64 and GP(64/F) proteins were shown to incorporate into HRSV-induced filaments at the cell surface. Antibody neutralization, ammonium chloride inhibition, and replication levels in cell culture showed that both GP64 proteins efficiently mediated infectivity of the respective viruses in a temperature-sensitive, low-pH-dependent manner. Furthermore, RSDeltaSH,G,F/GP64 and RSDeltaSH,G,F/GP(64/F) replicated to higher levels and had significantly higher stability of infectivity than HRSVs containing the homologous HRSV G and F proteins. Thus, GP64 and a GP64/HRSV F chimeric protein were functional and efficiently complemented an unrelated human virus in mammalian cells, producing stable, infectious virus stocks. These results demonstrate the potential of GP64 for both practical applications requiring stable pseudotypes in mammalian systems and for studies of viral glycoprotein requirements in assembly and pathogenesis.

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.

Membrane permeability changes at early stages of influenza hemagglutinin-mediated fusion

While biological membrane fusion is classically defined as the leak-free merger of membranes and contents, leakage is a finding in both experimental and theoretical studies. The fusion stages, if any, that allow membrane permeation are uncharted. In this study we monitored membrane ionic permeability at early stages of fusion mediated by the fusogenic protein influenza hemagglutinin (HA). HAb2 cells, expressing HA on their plasma membrane, fused with human red blood cells, cultured liver cells PLC/PRF/5, or planar phospholipid bilayer membranes. With a probability that depended upon the target membrane, an increase of the electrical conductance of the fusing membranes (leakage) by up to several nS was generally detected. This leakage was recorded at the initial stages of fusion, when fusion pores formed. This leakage usually accompanied the "flickering" stage of the early fusion pore development. As the pore widened, the leakage reduced; concomitantly, the lipid exchange between the fusing membranes accelerated. We conclude that during fusion pore formation, HA locally and temporarily increases the permeability of fusing membranes. Subsequent rearrangement in the fusion complex leads to the resealing of the leaky membranes and enlargement of the pore.

Protein-Lipid Interplay in Fusion and Fission of Biological Membranes

Disparate biological processes involve fusion of two membranes into one and fission of one membrane into two. To formulate the possible job description for the proteins that mediate remodeling of biological membranes, we analyze the energy price of disruption and bending of membrane lipid bilayers at the different stages of bilayer fusion. The phenomenology and the pathways of the well-characterized reactions of biological remodeling, such as fusion mediated by influenza hemagglutinin, are compared with those studied for protein-free bilayers. We briefly consider some proteins involved in fusion and fission, and the dependence of remodeling on the lipid composition of the membranes. The specific hypothetical mechanisms by which the proteins can lower the energy price of the bilayer rearrangement are discussed in light of the experimental data and the requirements imposed by the elastic properties of the bilayer.

Progressive truncations C terminal to the membrane-spanning domain of simian immunodeficiency virus Env reduce fusogenicity and increase concentration dependence of Env for fusion

The simian immunodeficiency virus (SIV) transmembrane (TM) protein, gp41, has multiple functions, which include anchoring the glycoprotein complex in the lipid envelope of the virus and mediating fusion of the virus and host cell membranes. Recently, a series of mutants of the SIVmac239 TM protein that have truncations at the carboxyl terminus of the membrane-spanning domain (MSD) have been characterized (J. T. West, P. Johnston, S. R. Dubay, and E. Hunter, J. Virol. 75:9601-9612, 2001). These mutants retained membrane anchorage but demonstrated reduced fusogenicity and infectivity as the MSD length was shortened. We have established a novel three-color fluorescence assay, which allows qualitative confocal and quantitative flow cytometric analyses, to further characterize the nature of the fusion defect in five of the MSD mutants: TM185, TM186, TM187, TM188, and TM189. Our analysis showed that each mutant could mediate complete lipid and aqueous dye transfer at early time points after effector and target cell mixing. No hemifusion with only lipid dye flux was detected. However, another intermediate fusion stage, which appears to involve small-fusion-pore formation that allowed small aqueous dye transfer but prevented the exchange of large cytoplasmic components, was identified infrequently in mutant-Env-expressing cell and target cell mixtures. Quantitative flow cytometric analysis of these mutants demonstrated that the TM187, TM188, and TM189 mutants were significantly more fusogenic than TM185 and TM186 but remained significantly impaired compared to the wild type. Moreover, fusion efficiency showed an increased dependence on the expression level of glycoproteins, suggesting that, for these mutants, formation of an active fusion complex was an increasingly stochastic event.

Large-scale production of pseudotyped lentiviral vectors using baculovirus GP64

Unlike oncoretroviruses, lentiviral vectors can insert large genes and can target both dividing and nondividing cells; thus they hold unique promise as gene transfer agents. To enhance target range, the native lentiviral envelope glycoprotein is replaced (pseudotyped) with vesicular stomatitis virus G (VSVG), and the genes of interest are packaged in nonreplicating vectors by transient transfection with three plasmids. However, because of cytotoxic effects of VSVG expression in producer cells (293T cells) it has been difficult to generate a packaging cell line, required for even modest scale-up of vector production. Here we introduce a pseudotyped lentivirus vector using the baculovirus GP64 envelope glycoprotein. Compared with VSVG, GP64 vectors exhibited a similar broad tropism and similar native titers. GP64-pseudotyped vectors were usually highly concentrated without much loss of titer. Because, unlike VSVG, GP64 expression does not kill cells, we generated 293T-based cell lines constitutively expressing GP64. Our results demonstrate that the baculovirus GP64 protein is an attractive alternative to VSVG for viral vectors used in the large-scale production of high-titer virus required for clinical and commercial applications.

Effects of membrane potential and sphingolipid structures on fusion of Semliki Forest virus

Cells expressing the E1 and E2 envelope proteins of Semliki Forest virus (SFV) were fused to voltage-clamped planar lipid bilayer membranes at low pH. Formation and evolution of fusion pores were electrically monitored by capacitance measurements, and membrane continuity was tracked by video fluorescence microscopy by including rhodamine-phosphatidylethanolamine in the bilayer. Fusion occurred without leakage for a negative potential applied to the trans side of the planar membrane. When a positive potential was applied, leakage was severe, obscuring the observation of any fusion. E1-mediated cell-cell fusion occurred without leakage for negative intracellular potentials but with substantial leakage for zero membrane potential. Thus, negative membrane potentials are generally required for nonleaky fusion. With planar bilayers as the target, the first fusion pore that formed almost always enlarged; pore flickering was a rare event. Similar to other target membranes, fusion required cholesterol and sphingolipids in the planar membrane. Sphingosine did not support fusion, but both ceramide, with even a minimal acyl chain (C(2)-ceramide), and lysosphingomyelin (lyso-SM) promoted fusion with the same kinetics. Thus, unrelated modifications to different parts of sphingosine yielded sphingolipids that supported fusion to the same degree. Fusion studies of pyrene-labeled SFV with cholesterol-containing liposomes showed that C(2)-ceramide supported fusion while lyso-SM did not, apparently due to its positive curvature effects. A model is proposed in which the hydroxyls of C-1 and C-3 as well as N of C-2 of the sphingosine backbone must orient so as to form multiple hydrogen bonds to amino acids of SFV E1 for fusion to proceed.

Characterization of the equilibrium between the native and fusion-inactive conformation of rabies virus glycoprotein indicates that the fusion complex is made of several trimers

Rabies virus-induced membrane fusion is triggered at low pH and is mediated by the trimeric viral glycoprotein (G). G assumes three conformations: the native state (N) detected above pH 7; the activated state (A), which initiates the fusion process; and the fusion-inactive conformation (I) observed after prolonged incubation at low pH. Differently from other viral fusogenic glycoproteins, G in the I state recovers its native conformation when reincubated above pH 7. Here, we demonstrate that there is a thermodynamic equilibrium between the different states of G between pH 6 and pH 7.5. The study of this equilibrium at various pH values indicated that the conformational change toward I is induced by the protonation of at least three residues per trimer. Finally, studies on the mechanism leading to low pH induced fusion inactivation indicated that a large number of G molecules is required for stable hydrophobic interaction of the virus with the target membrane.

Characterization of a truncated soluble form of the baculovirus (AcMNPV) major envelope protein Gp64

A truncated tagged form of the Autographica californica multiple nuclear polyhedrosis virus major surface glycoprotein, gp64, has been expressed using the baculovirus expression system and purified to homogeneity by immune-affinity chromatography. The protein, which is responsible for virus-cell fusion, was a trimer in solution and retained this oligomeric form at pH 5, the pH of fusion. Circular dichroism spectroscopy indicated a protein with mixed alpha-helix and beta-sheet content that did not undergo significant change at pH 5. The soluble protein showed no detectable binding to the insect cell surface. These data suggest a novel fusion mechanism for gp64 compared to models such as the influenza HA. In a crystal screen, deglycosylated, but not glycosylated, preparations of the protein were found to form small needle-shaped crystals that may form the basis of a dedicated structural study.

Furin is involved in baculovirus envelope fusion protein activation

The Spodoptera exigua multicapsid nucleopolyhedrovirus (SeMNPV) Se8 gene was recently shown to encode the viral envelope fusion (F) protein. A 60-kDa C-terminal subunit (F1) of the 76-kDa primary translation product of this gene was found to be the major envelope protein of SeMNPV budded virus (BV) (W. F. J. IJkel, M. Westenberg, R. W. Goldbach, G. W. Blissard, J. M. Vlak, and D. Zuidema, Virology 275:30-41, 2000). A specific inhibitor was used to show that furin is involved in cleavage of the precursor envelope fusion (F0) protein. BV produced in the presence of the inhibitor possesses the uncleaved F0 protein, while an F protein with a mutation in the furin cleavage site was translocated to the plasma membrane but lost its fusogenic activity. These results indicate that cleavage of F0 is required to activate the SeMNPV F protein and is necessary for BV infectivity. Specific antibodies against F1 and against the putative N terminus (F2) of the primary translation product were used to show that the F protein is BV specific and that BVs contain both the 60- (F1) and 21-kDa (F2) cleavage products. In nonreducing sodium dodecyl sulfate-polyacrylamide gel electrophoresis both subunits migrate as a single 80-kDa protein, indicating that the subunits remain associated by a disulfide linkage. In addition, the presence of the F protein predominantly as a monomer suggests that disulfide links are not involved in oligomerization. Thus, the envelope fusion protein from group II nucleopolyhedroviruses of baculoviruses has properties similar to those of proteins from a number of vertebrate viruses.

Synchronized activation and refolding of influenza hemagglutinin in multimeric fusion machines

At the time of fusion, membranes are packed with fusogenic proteins. Do adjacent individual proteins interact with each other in the plane of the membrane? Or does each of these proteins serve as an independent fusion machine? Here we report that the low pH-triggered transition between the initial and final conformations of a prototype fusogenic protein, influenza hemagglutinin (HA), involves a preserved interaction between individual HAs. Although the HAs of subtypes H3 and H2 show notably different degrees of activation, for both, the percentage of low pH-activated HA increased with higher surface density of HA, indicating positive cooperativity. We propose that a concerted activation of HAs, together with the resultant synchronized release of their conformational energy, is an example of a general strategy of coordination in biological design, crucial for the functioning of multiprotein fusion machines.

Measurement of membrane fusion activity from viral membrane fusion proteins based on a fusion-dependent promoter induction system in insect cells

A number of viral membrane fusion proteins can be expressed alone on the surface of host cells, and then triggered to induce cell-to-cell fusion or syncytium formation. Although rapid and easily observed, syncytium formation is not easily quantified and differences in fusion activity are not easily distinguished or measured. To address this problem, we developed a rapid and quantitative cell-to-cell fusion system that is useful for comparative analysis and may be suitable for high throughput screening. In this system, expression of a reporter protein, enhanced green fluorescent protein (EGFP), is dependent on cell-to-cell fusion. Spodoptera frugiperda (Sf9) insect cells expressing a chimeric Lac repressor-IE1 protein were fused to Sf9 cells containing an EGFP reporter construct under the control of a responsive lac operator-containing promoter. Membrane fusion efficiency was measured from the resulting EGFP fluorescence activity. Sf9 cells expressing the Orgyia pseudotsugata multicapsid nucleopolyhedrovirus (OpMNPV) GP64 envelope fusion protein were used as a model to test this fusion assay. Subtle changes in fusion activities of GP64 proteins containing single amino acid substitutions in a putative membrane fusion domain were distinguished, and decreases in EGFP fluorescence corresponded to decreases in the hydrophobicity in the small putative membrane fusion domain.

A GP64-null baculovirus pseudotyped with vesicular stomatitis virus G protein

The Autographa californica multiple nucleopolyhedrovirus (AcMNPV) GP64 protein is an essential virion protein that is involved in both receptor binding and membrane fusion during viral entry. Genetic studies have shown that GP64-null viruses are unable to move from cell to cell and this results from a defect in the assembly and production of budded virions (BV). To further examine requirements for virion budding, we asked whether a GP64-null baculovirus, vAc(64-), could be pseudotyped by introducing a heterologous viral envelope protein (vesicular stomatitis virus G protein [VSV-G]) into its membrane and whether the resulting virus was infectious. To address this question, we generated a stably transfected insect Sf9 cell line (Sf9(VSV-G)) that inducibly expresses the VSV-G protein upon infection with AcMNPV Sf9(VSV-G) and Sf9 cells were infected with vAc(64-), and cells were monitored for infection and for movement of infection from cell to cell. vAc(64-) formed plaques on Sf9(VSV-G) cells but not on Sf9 cells, and plaques formed on Sf9(VSV-G) cells were observed only after prolonged intervals. Passage and amplification of vAc(64-) on Sf9(VSV-G) cells resulted in pseudotyped virus particles that contained the VSV-G protein. Cell-to-cell propagation of vAc(64-) in the G-expressing cells was delayed in comparison to wild-type (wt) AcMNPV, and growth curves showed that pseudotyped vAc(64-) was generated at titers of approximately 10(6) to 10(7) infectious units (IU)/ml, compared with titers of approximately 10(8) IU/ml for wt AcMNPV. Propagation and amplification of pseudotyped vAc(64-) virions in Sf9(VSV-G) cells suggests that the VSV-G protein may either possess the signals necessary for baculovirus BV assembly and budding at the cell surface or may otherwise facilitate production of infectious baculovirus virions. The functional complementation of GP64-null viruses by VSV-G protein was further demonstrated by identification of a vAc(64-)-derived virus that had acquired the G gene through recombination with Sf9(VSV-G) cellular DNA. GP64-null viruses expressing the VSV-G gene were capable of productive infection, replication, and propagation in Sf9 cells.

Baculovirus infection of nondividing mammalian cells: mechanisms of entry and nuclear transport of capsids

We have studied the infection pathway of Autographa californica multinuclear polyhedrosis virus (baculovirus) in mammalian cells. By titration with a baculovirus containing a green fluorescent protein cassette, we found that several, but not all, mammalian cell types can be infected efficiently. In contrast to previous suggestions, our data show that the asialoglycoprotein receptor is not required for efficient infection. We demonstrate for the first time that this baculovirus can infect nondividing mammalian cells, which implies that the baculovirus is able to transport its genome across the nuclear membrane of mammalian cells. Our data further show that the virus enters via endocytosis, followed by an acid-induced fusion event, which releases the nucleocapsid into the cytoplasm. Cytochalasin D strongly reduces the infection efficiency but not the delivery of nucleocapsids to the cytoplasm, suggesting involvement of actin filaments in cytoplasmic transport of the capsids. Electron microscopic analysis shows the cigar-shaped nucleocapsids located at nuclear pores of nondividing cells. Under these conditions, we observed the viral genome, major capsid protein, and electron-dense capsids inside the nucleus. This suggests that the nucleocapsid is transported through the nuclear pore. This mode of transport seems different from viruses with large spherical capsids, such as herpes simplex virus and adenovirus, which are disassembled before nuclear transport of the genome. The implications for the application of baculovirus or its capsid proteins in gene therapy are discussed.