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

Measuring single-virus fusion kinetics using an assay for nucleic acid exposure

The kinetics by which individual enveloped viruses fuse with membranes provide an important window into viral-entry mechanisms. We have developed a real-time assay using fluorescent probes for single-virus genome exposure than can report on stages of viral entry including or subsequent to fusion pore formation and prior to viral genome trafficking. We accomplish this using oxazole yellow nucleic-acid-binding dyes, which can be encapsulated in the lumen of target membranes to permit specific detection of fusion events. Since increased fluorescence of the dye occurs only when it encounters viral genome via a fusion pore and binds, this assay excludes content leakage without fusion. Using this assay, we show that influenza virus fuses with liposomes of different sizes with indistinguishable kinetics by both testing liposomes extruded through pores of different radii and showing that the fusion kinetics of individual liposomes are uncorrelated with the size of the liposome. These results suggest that the starting curvature of such liposomes does not control the rate-limiting steps in influenza entry.

Common characteristics and unique features: A comparison of the fusion machinery of the alphaherpesviruses Pseudorabies virus and Herpes simplex virus

Membrane fusion is a fundamental biological process that allows different cellular compartments delimited by a lipid membrane to release or exchange their respective contents. Similarly, enveloped viruses such as alphaherpesviruses exploit membrane fusion to enter and infect their host cells. For infectious entry the prototypic human Herpes simplex viruses 1 and 2 (HSV-1 and -2, collectively termed HSVs) and the porcine Pseudorabies virus (PrV) utilize four different essential envelope glycoproteins (g): the bona fide fusion protein gB and the regulatory heterodimeric gH/gL complex that constitute the "core fusion machinery" conserved in all members of the Herpesviridae; and the subfamily specific receptor binding protein gD. These four components mediate attachment and fusion of the virion envelope with the host cell plasma membrane through a tightly regulated sequential activation process. Although PrV and the HSVs are closely related and employ the same set of glycoproteins for entry, they show remarkable differences in the requirements for fusion. Whereas the HSVs strictly require all four components for membrane fusion, PrV can mediate cell-cell fusion without gD. Moreover, in contrast to the HSVs, PrV provides a unique opportunity for reversion analyses of gL-negative mutants by serial cell culture passaging, due to a limited cell-cell spread capacity of gL-negative PrV not observed in the HSVs. This allows a more direct analysis of the function of gH/gL during membrane fusion. Unraveling the molecular mechanism of herpesvirus fusion has been a goal of fundamental research for years, and yet important mechanistic details remain to be uncovered. Nevertheless, the elucidation of the crystal structures of all key players involved in PrV and HSV membrane fusion, coupled with a wealth of functional data, has shed some light on this complex puzzle. In this review, we summarize and discuss the contemporary knowledge on the molecular mechanism of entry and membrane fusion utilized by the alphaherpesvirus PrV, and highlight similarities but also remarkable differences in the requirements for fusion between PrV and the HSVs.

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.

Membrane Fusion and Infection of the Influenza Hemagglutinin

The influenza virus is a major health concern associated with an estimated 5000 to 30,000 deaths every year (Reed et al. 2015) and a significant economic impact with the development of treatments, vaccinations and research (Molinari et al. 2007). The entirety of the influenza genome is comprised of only eleven coding genes. An enormous degree of variation in non-conserved regions leads to significant challenges in the development of inclusive inhibitors for treatment. The fusion peptide domain of the influenza A hemagglutinin (HA) is a promising candidate for treatment since it is one of the most highly conserved sequences in the influenza genome (Heiny et al. 2007), and it is vital to the viral life cycle. Hemagglutinin is a class I viral fusion protein that catalyzes the membrane fusion process during cellular entry and infection. Impediment of the hemagglutinin's function, either through incomplete post-translational processing (Klenk et al. 1975; Lazarowitz and Choppin 1975) or through mutations (Cross et al. 2001), leads to non-infective virus particles. This review will investigate current research on the role of hemagglutinin in the virus life cycle, its structural biology and mechanism as well as the central role of the hemagglutinin fusion peptide (HAfp) to influenza membrane fusion and infection.

Mutations of two transmembrane cysteines of hemagglutinin (HA) from influenza A H3N2 virus affect HA thermal stability and fusion activity

Influenza A H3N2 virus caused 1968 Hong Kong influenza pandemic, and has since been one of the most prevalent seasonal influenza viruses in global populations, representing a credible pandemic candidate in future. Previous studies have established that the hemagglutinin (HA) protein is the predominant antigen and executes receptor binding and membrane fusion. Homologous sequence analysis of all HA subtypes of influenza viruses revealed that two cysteine residues (540 and 544) are uniquely present in the transmembrane domain (TM) of HA proteins from all influenza A H3N2 viruses. However, the functions of these two cysteines have not been fully studied. Here, we generated three mutants (C540S, C544L, and 2C/SL) to investigate the effects of the two TM cysteines on the biological functions of H3 HA. We herein presented evidences that the mutations of one or two of the cysteines did not affect the proper expressions of HA proteins in cells, and more importantly all mutant H3 HAs showed decreased thermal stability but increased fusion activity in comparison with wildtype HA. Our results taken together demonstrated that the two TM cysteines are important for the biological functions of H3 HA proteins.

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.

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.

The final conformation of the complete ectodomain of the HA2 subunit of influenza hemagglutinin can by itself drive low pH-dependent fusion

One of the best characterized fusion proteins, the influenza virus hemagglutinin (HA), mediates fusion between the viral envelope and the endosomal membrane during viral entry into the cell. In the initial conformation of HA, its fusogenic subunit, the transmembrane protein HA2, is locked in a metastable conformation by the receptor-binding HA1 subunit of HA. Acidification in the endosome triggers HA2 refolding toward the final lowest energy conformation. Is the fusion process driven by this final conformation or, as often suggested, by the energy released by protein restructuring? Here we explored structural properties as well as the fusogenic activity of the full sized trimeric HA2(1-185) (here called HA2*) that presents the final conformation of the HA2 ectodomain. We found HA2* to mediate fusion between lipid bilayers and between biological membranes in a low pH-dependent manner. Two mutations known to inhibit HA-mediated fusion strongly inhibited the fusogenic activity of HA2*. At surface densities similar to those of HA in the influenza virus particle, HA2* formed small fusion pores but did not expand them. Our results confirm that the HA1 subunit responsible for receptor binding as well as the transmembrane and cytosolic domains of HA2 is not required for fusion pore opening and substantiate the hypothesis that the final form of HA2 is more important for fusion than the conformational change that generates this form.

Cooperative elastic stresses, the hydrophobic effect, and lipid tilt in membrane remodeling

One of the fundamental properties of biological membranes is the high lateral integrity provided by the lipid bilayer, the structural core and the foundation of their barrier function. This tensile strength is due to the intrinsic properties of amphiphilic lipid molecules, which spontaneously self-assemble into a stable bilayer structure due to the hydrophobic effect. In the highly dynamic life of cellular membranes systems, however, this integrity has to be regularly compromised. One of the emerging puzzles is the mechanism of localized rupture of lipid monolayer, the formation of tiny hydrophobic patches and flipping of lipid tails between closely apposed monolayers. The energy cost of such processes is prohibitively high, unless cooperative deformations in a small membrane patch are carefully organized. Here we review the latest experimental and theoretical data on how such deformations can be conducted, specifically describing how elastic stresses yield tilting of lipids leading to cooperative restructuring of lipid monolayers. Proteins specializing in membrane remodeling assemble into closely packed circular complexes to arrange these deformations in time and space.

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.