Sequential conformational rearrangements in flavivirus membrane fusion

Neurodegenerative disorders share common features of "loss of function" states of a proposed mechanism of nervous system functions

Neurodegenerative disorders are highly heterogeneous for the locations affected and the nature of the aggregated proteins. Nearly 80% of the neurodegenerative disorders occur sporadically, indicating that certain factors must combine to initiate the degenerative changes. The contiguous extension of degenerative changes from cell to cell, the association with viral fusion proteins, loss of dendritic spines (postsynaptic terminals), and the eventual degeneration of cells indicate the presence of a unique mechanism for inter-cellular spread of pathology. It is not known whether the "loss of function" states of the still unknown normal nervous system operations can lead to neurodegenerative disorders. Here, the possible loss of function states of a proposed normal nervous system function are examined. A reversible inter-postsynaptic functional LINK (IPL) mechanism, consisting of transient inter-postsynaptic membrane (IPM) hydration exclusion and partial to complete IPM hemifusions, was proposed as a critical step necessary for the binding process and the induction of internal sensations of higher brain functions. When various findings from different neurodegenerative disorders are systematically organized and examined, disease features match the effects of loss of function states of different IPLs. Changes in membrane composition, enlargement of dendritic spines by dopamine and viral fusion proteins are capable of altering the IPLs to form IPM fusion. The latter can lead to the observed lateral spread of pathology, inter-neuronal cytoplasmic content mixing and abnormal protein aggregation. Since both the normal mechanism of reversible IPM hydration exclusion and the pathological process of transient IPM fusion can evade detection, testing their occurrence may provide preventive and therapeutic opportunities for these disorders.

Structure of a phleboviral envelope glycoprotein reveals a consolidated model of membrane fusion

An emergent viral pathogen termed severe fever with thrombocytopenia syndrome virus (SFTSV) is responsible for thousands of clinical cases and associated fatalities in China, Japan, and South Korea. Akin to other phleboviruses, SFTSV relies on a viral glycoprotein, Gc, to catalyze the merger of endosomal host and viral membranes during cell entry. Here, we describe the postfusion structure of SFTSV Gc, revealing that the molecular transformations the phleboviral Gc undergoes upon host cell entry are conserved with otherwise unrelated alpha- and flaviviruses. By comparison of SFTSV Gc with that of the prefusion structure of the related Rift Valley fever virus, we show that these changes involve refolding of the protein into a trimeric state. Reverse genetics and rescue of site-directed histidine mutants enabled localization of histidines likely to be important for triggering this pH-dependent process. These data provide structural and functional evidence that the mechanism of phlebovirus-host cell fusion is conserved among genetically and patho-physiologically distinct viral pathogens.

Fusion of Enveloped Viruses in Endosomes

Ari Helenius launched the field of enveloped virus fusion in endosomes with a seminal paper in the Journal of Cell Biology in 1980. In the intervening years, a great deal has been learned about the structures and mechanisms of viral membrane fusion proteins as well as about the endosomes in which different enveloped viruses fuse and the endosomal cues that trigger fusion. We now recognize three classes of viral membrane fusion proteins based on structural criteria and four mechanisms of fusion triggering. After reviewing general features of viral membrane fusion proteins and viral fusion in endosomes, we delve into three characterized mechanisms for viral fusion triggering in endosomes: by low pH, by receptor binding plus low pH and by receptor binding plus the action of a protease. We end with a discussion of viruses that may employ novel endosomal fusion‐triggering mechanisms. A key take‐home message is that enveloped viruses that enter cells by fusing in endosomes traverse the endocytic pathway until they reach an endosome that has all of the environmental conditions (pH, proteases, ions, intracellular receptors and lipid composition) to (if needed) prime and (in all cases) trigger the fusion protein and to support membrane fusion.

Distinct functional determinants of influenza hemagglutinin-mediated membrane fusion

Membrane fusion is the critical step for infectious cell penetration by enveloped viruses. We have previously used single-virion measurements of fusion kinetics to study the molecular mechanism of influenza-virus envelope fusion. Published data on fusion inhibition by antibodies to the 'stem' of influenza virus hemagglutinin (HA) now allow us to incorporate into simulations the provision that some HAs are inactive. We find that more than half of the HAs are unproductive even for virions with no bound antibodies, but that the overall mechanism is extremely robust. Determining the fraction of competent HAs allows us to determine their rates of target-membrane engagement. Comparison of simulations with data from H3N2 and H1N1 viruses reveals three independent functional variables of HA-mediated membrane fusion closely linked to neutralization susceptibility. Evidence for compensatory changes in the evolved mechanism sets the stage for studies aiming to define the molecular constraints on HA evolvability.

Interplay between the Herpes Simplex Virus 1 gB Cytodomain and the gH Cytotail during Cell-Cell Fusion

Herpesvirus entry into cells is mediated by the viral fusogen gB, which is thought to refold from the prefusion to the postfusion form in a series of large conformational changes that energetically couple refolding to membrane fusion. In contrast to most viral fusogens, gB requires a conserved heterodimer, gH/gL, as well as other nonconserved proteins. In a further mechanistic twist, gB-mediated cell-cell fusion appears restricted by its intraviral or cytoplasmic domain (cytodomain) because mutations within it result in a hyperfusogenic phenotype. Here, we characterized a panel of hyperfusogenic HSV-1 gB cytodomain mutants and show that they are fully functional in cell-cell fusion at shorter coincubation times and at lower temperatures than those for wild-type (WT) gB, which suggests that these mutations reduce the kinetic energy barrier to fusion. Despite this, the mutants require both gH/gL and gD. We confirm previous observations that the gH cytotail is an essential component of the cell-cell fusion mechanism and show that the N-terminal portion of the gH cytotail is critical for this process. Moreover, the fusion levels achieved by all gB constructs, WT and mutant, were proportionate to the length of the gH cytotail. Putting these results together, we propose that the gH cytotail, in addition to the gH/gL ectodomain, plays an essential role in gB activation, potentially acting as a "wedge" to release the gB cytodomain "clamp" and enable gB activation.

IMPORTANCE

Herpesviruses infect their hosts for life and cause a substantial disease burden. Herpes simplex viruses cause oral and genital sores as well as rare yet severe encephalitis and a panoply of ocular ailments. Infection initiates when the viral envelope fuses with the host cell membrane in a process orchestrated by the viral fusogen gB, assisted by the viral glycoproteins gH, gL, and gD and a cellular gD receptor. This process is more complicated than that of most other viruses and is subject to multiple regulatory inputs. Antiviral and vaccine development would benefit from a detailed mechanistic knowledge of this process and how it is regulated.

Viral membrane fusion

Membrane fusion is an essential step when enveloped viruses enter cells. Lipid bilayer fusion requires catalysis to overcome a high kinetic barrier; viral fusion proteins are the agents that fulfill this catalytic function. Despite a variety of molecular architectures, these proteins facilitate fusion by essentially the same generic mechanism. Stimulated by a signal associated with arrival at the cell to be infected (e.g., receptor or co-receptor binding, proton binding in an endosome), they undergo a series of conformational changes. A hydrophobic segment (a "fusion loop" or "fusion peptide") engages the target-cell membrane and collapse of the bridging intermediate thus formed draws the two membranes (virus and cell) together. We know of three structural classes for viral fusion proteins. Structures for both pre- and postfusion conformations of illustrate the beginning and end points of a process that can be probed by single-virion measurements of fusion kinetics.

Physico-chemical requirements and kinetics of membrane fusion of flavivirus-like particles

Flaviviruses deliver their RNA genome into the host-cell cytoplasm by fusing their lipid envelope with a cellular membrane. Expression of the flavivirus pre-membrane and envelope glycoprotein genes in the absence of other viral genes results in the spontaneous assembly and secretion of virus-like particles (VLPs) with membrane fusion activity. Here, we examined the physico-chemical requirements for membrane fusion of VLPs from West Nile and Japanese encephalitis viruses. In a bulk fusion assay, optimal hemifusion (or lipid mixing) efficiencies were observed at 37 °C. Fusion efficiency increased with decreasing pH; half-maximal hemifusion was attained at pH 5.6. The anionic lipids bis(monoacylglycero)phosphate and phosphatidylinositol-3-phosphate, when present in the target membrane, significantly enhanced fusion efficiency, consistent with the emerging model that flaviviruses fuse with intermediate-to-late endosomal compartments, where these lipids are most abundant. In a single-particle fusion assay, VLPs catalysed membrane hemifusion, tracked as lipid mixing with the cellular membrane, on a timescale of 7–20 s after acidification. Lipid mixing kinetics suggest that hemifusion is a kinetically complex, multistep process.