Structure of the St. Louis encephalitis virus postfusion envelope trimer

Impact of structural dynamics on biological functions of flaviviruses

Flaviviruses comprise a number of mosquito- or tick-transmitted human pathogens of global public health importance. Advances in structural biology techniques have contributed substantially to our current understanding of the life cycle of these small enveloped RNA viruses and led to deep insights into details of virus assembly, maturation and cell entry. In addition to large-scale conformational changes and oligomeric rearrangements of envelope proteins during these processes, there is increasing evidence that smaller-scale protein dynamics (referred to as virus "breathing") can confer extra flexibility to these viruses for the fine-tuning of their interactions with the immune system and possibly with cellular factors they encounter in their complex ecological cycles in arthropod and vertebrate hosts. In this review, we discuss how work with tick-borne encephalitis virus has extended our view on flavivirus breathing, leading to the identification of a novel mechanism of antibody-mediated infection enhancement and demonstrating breathing intermediates of the envelope protein in the process of membrane fusion. These data are discussed in the context of other flaviviruses and the perspective of a potential role of virus breathing to cope with the requirements of adaptation and replication in evolutionarily very different hosts.

Isolation of a novel insect-specific flavivirus with immunomodulatory effects in vertebrate systems

We describe the isolation and characterization of a novel insect-specific flavivirus (ISFV), tentatively named Aripo virus (ARPV), that was isolated from Psorophora albipes mosquitoes collected in Trinidad. The ARPV genome was determined and phylogenetic analyses showed that it is a dual host associated ISFV, and clusters with the main mosquito-borne flaviviruses. ARPV antigen was significantly cross-reactive with Japanese encephalitis virus serogroup antisera, with significant cross-reactivity to Ilheus and West Nile virus (WNV). Results suggest that ARPV replication is limited to mosquitoes, as it did not replicate in the sandfly, culicoides or vertebrate cell lines tested. We also demonstrated that ARPV is endocytosed into vertebrate cells and is highly immunomodulatory, producing a robust innate immune response despite its inability to replicate in vertebrate systems. We show that prior infection or coinfection with ARPV limits WNV-induced disease in mouse models, likely the result of a robust ARPV-induced type I interferon response.

Identification of Zika Virus Inhibitors Using Homology Modeling and Similarity-Based Screening to Target Glycoprotein E

The World Health Organization has designated Zika virus (ZIKV) as a dangerous, mosquito-borne pathogen that can cause severe developmental defects. The primary goal of this work was identification of small molecules as potential ZIKV inhibitors that target the viral envelope glycoprotein (ZIKV E) involved in membrane fusion and viral entry. A homology model of ZIKV E containing the small molecule β-octyl glucoside (BOG) was constructed, on the basis of an analogous X-ray structure from dengue virus, and >4 million commercially available compounds were computationally screened using the program DOCK6. A key feature of the screen involved the use of similarity-based scoring to identify inhibitor candidates that make similar interaction energy patterns (molecular footprints) as the BOG reference. Fifty-three prioritized compounds underwent experimental testing using cytotoxicity, cell viability, and tissue culture infectious dose 50% (TCID50) assays. Encouragingly, relative to a known control (NITD008), six compounds were active in both the cell viability assay and the TCID50 infectivity assay, and they showed activity in a third caspase activity assay. In particular, compounds 8 and 15 (tested at 25 μM) and compound 43 (tested at 10 μM) appeared to provide significant protection to infected cells, indicative of anti-ZIKV activity. Overall, the study highlights how similarity-based scoring can be leveraged to computationally identify potential ZIKV E inhibitors that mimic a known reference (in this case BOG), and the experimentally verified hits provide a strong starting point for further refinement and optimization efforts.

The Hantavirus Surface Glycoprotein Lattice and Its Fusion Control Mechanism

Hantaviruses are rodent-borne viruses causing serious zoonotic outbreaks worldwide for which no treatment is available. Hantavirus particles are pleomorphic and display a characteristic square surface lattice. The envelope glycoproteins Gn and Gc form heterodimers that further assemble into tetrameric spikes, the lattice building blocks. The glycoproteins, which are the sole targets of neutralizing antibodies, drive virus entry via receptor-mediated endocytosis and endosomal membrane fusion. Here we describe the high-resolution X-ray structures of the heterodimer of Gc and the Gn head and of the homotetrameric Gn base. Docking them into an 11.4-Å-resolution cryoelectron tomography map of the hantavirus surface accounted for the complete extramembrane portion of the viral glycoprotein shell and allowed a detailed description of the surface organization of these pleomorphic virions. Our results, which further revealed a built-in mechanism controlling Gc membrane insertion for fusion, pave the way for immunogen design to protect against pathogenic hantaviruses.

Extensive flavivirus E trimer breathing accompanies stem zippering of the post-fusion hairpin

Flaviviruses enter cells by fusion with endosomal membranes through a rearrangement of the envelope protein E, a class II membrane fusion protein, into fusogenic trimers. The rod-like E subunits bend into "hairpins" to bring the fusion loops next to the C-terminal transmembrane (TM) anchors, with the TM-proximal "stem" element zippering the E trimer to force apposition of the membranes. The structure of the complete class II trimeric hairpin is known for phleboviruses but not for flaviviruses, for which the stem is only partially resolved. Here, we performed comparative analyses of E-protein trimers from the tick-borne encephalitis flavivirus with sequential stem truncations. Our thermostability and antibody-binding data suggest that the stem "zipper" ends at a characteristic flavivirus conserved sequence (CS) that cloaks the fusion loops, with the downstream segment not contributing to trimer stability. We further identified a highly dynamic behavior of E trimers C-terminally truncated upstream the CS, which, unlike fully stem-zippered trimers, undergo rapid deuterium exchange at the trimer interface. These results thus identify important "breathing" intermediates in the E-protein-driven membrane fusion process.

Double Lock of a Human Neutralizing and Protective Monoclonal Antibody Targeting the Yellow Fever Virus Envelope

Yellow fever virus (YFV), a deadly human pathogen, is the prototype of the genus Flavivirus. Recently, YFV re-emerged in Africa and Brazil, leading to hundreds of deaths, with some cases imported to China. Prophylactic or therapeutic countermeasures are urgently needed. Previously, several human monoclonal antibodies against YFV were screened out by phage display. Here, we find that one of them, 5A, exhibits high neutralizing potency and good protection. Crystallographic analysis of the YFV envelope (E) protein in its pre- and post-fusion states shows conformations similar to those observed in other E proteins of flaviviruses. Furthermore, the structures of 5A in complex with the E protein in both states are resolved, revealing an invariant recognition site. Structural analysis and functional data suggest that 5A has high neutralization potency because it interferes with virus entry by preventing both virus attachment and fusion. These findings will be instrumental for immunogen or inhibitor design.

CD4 T Cell Determinants in West Nile Virus Disease and Asymptomatic Infection

West Nile (WN) virus infection of humans is frequently asymptomatic, but can also lead to WN fever or neuroinvasive disease. CD4 T cells and B cells are critical in the defense against WN virus, and neutralizing antibodies, which are directed against the viral glycoprotein E, are an accepted correlate of protection. For the efficient production of these antibodies, B cells interact directly with CD4 helper T cells that recognize peptides from E or the two other structural proteins (capsid-C and membrane-prM/M) of the virus. However, the specific protein sites yielding such helper epitopes remain unknown. Here, we explored the CD4 T cell response in humans after WN virus infection using a comprehensive library of overlapping peptides covering all three structural proteins. By measuring T cell responses in 29 individuals with either WN virus disease or asymptomatic infection, we showed that CD4 T cells focus on peptides in specific structural elements of C and at the exposed surface of the pre- and postfusion forms of the E protein. Our data indicate that these immunodominant epitopes are recognized in the context of multiple different HLA molecules. Furthermore, we observed that immunodominant antigen regions are structurally conserved and similarly targeted in other mosquito-borne flaviviruses, including dengue, yellow fever, and Zika viruses. Together, these findings indicate a strong impact of virion protein structure on epitope selection and antigenicity, which is an important issue to consider in future vaccine design.

Roles of Pro-viral Host Factors in Mosquito-Borne Flavivirus Infections

Identification and analysis of viral host factors is a growing area of research which aims to understand the how viruses molecularly interface with the host cell. Investigations into flavivirus-host interactions has led to new discoveries in viral and cell biology, and will potentially bolster strategies to control the important diseases caused by these pathogens. Here, we address the current knowledge of prominent host factors required for the flavivirus life-cycle and mechanisms by which they promote infection.

Molecular Basis of a Protective/Neutralizing Monoclonal Antibody Targeting Envelope Proteins of both Tick-Borne Encephalitis Virus and Louping Ill Virus

Tick-borne encephalitis virus (TBEV) and louping ill virus (LIV) are members of the tick-borne flaviviruses (TBFVs) in the family Flaviviridae which cause encephalomeningitis and encephalitis in humans and other animals. Although vaccines against TBEV and LIV are available, infection rates are rising due to the low vaccination coverage. To date, no specific therapeutics have been licensed. Several neutralizing monoclonal antibodies (MAbs) show promising effectiveness in the control of TBFVs, but the underlying molecular mechanisms are yet to be characterized. Here, we determined the crystal structures of the LIV envelope (E) protein and report the comparative structural analysis of a TBFV broadly neutralizing murine MAb (MAb 4.2) in complex with either the LIV or TBEV E protein. The structures reveal that MAb 4.2 binds to the lateral ridge of domain III of the E protein (EDIII) of LIV or TBEV, an epitope also reported for other potently neutralizing MAbs against mosquito-borne flaviviruses (MBFVs), but adopts a unique binding orientation. Further structural analysis suggested that MAb 4.2 may neutralize flavivirus infection by preventing the structural rearrangement required for membrane fusion during virus entry. These findings extend our understanding of the vulnerability of TBFVs and other flaviviruses (including MBFVs) and provide an avenue for antibody-based TBFV antiviral development.IMPORTANCE Understanding the mechanism of antibody neutralization/protection against a virus is crucial for antiviral countermeasure development. Tick-borne encephalitis virus (TBEV) and louping ill virus (LIV) are tick-borne flaviviruses (TBFVs) in the family Flaviviridae They cause encephalomeningitis and encephalitis in humans and other animals. Although vaccines for both viruses are available, infection rates are rising due to low vaccination coverage. In this study, we solved the crystal structures of the LIV envelope protein (E) and a broadly neutralizing/protective TBFV MAb, MAb 4.2, in complex with E from either TBEV or LIV. Key structural features shared by TBFV E proteins were analyzed. The structures of E-antibody complexes showed that MAb 4.2 targets the lateral ridge of both the TBEV and LIV E proteins, a vulnerable site in flaviviruses for other potent neutralizing MAbs. Thus, this site represents a promising target for TBFV antiviral development. Further, these structures provide important information for understanding TBFV antigenicity.

Fusion surface structure, function, and dynamics of gamete fusogen HAP2

HAP2 is a class II gamete fusogen in many eukaryotic kingdoms. A crystal structure of Chlamydomonas HAP2 shows a trimeric fusion state. Domains D1, D2.1 and D2.2 line the 3-fold axis; D3 and a stem pack against the outer surface. Surprisingly, hydrogen-deuterium exchange shows that surfaces of D1, D2.2 and D3 closest to the 3-fold axis are more dynamic than exposed surfaces. Three fusion helices in the fusion loops of each monomer expose hydrophobic residues at the trimer apex that are splayed from the 3-fold axis, leaving a solvent-filled cavity between the fusion loops in each monomer. At the base of the two fusion loops, Arg185 docks in a carbonyl cage. Comparisons to other structures, dynamics, and the greater effect on Chlamydomonas gamete fusion of mutation of axis-proximal than axis-distal fusion helices suggest that the apical portion of each monomer could tilt toward the 3-fold axis with merger of the fusion helices into a common fusion surface.

Early Events in Japanese Encephalitis Virus Infection: Viral Entry

Japanese encephalitis virus (JEV), a mosquito-borne zoonotic flavivirus, is an enveloped positive-strand RNA virus that can cause a spectrum of clinical manifestations, ranging from mild febrile illness to severe neuroinvasive disease. Today, several killed and live vaccines are available in different parts of the globe for use in humans to prevent JEV-induced diseases, yet no antivirals are available to treat JEV-associated diseases. Despite the progress made in vaccine research and development, JEV is still a major public health problem in southern, eastern, and southeastern Asia, as well as northern Oceania, with the potential to become an emerging global pathogen. In viral replication, the entry of JEV into the cell is the first step in a cascade of complex interactions between the virus and target cells that is required for the initiation, dissemination, and maintenance of infection. Because this step determines cell/tissue tropism and pathogenesis, it is a promising target for antiviral therapy. JEV entry is mediated by the viral glycoprotein E, which binds virions to the cell surface (attachment), delivers them to endosomes (endocytosis), and catalyzes the fusion between the viral and endosomal membranes (membrane fusion), followed by the release of the viral genome into the cytoplasm (uncoating). In this multistep process, a collection of host factors are involved. In this review, we summarize the current knowledge on the viral and cellular components involved in JEV entry into host cells, with an emphasis on the initial virus-host cell interactions on the cell surface.

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

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

Structural Basis for CD4+ T Cell Epitope Dominance in Arbo-Flavivirus Envelope Proteins: A Meta-Analysis

A meta-analysis of CD4+ T cell epitope maps reveals clusters and gaps in envelope-protein (E protein) immunogenicity that can be explained by the likelihood of epitope processing, as determined by E protein three-dimensional structures. Differential processing may be at least partially responsible for variations in disease severity among arbo-flaviruses and points to structural features that modulate protection from disease.

The Ancient Gamete Fusogen HAP2 Is a Eukaryotic Class II Fusion Protein

Sexual reproduction is almost universal in eukaryotic life and involves the fusion of male and female haploid gametes into a diploid cell. The sperm-restricted single-pass transmembrane protein HAP2-GCS1 has been postulated to function in membrane merger. Its presence in the major eukaryotic taxa—animals, plants, and protists (including important human pathogens like Plasmodium)—suggests that many eukaryotic organisms share a common gamete fusion mechanism. Here, we report combined bioinformatic, biochemical, mutational, and X-ray crystallographic studies on the unicellular alga Chlamydomonas reinhardtii HAP2 that reveal homology to class II viral membrane fusion proteins. We further show that targeting the segment corresponding to the fusion loop by mutagenesis or by antibodies blocks gamete fusion. These results demonstrate that HAP2 is the gamete fusogen and suggest a mechanism of action akin to viral fusion, indicating a way to block Plasmodium transmission and highlighting the impact of virus-cell genetic exchanges on the evolution of eukaryotic life.

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The primordial gamete fusogen HAP2 exhibits homology to class II viral fusion proteins

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HAP2 inserts into the target gamete membrane via a hydrophobic fusion loop

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HAP2 links virus entry into target cells and the origins of sexual reproduction

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HAP2 is a sex-specific target for blocking fertilization in multiple kingdoms

Near-atomic structure of Japanese encephalitis virus reveals critical determinants of virulence and stability

Although several different flaviviruses may cause encephalitis, Japanese encephalitis virus is the most significant, being responsible for thousands of deaths each year in Asia. The structural and molecular basis of this encephalitis is not fully understood. Here, we report the cryo-electron microscopy structure of mature Japanese encephalitis virus at near-atomic resolution, which reveals an unusual “hole” on the surface, surrounded by five encephalitic-specific motifs implicated in receptor binding. Glu138 of E, which is highly conserved in encephalitic flaviviruses, maps onto one of these motifs and is essential for binding to neuroblastoma cells, with the E138K mutation abrogating the neurovirulence and neuroinvasiveness of Japanese encephalitis virus in mice. We also identify structural elements modulating viral stability, notably Gln264 of E, which, when replaced by His264 strengthens a hydrogen-bonding network, leading to a more stable virus. These studies unveil determinants of neurovirulence and stability in Japanese encephalitis virus, opening up new avenues for therapeutic interventions against neurotropic flaviviruses.

Japanese encephalitis virus (JEV) is a Flavivirus responsible for thousands of deaths every year for which there are no specific anti-virals. Here, Wang et al. report the cryo-EM structure of mature JEV at near-atomic resolution and identify structural elements that modulate stability and virulence.

Crystal Structure of Glycoprotein C from a Hantavirus in the Post-fusion Conformation

Hantaviruses are important emerging human pathogens and are the causative agents of serious diseases in humans with high mortality rates. Like other members in the Bunyaviridae family their M segment encodes two glycoproteins, G_N and G_C, which are responsible for the early events of infection. Hantaviruses deliver their tripartite genome into the cytoplasm by fusion of the viral and endosomal membranes in response to the reduced pH of the endosome. Unlike phleboviruses (e.g. Rift valley fever virus), that have an icosahedral glycoprotein envelope, hantaviruses display a pleomorphic virion morphology as G_N and G_C assemble into spikes with apparent four-fold symmetry organized in a grid-like pattern on the viral membrane. Here we present the crystal structure of glycoprotein C (G_C) from Puumala virus (PUUV), a representative member of the Hantavirus genus. The crystal structure shows G_C as the membrane fusion effector of PUUV and it presents a class II membrane fusion protein fold. Furthermore, G_C was crystallized in its post-fusion trimeric conformation that until now had been observed only in Flavi- and Togaviridae family members. The PUUV G_C structure together with our functional data provides intriguing evolutionary and mechanistic insights into class II membrane fusion proteins and reveals new targets for membrane fusion inhibitors against these important pathogens.

Hantaviruses (family: Bunyaviridae) encompass pathogens responsible to serious human diseases and economic burden worldwide. Following endocytosis, these enveloped RNA viruses are directed to an endosomal compartment where a sequence of pH-dependent conformational changes of the viral envelope glycoproteins mediates the fusion between the viral and endosomal membranes. The lack of high-resolution structural information for the entry of hantaviruses impair our ability to rationalize new treatments and prevention strategies. We determined the three-dimensional structure of a glycoprotein C from Puumala virus (PUUV) using X-ray crystallography. The two structures (at pH 6.0 and 8.0) were determined to 1.8 Å and 2.3 Å resolutions, respectively. Both structures reveal a class II membrane fusion protein in its post-fusion trimeric conformation with novel structural features in the trimer assembly and stabilization. Our structures suggest that neutralizing antibodies against G_C target its conformational changes as inhibition mechanism and highlight new molecular targets for hantavirus-specific membrane fusion inhibitors. Furthermore, combined with the available structures of other class II proteins, we remodeled the evolutionary relationships between virus families encompassing these proteins.

A humanized monoclonal antibody neutralizes yellow fever virus strain 17D-204 in vitro but does not protect a mouse model from disease

The yellow fever virus (YFV) vaccine 17D-204 is considered safe and effective, yet rare severe adverse events (SAEs), some resulting in death, have been documented following vaccination. Individuals exhibiting post-vaccinal SAEs are ideal candidates for antiviral monoclonal antibody (MAb) therapy; the time until appearance of clinical signs post-exposure is usually short and patients are quickly hospitalized. We previously developed a murine-human chimeric monoclonal antibody (cMAb), 2C9-cIgG, reactive with both virulent YFV and 17D-204, and demonstrated its ability to prevent and treat YF disease in both AG129 mouse and hamster models of infection. To counteract possible selection of 17D-204 variants that escape neutralization by treatment with a single MAb (2C9-cIgG), we developed a second cMAb, 864-cIgG, for use in combination with 2C9-cIgG in post-vaccinal therapy. MAb 864-cIgG recognizes/neutralizes only YFV 17D-204 vaccine substrain and binds to domain III (DIII) of the viral envelope protein, which is different from the YFV type-specific binding site of 2C9-cIgG in DII. Although it neutralized 17D-204 in vitro, administration of 864-cIgG had no protective capacity in the interferon receptor-deficient AG129 mouse model of 17D-204 infection. The data presented here show that although DIII-specific 864-cIgG neutralizes virus infectivity in vitro, it does not have the ability to abrogate disease in vivo. Therefore, combination of 864-cIgG with 2C9-cIgG for treatment of YF vaccination SAEs does not appear to provide an improvement on 2C9-cIgG therapy alone.

B cell response and mechanisms of antibody protection to West Nile virus

West Nile virus (WNV) has become the principal cause of viral encephalitis in North America since its introduction in New York in 1999. This emerging virus is transmitted to humans via the bite of an infected mosquito. While there have been several candidates in clinical trials, there are no approved vaccines or WNV-specific therapies for the treatment of WNV disease in humans. From studies with small animal models and convalescent human patients, a great deal has been learned concerning the immune response to infection with WNV. Here, we provide an overview of a subset of that information regarding the humoral and antibody response generated during WNV infection.

Relating structure to evolution in class II viral membrane fusion proteins

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Until 2013, class II proteins had only been found in flaviviruses and alphaviruses.

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A class II fusion protein was recently discovered in the unrelated phlebovirus genus.

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Within the same family as alphaviruses, rubella virus has a divergent class II fold.

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Pestiviruses, although they are Flaviviridae, have fusion proteins from a novel class.

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Viral class II proteins may originate from cellular class II fusion protein ancestors.

Enveloped viruses must fuse their lipid membrane to a cellular membrane to deliver the viral genome into the cytoplasm for replication. Viral envelope proteins catalyze this critical membrane fusion event. They fall into at least three distinct structural classes. Class II fusion proteins have a conserved three-domain architecture and are found in many important viral pathogens. Until 2013, class II proteins had only been found in flaviviruses and alphaviruses. However, in 2013 a class II fusion protein was discovered in the unrelated phlebovirus genus, and two unexpectedly divergent envelope proteins were identified in families that also contain prototypical class II proteins. The structural relationships of newly identified class II proteins, reviewed herein, shift the paradigm for how these proteins evolved.

Atomic-level functional model of dengue virus Envelope protein infectivity

A number of structures have been solved for the Envelope (E) protein from dengue virus and closely related flaviviruses, providing detailed pictures of the conformational states of the protein at different stages of infectivity. However, the key functional residues responsible for mediating the dynamic changes between these structures remain largely unknown. Using a comprehensive library of functional point mutations covering all 390 residues of the dengue virus E protein ectodomain, we identified residues that are critical for virus infectivity, but that do not affect E protein expression, folding, virion assembly, or budding. The locations and atomic interactions of these critical residues within different structures representing distinct fusogenic conformations help to explain how E protein (i) regulates fusion-loop exposure by shielding, tethering, and triggering its release; (ii) enables hinge movements between E domain interfaces during triggered structural transformations; and (iii) drives membrane fusion through late-stage zipper contacts with stem. These results provide structural targets for drug and vaccine development and integrate the findings from structural studies and isolated mutagenesis efforts into a cohesive model that explains how specific residues in this class II viral fusion protein enable virus infectivity.

Structure of a dengue virus envelope protein late-stage fusion intermediate

The final stages of dengue virus fusion are thought to occur when the membrane-proximal stem drives the transmembrane anchor of the viral envelope protein (E) toward the fusion loop, buried in the target cell membrane. Crystal structures of E have lacked this essential stem region. We expressed and crystallized soluble mutant forms of the dengue virus envelope protein (sE) that include portions of the juxtamembrane stem. Their structures represent late-stage fusion intermediates. The proximal part of the stem has both intra- and intermolecular interactions, so the chain "zips up" along the trimer seam. The penultimate interaction we detected involves the conserved residue F402, which has hydrophobic contacts with a conserved surface on domain II. These interactions do not require any larger-scale changes in trimer packing. The techniques for expression and crystallization of sE containing stem reported here may allow further characterization of the final stages of flavivirus fusion.