A novel membrane fusion protein family in Flaviviridae?
2014; Elsevier BV; Volume: 22; Issue: 4 Linguagem: Inglês
10.1016/j.tim.2014.01.008
ISSN1878-4380
Autores Tópico(s)Influenza Virus Research Studies
Resumo•Viral membrane fusion proteins fall into three distinct structural classes (I–III).•In 2013, the E2 glycoproteins of pesti- and hepaciviruses were found to have novel folds.•E1 and E2 proteins of pesti- and hepaciviruses define a new class of fusion machinery.•Structural data suggest that fusion proteins evolved by host–virus or virus–virus transfer. Enveloped viruses must fuse their lipid membrane to a cellular membrane to deliver their genome into the cytoplasm for replication. Viral envelope proteins catalyze this critical membrane fusion event. They fall into three distinct structural classes. In 2013, envelope proteins from a pestivirus and hepatitis C virus were found to have two distinct novel folds. This was unexpected because these viruses are in the same family as flaviviruses, which have class II fusion proteins. We propose that the membrane fusion machinery of the closely related pestiviruses and hepatitis C virus defines a new structural class. This and other recently identified structural relationships between viral fusion proteins shift the paradigm for how these proteins evolved. Enveloped viruses must fuse their lipid membrane to a cellular membrane to deliver their genome into the cytoplasm for replication. Viral envelope proteins catalyze this critical membrane fusion event. They fall into three distinct structural classes. In 2013, envelope proteins from a pestivirus and hepatitis C virus were found to have two distinct novel folds. This was unexpected because these viruses are in the same family as flaviviruses, which have class II fusion proteins. We propose that the membrane fusion machinery of the closely related pestiviruses and hepatitis C virus defines a new structural class. This and other recently identified structural relationships between viral fusion proteins shift the paradigm for how these proteins evolved. In many viruses, the genome is enveloped in a lipid membrane. Viral envelope proteins anchored in the membrane fulfill indispensable functions throughout the life cycle of the virus. Envelope proteins drive virus assembly, form the protective outer shell of the virus, mediate cellular attachment and tropism, and catalyze the fusion of the viral and host cell membranes to deliver the viral genome into the cytoplasm for replication. Envelope proteins also provide a shield against the immune system of the host and bear most of the neutralizing antibody epitopes against any given virus. In viruses with more than one envelope protein, the proteins responsible for cellular attachment are as varied as the host cell receptors that they recognize. By contrast, the viral envelope proteins that catalyze the essential membrane fusion step in cell entry fall into three broad yet distinct structural classes (Figure 1). The influenza virus hemagglutinin (HA) is the prototype of class I fusion proteins [1Skehel J.J. Wiley D.C. Receptor binding and membrane fusion in virus entry: the influenza hemagglutinin.Annu. Rev. Biochem. 2000; 69: 531-569Crossref PubMed Scopus (2304) Google Scholar], which encompass those of other orthomyxoviruses, and of paramyxoviruses [2Lamb R.A. Jardetzky T.S. Structural basis of viral invasion: lessons from paramyxovirus F.Curr. Opin. Struct. Biol. 2007; 17: 427-436Crossref PubMed Scopus (226) Google Scholar], retroviruses [3Chan D.C. et al.Core structure of gp41 from the HIV envelope glycoprotein.Cell. 1997; 89: 263-273Abstract Full Text Full Text PDF PubMed Scopus (1874) Google Scholar, 4Weissenhorn W. et al.Atomic structure of the ectodomain from HIV-1 gp41.Nature. 1997; 387: 426-430Crossref PubMed Scopus (1489) Google Scholar], filoviruses [5Weissenhorn W. et al.Crystal structure of the Ebola virus membrane fusion subunit, GP2, from the envelope glycoprotein ectodomain.Mol. Cell. 1998; 2: 605-616Abstract Full Text Full Text PDF PubMed Scopus (362) Google Scholar], and coronaviruses [6Xu Y. et al.Crystal structure of severe acute respiratory syndrome coronavirus spike protein fusion core.J. Biol. Chem. 2004; 279: 49414-49419Crossref PubMed Scopus (182) Google Scholar, 7Xu Y. et al.Structural basis for coronavirus-mediated membrane fusion. Crystal structure of mouse hepatitis virus spike protein fusion core.J. Biol. Chem. 2004; 279: 30514-30522Crossref PubMed Scopus (112) Google Scholar]. The unifying structural features of class I fusion proteins are a proteolytically generated N-terminal fusion peptide, and a core consisting of three bundled α-helices in the prefusion conformation, which refolds into a six-helix bundle in the postfusion conformation [8Schibli D.J. Weissenhorn W. Class I and class II viral fusion protein structures reveal similar principles in membrane fusion.Mol. Membr. Biol. 2004; 21: 361-371Crossref PubMed Scopus (80) Google Scholar]. Class II fusion proteins are a structurally unrelated class found in flaviviruses [9Rey F.A. et al.The envelope glycoprotein from tick-borne encephalitis virus at 2 A resolution.Nature. 1995; 375: 291-298Crossref PubMed Scopus (1279) Google Scholar], alphaviruses [10Lescar J. et al.The fusion glycoprotein shell of Semliki Forest virus: an icosahedral assembly primed for fusogenic activation at endosomal pH.Cell. 2001; 105: 137-148Abstract Full Text Full Text PDF PubMed Scopus (448) Google Scholar], and most recently in rubella virus (sole member of the rubivirus genus) [11DuBois R.M. et al.Functional and evolutionary insight from the crystal structure of rubella virus protein E1.Nature. 2013; 493: 552-556Crossref PubMed Scopus (80) Google Scholar] and Rift Valley fever virus (RVFV, from the phlebovirus genus) [12Dessau M. Modis Y. Crystal structure of glycoprotein C from Rift Valley fever virus.Proc. Natl. Acad. Sci. U.S.A. 2013; 110: 1696-1701Crossref PubMed Scopus (107) Google Scholar]. Class II proteins share a three-domain architecture consisting almost entirely of β-strands, with a tightly folded 'fusion loop' in the central domain serving as the anchor in the cellular membrane targeted for fusion (Figure 1) [13Modis Y. Relating structure to evolution in class II viral membrane fusion proteins.Curr. Opin. Virol. 2014; 5: 34-41Crossref Scopus (50) Google Scholar]. Class III fusion proteins, found in herpesviruses [14Heldwein E.E. et al.Crystal structure of glycoprotein B from herpes simplex virus 1.Science. 2006; 313: 217-220Crossref PubMed Scopus (484) Google Scholar], rhabdoviruses [15Roche S. et al.Crystal structure of the low-pH form of the vesicular stomatitis virus glycoprotein G.Science. 2006; 313: 187-191Crossref PubMed Scopus (364) Google Scholar], and baculoviruses [16Kadlec J. et al.The postfusion structure of baculovirus gp64 supports a unified view of viral fusion machines.Nat. Struct. Mol. Biol. 2008; 15: 1024-1030Crossref PubMed Scopus (193) Google Scholar], possess core helical bundles like class I proteins, and a central β-stranded domain bearing one or more fusion loops like class II proteins (Figure 1). However, similarities between class II and class III proteins are likely to have arisen from convergent evolution because the fold and connectivity of the fusion domains are different in the two classes. Notably, reoviruses encode a family of fusion-associated small transmembrane (FAST) proteins that function as dedicated cell–cell fusogens. The resulting multinuclear syncytia promote viral replication by obviating the need for cell-to-cell transmission (reviewed in [17Boutilier J. Duncan R. The reovirus fusion-associated small transmembrane (FAST) proteins: virus-encoded cellular fusogens.Curr. Top. Membr. 2011; 68: 107-140Crossref PubMed Scopus (28) Google Scholar]). Viruses from the Flaviviridae family, including flaviviruses, pestiviruses, and hepaciviruses (principally hepatitis C virus, HCV), share many key characteristics. Because flaviviruses contain prototypical class II membrane fusion proteins, pestiviruses and hepaciviruses had been expected to have similar class II fusion proteins [18Krey T. et al.The disulfide bonds in glycoprotein E2 of hepatitis C virus reveal the tertiary organization of the molecule.PLoS Pathog. 2010; 6: e1000762Crossref PubMed Scopus (204) Google Scholar, 19Garry R.F. Dash S. Proteomics computational analyses suggest that hepatitis C virus E1 and pestivirus E2 envelope glycoproteins are truncated class II fusion proteins.Virology. 2003; 307: 255-265Crossref PubMed Scopus (118) Google Scholar]. However, in 2013 the larger envelope protein, E2, from the pestivirus BVDV (bovine viral diarrhea virus) was unexpectedly found to have a novel fold [20Li Y. et al.Crystal structure of glycoprotein E2 from bovine viral diarrhea virus.Proc. Natl. Acad. Sci. U.S.A. 2013; 110: 6805-6810Crossref PubMed Scopus (103) Google Scholar, 21El Omari K. et al.Structure of a pestivirus envelope glycoprotein E2 clarifies its role in cell entry.Cell Rep. 2013; 3: 30-35Abstract Full Text Full Text PDF PubMed Scopus (119) Google Scholar]. The structure of a core fragment of E2 from HCV was subsequently found to have a novel fold unrelated to that of BVDV E2 [22Kong L. et al.Hepatitis C virus E2 envelope glycoprotein core structure.Science. 2013; 342: 1090-1094Crossref PubMed Scopus (331) Google Scholar]. Moreover, BVDV E2 and HCV E2 both lack the structural hallmarks of fusion proteins. Together, these discoveries suggest that E1 is the fusogen and that pesti- and hepaciviruses contain a new class (or classes) of membrane fusion machinery. The evolutionary implications of this and other recently identified unexpected structural relationships between fusion proteins across virus families are discussed. Structural studies of viral envelope proteins have revealed certain overarching commonalities in the membrane fusion mechanisms of viruses across different families. Crystal structures of fusion proteins from classes I, II, and III before and after the conformational change that catalyzes membrane fusion provide a molecular outline of their respective fusion mechanisms (reviewed in [1Skehel J.J. Wiley D.C. Receptor binding and membrane fusion in virus entry: the influenza hemagglutinin.Annu. Rev. Biochem. 2000; 69: 531-569Crossref PubMed Scopus (2304) Google Scholar, 8Schibli D.J. Weissenhorn W. Class I and class II viral fusion protein structures reveal similar principles in membrane fusion.Mol. Membr. Biol. 2004; 21: 361-371Crossref PubMed Scopus (80) Google Scholar, 23Kielian M. Rey F.A. Virus membrane-fusion proteins: more than one way to make a hairpin.Nat. Rev. Microbiol. 2006; 4: 67-76Crossref PubMed Scopus (464) Google Scholar, 24Baquero E. et al.Intermediate conformations during viral fusion glycoprotein structural transition.Curr. Opin. Virol. 2013; 3: 143-150Crossref PubMed Scopus (39) Google Scholar]). Complementing these pre- and postfusion structures, structures thought to represent fusion intermediates provide invaluable insights on the steps required for fusion [12Dessau M. Modis Y. Crystal structure of glycoprotein C from Rift Valley fever virus.Proc. Natl. Acad. Sci. U.S.A. 2013; 110: 1696-1701Crossref PubMed Scopus (107) Google Scholar, 24Baquero E. et al.Intermediate conformations during viral fusion glycoprotein structural transition.Curr. Opin. 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Fusion proteins from all three classes respond to one or more environmental cues – such as low pH, coreceptor binding, or disulfide bond exchange – by exposing a hydrophobic fusion motif previously shielded from the solvent (Figure 2B). The fusion motif, an N-terminal 'fusion peptide' in class I proteins or internal fusion loops in class II and class III proteins, spontaneously inserts into the outer bilayer leaflet of the host cell membrane (Figure 2C). This extended conformation, postulated for all viral fusion proteins and recently observed in a bunyavirus protein [12Dessau M. Modis Y. Crystal structure of glycoprotein C from Rift Valley fever virus.Proc. Natl. Acad. Sci. U.S.A. 2013; 110: 1696-1701Crossref PubMed Scopus (107) Google Scholar], is called the prehairpin intermediate (Figure 2C). The fusion protein then folds back on itself, directing its C-terminal transmembrane anchor towards the fusion motif (Figure 2D). This fold-back forces the host cell membrane (held by the fusion motif) and the viral membrane (held by the transmembrane anchor) against each other, resulting in fusion of the outer leaflets of the two membranes to form a hemifusion intermediate (Figure 2E), followed by fusion of the distal leaflets to form a fusion pore and complete fusion [33Chernomordik L.V. Kozlov M.M. Membrane hemifusion: crossing a chasm in two leaps.Cell. 2005; 123: 375-382Abstract Full Text Full Text PDF PubMed Scopus (305) Google Scholar] (Figure 2F). The oligomeric state of fusion proteins vary before fusion, but all fusion proteins undergo the fusogenic fold-back as trimers and are trimeric in the postfusion conformation (Figure 3). Moreover, postfusion trimers from all three classes have been reported to form interacting networks [34Gibbons D.L. et al.Visualization of the target-membrane-inserted fusion protein of Semliki Forest virus by combined electron microscopy and crystallography.Cell. 2003; 114: 573-583Abstract Full Text Full Text PDF PubMed Scopus (93) Google Scholar, 35Stiasny K. et al.Characterization of a membrane-associated trimeric low-pH-induced form of the class II viral fusion protein E from tick-borne encephalitis virus and its crystallization.J. Virol. 2004; 78: 3178-3183Crossref PubMed Scopus (51) Google Scholar, 36Libersou S. et al.Distinct structural rearrangements of the VSV glycoprotein drive membrane fusion.J. 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The conservation of trimeric postfusion states across structural classes may be coincidental, but it is possible that trimeric assemblies have been selected because they provide the optimal balance of stability and susceptibility to the first fold-back event. Indeed, because fusion requires multiple trimers to fold back cooperatively [38Ivanovic T. et al.Influenza-virus membrane fusion by cooperative fold-back of stochastically induced hemagglutinin intermediates.eLIFE. 2013; 2: e00333Crossref Scopus (125) Google Scholar], prehairpins cannot be too short-lived but should fold back rapidly once fold back has been initiated. Like a three-legged stool, a trimeric prehairpin intermediate may have a favorable degree of stability (necessary for multiple prehairpins to accumulate on the viral surface), but is rapidly destabilized once the first subunit begins to fold back (allowing multiple trimers to refold cooperatively).Figure 3Conformational changes associated with membrane fusion in the different structural classes of membrane fusion proteins. In all classes, a fusion motif (orange) that is shielded from the solvent in the prefusion conformation (left column) becomes exposed in response to environmental cues (e.g., low pH or coreceptor binding). The fusion motif inserts into the cell membrane and the protein folds back on itself, forcing the fusion motif and the C-terminal transmembrane domain (not shown) anchored in the viral membrane towards each other. The proteins are trimeric in their postfusion conformations (right column). (A) In class I fusion proteins, such as influenza A virus hemagglutinin (Flu HA) shown here, membrane fusion is catalyzed by extensive refolding and secondary structure rearrangements of prefusion trimers to form a six-helix bundle [Protein Data Bank (PDB) codes 2HMG, 1HTM, 1QU1]. (B) Class II proteins usually form icosahedral shells in infectious virions. The envelope proteins respond to the reduced pH of an endosome with a repositioning of the three domains with only minor changes in secondary structure. The proteins form trimers during the fusion transition and the fusion loop in the central domain is directed towards the viral transmembrane anchor. The pre- and postfusion conformations of dengue type 2 virus E (DEN E) are shown here (PDB codes 1OKE, 1OK8). (C) Class III proteins are trimeric before and after fusion and undergo extensive refolding during the fusion transition like class I fusion proteins, but they contain internal fusion loops like class II proteins. The pre- and postfusion structures of vesicular stomatitis virus G (VSV G) are shown here (PDB codes 2J6J, 2CMZ). (D) The structure of envelope glycoprotein E2 from the pestivirus bovine viral diarrhea virus (BVDV) has been proposed to serve as a molecular scaffold for E1, which may define a new structural class of fusion machinery (PDB code 4JNT). The structure of envelope protein E1 (gray) and the nature of the fusogenic conformational change remain unknown. The outer leaflets of the viral and cellular membranes are represented in green and cyan, respectively.View Large Image Figure ViewerDownload (PPT) The Flaviviridae family contains four genera: flavivirus, pestivirus, pegivirus (GB viruses), and hepacivirus (HCV). Among these genera, pestiviruses and pegiviruses are the most closely related to HCV, a serious and persistent global health threat [40Shepard C.W. et al.Global epidemiology of hepatitis C virus infection.Lancet Infect. Dis. 2005; 5: 558-567Abstract Full Text Full Text PDF PubMed Scopus (2303) Google Scholar]. Until 2013, envelope protein structures were available only from the flavivirus genus. Envelope proteins from pesti- and hepaciviruses had been predicted to have class II folds based on the disulfide-bonding pattern [18Krey T. et al.The disulfide bonds in glycoprotein E2 of hepatitis C virus reveal the tertiary organization of the molecule.PLoS Pathog. 2010; 6: e1000762Crossref PubMed Scopus (204) Google Scholar] and amino acid sequence analysis of the E1 and E2 envelope proteins [19Garry R.F. Dash S. Proteomics computational analyses suggest that hepatitis C virus E1 and pestivirus E2 envelope glycoproteins are truncated class II fusion proteins.Virology. 2003; 307: 255-265Crossref PubMed Scopus (118) Google Scholar]. It was therefore surprising when two groups discovered in 2013 that the larger envelope protein, E2, from the pestivirus BVDV is not a class II fusion protein [20Li Y. et al.Crystal structure of glycoprotein E2 from bovine viral diarrhea virus.Proc. Natl. Acad. Sci. U.S.A. 2013; 110: 6805-6810Crossref PubMed Scopus (103) Google Scholar, 21El Omari K. et al.Structure of a pestivirus envelope glycoprotein E2 clarifies its role in cell entry.Cell Rep. 2013; 3: 30-35Abstract Full Text Full Text PDF PubMed Scopus (119) Google Scholar]. Instead, the BVDV E2 ectodomain has a novel architecture, spanning a total of 140 Å and consisting of two immunoglobulin (Ig)-like domains followed by a unique elongated β-stranded domain (domain III) and a membrane anchor (Figure 1D). The overall fold and topology of BVDV E2 domain III bears no significant similarity to previously determined protein structures. The structure of a core fragment of HCV E2 was subsequently determined and found to have a novel globular architecture, distinct from that of BVDV E2, with an Ig-like β-sandwich at its core [22Kong L. et al.Hepatitis C virus E2 envelope glycoprotein core structure.Science. 2013; 342: 1090-1094Crossref PubMed Scopus (331) Google Scholar]. Notably, both BVDV E2 and HCV E2 lack an internal or terminal fusion motif with an obvious resemblance to those of other viral fusion proteins. Moreover, BVDV E2 forms tightly associated dimers but the dimerization interface, which contains a large cluster of conserved aromatic residues, is very different from that of flavivirus E proteins (Figure 3) [20Li Y. et al.Crystal structure of glycoprotein E2 from bovine viral diarrhea virus.Proc. Natl. Acad. Sci. U.S.A. 2013; 110: 6805-6810Crossref PubMed Scopus (103) Google Scholar]. The structures of BVDV E2 and HCV E2 provide striking examples of how structurally divergent viral envelope proteins can be within a single virus family. The novel structures of BVDV E2 and HCV E2 lack the structural hallmarks of membrane fusion proteins such as a hydrophobic fusion motif (found in all classes of fusion protein), a helical core (as in classes I and III), or a flexible multidomain structure (as in classes II and III). This suggests that E1 is the fusion protein in pesti- and hepaciviruses. Indeed, E1 has been proposed to bear the fusion motif in both genera [20Li Y. et al.Crystal structure of glycoprotein E2 from bovine viral diarrhea virus.Proc. Natl. Acad. Sci. U.S.A. 2013; 110: 6805-6810Crossref PubMed Scopus (103) Google Scholar, 21El Omari K. et al.Structure of a pestivirus envelope glycoprotein E2 clarifies its role in cell entry.Cell Rep. 2013; 3: 30-35Abstract Full Text Full Text PDF PubMed Scopus (119) Google Scholar, 41Drummer H.E. et al.Mutagenesis of a conserved fusion peptide-like motif and membrane-proximal heptad-repeat region of hepatitis C virus glycoprotein E1.J. Gen. Virol. 2007; 88: 1144-1148Crossref PubMed Scopus (83) Google Scholar]. As the fusion protein, E1 would have to at least transiently extend to span the distance between the cellular and viral membranes prior to membrane fusion, approximately 20 nm [28Kim Y.H. et al.Capture and imaging of a prehairpin fusion intermediate of the paramyxovirus PIV5.Proc. Natl. Acad. Sci. U.S.A. 2011; 108: 20992-20997Crossref PubMed Scopus (50) Google Scholar]. E1 would have to adopt a highly elongated fold in order to span 20 nm. With its ectodomain of less than 180 amino acids, E1 is too small to have a class II or class III fold that could span 20 nm. Additionally, secondary structure predictions suggest that approximately 30% of the E1 ectodomain is α-helical, which is inconsistent with a class II or class III architecture. Coiled-coils efficiently form rigid, highly elongated structures, thus it is possible that predicted α-helices in the central region of E1 form a helical bundle. However, E1 lacks a clear leucine zipper motif that would be indicative of a coiled coil. Moreover, E1 is unlikely to have a class I fold as it lacks key unifying features of class I proteins: E1 does not form trimers, is not subject to proteolytic activation, and does not appear to have an N-terminal fusion peptide. In fact, E1 cannot fold correctly or support cell entry without E2 [20Li Y. et al.Crystal structure of glycoprotein E2 from bovine viral diarrhea virus.Proc. Natl. Acad. Sci. U.S.A. 2013; 110: 6805-6810Crossref PubMed Scopus (103) Google Scholar, 21El Omari K. et al.Structure of a pestivirus envelope glycoprotein E2 clarifies its role in cell entry.Cell Rep. 2013; 3: 30-35Abstract Full Text Full Text PDF PubMed Scopus (119) Google Scholar, 42Ronecker S. et al.Formation of bovine viral diarrhea virus E1-E2 heterodimers is essential for virus entry and depends on charged residues in the transmembrane domains.J. Gen. Virol. 2008; 89: 2114-2121Crossref PubMed Scopus (58) Google Scholar, 43Patel J. et al.The transmembrane domain of the hepatitis C virus E2 glycoprotein is required for correct folding of the E1 glycoprotein and native complex formation.Virology. 2001; 279: 58-68Crossref PubMed Scopus (66) Google Scholar]. Thus, E2 appears to function as an essential molecular scaffold or chaperone to E1 as the fusogen [20Li Y. et al.Crystal structure of glycoprotein E2 from bovine viral diarrhea virus.Proc. Natl. Acad. Sci. U.S.A. 2013; 110: 6805-6810Crossref PubMed Scopus (103) Google Scholar, 21El Omari K. et al.Structure of a pestivirus envelope glycoprotein E2 clarifies its role in cell entry.Cell Rep. 2013; 3: 30-35Abstract Full Text Full Text PDF PubMed Scopus (119) Google Scholar]. Consistent with this view, fusogenic pesti- or hepacivirus particles contain E1–E2 disulfide links, and virus particles with mutations disrupting E1–E2 interactions (or lacking E2) are not fusogenic [42Ronecker S. et al.Formation of bovine viral diarrhea virus E1-E2 heterodimers is essential for virus entry and depends on charged residues in the transmembrane domains.J. Gen. Virol. 2008; 89: 2114-2121Crossref PubMed Scopus (58) Google Scholar, 44Durantel D. et al.Study of the mechanism of antiviral action of iminosugar derivatives against bovine viral diarrhea virus.J. Virol. 2001; 75: 8987-8998Crossref PubMed Scopus (149) Google Scholar, 45Weiland E. et al.Pestivirus glycoprotein which induces neutralizing antibodies forms part of a disulfide-linked heterodimer.J. Virol. 1990; 64: 3563-3569Crossref PubMed Google Scholar, 46Branza-Nichita N. et al.Antiviral effect of N-butyldeoxynojirimycin against bovine viral diarrhea virus correlates with misfolding of E2 envelope proteins and impairment of their association into E1-E2 heterodimers.J. Virol. 2001; 75: 3527-3536Crossref PubMed Scopus (75) Google Scholar, 47Vieyres G. et al.Characterization of the envelope glycoproteins associated with infectious hepatitis C virus.J. Virol. 2010; 84: 10159-10168Crossref PubMed Scopus (177) Google Scholar]. Based on the structures of BVDV E2 and HCV E2, on available biochemical data, and on the amino acid sequences of E1 proteins, the possibility that pesti- and hepaciviruses utilize class I, class II, or class III architectures can now be ruled out. Hence, we propose that in pesti- and hepaciviruses, E1 defines a new class (or two distinct classes) of membrane fusion protein, and that E2 plays an accessory role as a molecular scaffold for E1. The unexpected structural relationships between viral fusion proteins discovered in 2013 provide important insights on viral evolution. Viruses from the Flaviviridae family each have similar genetic organizations, coding strategies, morphologies, and cell entry pathways, suggesting that they might have evolved from a common virus ancestor (Box 1). However, the discovery of novel folds in BVDV E2 [20Li Y. et al.Crystal structure of glycoprotein E2 from bovine viral diarrhea virus.Proc. Natl. Acad. Sci. U.S.A. 2013; 110: 6805-6810Crossref PubMed Scopus (103) Google Scholar, 21El Omari K. et al.Structure of a pestivirus envelope glycoprotein E2 clarifies its role in cell entry.Cell Rep. 2013; 3: 30-35Abstract Full Text Full Text PDF PubMed Scopus (119) Google Scholar] and HCV E2 [22Kong L. et al.Hepatitis C virus E2 envelope glycoprotein core structure.Science. 2013; 342: 1090-1094Crossref PubMed Scopus (331) Google Scholar] implies that the envelope proteins of pestiviruses, hepaciviruses, and flaviviruses have different evolutionary origins
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