Non-replicating vaccinia vector efficiently expresses bacteriophage T7 RNA polymerase

From structure prediction to function: defining the domain on the African swine fever virus CD2v protein required for binding to erythrocytes

African swine fever virus (ASFV) is a high-consequence pathogen posing a substantial threat to global food security. This large DNA virus encodes more than 150 open reading frames, many of which are uncharacterized. The EP402R gene encodes CD2v, a glycoprotein expressed on the surface of infected cells and the only viral protein known to be present in the virus external envelope. This protein mediates binding of erythrocytes to both cells and virions. This interaction is known to prolong virus persistence in blood thus facilitating viral transmission. The sequence of the extracellular domain of CD2v shows similarity with that of mammalian CD2 proteins and is therefore likely to feature two immunoglobulin (Ig)-like domains. A combination of protein structure modeling and extensive mutagenesis was used to identify residues mediating binding of transiently expressed CD2v to erythrocytes. The N-terminal Ig-like domain AGFCC'C″ β sheet was identified as the putative CD2v erythrocyte-binding area. This region differed from the putative CD58 ligand binding site of host CD2, suggesting that CD2v may bind to a ligand(s) other than CD58. An attenuated genotype I ASFV was constructed by replacing the wild-type EP402R gene for a mutant form expressing CD2v bearing a single amino acid substitution, which abrogated the binding to erythrocytes. Pigs immunized with the recombinant virus developed early antibody and cellular responses, low levels of viremia, mild clinical signs post-immunization, and high levels of protection against challenge. These findings improve our understanding of virus-host interactions and provide a promising approach to modified live vaccine development.

IMPORTANCE

A better understanding of the interactions between viruses and their hosts is a crucial step in the development of strategies for controlling viral diseases, such as vaccines and antivirals. African swine fever, a pig disease with fatality rates approaching 100%, causes very substantial economic losses in affected countries, and new control measures are clearly needed. In this study, we characterized the interaction between the ASFV CD2v protein and host erythrocytes. The interaction plays a key role in viral persistence in blood since it can allow the virus to "hide" from the host immune system. We identified the amino acids in the viral protein that mediate the interaction with erythrocytes and used this information to construct a mutant virus that is no longer able to bind these cells. This virus induces strong immune responses that provide high levels of protection against infection with the deadly parental virus.

Implementation of an Immunoassay Based on the MVA-T7pol-Expression System for Rapid Identification of Immunogenic SARS-CoV-2 Antigens: A Proof-of-Concept Study

The emergence of hitherto unknown viral pathogens presents a great challenge for researchers to develop effective therapeutics and vaccines within a short time to avoid an uncontrolled global spread, as seen during the coronavirus disease 2019 (COVID-19) pandemic. Therefore, rapid and simple methods to identify immunogenic antigens as potential therapeutical targets are urgently needed for a better pandemic preparedness. To address this problem, we chose the well-characterized Modified Vaccinia virus Ankara (MVA)-T7pol expression system to establish a workflow to identify immunogens when a new pathogen emerges, generate candidate vaccines, and test their immunogenicity in an animal model. By using this system, we detected severe acute respiratory syndrome (SARS) coronavirus 2 (SARS-CoV-2) nucleoprotein (N)-, and spike (S)-specific antibodies in COVID-19 patient sera, which is in line with the current literature and our observations from previous immunogenicity studies. Furthermore, we detected antibodies directed against the SARS-CoV-2-membrane (M) and -ORF3a proteins in COVID-19 patient sera and aimed to generate recombinant MVA candidate vaccines expressing either the M or ORF3a protein. When testing our candidate vaccines in a prime-boost immunization regimen in humanized HLA-A2.1-/HLA-DR1-transgenic H-2 class I-/class II-knockout mice, we were able to demonstrate M- and ORF3a-specific cellular and humoral immune responses. Hence, the established workflow using the MVA-T7pol expression system represents a rapid and efficient tool to identify potential immunogenic antigens and provides a basis for future development of candidate vaccines.

Generation of Defective Interfering Particles of Morbilliviruses Using Reverse Genetics

RNA viruses generate defective genomes naturally during virus replication. Defective genomes that interfere with the infection dynamics either through resource competition or by interferon stimulation are known as defective interfering (DI) genomes. DI genomes can be successfully packaged into virus-like-particles referred to as defective interfering particles (DIPs). Such DIPs can sustainably coexist with the full-length virus particles and have been shown to negatively impact virus replication in vitro and in vivo. Here, we describe a method to generate a clonal DI genome population by reverse genetics. This method is applicable to other RNA viruses and will enable assessment of DIPs for their antiviral properties.

Packaging defects in pestiviral NS4A can be compensated by mutations in NS2 and NS3

The non-structural (NS) proteins of the Flaviviridae members play a dual role in genome replication and virion morphogenesis. For pestiviruses, like bovine viral diarrhea virus, the NS2-3 region and its processing by the NS2 autoprotease is of particular importance. While uncleaved NS2-3 in complex with NS4A is essential for virion assembly, it cannot replace free NS3/4A in the viral replicase. Furthermore, surface interactions between NS3 and the C-terminal cytosolic domain of NS4A were shown to serve as a molecular switch between RNA replication and virion morphogenesis. To further characterize the functionality of NS4A, we performed an alanine-scanning mutagenesis of two NS4A regions, a short highly conserved cytoplasmic linker downstream of the transmembrane domain and the C-terminal domain. NS4A residues critical for polyprotein processing, RNA replication, and/or virion morphogenesis were identified. Three double-alanine mutants, two in the linker region and one close to the C-terminus of NS4A, showed a selective effect on virion assembly. All three packaging defective mutants could be rescued by a selected set of two second-site mutations, located in NS2 and NS3, respectively. This phenotype was additionally confirmed by complementation studies providing the NS2-3/4A packaging molecules containing the rescue mutations in trans. This indicates that the linker region and the cytosolic C-terminal part of NS4A are critical for the formation of protein complexes required for virion morphogenesis. The ability of the identified sets of second-site mutations in NS2-3 to compensate for diverse NS4A defects highlights a surprising functional flexibility for pestiviral NS proteins. IMPORTANCE Positive-strand RNA viruses have a limited coding capacity due to their rather small genome size. To overcome this constraint, viral proteins often exhibit multiple functions that come into play at different stages during the viral replication cycle. The molecular basis for this multifunctionality is often unknown. For the bovine viral diarrhea virus, the non-structural protein (NS) 4A functions as an NS3 protease cofactor, a replicase building block, and a component in virion morphogenesis. Here, we identified the critical amino acids of its C-terminal cytosolic region involved in those processes and show that second-site mutations in NS2 and NS3 can compensate for diverse NS4A defects in virion morphogenesis. The ability to evolve alternative functional solutions by gain-of-function mutations highlights the astounding plasticity of the pestiviral system.

The M2 proteins of bat influenza A viruses reveal atypical features compared to conventional M2 proteins

The influenza A virus (IAV) M2 protein has proton channel activity, which plays a role in virus uncoating and may help to preserve the metastable conformation of the IAV hemagglutinin (HA). In contrast to the highly conserved M2 proteins of conventional IAV, the primary sequences of bat IAV H17N10 and H18N11 M2 proteins show remarkable divergence, suggesting that these proteins may differ in their biological function. We, therefore, assessed the proton channel activity of bat IAV M2 proteins and investigated its role in virus replication. Here, we show that the M2 proteins of bat IAV did not fully protect acid-sensitive HA of classical IAV from low pH-induced conformational change, indicating low proton channel activity. Interestingly, the N31S substitution not only rendered bat IAV M2 proteins sensitive to inhibition by amantadine but also preserved the metastable conformation of acid-sensitive HA to a greater extent. In contrast, the acid-stable HA of H18N11 did not rely on such support by M2 protein. When mutant M2(N31S) protein was expressed in the context of chimeric H18N11/H5N1(6:2) encoding HA and NA of avian IAV H5N1, amantadine significantly inhibited virus entry, suggesting that ion channel activity supported virus uncoating. Finally, the cytoplasmic domain of the H18N11 M2 protein mediated rapid internalization of the protein from the plasma membrane leading to low-level expression at the cell surface. However, cell surface levels of H18N11 M2 protein were significantly enhanced in cells infected with the chimeric H18N11/H5N1(6:2) virus. The potential role of the N1 sialidase in arresting M2 internalization is discussed. IMPORTANCE Bat IAV M2 proteins not only differ from the homologous proteins of classical IAV by their divergent primary sequence but are also unable to preserve the metastable conformation of acid-sensitive HA, indicating low proton channel activity. This unusual feature may help to avoid M2-mediated cytotoxic effects and inflammation in bats infected with H17N10 or H18N11. Unlike classical M2 proteins, bat IAV M2 proteins with the N31S substitution mediated increased protection of HA from acid-induced conformational change. This remarkable gain of function may help to understand how single point mutations can modulate proton channel activity. In addition, the cytoplasmic domain was found to be responsible for the low cell surface expression level of bat IAV M2 proteins. Given that the M2 cytoplasmic domain of conventional IAV is well known to participate in virus assembly at the plasma membrane, this atypical feature might have consequences for bat IAV budding and egress.

Characterization of a multipurpose NS3 surface patch coordinating HCV replicase assembly and virion morphogenesis

The hepatitis C virus (HCV) life cycle is highly regulated and characterized by a step-wise succession of interactions between viral and host cell proteins resulting in the assembly of macromolecular complexes, which catalyse genome replication and/or virus production. Non-structural (NS) protein 3, comprising a protease and a helicase domain, is involved in orchestrating these processes by undergoing protein interactions in a temporal fashion. Recently, we identified a multifunctional NS3 protease surface patch promoting pivotal protein-protein interactions required for early steps of the HCV life cycle, including NS3-mediated NS2 protease activation and interactions required for replicase assembly. In this work, we extend this knowledge by identifying further NS3 surface determinants important for NS5A hyperphosphorylation, replicase assembly or virion morphogenesis, which map to protease and helicase domain and form a contiguous NS3 surface area. Functional interrogation led to the identification of phylogenetically conserved amino acid positions exerting a critical function in virion production without affecting RNA replication. These findings illustrate that NS3 uses a multipurpose protein surface to orchestrate the step-wise assembly of functionally distinct multiprotein complexes. Taken together, our data provide a basis to dissect the temporal formation of viral multiprotein complexes required for the individual steps of the HCV life cycle.

Identification of NS2 determinants stimulating intrinsic HCV NS2 protease activity

Hepatitis C Virus NS2-NS3 cleavage is mediated by NS2 autoprotease (NS2pro) and this cleavage is important for genome replication and virus assembly. Efficient NS2-NS3 cleavage relies on the stimulation of an intrinsic NS2pro activity by the NS3 protease domain. NS2pro activation depends on conserved hydrophobic NS3 surface residues and yet unknown NS2-NS3 surface interactions. Guided by an in silico NS2-NS3 precursor model, we experimentally identified two NS2 surface residues, F103 and L144, that are important for NS2pro activation by NS3. When analyzed in the absence of NS3, a combination of defined amino acid exchanges, namely F103A and L144I, acts together to increase intrinsic NS2pro activity. This effect is conserved between different HCV genotypes. For mutation L144I its stimulatory effect on NS2pro could be also demonstrated for two other mammalian hepaciviruses, highlighting the functional significance of this finding. We hypothesize that the two exchanges stimulating the intrinsic NS2pro activity mimic structural changes occurring during NS3-mediated NS2pro activation. Introducing these activating NS2pro mutations into a NS2-NS5B replicon reduced NS2-NS3 cleavage and RNA replication, indicating their interference with NS2-NS3 surface interactions pivotal for NS2pro activation by NS3. Data from chimeric hepaciviral NS2-NS3 precursor constructs, suggest that NS2 F103 is involved in the reception or transfer of the NS3 stimulus by NS3 P115. Accordingly, fine-tuned NS2-NS3 surface interactions are a salient feature of HCV NS2-NS3 cleavage. Together, these novel insights provide an exciting basis to dissect molecular mechanisms of NS2pro activation by NS3.

Biparatopic sybodies neutralize SARS-CoV-2 variants of concern and mitigate drug resistance

The ongoing COVID‐19 pandemic represents an unprecedented global health crisis. Here, we report the identification of a synthetic nanobody (sybody) pair, Sb#15 and Sb#68, that can bind simultaneously to the SARS‐CoV‐2 spike RBD and efficiently neutralize pseudotyped and live viruses by interfering with ACE2 interaction. Cryo‐EM confirms that Sb#15 and Sb#68 engage two spatially discrete epitopes, influencing rational design of bispecific and tri‐bispecific fusion constructs that exhibit up to 100‐ and 1,000‐fold increase in neutralization potency, respectively. Cryo‐EM of the sybody‐spike complex additionally reveals a novel up‐out RBD conformation. While resistant viruses emerge rapidly in the presence of single binders, no escape variants are observed in the presence of the bispecific sybody. The multivalent bispecific constructs further increase the neutralization potency against globally circulating SARS‐CoV‐2 variants of concern. Our study illustrates the power of multivalency and biparatopic nanobody fusions for the potential development of therapeutic strategies that mitigate the emergence of new SARS‐CoV‐2 escape mutants.

Conformational changes in Lassa virus L protein associated with promoter binding and RNA synthesis activity

Lassa virus is endemic in West Africa and can cause severe hemorrhagic fever. The viral L protein transcribes and replicates the RNA genome via its RNA-dependent RNA polymerase activity. Here, we present nine cryo-EM structures of the L protein in the apo-, promoter-bound pre-initiation and active RNA synthesis states. We characterize distinct binding pockets for the conserved 3' and 5' promoter RNAs and show how full-promoter binding induces a distinct pre-initiation conformation. In the apo- and early elongation states, the endonuclease is inhibited by two distinct L protein peptides, whereas in the pre-initiation state it is uninhibited. In the early elongation state, a template-product duplex is bound in the active site cavity together with an incoming non-hydrolysable nucleotide and the full C-terminal region of the L protein, including the putative cap-binding domain, is well-ordered. These data advance our mechanistic understanding of how this flexible and multifunctional molecular machine is activated.

The Molecular Basis for Erns Dimerization in Classical Swine Fever Virus

The pestivirus classical swine fever virus (CSFV) represents one of the most important pathogens of swine. Its virulence is dependent on the RNase activity of the essential structural glycoprotein E^rns that uses an amphipathic helix as a membrane anchor and forms homodimers via disulfide bonds employing cysteine 171. Dimerization is not necessary for CSFV viability but for its virulence. Mutant E^rns proteins lacking cysteine 171 are still able to interact transiently as shown in crosslink experiments. Deletion analysis did not reveal the presence of a primary sequence-defined contact surface essential for dimerization, but indicated a general importance of an intact ectodomain for efficient establishment of dimers. Pseudoreverted viruses reisolated in earlier experiments from pigs with mutations Cys171Ser/Ser209Cys exhibited partially restored virulence and restoration of the ability to form E^rns homodimers. Dimer formation was also observed for experimentally mutated proteins, in which other amino acids at different positions of the membrane anchor region of E^rns were replaced by cysteine. However, with one exception of two very closely located residues, the formation of disulfide-linked dimers was only observed for cysteine residues located at the same position of the helix.

A novel NS3/4A protease dependent cleavage site within pestiviral NS2

Pestiviruses like bovine viral diarrhoea virus (BVDV) and classical swine fever virus (CSFV) belong to the family Flaviviridae. A special feature of the Flaviviridae is the importance of nonstructural (NS) proteins for both genome replication and virion morphogenesis. The NS2-3-4A region and its regulated processing by the NS2 autoprotease and the NS3/4A protease plays a central role in the pestiviral life cycle. We report the identification and characterization of a novel internal cleavage in BVDV NS2, which is mediated by the NS3/4A protease. Further mapping using the NS2 of BVDV-1 strain NCP7 showed that cleavage occurs between L188 and G189. This cleavage site represents a novel sequence motif recognized by the NS3/4A protease and is conserved between the pestivirus species A, B and D. Inhibition of this internal NS2 cleavage by mutating the cleavage site did not cause obvious effects on RNA replication or virion morphogenesis in cultured cell lines. Accordingly, this novel internal NS2 cleavage adds an additional layer to the already complex polyprotein processing of Pestiviruses and might further extend the repertoires of the multifunctional NS2. However, unravelling of the functional relevance of this novel processing event in NS2, therefore, awaits future in vivo studies.

Downregulation of mitochondrial biogenesis by virus infection triggers antiviral responses by cyclic GMP-AMP synthase

In general, in mammalian cells, cytosolic DNA viruses are sensed by cyclic GMP-AMP synthase (cGAS), and RNA viruses are recognized by retinoic acid-inducible gene I (RIG-I)-like receptors, triggering a series of downstream innate antiviral signaling steps in the host. We previously reported that measles virus (MeV), which possesses an RNA genome, induces rapid antiviral responses, followed by comprehensive downregulation of host gene expression in epithelial cells. Interestingly, gene ontology analysis indicated that genes encoding mitochondrial proteins are enriched among the list of downregulated genes. To evaluate mitochondrial stress after MeV infection, we first observed the mitochondrial morphology of infected cells and found that significantly elongated mitochondrial networks with a hyperfused phenotype were formed. In addition, an increased amount of mitochondrial DNA (mtDNA) in the cytosol was detected during progression of infection. Based on these results, we show that cytosolic mtDNA released from hyperfused mitochondria during MeV infection is captured by cGAS and causes consequent priming of the DNA sensing pathway in addition to canonical RNA sensing. We also ascertained the contribution of cGAS to the in vivo pathogenicity of MeV. In addition, we found that other viruses that induce downregulation of mitochondrial biogenesis as seen for MeV cause similar mitochondrial hyperfusion and cytosolic mtDNA-priming antiviral responses. These findings indicate that the mtDNA-activated cGAS pathway is critical for full innate control of certain viruses, including RNA viruses that cause mitochondrial stress.

Viruses exert their pathogenicity by targeting various cellular components in infected cells. In response, host cells have evolved strategies to sense intracellular pathogen-associated molecules, such as nucleic acids derived from infected virus, and trigger subsequent antiviral responses to counteract infection. Measles virus (MeV), the causative agent of human measles, is the most highly contagious virus, killing 300 children per day worldwide; thus MeV has been targeted for eradication by the World Health Organization. In the present study, we found that MeV causes downregulation of mitochondrial biogenesis accompanied with aberrant hyperfusion of mitochondria in the infected cells. Furthermore, we show that cytoplasmic release of mitochondrial DNA activates DNA sensor molecule, cGAS, in addition to the innate immune response induced by the viral component. Importantly, this phenomenon was also observed for viruses, both RNA and DNA, which target mitochondrial biogenesis. Our study provides new insights into the mitochondrial stress by virus infection and an important host defense system to suppress viral propagation.

Characterization of Membrane Topology and Retention Signal of Pestiviral Glycoprotein E1

Pestiviruses are members of the family Flaviviridae, a group of enveloped viruses that bud at intracellular membranes. Pestivirus particles contain three glycosylated envelope proteins, E^rns, E1, and E2. Among them, E1 is the least characterized concerning both biochemical features and function. E1 from bovine viral diarrhea virus (BVDV) strain CP7 was analyzed with regard to its intracellular localization and membrane topology. Here, it is shown that even in the absence of other viral proteins, E1 is not secreted or expressed at the cell surface but localizes predominantly in the endoplasmic reticulum (ER). Using engineered chimeric transmembrane domains with sequences from E1 and vesicular stomatitis virus G protein, the E1 ER-retention signal could be narrowed down to six fully conserved polar residues in the middle part of the transmembrane domain of E1. Retention was observed even when several of these polar residues were exchanged for alanine. Mutations with a strong impact on E1 retention prevented recovery of infectious viruses when tested in the viral context. Analysis of the membrane topology of E1 before and after the signal peptide cleavage via a selective permeabilization and an in vivo labeling approach revealed that mature E1 is a typical type I transmembrane protein with a single span transmembrane anchor at its C terminus, whereas it adopts a hairpin-like structure with the C terminus located in the ER lumen when the precleavage situation is mimicked by blocking the cleavage site between E1 and E2.

IMPORTANCE The shortage of specific antibodies against E1, making detection and further analysis of E1 difficult, resulted in a lack of knowledge on E1 compared to E^rns and E2 with regard to biosynthesis, structure, and function. It is known that pestiviruses bud intracellularly. Here, we show that E1 contains its own ER retention signal: six fully conserved polar residues in the middle part of the transmembrane domain are shown to be the determinants for ER retention of E1. Moreover, those six polar residues could serve as a functional group that intensely affect the generation of infectious viral particles. In addition, the membrane topology of E1 has been determined. In this context, we also identified dynamic changes in membrane topology of E1 with the carboxy terminus located on the luminal side of the ER in the precleavage state and relocation of this sequence upon signal peptidase cleavage. Our work provides the first systematic analysis of the pestiviral E1 protein with regard to its biochemical and functional characteristics.

Interaction of Pestiviral E1 and E2 Sequences in Dimer Formation and Intracellular Retention

Pestiviruses contain three envelope proteins: E^rns, E1, and E2. Expression of HA-tagged E1 or mutants thereof showed that E1 forms homodimers and -trimers. C123 and, to a lesser extent, C171, affected the oligomerization of E1 with a double mutant C123S/C171S preventing oligomerization completely. E1 also establishes disulfide linked heterodimers with E2, which are crucial for the recovery of infectious viruses. Co-expression analyses with the HA-tagged E1 wt/E1 mutants and E2 wt/E2 mutants demonstrated that C123 in E1 and C295 in E2 are the critical sites for E1/E2 heterodimer formation. Introduction of mutations preventing E1/E2 heterodimer formation into the full-length infectious clone of BVDV CP7 prevented the recovery of infectious viruses, proving that C123 in E1 and C295 in E2 play an essential role in the BVDV life cycle, and further support the conclusion that heterodimer formation is the crucial step. Interestingly, we found that the retention signal of E1 is mandatory for intracellular localization of the heterodimer, so that absence of the E1 retention signal directs the heterodimer to the cell surface even though the E2 retention signal is still present. The covalent linkage between E1 and E2 plays an essential role for this process.

The Erns Carboxyterminus: Much More Than a Membrane Anchor

Pestiviruses express the unique essential envelope protein E^rns, which exhibits RNase activity, is attached to membranes by a long amphipathic helix, and is partially secreted from infected cells. The RNase activity of E^rns is directly connected with pestivirus virulence. Formation of homodimers and secretion of the protein are hypothesized to be important for its role as a virulence factor, which impairs the host's innate immune response to pestivirus infection. The unusual membrane anchor of E^rns raises questions with regard to proteolytic processing of the viral polyprotein at the E^rns carboxy-terminus. Moreover, the membrane anchor is crucial for establishing the critical equilibrium between retention and secretion and ensures intracellular accumulation of the protein at the site of virus budding so that it is available to serve both as structural component of the virion and factor controlling host immune reactions. In the present manuscript, we summarize published as well as new data on the molecular features of E^rns including aspects of its interplay with the other two envelope proteins with a special focus on the biochemistry of the E^rns membrane anchor.

Membrane Topology of Pestiviral Non-Structural Protein 2 and determination of the minimal autoprotease domain

Pestiviruses like bovine viral diarrhea virus (BVDV) belong to the family Flaviviridae A distinctive feature of the Flaviviridae is the importance of non-structural (NS) proteins for RNA genome replication and virus morphogenesis. For pestiviruses, the NS2 protease-mediated release of NS3 is essential for RNA replication, whereas uncleaved NS2-3 is indispensable for producing viral progeny. Accordingly, in the pestiviral life cycle the switch from RNA replication to virion morphogenesis is temporally regulated by the extent of NS2-3 cleavage, which is catalyzed by the NS2 autoprotease. A detailed knowledge of the structural and functional properties of pestiviral NS2 and NS2-3 is mandatory for a better understanding of these processes.In the present study, we experimentally determined the membrane topology of NS2 of BVDV-1 strain NCP7 by the Substituted Cysteine Accessibility Method (SCAM) assay. According to the resulting model, the N terminus of NS2 resides in the ER lumen and is followed by three transmembrane segments (TM) and a cytoplasmic C-terminal protease domain. We used the resulting model for fine mapping of the minimal autoprotease domain. Only one TM segment was found to be essential for maintaining residual autoprotease activity. While the topology of pestiviral NS2 is overall comparable to the one of hepatitis C virus (HCV) NS2, our data also reveal potentially important differences between the two molecules. The improved knowledge about structural and functional properties of this protein will support future functional and structural studies on pestiviral NS2.ImportancePestiviral NS2 is central to the regulation of RNA replication and virion morphogenesis via its autoprotease activity. This activity is temporally regulated by the cellular DNAJC14 as a cofactor: while free NS3 is required for RNA replication as a component of the viral replicase, only uncleaved NS2-3 supports virion morphogenesis. For a better understanding of the underlying molecular interactions, topological and structural data are required. The topology-based determination of the minimal NS2-protease domain in the present study will facilitate future attempts to determine the structure of this unusual protease cofactor complex. In the hepatitis C virus system, NS2 functions as a hub in virion morphogenesis by interacting with structural as well as non-structural proteins. Our knowledge of the membrane topology will significantly support future detailed interaction studies for pestiviral NS2.

Charged Residues in the Membrane Anchor of the Pestiviral Erns Protein Are Important for Processing and Secretion of Erns and Recovery of Infectious Viruses

The pestivirus envelope protein E^rns is anchored in membranes via a long amphipathic helix. Despite the unusual membrane topology of the E^rns membrane anchor, it is cleaved from the following glycoprotein E1 by cellular signal peptidase. This was proposed to be enabled by a salt bridge-stabilized hairpin structure (so-called charge zipper) formed by conserved charged residues in the membrane anchor. We show here that the exchange of one or several of these charged residues reduces processing at the E^rns carboxy-terminus to a variable extend, but reciprocal mutations restoring the possibility to form salt bridges did not necessarily restore processing efficiency. When introduced into an E^rns-only expression construct, these mutations enhanced the naturally occurring E^rns secretion significantly, but again to varying extents that did not correlate with the number of possible salt bridges. Equivalent effects on both processing and secretion were also observed when the proteins were expressed in avian cells, which points at phylogenetic conservation of the underlying principles. In the viral genome, some of the mutations prevented recovery of infectious viruses or immediately (pseudo)reverted, while others were stable and neutral with regard to virus growth.

Development of a T7 RNA polymerase expressing cell line using lentivirus vectors for the recovery of recombinant Newcastle disease virus

The development of a T7 RNA polymerase (T7 RNAP) expressing cell line i.e. BSR T7/5 cells marks an improvement of reverse genetics for the recovery of recombinant Newcastle disease virus (rNDV). BSR T7/5 is developed by transient transfection of plasmid encoding T7 RNAP gene for rNDV rescue. However, the gene expression decreases gradually over multiple passages and eventually hinders the rescue of rNDV. To address this issue, lentiviral vector was used to develop T7 RNAP-expressing HEK293-TA (HEK293-TA-Lv-T7) and SW620 (SW620-Lv-T7) cell lines, evidenced by the expression of T7 RNAP after subsequent 20 passages. rNDV was rescued successfully using HEK293-TA-Lv-T7 clones (R1D3, R1D8, R5B9) and SW620-Lv-T7 clones (R1C11, R3C5) by reverse transfection, yielding comparable virus rescue efficiency and virus titres to that of BSR T7/5. This study provides new tools for rNDV rescue and insights into cell line development and virology by reverse genetics.

Downstream Sequences Control the Processing of the Pestivirus Erns-E1 Precursor

Like other enveloped viruses, pestiviruses employ cellular proteases for processing of their structural proteins. While typical signal peptidase cleavage motifs are present at the carboxy terminus of the signal sequence preceding E^rns and the E1/E2 and E2/P7 sites, the E^rns-E1 precursor is cleaved by signal peptidase at a highly unusual structure, in which the transmembrane sequence upstream of the cleavage site is replaced by an amphipathic helix. As shown before, the integrity of the amphipathic helix is crucial for efficient processing. The data presented here demonstrate that the E1 sequence downstream of this cleavage site is also important for the cleavage. Carboxy-terminal truncation of the E1 moiety as well as internal deletions in E1 reduced the cleavage efficiency to less than 30% of the wild-type (wt) level. Moreover, the C-terminal truncation by more than 30 amino acids resulted in strong secretion of the uncleaved fusion proteins. The reduced processing and increased secretion were even observed when 10 to 5 amino-terminal residues of E1 were left, whereas extensions by 1 or 3 E1 residues resulted in reduced processing but no significantly increased secretion. In contrast to the E1 sequences, a 10-amino-acid c-myc tag fused to the E^rns C terminus had only marginal effect on secretion but was also not processed efficiently. Mutation of the von Heijne sequence upstream of E2 not only blocked the cleavage between E1 and E2 but also prevented the processing between E^rns and E2. Thus, processing at the E^rns-E1 site is a highly regulated process.IMPORTANCE Cellular signal peptidase (SPase) cleavage represents an important step in maturation of viral envelope proteins. Fine tuning of this system allows for establishment of concerted folding and processing processes in different enveloped viruses. We report here on SPase processing of the E^rns-E1-E2 glycoprotein precursor of pestiviruses. E^rns-E1 cleavage is delayed and only executed efficiently when the complete E1 sequence is present. C-terminal truncation of the E^rns-E1 precursor impairs processing and leads to significant secretion of the protein. The latter is not detected when internal deletions preserving the E1 carboxy terminus are introduced, but also these constructs show impaired processing. Moreover, E^rns-E1 is only processed after cleavage at the E1/E2 site. Thus, processing of the pestiviral glycoprotein precursor by SPase is done in an ordered way and depends on the integrity of the proteins for efficient cleavage. The functional importance of this processing scheme is discussed in the paper.

Advancing Marburg virus antiviral screening: Optimization of a novel T7 polymerase-independent minigenome system

Marburg virus (MARV) is the only known pathogenic filovirus not belonging to the genus Ebolavirus. Minigenomes have proven a useful tool to study MARV, but all existing MARV minigenomes are dependent on the addition of an exogenous T7 RNA polymerase to drive minigenome expression. However, exogenous expression of a T7 polymerase is not always feasible and can act as a confounding factor in compound screening assays. We have developed an alternative minigenome that is controlled by the natively expressed RNA polymerase II. We demonstrate here the characteristics of this new system and its applicability in a wide range of cell types. Our system shows a clear concentration-dependent activity and shows comparable activity to the existing T7 polymerase-based system at higher concentrations, also in difficult-to-transfect cell lines. In addition, we show that our system can be used for compound screening in a 96-well format, thereby providing an attractive alternative to previously developed MARV minigenomes.

Major Capsid Protein Synthesis from the Genomic RNA of Feline Calicivirus

Caliciviruses have a positive-strand RNA genome with a length of about 7.5 kb that contains 2, 3, or 4 functional open reading frames (ORFs). A subgenomic mRNA (sg-RNA) is transcribed in the infected cell, and both major capsid protein viral protein 1 (VP1) and minor capsid protein VP2 are translated from the sg-RNA. Translation of proteins from the genomic RNA (g-RNA) and from the sg-RNA is mediated by the RNA-linked viral protein VPg (virus protein, genome linked). Most of the calicivirus genera have translation mechanisms leading to VP1 expression from the g-RNA. VP1 is part of the polyprotein for sapoviruses, lagoviruses, and neboviruses, and a termination/reinitiation mechanism was described for noroviruses. Vesiviruses have no known mechanism for the expression of VP1 from the g-RNA, and the Vesivirus genus is the only genus of the Caliciviridae that generates VP1 via a precursor capsid leader protein (LC-VP1). Analyses of feline calicivirus (FCV) g-RNA translation showed a low level of VP1 expression with an initiation downstream of the original start codon of LC-VP1, leading to a smaller, truncated LC-VP1 (tLC-VP1) protein. Deletion and substitution analyses of the region surrounding the LC-VP1 start codon allowed the identification of sequences within the leader protein coding region of FCV that have an impact on VP1 translation frequency from the g-RNA. Introduction of such mutations into the virus showed an impact of strongly reduced tLC-VP1 levels translated from the g-RNA on viral replication.IMPORTANCE Caliciviruses are a cause of important diseases in humans and animals. It is crucial to understand the prerequisites of efficient replication of these viruses in order to develop strategies for prevention and treatment of these diseases. It was shown before that all caliciviruses except vesiviruses have established mechanisms to achieve major capsid protein (VP1) translation from the genomic RNA. Here, we show for the first time that a member of the genus Vesivirus also has a translation initiation mechanism by which a precursor protein of the VP1 protein is expressed from the genomic RNA. This finding clearly points at a functional role of the calicivirus VP1 capsid protein in early replication, and we provide experimental data supporting this hypothesis.

Structure and function of the Toscana virus cap-snatching endonuclease

Toscana virus (TOSV) is an arthropod-borne human pathogen responsible for seasonal outbreaks of fever and meningoencephalitis in the Mediterranean basin. TOSV is a segmented negative-strand RNA virus (sNSV) that belongs to the genus phlebovirus (family Phenuiviridae, order Bunyavirales), encompassing other important human pathogens such as Rift Valley fever virus (RVFV). Here, we carried out a structural and functional characterization of the TOSV cap-snatching endonuclease, an N terminal domain of the viral polymerase (L protein) that provides capped 3′OH primers for transcription. We report TOSV endonuclease crystal structures in the apo form, in complex with a di-ketoacid inhibitor (DPBA) and in an intermediate state of inhibitor release, showing details on substrate binding and active site dynamics. The structure reveals substantial folding rearrangements absent in previously reported cap-snatching endonucleases. These include the relocation of the N terminus and the appearance of new structural motifs important for transcription and replication. The enzyme shows high activity rates comparable to other His+ cap-snatching endonucleases. Moreover, the activity is dependent on conserved residues involved in metal ion and substrate binding. Altogether, these results bring new light on the structure and function of cap-snatching endonucleases and pave the way for the development of specific and broad-spectrum antivirals.

Rescue of recombinant Newcastle disease virus: a promising vector with two decades of intensive vaccine research

Two decades have passed since the first reverse genetics system for the rescue of recombinant Newcastle disease virus was developed. Since then, the recombinant Newcastle disease virus vector has shown promising results as a safe and potent vector for development of many vaccines for both avian and human use. Herein, we review several technical topics that would be useful to further understanding of this technology. First, the effect of using helper plasmids encoding proteins belonging to strains other than the full-length cDNA and the possible incorporation of these expressed proteins into progeny virus will be discussed. Then, we will discuss the effect of removal of additional G residues from the T7 initiation sequence and finally, we will review different ways to improve rescue efficiency.

Determination of Critical Requirements for Classical Swine Fever Virus NS2-3-Independent Virion Formation

For members of the Flaviviridae, it is known that, besides the structural proteins, nonstructural (NS) proteins also play a critical role in virion formation. Pestiviruses, such as bovine viral diarrhea virus (BVDV), rely on uncleaved NS2-3 for virion formation, while its cleavage product, NS3, is selectively active in RNA replication. This dogma was recently challenged by the selection of gain-of-function mutations in NS2 and NS3 which allowed virion formation in the absence of uncleaved NS2-3 in BVDV type 1 (BVDV-1) variants encoding either a ubiquitin (Ubi) (NS2-Ubi-NS3) or an internal ribosome entry site (IRES) (NS2-IRES-NS3) between NS2 and NS3. To determine whether the ability to adapt to NS2-3-independent virion morphogenesis is conserved among pestiviruses, we studied the corresponding NS2 and NS3 mutations (2/T444-V and 3/M132-A) in classical swine fever virus (CSFV). We observed that these mutations were capable of restoring low-level NS2-3-independent virion formation only for CSFV NS2-Ubi-NS3. Interestingly, a second NS2 mutation (V439-D), identified by selection, was essential for high-titer virion production. Similar to previous findings for BVDV-1, these mutations in NS2 and NS3 allowed for low-titer virion production only in CSFV NS2-IRES-NS3. For efficient virion morphogenesis, additional exchanges in NS4A (A48-T) and NS5B (D280-G) were required, indicating that these proteins cooperate in NS2-3-independent virion formation. Interestingly, both NS5B mutations, selected independently for NS2-IRES-NS3 variants of BVDV-1 and CSFV, are located in the fingertip region of the viral RNA-dependent RNA polymerase, classifying this structural element as a novel determinant for pestiviral NS2-3-independent virion formation. Together, these findings will stimulate further mechanistic studies on the genome packaging of pestiviruses.IMPORTANCE For Flaviviridae members, the nonstructural proteins are essential for virion formation and thus exert a dual role in RNA replication and virion morphogenesis. However, it remains unclear how these proteins are functionalized for either process. In wild-type pestiviruses, the NS3/4A complex is selectively active in RNA replication, while NS2-3/4A is essential for virion formation. Mutations recently identified in BVDV-1 rendered NS3/4A capable of supporting NS2-3-independent virion morphogenesis. A comparison of NS3/4A complexes incapable/capable of supporting virion morphogenesis revealed that changes in NS3/NS4A surface interactions are decisive for the gain of function. However, so far, the role of the NS2 mutations as well as the accessory mutations additionally required in the NS2-IRES-NS3 virus variant has not been clarified. To unravel the course of genome packaging, the additional sets of mutations obtained for a second pestivirus species (CSFV) are of significant importance to develop mechanistic models for this complex process.

Structure-function relationship in the 'termination upstream ribosomal binding site' of the calicivirus rabbit hemorrhagic disease virus

Caliciviruses use a termination/reinitiation mechanism for translation of their minor capsid protein VP2. A sequence element of about 80 nucleotides denoted ‘termination upstream ribosomal binding site’ (TURBS) is crucial for reinitiation. RNA secondary structure probing and computer aided secondary structure prediction revealed a rather low degree of secondary structure determinants for the TURBS of the rabbit hermorrhagic disease virus. Mutation analysis showed that prevention of duplex formation had major impact on the VP2 expression levels. Restoration of complementarity of the respective sequences by reciprocal mutation at least partially restored reinitiating rates. Synthetic TURBS structures preserving only the secondary structure forming sequences and the known short motifs important for TURBS function were found to drive reinitiation when the altered sequence could be predicted to allow establishment of the crucial secondary structures of the TURBS.

Structure of a functional cap-binding domain in Rift Valley fever virus L protein

Rift Valley fever virus (RVFV) belongs to the family of Phenuiviridae within the order of Bunyavirales. The virus may cause fatal disease both in livestock and humans, and therefore, is of great economical and public health relevance. In analogy to the influenza virus polymerase complex, the bunyavirus L protein is assumed to bind to and cleave off cap structures of cellular mRNAs to prime viral transcription. However, even though the presence of an endonuclease in the N-terminal domain of the L protein has been demonstrated for several bunyaviruses, there is no evidence for a cap-binding site within the L protein. We solved the structure of a C-terminal 117 amino acid-long domain of the RVFV L protein by X-ray crystallography. The overall fold of the domain shows high similarity to influenza virus PB2 cap-binding domain and the putative non-functional cap-binding domain of reptarenaviruses. Upon co-crystallization with m⁷GTP, we detected the cap-analogue bound between two aromatic side chains as it has been described for other cap-binding proteins. We observed weak but specific interaction with m⁷GTP rather than GTP in vitro using isothermal titration calorimetry. The importance of m⁷GTP-binding residues for viral transcription was validated using a RVFV minigenome system. In summary, we provide structural and functional evidence for a cap-binding site located within the L protein of a virus from the Bunyavirales order.

Rift Valley fever virus (RVFV) is endemic to sub-Saharan Africa and the Arabian Peninsula and leads to abortions in and death of ruminants. The virus can also be transmitted to humans causing febrile illness up to hemorrhagic fever with the possibility of fatal outcome. As there is currently no human vaccine or specific treatment available and because of the high epidemic potential, WHO has listed RVFV on its R&D Blueprint for urgent development of medical countermeasures. In order to amplify, the virus needs to transcribe and replicate the viral genome inside the cell cytoplasm. For transcription, the virus uses a process called cap-snatching, which is essentially depending on two functions presumed to reside within the large viral L protein: the ability to bind cap-structures and the activity of cleaving them off from cellular mRNA. Both functions could serve as specific targets for antiviral drug design. We identified and solved the structure of the cap-binding domain of RVFV and provide the first evidence for the presence of a functional cap-binding site in the L protein of bunyaviruses. Comparison with cap-binding proteins of related viruses revealed similarities and important differences critical for the development of potential broad-spectrum antivirals.

Reverse Genetics for Peste des Petits Ruminants Virus: Current Status and Lessons to Learn from Other Non-segmented Negative-Sense RNA Viruses

Peste des petits ruminants (PPR) is a highly contagious transboundary animal disease with a severe socio-economic impact on the livestock industry, particularly in poor countries where it is endemic. Full understanding of PPR virus (PPRV) pathobiology and molecular biology is critical for effective control and eradication of the disease. To achieve these goals, establishment of stable reverse genetics systems for PPRV would play a key role. Unfortunately, this powerful technology remains less accessible and poorly documented for PPRV. In this review, we discussed the current status of PPRV reverse genetics as well as the recent innovations and advances in the reverse genetics of other non-segmented negative-sense RNA viruses that could be applicable to PPRV. These strategies may contribute to the improvement of existing techniques and/or the development of new reverse genetics systems for PPRV.

Neuraminidase-Inhibiting Antibody Titers Correlate with Protection from Heterologous Influenza Virus Strains of the Same Neuraminidase Subtype

Immune responses induced by currently licensed inactivated influenza vaccines are mainly directed against the hemagglutinin (HA) glycoprotein, the immunodominant antigen of influenza viruses. The resulting antigenic drift of HA requires frequent updating of the vaccine composition and annual revaccination. On the other hand, the levels of antibodies directed against the neuraminidase (NA) glycoprotein, the second major influenza virus antigen, vary greatly. To investigate the potential of the more conserved NA protein for the induction of subtype-specific protection, vesicular stomatitis virus-based replicons expressing a panel of N1 proteins from prototypic seasonal and pandemic H1N1 strains and human H5N1 and H7N9 isolates were generated. Immunization of mice and ferrets with the replicon carrying the matched N1 protein resulted in robust humoral and cellular immune responses and protected against challenge with the homologous influenza virus with an efficacy similar to that of the matched HA protein, illustrating the potential of the NA protein as a vaccine antigen. The extent of protection after immunization with mismatched N1 proteins correlated with the level of cross-reactive neuraminidase-inhibiting antibody titers. Passive serum transfer experiments in mice confirmed that these functional antibodies determine subtype-specific cross-protection. Our findings illustrate the potential of NA-specific immunity for achieving broader protection against antigenic drift variants or newly emerging viruses carrying the same NA but a different HA subtype.IMPORTANCE Despite the availability of vaccines, annual influenza virus epidemics cause 250,000 to 500,000 deaths worldwide. Currently licensed inactivated vaccines, which are standardized for the amount of the hemagglutinin (HA) antigen, primarily induce strain-specific antibodies, whereas the immune response to the neuraminidase (NA) antigen, which is also present on the viral surface, is usually low. Using NA-expressing single-cycle vesicular stomatitis virus replicons, we show that the NA antigen conferred protection of mice and ferrets against not only the matched influenza virus strains but also viruses carrying NA proteins from other strains of the same subtype. The extent of protection correlated with the level of cross-reactive NA-inhibiting antibodies. This highlights the potential of the NA antigen for the development of more broadly protective influenza vaccines. Such vaccines may also provide partial protection against newly emerging strains with the same NA but a different HA subtype.

A GXXXA Motif in the Transmembrane Domain of the Ebola Virus Glycoprotein Is Required for Tetherin Antagonism

The interferon-induced antiviral host cell protein tetherin can inhibit the release of several enveloped viruses from infected cells. The Ebola virus (EBOV) glycoprotein (GP) antagonizes tetherin, but the domains and amino acids in GP that are required for tetherin antagonism have not been fully defined. A GXXXA motif within the transmembrane domain (TMD) of EBOV-GP was previously shown to be important for GP-mediated cellular detachment. Here, we investigated whether this motif also contributes to tetherin antagonism. Mutation of the GXXXA motif did not impact GP expression or particle incorporation and only modestly reduced EBOV-GP-driven entry. In contrast, the GXXXA motif was required for tetherin antagonism in transfected cells. Moreover, alteration of the GXXXA motif increased tetherin sensitivity of a replication-competent vesicular stomatitis virus (VSV) chimera encoding EBOV-GP. Although these results await confirmation with authentic EBOV, they indicate that a GXXXA motif in the TMD of EBOV-GP is important for tetherin antagonism. Moreover, they provide the first evidence that GP can antagonize tetherin in the context of an infectious EBOV surrogate.IMPORTANCE The glycoprotein (GP) of Ebola virus (EBOV) inhibits the antiviral host cell protein tetherin and may promote viral spread in tetherin-positive cells. However, tetherin antagonism by GP has so far been demonstrated only with virus-like particles, and it is unknown whether GP can block tetherin in infected cells. Moreover, a mutation in GP that selectively abrogates tetherin antagonism is unknown. Here, we show that a GXXXA motif in the transmembrane domain of EBOV-GP, which was previously reported to be required for GP-mediated cell rounding, is also important for tetherin counteraction. Moreover, analysis of this mutation in the context of vesicular stomatitis virus chimeras encoding EBOV-GP revealed that GP-mediated tetherin counteraction is operative in infected cells. To our knowledge, these findings demonstrate for the first time that GP can antagonize tetherin in infected cells and provide a tool to study the impact of GP-dependent tetherin counteraction on EBOV spread.

Rescue of Sendai Virus from Cloned cDNA

Sendai virus (SeV) is a non-segment negative-sense RNA virus that naturally infects and causes pneumonia in mice. As a prototypic member of the family Paramyxoviridae, SeV has been characterized well, and these studies revealed numerous traits of paramyxovirus biology. The reverse genetics system to rescue SeV was first established in 1995. The virus was rescued from a cloned cDNA that contains full genome sequence flanked by T7 promoter and hepatitis delta virus ribozyme. To rescue SeV, it is necessary to infect cells with a recombinant vaccinia virus vTF7.3 that expresses T7 RNA polymerase, and transfect with the SeV full genome cDNAs together with supporting plasmids encoding NP, P, and L genes under the T7 promoter. Synthesized viral RNA by T7 RNA polymerase will be encapsidated with NP and associated with a polymerase complex composed of P and L. The polymerase complex transcribes and replicates the genome, and produces progeny virions. Rescued SeV needs to be plaque purified to exclude vTF7.3 from viral stock. Reverse genetics system of SeV is relatively efficient compared to other paramyxoviruses, but alternative approaches to rescue poorly growing mutant viruses are also available.

The glycoprotein of vesicular stomatitis virus promotes release of virus-like particles from tetherin-positive cells

Vesicular stomatitis virus (VSV) release from infected cells is inhibited by the interferon (IFN)-inducible antiviral host cell factor tetherin (BST-2, CD317). However, several viruses encode tetherin antagonists and it is at present unknown whether residual VSV spread in tetherin-positive cells is also promoted by a virus-encoded tetherin antagonist. Here, we show that the viral glycoprotein (VSV-G) antagonizes tetherin in transfected cells, although with reduced efficiency as compared to the HIV-1 Vpu protein. Tetherin antagonism did not involve alteration of tetherin expression and was partially dependent on a GXXXG motif in the transmembrane domain of VSV-G. However, mutation of the GXXXG motif did not modulate tetherin sensitivity of infectious VSV. These results identify VSV-G as a tetherin antagonist in transfected cells but fail to provide evidence for a contribution of tetherin antagonism to viral spread.

An RNA polymerase II-driven Ebola virus minigenome system as an advanced tool for antiviral drug screening

Ebola virus (EBOV) causes a severe disease in humans with the potential for significant international public health consequences. Currently, treatments are limited to experimental vaccines and therapeutics. Therefore, research into prophylaxis and antiviral strategies to combat EBOV infections is of utmost importance. The requirement for high containment laboratories to study EBOV infection is a limiting factor for conducting EBOV research. To overcome this issue, minigenome systems have been used as valuable tools to study EBOV replication and transcription mechanisms and to screen for antiviral compounds at biosafety level 2. The most commonly used EBOV minigenome system relies on the ectopic expression of the T7 RNA polymerase (T7), which can be limiting for certain cell types. We have established an improved EBOV minigenome system that utilizes endogenous RNA polymerase II (pol II) as a driver for the synthesis of minigenome RNA. We show here that this system is as efficient as the T7-based minigenome system, but works in a wider range of cell types, including biologically relevant cell types such as bat cells. Importantly, we were also able to adapt this system to a reliable and cost-effective 96-well format antiviral screening assay with a Z-factor of 0.74, indicative of a robust assay. Using this format, we identified JG40, an inhibitor of Hsp70, as an inhibitor of EBOV replication, highlighting the potential for this system as a tool for antiviral drug screening. In summary, this updated EBOV minigenome system provides a convenient and effective means of advancing the field of EBOV research.

Reovirus FAST Protein Enhances Vesicular Stomatitis Virus Oncolytic Virotherapy in Primary and Metastatic Tumor Models

The reovirus fusion-associated small transmembrane (FAST) proteins are the smallest known viral fusogens (∼100–150 amino acids) and efficiently induce cell-cell fusion and syncytium formation in multiple cell types. Syncytium formation enhances cell-cell virus transmission and may also induce immunogenic cell death, a form of apoptosis that stimulates immune recognition of tumor cells. These properties suggest that FAST proteins might serve to enhance oncolytic virotherapy. The oncolytic activity of recombinant VSVΔM51 (an interferon-sensitive vesicular stomatitis virus [VSV] mutant) encoding the p14 FAST protein (VSV-p14) was compared with a similar construct encoding GFP (VSV-GFP) in cell culture and syngeneic BALB/c tumor models. Compared with VSV-GFP, VSV-p14 exhibited increased oncolytic activity against MCF-7 and 4T1 breast cancer spheroids in culture and reduced primary 4T1 breast tumor growth in vivo. VSV-p14 prolonged survival in both primary and metastatic 4T1 breast cancer models, and in a CT26 metastatic colon cancer model. As with VSV-GFP, VSV-p14 preferentially replicated in vivo in tumors and was cleared rapidly from other sites. Furthermore, VSV-p14 increased the numbers of activated splenic CD4, CD8, natural killer (NK), and natural killer T (NKT) cells, and increased the number of activated CD4 and CD8 cells in tumors. FAST proteins may therefore provide a multi-pronged approach to improving oncolytic virotherapy via syncytium formation and enhanced immune stimulation.

Rescue of recombinant Newcastle disease virus: a short history of how it all started

Reverse genetics of viruses has come a long way, and many recombinant viruses have been generated since the first successful "rescues" were reported in the late 1970s. Recombinant Newcastle disease virus (rNDV), a non-segmented negative-sense RNA virus (NSNSV), was first rescued in 1999 using a reverse genetics approach similar to that reported for other recombinant viruses of the order Mononegavirales a few years before. The route from an original NDV isolate to the generation of its recombinant counterpart requires many steps that have to be sequentially and carefully completed. Background knowledge of each of these steps is essential because it allows one to make the best choices for fulfilling the specific requirements of the final recombinant virus. We have previously reviewed the latest strategies in cloning the NDV full-length cDNA into transcription vectors and the use of different RNA polymerase systems for the generation of viral RNA from plasmid DNA. In this article, we review a number of discoveries on the mechanism of transcription and replication of NDV, including a brief history behind the discovery of its RNP complex. This includes the generation of artificial and functional RNP constructs, in combination with the smart use of available knowledge and technologies that ultimately resulted in rescue of the first rNDV.

A positive-strand RNA virus uses alternative protein-protein interactions within a viral protease/cofactor complex to switch between RNA replication and virion morphogenesis

The viruses of the family Flaviviridae possess a positive-strand RNA genome and express a single polyprotein which is processed into functional proteins. Initially, the nonstructural (NS) proteins, which are not part of the virions, form complexes capable of genome replication. Later on, the NS proteins also play a critical role in virion formation. The molecular basis to understand how the same proteins form different complexes required in both processes is so far unknown. For pestiviruses, uncleaved NS2-3 is essential for virion morphogenesis while NS3 is required for RNA replication but is not functional in viral assembly. Recently, we identified two gain of function mutations, located in the C-terminal region of NS2 and in the serine protease domain of NS3 (NS3 residue 132), which allow NS2 and NS3 to substitute for uncleaved NS2-3 in particle assembly. We report here the crystal structure of pestivirus NS3-4A showing that the NS3 residue 132 maps to a surface patch interacting with the C-terminal region of NS4A (NS4A-kink region) suggesting a critical role of this contact in virion morphogenesis. We show that destabilization of this interaction, either by alanine exchanges at this NS3/4A-kink interface, led to a gain of function of the NS3/4A complex in particle formation. In contrast, RNA replication and thus replicase assembly requires a stable association between NS3 and the NS4A-kink region. Thus, we propose that two variants of NS3/4A complexes exist in pestivirus infected cells each representing a basic building block required for either RNA replication or virion morphogenesis. This could be further corroborated by trans-complementation studies with a replication-defective NS3/4A double mutant that was still functional in viral assembly. Our observations illustrate the presence of alternative overlapping surfaces providing different contacts between the same proteins, allowing the switch from RNA replication to virion formation.

Many positive-strand RNA viruses replicate without transcribing subgenomic RNAs otherwise often used to temporally coordinate the expression of proteins involved either in genome replication (early) or virion formation (late). Instead, the RNA genomes of the Flaviviridae are translated into a single polyprotein. Their nonstructural proteins (NS), while not present in the virions, are known to be crucially involved in RNA replication and virion formation. The important question how the same proteins form specific complexes required for fundamentally different aspects of the viral replication cycle is not solved yet. For pestiviruses the mature NS3/4A complex is an essential component of the viral RNA-replicase but is incapable of participating in virion morphogenesis which in turn depends on uncleaved NS2-3 in complex with NS4A. However, a gain of function mutation in NS3 enabled the NS3/4A complex to function in virion assembly. Using structure guided mutagenesis in combination with functional studies we identified the interface between NS3 and the C-terminal NS4A region as a module critical for the decision whether a NS3/4A complex serves in RNA replication or as a packaging component. Thus, we propose that subtle changes in local protein interactions represent decisive switches in viral complex formation pathways.

Rescue of recombinant Newcastle disease virus: current cloning strategies and RNA polymerase provision systems

Since the first rescue of a recombinant Newcastle disease virus (rNDV) in the late 1990s, many more rNDVs have been rescued by researchers around the world. Regardless of methodology, the main principle behind rescue of the virus has remained the same, i.e., the formation of a functional replication complex by simultaneously providing the full-length viral RNA and the viral NP, P and L proteins. However, different strategies have been reported for the insertion of the full-length genome into a suitable transcription vector, which remains the most challenging step of the rescue. Moreover, several systems have been published for provision of the DNA-dependent RNA polymerase, which is needed for transcription of viral RNA (vRNA) from the transfected plasmid DNA. The aim of this article is to consolidate all of the current cDNA assembly strategies and transcription systems used in rescue of rNDV in order to attain a better understanding of the advantages and disadvantages of each approach.

Modified Vaccinia Virus Ankara: History, Value in Basic Research, and Current Perspectives for Vaccine Development

Safety tested Modified Vaccinia virus Ankara (MVA) is licensed as third-generation vaccine against smallpox and serves as a potent vector system for development of new candidate vaccines against infectious diseases and cancer. Historically, MVA was developed by serial tissue culture passage in primary chicken cells of vaccinia virus strain Ankara, and clinically used to avoid the undesirable side effects of conventional smallpox vaccination. Adapted to growth in avian cells MVA lost the ability to replicate in mammalian hosts and lacks many of the genes orthopoxviruses use to conquer their host (cell) environment. As a biologically well-characterized mutant virus, MVA facilitates fundamental research to elucidate the functions of poxvirus host-interaction factors. As extremely safe viral vectors MVA vaccines have been found immunogenic and protective in various preclinical infection models. Multiple recombinant MVA currently undergo clinical testing for vaccination against human immunodeficiency viruses, Mycobacterium tuberculosis or Plasmodium falciparum. The versatility of the MVA vector vaccine platform is readily demonstrated by the swift development of experimental vaccines for immunization against emerging infections such as the Middle East Respiratory Syndrome. Recent advances include promising results from the clinical testing of recombinant MVA-producing antigens of highly pathogenic avian influenza virus H5N1 or Ebola virus. This review summarizes our current knowledge about MVA as a unique strain of vaccinia virus, and discusses the prospects of exploiting this virus as research tool in poxvirus biology or as safe viral vector vaccine to challenge existing and future bottlenecks in vaccinology.

Partial Protection against Porcine Influenza A Virus by a Hemagglutinin-Expressing Virus Replicon Particle Vaccine in the Absence of Neutralizing Antibodies

This work was initiated by previous reports demonstrating that mismatched influenza A virus (IAV) vaccines can induce enhanced disease, probably mediated by antibodies. Our aim was, therefore, to investigate if a vaccine inducing opsonizing but not neutralizing antibodies against the hemagglutinin (HA) of a selected heterologous challenge virus would enhance disease or induce protective immune responses in the pig model. To this end, we immunized pigs with either whole inactivated virus (WIV)-vaccine or HA-expressing virus replicon particles (VRP) vaccine based on recombinant vesicular stomatitis virus (VSV). Both types of vaccines induced virus neutralizing and opsonizing antibodies against homologous virus as shown by a highly sensitive plasmacytoid dendritic cell-based opsonization assay. Opsonizing antibodies showed a broader reactivity against heterologous IAV compared with neutralizing antibodies. Pigs immunized with HA-recombinant VRP vaccine were partially protected from infection with a mismatched IAV, which was not neutralized but opsonized by the immune sera. The VRP vaccine reduced lung lesions, lung inflammatory cytokine responses, serum IFN-α responses, and viral loads in the airways. Only the VRP vaccine was able to prime IAV-specific IFNγ/TNFα dual secreting CD4(+) T cells detectable in the peripheral blood. In summary, this work demonstrates that with the virus pair selected, a WIV vaccine inducing opsonizing antibodies against HA which lack neutralizing activity, is neither protective nor does it induce enhanced disease in pigs. In contrast, VRP-expressing HA is efficacious vaccines in swine as they induced both potent antibodies and T-cell immunity resulting in a broader protective value.

Marburg Virus Reverse Genetics Systems

The highly pathogenic Marburg virus (MARV) is a member of the Filoviridae family and belongs to the group of nonsegmented negative-strand RNA viruses. Reverse genetics systems established for MARV have been used to study various aspects of the viral replication cycle, analyze host responses, image viral infection, and screen for antivirals. This article provides an overview of the currently established MARV reverse genetic systems based on minigenomes, infectious virus-like particles and full-length clones, and the research that has been conducted using these systems.

Comparative Structural and Functional Analysis of Bunyavirus and Arenavirus Cap-Snatching Endonucleases

Segmented negative strand RNA viruses of the arena-, bunya- and orthomyxovirus families uniquely carry out viral mRNA transcription by the cap-snatching mechanism. This involves cleavage of host mRNAs close to their capped 5′ end by an endonuclease (EN) domain located in the N-terminal region of the viral polymerase. We present the structure of the cap-snatching EN of Hantaan virus, a bunyavirus belonging to hantavirus genus. Hantaan EN has an active site configuration, including a metal co-ordinating histidine, and nuclease activity similar to the previously reported La Crosse virus and Influenza virus ENs (orthobunyavirus and orthomyxovirus respectively), but is more active in cleaving a double stranded RNA substrate. In contrast, Lassa arenavirus EN has only acidic metal co-ordinating residues. We present three high resolution structures of Lassa virus EN with different bound ion configurations and show in comparative biophysical and biochemical experiments with Hantaan, La Crosse and influenza ENs that the isolated Lassa EN is essentially inactive. The results are discussed in the light of EN activation mechanisms revealed by recent structures of full-length influenza virus polymerase.

Segmented negative strand viruses (sNSV) such as Influenza, Lassa or Hantaan viruses are responsible for a large number of severe human infectious diseases. Currently, there are vaccines and antiviral treatments available for influenza but none for the infections caused by other sNSV. All carry out transcription by the cap-snatching mechanism, which requires the action of a metal ion dependent endonuclease (EN), a domain within their large viral polymerases. Here we provide the crystal structure of the Hantaan virus (family Bunyaviridae) and Lassa virus (family Arenaviridae) cap-snatching ENs in complex with manganese and a comparative functional study of their catalytic activity. Despite the high structural homology between the two ENs a few changes in the active site, involving a catalytic histidine, cause a different binding of the metal ions with dramatic consequences for their in vitro activity. Hantaan EN binds the metal ions as Influenza A (family Orthomyxoviridae) and LACV (family Bunyaviridae) ENs and all three are active in vitro. In contrast Lassa virus EN is inactive in the same experimental conditions. We can now classify sNSV into two functionally distinct groups (His+ and His- ENs), providing a broad view of the sNSV cap-snatching ENs properties that will be determinant for the comprehensive development of broad-spectrum antiviral drugs. These results also have implications for the viral transcription regulation in the light of recent studies on full-length sNSV polymerases.

Characterization of the Determinants of NS2-3-Independent Virion Morphogenesis of Pestiviruses

A peculiarity of the Flaviviridae is the critical function of nonstructural (NS) proteins for virus particle formation. For pestiviruses, like bovine viral diarrhea virus (BVDV), uncleaved NS2-3 represents an essential factor for virion morphogenesis, while NS3 is an essential component of the viral replicase. Accordingly, in natural pestivirus isolates, processing at the NS2-3 cleavage site is not complete, to allow for virion morphogenesis. Virion morphogenesis of the related hepatitis C virus (HCV) shows a major deviation from that of pestiviruses: while RNA replication also requires free NS3, virion formation does not depend on uncleaved NS2-NS3. Recently, we described a BVDV-1 chimera based on strain NCP7 encompassing the NS2-4B*-coding region of strain Osloss (E. Lattwein, O. Klemens, S. Schwindt, P. Becher, and N. Tautz, J Virol 86:427-437, 2012, doi:10.1128/JVI.06133-11). This chimera allowed for the production of infectious virus particles in the absence of uncleaved NS2-3. The Osloss sequence deviates in the NS2-4B* part from NCP7 in 48 amino acids and also has a ubiquitin insertion between NS2 and NS3. The present study demonstrates that in the NCP7 backbone, only two amino acid exchanges in NS2 (E1576V) and NS3 (V1721A) are sufficient and necessary to allow for efficient NS2-3-independent virion morphogenesis. The adaptation of a bicistronic virus encompassing an internal ribosomal entry site element between the NS2 and NS3 coding sequences to efficient virion morphogenesis led to the identification of additional amino acids in E2, NS2, and NS5B that are critically involved in this process. The surprisingly small requirements for approximating the packaging schemes of pestiviruses and HCV with respect to the NS2-3 region is in favor of a common mechanism in an ancestral virus.

IMPORTANCE

For positive-strand RNA viruses, the processing products of the viral polyprotein serve in RNA replication as well as virion morphogenesis. For bovine viral diarrhea virus, nonstructural protein NS2-3 is of critical importance to switch between these processes. While free NS3 is essential for RNA replication, uncleaved NS2-3, which accumulates over time in the infected cell, is required for virion morphogenesis. In contrast, the virion morphogenesis of the related hepatitis C virus is independent from uncleaved NS2-NS3. Here, we demonstrate that pestiviruses can adapt to virion morphogenesis in the absence of uncleaved NS2-3 by just two amino acid exchanges. While the mechanism behind this gain of function remains elusive, the fact that it can be achieved by such minor changes is in line with the assumption that an ancestral virus already used this mechanism but lost it in the course of adapting to a new host/infection strategy.

Use of Recombinant Virus Replicon Particles for Vaccination against Mycobacterium ulcerans Disease

Buruli ulcer, caused by infection with Mycobacterium ulcerans, is a necrotizing disease of the skin and subcutaneous tissue, which is most prevalent in rural regions of West African countries. The majority of clinical presentations seen in patients are ulcers on limbs that can be treated by eight weeks of antibiotic therapy. Nevertheless, scarring and permanent disabilities occur frequently and Buruli ulcer still causes high morbidity. A vaccine against the disease is so far not available but would be of great benefit if used for prophylaxis as well as therapy. In the present study, vesicular stomatitis virus-based RNA replicon particles encoding the M. ulcerans proteins MUL2232 and MUL3720 were generated and the expression of the recombinant antigens characterized in vitro. Immunisation of mice with the recombinant replicon particles elicited antibodies that reacted with the endogenous antigens of M. ulcerans cells. A prime-boost immunization regimen with MUL2232-recombinant replicon particles and recombinant MUL2232 protein induced a strong immune response but only slightly reduced bacterial multiplication in a mouse model of M. ulcerans infection. We conclude that a monovalent vaccine based on the MUL2232 antigen will probably not sufficiently control M. ulcerans infection in humans.

Infection with Mycobacterium ulcerans can lead to a slow progressing, ulcerative disease of the skin and underlying soft tissue called Buruli ulcer. The disease is most prevalent in rural African communities with limited access to health care facilities. The most efficient means to prevent the disease, a vaccine against Buruli ulcer is not available to date. In the present study we investigated the immunogenicity and protective potential of a single cycle virus system expressing the two M. ulcerans antigens MUL2232 and MUL3720. Immunization of mice with those vesicular stomatitis virus replicon particles led to the induction of humoral as well as cellular immune responses in the immunized animals. Subsequent challenge experiments in a mouse model of M. ulcerans infection demonstrated only a limited reduction of bacterial burden in mice immunized with a prime-boost approach with MUL2232. Most probably, a vaccine formulation with only one antigen will not be able to provide protection against Buruli ulcer in humans.

A G-quadruplex-binding macrodomain within the "SARS-unique domain" is essential for the activity of the SARS-coronavirus replication-transcription complex

The multi-domain non-structural protein 3 of SARS-coronavirus is a component of the viral replication/transcription complex (RTC). Among other domains, it contains three sequentially arranged macrodomains: the X domain and subdomains SUD-N as well as SUD-M within the “SARS-unique domain”. The X domain was proposed to be an ADP-ribose-1”-phosphatase or a poly(ADP-ribose)-binding protein, whereas SUD-NM binds oligo(G)-nucleotides capable of forming G-quadruplexes. Here, we describe the application of a reverse genetic approach to assess the importance of these macrodomains for the activity of the SARS-CoV RTC. To this end, Renilla luciferase-encoding SARS-CoV replicons with selectively deleted macrodomains were constructed and their ability to modulate the RTC activity was examined. While the SUD-N and the X domains were found to be dispensable, the SUD-M domain was crucial for viral genome replication/transcription. Moreover, alanine replacement of charged amino-acid residues of the SUD-M domain, which are likely involved in G-quadruplex-binding, caused abrogation of RTC activity.

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A SARS-CoV replicon encoding Renilla luciferase as reporter protein is constructed.

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The role of three macrodomains for the replication/transcription complex is analyzed.

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In contrast to macrodomains X and SUD-N, SUD-M is found indispensable for replication.

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Site-directed mutagenesis identifies charged SUD-M residues required for replication.

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These residues have previously been shown to be involved in G-quadruplex binding.

A conserved NS3 surface patch orchestrates NS2 protease stimulation, NS5A hyperphosphorylation and HCV genome replication

Hepatitis C virus (HCV) infection is a leading cause of liver disease worldwide. The HCV RNA genome is translated into a single polyprotein. Most of the cleavage sites in the non-structural (NS) polyprotein region are processed by the NS3/NS4A serine protease. The vital NS2-NS3 cleavage is catalyzed by the NS2 autoprotease. For efficient processing at the NS2/NS3 site, the NS2 cysteine protease depends on the NS3 serine protease domain. Despite its importance for the viral life cycle, the molecular details of the NS2 autoprotease activation by NS3 are poorly understood. Here, we report the identification of a conserved hydrophobic NS3 surface patch that is essential for NS2 protease activation. One residue within this surface region is also critical for RNA replication and NS5A hyperphosphorylation, two processes known to depend on functional replicase assembly. This dual function of the NS3 surface patch prompted us to reinvestigate the impact of the NS2-NS3 cleavage on NS5A hyperphosphorylation. Interestingly, NS2-NS3 cleavage turned out to be a prerequisite for NS5A hyperphosphorylation, indicating that this cleavage has to occur prior to replicase assembly. Based on our data, we propose a sequential cascade of molecular events: in uncleaved NS2-NS3, the hydrophobic NS3 surface patch promotes NS2 protease stimulation; upon NS2-NS3 cleavage, this surface region becomes available for functional replicase assembly. This model explains why efficient NS2-3 cleavage is pivotal for HCV RNA replication. According to our model, the hydrophobic surface patch on NS3 represents a module critically involved in the temporal coordination of HCV replicase assembly.

Fine mapping and characterization of the L-polymerase-binding domain of the respiratory syncytial virus phosphoprotein

The minimum requirement for an active RNA-dependent RNA polymerase of respiratory syncytial virus (RSV) is a complex made of two viral proteins, the polymerase large protein (L) and the phosphoprotein (P). Here we have investigated the domain on P that is responsible for this critical P-L interaction. By use of recombinant proteins and serial deletions, an L binding site was mapped in the C-terminal region of P, just upstream of the N-RNA binding site. The role of this molecular recognition element of about 30 amino acid residues in the L-P interaction and RNA polymerase activity was evaluated in cellula using an RSV minigenome system and site-directed mutagenesis. The results highlighted the critical role of hydrophobic residues located in this region.

IMPORTANCE

Respiratory syncytial virus (RSV) is the leading cause of lower respiratory tract illness in infants. Since no vaccine and no good antivirals against RSV are available, it is essential to better understand how the viral machinery functions in order to develop new antiviral strategies. Like all negative-strand RNA viruses, RSV codes for its own machinery to replicate and transcribe its genome. The core of this machinery is composed of two proteins, the phosphoprotein (P) and the large protein (L). Here, using recombinant proteins, we have mapped and characterized the P domain responsible for this L-P interaction and the formation of an active L-P complex. These findings extend our understanding of the mechanism of action of RSV RNA polymerase and allow us to define a new target for the development of drugs against RSV.

The interaction between the hepatitis C proteins NS4B and NS5A is involved in viral replication

Hepatitis C virus (HCV) replicates in membrane associated, highly ordered replication complexes (RCs). These complexes include viral and host proteins necessary for viral RNA genome replication. The interaction network among viral and host proteins underlying the formation of these RCs is yet to be thoroughly characterized. Here, we investigated the association between NS4B and NS5A, two critical RC components. We characterized the interaction between these proteins using fluorescence resonance energy transfer and a mammalian two-hybrid system. Specific tryptophan residues within the C-terminal domain (CTD) of NS4B were shown to mediate this interaction. Domain I of NS5A, was sufficient to mediate its interaction with NS4B. Mutations in the NS4B CTD tryptophan residues abolished viral replication. Moreover, one of these mutations also affected NS5A hyperphosphorylation. These findings provide new insights into the importance of the NS4B-NS5A interaction and serve as a starting point for studying the complex interactions between the replicase subunits.

Biological and protective properties of immune sera directed to the influenza virus neuraminidase

The envelope of influenza A viruses contains two large antigens, hemagglutinin (HA) and neuraminidase (NA). Conventional influenza virus vaccines induce neutralizing antibodies that are predominantly directed to the HA globular head, a domain that is subject to extensive antigenic drift. Antibodies directed to NA are induced at much lower levels, probably as a consequence of the immunodominance of the HA antigen. Although antibodies to NA may affect virus release by inhibiting the sialidase function of the glycoprotein, the antigen has been largely neglected in past vaccine design. In this study, we characterized the protective properties of monospecific immune sera that were generated by vaccination with recombinant RNA replicon particles encoding NA. These immune sera inhibited hemagglutination in an NA subtype-specific and HA subtype-independent manner and interfered with infection of MDCK cells. In addition, they inhibited the sialidase activities of various influenza viruses of the same and even different NA subtypes. With this, the anti-NA immune sera inhibited the spread of H5N1 highly pathogenic avian influenza virus and HA/NA-pseudotyped viruses in MDCK cells in a concentration-dependent manner. When chickens were immunized with NA recombinant replicon particles and subsequently infected with low-pathogenic avian influenza virus, inflammatory serum markers were significantly reduced and virus shedding was limited or eliminated. These findings suggest that NA antibodies can inhibit virus dissemination by interfering with both virus attachment and egress. Our results underline the potential of high-quality NA antibodies for controlling influenza virus replication and place emphasis on NA as a vaccine antigen.

IMPORTANCE

The neuraminidase of influenza A viruses is a sialidase that acts as a receptor-destroying enzyme facilitating the release of progeny virus from infected cells. Here, we demonstrate that monospecific anti-NA immune sera inhibited not only sialidase activity, but also influenza virus hemagglutination and infection of MDCK cells, suggesting that NA antibodies can interfere with virus attachment. Inhibition of both processes, virus release and virus binding, may explain why NA antibodies efficiently blocked virus dissemination in vitro and in vivo. Anti-NA immune sera showed broader reactivity than anti-HA sera in hemagglutination inhibition tests and demonstrated cross-subtype activity in sialidase inhibition tests. These remarkable features of NA antibodies highlight the importance of the NA antigen for the development of next-generation influenza virus vaccines.

Role of the C terminus of Lassa virus L protein in viral mRNA synthesis

The N terminus of arenavirus L protein contains an endonuclease presumably involved in "cap snatching." Here, we employed the Lassa virus replicon system to map other L protein sites that might be involved in this mechanism. Residues Phe-1979, Arg-2018, Phe-2071, Asp-2106, Trp-2173, Tyr-2179, Arg-2200, and Arg-2204 were important for viral mRNA synthesis but dispensable for genome replication. Thus, the C terminus of L protein is involved in the mRNA synthesis process, potentially by mediating cap binding.

Molecular determinants defining the triggering range of prefusion F complexes of canine distemper virus

The morbillivirus cell entry machinery consists of a fusion (F) protein trimer that refolds to mediate membrane fusion following receptor-induced conformational changes in its binding partner, the tetrameric attachment (H) protein. To identify molecular determinants that control F refolding, we generated F chimeras between measles virus (MeV) and canine distemper virus (CDV). We located a central pocket in the globular head domain of CDV F that regulates the stability of the metastable, prefusion conformational state of the F trimer. Most mutations introduced into this "pocket'" appeared to mediate a destabilizing effect, a phenotype associated with enhanced membrane fusion activity. Strikingly, under specific triggering conditions (i.e., variation of receptor type and H protein origin), some F mutants also exhibited resistance to a potent morbillivirus entry inhibitor, which is known to block F triggering by enhancing the stability of prefusion F trimers. Our data reveal that the molecular nature of the F stimulus and the intrinsic stability of metastable prefusion F both regulate the efficiency of F refolding and escape from small-molecule refolding blockers.

IMPORTANCE

With the aim to better characterize the thermodynamic basis of morbillivirus membrane fusion for cell entry and spread, we report here that the activation energy barrier of prefusion F trimers together with the molecular nature of the triggering "stimulus" (attachment protein and receptor types) define a "triggering range," which governs the initiation of the membrane fusion process. A central "pocket" microdomain in the globular F head contributes substantially to the regulation of the conformational stability of the prefusion complexes. The triggering range also defines the mechanism of viral escape from entry inhibitors and describes how the cellular environment can affect membrane fusion efficiency.