Paramyxovirus membrane fusion: lessons from the F and HN atomic structures

Low-pH-induced membrane fusion mediated by human metapneumovirus F protein is a rare, strain-dependent phenomenon

Membrane fusion promoted by human metapneumovirus (HMPV) fusion (F) protein was suggested to require low pH (R. M. Schowalter, S. E. Smith, and R. E. Dutch, J. Virol. 80:10931-10941, 2006). Using prototype F proteins representing the four HMPV genetic lineages, we detected low-pH-dependent fusion only with some lineage A proteins and not with lineage B proteins. A glycine at position 294 was found responsible for the low-pH requirement in lineage A proteins. Only 6% of all HMPV lineage A F sequences have 294G, and none of the lineage B sequences have 294G. Thus, acidic pH is not a general trigger of HMPV F proteins for activity.

Reprogrammed viruses as cancer therapeutics: targeted, armed and shielded

Virotherapy is currently undergoing a renaissance, based on our improved understanding of virus biology and genetics and our better knowledge of many different types of cancer. Viruses can be reprogrammed into oncolytic vectors by combining three types of modification: targeting, arming and shielding. Targeting introduces multiple layers of cancer specificity and improves safety and efficacy; arming occurs through the expression of prodrug convertases and cytokines; and coating with polymers and the sequential usage of different envelopes or capsids provides shielding from the host immune response. Virus-based therapeutics are beginning to find their place in cancer clinical practice, in combination with chemotherapy and radiation.

Functional interaction between paramyxovirus fusion and attachment proteins

Paramyxovirinae envelope glycoproteins constitute a premier model to dissect how specific and dynamic interactions in multisubunit membrane protein complexes can control deep-seated conformational rearrangements. However, individual residues that determine reciprocal specificity of the viral attachment and fusion (F) proteins have not been identified. We have developed an assay based on a pair of canine distemper virus (CDV) F proteins (strains Onderstepoort (ODP) and Lederle) that share approximately 95% identity but differ in their ability to form functional complexes with the measles virus (MV) attachment protein (H). Characterization of CDV F chimeras and mutagenesis reveals four residues in CDV F-ODP (positions 164, 219, 233, and 317) required for productive interaction with MV H. Mutating these residues to the Lederle type disrupts triggering of F-ODP by MV H without affecting functionality when co-expressed with CDV H. Co-immunoprecipitation shows a stronger physical interaction of F-ODP than F-Lederle with MV H. Mutagenesis of MV F highlights the MV residues homologous to CDV F residues 233 and 317 as determinants for physical glycoprotein interaction and fusion activity under homotypic conditions. In assay reversal, the introduction of sections of the CDV H stalk into MV H shows a five-residue fragment (residues 110-114) to mediate specificity for CDV F-Lederle. All of the MV H stalk chimeras are surface-expressed, show hemadsorption activity, and trigger MV F. Combining the five-residue H chimera with the CDV F-ODP quadruple mutant partially restores activity, indicating that the residues identified in either glycoprotein contribute interdependently to the formation of functional complexes. Their localization in structural models of F and H suggests that placement in particular of F residue 233 in close proximity to the 110-114 region of H is structurally conceivable.

Insertion of the two cleavage sites of the respiratory syncytial virus fusion protein in Sendai virus fusion protein leads to enhanced cell-cell fusion and a decreased dependency on the HN attachment protein for activity

Cell entry by paramyxoviruses requires fusion of the viral envelope with the target cell membrane. Fusion is mediated by the viral fusion (F) glycoprotein and usually requires the aid of the attachment glycoprotein (G, H or HN, depending on the virus). Human respiratory syncytial virus F protein (F(RSV)) is able to mediate membrane fusion in the absence of the attachment G protein and is unique in possessing two multibasic furin cleavage sites, separated by a region of 27 amino acids (pep27). Cleavage at both sites is required for cell-cell fusion. We have investigated the significance of the two cleavage sites and pep27 in the context of Sendai virus F protein (F(SeV)), which possesses a single monobasic cleavage site and requires both coexpression of the HN attachment protein and trypsin in order to fuse cells. Inclusion of both F(RSV) cleavage sites in F(SeV) resulted in a dramatic increase in cell-cell fusion activity in the presence of HN. Furthermore, chimeric F(SeV) mutants containing both F(RSV) cleavage sites demonstrated cell-cell fusion in the absence of HN. The presence of two multibasic cleavage sites may therefore represent a strategy to regulate activation of a paramyxovirus F protein for cell-cell fusion in the absence of an attachment protein.

Role of endocytosis and cathepsin-mediated activation in Nipah virus entry

The recent discovery that the Nipah virus (NiV) fusion protein (F) is activated by endosomal cathepsin L raised the question if NiV utilize pH- and protease-dependent mechanisms of entry. We show here that the NiV receptor ephrin B2, virus-like particles and infectious NiV are internalized from the cell surface. However, endocytosis, acidic pH and cathepsin-mediated cleavage are not necessary for the initiation of infection of new host cells. Our data clearly demonstrate that proteolytic activation of the NiV F protein is required before incorporation into budding virions but not after virus entry.

A virus-encoded cell-cell fusion machine dependent on surrogate adhesins

The reovirus fusion-associated small transmembrane (FAST) proteins function as virus-encoded cellular fusogens, mediating efficient cell–cell rather than virus–cell membrane fusion. With ectodomains of only ∼20–40 residues, it is unclear how such diminutive viral fusion proteins mediate the initial stages (i.e. membrane contact and close membrane apposition) of the fusion reaction that precede actual membrane merger. We now show that the FAST proteins lack specific receptor-binding activity, and in their natural biological context of promoting cell–cell fusion, rely on cadherins to promote close membrane apposition. The FAST proteins, however, are not specifically reliant on cadherin engagement to mediate membrane apposition as indicated by their ability to efficiently utilize other adhesins in the fusion reaction. Results further indicate that surrogate adhesion proteins that bridge membranes as close as 13 nm apart enhance FAST protein-induced cell–cell fusion, but active actin remodelling is required for maximal fusion activity. The FAST proteins are the first example of membrane fusion proteins that have specifically evolved to function as opportunistic fusogens, designed to exploit and convert naturally occurring adhesion sites into fusion sites. The capacity of surrogate, non-cognate adhesins and active actin remodelling to enhance the cell–cell fusion activity of the FAST proteins are features perfectly suited to the structural and functional evolution of these fusogens as the minimal fusion component of a virus-encoded cellular fusion machine. These results also provide a basis for reconciling the rudimentary structure of the FAST proteins with their capacity to fuse cellular membranes.

Much of our current understanding of how proteins mediate membrane fusion derives from the study of enveloped virus fusion proteins. These fusion protein complexes function autonomously to co-ordinately regulate virus–cell attachment and subsequent membrane merger. In contrast, the reovirus Fusion-Associated Small Transmembrane (FAST) proteins are the only example of virus-encoded cellular fusogens, specifically designed to mediate cell–cell rather than virus–cell membrane fusion. In view of their small size, it was unclear if, or how, the FAST proteins are responsible for promoting the membrane attachment and close apposition stages of the fusion reaction. We now show that the FAST proteins have specifically evolved to function as the fusion component in a biphasic cell–cell fusion reaction, where the membrane attachment and membrane merger stages represent two distinct, uncoupled phases. Exploiting cadherins as surrogate adhesins, the FAST proteins have retained within their rudimentary structures the minimal determinants required to convert pre-existing adherens junctions into sites of cell–cell membrane fusion. These results raise the interesting possibility that other, yet to be identified cellular fusion proteins may resemble the FAST proteins, using separate adhesins and less complex fusion proteins in a similar biphasic membrane fusion reaction.

Characterization of equine arteritis virus particles and demonstration of their hemolytic activity

Equine arteritis virus (EAV), a member of the newly established family Arteriviridae, is a small, positive-stranded RNA virus. It carries two protein complexes in its envelope, gp5/M and the recently described gp2b/gp3/gp4 complex. We report here on several basic features of EAV replication in cell culture and on the protein composition of virus particles. We have also characterized gp2b, gp3, and gp4 expressed using a baculovirus system in insect cells. Finally, we provide evidence that EAV possess hemagglutinating and hemolytic activity. The hemolysis assay might be useful for determining which of the surface proteins carries the receptor-binding and membrane fusion activity of EAV.

Genetic characterization of RS-12 (S-12), an Iranian isolate of mumps virus, by sequence analysis and comparative genomics of F, SH, and HN genes

RS-12 mumps virus strain was isolated in 1986, in monkey kidney cells, from the throat-washing of an Iranian patient and developed to RS-12 vaccine by serial passage of the pathogen in MRC-5 cells. During the present study, an early passage RS-12 containing its virulent pathogenic phenotype, was characterized genetically. Its F, SH and HN genes were isolated by RT-PCR amplification and sequenced. It is quite evident that RS-12 belongs to genotype H, closely related to European strains but distinguishable from Asian strains. The deduced amino acid sequences of HN and F proteins that comprise immunogenic epitopes, were compared to other vaccine and wild strains. The multiple sequence alignment revealed that the RS-12 has isoleucine and aspartic acid at positions 269 and 523 of its F and HN proteins, respectively, which could differentiate RS-12 from other available sequences. This isolate has trivial variations in the major antigenic sites of HN protein. The frequency and pattern of F and HN glycosylation sites seems to be similar to most other strains. It seems that the mumps regional outbreak during 1986 in Iran was caused by genotype H and this strain has been spreading in countries surrounding the Caspian sea for over 17 years. These data support the previous results that RS-12 could be an efficient vaccine, especially in the Middle East. This is the first genotype report from Iranian isolates and provides strong data on the molecular epidemiology of mumps in Iran, the Middle East, Central Asia, Russia and other countries of this region.

Membrane Fusion

Membrane fusion of enveloped viruses with cellular membranes is mediated by viral glycoproteins, which are activated by proteolytic cleavage and/or interaction with cellular receptors. Fusion entails extensive conformational changes of the fusion protein that initially anchors to cellular membranes thus bridging two bilayers. Further refolding into a hairpin-like structure pulls viral and cellular membranes into close proximity to permit lipid bilayer fusion. Refolding provides the energy for fusion, which follows several defined lipidic intermediate states occurring concomitantly with refolding. Although different classes of fusion proteins use different structural motifs, the principle of repositioning both membrane anchors, the transmembrane and the fusion peptide region, at the same end of an elongated hairpin structure generated by receptor binding is maintained in all known viral fusion protein structures, suggesting that they follow similar principles to achieve lipid bilayer fusion.

Identification and evaluation of a highly effective fusion inhibitor for human metapneumovirus

Human metapneumovirus (hMPV) can cause acute upper and lower respiratory tract infections that are particularly severe in young children, elderly subjects, and immunocompromised patients. To date, no treatments or vaccines are available for hMPV infections. Our objective was to assess the inhibitory potential of several peptides derived from the heptad repeat A and B (HRA and HRB) domains of the hMPV fusion protein. Nine candidate peptides were expressed in Escherichia coli or obtained synthetically and tested in vitro and in an animal model. Excellent in vitro inhibition of an hMPV strain of the A1 subgroup was obtained with five peptides, with 50% inhibitory concentrations ranging from 1.4 nM to 3.3 microM. One peptide, HRA2, displayed very potent activity against all four hMPV subgroups. It was also moderately active against human respiratory syncytial virus (strain A2) but displayed no activity against human parainfluenza virus type 3. BALB/c mice that received the HRA2 peptide and a lethal hMPV intranasal challenge simultaneously were completely protected from clinical symptoms and mortality. On day 5 postinfection, HRA2-treated mice had undetectable lung viral loads which were significantly less than those of untreated mice (3 x 10(4) 50% tissue culture infective doses/lung). Pulmonary inflammation, levels of proinflammatory cytokines/chemokines (RANTES, gamma interferon, and monocyte chemoattractant protein 1) and airway obstruction were also significantly decreased in HRA2-treated mice. The results of this study demonstrate that potent antivirals can be derived from the hMPV fusion protein HR domains. Moreover, hMPV, compared to other paramyxoviruses and to the human immunodeficiency virus, seems to be more susceptible to HRA- than HRB-derived peptides.

Immunization of Syrian golden hamsters with F subunit vaccine of human metapneumovirus induces protection against challenge with homologous or heterologous strains

Human metapneumovirus (hMPV), a newly discovered paramyxovirus, is associated with acute respiratory-tract illness, primarily in young children, individuals with underlying disease and the elderly. Two genetic lineages of hMPV circulate around the world, and viruses from these two lineages demonstrate antigenic differences. The clinical impact of hMPV warrants the development of vaccines. Recombinant soluble fusion (F) proteins of prototype viruses of the two main lineages of hMPV that can be produced in high yields have been constructed. In this study, the antigenicity, immunogenicity and protective efficacy of these soluble F subunit vaccines were evaluated in Syrian golden hamsters (Mesocricetus auratus). Immunization of hamsters with the soluble F proteins, adjuvanted with Specol or iscom matrix, induced high virus-neutralization titres, with higher titres against the homologous than the heterologous virus. The neutralizing antibodies protected from subsequent infection of the lungs with both homologous and heterologous virus. Upon challenge, viral titres in the nasal turbinates of immunized animals were reduced significantly compared with those of PBS-immunized animals. In conclusion, a soluble F subunit vaccine for hMPV that induces cross-protective immunity for infection of the lower respiratory tract in Syrian golden hamsters has been generated.

Respiratory syncytial virus and innate immunity: a complex interplay of exploitation and subversion

Respiratory syncytial virus causes significant disease in infants, the elderly and select groups of immunocompromised patients. Healthy individuals are also naturally infected with respiratory syncytial virus repeatedly throughout life. Therefore, safe and effective vaccines and therapies are needed. However, a number of factors have prevented development of such antiviral interventions to date. These include a failed vaccine trial, the very young age of the primary target population (neonates), the inability of natural infection to induce long-term protective immunity, and an incomplete understanding of virus-host interactions. The identification of pattern recognition receptors has led to significant increases in our understanding of induction and regulation of innate immune responses. This review will address the impact of these findings on respiratory syncytial virus research.

Analysis of residues near the fusion peptide in the influenza hemagglutinin structure for roles in triggering membrane fusion

Influenza virus entry occurs in endosomes, where acidification triggers irreversible conformational changes of the hemagglutinin glycoprotein (HA) that are required for membrane fusion. The acid-induced HA structural rearrangements have been well documented, and several models have been proposed to relate these to the process of membrane fusion. However, details regarding the role of specific residues in the initiation of structural rearrangements and membrane fusion are lacking. Here we report the results of studies on the HA of A/Aichi/2/68 virus (H3 subtype), in which mutants with changes at several ionizable residues in the vicinity of the "fusion peptide" were analyzed for their effects on the pH at which conformational changes and membrane fusion occur. A variety of phenotypes was obtained, including examples of substitutions that lead to an increase in HA stability at reduced pH. Of particular note was the observation that a histidine to tyrosine substitution at HA1 position 17 resulted in a decrease in pH at which HA structural changes and membrane fusion take place by 0.3 relative to WT. The results are discussed in relation to possible mechanisms by which HA structural rearrangements are initiated at low pH and clade-specific differences near the fusion peptide.

Affinity thresholds for membrane fusion triggering by viral glycoproteins

Enveloped viruses trigger membrane fusion to gain entry into cells. The receptor affinities of their attachment proteins vary greatly, from 10(-4) M to 10(-9) M, but the significance of this is unknown. Using six retargeted measles viruses that bind to Her-2/neu with a 5-log range in affinity, we show that receptor affinity has little impact on viral attachment but is nevertheless a key determinant of infectivity and intercellular fusion. For a given cell surface receptor density, there is an affinity threshold above which cell-cell fusion proceeds efficiently. Suprathreshold affinities do not further enhance the efficiency of membrane fusion.

Evaluation of Newcastle disease virus chimeras expressing the Hemagglutinin-Neuraminidase protein of velogenic strains in the context of a mesogenic recombinant virus backbone

A major factor in the pathogenicity of Newcastle disease virus (NDV) is the amino acid sequence of the fusion protein cleavage site, but the role of other viral genes that contribute to virulence and different clinical forms of the disease remain undefined. To assess the role of other NDV genes in virus pathogenicity, a reverse genetics system was developed using the mesogenic NDV Anhinga strain to provide a backbone for generating gene mutations or gene exchanges in attempts to enhance or attenuate the virulence of that virus. Chimeras created by exchange of the Anhinga Hemagglutinin-Neuraminidase (HN) gene with HN genes of neurotropic and viscerotropic velogenic viruses produced no significant change in virus pathogenicity as assessed by conducting the mean death time and intracerebral pathogenicity index assays and by inoculation of susceptible day-old SPF chickens. Inclusion in the recombinant construct of homotypic F genes, obtained from the parental viruses, also failed to enhance the pathotype of the recombinant viruses to a velogenic pathotype. A HN gene exchange alone within the context of the NDV Anhinga backbone failed to increase virus virulence from mesogenic to velogenic pathotype and suggests a multigenic role for NDV pathogenicity.

Single amino acid changes in the Nipah and Hendra virus attachment glycoproteins distinguish ephrinB2 from ephrinB3 usage

The henipaviruses, Nipah virus (NiV) and Hendra virus (HeV), are lethal emerging paramyxoviruses. EphrinB2 and ephrinB3 have been identified as receptors for henipavirus entry. NiV and HeV share similar cellular tropisms and likely use an identical receptor set, although a quantitative comparison of receptor usage by NiV and HeV has not been reported. Here we show that (i) soluble NiV attachment protein G (sNiV-G) bound to cell surface-expressed ephrinB3 with a 30-fold higher affinity than that of sHeV-G, (ii) NiV envelope pseudotyped reporter virus (NiVpp) entered ephrinB3-expressing cells much more efficiently than did HeV pseudotyped particles (HeVpp), and (iii) NiVpp but not HeVpp entry was inhibited efficiently by soluble ephrinB3. These data underscore the finding that NiV uses ephrinB3 more efficiently than does HeV. Henipavirus G chimeric protein analysis implicated residue 507 in the G ectodomain in efficient ephrinB3 usage. Curiously, alternative versions of published HeV-G sequences show variations at residue 507 that can clearly affect ephrinB3 but not ephrinB2 usage. We further defined surrounding mutations (W504A and E505A) that diminished ephrinB3-dependent binding and viral entry without compromising ephrinB2 receptor usage and another mutation (E533Q) that abrogated both ephrinB2 and -B3 usage. Our results suggest that ephrinB2 and -B3 binding determinants on henipavirus G are distinct and dissociable. Global expression analysis showed that ephrinB3, but not ephrinB2, is expressed in the brain stem. Thus, ephrinB3-mediated viral entry and pathology may underlie the severe brain stem neuronal dysfunction seen in fatal Nipah viral encephalitis. Characterizing the determinants of ephrinB2 versus -B3 usage will further our understanding of henipavirus pathogenesis.

Functional links between the fusion peptide-proximal polar segment and membrane-proximal region of human immunodeficiency virus gp41 in distinct phases of membrane fusion

The binding of CD4 and chemokine receptors to the gp120 attachment glycoprotein of human immunodeficiency virus triggers refolding of the associated gp41 fusion glycoprotein into a trimer of hairpins with a 6-helix bundle (6HB) core. These events lead to membrane fusion and viral entry. Here, we examined the functions of the fusion peptide-proximal polar segment and membrane-proximal Trp-rich region (MPR), which are exterior to the 6HB. Alanine substitution of Trp(666), Trp(672), Phe(673), and Ile(675) in the MPR reduced entry by up to 120-fold without affecting gp120-gp41 association or cell-cell fusion. The L537A polar segment mutation led to the loss of gp120 from the gp120-gp41 complex, reduced entry by approximately 10-fold, but did not affect cell-cell fusion. Simultaneous Ala substitution of Leu(537) with Trp(666), Trp(672), Phe(673), or Ile(675) abolished entry with 50-80% reductions in cell-cell fusion. gp120-gp41 complexes of fusion-defective double mutants were resistant to soluble CD4-induced shedding of gp120, suggesting that their ability to undergo receptor-induced conformational changes was compromised. Consistent with this idea, a representative mutation, L537A/W666A, led to an approximately 80% reduction in lipophilic fluorescent dye transfer between gp120-gp41-expressing cells and receptor-expressing targets, indicating a block prior to the lipid-mixing phase. The L537A/W666A double mutation increased the chymotrypsin sensitivity of the polar segment in a trimer of hairpins model, comprising the 6HB core, the polar segment, and MPR linked N-terminally to maltose-binding protein. The data indicate that the polar segment and MPR of gp41 act synergistically in forming a fusion-competent gp120-gp41 complex and in stabilizing the membrane-interactive end of the trimer of hairpins.

A conserved region in the F(2) subunit of paramyxovirus fusion proteins is involved in fusion regulation

Paramyxoviruses utilize both an attachment protein and a fusion (F) protein to drive virus-cell and cell-cell fusion. F exists functionally as a trimer of two disulfide-linked subunits: F(1) and F(2). Alignment and analysis of a set of paramyxovirus F protein sequences identified three conserved blocks (CB): one in the fusion peptide/heptad repeat A domain, known to play important roles in fusion promotion, one in the region between the heptad repeats of F(1) (CBF(1)) (A. E. Gardner, K. L. Martin, and R. E. Dutch, Biochemistry 46:5094-5105, 2007), and one in the F(2) subunit (CBF(2)). To analyze the functions of CBF(2), alanine substitutions at conserved positions were created in both the simian virus 5 (SV5) and Hendra virus F proteins. A number of the CBF(2) mutations resulted in folding and expression defects. However, the CBF(2) mutants that were properly expressed and trafficked had altered fusion promotion activity. The Hendra virus CBF(2) Y79A and P89A mutants showed significantly decreased levels of fusion, whereas the SV5 CBF(2) I49A mutant exhibited greatly increased cell-cell fusion relative to that for wild-type F. Additional substitutions at SV5 F I49 suggest that both side chain volume and hydrophobicity at this position are important in the folding of the metastable, prefusion state and the subsequent triggering of membrane fusion. The recently published prefusogenic structure of parainfluenza virus 5/SV5 F (H. S. Yin et al., Nature 439:38-44, 2006) places CBF(2) in direct contact with heptad repeat A. Our data therefore indicate that this conserved region plays a critical role in stabilizing the prefusion state, likely through interactions with heptad repeat A, and in triggering membrane fusion.

Pathobiology of henipavirus entry: insights into therapeutic strategies

The recently emerged paramyxoviruses, Nipah (NiV) and Hendra (HeV), designated as Biosafety Level 4 pathogens, can cause lethal respiratory and neurological disease in both animals and humans. NiV outbreaks have been associated with efficient transmission amongst livestock (pigs) and mortality rates exceeding 70%, with documented cases of human-to-human transmission. Without vaccines or effective therapeutics, NiV and HeV continue to present an impending threat to global health and economies. The current understanding of henipavirus pathobiology has led to the development of small animal models reflecting certain aspects of the human pathology. In this review, we discuss how these animal models have been critical in testing vaccination strategies and in eliciting neutralizing antibodies against the envelope glycoproteins. Additionally, the discovery of the viral receptor and development of pseudotyped-viral systems have allowed us to explore the multiple opportunities for therapeutic intervention existing within the individual steps of the henipavirus entry pathway. Current research shows promise for the future development of effective strategies to limit the impact of these biological threats.

Nonnucleoside inhibitor of measles virus RNA-dependent RNA polymerase complex activity

Paramyxoviruses comprise several major human pathogens. Although a live-attenuated vaccine protects against measles virus (MV), a member of the paramyxovirus family, the virus remains a principal cause of worldwide mortality and accounts for approximately 21 million cases and 300,000 to 400,000 deaths annually. The development of novel antivirals that allow improved case management of severe measles and silence viral outbreaks is thus highly desirable. We have previously described the development of novel MV fusion inhibitors. The potential for preexisting or emerging resistance in the field constitutes the rationale for the identification of additional MV inhibitors with a diverse target spectrum. Here, we report the development and implementation of a cell-based assay for high-throughput screening of MV antivirals, which has yielded several hit candidates. Following confirmation by secondary assays and chemical synthesis, the most potent hit was found to act as a target-specific inhibitor of MV replication with desirable drug-like properties. The compound proved highly active against multiple primary isolates of diverse MV genotypes currently circulating worldwide, showing active concentrations of 35 to 145 nM. Significantly, it does not interfere with viral entry and lacks cross-resistance with the MV fusion inhibitor class. Mechanistic characterization on a subinfection level revealed that the compound represents a first-in-class nonnucleoside inhibitor of MV RNA-dependent RNA polymerase complex activity. Singly or in combination with the fusion inhibitors, this novel compound class has high developmental potential as a potent therapeutic against MV and will likely further the mechanistic characterization of the viral polymerase complex.

Mapping antigenic diversity and strain specificity of mumps virus: A bioinformatics approach

Mumps is an acute infectious disease caused by mumps virus, a member of the family Paramyxoviridae. With the implementation of vaccination programs, mumps infection is under control. However, due to resurgence of mumps epidemics, there is a renewed interest in understanding the antigenic diversity of mumps virus. Hemagglutinin-neuraminidase (HN) is the major surface antigen and is known to elicit neutralizing antibodies. Mutational analysis of HN of wild-type and vaccine strains revealed that the hypervariable positions are distributed over the entire length with no detectable pattern. In the absence of experimentally derived 3D structure data, the structure of HN protein of mumps virus was predicted using homology modeling. Mutations mapped on the predicted structures were found to cluster on one of the surfaces. A predicted conformational epitope encompasses experimentally characterized epitopes suggesting that it is a major site for neutralization. These analyses provide rationale for strain specificity, antigenic diversity and varying efficacy of mumps vaccines.

Mutational evidence of internal fusion loops in herpes simplex virus glycoprotein B

Herpes simplex virus type 1 (HSV-1) glycoprotein B (gB) is one of four glycoproteins necessary and sufficient for HSV cellular entry. Recently, the crystal structures of HSV-1 gB and vesicular stomatitis virus glycoprotein G were determined. Surprisingly, the two proteins share remarkable structural homology. Both proteins are homotrimeric and center about a long alpha-helix, features reminiscent of class I fusion proteins, such as influenza virus hemagglutinin or paramyxovirus F. However, these structures revealed that G has internal fusion loops, similar to the fusion loops of the class II fusion proteins, and that these loops are structurally conserved in gB. To examine whether these putative fusion loops are important for gB function, we mutated potential membrane-interacting (hydrophobic) residues to charged amino acids. Of most interest were mutant gB proteins that were expressed on the cell surface and were recognized by monoclonal antibodies against conformational epitopes but lacked the ability to function in cell-cell fusion assays. We find that three of the five hydrophobic amino acids targeted in these loops, tryptophan 174, tyrosine 179, and alanine 261, are integral in the function of gB. Our data suggest that they are part of an important functional domain. We hypothesize that two loops in domain 1 of HSV gB function as fusion loops. Our data are further evidence that gB is a viral fusogen and suggest clues as to how gB may function.

Virus membrane fusion

Membrane fusion of enveloped viruses with cellular membranes is mediated by viral glycoproteins (GP). Interaction of GP with cellular receptors alone or coupled to exposure to the acidic environment of endosomes induces extensive conformational changes in the fusion protein which pull two membranes into close enough proximity to trigger bilayer fusion. The refolding process provides the energy for fusion and repositions both membrane anchors, the transmembrane and the fusion peptide regions, at the same end of an elongated hairpin structure in all fusion protein structures known to date. The fusion process follows several lipidic intermediate states, which are generated by the refolding process. Although the major principles of viral fusion are understood, the structures of fusion protein intermediates and their mode of lipid bilayer interaction, the structures and functions of the membrane anchors and the number of fusion proteins required for fusion, necessitate further investigations.

Canine distemper virus infection requires cholesterol in the viral envelope

Cholesterol is known to play an important role in stabilizing particular cellular membrane structures, so-called lipid or membrane rafts. For several viruses, a dependence on cholesterol for virus entry and/or morphogenesis has been shown. Using flow cytometry and fluorescence microscopy, we demonstrate that infection of cells by canine distemper virus (CDV) was not impaired after cellular cholesterol had been depleted by the drug methyl-beta-cyclodextrin. This effect was independent of the multiplicity of infection and the cellular receptor used for infection. However, cholesterol depletion of the viral envelope significantly reduced CDV infectivity. Replenishment by addition of exogenous cholesterol restored infectivity up to 80%. Thus, we conclude that CDV entry is dependent on cholesterol in the viral envelope. Furthermore, reduced syncytium formation was observed when the cells were cholesterol depleted during the course of the infection. This may be related to the observation that CDV envelope proteins H and F partitioned into cellular detergent-resistant membranes. Therefore, a role for lipid rafts during virus assembly and release as well is suggested.

Spring-loaded heptad repeat residues regulate the expression and activation of paramyxovirus fusion protein

During viral entry, the paramyxovirus fusion (F) protein fuses the viral envelope to a cellular membrane. Similar to other class I viral fusion glycoproteins, the F protein has two heptad repeat regions (HRA and HRB) that are important in membrane fusion and can be targeted by antiviral inhibitors. Upon activation of the F protein, HRA refolds from a spring-loaded, crumpled structure into a coiled coil that inserts a hydrophobic fusion peptide into the target membrane and binds to the HRB helices to form a fusogenic hairpin. To investigate how F protein conformational changes are regulated, we mutated in the Sendai virus F protein a highly conserved 10-residue sequence in HRA that undergoes major structural changes during protein refolding. Nine of the 15 mutations studied caused significant defects in F protein expression, processing, and fusogenicity. Conversely, the remaining six mutations enhanced the fusogenicity of the F protein, most likely by helping spring the HRA coil. Two of the residues that were neither located at "a" or "d" positions in the heptad repeat nor conserved among the paramyxoviruses were key regulators of the folding and fusion activity of the F protein, showing that residues not expected to be important in coiled-coil formation may play important roles in regulating membrane fusion. Overall, the data support the hypothesis that regions in the F protein that undergo dramatic changes in secondary and tertiary structure between the prefusion and hairpin conformations regulate F protein expression and activation.

Synthesis and evaluation of 4-O-alkylated 2-deoxy-2,3-didehydro-N-acetylneuraminic acid derivatives as inhibitors of human parainfluenza virus type-3 sialidase activity

The X-ray crystal structure of the paramyxoviral surface glycoprotein haemagglutinin-neuraminidase (HN) from Newcastle Disease virus was used as a template to design inhibitors of the HN from human parainfluenza virus type-3 (hPIV-3). 4-O-Alkylated derivatives of 2-deoxy-2,3-didehydro-N-acetylneuraminic acid (Neu5Ac2en), accessed from 8,9-O-isopropylidenated-Neu5Ac2en1Me, were found to inhibit the sialidase (neuraminidase) activity of hPIV-3 (strain C243) in the range of 3-30muM. This is comparable or improved activity compared to the parent 4-hydroxy compound.

The multipartite system that mediates entry of herpes simplex virus into the cell

The multipartite entry-fusion system of herpes simplex virus is made of a quartet of glycoproteins-gD, gB, gH.gL-and three alternative gD receptors, herpesvirus entry mediator (HVEM), nectin1 and modified sites on heparan sulphate. This multipartite system recapitulates the basic steps of virus-cell fusion, i.e. receptor recognition, triggering of fusion and fusion execution. Specifically, in addition to serving as the receptor-binding glycoprotein, gD triggers fusion through a specialised domain, named pro-fusion domain (PFD), located C-terminally in the ectodomain. In the unliganded gD the C-terminal region folds around the N-terminal region, such that gD adopts a closed autoinhibited conformation. In HVEM- and nectin1-bound gD the C-terminal region is displaced (opened conformation). gD is the tool for modification of HSV tropism, through insertion of ligands to heterologous tumour-specific receptors. It is discussed whether gD responds to the interaction with the natural and the heterologous receptors by adopting similar conformations, and whether the closed-to-open switch in conformation is a generalised mechanism of activation. A peculiar recombinant highlighted that the central Ig-folded core of gD may not encode executable functions for entry and that the 219-314 aa segment may be sufficient to trigger fusion. With respect to fusion execution, gB appears to be a prospective fusogen based on its coiled-coil trimeric structure, similar to that of another fusion glycoprotein. On the other hand, gH exhibits molecular elements typical of class 1 fusion glycoproteins, in particular heptad repeats and strong tendency to interact with lipids. Whether fusion execution is carried out by gB or gH.gL, or both glycoproteins in complex or sequentially remains to be determined.

Effects of Ectodomain Sequences between HR1 and HR2 of F1 Protein on the Specific Membrane Fusion in Paramyxoviruses

OBJECTIVES

To explore the effects of ectodomain sequences between HR1 and HR2 of F1 protein on the specific interaction with its homologous hemagglutinin-neuraminidase (HN) in paramyxoviruses.

METHODS

Site-directed mutagenesis was used to obtain mutants containing new enzyme sites on the F genes of Newcastle disease virus (NDV) and human parainfluenza virus (hPIV), and four DNA segments located between the HR1 and HR2 (NDV F-1, hPIV F-1, NDV F-2 and hPIV F-2) were obtained by cutting mutant F genes with specific endonucleases. Gene recombination was used to get chimeric F proteins NDV-C1 and hPIV-C1 by exchanging NDV F-1 and hPIV F-1 each other, and NDV-C2 and hPIV-C2 were also obtained by the same way. All the mutants and chimeric F proteins were co-expressed with their homologous or heterologous HN proteins in eukaryocytes. The fusion functions were assayed with Giemsa staining and reporter gene method for qualitative and quantitative analyses, respectively. The cell surface expression of F proteins was assayed with fluorescence-activated cell sorter (FACS) for quantitative analysis.

RESULTS

All the mutants of F proteins had the same functions as their relevant wild types. Chimeric F proteins NDV-C1 and hPIV-C1 had 76.34 and 65.82% of fusion activities, and NDV-C2 and hPIV-C2 had 96.25 and 93.78% of fusion activities, respectively, as compared with their relevant wild types. The analysis of FACS indicated that all the mutants and chimeric F proteins had almost the same expression efficiencies as their relevant wild types.

CONCLUSIONS

The segments of NDV F-1 and hPIV F-1 were important for their specific membrane fusion, but NDV F-2 and hPIV F-2 were not.

Paramyxovirus fusion: real-time measurement of parainfluenza virus 5 virus-cell fusion

Although cell-cell fusion assays are useful surrogate methods for studying virus fusion, differences between cell-cell and virus-cell fusion exist. To examine paramyxovirus fusion in real time, we labeled viruses with fluorescent lipid probes and monitored virus-cell fusion by fluorimetry. Two parainfluenza virus 5 (PIV5) isolates (W3A and SER) and PIV5 containing mutations within the fusion protein (F) were studied. Fusion was specific and temperature-dependent. Compared to many low pH-dependent viruses, the kinetics of PIV5 fusion was slow, approaching completion within several minutes. As predicted from cell-cell fusion assays, virus containing an F protein with an extended cytoplasmic tail (rSV5 F551) had reduced fusion compared to wild-type virus (W3A). In contrast, virus-cell fusion for SER occurred at near wild-type levels, despite the fact that this isolate exhibits a severely reduced cell-cell fusion phenotype. These results support the notion that virus-cell and cell-cell fusion have significant differences.

Refolding of a paramyxovirus F protein from prefusion to postfusion conformations observed by liposome binding and electron microscopy

For paramyxoviruses, two viral glycoproteins are key to the entry process: an attachment protein (HN, H, or G) and the fusion protein (F). The F protein folds to a metastable state that can be triggered to undergo large conformational rearrangements to a fusogenic intermediate and a more stable postfusion state. The triggering mechanism that controls paramyxovirus fusion has not been elucidated. To correlate the molecular structure of a soluble form of the prefusion F (PIV5 F-GCNt) with the biological function of F, soluble F protein was triggered to refold. In the absence of HN, heat was found to function as a surrogate F trigger, and F associated with liposomes and aggregated on sucrose density gradients. Electron microscopy data showed that triggered F formed rosettes. Taken together these data suggest that release and membrane insertion of the hydrophobic fusion peptide require both cleavage of F and heat. Heating of cleaved F causes conversion to a postfusion form as judged by its "golf tee" morphology in the electron microscope. Heating of uncleaved F also causes conversion of F to a morphologically similar form. The reactivity of the F protein with conformation-specific mAbs and peptide binding suggest that soluble F-GCNt and membrane-bound F proteins refold through a comparable pathway.

Identification of linear heparin-binding peptides derived from human respiratory syncytial virus fusion glycoprotein that inhibit infectivity

It has been shown previously that the fusion glycoprotein of human respiratory syncytial virus (RSV-F) interacts with cellular heparan sulfate. Synthetic overlapping peptides derived from the F-protein sequence of RSV subtype A (strain A2) were tested for their ability to bind heparin using heparin-agarose affinity chromatography (HAAC). This evaluation identified 15 peptides representing eight linear heparin-binding domains (HBDs) located within F1 and F2 and spanning the protease cleavage activation site. All peptides bound to Vero and A549 cells, and binding was inhibited by soluble heparins and diminished by either enzymatic treatment to remove cell surface glycosaminoglycans or by treatment with sodium chlorate to decrease cellular sulfation. RSV-F HBD peptides were less likely to bind to glycosaminoglycan-deficient CHO-745 cells than parental CHO-K1 cells that express these molecules. Three RSV-F HBD peptides (F16, F26, and F55) inhibited virus infectivity; two of these peptides (F16 and F55) inhibited binding of virus to Vero cells, while the third (F26) did not. These studies provided evidence that two of the linear HBDs mapped by peptides F16 and F55 may mediate one of the first steps in the attachment of virus to cells while the third, F26, inhibited infectivity at a postattachment step, suggesting that interactions with cell surface glycosaminoglycans may play a role in infectivity of some RSV strains.

Hydrophobic alpha-helices 1 and 2 of herpes simplex virus gH interact with lipids, and their mimetic peptides enhance virus infection and fusion

Entry of herpes simplex virus into cells occurs by fusion and requires four glycoproteins. gD serves as the receptor binding glycoprotein. Of the remaining glycoproteins, gH carries structural and functional elements typical of class 1 fusion glycoproteins, in particular alpha-helix 1 (alpha-H1), with properties of a candidate fusion peptide, and two heptad repeats. Here, we characterized alpha-H2 and compared it to alpha-H1. alpha-H2 (amino acids 513 to 531) is of lower hydrophobicity than alpha-H1. Its deletion or mutation decreased virus infection and cell fusion. Its replacement with heterologous fusion peptides did not rescue infection and cell fusion beyond the levels exhibited by the alpha-H2-deleted gH. This contrasts with alpha-H1, which cannot be deleted and can be functionally replaced with heterologous fusion peptides (T. Gianni et al., J. Virol. 79:2931-2940, 2005). Synthetic peptides mimicking alpha-H1 and alpha-H2 induced fusion of nude lipid vesicles. Importantly, they increased infection of herpes simplex virus, pseudorabies virus, bovine herpesvirus 1, and vesicular stomatitis virus. The alpha-H1 mimetic peptide was more effective than the alpha-H2 peptide. Consistent with the findings that gH carries membrane-interacting segments, a soluble form of gH, but not of gD or gB, partitioned with lipid vesicles. Current findings highlight that alpha-H2 is an important albeit nonessential region for virus entry and fusion. alpha-H1 and alpha-H2 share the ability to target the membrane lipids; they contribute to virus entry and fusion, possibly by destabilizing the membranes. However, alpha-H2 differs from alpha-H1 in that it is of lower hydrophobicity and cannot be replaced with heterologous fusion peptides.

The structural basis of paramyxovirus invasion

To deliver their genetic material into host cells, enveloped viruses have surface glycoproteins that actively cause the fusion of the viral and cellular membranes. Recently determined X-ray crystal structures of the paramyxovirus fusion (F) protein in its pre-fusion and post-fusion conformations reveal the dramatic structural transformation that this protein undergoes while causing membrane fusion. Conformational changes in key regions of the F protein suggest the mechanism by which the F protein is activated and refolds.

Analysis of the pH requirement for membrane fusion of different isolates of the paramyxovirus parainfluenza virus 5

Paramyxoviruses enter cells by fusing their envelopes with the plasma membrane, a process that occurs at neutral pH. Recently, it has been found that there is an exception to this dogma in that a porcine isolate of the paramyxovirus parainfluenza virus 5 (PIV5), known as SER, requires a low-pH step for fusion (S. Seth, A. Vincent, and R. W. Compans, J. Virol. 77: 6520-6527, 2003). As a low-pH activation mechanism for fusion would greatly facilitate biophysical studies of paramyxovirus-mediated membrane fusion, we have reexamined the triggering of the PIV5 SER fusion protein. Using multiple assays, we could not find a requirement for low-pH triggering of PIV5 SER fusion. The challenge of discovering how the paramyxovirus receptor binding protein (HN, H, or G) activates the metastable fusion protein to cause membrane fusion at neutral pH remains.

Solution structure of the severe acute respiratory syndrome-coronavirus heptad repeat 2 domain in the prefusion state

The envelope glycoprotein, termed the spike protein, of severe acute respiratory syndrome coronavirus (SARS-CoV) is known to mediate viral entry. Similar to other class 1 viral fusion proteins, the heptad repeat regions of SARS-CoV spike are thought to undergo conformational changes from a prefusion form to a subsequent post-fusion form that enables fusion of the viral and host membranes. Recently, the structure of a post-fusion form of SARS-CoV spike, which consists of isolated domains of heptad repeats 1 and 2 (HR1 and HR2), has been determined by x-ray crystallography. To date there is no structural information for the prefusion conformations of SARS-CoV HR1 and HR2. In this work we present the NMR structure of the HR2 domain (residues 1141-1193) from SARS-CoV (termed S2-HR2) in the presence of the co-solvent trifluoroethanol. We find that in the absence of HR1, S2-HR2 forms a coiled coil symmetric trimer with a complex molecular mass of 18 kDa. The S2-HR2 structure, which is the first example of the prefusion form of coronavirus envelope, supports the current model of viral membrane fusion and gives insight into the design of structure-based antagonists of SARS.