Cloning, expression, and crystallization of the fusion protein of Newcastle disease virus

Adaptation of Newcastle Disease Virus (NDV) in Feral Birds and their Potential Role in Interspecies Transmission

Introduction:

Newcastle Disease (ND), caused by Avian avulavirus 1 (AAvV 1, avulaviruses), is a notifiable disease throughout the world due to the economic impact on trading restrictions and its embargoes placed in endemic regions. The feral birds including aquatic/migratory birds and other wild birds may act as natural reservoir hosts of ND Viruses (NDVs) and may play a remarkable role in the spread of the virus in environment. In addition, other 19 avulaviruses namely: AAvV 2 to 20, have been potentially recognized from feral avian species.

Expalantion:

Many previous studies have investigated the field prevailing NDVs to adapt a wide range of susceptible host. Still the available data is not enough to declare the potential role of feral birds in transmission of the virus to poultry and/or other avian birds. In view of the latest evidence related to incidences of AAvVs in susceptible avian species, it is increasingly important to understand the potential of viruses to transmit within the domestic poultry and other avian hosts. Genomic and phylogenomic analysis of several investigations has shown the same (RK/RQRR↓F) motif cleavage site among NDV isolates with same genotypes from domestic poultry and other wild hosts. So, the insight of this, various semi-captive/free-ranging wild avian species could play a vital role in the dissemination of the virus, which is an important consideration to control the disease outbreaks. Insufficient data on AAvV 1 transmission from wild birds to poultry and vice versa is the main constraint to understand about its molecular biology and genomic potential to cause infection in all susceptible hosts.

Conclusion:

The current review details the pertinent features of several historical and contemporary aspects of NDVs and the vital role of feral birds in its molecular epidemiology and ecology.

Chimeric rabies glycoprotein with a transmembrane domain and cytoplasmic tail from Newcastle disease virus fusion protein incorporates into the Newcastle disease virion at reduced levels

Rabies remains an important worldwide health problem. Newcastle disease virus (NDV) was developed as a vaccine vector in animals by using a reverse genetics approach. Previously, our group generated a recombinant NDV (LaSota strain) expressing the complete rabies virus G protein (RVG), named rL-RVG. In this study, we constructed the variant rL-RVGTM, which expresses a chimeric rabies virus G protein (RVGTM) containing the ectodomain of RVG and the transmembrane domain (TM) and a cytoplasmic tail (CT) from the NDV fusion glycoprotein to study the function of RVG's TM and CT. The RVGTM did not detectably incorporate into NDV virions, though it was abundantly expressed at the surface of infected BHK-21 cells. Both rL-RVG and rL-RVGTM induced similar levels of NDV virus-neutralizing antibody (VNA) after initial and secondary vaccination in mice, whereas rabies VNA induction by rL-RVGTM was markedly lower than that induced by rL-RVG. Though rL-RVG could spread from cell to cell like that in rabies virus, rL-RVGTM lost this ability and spread in a manner similar to the parental NDV. Our data suggest that the TM and CT of RVG are essential for its incorporation into NDV virions and for spreading of the recombinant virus from the initially infected cells to surrounding cells.

Measles Virus Fusion Protein: Structure, Function and Inhibition

Measles virus (MeV), a highly contagious member of the Paramyxoviridae family, causes measles in humans. The Paramyxoviridae family of negative single-stranded enveloped viruses includes several important human and animal pathogens, with MeV causing approximately 120,000 deaths annually. MeV and canine distemper virus (CDV)-mediated diseases can be prevented by vaccination. However, sub-optimal vaccine delivery continues to foster MeV outbreaks. Post-exposure prophylaxis with antivirals has been proposed as a novel strategy to complement vaccination programs by filling herd immunity gaps. Recent research has shown that membrane fusion induced by the morbillivirus glycoproteins is the first critical step for viral entry and infection, and determines cell pathology and disease outcome. Our molecular understanding of morbillivirus-associated membrane fusion has greatly progressed towards the feasibility to control this process by treating the fusion glycoprotein with inhibitory molecules. Current approaches to develop anti-membrane fusion drugs and our knowledge on drug resistance mechanisms strongly suggest that combined therapies will be a prerequisite. Thus, discovery of additional anti-fusion and/or anti-attachment protein small-molecule compounds may eventually translate into realistic therapeutic options.

Isolation, Identification, and Sequencing of a Goose-Derived Newcastle Disease Virus and Determination of Its Pathogenicity

In August 2010, geese in the Meihekou area of Jilin province in China were found to be infected by a pathogen that caused a disease similar to Newcastle disease. To determine the causative agent of the infections, a virus was isolated from liver tissues of infected geese, followed by a pathogenicity determination. The isolated virus was named NDV/White Goose/China/Jilin(Meihekou)/MHK-1/2010. Specific primers were designed to amplify the whole genome of the MHK-1 virus, followed by sequencing and splicing of the entire genome. Sequencing and phylogenetic analysis of MHK-1 showed that the isolate was a virulent strain of Newcastle disease virus. The MHK-1 genome is 15,192 nucleotides long, and it belongs to the class II branch of Newcastle disease viruses, as evidenced by the amino acid sequence (112R-R-Q-K-R-F117) of the F protein. The hemagglutinin titer was 1:128 to 1:512. The chicken embryo mean death time, the intracerebral pathogenicity index, and the median lethal dose of chicken embryos of MHK-1 were 43 hr, 1.63, and 10(9)/ml, respectively, which revealed that the newly isolated MHK-1 strain is strongly pathogenic to geese.

Genotype characterization of commonly used Newcastle disease virus vaccine strains of India

Newcastle disease is an avian pathogen causing severe economic losses to the Indian poultry industry due to recurring outbreaks in vaccinated and unvaccinated flocks. India being an endemic country, advocates vaccination against the virus using lentogenic and mesogenic strains. Two virus strains which are commonly used for vaccination are strain F (a lentogenic virus) and strain R2B (a mesogenic virus). Strain F is given to 0-7 days old chicks and R2B is given to older birds which are around 6-8 weeks old. To understand the genetic makeup of these two strains, a complete genome study and phylogenetic analysis of the F, HN genes of these vaccine strains were carried out. Both the viral strains had a genome length of 15,186 nucleotides and consisted of six genes with conserved complimentary 3' leader and 5' trailer regions. The fusion protein cleavage site of strain F is GGRQGRL and strain R2B is RRQKRF. Although both the viral strains had different virulence attributes, the length of the HN protein was similar with 577 amino acids. Phylogenetic analysis of F, HN and complete genome sequences grouped these two strains in genotype II category which are considered as early genotypes and corroborated with their years of isolation.

Biochemical, conformational, and immunogenic analysis of soluble trimeric forms of henipavirus fusion glycoproteins

The henipaviruses, Hendra virus (HeV) and Nipah virus (NiV), are paramyxoviruses discovered in the mid- to late 1990s that possess a broad host tropism and are known to cause severe and often fatal disease in both humans and animals. HeV and NiV infect cells by a pH-independent membrane fusion mechanism facilitated by their attachment (G) and fusion (F) glycoproteins. Here, several soluble forms of henipavirus F (sF) were engineered and characterized. Recombinant sF was produced by deleting the transmembrane (TM) and cytoplasmic tail (CT) domains and appending a glycosylphosphatidylinositol (GPI) anchor signal sequence followed by GPI-phospholipase D digestion, appending a trimeric coiled-coil (GCNt) domain (sF(GCNt)), or deleting the TM, CT, and fusion peptide domain. These sF glycoproteins were produced as F(0) precursors, and all were apparent stable trimers recognized by NiV-specific antisera. Surprisingly, however, only the GCNt-appended constructs (sF(GCNt)) could elicit cross-reactive henipavirus-neutralizing antibody in mice. In addition, sF(GCNt) constructs could be triggered in vitro by protease cleavage and heat to transition from an apparent prefusion to postfusion conformation, transitioning through an intermediate that could be captured by a peptide corresponding to the C-terminal heptad repeat domain of F. The pre- and postfusion structures of sF(GCNt) and non-GCNt-appended sF could be revealed by electron microscopy and were distinguishable by F-specific monoclonal antibodies. These data suggest that only certain sF constructs could serve as potential subunit vaccine immunogens against henipaviruses and also establish important tools for further structural, functional, and diagnostic studies on these important emerging viruses.

Characterization of newly emerging Newcastle disease viruses isolated during 2002-2008 in Taiwan

To elucidate the epidemiological relationships between ND outbreaks and genetic lineages, a portion of the F gene (535 bp) and the full-length HN gene (1922 bp) of recent Taiwanese NDVs isolated in 2002-2008 was amplified by reverse transcription (RT)-polymerase chain reaction (PCR) and sequenced. Only a portion of above amplified PCR products of the F and HN genes (374 and 1713 bp) and their deduced amino acid residues were compared with the other 60 NDVs retrieved from GenBank. Most (29/30) of the recent Taiwanese isolates were clustered in subgenotype VIIe while only one isolate was classified as subgenotype VIIc. All the 29 isolates of subgenotype VIIe were further subclassified and termed provisionally as sub-subgenotypes VIIe2 (13 isolates), VIIe3 (5 isolates), and VIIe4 (11 isolates). The sub-subgenotype VIIe2 isolates possessing the motif (112)R-R-Q-K-R-F(117) and amino acid residue substitutions at positions 23 (L to F) and 90 (T to A) were collected during 2002-2005. The sub-subgenotype VIIe3 isolates possessing the motif (112)R-R-K-K-R-F(117) and amino acid residue substitutions at positions 74 (E to G) and 75 (A to G) within epitopes and 114 (Q to K) within cleavage site of F protein were collected during 2003-2006. The sub-subgenotype VIIe4 isolates possessing the motif (112)R-R-Q-K-R-F(117) and amino acid residue substitutions at positions 23 (L to F), 26 (I to T), and 90 (T to A) were collected during 2007-2008. All the NDV isolates in this study exhibited a high intra-cerebral pathogenicity index (ICPI), they were all classified as velogenic type of NDVs. The sub-subgenotype VIIe2 and VIIe4 viruses are now dominant and have been implicated in most of the recent ND outbreaks in Taiwan. Phylogenetic analysis of these isolates revealed that they may have evolved from previously reported local strains (VIIe1). This finding is essential for improving the disease control strategies and development of vaccines for ND.

Crystal structure and carbohydrate analysis of Nipah virus attachment glycoprotein: a template for antiviral and vaccine design

Two members of the paramyxovirus family, Nipah virus (NiV) and Hendra virus (HeV), are recent additions to a growing number of agents of emergent diseases which use bats as a natural host. Identification of ephrin-B2 and ephrin-B3 as cellular receptors for these viruses has enabled the development of immunotherapeutic reagents which prevent virus attachment and subsequent fusion. Here we present the structural analysis of the protein and carbohydrate components of the unbound viral attachment glycoprotein of NiV glycoprotein (NiV-G) at a 2.2-A resolution. Comparison with its ephrin-B2-bound form reveals that conformational changes within the envelope glycoprotein are required to achieve viral attachment. Structural differences are particularly pronounced in the 579-590 loop, a major component of the ephrin binding surface. In addition, the 236-245 loop is rather disordered in the unbound structure. We extend our structural characterization of NiV-G with mass spectrometric analysis of the carbohydrate moieties. We demonstrate that NiV-G is largely devoid of the oligomannose-type glycans that in viruses such as human immunodeficiency virus type 1 and Ebola virus influence viral tropism and the host immune response. Nevertheless, we find putative ligands for the endothelial cell lectin, LSECtin. Finally, by mapping structural conservation and glycosylation site positions from other members of the paramyxovirus family, we suggest the molecular surface involved in oligomerization. These results suggest possible pathways of virus-host interaction and strategies for the optimization of recombinant vaccines.

Serum-free transient protein production system based on adenoviral vector and PER.C6 technology: high yield and preserved bioactivity

Stable E1 transformed cells, like PER.C6, are able to grow at scale and to high cell densities. E1-deleted adenoviruses replicate to high titer in PER.C6 cells whereas subsequent deletion of E2A from the vector results in absence of replication in PER.C6 cells and drastically lowers the expression of adenovirus proteins in such cells. We therefore considered the use of an DeltaE1/DeltaE2 type 5 vector (Ad5) to deliver genes to PER.C6 cells growing in suspension with the aim to achieve high protein yield. To evaluate the utility of this system we constructed DeltaE1/DeltaE2 vector carrying different classes of protein, that is, the gene coding for spike protein derived from the Coronavirus causing severe acute respiratory syndrome (SARS-CoV), a gene coding for the SARS-CoV receptor or the genes coding for an antibody shown to bind and neutralize SARS-CoV (SARS-AB). The DeltaE1/DeltaE2A-vector backbones were rescued on a PER.C6 cell line engineered to constitutively over express the Ad5 E2A protein. Exposure of PER.C6 cells to low amounts (30 vp/cell) of DeltaE1/DeltaE2 vectors resulted in highly efficient (>80%) transduction of PER.C6 cells growing in suspension. The efficient cell transduction resulted in high protein yield (up to 60 picogram/cell/day) in a 4 day batch production protocol. FACS and ELISA assays demonstrated the biological activity of the transiently produced proteins. We therefore conclude that DeltaE1/DeltaE2 vectors in combination with the PER.C6 technology may provide a viable answer to the increasing demand for high quality, high yield recombinant protein.

Structural basis of viral invasion: lessons from paramyxovirus F

The structures of glycoproteins that mediate enveloped virus entry into cells have revealed dramatic structural changes that accompany membrane fusion and provided mechanistic insights into this process. The group of class I viral fusion proteins includes the influenza hemagglutinin, paramyxovirus F, HIV env, and other mechanistically related fusogens, but these proteins are unrelated in sequence and exhibit clearly distinct structural features. Recently determined crystal structures of the paramyxovirus F protein in two conformations, representing pre-fusion and post-fusion states, reveal a novel protein architecture that undergoes large-scale, irreversible refolding during membrane fusion, extending our understanding of this diverse group of membrane fusion machines.

Signal peptide and helical bundle domains of virulent canine distemper virus fusion protein restrict fusogenicity

Persistence in canine distemper virus (CDV) infection is correlated with very limited cell-cell fusion and lack of cytolysis induced by the neurovirulent A75/17-CDV compared to that of the cytolytic Onderstepoort vaccine strain. We have previously shown that this difference was at least in part due to the amino acid sequence of the fusion (F) protein (P. Plattet, J. P. Rivals, B. Zuber, J. M. Brunner, A. Zurbriggen, and R. Wittek, Virology 337:312-326, 2005). Here, we investigated the molecular mechanisms of the neurovirulent CDV F protein underlying limited membrane fusion activity. By exchanging the signal peptide between both F CDV strains or replacing it with an exogenous signal peptide, we demonstrated that this domain controlled intracellular and consequently cell surface protein expression, thus indirectly modulating fusogenicity. In addition, by serially passaging a poorly fusogenic virus and selecting a syncytium-forming variant, we identified the mutation L372W as being responsible for this change of phenotype. Intriguingly, residue L372 potentially is located in the helical bundle domain of the F(1) subunit. We showed that this mutation drastically increased fusion activity of F proteins of both CDV strains in a signal peptide-independent manner. Due to its unique structure even among morbilliviruses, our findings with respect to the signal peptide are likely to be specifically relevant to CDV, whereas the results related to the helical bundle add new insights to our growing understanding of this class of F proteins. We conclude that different mechanisms involving multiple domains of the neurovirulent A75/17-CDV F protein act in concert to limit fusion activity, preventing lysis of infected cells, which ultimately may favor viral persistence.

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.

Effects of Heptad Repeat Regions of F Protein on the Specific Membrane Fusion in Paramyxoviruses

OBJECTIVES

To identify the effects of heptad repeat regions (HR1 and HR2) of F on the specific membrane fusion in paramyxoviruses.

METHODS

Site-directed mutagenesis was used to create same enzyme sites on the F genes of Newcastle disease virus (NDV) and human parainfluenza virus (hPIV). Gene recombination was used to get chimeric F proteins NDV C-HR1 and hPIV C-HR1 by exchanging HR1 fragments each other; NDV C-HR2 and hPIV C-HR2 were also obtained by the same way. All the chimeric F proteins were co-expressed with their homologous or heterogeneous HN in eukaryocytes. Cell fusion functions were assayed by Giemsa staining and reporter gene method. The expression efficiencies of F proteins were assayed with fluorescence-activated cell sorter (FACS).

RESULTS

NDV C-HR1 and hPIV C-HR1 had 53.91 and 83.15% of fusion activities, and NDV C-HR2 and hPIV C-HR2 had 107.23 and 12.01% of fusion activities, respectively, as compared with their relevant wild types. The analysis of FACS indicated that the expression efficiencies of all the chimeric F proteins except NDV C-HR2 were lower than those of their relevant wild types.

CONCLUSIONS

HR1 of NDV F might be important for its specific membrane fusion, but HR2 of NDV F may not; both HR1 and HR2 of hPIV F may be important for its specific membrane fusion.

Identification of an N-terminal trimeric coiled-coil core within arenavirus glycoprotein 2 permits assignment to class I viral fusion proteins

The lymphocytic choriomeningitis virus (LCMV) glycoprotein (GP) consists of the transmembrane subunit GP-2 and the receptor binding subunit GP-1. Both are synthesized as one precursor protein and stay noncovalently attached after cleavage. In this study, we determined the oligomeric state of the LCMV GP and expressed it in two different conformations suitable for structural analysis. Sequence analysis of GP-2 identified a trimeric heptad repeat pattern containing an N-terminal alpha-helix. An alpha-helical peptide matching this region formed a stable oligomer as revealed by gel filtration chromatography and dynamic light scattering. In contrast, a second alpha-helical peptide corresponding to a predicted C-terminal alpha-helix within GP-2 did not oligomerize. Refolding of the complete GP-2 ectodomain revealed trimeric all-alpha complexes probably representing the six-helix bundle state that is considered a hallmark of class I viral fusion proteins. Based on these results, we generated a construct consisting of the complete uncleavable LCMV GP ectodomain fused C-terminally to the trimeric motif of fibritin. Gel filtration analysis of the secreted fusion protein identified two complexes of approximately 230 and approximately 440 kDa. Both complexes bound to a set of conformational and linear antibodies. Cross-linking confirmed the 230-kDa complex to be a trimer. The 440-kDa complexes were found to represent disulfide-linked pairs of trimers, since partial reduction converted them to a complex species migrating at 250 kDa. By electron microscopy, the 230-kDa complexes appeared as single spherical particles and showed no signs of rosette formation. Our results clearly demonstrate that the arenavirus GP is a trimer and must be considered a member of the class I viral fusion protein family.

Contribution of cysteine residues in the extracellular domain of the F protein of human respiratory syncytial virus to its function

The mature F protein of all known isolates of human respiratory syncytial virus (HRSV) contains fifteen absolutely conserved cysteine (C) residues that are highly conserved among the F proteins of other pneumoviruses as well as the paramyxoviruses. To explore the contribution of the cysteines in the extracellular domain to the fusion activity of HRSV F protein, each cysteine was changed to serine. Mutation of cysteines 37, 313, 322, 333, 343, 358, 367, 393, 416, and 439 abolished or greatly reduced cell surface expression suggesting these residues are critical for proper protein folding and transport to the cell surface. As expected, the fusion activity of these mutations was greatly reduced or abolished. Mutation of cysteine residues 212, 382, and 422 had little to no effect upon cell surface expression or fusion activity at 32 degrees C, 37 degrees C, or 39.5 degrees C. Mutation of C37 and C69 in the F2 subunit either abolished or reduced cell surface expression by 75% respectively. None of the mutations displayed a temperature sensitive phenotype.

Structure of the uncleaved ectodomain of the paramyxovirus (hPIV3) fusion protein

Class I viral fusion proteins share common mechanistic and structural features but little sequence similarity. Structural insights into the protein conformational changes associated with membrane fusion are based largely on studies of the influenza virus hemagglutinin in pre- and postfusion conformations. Here, we present the crystal structure of the secreted, uncleaved ectodomain of the paramyxovirus, human parainfluenza virus 3 fusion (F) protein, a member of the class I viral fusion protein group. The secreted human parainfluenza virus 3 F forms a trimer with distinct head, neck, and stalk regions. Unexpectedly, the structure reveals a six-helix bundle associated with the postfusion form of F, suggesting that the anchor-minus ectodomain adopts a conformation largely similar to the postfusion state. The transmembrane anchor domains of F may therefore profoundly influence the folding energetics that establish and maintain a metastable, prefusion state.

Amino acid substitutions in the F-specific domain in the stalk of the newcastle disease virus HN protein modulate fusion and interfere with its interaction with the F protein

The hemagglutinin-neuraminidase (HN) protein of Newcastle disease virus mediates attachment to sialic acid receptors, as well as cleavage of the same moiety. HN also interacts with the other viral glycoprotein, the fusion (F) protein, to promote membrane fusion. The ectodomain of the HN spike consists of a stalk and a terminal globular head. The most conserved part of the stalk consists of two heptad repeats separated by a nonhelical intervening region (residues 89 to 95). Several amino acid substitutions for a completely conserved proline residue in this region not only impair fusion and the HN-F interaction but also decrease neuraminidase activity in the globular domain, suggesting that the substitutions may alter HN structure. Substitutions for L94 also interfere with fusion and the HN-F interaction but have no significant effect on any other HN function. Amino acid substitutions at other positions in the intervening region also modulate only fusion. In all cases, diminished fusion correlates with a decreased ability of the mutated HN protein to interact with F at the cell surface. These findings indicate that the intervening region is critical to the role of HN in the promotion of fusion and may be directly involved in its interaction with the homologous F protein.

The 3D structure of the fusion primed Sendai F-protein determined by electron cryomicroscopy

The three dimensional (3D) structure of the ectodomain of the entire fusion mediating F protein from Sendai virus [MW (trimer) approximately 177 kDa] has been determined by cryoelectron microscopy of single molecules and subsequent 3D reconstruction at a resolution of approximately 16 A. The reconstruction, which has been obtained from the native, proteolytic processed fusion primed F1+F2 form, shows the protein protruding approximately 170 A out of the membrane in a homotrimeric association. It consists of a defined approximately 65 A wide distal head and an adjacent neck, which is connected to an 70 A elongated stalk. Although the overall shape appears to be similar to the recently reported X-ray structure of the Newcastle disease virus F protein, a closer comparison reveals structural differences suggesting that the investigated Sendai F structure represents an advanced state towards the fusion active conformation.

Structure and function of a paramyxovirus fusion protein

Paramyxoviruses initiate infection by attaching to cell surface receptors and fusing viral and cell membranes. Viral attachment proteins, hemagglutinin-neuraminidase (HN), hemagglutinin (HA), or glycoprotein (G), bind receptors while fusion (F) proteins direct membrane fusion. Because paramyxovirus fusion is pH independent, virus entry occurs at host cell plasma membranes. Paramyxovirus fusion also usually requires co-expression of both the attachment protein and the fusion (F) protein. Newcastle disease virus (NDV) has assumed increased importance as a prototype paramyxovirus because crystal structures of both the NDV F protein and the attachment protein (HN) have been determined. Furthermore, analysis of structure and function of both viral glycoproteins by mutation, reactivity of antibody, and peptides have defined domains of the NDV F protein important for virus fusion. These domains include the fusion peptide, the cytoplasmic domain, as well as heptad repeat (HR) domains. Peptides with sequences from HR domains inhibit fusion, and characterization of the mechanism of this inhibition provides evidence for conformational changes in the F protein upon activation of fusion. Both proteolytic cleavage of the F protein and interactions with the attachment protein are required for fusion activation in most systems. Subsequent steps in membrane merger directed by F protein are poorly understood.

The structural biology of type I viral membrane fusion

The fusion of viral membranes with target-cell membranes is an essential step in the entry of enveloped viruses into cells, and recent X-ray structures of paramyxoviral envelope proteins have provided new insights into protein-mediated plasma-membrane fusion. Here, we review our understanding of the structural transitions that are involved in this fusion pathway, compare it to our understanding of influenza virus membrane fusion, and discuss the implications for retroviral membrane fusion.

Effect of proteolytic processing at two distinct sites on shape and aggregation of an anchorless fusion protein of human respiratory syncytial virus and fate of the intervening segment

We have examined the consequences of cleaving the fusion glycoprotein (F) of human respiratory syncytial virus (HRSV) at two distinct furin-recognition sites. Purified anchorless F is a mixture of unaggregated cone-shaped molecules and rosettes of lollipop-shaped spikes. The unaggregated molecules contain a proportion of uncleaved F0 and an intermediate, F(delta1-109), cleaved only at site I, residues 106-109. Inhibition of cleavage at site I, by two amino acid changes (R108N/R109N), reduces the proportion of aggregated molecules with a concomitant increase in the amount of unprocessed F0. Inhibition of cleavage at site II, residues 131-136, by deletion of four amino acids (delta131-134), abrogates aggregation of anchorless F and all molecules are seen as individual cone-shaped rods. In vitro cleavage of anchorless F, or mutant delta131-134, with trypsin at 4, 20, or 37 degrees C, under conditions in which cleavage at site II is complete in all molecules, leads to their aggregation in rosettes of lollipop-shaped spikes. Thus, cleavage at site II is required for the structural changes in anchorless F that lead to changes in shape and to aggregation. The segment between sites I and II, residues 110-136, is not associated with anchorless F in the supernatant of infected cell cultures, indicating that it is released from the processed protein when cleavage at sites I and II is completed.