Carbohydrate modifications of the NDV fusion protein heptad repeat domains influence maturation and fusion activity

SLC35A2 modulates paramyxovirus fusion events during infection

Paramyxoviruses are significant human and animal pathogens that include mumps virus (MuV), Newcastle disease virus (NDV) and the murine parainfluenza virus Sendai (SeV). Despite their importance, few host factors implicated in paramyxovirus infection are known. Using a recombinant SeV expressing destabilized GFP (rSeVCdseGFP) in a loss-of-function CRISPR screen, we identified the CMP-sialic acid transporter (CST) gene SLC35A1 and the UDP-galactose transporter (UGT) gene SLC35A2 as essential for paramyxovirus infection. SLC35A1 knockout (KO) cells showed significantly reduced binding and infection of SeV, NDV and MuV due to the lack of cell surface sialic acids, which act as their receptors. However, SLC35A2 KO cells revealed unknown critical roles for this factor in virus-cell and cell-to-cell fusion events during infection with different paramyxoviruses. While the UGT was essential for virus-cell fusion during SeV entry to the cell, it was not required for NDV or MuV entry. Importantly, the UGT promoted the formation of larger syncytia during MuV infection, suggesting a role in cell-to-cell virus spread. Our findings demonstrate that paramyxoviruses can bind to or enter A549 cells in the absence of canonical galactose-bound sialic-acid decorations and show that the UGT facilitates paramyxovirus fusion processes involved in entry and spread.

Virus-like particles (VLPs): A promising platform for combating against Newcastle disease virus

The global poultry industry plays a pivotal role in providing eggs and meat for human consumption. However, outbreaks of viral disease, especially Newcastle virus disease (NDV), within poultry farms have detrimental effects on various zootechnical parameters, such as body weight gain, feed intake, feed conversion ratio, as well as the quality of egg and meat production. Cases of vaccine failure have been reported in regions where highly pathogenic strains of NDV are prevalent. To tackle this challenge, virus-like particles (VLPs) have emerged as a potential solution. VLPs closely resemble natural viruses, offering biocompatibility and immune-stimulating properties that make them highly promising for therapeutic applications against NDV. Hence, this review emphasizes the significance of NDV and the need for effective treatments. The manuscript will contain several key aspects, starting with an exploration of the structure and properties of NDV. Subsequently, the paper will delve into the characteristics and benefits of VLPs compared to conventional drug delivery systems. A comprehensive analysis of VLPs as potential vaccine candidates targeting NDV will be presented, along with a discussion on strategies for loading cargo into these NDV-targeting VLPs. The review will also examine various expression systems utilized in the production of NDV-targeting VLPs. Additionally, the manuscript will address future prospects and challenges in the field, concluding with recommendations for further research.

SNARE protein USE1 is involved in the glycosylation and the expression of mumps virus fusion protein and important for viral propagation

Mumps virus (MuV) is the etiological agent of mumps, a disease characterized by painful swelling of the parotid glands and often accompanied by severe complications. To understand the molecular mechanism of MuV infection, a functional analysis of the involved host factors is required. However, little is known about the host factors involved in MuV infection, especially those involved in the late stage of infection. Here, we identified 638 host proteins that have close proximity to MuV glycoproteins, which are a major component of the viral particles, by proximity labeling and examined comprehensive protein-protein interaction networks of the host proteins. From siRNA screening and immunoprecipitation results, we found that a SNARE subfamily protein, USE1, bound specifically to the MuV fusion (F) protein and was important for MuV propagation. In addition, USE1 plays a role in complete N-linked glycosylation and expression of the MuV F protein.

The role of N-linked glycosylation in proteolytic processing and cell surface transport of the Cedar virus fusion protein

Background

N-linked glycans on viral glycoproteins have been shown to be important for protein expression, processing and intracellular transport. The fusion glycoprotein F of Cedar virus (CedV) contains six potential N-glycosylation sites.

Findings

To investigate their impact on cell surface transport, proteolytic cleavage and biological activity, we disrupted the consensus sequences by conservative mutations (Asn to Gln) and found that five of the six potential N-glycosylation sites are actually utilized. The individual removal of N-glycan g1 (N66), g2 (N79) and g3 (N98) in the CedV F₂ subunit had no or only little effect on cell surface transport, proteolytic cleavage and fusion activity of CedV F. Interestingly, removal of N-linked glycan g6 (N463) in the F₁ subunit resulted in reduced cell surface expression but slightly increased fusogenicity upon co-expression with the CedV receptor-binding protein G. Most prominent effects however were observed for the disruption of N-glycosylation motif g4 (N413), which significantly impaired the transport of CedV F to the cell surface, thereby also affecting proteolytic cleavage and fusion activity.

Conclusions

Our findings indicate that the individual N-linked modifications, with the exception of glycan g4, are dispensable for processing of CedV F protein in transfection experiments. However, removal of g4 led to a phenotype that was strongly impaired concerning cell surface expression and proteolytic activation.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12985-022-01864-5.

Molecular characterisation and phylogenetic study of the fusion gene of Newcastle disease viruses isolated from broiler farms of Iran in 2018-2019

Avian orthoavulavirus, commonly known as Newcastle disease virus (NDV) has been a constant threat for the poultry industry of Iran for decades. Recently, a couple of preliminary studies on backyard and commercial chicken suggested that a major subgenotype circulating in Iran may be VII(L) subgenotype, which is now known as VII.1.1 according to the new classification system. The unique subgenotype was not reported from other parts of the world and was slightly (≥3%) different from the closest group that was VIId. The study was conducted between July 2018 and March 2019 to determine the exact NDV genotypes/subgenotypes circulating in Iranian broiler poultry farms; five-hundred and forty chickens were sampled from thirty-six broiler farms located in eighteen provinces of Iran. As other genotypes/subgenotypes such as XIII and VI.2 are circulating in neighbouring countries, border provinces were also sampled. The F gene of the NDV isolates was sequenced and phylogenetic analysis was conducted. All the isolates clustered under the VII.1.1 group. The evolutionary analysis also revealed that the distances were between 0.0 and 0.7% meaning that the Iranian NDV circulating in broiler farms were not only of VII.1.1 sub-genotype, but also genetically very identical, indicating that the routine control measures for ND in Iran were not able to prevent the circulating NDVs. Although stricter biosecurity measures have been really effective in developed countries, surveillance of NDV to determine the circulating genotypes might also help us to implement better vaccination strategies in the future.

The effect of the HRB linker of Newcastle disease virus fusion protein on the fusogenic activity

Newcastle disease, designated a class A disease of poultry by the Office international des epizooties (OIE), is an acute infection caused by Newcastle disease virus (NDV). The merging of the envelope of NDV with the membrane of a target host cell is the key step in the infection pathway, which is driven by the concerted action of two glycoproteins: haemagglutinin-neuraminidase (HN) and fusion (F) protein. When the HN protein binds to the host cell surface receptor, the F protein is activated to mediate fusion. The three-dimensional structure of the F protein has been reported to have low electron density between the DIII domain and the HRB domain, and this electron-poor region is defined as the HRB linker. To clarify the contributing role of the HRB linker in the NDV F protein-mediated fusion process, 6 single amino acid mutants were obtained by site-directed mutagenesis of the HRB linker. The expression of the mutants and their abilities to mediate fusion were analysed, and the key amino acids in the HRB linker were identified as L436, E439, I450, and S453, as they can modulate the fusion ability or expression of the active form to a certain extent. The data shed light on the crucial role of the F protein HRB linker in the acquisition of a normal fusogenic phenotype.

Novel Roles of the N1 Loop and N4 Alpha-Helical Region of the Nipah Virus Fusion Glycoprotein in Modulating Early and Late Steps of the Membrane Fusion Cascade

Nipah virus (NiV) is a zoonotic bat henipavirus in the family Paramyxoviridae NiV is deadly to humans, infecting host cells by direct fusion of the viral and host cell plasma membranes. This membrane fusion process is coordinated by the receptor-binding attachment (G) and fusion (F) glycoproteins. Upon G-receptor binding, F fuses membranes via a cascade that sequentially involves F-triggering, fusion pore formation, and viral or genome entry into cells. Using NiV as an important paramyxoviral model, we identified two novel regions in F that modulate the membrane fusion cascade. For paramyxoviruses and other viral families with class I fusion proteins, the heptad repeat 1 (HR1) and HR2 regions in the fusion protein prefusion conformation bind to form a six-helix bundle in the postfusion conformation. Here, structural comparisons between the F prefusion and postfusion conformations revealed that a short loop region (N1) undergoes dramatic spatial reorganization and a short alpha helix (N4) undergoes secondary structural changes. The roles of the N1 and N4 regions during the membrane fusion cascade, however, remain unknown for henipaviruses and paramyxoviruses. By performing alanine scanning mutagenesis and various functional analyses, we report that specific residues within these regions alter various steps in the membrane fusion cascade. While the N1 region affects early F-triggering, the N4 region affects F-triggering, F thermostability, and extensive fusion pore expansion during syncytium formation, also uncovering a link between F-G interactions and F-triggering. These novel mechanistic roles expand our understanding of henipaviral and paramyxoviral F-triggering, viral entry, and cell-cell fusion (syncytia), a pathognomonic feature of paramyxoviral infections.IMPORTANCE Henipaviruses infect bats, agriculturally important animals, and humans, with high mortality rates approaching ∼75% in humans. Known human outbreaks have been concentrated in Southeast Asia and Australia. Furthermore, about 20 new henipaviral species have been recently discovered in bats, with geographical spans in Asia, Africa, and South America. The development of antiviral therapeutics requires a thorough understanding of the mechanism of viral entry into host cells. In this study, we discovered novel roles of two regions within the fusion protein of the deadly henipavirus NiV. Such roles were in allowing viral entry into host cells and cell-cell fusion, a pathological hallmark of this and other paramyxoviruses. These novel roles were in the previously undescribed N1 and N4 regions within the fusion protein, modulating early and late steps of these important processes of viral infection and henipaviral disease. Notably, this knowledge may apply to other henipaviruses and more broadly to other paramyxoviruses.

Third Helical Domain of the Nipah Virus Fusion Glycoprotein Modulates both Early and Late Steps in the Membrane Fusion Cascade

Medically important paramyxoviruses, such as measles, mumps, parainfluenza, Nipah, and Hendra viruses, infect host cells by directing fusion of the viral and cellular plasma membranes. Upon infection, paramyxoviruses cause a second type of membrane fusion, cell-cell fusion (syncytium formation), which is linked to pathogenicity. Host cell receptor binding causes conformational changes in the attachment glycoprotein (HN, H, or G) that trigger a conformational cascade in the fusion (F) glycoprotein that mediates membrane fusion. F, a class I fusion protein, contains the archetypal heptad repeat regions 1 (HR1) and 2 (HR2). It is well established that binding of HR1 and HR2 is key to fusing viral and cellular membranes. In this study, we uncovered a novel fusion-modulatory role of a third structurally conserved helical region (HR3) in F. Based on its location within the F structure, and structural differences between its prefusion and postfusion conformations, we hypothesized that the HR3 modulates triggering of the F conformational cascade (still requiring G). We used the deadly Nipah virus (NiV) as an important paramyxoviral model to perform alanine scan mutagenesis and a series of multidisciplinary structural/functional analyses that dissect the various states of the membrane fusion cascade. Remarkably, we found that specific residues within the HR3 modulate not only early F-triggering but also late extensive fusion pore expansion steps in the membrane fusion cascade. Our results characterize these novel fusion-modulatory roles of the F HR3, improving our understanding of the membrane fusion process for NiV and likely for the related Henipavirus genus and possibly Paramyxoviridae family members.IMPORTANCE The Paramyxoviridae family includes important human and animal pathogens, such as measles, mumps, and parainfluenza viruses and the deadly henipaviruses Nipah (NiV) and Hendra (HeV) viruses. Paramyxoviruses infect the respiratory tract and the central nervous system (CNS) and can be highly infectious. Most paramyxoviruses have a limited host range. However, the biosafety level 4 NiV and HeV are highly pathogenic and have a wide mammalian host range. Nipah viral infections result in acute respiratory syndrome and severe encephalitis in humans, leading to 40 to 100% mortality rates. The lack of licensed vaccines or therapeutic approaches against NiV and other important paramyxoviruses underscores the need to understand viral entry mechanisms. In this study, we uncovered a novel role of a third helical region (HR3) of the NiV fusion glycoprotein in the membrane fusion process that leads to viral entry. This discovery sets HR3 as a new candidate target for antiviral strategies for NiV and likely for related viruses.

Addicted to sugar: roles of glycans in the order Mononegavirales

Glycosylation is a biologically important protein modification process by which a carbohydrate chain is enzymatically added to a protein at a specific amino acid residue. This process plays roles in many cellular functions, including intracellular trafficking, cell-cell signaling, protein folding and receptor binding. While glycosylation is a common host cell process, it is utilized by many pathogens as well. Protein glycosylation is widely employed by viruses for both host invasion and evasion of host immune responses. Thus better understanding of viral glycosylation functions has potential applications for improved antiviral therapeutic and vaccine development. Here, we summarize our current knowledge on the broad biological functions of glycans for the Mononegavirales, an order of enveloped negative-sense single-stranded RNA viruses of high medical importance that includes Ebola, rabies, measles and Nipah viruses. We discuss glycobiological findings by genera in alphabetical order within each of eight Mononegavirales families, namely, the bornaviruses, filoviruses, mymonaviruses, nyamiviruses, paramyxoviruses, pneumoviruses, rhabdoviruses and sunviruses.

Molecular characterization of a Class I Newcastle disease virus strain isolated from a pigeon in China

Constant monitoring is performed to elucidate the role of natural hosts in the ecology of Newcastle disease virus (NDV). In this study, an NDV strain isolated from an asymptomatic pigeon was sequenced and analysed. Results showed that the full-length genomes of this isolate were 15,198 nucleotides with the gene order of 3'-NP-P-M-F-HN-L-5'. This NDV isolate was lentogenic, with an intracerebral pathogenicity index of 0.00 and a mean time of death more than 148 h. The isolate possessed a motif of -(112)E-R-Q-E-R-L(117)- at the F protein cleavage site. In addition, 7 and 13 amino acid substitutions were identified in the functional domains of fusion protein (F) and haemagglutinin-neuraminidase protein (HN) proteins, respectively. Analysis of the amino acids of neutralizing epitopes of F and HN proteins showed 3 and 10 amino acid substitutions, respectively, in the isolate. Phylogenetic analysis classified the isolate into genotype Ib in Class I. This isolate shared high homologies with the NDV strains isolated from wild birds and waterfowl in southern and eastern parts of China from 2005 to 2013. To our knowledge, this study is the first to report a NDV strain isolated from pigeon that belongs to genotype Ib in Class I, rather than to the traditional genotype VI or other sub-genotypes in Class II. This study provides information to elucidate the distribution and evolution of Class I viruses for further NDV prevention.

Recombinant mumps viruses expressing the batMuV fusion glycoprotein are highly fusion active and neurovirulent

A recent study reported the detection of a bat-derived virus (BatPV/Epo_spe/AR1/DCR/2009, batMuV) with phylogenetic relatedness to human mumps virus (hMuV). Since all efforts to isolate infectious batMuV have reportedly failed, we generated recombinant mumps viruses (rMuVs) in which the open reading frames (ORFs) of the fusion (F) and haemagglutinin-neuraminidase (HN) glycoproteins of an hMuV strain were replaced by the corresponding ORFs of batMuV. The batMuV F and HN proteins were successfully incorporated into viral particles and the resultant chimeric virus was able to mediate infection of Vero cells. Distinct differences were observed between the fusogenicity of rMuVs expressing one or both batMuV glycoproteins: viruses expressing batMuV F were highly fusogenic, regardless of the origin of HN. In contrast, rMuVs expressing human F and bat-derived HN proteins were less fusogenic compared to hMuV. The growth kinetics of chimeric MuVs expressing batMuV HN in combination with either hMuV or batMuV F were similar to that of the backbone virus, whereas a delay in virus replication was obtained for rMuVs harbouring batMuV F and hMuV HN. Replacement of the hMuV F and HN genes or the HN gene alone by the corresponding batMuV genes led to a slight reduction in neurovirulence of the highly neurovirulent backbone strain. Neutralizing antibodies inhibited infection mediated by all recombinant viruses generated. Furthermore, group IV anti-MuV antibodies inhibited the neuraminidase activity of bat-derived HN. Our study reports the successful generation of chimeric MuVs expressing the F and HN proteins of batMuV, providing a means for further examination of this novel batMuV.

Mutations in the DI-DII Linker of Human Parainfluenza Virus Type 3 Fusion Protein Result in Diminished Fusion Activity

Human parainfluenza virus type 3 (HPIV3) can cause severe respiratory tract diseases in infants and young children, but no licensed vaccines or antiviral agents are currently available for treatment. Fusing the viral and target cell membranes is a prerequisite for its entry into host cells and is directly mediated by the fusion (F) protein. Although several domains of F are known to have important effects on regulating the membrane fusion activity, the roles of the DI-DII linker (residues 369–374) of the HPIV3 F protein in the fusogenicity still remains ill-defined. To facilitate our understanding of the role of this domain might play in F-induced cell-cell fusion, nine single mutations were engineered into this domain by site-directed mutagenesis. A vaccinia virus-T7 RNA polymerase transient expression system was employed to express the wild-type or mutated F proteins. These mutants were analyzed for membrane fusion activity, cell surface expression, and interaction between F and HN protein. Each of the mutated F proteins in this domain has a cell surface expression level similar to that of wild-type F. All of them resulted in a significant reduction in fusogenic activity in all steps of membrane fusion. Furthermore, all these fusion-deficient mutants reduced the amount of the HN-F complexes at the cell surface. Together, the results of our work suggest that this region has an important effect on the fusogenic activity of F.

The Role of Conserved N-Linked Glycans on Ebola Virus Glycoprotein 2

BACKGROUND

N-linked glycosylation is a common posttranslational modification found on viral glycoproteins (GPs) and involved in promoting expression, cellular attachment, protection from proteases, and antibody evasion. The GP subunit GP2 of filoviruses contains 2 completely conserved N-linked glycosylation sites (NGSs) at N563 and N618, suggesting that they have been maintained through selective pressures.

METHODS

We assessed mutants lacking these glycans for expression and function to understand the role of these sites during Ebola virus entry.

RESULTS

Elimination of either GP2 glycan individually had a modest effect on GP expression and no impact on antibody neutralization of vesicular stomatitis virus pseudotyped with Ebola virus GP. However, loss of the N563 glycan enhanced entry by 2-fold and eliminated GP detection by a well-characterized monoclonal antibody KZ52. Loss of both sites dramatically decreased GP expression and abolished entry. Surprisingly, a GP that retained a single NGS at N563, eliminating the remaining 16 NGSs from GP1 and GP2, had detectable expression, a modest increase in entry, and pronounced sensitivity to antibody neutralization.

CONCLUSIONS

Our findings support the importance of the GP2 glycans in GP expression/structure, transduction efficiency, and antibody neutralization, particularly when N-linked glycans are also removed from GP1.

Novel Functions of Hendra Virus G N-Glycans and Comparisons to Nipah Virus

Hendra virus (HeV) and Nipah virus (NiV) are reportedly the most deadly pathogens within the Paramyxoviridae family. These two viruses bind the cellular entry receptors ephrin B2 and/or ephrin B3 via the viral attachment glycoprotein G, and the concerted efforts of G and the viral fusion glycoprotein F result in membrane fusion. Membrane fusion is essential for viral entry into host cells and for cell-cell fusion, a hallmark of the disease pathobiology. HeV G is heavily N-glycosylated, but the functions of the N-glycans remain unknown. We disrupted eight predicted N-glycosylation sites in HeV G by conservative mutations (Asn to Gln) and found that six out of eight sites were actually glycosylated (G2 to G7); one in the stalk (G2) and five in the globular head domain (G3 to G7). We then tested the roles of individual and combined HeV G N-glycan mutants and found functions in the modulation of shielding against neutralizing antibodies, intracellular transport, G-F interactions, cell-cell fusion, and viral entry. Between the highly conserved HeV and NiV G glycoproteins, similar trends in the effects of N-glycans on protein functions were observed, with differences in the levels at which some N-glycan mutants affected such functions. While the N-glycan in the stalk domain (G2) had roles that were highly conserved between HeV and NiV G, individual N-glycans in the head affected the levels of several protein functions differently. Our findings are discussed in the context of their contributions to our understanding of HeV and NiV pathogenesis and immune responses.

IMPORTANCE

Viral envelope glycoproteins are important for viral pathogenicity and immune evasion. N-glycan shielding is one mechanism by which immune evasion can be achieved. In paramyxoviruses, viral attachment and membrane fusion are governed by the close interaction of the attachment proteins H/HN/G and the fusion protein F. In this study, we show that the attachment glycoprotein G of Hendra virus (HeV), a deadly paramyxovirus, is N-glycosylated at six sites (G2 to G7) and that most of these sites have important roles in viral entry, cell-cell fusion, G-F interactions, G oligomerization, and immune evasion. Overall, we found that the N-glycan in the stalk domain (G2) had roles that were very conserved between HeV G and the closely related Nipah virus G, whereas individual N-glycans in the head quantitatively modulated several protein functions differently between the two viruses.

Pigeon paramyxovirus type 1 variants with polybasic F protein cleavage site but strikingly different pathogenicity

Newcastle disease viruses (NDV) isolated from pigeons (pigeon paramyxovirus type 1; PPMV-1) are mostly of mesogenic pathotype and characterized by a polybasic amino acid sequence motif at the fusion protein (F) cleavage site. This feature also applies to isolate R75/98 from Germany. Its genome consists of 15,192 nucleotides and it specifies an intracerebral pathogenicity index (ICPI) of 1.1, as is typical for mesogenic NDV. Recombinant R75/98 (rR75/98) derived by reverse genetics also possesses a polybasic F protein cleavage site but exhibits ICPI of 0.28, indicating a lentogenic virus. While ten virus passages of rR75/98 on embryonated chicken eggs did not result in any alteration of virus characteristics, virus which had been re-isolated from the brain of an intracerebrally inoculated chicken showed an increase in virulence, characterized by an ICPI of 0.93. Comparison of whole genome sequences of rR75/98 and re-isolated rR75/98 (RrR75/98) demonstrated only two amino acid differences, one in the F protein (N472 K) and one in the polymerase protein (K2168R). This result indicates that only very few amino acid alterations are sufficient to modulate virus virulence in the presence of a polybasic amino acid sequence at the proteolytic F protein cleavage site.

Paramyxovirus glycoprotein incorporation, assembly and budding: a three way dance for infectious particle production

Paramyxoviruses are a family of negative sense RNA viruses whose members cause serious diseases in humans, such as measles virus, mumps virus and respiratory syncytial virus; and in animals, such as Newcastle disease virus and rinderpest virus. Paramyxovirus particles form by assembly of the viral matrix protein, the ribonucleoprotein complex and the surface glycoproteins at the plasma membrane of infected cells and subsequent viral budding. Two major glycoproteins expressed on the viral envelope, the attachment protein and the fusion protein, promote attachment of the virus to host cells and subsequent virus-cell membrane fusion. Incorporation of the surface glycoproteins into infectious progeny particles requires coordinated interplay between the three viral structural components, driven primarily by the matrix protein. In this review, we discuss recent progress in understanding the contributions of the matrix protein and glycoproteins in driving paramyxovirus assembly and budding while focusing on the viral protein interactions underlying this process and the intracellular trafficking pathways for targeting viral components to assembly sites. Differences in the mechanisms of particle production among the different family members will be highlighted throughout.

Comprehensive functional analysis of N-linked glycans on Ebola virus GP1

Ebola virus (EBOV) entry requires the virion surface-associated glycoprotein (GP) that is composed of a trimer of heterodimers (GP1/GP2). The GP1 subunit contains two heavily glycosylated domains, the glycan cap and the mucin-like domain (MLD). The glycan cap contains only N-linked glycans, whereas the MLD contains both N- and O-linked glycans. Site-directed mutagenesis was performed on EBOV GP1 to systematically disrupt N-linked glycan sites to gain an understanding of their role in GP structure and function. All 15 N-glycosylation sites of EBOV GP1 could be removed without compromising the expression of GP. The loss of these 15 glycosylation sites significantly enhanced pseudovirion transduction in Vero cells, which correlated with an increase in protease sensitivity. Interestingly, exposing the receptor-binding domain (RBD) by removing the glycan shield did not allow interaction with the endosomal receptor, NPC1, indicating that the glycan cap/MLD domains mask RBD residues required for binding. The effects of the loss of GP1 N-linked glycans on Ca(2+)-dependent (C-type) lectin (CLEC)-dependent transduction were complex, and the effect was unique for each of the CLECs tested. Surprisingly, EBOV entry into murine peritoneal macrophages was independent of GP1 N-glycans, suggesting that CLEC-GP1 N-glycan interactions are not required for entry into this important primary cell. Finally, the removal of all GP1 N-glycans outside the MLD enhanced antiserum and antibody sensitivity. In total, our results provide evidence that the conserved N-linked glycans on the EBOV GP1 core protect GP from antibody neutralization despite the negative impact the glycans have on viral entry efficiency.

IMPORTANCE

Filovirus outbreaks occur sporadically throughout central Africa, causing high fatality rates among the general public and health care workers. These unpredictable hemorrhagic fever outbreaks are caused by multiple species of Ebola viruses, as well as Marburg virus. While filovirus vaccines and therapeutics are being developed, there are no licensed products. The sole viral envelope glycoprotein, which is a principal immunogenic target, contains a heavy shield of glycans surrounding the conserved receptor-binding domain. We find that disruption of this shield through targeted mutagenesis leads to an increase in cell entry, protease sensitivity, and antiserum/antibody sensitivity but is not sufficient to allow virion binding to the intracellular receptor NPC1. Therefore, our studies provide evidence that filoviruses maintain glycoprotein glycosylation to protect against proteases and antibody neutralization at the expense of efficient entry. Our results unveil interesting insights into the unique entry process of filoviruses and potential immune evasion tactics of the virus.

High genetic diversity of Newcastle disease virus in poultry in West and Central Africa: cocirculation of genotype XIV and newly defined genotypes XVII and XVIII

Despite rampant Newcastle disease virus (NDV) outbreaks in Africa for decades, the information about the genetic characteristics of the virulent strains circulating in West and Central Africa is still scarce. In this study, 96 complete NDV fusion gene sequences were obtained from poultry sampled in Cameroon, Central African Republic, Côte d'Ivoire, and Nigeria between 2006 and 2011. Based on rational criteria recently proposed for the classification of NDV strains into classes, genotypes, and subgenotypes, we revisited the classification of virulent strains, in particular those from West and Central Africa, leading to their grouping into genotype XIV and newly defined genotypes XVII and XVIII, each with two subgenotypes. Phylogenetic analyses revealed that several (sub)genotypes are found in almost every country. In Cameroon, most strains were related to vaccine strains, but a single genotype XVII strain was also found. Only three highly similar genotype XVII strains were detected in Central African Republic. Subgenotypes XVIIa, XVIIIa, and XVIIIb cocirculated in Côte d'Ivoire, while subgenotypes XIVa, XIVb, XVIIa, XVIIb, and XVIIIb were found in Nigeria. While these genotypes are so far geographically restricted, local and international trade of domestic and exotic birds may lead to their spread beyond West and Central Africa. A high genetic diversity, mutations in important neutralizing epitopes paired with suboptimal vaccination, various levels of clinical responses of poultry and wild birds to virulent strains, strains with new cleavage sites, and other genetic modifications found in these genotypes tend to undermine and complicate NDV management in Africa.

Complete genome sequence of a Newcastle disease virus strain isolated from broiler breeder flocks in China

In 2010 and 2011, several devastating Newcastle disease (ND) outbreaks occurred in China, affecting broilers, layers, and breeders. The CK-JSX1-201005 virus was isolated from broiler breeder flocks vaccinated with the classical ND virus (NDV) vaccine program, but laying rate decreased from 80% to 30 to 40% in the clinic. Here, we report the complete genome sequence and molecular characteristic of the CK-JSX1-201005 NDV. These findings provide additional insights into the genetic variation of NDV circulating in China and are useful for vaccine development for NDV.

N-Glycans on the Nipah virus attachment glycoprotein modulate fusion and viral entry as they protect against antibody neutralization

Nipah virus (NiV) is the deadliest known paramyxovirus. Membrane fusion is essential for NiV entry into host cells and for the virus' pathological induction of cell-cell fusion (syncytia). The mechanism by which the attachment glycoprotein (G), upon binding to the cell receptors ephrinB2 or ephrinB3, triggers the fusion glycoprotein (F) to execute membrane fusion is largely unknown. N-glycans on paramyxovirus glycoproteins are generally required for proper protein conformational integrity, transport, and sometimes biological functions. We made conservative mutations (Asn to Gln) at the seven potential N-glycosylation sites in the NiV G ectodomain (G1 to G7) individually or in combination. Six of the seven N-glycosylation sites were found to be glycosylated. Moreover, pseudotyped virions carrying these N-glycan mutants had increased antibody neutralization sensitivities. Interestingly, our results revealed hyperfusogenic and hypofusogenic phenotypes for mutants that bound ephrinB2 at wild-type levels, and the mutant's cell-cell fusion phenotypes generally correlated to viral entry levels. In addition, when removing multiple N-glycans simultaneously, we observed synergistic or dominant-negative membrane fusion phenotypes. Interestingly, our data indicated that 4- to 6-fold increases in fusogenicity resulted from multiple mechanisms, including but not restricted to the increase of F triggering. Altogether, our results suggest that NiV-G N-glycans play a role in shielding virions against antibody neutralization, while modulating cell-cell fusion and viral entry via multiple mechanisms.

Henipavirus membrane fusion and viral entry

Nipah (NiV) and Hendra (HeV) viruses cause cell-cell fusion (syncytia) in brain, lung, heart, and kidney tissues, leading to encephalitis, pneumonia, and often death. Membrane fusion is essential to both viral entry and virus-induced cell-cell fusion, a hallmark of henipavirus infections. Elucidiation of the mechanism(s) of membrane fusion is critical to understanding henipavirus pathobiology and has the potential to identify novel strategies for the development of antiviral therapeutic agents. Henipavirus membrane fusion requires the coordinated actions of the viral attachment (G) and fusion (F) glycoproteins. Current henipavirus fusion models posit that attachment of NiV or HeV G to its cell surface receptors releases F from its metastable pre-fusion conformation to mediate membrane fusion. The identification of ephrinB2 and ephrinB3 as henipavirus receptors has paved the way for recent advances in our understanding of henipavirus membrane fusion. These advances highlight mechanistic similarities and differences between membrane fusion for the henipavirus and other genera within the Paramyxoviridae family. Here, we review these mechanisms and the current gaps in our knowledge in the field.

Molecular virology of the henipaviruses

Nipah (NiV) and Hendra (HeV) viruses comprise the genus Henipavirus and are highly pathogenic paramyxoviruses, which cause fatal encephalitis and respiratory disease in humans. Since their respective initial outbreaks in 1998 and 1994, they have continued to cause sporadic outbreaks resulting in fatal disease. Due to their designation as Biosafety Level 4 pathogens, the level of containment required to work with live henipaviruses is available only to select laboratories around the world. This chapter provides an overview of the molecular virology of NiV and HeV including comparisons to other, well-characterized paramyxoviruses. This chapter also describes the sequence diversity present among the henipaviruses.

Coordinate deletion of N-glycans from the heptad repeats of the fusion F protein of Newcastle disease virus yields a hyperfusogenic virus with increased replication, virulence, and immunogenicity

The role of N-linked glycosylation of the Newcastle disease virus (NDV) fusion (F) protein in viral replication and pathogenesis was examined by eliminating potential acceptor sites using a reverse genetics system for the moderately pathogenic strain Beaudette C (BC). The NDV-BC F protein contains six potential acceptor sites for N-linked glycosylation at residues 85, 191, 366, 447, 471, and 541 (sites Ng1 to Ng6, respectively). The sites at Ng2 and Ng5 are present in heptad repeat (HR) domains HR1 and HR2, respectively, and thus might affect fusion. Each N-glycosylation site was eliminated individually by replacing asparagine (N) with glutamine (Q), and a double mutant (Ng2 + 5) involving the two HR domains was also made. Each mutant was successfully recovered by reverse genetics except for the one involving Ng6, which is present in the cytoplasmic domain. All of the F proteins expressed by the recovered mutant viruses were efficiently cleaved and transported to the infected-cell surface. None of the individual mutations affected viral fusogenicity, but the double mutation at Ng2 and Ng5 in HR1 and HR2 increased fusogenicity >12-fold. The single mutations at sites Ng1, Ng2, and Ng5 resulted in modestly reduced multicycle growth in vitro. These three single mutations were also the most attenuating in eggs and 1-day-old chicks and were associated with decreased replication and spread in 2-week-old chickens. In contrast, the combination of the mutations at Ng2 and Ng5 yielded a virus that, compared to the BC parent, replicated >100-fold more efficiently in vitro, was more virulent in eggs and chicks, replicated more efficiently in chickens with enhanced tropism for the brain and gut, and elicited stronger humoral cell responses. These results illustrate the effects of N-glycosylation of the F protein on NDV pathobiology and suggest that the N-glycans in HR1 and HR2 coordinately downregulate viral fusion and virulence.

Modes of paramyxovirus fusion: a Henipavirus perspective

Henipavirus is a new genus of Paramyxoviridae that uses protein-based receptors (ephrinB2 and ephrinB3) for virus entry. Paramyxovirus entry requires the coordinated action of the fusion (F) and attachment viral envelope glycoproteins. Receptor binding to the attachment protein triggers F to undergo a conformational cascade that results in membrane fusion. The accumulation of structural and functional studies on many paramyxoviral fusion and attachment proteins, including the recent elucidation of structures of Nipah virus (NiV) and Hendra virus (HeV) G glycoproteins bound and unbound to cognate ephrinB receptors, indicate that henipavirus entry and fusion could differ mechanistically from paramyxoviruses that use glycan-based receptors.

The transmembrane domain sequence affects the structure and function of the Newcastle disease virus fusion protein

The role of specific sequences in the transmembrane (TM) domain of Newcastle disease virus (NDV) fusion (F) protein in the structure and function of this protein was assessed by replacing this domain with the F protein TM domains from two other paramyxoviruses, Sendai virus (SV) and measles virus (MV), or the TM domain of the unrelated glycoprotein (G) of vesicular stomatitis virus (VSV). Mutant proteins with the SV or MV F protein TM domains were expressed, transported to cell surfaces, and proteolytically cleaved at levels comparable to that of the wild-type protein, while mutant proteins with the VSV G protein TM domain were less efficiently expressed on cell surfaces and proteolytically cleaved. All mutant proteins were defective in all steps of membrane fusion, including hemifusion. In contrast to the wild-type protein, the mutant proteins did not form detectable complexes with the NDV hemagglutinin-neuraminidase (HN) protein. As determined by binding of conformation-sensitive antibodies, the conformations of the ectodomains of the mutant proteins were altered. These results show that the specific sequence of the TM domain of the NDV F protein is important for the conformation of the preactivation form of the ectodomain, the interactions of the protein with HN protein, and fusion activity.

Canine distemper viruses expressing a hemagglutinin without N-glycans lose virulence but retain immunosuppression

Paramyxovirus glycoproteins are posttranslationally modified by the addition of N-linked glycans, which are often necessary for correct folding, processing, and cell surface expression. To establish the contribution of N glycosylation to morbillivirus attachment (H) protein function and overall virulence, we first determined the use of the potential N-glycosylation sites in the canine distemper virus (CDV) H proteins. Biochemical characterization revealed that the three sites conserved in all strains were N glycosylated, whereas only two of the up to five additional sites present in wild-type strains are used. A wild-type virus with an H protein reproducing the vaccine strain N-glycosylation pattern remained lethal in ferrets but with a prolonged course of disease. In contrast, introduction of the vaccine H protein in the wild-type context resulted in complete attenuation. To further characterize the role of N glycosylation in CDV pathogenesis, the N-glycosylation sites of wild-type H proteins were successively deleted, including a nonstandard site, to ultimately generate a nonglycosylated H protein. Despite reduced expression levels, this protein remained fully functional. Recombinant viruses expressing N-glycan-deficient H proteins no longer caused disease, even though their immunosuppressive capacities were retained, indicating that reduced N glycosylation contributes to attenuation without affecting immunosuppression.

Role of thiol/disulfide exchange in newcastle disease virus entry

Newcastle disease virus (NDV) entry into host cells is mediated by the hemagglutinin-neuraminidase (HN) and fusion (F) glycoproteins. We previously showed that production of free thiols in F protein is required for membrane fusion directed by F protein (S. Jain et al., J. Virol. 81:2328-2339, 2007). In the present study we evaluated the oxidation state of F protein in virions and virus-like particles and its relationship to activation of F protein by HN protein, F protein conformational intermediates, and virus-cell fusion. F protein, in particles, does not have free thiols, but free thiols were produced upon binding of particles to target cells. Free thiols were produced at 16 degrees C in F protein in virions bound to the target cells. They also appeared in different fusion defective mutant F proteins. Free thiols were produced in the presence of mutant HN proteins that are defective in F protein activation but are attachment competent. These results suggest that free thiols appear prior to any of the proposed major conformational changes in F protein which accompany fusion activation. These results also indicate that HN protein binding to its receptor likely facilitates the interaction between F protein and host cell isomerases, leading to reduction of disulfide bonds in F protein. Taken together, these results show that free thiols are produced in F protein at a very early stage during the onset of fusion and that the production of free thiols is required for fusion in addition to activation by HN protein.

Functional consequences of attenuating mutations in the haemagglutinin neuraminidase, fusion and polymerase proteins of a wild-type mumps virus strain

Wild-type mumps viruses (MuVs) are highly neurotropic and, prior to widespread vaccination programmes, were a major cause of viral meningitis and encephalitis in most developed countries. At present, there are no markers for virus attenuation, apart from the failure of a passaged isolate to produce clinical symptoms in vaccinees. Indeed, some MuV vaccines have retained residual neurovirulence properties and have caused aseptic meningitis in vaccinees. Three amino acid changes associated with the neuroattenuation of a wild-type MuV strain were identified previously. This study evaluated the impact of these changes on the function of the respective proteins. The data demonstrated that the Ser-->Asp amino acid substitution at position 466 in the haemagglutinin-neuraminidase protein resulted in decreased receptor binding and neuraminidase activity, the Ala/Thr-->Thr selection in the fusion protein resulted in decreased fusion activity, and the Ile-->Val substitution in the polymerase resulted in increased replicative/transcriptional activity. These data suggest a polygenic component (i.e. specific and inter-related roles of these amino acid changes) to MuV neuroattenuation.

Molecular characteristics of the Nipah virus glycoproteins

Nipah virus (NiV) is a highly pathogenic paramyxovirus, which emerged in 1998 from fruit bats in Malaysia and caused an outbreak of severe respiratory disease in pigs and fatal encephalitis in humans with high mortality rates. In contrast to most paramyxoviruses, NiV can infect a large variety of mammalian species. Due to this broad host range, its zoonotic potential, its high pathogenicity for humans, and the lack of effective vaccines or therapeutics, NiV was classified as a biosafety level 4 pathogen. This article provides an overview of the molecular characteristics of NiV focusing on the structure, functions, and unique biological properties of the two NiV surface glycoproteins, the receptor-binding G protein, and the fusion protein F. Since viral glycoproteins are major determinants for cell tropism and virus spread, a detailed knowledge of these proteins can help to understand the molecular basis of viral pathogenicity.

Protein N-glycosylation in the baculovirus-insect cell expression system and engineering of insect cells to produce "mammalianized" recombinant glycoproteins

Baculovirus expression vectors are frequently used to express glycoproteins, a subclass of proteins that includes many products with therapeutic value. The insect cells that serve as hosts for baculovirus vector infection are capable of transferring oligosaccharide side chains (glycans) to the same sites in recombinant proteins as those that are used for native protein N-glycosylation in mammalian cells. However, while mammalian cells produce compositionally more complex N-glycans containing terminal sialic acids, insect cells mostly produce simpler N-glycans with terminal mannose residues. This structural difference between insect and mammalian N-glycans compromises the in vivo bioactivity of glycoproteins and can potentially induce allergenic reactions in humans. These features obviously compromise the biomedical value of recombinant glycoproteins produced in the baculovirus expression vector system. Thus, much effort has been expended to characterize the potential and limits of N-glycosylation in insect cell systems. Discoveries from this research have led to the engineering of insect N-glycosylation pathways for assembly of mammalian-style glycans on baculovirus-expressed glycoproteins. This chapter summarizes our knowledge of insect N-glycosylation pathways and describes efforts to engineer baculovirus vectors and insect cell lines to overcome the limits of insect cell glycosylation. In addition, we consider other possible strategies for improving glycosylation in insect cells.

Characterization of human metapneumovirus F protein-promoted membrane fusion: critical roles for proteolytic processing and low pH

Human metapneumovirus (HMPV) is a recently described human pathogen of the pneumovirus subfamily within the paramyxovirus family. HMPV infection is prevalent worldwide and is associated with severe respiratory disease, particularly in infants. The HMPV fusion protein (F) amino acid sequence contains features characteristic of other paramyxovirus F proteins, including a putative cleavage site and potential N-linked glycosylation sites. Propagation of HMPV in cell culture requires exogenous trypsin, which cleaves the F protein, and HMPV, like several other pneumoviruses, is infectious in the absence of its attachment protein (G). However, little is known about HMPV F-promoted fusion, since the HMPV glycoproteins have yet to be analyzed separately from the virus. Using syncytium and luciferase reporter gene fusion assays, we determined the basic requirements for HMPV F protein-promoted fusion in transiently transfected cells. Our data indicate that proteolytic cleavage of the F protein is a stringent requirement for fusion and that the HMPV G protein does not significantly enhance fusion. Unexpectedly, we also found that fusion can be detected only when transfected cells are treated with trypsin and exposed to low pH, indicating that this viral fusion protein may function in a manner unique among the paramyxoviruses. We also analyzed the F protein cleavage site and three potential N-linked glycosylation sites by mutagenesis. Mutations in the cleavage site designed to facilitate endogenous cleavage did so with low efficiency, and our data suggest that all three N-glycosylation sites are utilized and that each affects cleavage and fusion to various degrees.

Expression of the Newcastle disease virus fusion protein in transgenic maize and immunological studies

Transgenic plants have been employed successfully as a low-cost system for the production of therapeutically valuable proteins, including antibodies, antigens and hormones. Here, we report the expression of the fusion (F) gene of the Newcastle disease virus (NDV) in transgenic maize plants. The expression of the transgene, driven by the maize ubiquitin promoter, caused accumulation of the F protein in maize kernels. The presence of the transgene was verified by Southern and western blots. Feeding chickens with kernels containing the F protein induced the production of antibodies, which conferred protection against a viral challenge. This protection was comparable to that conferred by a commercial vaccine. Possible uses of this plant-based F protein as a potential mucosal vaccine are discussed.

N-glycans on Nipah virus fusion protein protect against neutralization but reduce membrane fusion and viral entry

Nipah virus (NiV) is a deadly emerging paramyxovirus. The NiV attachment (NiV-G) and fusion (NiV-F) envelope glycoproteins mediate both syncytium formation and viral entry. Specific N-glycans on paramyxovirus fusion proteins are generally required for proper conformational integrity and biological function. However, removal of individual N-glycans on NiV-F had little negative effect on processing or fusogenicity and has even resulted in slightly increased fusogenicity. Here, we report that in both syncytium formation and viral entry assays, removal of multiple N-glycans on NiV-F resulted in marked increases in fusogenicity (>5-fold) but also resulted in increased sensitivity to neutralization by NiV-F-specific antisera. The mechanism underlying the hyperfusogenicity of these NiV-F N-glycan mutants is likely due to more-robust six-helix bundle formation, as these mutants showed increased fusion kinetics and were more resistant to neutralization by a fusion-inhibitory reagent based on the C-terminal heptad repeat region of NiV-F. Finally, we demonstrate that the fusogenicities of the NiV-F N-glycan mutants were inversely correlated with the relative avidities of NiV-F's interactions with NiV-G, providing support for the attachment protein "displacement" model of paramyxovirus fusion. Our results indicate that N-glycans on NiV-F protect NiV from antibody neutralization, suggest that this "shielding" role comes together with limiting cell-cell fusion and viral entry efficiencies, and point to the mechanisms underlying the hyperfusogenicity of these N-glycan mutants. These features underscore the varied roles that N-glycans on NiV-F play in the pathobiology of NiV entry but also shed light on the general mechanisms of paramyxovirus fusion with host cells.

Inhibition of receptor binding stabilizes Newcastle disease virus HN and F protein-containing complexes

Receptor binding of paramyxovirus attachment proteins and the interactions between attachment and fusion (F) proteins are thought to be central to activation of the F protein activity; however, mechanisms involved are unclear. To explore the relationships between Newcastle disease virus (NDV) HN and F protein interactions and HN protein attachment to sialic acid receptors, HN and F protein-containing complexes were detected and quantified by reciprocal coimmunoprecipitation from extracts of transfected avian cells. To inhibit HN protein receptor binding, cells transfected with HN and F protein cDNAs were incubated with neuraminidase from the start of transfection. Under these conditions, no fusion was observed, but amounts of HN and F protein complexes increased twofold over amounts detected in extracts of untreated cells. Stimulation of attachment by incubation of untransfected target cells with neuraminidase-treated HN and F protein-expressing cells resulted in a twofold decrease in amounts of HN and F protein complexes. In contrast, high levels of complexes containing HN protein and an uncleaved F protein (F-K115Q) were detected, and those levels were unaffected by neuraminidase treatment of cell monolayers or by incubation with target cells. These results suggest that HN and F proteins reside in a complex in the absence of receptor binding. Furthermore, the results show that not only receptor binding but also F protein cleavage are necessary for disassociation of the HN and F protein-containing complexes.

Evidence that maturation of the N-linked glycans of the respiratory syncytial virus (RSV) glycoproteins is required for virus-mediated cell fusion: The effect of alpha-mannosidase inhibitors on RSV infectivity

Glycan heterogeneity of the respiratory syncytial virus (RSV) fusion (F) protein was demonstrated by proteomics. The effect of maturation of the virus glycoproteins-associated glycans on virus infectivity was therefore examined using the alpha-mannosidase inhibitors deoxymannojirimycin (DMJ) and swainsonine (SW). In the presence of SW the N-linked glycans on the F protein appeared in a partially mature form, whereas in the presence of DMJ no maturation of the glycans was observed. Neither inhibitor had a significant effect on G protein processing or on the formation of progeny virus. Although the level of infectious virus and syncytia formation was not significantly affected by SW-treatment, DMJ-treatment correlated with a one hundred-fold reduction in virus infectivity. Our data suggest that glycan maturation of the RSV glycoproteins, in particular those on the F protein, is an important step in virus maturation and is required for virus infectivity.

The C terminus of the B5 receptor for herpes simplex virus contains a functional region important for infection

The expression of a previously uncharacterized human hfl-B5 cDNA confers susceptibility for herpes simplex virus (HSV) to porcine cells and fulfills criteria as an HSV entry receptor (A. Perez, Q.-X. Li, P. Perez-Romero, G. DeLassus, S. R. Lopez, S. Sutter, N. McLaren, and A. Oveta Fuller, J. Virol. 79:7419-7430, 2005). Heptad repeats found in the B5 C terminus are predicted to form an alpha-helix for coiled coil structure. We used mutagenesis and synthetic peptides with wild-type and mutant sequences to examine the function of the heptad repeat motif in HSV binding and entry into porcine cells that express B5 and for infection of naturally susceptible human HEp-2 cells. B5 with point mutations predicted to disrupt the putative C-terminal coiled coil failed to mediate HSV binding and entry into porcine cells. Synthetic peptides that contain the single amino acid changes lose the blocking activity of HSV entry. We concluded that the C terminus of B5 contains a functional region that is important for the B5 receptor to mediate events in HSV entry. Structural evidence that this functional region forms coiled coil structures is under investigation. Blocking of HSV interaction with the C-terminal region of the B5 receptor is a new potential target site to intervene in the virus infection of human cells.

Role of N-linked glycosylation of the Hendra virus fusion protein

The Hendra virus fusion (F) protein contains five potential sites for N-linked glycosylation in the ectodomain. Examination of F protein mutants with single asparagine-to-alanine mutations indicated that two sites in the F(2) subunit (N67 and N99) and two sites in the F(1) subunit (N414 and N464) normally undergo N-linked glycosylation. While N-linked modification at N414 is critical for protein folding and transport, F proteins lacking carbohydrates at N67, N99, or N464 remained fusogenically active. As N464 lies within heptad repeat B, these results contrast with those seen for several paramyxovirus F proteins.

Influence of N-glycans on processing and biological activity of the nipah virus fusion protein

Nipah virus (NiV), a new member of the Paramyxoviridae, codes for a fusion (F) protein with five potential N-glycosylation sites. Because glycans are known to be important structural components affecting the conformation and function of viral glycoproteins, we analyzed the effect of the deletion of N-linked oligosaccharides on cell surface transport, proteolytic cleavage, and the biological activity of the NiV F protein. Each of the five potential glycosylation sites was removed either individually or in combination, revealing that four sites are actually utilized (g2 and g3 in the F(2) subunit and g4 and g5 in the F(1) subunit). While the removal of g2 and/or g3 had no or little effect on cleavage, surface transport, and fusion activity, the elimination of g4 or g5 reduced the surface expression by more than 80%. Similar to a mutant lacking all N-glycans, g4 deletion mutants in which the potential glycosylation site was destroyed by introducing a glycine residue were neither cleaved nor transported to the cell surface and consequently were not able to mediate cell-to-cell fusion. This finding indicates that in the absence of g4, the amino acid sequence around position 414 is important for folding and transport.

Role of the cytoplasmic domain of the Newcastle disease virus fusion protein in association with lipid rafts

To explore the association of the Newcastle disease virus (NDV) fusion (F) protein with cholesterol-rich membrane domains, its localization in detergent-resistant membranes (DRMs) in transfected cells was characterized. After solubilization of cells expressing the F protein with 1% Triton X-100 at 4 degrees C, ca. 40% of total, cell-associated F protein fractionated with classical DRMs with densities of 1.07 to l.14 as defined by flotation into sucrose density gradients. Association of the F protein with this cell fraction was unaffected by the cleavage of F(0) to F(1) and F(2) or by coexpression of the NDV attachment protein, the hemagglutinin-neuraminidase protein (HN). Furthermore, elimination by mutation, of potential palmitate addition sites in and near the F-protein transmembrane domain had no effect on F-protein association with DRMs. Rather, specific deletions of the cytoplasmic domain of the F protein eliminated association with classical DRMs. Comparisons of deletions that affected fusion activity of the protein and deletions that affected DRM association suggested that there is no direct link between the cell-cell fusion activity of the F protein and DRM association. Furthermore, depletion of cholesterol from cells expressing F and HN protein, while eliminating DRM association, had no effect on the ability of these cells to fuse with avian red blood cells. These results suggest that specific localization of the F protein in cholesterol-rich membrane domains is not required for cell-to-cell fusion. Paramyxovirus F-protein cytoplasmic domains have been implicated in virus assembly. The results presented here raise the possibility that the cytoplasmic domain is important in virus assembly at least in part because it directs the protein to cholesterol-rich membrane domains.

Interacting domains of the HN and F proteins of newcastle disease virus

The activation of most paramyxovirus fusion proteins (F proteins) requires not only cleavage of F(0) to F(1) and F(2) but also coexpression of the homologous attachment protein, hemagglutinin-neuraminidase (HN) or hemagglutinin (H). The type specificity requirement for HN or H protein coexpression strongly suggests that an interaction between HN and F proteins is required for fusion, and studies of chimeric HN proteins have implicated the membrane-proximal ectodomain in this interaction. Using biotin-labeled peptides with sequences of the Newcastle disease virus (NDV) F protein heptad repeat 2 (HR2) domain, we detected a specific interaction with amino acids 124 to 152 from the NDV HN protein. Biotin-labeled HR2 peptides bound to glutathione S-transferase (GST) fusion proteins containing these HN protein sequences but not to GST or to GST containing HN protein sequences corresponding to amino acids 49 to 118. To verify the functional significance of the interaction, two point mutations in the HN protein gene, I133L and L140A, were made individually by site-specific mutagenesis to produce two mutant proteins. These mutations inhibited the fusion promotion activities of the proteins without significantly affecting their surface expression, attachment activities, or neuraminidase activities. Furthermore, these changes in the sequence of amino acids 124 to 152 in the GST-HN fusion protein that bound HR2 peptides affected the binding of the peptides. These results are consistent with the hypothesis that HN protein binds to the F protein HR2 domain, an interaction important for the fusion promotion activity of the HN protein.

N-linked glycans with similar location in the fusion protein head modulate paramyxovirus fusion

N-linked glycans not only orchestrate the folding and intracellular transport of viral glycoproteins but also modulate their function. We have characterized the three glycans attached to fusion (F) proteins of the morbilliviruses canine distemper virus and measles virus. The individual Morbillivirus glycans have similar functional properties: the glycan at position 68 is essential for protein transport, and those at positions 36 and 75 modulate fusion (numbering according to the Newcastle disease virus [NDV] F protein sequence). Based on the crystal structure of the NDV F protein, we then predicted the locations of the Morbillivirus glycans: the glycan at position 36 is located in the F protein head, and those at positions 68 and 75 are located near the neck-stalk interface. NDV position 36 is not occupied by a glycan; the only glycan in that F protein head also has a fusion control function and grows from residue 366, located only 6 A from residue 36. We then exchanged the glycan at position 36 with the glycan at position 366 and showed functional complementation. Thus, structural information about the F proteins of Paramyxoviridae coupled with functional analysis disclosed a location in the protein head into which fusion-modulating glycans independently evolved.

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.

Evidence for mixed membrane topology of the newcastle disease virus fusion protein

The synthesis of the Newcastle disease virus (NDV) fusion (F) protein in a cell-free protein-synthesizing system containing membranes was characterized. The membrane-associated products were in at least two different topological forms with respect to the membranes. The properties of one form were consistent with the expected membrane insertion as a classical type 1 glycoprotein. This form of the protein was fully glycosylated, and sequences amino terminal to the transmembrane domain were protected from protease digestion by the membranes. The second form of membrane-associated F protein was partially glycosylated and partially protected from protease digestion by the membranes. Protease digestion resulted in a 23-kDa protease-protected polypeptide derived from F2 sequences and sequences from the amino-terminal end of the F1 domain. Furthermore, a 10-kDa polypeptide derived from the cytoplasmic domain (CT) was also protected from protease digestion by the membranes. Protease resistance of the 23- and 10-kDa polypeptides suggested that this second form of F protein inserted in membranes in a polytopic conformation with both the amino-terminal end and the carboxyl-terminal end translocated across membranes. To determine if this second form of the fusion protein could be found in cells expressing the F protein, two different approaches were taken. A polypeptide with the size of the partially translocated F protein was detected by Western analysis of proteins in total-cell extracts of NDV strain B1 (avirulent)-infected Cos-7 cells. Using antibodies raised against a peptide with sequences from the cytoplasmic domain, CT sequences were detected on surfaces of F protein-expressing Cos-7 cells by immunofluorescence and by flow cytometry. This antibody also inhibited the fusion of red blood cells to cells expressing F and HN proteins. These results suggest that NDV F protein made both in a cell-free system and in Cos-7 cells may exist in two topological forms with respect to membranes and that the second form of the protein may be involved in cell-cell fusion.

Mutational analysis of the membrane proximal heptad repeat of the newcastle disease virus fusion protein

Paramyxovirus fusion proteins have two heptad repeat domains, HR1 and HR2, that have been implicated in the fusion activity of the protein. Peptides from these two domains form a six-stranded, coiled-coil with the HR1 sequences forming a central trimer and three molecules of the HR2 helix located within the grooves in the central trimer (Baker et al., 1999, Mol. Cell 3, 309; Zhao et al. 2000, Proc. Natl. Acad. Sci. USA 97, 14172). Nonconservative mutations were made in the HR2 domain of the Newcastle disease virus fusion protein in residues that are likely to form contacts with the HR1 core trimer. These residues form the hydrophobic face of the helix and adjacent residues ("a" and "g" positions in the HR2 helical wheel structure). Mutant proteins were characterized for effects on synthesis, steady-state levels, proteolytic cleavage, and surface expression as well as fusion activity as measured by syncytia formation, content mixing, and lipid mixing. While all mutant proteins were transport competent and proteolytically cleaved, these mutations did variously affect fusion activity of the protein. Nonconservative mutations in the "g" position had no effect on fusion. In contrast, single changes in the middle "a" position of HR2 inhibited lipid mixing, content mixing, and syncytia formation. A single mutation in the more carboxyl-terminal "a" position had minimal effects on lipid mixing but did inhibit content mixing and syncytia formation. These results are consistent with the idea that the HR2 domain is involved in posttranslational interactions with HR1 that mediate the close approach of membranes. These results also suggest that the HR2 domain, particularly the carboxyl-terminal region, plays an additional role in fusion, a role related to content mixing and syncytia formation.

Mutations in the fusion peptide and adjacent heptad repeat inhibit folding or activity of the Newcastle disease virus fusion protein

Paramyxovirus fusion proteins have two heptad repeat domains, HR1 and HR2, which have been implicated in the fusion activity of the protein. Peptides with sequences from these two domains form a six-stranded coiled coil, with the HR1 sequences forming a central trimer (K. A. Baker, R. E. Dutch, R. A. Lamb, and T. S. Jardetzky, Mol. Cell 3:309-319, 1999; X. Zhao, M. Singh, V. N. Malashkevich, and P. S. Kim, Proc. Natl. Acad. Sci. USA 97:14172-14177, 2000). We have extended our previous mutational analysis of the HR1 domain of the Newcastle disease virus fusion protein, focusing on the role of the amino acids forming the hydrophobic core of the trimer, amino acids in the "a" and "d" positions of the helix from amino acids 123 to 182. Both conservative and nonconservative point mutations were characterized for their effects on synthesis, stability, proteolytic cleavage, and surface expression. Mutant proteins expressed on the cell surface were characterized for fusion activity by measuring syncytium formation, content mixing, and lipid mixing. We found that all mutations in the "a" position interfered with proteolytic cleavage and surface expression of the protein, implicating the HR1 domain in the folding of the F protein. However, mutation of five of seven "d" position residues had little or no effect on surface expression but, with one exception at residue 175, did interfere to various extents with the fusion activity of the protein. One of these "d" mutations, at position 154, interfered with proteolytic cleavage, while the rest of the mutants were cleaved normally. That most "d" position residues do affect fusion activity argues that a stable HR1 trimer is required for formation of the six-stranded coiled coil and, therefore, optimal fusion activity. That most of the "d" position mutations do not block folding suggests that formation of the core trimer may not be required for folding of the prefusion form of the protein. We also found that mutations within the fusion peptide, at residue 128, can interfere with folding of the protein, implicating this region in folding of the molecule. No characterized mutation enhanced fusion.