Structure-guided mutagenesis of Henipavirus receptor-binding proteins reveals molecular determinants of receptor usage and antibody-binding epitopes

Nipah virus (NiV) is a highly lethal, zoonotic Henipavirus (HNV) that causes respiratory and neurological signs and symptoms in humans. Similar to other paramyxoviruses, HNVs mediate entry into host cells through the concerted actions of two surface glycoproteins: a receptor-binding protein (RBP) that mediates attachment and a fusion glycoprotein (F) that triggers fusion in an RBP-dependent manner. NiV uses ephrin-B2 (EFNB2) and ephrin-B3 (EFNB3) as entry receptors. Ghana virus (GhV), a novel HNV identified in a Ghanaian bat, uses EFNB2 but not EFNB3. In this study, we employ a structure-informed approach to identify receptor-interfacing residues and systematically introduce GhV-RBP residues into a NiV-RBP backbone to uncover the molecular determinants of EFNB3 usage. We reveal two regions that severely impair EFNB3 binding by NiV-RBP and EFNB3-mediated entry by NiV pseudotyped viral particles. Further analyses uncovered two-point mutations (NiVN557SGhV and NiVY581TGhV) pivotal for this phenotype. Moreover, we identify NiV interaction with Y120 of EFNB3 as important for the usage of this receptor. Beyond these EFNB3-related findings, we reveal two domains that restrict GhV binding of EFNB2, confirm the HNV-head as an immunodominant target for polyclonal and monoclonal antibodies, and describe putative epitopes for GhV- and NiV-specific monoclonal antibodies. Cumulatively, the work presented here generates useful reagents and tools that shed insight to residues important for NiV usage of EFNB3, reveal regions critical for GhV binding of EFNB2, and describe putative HNV antibody-binding epitopes.

IMPORTANCE

Hendra virus and Nipah virus (NiV) are lethal, zoonotic Henipaviruses (HNVs) that cause respiratory and neurological clinical features in humans. Since their initial outbreaks in the 1990s, several novel HNVs have been discovered worldwide, including Ghana virus. Additionally, there is serological evidence of zoonotic transmission, lending way to concerns about future outbreaks. HNV infection of cells is mediated by the receptor-binding protein (RBP) and the Fusion protein (F). The work presented here identifies NiV RBP amino acids important for the usage of ephrin-B3 (EFNB3), a receptor highly expressed in neurons and predicted to be important for neurological clinical features caused by NiV. This study also characterizes epitopes recognized by antibodies against divergent HNV RBPs. Together, this sheds insight to amino acids critical for HNV receptor usage and antibody binding, which is valuable for future studies investigating determinants of viral pathogenesis and developing antibody therapies.

Henipaviruses and lyssaviruses target nucleolar treacle protein and regulate ribosomal RNA synthesis

The nucleolus is a common target of viruses and viral proteins, but for many viruses the functional outcomes and significance of this targeting remains unresolved. Recently, the first intranucleolar function of a protein of a cytoplasmically-replicating negative-sense RNA virus (NSV) was identified, with the finding that the matrix (M) protein of Hendra virus (HeV) (genus Henipavirus, family Paramyxoviridae) interacts with Treacle protein within nucleolar subcompartments and mimics a cellular mechanism of the nucleolar DNA-damage response (DDR) to suppress ribosomal RNA (rRNA) synthesis. Whether other viruses utilise this mechanism has not been examined. We report that sub-nucleolar Treacle targeting and modulation is conserved between M proteins of multiple Henipaviruses, including Nipah virus and other potentially zoonotic viruses. Furthermore, this function is also evident for P3 protein of rabies virus, the prototype virus of a different RNA virus family (Rhabdoviridae), with Treacle depletion in cells also found to impact virus production. These data indicate that unrelated proteins of viruses from different families have independently developed nucleolar/Treacle targeting function, but that modulation of Treacle has distinct effects on infection. Thus, subversion of Treacle may be an important process in infection by diverse NSVs, and so could provide novel targets for antiviral approaches with broad specificity.

Identification of a Region in the Common Amino-terminal Domain of Hendra Virus P, V, and W Proteins Responsible for Phase Transition and Amyloid Formation

Henipaviruses are BSL-4 zoonotic pathogens responsible in humans for severe encephalitis. Their V protein is a key player in the evasion of the host innate immune response. We previously showed that the Henipavirus V proteins consist of a long intrinsically disordered N-terminal domain (NTD) and a β-enriched C-terminal domain (CTD). These terminals are critical for V binding to DDB1, which is a cellular protein that is a component of the ubiquitin ligase E3 complex, as well as binding to MDA5 and LGP2, which are two host sensors of viral RNA. Here, we serendipitously discovered that the Hendra virus V protein undergoes a liquid-to-hydrogel phase transition and identified the V region responsible for this phenomenon. This region, referred to as PNT3 and encompassing residues 200-310, was further investigated using a combination of biophysical and structural approaches. Congo red binding assays, together with negative-staining transmisison electron microscopy (TEM) studies, show that PNT3 forms amyloid-like fibrils. Fibrillation abilities are dramatically reduced in a rationally designed PNT3 variant in which a stretch of three contiguous tyrosines, falling within an amyloidogenic motif, were replaced by three alanines. Worthy to note, Congo red staining experiments provided hints that these amyloid-like fibrils form not only in vitro but also in cellula after transfection or infection. The present results set the stage for further investigations aimed at assessing the functional role of phase separation and fibrillation by the Henipavirus V proteins.

Henipavirus W Proteins Interact with 14-3-3 To Modulate Host Gene Expression

Nipah virus (NiV) and Hendra virus (HeV), members of the Henipavirus genus in the Paramyxoviridae family, are recently emerged, highly lethal zoonotic pathogens. The NiV and HeV nonsegmented, negative-sense RNA genomes encode nine proteins, including the W protein. Expressed from the P gene through mRNA editing, W shares a common N-terminus with P and V but has a unique C-terminus. Expressed alone, W modulates innate immune responses by several mechanisms, and elimination of W from NiV alters the course of infection in experimentally infected ferrets. However, the specific host interactions that allow W to modulate innate immunity are incompletely understood. This study demonstrates that the NiV and HeV W proteins interact with all seven isoforms of the 14-3-3 family, regulatory molecules that preferentially bind phosphorylated target proteins to regulate a wide range of cellular functions. The interaction is dependent on the penultimate amino acid residue in the W sequence, a conserved, phosphorylated serine. The cocrystal structure of the W C-terminal binding motif with 14-3-3 provides only the second structure of a complex containing a mode III interactor, which is defined as a 14-3-3 interaction with a phosphoserine/phosphothreonine at the C-termini of the target protein. Transcriptomic analysis of inducible cell lines infected with an RNA virus and expressing either wild-type W or W lacking 14-3-3 binding, identifies new functions for W. These include the regulation of cellular metabolic processes, extracellular matrix organization, and apoptosis.IMPORTANCE Nipah virus (NiV) and Hendra virus (HeV), members of the Henipavirus genus, are recently emerged, highly lethal zoonotic pathogens that cause yearly outbreaks. NiV and HeV each encode a W protein that has roles in regulating host signaling pathways, including antagonism of the innate immune response. However, the mechanisms used by W to regulate these host responses are not clear. Here, characterization of the interaction of NiV and HeV W with 14-3-3 identifies modulation of 14-3-3-regulated host signaling pathways not previously associated with W, suggesting new avenues of research. The cocrystal structure of the NiV W:14-3-3 complex, as only the second structure of a 14-3-3 mode III interactor, provides further insight into this less-well-understood 14-3-3 binding motif.

Structural basis for importin alpha 3 specificity of W proteins in Hendra and Nipah viruses

Seven human isoforms of importin α mediate nuclear import of cargo in a tissue- and isoform-specific manner. How nuclear import adaptors differentially interact with cargo harbouring the same nuclear localisation signal (NLS) remains poorly understood, as the NLS recognition region is highly conserved. Here, we provide a structural basis for the nuclear import specificity of W proteins in Hendra and Nipah viruses. We determine the structural interfaces of these cargo bound to importin α1 and α3, identifying a 2.4-fold more extensive interface and > 50-fold higher binding affinity for importin α3. Through the design of importin α1 and α3 chimeric and mutant proteins, together with structures of cargo-free importin α1 and α3 isoforms, we establish that the molecular basis of specificity resides in the differential positioning of the armadillo repeats 7 and 8. Overall, our study provides mechanistic insights into a range of important nucleocytoplasmic transport processes reliant on isoform adaptor specificity.

Indirect ELISA based on Hendra and Nipah virus proteins for the detection of henipavirus specific antibodies in pigs

Hendra virus (HeV) and Nipah virus (NiV) belong to the genus Henipavirus in the family Paramyxoviridae. Henipavirus infections were first reported in the 1990’s causing severe and often fatal outbreaks in domestic animals and humans in Southeast Asia and Australia. NiV infections were observed in humans in Bangladesh, India and in the first outbreak in Malaysia, where pigs were also infected. HeV infections occurred in horses in the North-Eastern regions of Australia, with singular transmission events to humans. Bats of the genus Pteropus have been identified as the reservoir hosts for henipaviruses. Molecular and serological indications for the presence of henipa-like viruses in African fruit bats, pigs and humans have been published recently. In our study, truncated forms of HeV and NiV attachment (G) proteins as well as the full-length NiV nucleocapsid (N) protein were expressed using different expression systems. Based on these recombinant proteins, Enzyme-linked Immunosorbent Assays (ELISA) were developed for the detection of HeV or NiV specific antibodies in porcine serum samples. We used the NiV N ELISA for initial serum screening considering the general reactivity against henipaviruses. The G protein based ELISAs enabled the differentiation between HeV and NiV infections, since as expected, the sera displayed higher reactivity with the respective homologous antigens. In the future, these assays will present valuable tools for serosurveillance of swine and possibly other livestock or wildlife species in affected areas. Such studies will help assessing the potential risk for human and animal health worldwide by elucidating the distribution of henipaviruses.

Genome-wide siRNA Screening at Biosafety Level 4 Reveals a Crucial Role for Fibrillarin in Henipavirus Infection

Hendra and Nipah viruses (genus Henipavirus, family Paramyxoviridae) are highly pathogenic bat-borne viruses. The need for high biocontainment when studying henipaviruses has hindered the development of therapeutics and knowledge of the viral infection cycle. We have performed a genome-wide siRNA screen at biosafety level 4 that identified 585 human proteins required for henipavirus infection. The host protein with the largest impact was fibrillarin, a nucleolar methyltransferase that was also required by measles, mumps and respiratory syncytial viruses for infection. While not required for cell entry, henipavirus RNA and protein syntheses were greatly impaired in cells lacking fibrillarin, indicating a crucial role in the RNA replication phase of infection. During infection, the Hendra virus matrix protein co-localized with fibrillarin in cell nucleoli, and co-associated as a complex in pulldown studies, while its nuclear import was unaffected in fibrillarin-depleted cells. Mutagenesis studies showed that the methyltransferase activity of fibrillarin was required for henipavirus infection, suggesting that this enzyme could be targeted therapeutically to combat henipavirus infections.

The henipaviruses Hendra and Nipah are bat-borne paramyxoviruses that are highly pathogenic in humans. The need for high biocontainment when studying Hendra and Nipah virus biology has hindered the development of therapeutics and knowledge of the viral infection cycle. This study describes a genome-wide functional genomics screen of human host genes required for henipavirus infection, to our knowledge the first such study conducted at biosafety level 4. Our study demonstrates that henipavirus infection is critically reliant on fibrillarin, a methyltransferase enzyme residing in the cell nucleolus. Despite henipavirus genome replication occurring in the cytoplasm of infected cells, viral RNA synthesis was greatly impaired in cells lacking fibrillarin. Furthermore during the early stages of infection the Hendra virus matrix protein shuttles to the nucleolus and binds fibrillarin. Collectively these results suggest a hitherto unappreciated role for nucleolar host-virus interactions in the early replication phase of henipavirus infection. Finally, mutating the catalytic activity of fibrillarin inhibits henipavirus infection, suggesting that this enzyme could be targeted therapeutically to combat henipavirus infections.

Atomic resolution description of the interaction between the nucleoprotein and phosphoprotein of Hendra virus

Hendra virus (HeV) is a recently emerged severe human pathogen that belongs to the Henipavirus genus within the Paramyxoviridae family. The HeV genome is encapsidated by the nucleoprotein (N) within a helical nucleocapsid. Recruitment of the viral polymerase onto the nucleocapsid template relies on the interaction between the C-terminal domain, N(TAIL), of N and the C-terminal X domain, XD, of the polymerase co-factor phosphoprotein (P). Here, we provide an atomic resolution description of the intrinsically disordered N(TAIL) domain in its isolated state and in intact nucleocapsids using nuclear magnetic resonance (NMR) spectroscopy. Using electron microscopy, we show that HeV nucleocapsids form herringbone-like structures typical of paramyxoviruses. We also report the crystal structure of XD of P that consists of a three-helix bundle. We study the interaction between N(TAIL) and XD using NMR titration experiments and provide a detailed mapping of the reciprocal binding sites. We show that the interaction is accompanied by α-helical folding of the molecular recognition element of N(TAIL) upon binding to a hydrophobic patch on the surface of XD. Finally, using solution NMR, we investigate the interaction between intact nucleocapsids and XD. Our results indicate that monomeric XD binds to N(TAIL) without triggering an additional unwinding of the nucleocapsid template. The present results provide a structural description at the atomic level of the protein-protein interactions required for transcription and replication of HeV, and the first direct observation of the interaction between the X domain of P and intact nucleocapsids in Paramyxoviridae.

Residues in the hendra virus fusion protein transmembrane domain are critical for endocytic recycling

Hendra virus is a highly pathogenic paramyxovirus classified as a biosafety level four agent. The fusion (F) protein of Hendra virus is critical for promoting viral entry and cell-to-cell fusion. To be fusogenically active, Hendra virus F must undergo endocytic recycling and cleavage by the endosomal/lysosomal protease cathepsin L, but the route of Hendra virus F following internalization and the recycling signals involved are poorly understood. We examined the intracellular distribution of Hendra virus F following endocytosis and showed that it is primarily present in Rab5- and Rab4-positive endosomal compartments, suggesting that cathepsin L cleavage occurs in early endosomes. Hendra virus F transmembrane domain (TMD) residues S490 and Y498 were found to be important for correct Hendra virus F recycling, with the hydroxyl group of S490 and the aromatic ring of Y498 important for this process. In addition, changes in association of isolated Hendra virus F TMDs correlated with alterations to Hendra virus F recycling, suggesting that appropriate TMD interactions play an important role in endocytic trafficking.

C-terminal tyrosine residues modulate the fusion activity of the Hendra virus fusion protein

The paramyxovirus family includes important human pathogens such as measles, mumps, respiratory syncytial virus, and the recently emerged, highly pathogenic Hendra and Nipah viruses. The viral fusion (F) protein plays critical roles in infection, promoting both the virus-cell membrane fusion events needed for viral entry as well as cell-cell fusion events leading to syncytia formation. We describe the surprising finding that addition of the short epitope HA tag to the cytoplasmic tail (CT) of the Hendra virus F protein leads to a significant increase in the extent of cell-cell membrane fusion. This increase was not due to alterations in surface expression, cleavage state, or association with lipid microdomains. Addition of a Myc tag of similar length did not alter Hendra F protein fusion activity, indicating that the observed stimulation was not solely a result of lengthening the CT. Three tyrosine residues within the HA tag were critical for the increase in the extent of fusion, suggesting C-terminal tyrosines may modulate Hendra fusion activity. The effects of addition of the HA tag varied with other fusion proteins, as parainfluenza virus 5 F-HA showed a decreased level of surface expression and no stimulation of fusion. These results indicate that additions to the C-terminal end of the F protein CT can modulate protein function in a sequence specific manner, reinforcing the need for careful analysis of epitope-tagged glycoproteins. In addition, our results implicate C-terminal tyrosine residues in the modulation of the membrane fusion reaction promoted by these viral glycoproteins.

Neutralization assays for differential henipavirus serology using Bio-Plex Protein Array Systems

Hendra virus (HeV) and Nipah virus (NiV) are related emerging paramyxoviruses classified in the genus Henipavirus. Both cause fatal disease in animals and humans and are classified as biosafety level 4 pathogens. Here we detail two new multiplexed microsphere assays, one for antibody detection and differentiation and another designed as a surrogate for virus neutralization. Both assays utilize recombinant soluble attachment glycoproteins (sG) whereas the latter incorporates the cellular receptor, recombinant ephrin-B2. Spectrally distinct sG(HeV)- and sG(NiV)-coupled microspheres preferentially bound antibodies from HeV- and NiV-seropositive animals, demonstrating a simple procedure to differentiate antibodies to these closely related viruses. Soluble ephrin-B2 bound sG-coupled microspheres in a dose-dependent fashion. Specificity of binding was further evaluated with henipavirus G-specific sera and MAbs. Sera from henipavirus-seropositive animals differentially blocked ephrin-B2 binding, suggesting that detection and differentiation of HeV and NiV neutralizing antibodies can be done simultaneously in the absence of live virus.

Identification of Hendra virus G glycoprotein residues that are critical for receptor binding

Hendra virus (HeV) is an emerging paramyxovirus capable of infecting and causing disease in a variety of mammalian species, including humans. The virus infects its host cells through the coordinated functions of its fusion (F) and attachment (G) glycoproteins, the latter of which is responsible for binding the virus receptors ephrinB2 and ephrinB3. In order to identify the receptor binding site, a panel of G glycoprotein constructs containing mutations was generated using an alanine-scanning mutagenesis strategy. Based on a predicted G structure, charged amino acids residing in regions that could be homologous to those in the measles virus H attachment glycoprotein known to be involved in its protein receptor interaction were targeted. Using a coprecipitation-based assay, seven single-amino-acid substitutions in HeV G were identified as having significantly impaired binding to both the ephrinB2 and ephrinB3 viral receptors: D257A, D260A, G439A, K443A, G449A, K465A, and D468A. The impairment of receptor interaction conferred a concomitant diminution in their abilities to promote membrane fusion when coexpressed with F. The G glycoprotein mutants were also recognized by three or more conformation-dependent monoclonal antibodies of a panel of five, were expressed on the cell surface, and retained their abilities to bind and coprecipitate F. Interestingly, some of these mutant G glycoproteins coprecipitated with F more efficiently than wild-type G. Taken together, these data provide strong biochemical and functional evidence that some of these residues could be part of a conformation-dependent, discontinuous, and overlapping ephrinB2 and -B3 binding domain within the HeV G glycoprotein.

Expression of truncated phosphoproteins of Nipah virus and Hendra virus in Escherichia coli for the differentiation of henipavirus infections

The genus Henipavirus in the family Paramyxoviridae compromises two newly identified dangerous pathogens, Nipah virus and Hendra virus. Phosphoprotein of the two viruses is one of the major immunodominant antigens and the most divergent protein in the viral genomes. We have now expressed two pairs of truncated phosphoproteins of the two viruses in Escherichia coli in a soluble form using a vector tailored from pET32a. The truncated recombinant phosphoproteins were purified with His-Tag affinity chromatography and their antigenicity was determined by western blotting and ELISA. The longer pair of truncated recombinant phosphoproteins, covering amino acid residues 4-550, was more antigenic than the shorter one and of potential utility in the serological differentiation of henipavirus infections.

Cathepsin L is involved in proteolytic processing of the Hendra virus fusion protein

Proteolytic processing of paramyxovirus fusion (F) proteins is essential for the generation of a mature and fusogenic form of the F protein. Although many paramyxovirus F proteins are proteolytically processed by the cellular protease furin at a multibasic cleavage motif, cleavage of the newly emerged Hendra virus F protein occurs by a previously unidentified cellular protease following a single lysine at residue 109. We demonstrate here that the cellular protease cathepsin L is involved in converting the Hendra virus precursor F protein (F(0)) to the active F(1) + F(2) disulfide-linked heterodimer. To initially identify the class of protease involved in Hendra virus F protein cleavage, Vero cells transfected with pCAGGS-Hendra F or pCAGGS-SV5 F (known to be proteolytically processed by furin) were metabolically labeled and chased in the absence or presence of serine, cysteine, aspartyl, and metalloprotease inhibitors. Nonspecific and specific protease inhibitors known to decrease cathepsin activity inhibited proteolytic processing of Hendra virus F but had no effect on simian virus 5 F processing. We next designed shRNA oligonucleotides to cathepsin L which dramatically reduced cathepsin L protein expression and enzyme activity. Cathepsin L shRNA-expressing Vero cells transfected with pCAGGS-Hendra F demonstrated a nondetectable amount of cleavage of the Hendra virus F protein and significantly decreased membrane fusion activity. Additionally, we found that purified human cathepsin L processed immunopurified Hendra virus F(0) into F(1) and F(2) fragments. These studies introduce a novel mechanism for primary proteolytic processing of viral glycoproteins and also suggest a previously unreported biological role for cathepsin L.

Endocytosis plays a critical role in proteolytic processing of the Hendra virus fusion protein

The Hendra virus fusion (F) protein is synthesized as a precursor protein, F(0), which is proteolytically processed to the mature form, F(1) + F(2). Unlike the case for the majority of paramyxovirus F proteins, the processing event is furin independent, does not require the addition of exogenous proteases, is not affected by reductions in intracellular Ca(2+), and is strongly affected by conditions that raise the intracellular pH (C. T. Pager, M. A. Wurth, and R. E. Dutch, J. Virol. 78:9154-9163, 2004). The Hendra virus F protein cytoplasmic tail contains a consensus motif for endocytosis, YXXPhi. To analyze the potential role of endocytosis in the processing and membrane fusion promotion of the Hendra virus F protein, mutation of tyrosine 525 to alanine (Hendra virus F Y525A) or phenylalanine (Hendra virus F Y525F) was performed. The rate of endocytosis of Hendra virus F Y525A was significantly reduced compared to that of the wild-type (wt) F protein, confirming the functional importance of the endocytosis motif. An intermediate level of endocytosis was observed for Hendra virus F Y525F. Surprisingly, dramatic reductions in the rate of proteolytic processing were observed for Hendra virus F Y525A, although initial transport to the cell surface was not affected. The levels of surface expression for both Hendra virus F Y525A and Hendra virus F Y525F were higher than that of the wt protein, and these mutants displayed enhanced syncytium formation. These results suggest that endocytosis is critically important for Hendra virus F protein cleavage, representing a new paradigm for proteolytic processing of paramyxovirus F proteins.

Sequence motif upstream of the Hendra virus fusion protein cleavage site is not sufficient to promote efficient proteolytic processing

The Hendra virus fusion (HeV F) protein is synthesized as a precursor, F(0), and proteolytically cleaved into the mature F(1) and F(2) heterodimer, following an HDLVDGVK(109) motif. This cleavage event is required for fusogenic activity. To determine the amino acid requirements for processing of the HeV F protein, we constructed multiple mutants. Individual and simultaneous alanine substitutions of the eight residues immediately upstream of the cleavage site did not eliminate processing. A chimeric SV5 F protein in which the furin site was substituted for the VDGVK(109) motif of the HeV F protein was not processed but was expressed on the cell surface. Another chimeric SV5 F protein containing the HDLVDGVK(109) motif of the HeV F protein underwent partial cleavage. These data indicate that the upstream region can play a role in protease recognition, but is neither absolutely required nor sufficient for efficient processing of the HeV F protein.

Ephrin-B2 ligand is a functional receptor for Hendra virus and Nipah virus

Hendra virus (HeV) and Nipah virus (NiV) belong to the genus Henipavirus of the family Paramyxoviridae and are unique in that they exhibit a broad species tropism and cause fatal disease in both animals and humans. They infect cells through a pH-independent membrane fusion process mediated by their fusion and attachment glycoproteins. Previously, we demonstrated identical cell fusion tropisms for HeV and NiV and the protease-sensitive nature of their unknown cell receptor and identified a human cell line (HeLa-USU) that was nonpermissive for fusion and virus infection. Here, a microarray analysis was performed on the HeLa-USU cells, permissive HeLa-CCL2 cells, and two other permissive human cell lines. From this analysis, we identified a list of genes encoding known and predicted plasma membrane surface-expressed proteins that were highly expressed in all permissive cells and absent from the HeLa-USU cells and rank-ordered them based on their relative levels. Available expression vectors containing the first 10 genes were obtained and individually transfected into HeLa-USU cells. One clone, encoding human ephrin-B2 (EFNB2), was found capable of rendering HeLa-USU cells permissive for HeV- and NiV-mediated cell fusion as well as infection by live virus. A soluble recombinant EFNB2 could potently block fusion and infection and bind soluble recombinant HeV and NiV attachment glycoproteins with high affinity. Together, these data indicate that EFNB2 serves as a functional receptor for both HeV and NiV. The highly conserved nature of EFNB2 in humans and animals is consistent with the broad tropism exhibited by these emerging zoonotic viruses.

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.

Subcellular localization and calcium and pH requirements for proteolytic processing of the Hendra virus fusion protein

Proteolytic cleavage of the Hendra virus fusion (F) protein results in the formation of disulfide-linked F1 and F2 subunits, with cleavage occurring after residue K109 in the sequence GDVK/L. This unusual cleavage site and efficient propagation of Hendra virus in a furin-deficient cell line indicate that the Hendra F protein is not cleaved by furin, the protease responsible for proteolytic activation of many viral fusion proteins. To identify the subcellular site of Hendra F processing, Vero cells transfected with pCAGGS-Hendra F or pCAGGS-SV5 F were metabolically labeled and chased in the absence and presence of inhibitors of exocytosis. The addition of carbonyl-cyanide-3-chlorophenylhydrazone, monensin, brefeldin A, or NaF-AlCl3 or incubation of cells at 20 degrees C all inhibited processing of the Hendra F protein, suggesting that cleavage of Hendra F occurs either in secretory vesicles budding from the trans-Golgi network or at the cell surface. In contrast to proteolytic cleavage of the simian virus 5 (SV5) F protein by the Ca(2+)-dependent protease furin, proteolytic cleavage of the Hendra F protein was not significantly inhibited by decreases in Ca2+ levels following incubation with EGTA or A23187. However, in the presence of weak amines and H+ V-ATPase inhibitors, known to raise intracellular pH, cleavage of Hendra F protein was inhibited while processing of the SV5 F protein was not significantly affected. The subcellular location, sensitivity to pH changes, and decreased Ca2+ requirement suggest that the protease responsible for cleavage of Hendra F protein differs from proteases previously shown to be involved in the processing of other viral glycoproteins.

Crystallization and preliminary crystallographic analysis of the fusion core from two new zoonotic paramyxoviruses, Nipah virus and Hendra virus

Highly conserved heptad-repeat (HR1 and HR2) regions in class I viral fusion (F) proteins, including the F protein from paramyxovirus, interact with each other post-fusion to form a six-helix bundle called a fusion core. Crystals of the fusion core of Nipah virus have been grown at 291 K using PEG 4000 as precipitant. The diffraction pattern of the crystal extends to 2.1 angstroms resolution at 100 K in-house. The crystals have unit-cell parameters a = 31.664, b = 31.725, c = 51.256 angstroms, alpha = 80.706, beta = 86.343, gamma = 65.812 degrees and belong to space group P1. Crystals of the fusion core of Hendra virus have also been grown at 291 K using PEG 4000 as precipitant. The diffraction pattern of the crystal extends to 2.0 angstroms resolution at 100 K in-house. A selenomethionine (SeMet) derivative of the HeV fusion core was overexpressed using the same Escherichia coli expression system and purified. The derivative crystals were obtained under similar conditions and three different wavelength data sets were collected to 2.0 angstroms resolution from the derivative crystal at BSRF (Beijing Synchrotron Radiation Facility). The crystals have unit-cell parameters a = 31.997, b = 31.970, c = 53.865 angstroms, alpha = 85.990, beta = 85.842, gamma = 68.245 degrees and belong to space group P1.

Basis for fusion inhibition by peptides: analysis of the heptad repeat regions of the fusion proteins from Nipah and Hendra viruses, newly emergent zoonotic paramyxoviruses

Nipah virus (NiV) and Hendra virus (HeV) are novel zoonotic members of the Paramyxoviridae family and are the prototypes for a newly designated genus, Genus Henipavirus. Recent studies have shown that paramyxovirus might adopt a similar mechanism of virus fusion-entry. Under this mechanism, the two highly conserved heptad repeat (HR) regions, HR1 and HR2, in the fusion (F) protein, seem to show characteristic structure in the fusion core: the formation of a 6-helix coiled-coil bundle. The three HR1s form the alpha-helix coiled-coil surrounded by three HR2s. In this study, the two HR regions of NiV or HeV were expressed in an Escherichia coli system as a single chain and the results do show that HR1 and HR2 interact with each other in both NiV and HeV and form typical 6-helix coiled-coil bundles. This provides the molecular basis of HR2 inhibition to NiV and HeV fusion as observed in an earlier report.