The structure of the fusion glycoprotein of Newcastle disease virus suggests a novel paradigm for the molecular mechanism of membrane fusion

Fusogenic variants of a noncytopathic paramyxovirus

SER virus is a type 5 parainfluenza virus that does not exhibit syncytium formation, in contrast to most other paramyxoviruses. This property has been attributed, at least in part, to the presence of an extension of the cytoplasmic tail (CT) of the SER F protein, as truncations or mutations of this region resulted in enhanced fusion. In this study we used repeated passage to select for mutant SER viruses, which were found to be fusogenic. The mutant viruses replicated at levels comparable to or higher than the wild-type SER virus and caused plaque formation, in contrast to the wild-type virus which does not form plaques. The mutants differed strikingly in their plaque sizes. The F genes of mutant viruses were cloned and sequenced and shared some mutations, including a proline-to-leucine change at position 22 and an isoleucine-to-leucine substitution at position 191; other changes that were specific to each mutant were also found. The HN proteins of mutant viruses also showed mutations spanning the length of the protein whereas the M protein showed a consistent mutation, threonine to isoleucine, at position 129. The structure of the F protein was used to identify residues involved in the mutant phenotypes in terms of their location and proximity to heptad repeat domains.

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

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

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

OBJECTIVES

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

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

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

Examination of a fusogenic hexameric core from human metapneumovirus and identification of a potent synthetic peptide inhibitor from the heptad repeat 1 region

Paramyxoviruses are a leading cause of childhood illness worldwide. A recently discovered paramyxovirus, human metapneumovirus (hMPV), has been studied by our group in order to determine the structural relevance of its fusion (F) protein to other well-characterized viruses utilizing type I integral membrane proteins as fusion aids. Sequence analysis and homology models suggested the presence of requisite heptad repeat (HR) regions. Synthetic peptides from HR regions 1 and 2 (HR-1 and -2, respectively) were induced to form a thermostable (melting temperature, approximately 90 degrees C) helical structure consistent in mass with a hexameric coiled coil. Inhibitory studies of hMPV HR-1 and -2 indicated that the synthetic HR-1 peptide was a significant fusion inhibitor with a 50% inhibitory concentration and a 50% effective concentration of approximately 50 nM. Many viral fusion proteins are type I integral membrane proteins utilizing the formation of a hexameric coiled coil of HR peptides as a major driving force for fusion. Our studies provide evidence that hMPV also uses a coiled-coil structure as a major player in the fusion process. Additionally, viral HR-1 peptide sequences may need further investigation as potent fusion inhibitors.

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

OBJECTIVES

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Nonpeptide Inhibitors of Measles Virus Entry

Measles virus (MV) is one of the most infectious pathogens known. Despite the existence of a vaccine, over 500,000 deaths/year result from MV or associated complications. Anti-measles compounds could conceivably reverse these statistics. Previously, we described a homology model of the MV fusion protein trimer and a putative binding site near the head-neck region. The resulting model permitted the identification of two nonpeptidic entry inhibitors. Here, we present the design, synthesis, and bioevaluation of several series of fusion inhibitors and describe their structure-activity relationships (SAR). Five simply substituted anilides show low-microM blockade of the MV, one of which (AS-48) exhibits IC50 = 0.6-3.0 microM across a panel of wild-type MV strains found in the field. Molecular field topology analysis (MFTA), a 2D QSAR approach based on local molecular properties (atomic charges, hydrogen-bonding capacity and local lipophilicity), applied to the anilide series suggests structural modifications to improve potency.

Crystal structure of the low-pH form of the vesicular stomatitis virus glycoprotein G

The vesicular stomatitis virus has an atypical membrane fusion glycoprotein (G) exhibiting a pH-dependent equilibrium between two forms at the virus surface. Membrane fusion is triggered during the transition from the high- to low-pH form. The structure of G in its low-pH form shows the classic hairpin conformation observed in all other fusion proteins in their postfusion conformation, in spite of a novel fold combining features of fusion proteins from classes I and II. The structure provides a framework for understanding the reversibility of the G conformational change. Unexpectedly, G is homologous to gB of herpesviruses, which raises important questions on viral evolution.

Architecture of the SARS coronavirus prefusion spike

The emergence in 2003 of a new coronavirus (CoV) responsible for the atypical pneumonia termed severe acute respiratory syndrome (SARS) was a stark reminder that hitherto unknown viruses have the potential to cross species barriers to become new human pathogens. Here we describe the SARS-CoV 'spike' structure determined by single-particle cryo-EM, along with the docked atomic structures of the receptor-binding domain and prefusion core.

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

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

The structural basis of paramyxovirus invasion

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

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.

[Viral fusion mechanisms]

The majority of viral fusion proteins can be divided into two classes. The influenza hemagglutinin (HA) belongs to the class I fusion proteins and undergoes a series of conformational changes at acidic pH, leading to membrane fusion. The crystal structures of the prefusion and the postfusion forms of HA have been revealed in 1981 and 1994, respectively. On the basis of these structures, a model for the mechanism of membrane fusion mediated by the conformational changes of HA has been proposed. The flavivirus E and alphavirus E1 proteins belong to the class II fusion proteins and mediate membrane fusion at acidic pH. Their prefusion structures are distinct from that of HA. Last year, however, it has become evident that the postfusion structures of these class I and class II fusion proteins are similar. The paramyxovirus F protein belongs to the class I fusion proteins. In contrast to HA, an interaction between F and its homologous attachment protein is required for F to undergo the conformational changes. Since F mediates fusion at neutral pH, the infected cells can fuse with neighboring uninfected cells. The crystal structures of F and the attachment protein HN have recently been clarified, which will facilitate studies of the molecular mechanism of F-mediated membrane fusion.

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

Paramyxoviruses enter cells by fusion of their lipid envelope with the target cell plasma membrane. Fusion of the viral membrane with the plasma membrane allows entry of the viral genome into the cytoplasm. For paramyxoviruses, membrane fusion occurs at neutral pH, but the trigger mechanism that controls the viral entry machinery such that it occurs at the right time and in the right place remains to be elucidated. Two viral glycoproteins are key to the infection process—an attachment protein that varies among different paramyxoviruses and the fusion (F) protein, which is found in all paramyxoviruses. For many of the paramyxoviruses (parainfluenza viruses 1–5, mumps virus, Newcastle disease virus and others), the attachment protein is the hemagglutinin/neuraminidase (HN) protein. In the last 5 years, atomic structures of paramyxovirus F and HN proteins have been reported. The knowledge gained from these structures towards understanding the mechanism of viral membrane fusion is described.

Vaccination of chicken embryos with escape mutants of La Sota Newcastle disease virus induces a protective immune response

To reduce the embryonic pathogenicity of Newcastle disease virus (NDV), escape mutants of the La Sota strain were produced with selected monoclonal antibodies. Immunoselection resulted in the elimination of an epitope by single amino acid substitution (F and HN molecule) or in a conformational change (HN molecule). The embryonic pathogenicity of these escape mutants was reduced and their dose was optimised for in ovo vaccination. Because antibody responses and protection of in ovo vaccinated chicks were similar to controls vaccinated at hatch with the La Sota strain, immunoselection appears a valuable technique to produce attenuated NDV strains, which are candidate in ovo vaccines.

Two domains that control prefusion stability and transport competence of the measles virus fusion protein

Most viral glycoproteins mediating membrane fusion adopt a metastable native conformation and undergo major conformational changes during fusion. We previously described a panel of compounds that specifically prevent fusion induced by measles virus (MV), most likely by interfering with conformational rearrangements of the MV fusion (F) protein. To further elucidate the basis of inhibition and better understand the mechanism of MV glycoprotein-mediated fusion, we generated and characterized resistant MV variants. Spontaneous mutations conferring drug resistance were confirmed in transient assays and in the context of recombinant virions and were in all cases located in the fusion protein. Several mutations emerged independently at F position 462, which is located in the C-terminal heptad repeat (HR-B) domain. In peptide competition assays, all HR-B mutants at residue 462 revealed reduced affinity for binding to the HR-A core complex compared to unmodified HR-B. Combining mutations at residue 462 with mutations in the distal F head region, which we had previously identified as mediating drug resistance, causes intracellular retention of the mutant proteins. The transport competence and activity of the mutants can be restored, however, by incubation at reduced temperature or in the presence of the inhibitory compounds, indicating that the F escape mutants have a reduced conformational stability and that the inhibitors stabilize a transport-competent conformation of the F trimer. The data support the conclusion that residues located in the head domain of the F trimer and the HR-B region contribute jointly to controlling F conformational stability.

Changes in mumps virus neurovirulence phenotype associated with quasispecies heterogeneity

Mumps virus is a highly neurotropic virus with evidence of central nervous system invasion (CNS) in approximately half of all cases of infection. In countries where live attenuated mumps virus vaccines were introduced, the number of mumps cases declined dramatically; however, recently, the safety of some vaccine strains has been questioned. For example, one of the most widely used vaccines, the Urabe AM9 strain, was causally associated with meningitis, leading to the withdrawal of this product from the market in several countries. This highlights the need for a better understanding of the attenuation process and the identification of markers of attenuation. To this end, we further attenuated the Urabe AM9 strain by serial passage in cell culture and compared the complete nucleotide sequences of the parental and passaged viruses. Interestingly, despite a dramatic decrease in virus virulence (as assayed in rats), the only genomic changes were in the form of changes in the level of genetic heterogeneity at specific genome sites, i.e., either selection of one nucleotide variant at positions where the starting material exhibited nucleotide heterogeneity or the evolution of an additional nucleotide to create a heterogenic site. This finding suggests that changes in the level of genetic heterogeneity at specific genome sites can have profound neurovirulence phenotypic consequences and, therefore, caution should be exercised when evaluating genetic markers of virulence or attenuation based only on a consensus sequence.

Structure of the parainfluenza virus 5 F protein in its metastable, prefusion conformation

Enveloped viruses have evolved complex glycoprotein machinery that drives the fusion of viral and cellular membranes, permitting entry of the viral genome into the cell. For the paramyxoviruses, the fusion (F) protein catalyses this membrane merger and entry step, and it has been postulated that the F protein undergoes complex refolding during this process. Here we report the crystal structure of the parainfluenza virus 5 F protein in its prefusion conformation, stabilized by the addition of a carboxy-terminal trimerization domain. The structure of the F protein shows that there are profound conformational differences between the pre- and postfusion states, involving transformations in secondary and tertiary structure. The positions and structural transitions of key parts of the fusion machinery, including the hydrophobic fusion peptide and two helical heptad repeat regions, clarify the mechanism of membrane fusion mediated by the F protein.

A mutant fusion (F) protein of simian virus 5 induces hemagglutinin-neuraminidase-independent syncytium formation despite the internalization of the F protein

The fusion (F) protein of simian virus 5 strain W3A induces syncytium formation independently of coexpression of the hemagglutinin-neuraminidase protein. This property can be transferred to the F protein of strain WR by replacing the leucine at position 22 with the W3A F counterpart, proline. The resulting mutant L22P has a conformation that is distinct from that of the WR F protein. Se-L22P is a cleavage site mutant of L22P that is cleavable only by addition of exogenous trypsin. We showed here that the cell surface-localized L22P was internalized with a t1/2 of 25 min and degraded in the cell, while the WR F protein was not. The cell surface-localized Se-L22P underwent a significant conformational change upon cleavage. Intriguingly, it disappeared from the cell surface due to its internalization, while inducing extensive syncytium formation. These results indicate that L22P may display an internalization signal during the course of fusion induction.

Sequence motifs and prokaryotic expression of the reptilian paramyxovirus fusion protein

Fourteen reptilian paramyxovirus isolates were chosen to represent the known extent of genetic diversity among this novel group of viruses. Selected regions of the fusion (F) gene were sequenced, analyzed and compared. The F gene of all isolates contained conserved motifs homologous to those described for other members of the family Paramyxoviridae including: signal peptide, transmembrane domain, furin cleavage site, fusion peptide, N-linked glycosylation sites, and two heptad repeats, the second of which (HRB-LZ) had the characteristics of a leucine zipper. Selected regions of the fusion gene of isolate Gono-GER85 were inserted into a prokaryotic expression system to generate three recombinant protein fragments of various sizes. The longest recombinant protein was cleaved by furin into two fragments of predicted length. Western blot analysis with virus-neutralizing rabbit-antiserum against this isolate demonstrated that only the longest construct reacted with the antiserum. This construct was unique in containing 30 additional C-terminal amino acids that included most of the HRB-LZ. These results indicate that the F genes of reptilian paramyxoviruses contain highly conserved motifs typical of other members of the family and suggest that the HRB-LZ domain of the reptilian paramyxovirus F protein contains a linear antigenic epitope.

Alterations of lymphocyte membranes during HIV-1 infection via multiple and simultaneous entry strategies

Human immunodeficiency virus type 1 (HIV-1) must bind to and enter lymphocytes to replicate and cause the acquired immunodeficiency syndrome. The association of viral particles with the lymphocyte plasma membrane may vary according to a multitude of unknown variables, including lymphocyte membrane receptor mobilization, lipid raft aggregation, clathrin, caveolin, endosomes, microendosome-mediated penetration or penetration through a hole in the membrane. The time course of this delivery appears to be short. Fusion of the virion membrane and lymphocyte plasma membrane leads to destabilization of the lymphocyte membrane. Five morphological stages of membrane alteration were observed in the infected lymphocytes: (1) swelling, (2) splitting, (3) fusion, (4) breaking, and (5) thinning of the lipid bilayer. These plasma membrane alterations were not contributed by fixation artifacts, because the dimensions and distance between the subunits of the surface glycoprotein (SU, gp120) and the transmembrane glycoprotein (gp41) of the viral particles adjacent to the infected cells and processed at the same time remained unchanged. Destabilization of lipid raft patches in the lymphocyte plasma membrane by unknown variables may facilitate HIV-1 penetration of lymphocyte, and other cell types. This a combined review of the pertinent literature with our data showing that HIV-1 may take advantage of multiple penetration approaches simultaneously in the same cell type (H9) to overwhelm the infected cells. The ultrastructural details of H9 cultured cells infected in vitro with HIV-1 contribute to our understanding of viral particle association with the plasma membrane of infected cells.

Crystallization and preliminary X-ray diffraction analysis of central structure domains from mumps virus F protein

Fusion of members of the Paramyxoviridae family involves two glycoproteins: the attachment protein and the fusion protein. Changes in the fusion-protein conformation were caused by binding of the attachment protein to the cellular receptor. In the membrane-fusion process, two highly conserved heptad-repeat (HR) regions, HR1 and HR2, are believed to form a stable six-helix coiled-coil bundle. However, no crystal structure has yet been determined for this state in the mumps virus (MuV, a member of the Paramyxoviridae family). In this study, a single-chain protein consisting of two HR regions connected by a flexible amino-acid linker (named 2-Helix) was expressed, purified and crystallized by the hanging-drop vapour-diffusion method. A complete X-ray data set was obtained in-house to 2.2 A resolution from a single crystal. The crystal belongs to space group C2, with unit-cell parameters a = 161.2, b = 60.8, c = 40.1 A, beta = 98.4 degrees. The crystal structure will help in understanding the molecular mechanism of Paramyxoviridae family membrane fusion.

Small molecules VP-14637 and JNJ-2408068 inhibit respiratory syncytial virus fusion by similar mechanisms

Here we present data on the mechanism of action of VP-14637 and JNJ-2408068 (formerly R-170591), two small-molecule inhibitors of respiratory syncytial virus (RSV). Both inhibitors exhibited potent antiviral activity with 50% effective concentrations (EC50s) of 1.4 and 2.1 nM, respectively. A similar inhibitory effect was observed in a RSV-mediated cell fusion assay (EC50=5.4 and 0.9 nM, respectively). Several drug-resistant RSV variants were selected in vitro in the presence of each compound. All selected viruses exhibited significant cross-resistance to both inhibitors and contained various single amino acid substitutions in two distinct regions of the viral F protein, the heptad repeat 2 (HR2; mutations D486N, E487D, and F488Y), and the intervening domain between HR1 and HR2 (mutation K399I and T400A). Studies using [3H]VP-14637 revealed a specific binding of the compound to RSV-infected cells that was efficiently inhibited by JNJ-2408068 (50% inhibitory concentration=2.9 nM) but not by the HR2-derived peptide T-118. Further analysis using a transient T7 vaccinia expression system indicated that RSV F protein is sufficient for this interaction. F proteins containing either the VP-14637 or JNJ-2408068 resistance mutations exhibited greatly reduced binding of [3H]VP-14637. Molecular modeling analysis suggests that both molecules may bind into a small hydrophobic cavity in the inner core of F protein, interacting simultaneously with both the HR1 and HR2 domains. Altogether, these data indicate that VP-14637 and JNJ-2408068 interfere with RSV fusion through a mechanism involving a similar interaction with the F protein.

Inhibition of Henipavirus fusion and infection by heptad-derived peptides of the Nipah virus fusion glycoprotein

Background

The recent emergence of four new members of the paramyxovirus family has heightened the awareness of and re-energized research on new and emerging diseases. In particular, the high mortality and person to person transmission associated with the most recent Nipah virus outbreaks, as well as the very recent re-emergence of Hendra virus, has confirmed the importance of developing effective therapeutic interventions. We have previously shown that peptides corresponding to the C-terminal heptad repeat (HR-2) of the fusion envelope glycoprotein of Hendra virus and Nipah virus were potent inhibitors of both Hendra virus and Nipah virus-mediated membrane fusion using recombinant expression systems. In the current study, we have developed shorter, second generation HR-2 peptides which include a capped peptide via amidation and acetylation and two poly(ethylene glycol)-linked (PEGylated) peptides, one with the PEG moity at the C-terminus and the other at the N-terminus. Here, we have evaluated these peptides as well as the corresponding scrambled peptide controls in Nipah virus and Hendra virus-mediated membrane fusion and against infection by live virus in vitro.

Results

Unlike their predecessors, the second generation HR-2 peptides exhibited high solubility and improved synthesis yields. Importantly, both Nipah virus and Hendra virus-mediated fusion as well as live virus infection were potently inhibited by both capped and PEGylated peptides with IC₅₀ concentrations similar to the original HR-2 peptides, whereas the scrambled modified peptides had no inhibitory effect. These data also indicate that these chemical modifications did not alter the functional properties of the peptides as inhibitors.

Conclusion

Nipah virus and Hendra virus infection in vitro can be potently blocked by specific HR-2 peptides. The improved synthesis and solubility characteristics of the second generation HR-2 peptides will facilitate peptide synthesis for pre-clinical trial application in an animal model of Henipavirus infection. The applied chemical modifications are also predicted to increase the serum half-life in vivo and should increase the chance of success in the development of an effective antiviral therapy.

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

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

Retrovirus envelope protein complex structure in situ studied by cryo-electron tomography

We used cryo-electron tomography in conjunction with single-particle averaging techniques to study the structures of frozen-hydrated envelope glycoprotein (Env) complexes on intact Moloney murine leukemia retrovirus particles. Cryo-electron tomography allows 3D imaging of viruses in toto at a resolution sufficient to locate individual macromolecules, and local averaging of abundant complexes substantially improves the resolution. The averaging of repetitive features in electron tomograms is hampered by a low signal-to-noise ratio and anisotropic resolution, which results from the "missing-wedge" effect. We developed an iterative 3D averaging algorithm that compensates for this effect and used it to determine the trimeric structure of Env to a resolution of 2.7 nm, at which individual domains can be resolved. Strikingly, the 3D reconstruction is shaped like a tripod in which the trimer penetrates the membrane at three distinct locations approximately 4.5 nm apart from one another. The Env reconstruction allows tentative docking of the x-ray crystal structure of the receptor-binding domain. This study thus provides 3D structural information regarding the prefusion conformation of an intact unstained retrovirus surface protein.

Role of the simian virus 5 fusion protein N-terminal coiled-coil domain in folding and promotion of membrane fusion

Formation of a six-helix bundle comprised of three C-terminal heptad repeat regions in antiparallel orientation in the grooves of an N-terminal coiled-coil is critical for promotion of membrane fusion by paramyxovirus fusion (F) proteins. We have examined the effect of mutations in four residues of the N-terminal heptad repeat in the simian virus 5 (SV5) F protein on protein folding, transport, and fusogenic activity. The residues chosen have previously been shown from study of isolated peptides to have differing effects on stability of the N-terminal coiled-coil and six-helix bundle (R. E. Dutch, G. P. Leser, and R. A. Lamb, Virology 254:147-159, 1999). The mutant V154M showed reduced proteolytic cleavage and surface expression, indicating a defect in intracellular transport, though this mutation had no effect when studied in isolated peptides. The mutation I137M, previously shown to lower thermostability of the six-helix bundle, resulted in an F protein which was properly processed and transported to the cell surface but which had reduced fusogenic activity. Finally, mutations at L140M and L161M, previously shown to disrupt alpha-helix formation of isolated N-1 peptides but not to affect six-helix bundle formation, resulted in F proteins that were properly processed. Interestingly, the L161M mutant showed increased syncytium formation and promoted fusion at lower temperatures than the wild-type F protein. These results indicate that interactions separate from formation of an N-terminal coiled-coil or six-helix bundle are important in the initial folding and transport of the SV5 F protein and that mutations that destabilize the N-terminal coiled-coil can result in stimulation of membrane fusion.

Selective recognition of oligomeric HIV-1 primary isolate envelope glycoproteins by potently neutralizing ligands requires efficient precursor cleavage

A critical component of an effective HIV vaccine will be the induction of broadly neutralizing antibodies. Comprising the HIV spike, the exterior envelope glycoprotein gp120 and the transmembrane glycoprotein gp41 mediate receptor binding, viral entry, and are the targets for neutralizing antibodies. The gp120 and gp41 glycoproteins are derived from the gp160 precursor glycoprotein and following gp160 glycosylation, oligomerization and cleavage in the endoplasmic reticulum and Golgi, remain as non-covalently associated trimers of heterodimers. Previously, using cell-surface envelope glycoproteins derived from infection of a laboratory-adapted HIV-1 strain, a correlation had been established between the binding of gp120-directed antibodies to the viral glycoprotein and the ability of the antibodies to neutralize laboratory-adapted isolates. However, this has been more difficult to demonstrate for glycoproteins derived from primary patient isolates. Here, using a FACS-based method, we report that only gp120-directed neutralizing antibodies and the neutralizing ligand soluble CD4 efficiently bind to glycoproteins derived from the JR-FL primary isolate provided that the gp160 precursor protein is efficiently cleaved. Precursor cleavage was demonstrated by cell-surface biotinylation and Western blotting. In stark contrast, both non-neutralizing and neutralizing antibodies bind non-cleaved envelope glycoproteins from JR-FL and YU2 isolates. These data imply that significant changes in Env spike structure are dependent upon precursor gp160 cleavage and are consistent with a restricted-binding-to-Env model of neutralization. The data also have implications in regards to the use and design of non-cleaved envelope glycoprotein trimeric immunogens as a means to selectively and preferentially present neutralizing epitopes to the host immune system.

Decreased dependence on receptor recognition for the fusion promotion activity of L289A-mutated newcastle disease virus fusion protein correlates with a monoclonal antibody-detected conformational change

It has been shown that the L289A-mutated Newcastle disease virus (NDV) fusion (F) protein gains the ability to promote fusion of Cos-7 cells independent of the viral hemagglutinin-neuraminidase (HN) protein and exhibits a 50% enhancement in HN-dependent fusion over wild-type (wt) F protein. Here, we show that HN-independent fusion by L289A-F is not exhibited in BHK cells or in several other cell lines. However, similar to the results in Cos-7 cells, the mutated protein plus HN does promote 50 to 70% more fusion above wt levels in all of the cell lines tested. L289A-F protein exhibits the same specificity as the wt F protein for the homologous HN protein, as well as NDV-human parainfluenza virus 3 HN chimeras. The mutated F protein promotes fusion more effectively than the wt when it is coexpressed with either the chimeras or HN proteins deficient in receptor recognition activity. In addition, its fusogenic activity is significantly more resistant to removal of sialic acid on target cells. These findings are consistent with the demonstration that L289A-F interacts more efficiently with wt and mutated HN proteins than does wt F by a cell surface coimmunoprecipitation assay. Taken together, these findings indicate that L289A-F promotes fusion by a mechanism analogous to that of the wt protein with respect to the HN-F interaction but is less dependent on the attachment activity of HN. The phenotype of the mutated F protein correlates with a conformational change in the protein detectable by two different monoclonal antibodies. This conformational change may reflect a destabilization of F structure induced by the L289A substitution, which may in turn indicate a lower energy requirement for fusion activation.

The many mechanisms of viral membrane fusion proteins

Every enveloped virus fuses its membrane with a host cell membrane, thereby releasing its genome into the cytoplasm and initiating the viral replication cycle. In each case, one or a small set of viral surface transmembrane glycoproteins mediates fusion. Viral fusion proteins vary in their mode of activation and in structural class. These features combine to yield many different fusion mechanisms. Despite their differences, common principles for how fusion proteins function are emerging: In response to an activating trigger, the metastable fusion protein converts to an extended, in some cases rodlike structure, which inserts into the target membrane via its fusion peptide. A subsequent conformational change causes the fusion protein to fold back upon itself, thereby bringing its fusion peptide and its transmembrane domain-and their attached target and viral membranes-into intimate contact. Fusion ensues as the initial lipid stalk progresses through local hemifusion, and then opening and enlargement of a fusion pore. Here we review recent advances in our understanding of how fusion proteins are activated, how fusion proteins change conformation during fusion, and what is happening to the lipids during fusion. We also briefly discuss the therapeutic potential of fusion inhibitors in treating viral infections.

Conserved glycine residues in the fusion peptide of the paramyxovirus fusion protein regulate activation of the native state

Hydrophobic fusion peptides (FPs) are the most highly conserved regions of class I viral fusion-mediating glycoproteins (vFGPs). FPs often contain conserved glycine residues thought to be critical for forming structures that destabilize target membranes. Unexpectedly, a mutation of glycine residues in the FP of the fusion (F) protein from the paramyxovirus simian parainfluenza virus 5 (SV5) resulted in mutant F proteins with hyperactive fusion phenotypes (C. M. Horvath and R. A. Lamb, J. Virol. 66:2443-2455, 1992). Here, we constructed G3A and G7A mutations into the F proteins of SV5 (W3A and WR isolates), Newcastle disease virus (NDV), and human parainfluenza virus type 3 (HPIV3). All of the mutant F proteins, except NDV G7A, caused increased cell-cell fusion despite having slight to moderate reductions in cell surface expression compared to those of wild-type F proteins. The G3A and G7A mutations cause SV5 WR F, but not NDV F or HPIV3 F, to be triggered to cause fusion in the absence of coexpression of its homotypic receptor-binding protein hemagglutinin-neuraminidase (HN), suggesting that NDV and HPIV3 F have stricter requirements for homotypic HN for fusion activation. Dye transfer assays show that the G3A and G7A mutations decrease the energy required to activate F at a step in the fusion cascade preceding prehairpin intermediate formation and hemifusion. Conserved glycine residues in the FP of paramyxovirus F appear to have a primary role in regulating the activation of the metastable native form of F. Glycine residues in the FPs of other class I vFGPs may also regulate fusion activation.

Targeting a binding pocket within the trimer-of-hairpins: small-molecule inhibition of viral fusion

Trimeric class I virus fusion proteins undergo a series of conformational rearrangements that leads to the association of C- and N-terminal heptad repeat domains in a "trimer-of-hairpins" structure, facilitating the apposition of viral and cellular membranes during fusion. This final fusion hairpin structure is sustained by protein-protein interactions, associations thought initially to be refractory to small-molecule inhibition because of the large surface area involved. By using a photoaffinity analog of a potent respiratory syncytial virus fusion inhibitor, we directly probed the interaction of the inhibitor with its fusion protein target. Studies have shown that these inhibitors bind within a hydrophobic cavity formed on the surface of the N-terminal heptad-repeat trimer. In the fusogenic state, this pocket is occupied by key amino acid residues from the C-terminal heptad repeat that stabilize the trimer-of-hairpins structure. The results indicate that a low-molecular-weight fusion inhibitor can interfere with the formation or consolidation of key structures within the hairpin moiety that are essential for membrane fusion. Because analogous cavities are present in many class I viruses, including HIV, these results demonstrate the feasibility of this approach as a strategy for drug discovery.

Ubiquitous activation of the Nipah virus fusion protein does not require a basic amino acid at the cleavage site

Nipah virus (NiV), a highly pathogenic paramyxovirus, causes a systemic infection in vivo and is able to replicate in cultured cells of many species and organs. Such pantropic paramyxoviruses generally encode fusion (F) proteins with multibasic cleavage sites activated by furin or other ubiquitous intracellular host cell proteases. In contrast, NiV has an F protein with a single arginine (R109) at the cleavage site, as is the case with paramyxoviruses that are activated by trypsin-like proteases only present in specific cells or tissues and therefore only cause localized infections. Unlike these viruses, cleavage of the NiV F protein is ubiquitous and does not require the addition of exogenous proteases in cell culture. To determine the importance of the amino acid sequence at the NiV F protein cleavage site for ubiquitous activation, we generated NiV F proteins with mutations around R109. Surprisingly, neither the exchange of amino acids upstream of R109 nor replacement of the basic residue itself interfered with F cleavage. Thus, R109 is not essential for F cleavage and activation. Our data demonstrate that NiV F-protein activation depends on a novel type of proteolytic cleavage that has not yet been described for any other paramyxovirus F protein. NiV F activation is mediated by a ubiquitous protease that requires neither a monobasic nor a multibasic cleavage site and therefore differs from the furin- or trypsin-like proteases known to activate other ortho- and paramyxovirus fusion proteins.

Mutations in multiple domains activate paramyxovirus F protein-induced fusion

SER virus, a paramyxovirus that is closely related to simian virus 5 (SV5), is unusual in that it fails to induce syncytium formation. The SER virus F protein has an unusually long cytoplasmic tail (CT), and it was previously observed that truncations or specific mutations of this domain result in enhanced syncytium formation. In addition to the long CT, the SER F protein has nine amino acid differences from the F protein of SV5. We previously observed only a partial suppression of fusion in a chimeric SV5 F protein with a CT derived from SER virus, indicating that these other amino acid differences between the SER and SV5 F proteins also play a role in regulating the fusion phenotype. To examine the effects of individual amino acid differences, we mutated the nine SER residues individually to the respective residues of the SV5 F protein. We found that most of the mutants were expressed well and were transported to the cell surface at levels comparable to that of the wild-type SER F protein. Many of the mutants showed enhanced lipid mixing, calcein transfer, and syncytium formation even in the presence of the long SER F protein CT. Some mutants, such as the I310 M, T438S, M489I, T516V, and N529K mutants, also showed fusion at lower temperatures of 32, 25, and 18 degrees C. The residue Asn529 plays a critical role in the suppression of fusion activity, as the mutation of this residue to lysine caused a marked enhancement of fusion. The effect of the N529K mutation on the enhancement of fusion by a previously described mutant, L539,548A, as well as by chimeric SV5/SER F proteins was also dramatic. These results indicate that activation to a fusogenic conformation is dependent on the interplay of residues in the ectodomain, the transmembrane domain, and the CT domain of paramyxovirus F proteins.

A synthetic peptide from a heptad repeat region of herpesvirus glycoprotein B inhibits virus replication

Glycoprotein B (gB) is the most conserved glycoprotein of herpesviruses and plays important roles in virus infectivity. Two intervening heptad repeat (HR) sequences were found in the C-terminal half of all herpesvirus gBs analysed. A synthetic peptide derived from the HR region (aa 477-510) of bovine herpesvirus type 1 (BoHV-1) gB was studied for its ability to inhibit virus replication. The peptide interfered with cell-to-cell spread and consistently inhibited replication of BoHV-1, with a 50 % effective concentration value (EC(50)) of 5 microM. Inhibition of replication was obtained not only with herpesviruses including pseudorabies virus and herpes simplex virus type 1 but also partly with Newcastle disease virus. Possible mechanisms of membrane fusion inhibition by the peptide are discussed.

Canine distemper virus and measles virus fusion glycoprotein trimers: partial membrane-proximal ectodomain cleavage enhances function

The trimeric fusion (F) glycoproteins of morbilliviruses are activated by furin cleavage of the precursor F(0) into the F(1) and F(2) subunits. Here we show that an additional membrane-proximal cleavage occurs and modulates F protein function. We initially observed that the ectodomain of approximately one in three measles virus (MV) F proteins is cleaved proximal to the membrane. Processing occurs after cleavage activation of the precursor F(0) into the F(1) and F(2) subunits, producing F(1a) and F(1b) fragments that are incorporated in viral particles. We also detected the F(1b) fragment, including the transmembrane domain and cytoplasmic tail, in cells expressing the canine distemper virus (CDV) or mumps virus F protein. Six membrane-proximal amino acids are necessary for efficient CDV F(1a/b) cleavage. These six amino acids can be exchanged with the corresponding MV F protein residues of different sequence without compromising function. Thus, structural elements of different sequence are functionally exchangeable. Finally, we showed that the alteration of a block of membrane-proximal amino acids results in diminished fusion activity in the context of a recombinant CDV. We envisage that selective loss of the membrane anchor in the external subunits of circularly arranged F protein trimers may disengage them from pulling the membrane centrifugally, thereby facilitating fusion pore formation.

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.

Structural characterization of the fusion-active complex of severe acute respiratory syndrome (SARS) coronavirus

The causative agent of a recent outbreak of an atypical pneumonia, known as severe acute respiratory syndrome (SARS), has been identified as a coronavirus (CoV) not belonging to any of the previously identified groups. Fusion of coronaviruses with the host cell is mediated by the envelope spike protein. Two regions within the spike protein of SARS-CoV have been identified, showing a high degree of sequence conservation with the other CoV, which are characterized by the presence of heptad repeats (HR1 and HR2). By using synthetic and recombinant peptides corresponding to the HR1 and HR2 regions, we were able to characterize the fusion-active complex formed by this novel CoV by CD, native PAGE, proteolysis protection analysis, and size-exclusion chromatography. HR1 and HR2 of SARS-CoV associate into an antiparallel six-helix bundle, with structural features typical of the other known class I fusion proteins. We have also mapped the specific boundaries of the region, within the longer HR1 domain, making contact with the shorter HR2 domain. Notably, the inner HR1 coiled coil is a stable alpha-helical domain even in the absence of interaction with the HR2 region. Inhibitors binding to HR regions of fusion proteins have been shown to be efficacious against many viruses, notably HIV. Our results may help in the design of anti-SARS therapeutics.

A target site for template-based design of measles virus entry inhibitors

Measles virus (MV) constitutes a principal cause of worldwide mortality, accounting for almost 1 million deaths annually. Although a live-attenuated vaccine protects against MV, vaccination efficiency of young infants is low because of interference by maternal antibodies. Parental concerns about vaccination safety further contribute to waning herd immunity in developed countries, resulting in recent MV outbreaks. The development of novel antivirals that close the vaccination gap in infants and silence viral outbreaks is thus highly desirable. We previously identified a microdomain in the MV fusion protein (F protein) that is structurally conserved in the paramyxovirus family and constitutes a promising target site for rationally designed antivirals. Here we report the template-based development of a small-molecule MV inhibitor, providing proof-of-concept for our approach. This lead compound specifically inhibits fusion and spread of live MV and MV glycoprotein-induced membrane fusion. The inhibitor induces negligible cytotoxicity and does not interfere with receptor binding or F protein biosynthesis or transport but prevents F protein-induced lipid mixing. Mutations in the postulated target site alter viral sensitivity to inhibition. In silico docking of the compound in this microdomain suggests a binding model that is experimentally corroborated by a structure-activity analysis of the compound and the inhibition profile of mutated F proteins. A second-generation compound designed on the basis of the interaction model shows a 200-fold increase in antiviral activity, creating the basis for novel MV therapeutics. This template-based design approach for MV may be applicable to other clinically relevant members of the paramyxovirus family.

Purification and crystallization reveal two types of interactions of the fusion protein homotrimer of Semliki Forest virus

The fusion proteins of the alphaviruses and flaviviruses have a similar native structure and convert to a highly stable homotrimer conformation during the fusion of the viral and target membranes. The properties of the alpha- and flavivirus fusion proteins distinguish them from the class I viral fusion proteins, such as influenza virus hemagglutinin, and establish them as the first members of the class II fusion proteins. Understanding how this new class carries out membrane fusion will require analysis of the structural basis for both the interaction of the protein subunits within the homotrimer and their interaction with the viral and target membranes. To this end we report a purification method for the E1 ectodomain homotrimer from the alphavirus Semliki Forest virus. The purified protein is trimeric, detergent soluble, retains the characteristic stability of the starting homotrimer, and is free of lipid and other contaminants. In contrast to the postfusion structures that have been determined for the class I proteins, the E1 homotrimer contains the fusion peptide region responsible for interaction with target membranes. This E1 trimer preparation is an excellent candidate for structural studies of the class II viral fusion proteins, and we report conditions that generate three-dimensional crystals suitable for analysis by X-ray diffraction. Determination of the structure will provide our first high-resolution views of both the low-pH-induced trimeric conformation and the target membrane-interacting region of the alphavirus fusion protein.

Virus entry: molecular mechanisms and biomedical applications

Viruses have evolved to enter cells from all three domains of life--Bacteria, Archaea and Eukaryotes. Of more than 3,600 known viruses, hundreds can infect human cells and most of those are associated with disease. To gain access to the cell interior, animal viruses attach to host-cell receptors. Advances in our understanding of how viral entry proteins interact with their host-cell receptors and undergo conformational changes that lead to entry offer unprecedented opportunities for the development of novel therapeutics and vaccines.

Second sialic acid binding site in Newcastle disease virus hemagglutinin-neuraminidase: implications for fusion

Paramyxoviruses are the leading cause of respiratory disease in children. Several paramyxoviruses possess a surface glycoprotein, the hemagglutinin-neuraminidase (HN), that is involved in attachment to sialic acid receptors, promotion of fusion, and removal of sialic acid from infected cells and progeny virions. Previously we showed that Newcastle disease virus (NDV) HN contained a pliable sialic acid recognition site that could take two states, a binding state and a catalytic state. Here we present evidence for a second sialic acid binding site at the dimer interface of HN and present a model for its involvement in cell fusion. Three different crystal forms of NDV HN now reveal identical tetrameric arrangements of HN monomers, perhaps indicative of the tetramer association found on the viral surface.

Cross-strand disulphides in cell entry proteins: poised to act

Cross-strand disulphides (CSDs) are unusual bonds that link adjacent strands in the same beta-sheet. Their peculiarity relates to the high potential energy stored in these bonds, both as torsional energy in the highly strained disulphide linkage and as deformation energy stored in the sheet itself. CSDs are relatively rare in protein structures but are conspicuous by their presence in proteins that are involved in cell entry. The finding that entry of botulinum neurotoxin and HIV into mammalian cells involves cleavage of CSDs suggests that the activity of other cell entry proteins may likewise involve cleavage of these bonds. We examine emerging evidence of the involvement of these unusual disulphides in cell entry events.

Structural characterization of the SARS-coronavirus spike S fusion protein core

The spike (S) glycoprotein of coronaviruses mediates viral entry into host cells. It is a type 1 viral fusion protein that characteristically contains two heptad repeat regions, denoted HR-N and HR-C, that form coiled-coil structures within the ectodomain of the protein. Previous studies have shown that the two heptad repeat regions can undergo a conformational change from their native state to a 6-helix bundle (trimer of dimers), which mediates fusion of viral and host cell membranes. Here we describe the biophysical analysis of the two predicted heptad repeat regions within the severe acute respiratory syndrome coronavirus S protein. Our results show that in isolation the HR-N region forms a stable α-helical coiled coil that associates in a tetrameric state. The HR-C region in isolation formed a weakly stable trimeric coiled coil. When mixed together, the two peptide regions (HR-N and HR-C) associated to form a very stable α-helical 6-stranded structure (trimer of heterodimers). Systematic peptide mapping showed that the site of interaction between the HR-N and HR-C regions is between residues 916–950 of HR-N and residues 1151–1185 of HR-C. Additionally, interchain disulfide bridge experiments showed that the relative orientation of the HR-N and HR-C helices in the complex was antiparallel. Overall, the structure of the hetero-stranded complex is consistent with the structures observed for other type 1 viral fusion proteins in their fusion-competent state.

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.