The paramyxovirus fusion protein forms an extremely stable core trimer: structural parallels to influenza virus haemagglutinin and HIV-1 gp41

Cleavage of the syncytial protein of J paramyxovirus is required for its ability to promote cell-cell fusion

Cell-cell fusion mediated by most paramyxovirus requires fusion protein (F) and attachment protein (H, HN, or G). The F protein is proteolytic cleaved to be fusogenically active. J paramyxovirus (JPV) has a unique feature in the family Paramyxoviridae: It encodes an integral membrane protein, syncytial protein (SP, formerly known as transmembrane protein, TM), which is essential in JPV-promoted cell-cell fusion (i.e., syncytial). In this study, we report that cleavage of SP is essential for its syncytial-promoting activity. We have identified the cleavage site of SP at amino acid residues 172 to 175, LKTG, and deletion of the "LKTG" residues abolished SP protein cleavage and its ability to promote cell-cell fusion. Replacing the cleavage site LKTG with a factor Xa protease cleavage site allows cleavage of the SP with factor Xa protease and restores its ability to promote cell-cell fusion. Furthermore, results from a hemifusion assay indicate that cleavage of SP plays an important role in the progression from the intermediate hemifusion state to a complete fusion. This work indicates that SP has many characteristics of a fusion protein. We propose that SP is likely a cell-cell fusion-promoting protein.

A Survey of Henipavirus Tropism-Our Current Understanding from a Species/Organ and Cellular Level

Henipaviruses are single-stranded RNA viruses that have been shown to be virulent in several species, including humans, pigs, horses, and rodents. Isolated nearly 30 years ago, these viruses have been shown to be of particular concern to public health, as at least two members (Nipah and Hendra viruses) are highly virulent, as well as zoonotic, and are thus classified as BSL4 pathogens. Although only 5 members of this genus have been isolated and characterized, metagenomics analysis using animal fluids and tissues has demonstrated the existence of other novel henipaviruses, suggesting a far greater degree of phylogenetic diversity than is currently known. Using a variety of molecular biology techniques, it has been shown that these viruses exhibit varying degrees of tropism on a species, organ/tissue, and cellular level. This review will attempt to provide a general overview of our current understanding of henipaviruses, with a particular emphasis on viral tropism.

Targeting RSV with vaccines and small molecule drugs

Respiratory syncytial virus (RSV) is the most significant cause of pediatric respiratory infections. Palivizumab (Synagis®), a humanized monoclonal antibody, has been used successfully for a number of years to prevent severe RSV disease in at-risk infants. However, despite intense efforts, there is no approved vaccine or small molecule drug for RSV. As an enveloped virus, RSV must fuse its envelope with the host cell membrane, which is accomplished through the actions of the fusion (F) glycoprotein, with attachment help from the G glycoprotein. Because of their integral role in initiation of infection and their accessibility outside the lipid bilayer, these proteins have been popular targets in the discovery and development of antiviral compounds and vaccines against RSV. This review examines advances in the development of antiviral compounds and vaccine candidates.

Side chain packing below the fusion peptide strongly modulates triggering of the Hendra virus F protein

Triggering of the Hendra virus fusion (F) protein is required to initiate the conformational changes which drive membrane fusion, but the factors which control triggering remain poorly understood. Mutation of a histidine predicted to lie near the fusion peptide to alanine greatly reduced fusion despite wild-type cell surface expression levels, while asparagine substitution resulted in a moderate restoration in fusion levels. Slowed kinetics of six-helix bundle formation, as judged by sensitivity to heptad repeat B-derived peptides, was observed for all H372 mutants. These data suggest that side chain packing beneath the fusion peptide is an important regulator of Hendra virus F triggering.

Crystal structures of Nipah and Hendra virus fusion core proteins

The Nipah and Hendra viruses are highly pathogenic paramyxoviruses that recently emerged from flying foxes to cause serious disease outbreaks in humans and livestock in Australia, Malaysia, Singapore and Bangladesh. Their unique genetic constitution, high virulence and wide host range set them apart from other paramyxoviruses. These characteristics have led to their classification into the new genus Henpavirus within the family Paramyxoviridae and to their designation as Biosafety Level 4 pathogens. The fusion protein, an enveloped glycoprotein essential for viral entry, belongs to the family of class I fusion proteins and is characterized by the presence of two heptad repeat (HR) regions, HR1 and HR2. These two regions associate to form a fusion-active hairpin conformation that juxtaposes the viral and cellular membranes to facilitate membrane fusion and enable subsequent viral entry. The Hendra and Nipah virus fusion core proteins were crystallized and their structures determined to 2.2 A resolution. The Nipah and Hendra fusion core structures are six-helix bundles with three HR2 helices packed against the hydrophobic grooves on the surface of a central coiled coil formed by three parallel HR1 helices in an oblique antiparallel manner. Because of the high level of conservation in core regions, it is proposed that the Nipah and Hendra virus fusion cores can provide a model for membrane fusion in all paramyxoviruses. The relatively deep grooves on the surface of the central coiled coil represent a good target site for drug discovery strategies aimed at inhibiting viral entry by blocking hairpin formation.

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.

Design and characterization of viral polypeptide inhibitors targeting Newcastle disease virus fusion

Paramyxovirus infections can be detected worldwide with some emerging zoonotic viruses and currently there are no specific therapeutic treatments or vaccines available for many of these diseases. Recent studies have demonstrated that peptides derived from the two heptad repeat regions (HR1 and HR2) of paramyxovirus fusion proteins could be used as inhibitors of virus fusion. The mechanism underlying this activity is in accordance with that of class I virus fusion proteins, of which human immunodeficiency virus (HIV) and influenza virus fusion proteins are members. For class I virus fusion proteins, the HR1 fragment binds to HR2 to form a six-helix bundle with three HR1 fragments forming the central coiled bundle surrounded by three coiled HR2 fragments in the post fusion conformational state (fusion core). It is hypothesized that the introduced exogenous HR1 or HR2 can compete against their endogenous counterparts, which results in fusion inhibition. Using Newcastle disease virus (NDV) as a model, we designed several protein inhibitors, denoted HR212 as well asHR121 and 5-Helix, which could bind the HR1 or HR2 region of fusion protein, respectively. All the proteins were expressed and purified using a GST-fusion expression system in Escherichia coli. The HR212 or GST-HR212 protein, which binds the HR1 peptide in vitro, displayed inhibitory activity against NDV-mediated cell fusion, while the HR121 and 5-Helix proteins, which bind the HR2 peptide in vitro, inhibited virus fusion from the avirulent NDV strain when added before the cleavage of the fusion protein. These results showed that the designed HR212, HR121 or 5-Helix protein could serve as specific antiviral agents. These data provide additional insight into the difference between the virulent and avirulent strains of NDV.

Design and Characterization of Human Respiratory Syncytial Virus Entry Inhibitors

Human respiratory syncytial virus (hRSV) is a pathogen of worldwide health concern. The crucial membrane fusion event during viral entry into host cells involves a ‘trimer-of-hairpins’ structure that brings the amino (N)- and carboxy (C)-terminal regions of the viral fusion glycoprotein (F protein) into close proximity. Two heptad repeat regions that are highly conserved in the F protein - HR1 (N-terminal) and HR2 (C-terminal) - have an important role in this process. It has been shown that both HR1-and HR2-based peptides can inhibit viral entry. However, these proteins, and the HR1 peptides in particular, are liable to aggregation. We designed three peptides containing multiple copies of alternating HR1 and HR2 sequences denoted 5-Helix, HR121 and HR212, respectively. The 5-Helix, HR121 and HR212 proteins were functionally analogous to single HR1, HR1 and HR2 sequences, respectively. All three proteins were expressed in soluble form and biophysical analysis showed that they exhibited α-helical secondary structures. The three proteins were potent fusion inhibitors in vitro, at the micromolar scale, with the HR1 analogues being approximately two times more effective than the HR2 analogue. Our results suggest that these rationally designed protein inhibitors could serve as a new class of anti-hRSV agents.

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.

Construct design, biophysical, and biochemical characterization of the fusion core from mouse hepatitis virus (a coronavirus) spike protein

Membrane fusion between virus and host cells is the key step for enveloped virus entry and is mediated by the viral envelope fusion protein. In murine coronavirus, mouse hepatitis virus (MHV), the spike (S) protein mediates this process. Recently, the formation of anti-parallel 6-helix bundle of the MHV S protein heptad repeat (HR) regions (HR1 and HR2) has been confirmed, implying coronavirus has a class I fusion protein. This bundle is also called fusion core. To facilitate the solution of the crystal structure of this fusion core, we deployed an Escherichia coli in vitro expression system to express the HR1 and HR2 regions linked together by a flexible linker as a single chain (named 2-helix). This 2-helix polypeptide subsequently assembled into a typical 6-helix bundle. This bundle has been analyzed by a series of biophysical and biochemical techniques and confirmed that the design technique can be used for coronavirus as we successfully used for members of paramyxoviruses.

Suppression of SARS-CoV entry by peptides corresponding to heptad regions on spike glycoprotein

Heptad repeat regions (HR1 and HR2) are highly conserved sequences located in the glycoproteins of enveloped viruses. They form a six-helix bundle structure and are important in the process of virus fusion. Peptides derived from the HR regions of some viruses have been shown to inhibit the entry of these viruses. SARS-CoV was also predicted to have HR1 and HR2 regions in the S2 protein. Based on this prediction, we designed 25 peptides and screened them using a HIV-luc/SARS pseudotyped virus assay. Two peptides, HR1-1 and HR2-18, were identified as potential inhibitors, with EC₅₀ values of 0.14 and 1.19 μM, respectively. The inhibitory effects of these peptides were validated by the wild-type SARS-CoV assay. HR1-1 and HR2-18 can serve as functional probes for dissecting the fusion mechanism of SARS-CoV and also provide the potential of further identifying potent inhibitors for SARS-CoV entry.

Following the rule: formation of the 6-helix bundle of the fusion core from severe acute respiratory syndrome coronavirus spike protein and identification of potent peptide inhibitors

Severe acute respiratory syndrome (SARS) coronavirus (SARS-CoV) is a newly identified member of Family Coronaviridae. Coronavirus envelope spike protein S is a class I viral fusion protein which is characterized by the existence of two heptad repeat regions (HR1 and HR2) (forming a complex called fusion core). Here we report that by using in vitro bio-engineering techniques, SARS-CoV HR1 and HR2 bind to each other and form a typical 6-helix bundle. The HR2, either as a synthetic peptide or as a GST-fusion polypeptide, is a potent inhibitor of virus entry. The results do show that SARS-CoV follows the general fusion mechanism of class I viruses and this lays the ground for identification of virus fusion/entry inhibitors for this devastating emerging virus.

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.

Six-helix bundle assembly and analysis of the central core of mumps virus fusion protein

The fusion protein of enveloped viruses mediates the fusion between the viral and cellular membranes, allowing the penetration of the viral genomes into the host cell. Many of these proteins share a common fold comprising a central core trimer of anti-parallel coiled-coil heterodimers, which are formed by two discontinuous heptad repeat (HR) motifs located at the ectodomain of the fusion proteins. In this study, we constructed and purified the corresponding regions (HR1 and HR2) of mumps virus fusion protein that are predicted to form coiled coil. The HR1 and HR2 were expressed and purified separately or as a single chain connected by an amino acid linker (HR1-linker-HR2, named 2-Helix). Series of biochemical and biophysical analyses of the expressed proteins have shown that HR1 and HR2 of mumps virus fusion protein share the common features of other enveloped virus fusion proteins. CD spectral results show that HR1 forms an alpha-helical coil structure while HR2 exists as an unstructured monomer in PBS in nature. Mixtures of HR1 and HR2 could form a stable six-helix bundle, indicating the interaction of HR1 and HR2. The 2-Helix protein also shows characteristic properties of the 6-helix bundle. Therefore, mumps virus fusion protein has a common core architecture and its HR regions could be used as a drug target for virus fusion inhibitors.

Modulation of entry of enveloped viruses by cholesterol and sphingolipids (Review)

Enveloped animal viruses infect host cells by fusion of viral and target membranes. This crucial fusion event occurs either with the plasma membrane of the host cells at the physiological pH or with the endosomal membranes at low pH and is triggered by specific glycoproteins in the virus envelope. Both lipids and proteins play critical and co-operative roles in the fusion process. Interactions of viral proteins with their receptors direct which membranes fuse and viral fusion proteins then drive the process. These fusion proteins operate on lipid assemblies, whose physical and mechanical properties are equally important to the proper functioning of the process. Lipids contribute to the viral fusion process by virtue of their distinct chemical structure, composition and/or their preferred partitioning into specific microdomains in the plasma membrane called 'rafts'. An involvement of lipid rafts in viral entry and membrane fusion has been examined recently. However, the mechanism(s) by which lipids as dynamic raft components control viral envelope-glycoprotein-triggered fusion is not clear. This paper will review literature findings on the contribution of the two raft-associated lipids, cholesterol and sphingolipids in viral entry.

Both heptad repeats of human respiratory syncytial virus fusion protein are potent inhibitors of viral fusion

Heptad repeat regions (HR1 and HR2) are highly conserved peptides located in F(1) of paramyxovirus envelope proteins. They are important in the process of virus fusion and form six-helix bundle structure (trimer of HR1 and HR2 heterodimer) post-fusion, similar to those found in the fusion proteins of other enveloped viruses, such as retrovirus HIV. Both HR1 and HR2 show potent inhibition for virus fusion in some members of paramyxovirus. However, in other members, only HR2 gives strong inhibition whereas HR1 does not. Human respiratory syncytial virus (hRSV) is a member of paramyxovirus and its crystal structure of HR1 and HR2 six-helix bundle was solved lately. Although hRSV HR2 inhibition was reported, nevertheless the effect of HR1 on virus fusion is not known. In this study, hRSV HR1 and HR2 were expressed as fusion protein separately in Escherichia coli system and their complex assembly and virus fusion inhibition effect have been analysed. It shows that both HR1 and HR2 (in the fusion form with 50-amino-acid fusion partner) of hRSV F protein give strong inhibition on virus fusion (IC(50) values are 1.68 and 2.93 microM, respectively) and they form stable six-helix bundle in vitro with both in the fusion protein form.

The fusion protein core of measles virus forms stable coiled-coil trimer

Recent studies have shown that paramyxovirus might adopt a similar molecular mechanism of virus entry and fusion in which the attachment glycoprotein binds receptor/s and triggers the conformational changes of the fusion protein. There are two conserved regions of heptad repeat (HR1 and HR2) in the fusion protein and they were shown with fusion-inhibition effects in many paramyxoviruses, including measles virus. They also appear to show characteristic structure in the fusion core: the HR1/HR2 forms stable six-helix coiled-coil centered by HR1 and is surrounded by HR2 (trimer of HR1/HR2), which represents the post-fusion conformational structure. In this study, we expressed the HR1 and HR2 of measles virus fusion protein as a single chain (named 2-Helix) and subsequently tested its formation of trimer. Indeed, the results do show that the HR1 and HR2 interact with each other and form stable six-helix coiled-coil bundle. This is the first member in genus Morbillivirus of family Paramyxoviridae to be confirmed with this characteristic structure and provides the basis for the HR2-inhibition effects on virus fusion/entry for measles virus.

Membrane fusion tropism and heterotypic functional activities of the Nipah virus and Hendra virus envelope glycoproteins

Nipah virus (NiV) and Hendra virus (HeV) are novel paramyxoviruses from pigs and horses, respectively, that are responsible for fatal zoonotic infections of humans. The unique genetic and biological characteristics of these emerging agents has led to their classification as the prototypic members of a new genus within the Paramyxovirinae subfamily called HENIPAVIRUS: These viruses are most closely related to members of the genus Morbillivirus and infect cells through a pH-independent membrane fusion event mediated by the actions of their attachment (G) and fusion (F) glycoproteins. Understanding their cell biological features and exploring the functional characteristics of the NiV and HeV glycoproteins will help define important properties of these emerging viruses and may provide new insights into paramyxovirus membrane fusion mechanisms. Using a recombinant vaccinia virus system and a quantitative assay for fusion, we demonstrate NiV glycoprotein function and the same pattern of cellular tropism recently reported for HeV-mediated fusion, suggesting that NiV likely uses the same cellular receptor for infection. Fusion specificity was verified by inhibition with a specific antiserum or peptides derived from the alpha-helical heptads of NiV or HeV F. Like that of HeV, NiV-mediated fusion also requires both F and G. Finally, interactions between the glycoproteins of the paramyxoviruses have not been well defined, but here we show that the NiV and HeV glycoproteins are capable of highly efficient heterotypic functional activity with each other. However, no heterotypic activity was observed with envelope glycoproteins of the morbilliviruses Measles virus and Canine distemper virus.

A CD46CD[55-46] chimeric receptor, eight short consensus repeats long, acts as an inhibitor of both CD46 (MCP)- and CD150 (SLAM)-mediated cell-cell fusion induced by CD46-using measles virus

According to their cellular receptor use, measles virus (MV) strains can be separated into two phenotypes, CD46-using and CD46-non-using. A long chimeric receptor, CD46CD[55-46], was generated from the CD46 backbone, encompassing the four short consensus repeat (SCR) domains of CD46 linked via a flexible glycine hinge to SCR1 and SCR2 of CD55, SCR3 and SCR4 of CD46 and the STP, transmembrane and cytoplasmic tail of CD46. This chimeric receptor was proficient for MV binding but deficient in mediating MV-induced cell-to-cell fusion and virus replication, possibly due to the extended distance between the MV haemagglutinin (H) binding site (CD46 SCR1-SCR2) and the cell membrane. When coexpressed with either wild-type CD46 or CD150, this fusion-incompetent receptor exerted a dominant negative effect and inhibited both cell-to-cell fusion and entry of MV with CD46-using, but not CD46-non-using, phenotype. A soluble octameric CD46-C4bpalpha exhibited similar CD46- and CD150-mediated fusion inhibition properties only against CD46-using MV. This suggests that the long CD46CD[55-46] receptor acts by sequestering incoming MV prior to its binding to the shorter functional CD46 or CD150 receptor.

Poliovirus cell entry: common structural themes in viral cell entry pathways

Structural studies of polio- and closely related viruses have provided a series of snapshots along their cell entry pathways. Based on the structures and related kinetic, biochemical, and genetic studies, we have proposed a model for the cell entry pathway for polio- and closely related viruses. In this model a maturation cleavage of a capsid protein precursor locks the virus in a metastable state, and the receptor acts like a transition-state catalyst to overcome an energy barrier and release the mature virion from the metastable state. This initiates a series of conformational changes that allow the virus to attach to membranes, form a pore, and finally release its RNA genome into the cytoplasm. This model has striking parallels with emerging models for the maturation and cell entry of more complex enveloped viruses such as influenza virus and HIV.

Current overview of the pathogenesis and prophylaxis of measles with focus on practical implications

Measles is one of the most important diseases of mankind, which is so highly contagious and evokes such persistent immunity that the virus cannot be sustained in a population of less than about 500,000 persons. The first of the licensed live virus vaccines against measles was developed empirically and was approved in 1963. It provides high level and lasting immunity and is a paradigm for solving major medical problems without really understanding them. In spite of means for control by prophylactic immunization, research on measles infection continues to be part of the effort to understand the pathogenesis of many different viruses, which may have important similarities and differences and provide important insights. Measles, usually, is spontaneously reversible and is a prime model for understanding virus-induced immunodeficiency disease (AIDS) which is rarely reversible. Much has been learned of basic immunology and vaccinology in measles through observation of the inappropriate use of vaccines of appropriate composition, and through inappropriate host response to measles vaccines of inappropriate composition. This review provides a current overview of selected highlights of measles, the virus, its immunopathogenesis, and its control by use of live virus vaccine which may lead to elimination of the disease and eventually to eradication of the virus.

Functional expression and membrane fusion tropism of the envelope glycoproteins of Hendra virus

Hendra virus (HeV) is an emerging paramyxovirus first isolated from cases of severe respiratory disease that fatally affected both horses and humans. Understanding the mechanisms of host cell infection and cross-species transmission is an important step in addressing the risk posed by such emerging pathogens. We have initiated studies to characterize the biological properties of the HeV envelope glycoproteins. Recombinant vaccinia viruses encoding the HeV F and G open reading frames were generated and glycoprotein expression was verified by metabolic labeling and detection using specific antisera. Glycoprotein function and cellular tropism were examined with a quantitative assay for HeV-mediated membrane fusion. Fusion specificity was verified through specific inhibition by anti-HeV antiserum and a peptide corresponding to one of the alpha-helical heptad repeats of F. HeV requires both F and G to mediate fusion. Permissive target cells have been identified, including cell lines derived from cat, bat, horse, human, monkey, mouse, and rabbit. Fusion negative cell types have also been identified. Protease treatments of the target cells abolished fusion activity, suggesting that the virus is employing a cell-surface protein as its receptor.

Herpes simplex virus type 1 entry is inhibited by the cobalt chelate complex CTC-96

The CTC series of cobalt chelates display in vitro and in vivo activity against herpes simplex virus types 1 and 2 (HSV-1 and HSV-2). The experiments described here identify the stage in the virus life cycle where CTC-96 acts and demonstrate that the drug inhibits infection of susceptible cells. CTC-96 at 50 microg/ml has no effect on adsorption of virions to Vero cell monolayers. Penetration assays reveal that CTC-96 inhibits entry of the virus independent of gC and cellular entry receptors. This observation was supported by the failure to detect the accumulation of virus-specified proteins and alpha mRNA transcripts when CTC-96 is present at the onset of infection. Moreover, virion-associated alphaTIF does not accumulate in the nucleus of cells infected in the presence of CTC-96. CTC-96 targets the initial fusion event between the virus and the cell and also inhibits cell-to-cell spread and syncytium formation. Furthermore, CTC-96 inhibits plaque formation by varicella-zoster virus and vesicular stomatitis virus as efficiently as by HSV-1. Collectively, these experiments suggest that CTC-96 is a broad-spectrum inhibitor of infection by enveloped viruses and that it inhibits HSV-1 infection at the point of membrane fusion independent of the type of virus and cellular receptors present.

Paramyxovirus fusion (F) protein: a conformational change on cleavage activation

The fusion (F) protein of the paramyxovirus SV5 promotes both virus-cell and cell-cell fusion. Recently, the atomic structure at 1.4 A of an extremely thermostable six-helix bundle core complex consisting of two heptad repeat regions of the F protein has been described (K. A. Baker, R. E. Dutch, R. A. Lamb, and T. S Jardetsky, Mol. Cell 3, 309-319, 1999). To analyze the conformations of the F protein at various stages of the membrane fusion process and to understand further the role of formation of the six-helix bundle core complex in promotion of membrane fusion, antibodies to peptides corresponding to regions of the F protein were obtained. Major changes in F protein antibody recognition were found after cleavage of the precursor protein F(0) to the fusogenically active disulfide-linked heterodimer, F(1) + F(2), and antibodies directed against the heptad repeat regions recognized only the uncleaved form. A monoclonal antibody directed against the F protein showed increased recognition at the cell surface of the cleaved form of the F protein as compared to uncleaved F protein, again indicating changes in conformation between the uncleaved and cleaved forms of the F protein. Anti-peptide antibodies specific for the heptad repeat regions were unable to precipitate a synthetic protein that consisted of the heptad repeat regions separated only by a small spacer, suggesting that the antibodies are unable to recognize their target regions when the heptad repeats are present in the six-helix bundle core complex. Taken together, these data indicate that the six-helix bundle core complex is not present in the precursor molecule F(0) and that significant conformational changes occur subsequent to cleavage of the F protein.

Receptor binding and membrane fusion in virus entry: the influenza hemagglutinin

Hemagglutinin (HA) is the receptor-binding and membrane fusion glycoprotein of influenza virus and the target for infectivity-neutralizing antibodies. The structures of three conformations of the ectodomain of the 1968 Hong Kong influenza virus HA have been determined by X-ray crystallography: the single-chain precursor, HA0; the metastable neutral-pH conformation found on virus, and the fusion pH-induced conformation. These structures provide a framework for designing and interpreting the results of experiments on the activity of HA in receptor binding, the generation of emerging and reemerging epidemics, and membrane fusion during viral entry. Structures of HA in complex with sialic acid receptor analogs, together with binding experiments, provide details of these low-affinity interactions in terms of the sialic acid substituents recognized and the HA residues involved in recognition. Neutralizing antibody-binding sites surround the receptor-binding pocket on the membrane-distal surface of HA, and the structures of the complexes between neutralizing monoclonal Fabs and HA indicate possible neutralization mechanisms. Cleavage of the biosynthetic precursor HA0 at a prominent loop in its structure primes HA for subsequent activation of membrane fusion at endosomal pH (Figure 1). Priming involves insertion of the fusion peptide into a charged pocket in the precursor; activation requires its extrusion towards the fusion target membrane, as the N terminus of a newly formed trimeric coiled coil, and repositioning of the C-terminal membrane anchor near the fusion peptide at the same end of a rod-shaped molecule. Comparison of this new HA conformation, which has been formed for membrane fusion, with the structures determined for other virus fusion glycoproteins suggests that these molecules are all in the fusion-activated conformation and that the juxtaposition of the membrane anchor and fusion peptide, a recurring feature, is involved in the fusion mechanism. Extension of these comparisons to the soluble N-ethyl-maleimide-sensitive factor attachment protein receptor (SNARE) protein complex of vesicle fusion allows a similar conclusion.

Three membrane-proximal amino acids in the human parainfluenza type 2 (HPIV 2) F protein are critical for fusogenic activity

In this study, we investigated the role of the membrane-proximal region of the human parainfluenza virus type 2 (HPIV2) F protein by mutational analysis, including deletion, insertion, and substitution. Deletion or replacement of the entire 12 amino acid region (aa 474-485) of the HPIV2 F protein completely abolished its fusion activity when coexpressed with the HPIV2 HN protein. Deletion of groups of four of aa 478-485, single alanine, or other amino acid substitutions among aa 478-485 had minimal or limited effects on HPIV2 F/HN-induced cell fusion. However, a significant reduction in, or complete inhibition of, fusion activity was observed when aa 474-477 were deleted, or the N475, F476, or F477 residues were singly substituted with alanine. In addition, insertions of four amino acids at this region or deletion of eight or more amino acids significantly reduced F protein fusion activity. The oligomerization patterns and levels of cell surface expression of the mutant F proteins were compared to those of the wild-type HPIV2 F protein. The mutant HPIV2 F proteins defective in fusion were also found to be unable to initiate hemifusion, indicating that there is a specific requirement for three specific amino acids as well as the spacing in this region for initiating lipid mixing.

The structure of an HIV-1 specific cell entry inhibitor in complex with the HIV-1 gp41 trimeric core

The three-dimensional structure of the complex between an HIV-1 cell-entry inhibitor selected from screening a combinatorial library of non-natural building blocks and the central, trimeric, coiled-coil core of HIV-1 gp41 has been determined by X-ray crystallography. The biased combinatorial library was designed to identify ligands binding in nonpolar pockets on the surface of the coiled-coil core of gp41. The crystal structure shows that the non-peptide moiety of the inhibitor binds to the targeted cavity in two different binding modes. This result suggests a strategy for increasing inhibitor potency by use of a second-generation combinatorial library designed to give simultaneous occupancy of both binding sites.

Close is not enough: SNARE-dependent membrane fusion requires an active mechanism that transduces force to membrane anchors

Is membrane fusion an essentially passive or an active process? It could be that fusion proteins simply need to pin two bilayers together long enough, and the bilayers could do the rest spontaneously. Or, it could be that the fusion proteins play an active role after pinning two bilayers, exerting force in the bilayer in one or another way to direct the fusion process. To distinguish these alternatives, we replaced one or both of the peptidic membrane anchors of exocytic vesicle (v)- and target membrane (t)-SNAREs (soluble N-ethylmaleimide-sensitive fusion protein [NSF] attachment protein [SNAP] receptor) with covalently attached lipids. Replacing either anchor with a phospholipid prevented fusion of liposomes by the isolated SNAREs, but still allowed assembly of trans-SNARE complexes docking vesicles. This result implies an active mechanism; if fusion occurred passively, simply holding the bilayers together long enough would have been sufficient. Studies using polyisoprenoid anchors ranging from 15-55 carbons and multiple phospholipid-containing anchors reveal distinct requirements for anchors of v- and t-SNAREs to function: v-SNAREs require anchors capable of spanning both leaflets, whereas t-SNAREs do not, so long as the anchor is sufficiently hydrophobic. These data, together with previous results showing fusion is inhibited as the length of the linker connecting the helical bundle-containing rod of the SNARE complex to the anchors is increased (McNew, J.A., T. Weber, D.M. Engelman, T.H. Sollner, and J.E. Rothman, 1999. Mol. Cell. 4:415-421), suggests a model in which one activity of the SNARE complex promoting fusion is to exert force on the anchors by pulling on the linkers. This motion would lead to the simultaneous inward movement of lipids from both bilayers, and in the case of the v-SNARE, from both leaflets.

Oligomerization, secretion, and biological function of an anchor-free parainfluenza virus type 2 (PI2) fusion protein

A number of studies indicate that the transmembrane domain, the cytoplasmic domain, or both regions of viral surface glycoproteins are involved in quaternary structure formation. In this report, the transmembrane domain and cytoplasmic tail coding sequence of the fusion (F) glycoprotein gene from parainfluenza type 2 virus was truncated by PCR and the resulting gene (PI2F') was expressed in HeLa-T4 cells by using the vaccinia virus-T7 transient expression system. Pulse-chase experiments indicated that the anchor-free PI2F' was expressed and processed into F(1) and F(2) subunits. Both the processed and the unprocessed anchor-free PI2F' proteins were found to be efficiently secreted into the culture medium. Examination of the oligomeric form of the anchor-free PI2F' by chemical cross-linking demonstrated that it assembles posttranslationally into dimers and trimers with a pattern similar to that of the wild-type PI2F protein. In an effort to better understand the biological properties of the truncated form of PI2F', we anchored PI2F' by a glycosyl-phosphatidylinositol (GPI) linkage. The GPI-anchored PI2F' protein, when coexpressed with PI2HN, did not induce cell fusion seen as syncytium formation, but was found to initiate lipid mixing (hemifusion) as observed by transfer of R-18 rhodamine from red blood cells to the GPI-PI2F'/PI2HN cotransfected cells. The results therefore indicate that the extracellular domain of the PI2 fusion protein contains not only the structural information sufficient to direct assembly into higher oligomers, but also is competent to initiate membrane fusion, suggesting that the anchor-free PI2F' may be useful for further structural studies.