Flu virus invasion: halfway there

Positive charge of Arg-201 on hemagglutinin is required for the binding of H6N1 avian influenza virus to its target through a two-step process

In our previous study, we produced a monoclonal antibody EB2 that recognized an epitope in the HA1 domain on the hemagglutinin (HA) of H6N1 influenza virus (A/chicken/Taiwan/2838 V/00). The residue Arg-201 (R201) on this epitope was protected by the glycan at Asn-167 (N167) from tryptic digestion; therefore, the infectivity of the virus was retained. R201 was extremely conserved in various subtypes of the influenza virus. To explore the role of R201 and the protecting glycan, we developed a bi-cistronic baculovirus expression system for the production of H6HA1 and H6HA0 (nearly full-length HA), which were glycosylated in insect cells. The expressed H6HA1 was mostly found in the trimeric form, and the H6HA0 protein was only found in the monomeric form. The trimeric H6HA1 was resistant to tryptic digestion; however, it could not bind to fetuin, a glycoprotein containing sialylated N-linked and O-linked glycans. By contrast, the monomeric H6HA0 could bind to fetuin but was sensitive to tryptic digestion. We found that the positive charge on R201 was critical for binding HA to the negatively charged surface of host cells because the mutant R201A of H6HA0 lost its binding capacity substantially. Moreover, this binding capacity was dependent on the pH value and inhibited by free electrically charged amino acids. We propose a two-step model for binding the influenza virus with a host cell. The first step involved the specific recognition of the receptor binding site on HA to the sialylated glycan on the host cell. After the virus is engulfed by the acidic endosome, R201 could bind to the cell surface with stronger interactions and trigger the fusion process.

Influenza Virus: A Master Tactician in Innate Immune Evasion and Novel Therapeutic Interventions

Influenza is a contagion that has plagued mankind for many decades, and continues to pose concerns every year, with millions of infections globally. The frequent mutations and recombination of the influenza A virus (IAV) cast a looming threat that antigenically novel strains/subtypes will rise with unpredictable pathogenicity and fear of it evolving into a pandemic strain. There have been four major influenza pandemics, since the beginning of twentieth century, with the great 1918 pandemic being the most severe, killing more than 50 million people worldwide. The mechanisms of IAV infection, host immune responses, and how viruses evade from such defensive responses at the molecular and structural levels have been greatly investigated in the past 30 years. While this has advanced our understanding of virus–host interactions and human immunology, and has led to the development of several antiviral drugs, they have minimal impact on the clinical outcomes of infection. The heavy use of these drugs has also imposed selective pressure on IAV to evolve and develop resistance. Vaccination remains the cornerstone of public health efforts to protect against influenza; however, rapid mass-production of sufficient vaccines is unlikely to occur immediately after the beginning of a pandemic. This, therefore, requires novel therapeutic strategies against this continually emerging infectious virus with higher specificity and cross-reactivity against multiple strains/subtypes of IAVs. This review discusses essential virulence factors of IAVs that determine sustainable human-to-human transmission, the mechanisms of viral hijacking of host cells and subversion of host innate immune responses, and novel therapeutic interventions that demonstrate promising antiviral properties against IAV. This hopefully will promote discussions and investigations on novel avenues of prevention and treatment strategies of influenza, that are effective and cross-protective against multiple strains/subtypes of IAV, in preparation for the advent of future IAVs and pandemics.

Analysis of the glycoproteins of Seoul orthohantavirus strain B1 associated with fusion activity

We analyzed two virus variants (S1 and L1) from Seoul orthohantavirus strain B1. Strain B1 produces large opaque plaques when plated on Vero E6 cell monolayers. However, although the L1 variant produced the large opaque plaques common to the strain, the variant S1 produced small clear ones on Vero E6 cells. Five days after Vero E6 cells were infected with the S1 variant, polykaryons formed spontaneously. However, the cells infected with the L1 variant did not show the formation of syncytia. An analysis of the pH dependency of the cell fusion demonstrated that the L1 variant could induce cell fusion, but only at a pH that was 0.2 units lower than the pH at which the S1 variant induced it. Sequencing of the M genome segment of the two virus variants revealed amino acid substitutions at 4 positions in the Gn and Gc gene products of the S1 variant. Two of these substitutions occurred in the extracellular domain of Gn and changed the charge of the protein. Our findings suggest that these amino acid substitutions caused the S1 variant Gn protein to induce fusion at an elevated pH.

The Redox State Regulates the Conformation of Rv2466c to Activate the Antitubercular Prodrug TP053

Rv2466c is a key oxidoreductase that mediates the reductive activation of TP053, a thienopyrimidine derivative that kills replicating and non-replicating Mycobacterium tuberculosis, but whose mode of action remains enigmatic. Rv2466c is a homodimer in which each subunit displays a modular architecture comprising a canonical thioredoxin-fold with a Cys(19)-Pro(20)-Trp(21)-Cys(22) motif, and an insertion consisting of a four α-helical bundle and a short α-helical hairpin. Strong evidence is provided for dramatic conformational changes during the Rv2466c redox cycle, which are essential for TP053 activity. Strikingly, a new crystal structure of the reduced form of Rv2466c revealed the binding of a C-terminal extension in α-helical conformation to a pocket next to the active site cysteine pair at the interface between the thioredoxin domain and the helical insertion domain. The ab initio low-resolution envelopes obtained from small angle x-ray scattering showed that the fully reduced form of Rv2466c adopts a "closed" compact conformation in solution, similar to that observed in the crystal structure. In contrast, the oxidized form of Rv2466c displays an "open" conformation, where tertiary structural changes in the α-helical subdomain suffice to account for the observed conformational transitions. Altogether our structural, biochemical, and biophysical data strongly support a model in which the formation of the catalytic disulfide bond upon TP053 reduction triggers local structural changes that open the substrate binding site of Rv2466c allowing the release of the activated, reduced form of TP053. Our studies suggest that similar structural changes might have a functional role in other members of the thioredoxin-fold superfamily.

Complete dissociation of the HIV-1 gp41 ectodomain and membrane proximal regions upon phospholipid binding

The envelope glycoprotein gp41 mediates the process of membrane fusion that enables entry of the HIV-1 virus into the host cell. Strong lipid affinity of the ectodomain suggests that its heptad repeat regions play an active role in destabilizing membranes by directly binding to the lipid bilayers and thereby lowering the free-energy barrier for membrane fusion. In such a model, immediately following the shedding of gp120, the N-heptad and C-heptad helices dissociate and melt into the host cell and viral membranes, respectively, pulling the destabilized membranes into juxtaposition, ready for fusion. Post-fusion, reaching the final 6-helix bundle (6 HB) conformation then involves competition between intermolecular interactions needed for formation of the symmetric 6 HB trimer and the membrane affinity of gp41's ectodomain, including its membrane-proximal regions. Our solution NMR study of the structural and dynamic properties of three constructs containing the ectodomain of gp41 with and without its membrane-proximal regions suggests that these segments do not form inter-helical interactions until the very late steps of the fusion process. Interactions between the polar termini of the heptad regions, which are not associating with the lipid surface, therefore may constitute the main driving force initiating formation of the final post-fusion states. The absence of significant intermolecular ectodomain interactions in the presence of dodecyl phosphocholine highlights the importance of trimerization of gp41's transmembrane helix to prevent complete dissociation of the trimer during the course of fusion.

A host susceptibility gene, DR1, facilitates influenza A virus replication by suppressing host innate immunity and enhancing viral RNA replication

Influenza A virus (IAV) depends on cellular factors to complete its replication cycle; thus, investigation of the factors utilized by IAV may facilitate antiviral drug development. To this end, a cellular transcriptional repressor, DR1, was identified from a genome-wide RNA interference (RNAi) screen. Knockdown (KD) of DR1 resulted in reductions of viral RNA and protein production, demonstrating that DR1 acts as a positive host factor in IAV replication. Genome-wide transcriptomic analysis showed that there was a strong induction of interferon-stimulated gene (ISG) expression after prolonged DR1 KD. We found that beta interferon (IFN-β) was induced by DR1 KD, thereby activating the JAK-STAT pathway to turn on ISG expression, which led to a strong inhibition of IAV replication. This result suggests that DR1 in normal cells suppresses IFN induction, probably to prevent undesired cytokine production, but that this suppression may create a milieu that favors IAV replication once cells are infected. Furthermore, biochemical assays of viral RNA replication showed that DR1 KD suppressed viral RNA replication. We also showed that DR1 associated with all three subunits of the viral RNA-dependent RNA polymerase (RdRp) complex, indicating that DR1 may interact with individual components of the viral RdRp complex to enhance viral RNA replication. Thus, DR1 may be considered a novel host susceptibility gene for IAV replication via a dual mechanism, not only suppressing the host defense to indirectly favor IAV replication but also directly facilitating viral RNA replication.

IMPORTANCE

Investigations of virus-host interactions involved in influenza A virus (IAV) replication are important for understanding viral pathogenesis and host defenses, which may manipulate influenza virus infection or prevent the emergence of drug resistance caused by a high error rate during viral RNA replication. For this purpose, a cellular transcriptional repressor, DR1, was identified from a genome-wide RNAi screen as a positive regulator in IAV replication. In the current studies, we showed that DR1 suppressed the gene expression of a large set of host innate immunity genes, which indirectly facilitated IAV replication in the event of IAV infection. Besides this scenario, DR1 also directly enhanced the viral RdRp activity, likely through associating with individual components of the viral RdRp complex. Thus, DR1 represents a novel host susceptibility gene for IAV replication via multiple functions, not only suppressing the host defense but also enhancing viral RNA replication. DR1 may be a potential target for drug development against influenza virus infection.

Innate immunity to influenza in chronic airways diseases

Influenza presents a unique human infectious disease that has a substantial impact on the public health, in general, and especially for those with chronic airways diseases. People with asthma and chronic obstructive pulmonary disease (COPD) are particularly vulnerable to influenza infection and experience more severe symptoms with the worsening of their pre-existing conditions. Recent advances in reverse genetics and innate immunity has revealed several influenza virulence factors and host factors involved in influenza pathogenesis and the immune responses to infection. Early innate immunity plays a critical role of limiting viral infection and spread; however, the underlying mechanisms that lead to enhanced susceptibility to influenza infection and severe symptoms in those with asthma and COPD to infection remain un-investigated. This review will explore the importance of early innate antiviral responses to influenza infection and how these responses are altered by influenza virus and in those with chronic airways diseases.

Mechanisms by which ambient humidity may affect viruses in aerosols

Many airborne viruses have been shown to be sensitive to ambient humidity, yet the mechanisms responsible for this phenomenon remain elusive. We review multiple hypotheses, including water activity, surface inactivation, and salt toxicity, that may account for the association between humidity and viability of viruses in aerosols. We assess the evidence and limitations for each hypothesis based on findings from virology, aerosol science, chemistry, and physics. In addition, we hypothesize that changes in pH within the aerosol that are induced by evaporation may trigger conformational changes of the surface glycoproteins of enveloped viruses and subsequently compromise their infectivity. This hypothesis may explain the differing responses of enveloped viruses to humidity. The precise mechanisms underlying the relationship remain largely unverified, and attaining a complete understanding of them will require an interdisciplinary approach.

The fight against new types of influenza virus

In 1997, during an outbreak in chickens in Hong Kong the avian H5N1 influenza virus crossed the species barrier and infected 18 people, of which 6 cases were fatal. The virus also infected wild birds and continued to circulate and mutate in geese and ducks in southeastern China. Since this occurrence, new antigenic variants that are highly pathogenic for humans as well as wild, domestic, and exotic waterfowl continue to appear in Hong Kong. This virus is spreading across Asia, and is encroaching upon Europe and other continents. Wild birds are now considered as the main reservoir of H5N1 virus. Humans become infected with this H5N1 virus usually via close contact with infected birds or a highly contaminated environment. The very low transmissibility of this virus prevented further person-to-person dissemination in spite of the complete absence of immunity in the human population to H5N1 viruses. Viruses of the H5N1 subtype are characterized by an exceptionally high pathogenicity for humans. The cause of the viral virulence is not known so far; however, several virulence factors are considered. The unprecedented capability of H5N1 viruses to kill humans intensifies the concern about its pandemic potential with catastrophic consequences. The effectiveness of existing antivirals as well as vaccines for humans and birds are reviewed.

Subversion of chemokine receptors by HIV: how can we exploit this?

The recent discovery that the chemokines RANTES (regulated upon activation, normal T-cell-expressed and secreted), macrophage inflammatory protein-1alpha (MIP-1a), and macrophage inflammatory protein-1beta (MIP-1b) are important modulators of HIV-1 infection and the subsequent identification of the essential role that chemokine receptors play as co-receptors for HIV-1 infection have provided new insight into the pathogenesis of HIV-1. On May 10, 1996, Ed Berger's group of the NIH announced the identity of one of the elusive ;co-receptors' for HIV-1. This, and subsequent papers showing that the expression of specific seven transmembrane (7TM) chemokine receptors and CD4 confers susceptibility to infection by HIV-1, has led to rapid and exciting advances in both chemokine and HIV research. During the year since then, a number of elegant studies by different groups have identified additional co-receptors for HIV-1 and the receptor involved in HIV-2 infection, as well as extended our understanding of the mechanism HIV utilises to enter cells. A number of researchers at academic institutes, pharmaceutical and biotech companies are gambling that this will translate into new therapeutics for the treatment of HIV infection and AIDS.

Screening of inhibitors for influenza A virus using high-performance affinity chromatography and combinatorial peptide libraries

The affinity inhibitor of fusion peptide of influenza A virus has been studied using a combination of high-performance affinity chromatography (HPAC) and combinatorial peptide libraries. Fusion peptide (FP) (1-11) of influenza A virus was used as the affinity ligand and immobilized onto the poly(glycidyl methacrylate) (PGMA) beads. Positional scanning peptide libraries based on antisense peptide strategy and extended peptide libraries were designed and synthesized. The screening was carried out at acidic pH (5.5) in order to imitate the environment of virus fusion. A hendecapeptide FHRKKGRGKHK was identified to have a strong affinity to the FP (1-11). The dissociation constant of the complex of the hendecapeptide and the FP (1-11) is 3.10 x 10(-6) mol l(-1) in a physiological buffer condition. The polypeptide has a fairly inhibitory effect on three different strains of influenza A virus H1N1 subtype.

Identification of conformational and cold-sensitive mutations in the MuLV envelope protein

The structure and function of the C-terminal domain of the murine leukemia virus Surface protein (MuLV SU) is not well defined. Passage of chimeric ecotropic-amphotropic MuLV viruses with junctions within the SU C-terminus results in the selection of specific point mutations which improve virus viability and Env function. Point mutations were characterized that alter the conformation of the SU/TM heterodimers on the viral particles. Mutation of position E311 within the Moloney MuLV SU protein alters the conformation of the TM protein and its recognition by antibody 42-114 in immunoprecipitation reactions. Mutation of either G541R in the amphotropic 4070A TM, V421M in the 4070A SU, or deletion of S39 and P40 at the N-terminus of the M-MuLV SU results in an irreversible cold-sensitive phenotype at 4 degrees C. This loss of viral titer can be restored by incorporating V421M plus G541R or del S39 P40 plus G541R in cis within the SU/TM.

Polycation gene delivery systems: escape from endosomes to cytosol

Clinical success of gene therapy based on oligonucleotides (ODNs), ribozymes, RNA and DNA will be greatly dependent on the availability of effective delivery systems. Polycations have gained increasing attention as a non-viral gene delivery vector in the past decades. Significant progress has been made in understanding complex formation between polycations and nucleic acids, entry of the complex into the cells and subsequent entry into the nucleus. Sophisticated molecular architectures of cationic polymers have made the vectors more stable and less susceptible to binding by enzymes or proteins. Incorporation of specific ligands to polycations has resulted in more cell-specific uptake by receptor-mediated mechanisms. However, there are still other barriers limiting the transfection efficiency of polycation gene delivery systems. There is a consensus that polycation-DNA complexes (polyplexes) enter cells via the endocytotic pathway. It is not clearly understood, however, how the polyplexes escape (if they do) from endosomes, how DNA is released from the polyplexes or how the released DNA is expressed. The primary focus of this article is to review various polycation gene delivery systems, which are designed to translocate DNA from endosomes into cytosol. Many polycation gene delivery systems have tried to mimic the mechanisms that viruses use for the endosomal escape. Polycation gene delivery systems are usually coupled with synthetic amphipathic peptides mimicking viral fusogenic peptides, histidine-based gene delivery systems for pH-responsive endosomal escape, polycations with intrinsic endosomolytic activity by the proton sponge mechanism and polyanions to mimic the anionic amphiphilic peptides.

Structure-activity relationships for a series of thiobenzamide influenza fusion inhibitors derived from 1,3,3-trimethyl-5-hydroxy-cyclohexylmethylamine

The anti-influenza activity of a series of thiobenzamide fusion inhibitors derived from 1,3,3-trimethyl-5-hydroxy-cyclohexylmethylamine is profiled. Axial disposition of the thioamide moiety is essential for potent influenza inhibitory activity.

Long-range effects on the binding of the influenza HA to receptors are mediated by changes in the stability of a metastable HA conformation

X-ray studies show that influenza hemagglutinin (HA) forms an elongated structure connecting the influenza virus at one end to cell-surface receptors at the other. At neutral pH, the 20 N-terminal residues of HA2-referred to as the fusion peptide-are buried in a hydrophobic pocket, about 100 A away from the receptor-binding site, and thus seem unlikely to affect HA binding to the receptor. To test this assumption, we mutated residues in the fusion peptide, heterologically expressed the mutated proteins in COS7 cells, and examined their ability to bind fluorescently labeled red blood cells (RBCs). Surprisingly, a significantly reduced binding was recorded for some of the mutants. Ample experimental data indicate that HA has at least two forms: the native structure at neutral pH (N) that is metastable and the fusogenic form (F), observed at low pH, which is stable. Thus, a simple interpretation of our data is that HA can bind to its receptors at the RBC surface in the N form much more effectively than in the F (or in any other stable) form and that the altered binding properties are due to destabilizing effects of the mutations on the N form. That is, some of the mutations involve reduction in the free energy barrier between the N and F forms. This, in turn, leads to reduction in the population of the N form, which is the only form capable of binding to the cell-surface receptors. To explore this possibility, we estimated the stability free energy difference between HA wild-type (wt) and mutants in the N form using an empirical surface tension coefficient. The calculated stability differences correlated well with the measured binding, supporting the above interpretation. Our results are examined taking into account the available experimental data on the affinity of different soluble and membrane-attached forms of HA to its receptors.

N-Glycans attached to the stem domain of haemagglutinin efficiently regulate influenza A virus replication

The haemagglutinin (HA) protein of fowl plague virus A/FPV/Rostock/34 (H7N1) contains three N-linked oligosaccharide side chains in its stem domain. These stem glycans, which are attached to the Asn residues at positions 12, 28 and 478, are highly conserved throughout all HA protein sequences analysed to date. In a previous study, in which mutant HA proteins lacking individual stem glycosylation sites had been expressed from an SV-40 vector, it was shown that these glycans maintain the HA protein in the metastable form required for fusion activity. In the present study, the functional role of the stem N-glycans for virus replication was investigated using recombinant influenza viruses generated by an RNA polymerase I-based system. Studies in Madin-Darby canine kidney cells and embryonated chickens' eggs revealed that the N-glycan at Asn(12) is crucial for virus replication. In both culture systems, growth of virus lacking this glycan (mutant cg1) was completely blocked at 37 degrees C and inhibited at 33 degrees C. Loss of the glycan from Asn(478) (mutant cg3) caused less striking, but still measurable, effects. Interestingly, it was not possible to generate mutant viruses containing the HA protein lacking the N-glycan at Asn(28). It is concluded from this that the N-glycan at Asn(28) is indispensable for the formation of replication-competent influenza viruses. When compared to viruses containing wild-type HA protein, mutants cg1 and cg3 showed a significantly decreased pH stability. Taken together, these data show that the HA stem glycans are potent regulators of influenza virus replication.

Implicit solvent model studies of the interactions of the influenza hemagglutinin fusion peptide with lipid bilayers

The "fusion peptide," a segment of approximately 20 residues of the influenza hemagglutinin (HA), is necessary and sufficient for HA-induced membrane fusion. We used mean-field calculations of the free energy of peptide-membrane association (DeltaG(tot)) to deduce the most probable orientation of the fusion peptide in the membrane. The main contributions to DeltaG(tot) are probably from the electrostatic (DeltaG(el)) and nonpolar (DeltaG(np)) components of the solvation free energy; these were calculated using continuum solvent models. The peptide was described in atomic detail and was modeled as an alpha-helix based on spectroscopic data. The membrane's hydrocarbon region was described as a structureless slab of nonpolar medium embedded in water. All the helix-membrane configurations, which were lower in DeltaG(tot) than the isolated helix in the aqueous phase, were in the same (wide) basin in configurational space. In each, the helix was horizontally adsorbed at the water-bilayer interface with its principal axis parallel to the membrane plane, its hydrophobic face dissolved in the bilayer, and its polar face in the water. The associated DeltaG(tot) value was approximately -8 to -10 kcal/mol (depending on the rotameric state of one of the phenylalanine residues). In contrast, the DeltaG(tot) values associated with experimentally observed oblique orientations were found to be near zero, suggesting they are marginally stable at best. The theoretical model did not take into account the interactions of the polar headgroups with the peptide and peptide-induced membrane deformation effects. Either or both may overcompensate for the DeltaG(tot) difference between the horizontal and oblique orientations.

Fusomorphogenesis: cell fusion in organ formation

Cell fusion is a universal process that occurs during fertilization and in the formation of organs such as muscles, placenta, and bones. Very little is known about the molecular and cellular mechanisms of cell fusion during pattern formation. Here we review the dynamic anatomy of all cell fusions during embryonic and postembryonic development in an organism. Nearly all the cell fates and cell lineages are invariant in the nematode C. elegans and one third of the cells that are born fuse to form 44 syncytia in a reproducible and stereotyped way. To explain the function of cell fusion in organ formation we propose the fusomorphogenetic model as a simple cellular mechanism to efficiently redistribute membranes using a combination of cell fusion and polarized membrane recycling during morphogenesis. Thus, regulated intercellular and intracellular membrane fusion processes may drive elongation of the embryo as well as postembryonic organ formation in C. elegans. Finally, we use the fusomorphogenetic hypothesis to explain the role of cell fusion in the formation of organs like muscles, bones, and placenta in mammals and other species and to speculate on how the intracellular machinery that drive fusomorphogenesis may have evolved.

Second-site changes affect viability of amphotropic/ecotropic chimeric enveloped murine leukemia viruses

Chimeras were previously generated between the ecotropic (Moloney-MuLV) and amphotropic (4070A) SU and TM proteins of murine leukemia virus (MuLV). After passage in D17 cells, three chimeras with junctions in the C terminus of SU (AE5, AE6, and AE7), showed improved kinetics of viral spreading, suggesting that they had adapted. Sequencing of the viruses derived from the D17 cell lines revealed second-site changes within the env gene. Changes were detected in the receptor binding domain, the proline-rich region, the C terminus of SU, and the ectodomain of TM. Second-site changes were subcloned into the parental DNA, singly and in combination, and tested for viability. All viruses had maintained their original cloned mutations and junctions. Reconstruction and passage of AE7 or AE6 virus with single point mutations recovered the additional second-site changes identified in the parental population. The AE5 isolate required changes in the VRA, the VRC, the VRB-hinge region, and the C terminus of SU for efficient infection. Passage of virus, including the parental 4070A, in D17 cells resulted in a predominant G100R mutation within the receptor binding domain. Viruses were subjected to titer determination in three cell types, NIH 3T3, canine D17, and 293T. AE6 viruses with changes in the proline-rich region initially adapted for growth on D17 cells could infect all cell types tested. AE6-based chimeras with additional mutations in the C terminus of SU could infect D17 and 293T cells. Infection of NIH 3T3 cells was dependent on the proline-rich mutation. AE7-based chimeras encoding L538Q and G100R were impaired in infecting NIH 3T3 and 293T cells.

Identification of functional domains in the 14-kilodalton envelope protein (A27L) of vaccinia virus

The mechanism of entry of vaccinia virus (VV) into cells is still a poorly understood process. A 14-kDa protein (encoded by the A27L gene) in the envelope of intracellular mature virus (IMV) has been implicated in virus-cell attachment, virus-cell fusion, and virus release from cells. We have previously described the structural organization of the VV 14-kDa protein, consisting of a triple-stranded coiled-coil region responsible for oligomer formation and a predicted Leu zipper-like third alpha helix with an important role in the interaction with a 21-kDa membrane protein (encoded by the A17L gene) thought to anchor the 14-kDa protein to the envelope of IMV (M.-I. Vázquez, G. Rivas, D. Cregut, L. Serrano, and M. Esteban, J. Virol. 72:10126-10137, 1998). To identify the functional domains important for virus entry and release, we have generated VV recombinants containing a copy of the A27L gene regulated by the lacI operator-repressor system of Escherichia coli (VVIndA27L) in the thymidine kinase locus and a mutant form of the A27L gene in the hemagglutinin locus but expressed constitutively under the control of an early-late VV promoter. Cells infected with a VV recombinant that expresses a mutant 14-kDa form lacking the first 29 amino acids at the N terminus failed to form extracellular enveloped virus (EEV). Fusion-from-without assays with purified virus confirmed that the fusion process was mediated by the 14-kDa protein and the fusion domain to be contained within amino acids 29 to 43 of the N-terminal region. Competitive inhibition of the infection process with soluble heparin and synthetic peptides and in vitro experiments with purified mutant proteins identified the heparin binding domain within amino acids 21 to 33, suggesting that this domain is involved in virus-cell binding via heparan sulfate. Thus, the N terminus of the 14-kDa protein contains a heparin binding domain, a fusion domain, and a domain responsible for interacting with proteins or lipids in the Golgi stacks for EEV formation and virus spread.

Enhanced expression, native purification, and characterization of CCR5, a principal HIV-1 coreceptor

Seven-transmembrane segment, G protein-coupled receptors (GPCRs) play important roles in many biological processes in which pharmaceutical intervention may be useful. High level expression and native purification of GPCRs are important steps in the biochemical and structural characterization of these molecules. Here, we describe enhanced mammalian cell expression and purification of a codon-optimized variant of the chemokine receptor CCR5, a GPCR that plays a central role in the entry of the human immunodeficiency virus-1 (HIV-1) into immune cells. CCR5 could be solubilized in its native state as determined by its ability to be precipitated by 2D7, a conformation-dependent anti-CCR5 antibody, and by the HIV-1 gp120 envelope glycoprotein. The 2D7 antibody recognized immature and mature forms of CCR5 equally, whereas gp120 preferentially recognized the mature form, a result that underscores a role for posttranslational modification of CCR5 in its HIV-1 coreceptor function. The methods described herein contribute to the analysis of CCR5 and are likely to be applicable to many other GPCRs.

pH-dependent changes in photoaffinity labeling patterns of the H1 influenza virus hemagglutinin by using an inhibitor of viral fusion

The hemagglutinin (HA) protein undergoes a low-pH-induced conformational change in the acidic milieu of the endosome, resulting in fusion of viral and cellular membranes. A class of compounds that specifically interact with the HA protein of H1 and H2 subtype viruses and inhibit this conformational change was recently described (G. X. Luo et al., Virology 226:66-76, 1996, and J. Virol. 71:4062-4070, 1997). In this study, purified HA trimers (bromelain-cleaved HA [BHA]) are used to examine the properties and binding characteristics of these inhibitors. Compounds were able to inhibit the low-pH-induced change of isolated trimers, as detected by resistance to digestion with trypsin. Protection from digestion was extremely stable, as BHA-inhibitor complexes could be incubated for 24 h in low pH with almost no change in BHA structure. One inhibitor was prepared as a radiolabeled photoaffinity analog and used to probe for specific drug interactions with the HA protein. Analysis of BHA after photoaffinity analog binding and UV cross-linking revealed that the HA2 subunit of the HA was specifically radiolabeled. Cross-linking of the photoaffinity analog to BHA under neutral (native) pH conditions identified a stretch of amino acids within the alpha-helix of HA2 that interact with the inhibitor. Interestingly, cross-linking of the analog under acidic conditions identified a different region within the HA2 N terminus which interacts with the photoaffinity compound. These attachment sites help to delineate a potential binding pocket and suggest a model whereby the BHA is able to undergo a partial, reversible structural change in the presence of inhibitor compound.

Inhibition of influenza A virus replication by compounds interfering with the fusogenic function of the viral hemagglutinin

Several compounds that specifically inhibited replication of the H1 and H2 subtypes of influenza virus type A were identified by screening a chemical library for antiviral activity. In single-cycle infections, the compounds inhibited virus-specific protein synthesis when added before or immediately after infection but were ineffective when added 30 min later, suggesting that an uncoating step was blocked. Sequencing of hemagglutinin (HA) genes of several independent mutant viruses resistant to the compounds revealed single amino acid changes that clustered in the stem region of the HA trimer in and near the HA2 fusion peptide. One of the compounds, an N-substituted piperidine, could be docked in a pocket in this region by computer-assisted molecular modeling. This compound blocked the fusogenic activity of HA, as evidenced by its inhibition of low-pH-induced cell-cell fusion in infected cell monolayers. An analog which was more effective than the parent compound in inhibiting virus replication was synthesized. It was also more effective in blocking other manifestations of the low-pH-induced conformational change in HA, including virus inactivation, virus-induced hemolysis of erythrocytes, and susceptibility of the HA to proteolytic degradation. Both compounds inhibited viral protein synthesis and replication more effectively in cells infected with a virus mutated in its M2 protein than with wild-type virus. The possible functional relationship between M2 and HA suggested by these results is discussed.

Nuclear import and export of viruses and virus genomes

Many viruses replicate in the nucleus of their animal and plant host cells. Nuclear import, export, and nucleo-cytoplasmic shuttling play a central role in their replication cycle. Although the trafficking of individual virus proteins into and out of the nucleus has been well studied for some virus systems, the nuclear transport of larger entities such as viral genomes and capsids has only recently become a subject of molecular analysis. In this review, the general concepts emerging are discussed and a survey is provided of current information on both plant and animal viruses. Summarizing the main findings in this emerging field, it is evident that most viruses that enter or exit the nucleus take advantage of the cell's nuclear import and export machinery. With a few exceptions, viruses seem to cross the nuclear envelope through the nuclear pore complexes, making use of cellular nuclear import and export signals, receptors, and transport factors. In many cases, they capitalize on subtle control systems such as phosphorylation that regulate traffic of cellular components into and out of the nucleus. The large size of viral capsids and their composition (they contain large RNA and DNA molecules for which there are few precedents in normal nuclear transport) make the processes unique and complicated. Prior capsid disassembly (or deformation) is required before entry of viral genomes and accessory proteins can occur through nuclear pores. Capsids of different virus families display diverse uncoating programs which culminate in genome transfer through the nuclear pores.

The pathway of membrane fusion catalyzed by influenza hemagglutinin: restriction of lipids, hemifusion, and lipidic fusion pore formation

The mechanism of bilayer unification in biological fusion is unclear. We reversibly arrested hemagglutinin (HA)-mediated cell-cell fusion right before fusion pore opening. A low-pH conformation of HA was required to form this intermediate and to ensure fusion beyond it. We present evidence indicating that outer monolayers of the fusing membranes were merged and continuous in this intermediate, but HA restricted lipid mixing. Depending on the surface density of HA and the membrane lipid composition, this restricted hemifusion intermediate either transformed into a fusion pore or expanded into an unrestricted hemifusion, without pores but with unrestricted lipid mixing. Our results suggest that restriction of lipid flux by a ring of activated HA is necessary for successful fusion, during which a lipidic fusion pore develops in a local and transient hemifusion diaphragm.

Receptor-triggered membrane association of a model retroviral glycoprotein

Current models of retroviral entry hypothesize that interactions between the viral envelope protein and the host receptor(s) induce conformational changes in the envelope protein that activate the envelope protein and initiate fusion. We employed a liposome-binding assay to demonstrate directly and characterize the activation of a model retroviral envelope protein (EnvA) from Rous sarcoma virus (RSV). In the presence of purified viral receptor, the trimeric ectodomain of EnvA was converted from a water-soluble form to a membrane-associated form, consistent with conversion of the envelope protein to its fusogenic state. This activation was nonlinear with respect to receptor concentration, suggesting cooperativity within the trimeric envelope protein. The activated EnvA was stably associated with the target membrane through hydrophobic interactions, behaving like an intrinsic membrane protein. The ability of EnvA to associate with membrane was coincident with a loss of receptor-binding activity, suggesting that during viral entry activated EnvA dissociates from the receptor to facilitate membrane fusion. These results provide direct evidence that receptor binding triggers conversion of the EnvA protein to a membrane-binding form, illustrating that RSV is a useful model for the study of retroviral entry and activation of pH-independent fusion proteins.

Morphological changes and fusogenic activity of influenza virus hemagglutinin

The kinetics of low-pH induced fusion of influenza virus with liposomes have been compared to changes in the morphology of influenza hemagglutinin (HA). At pH 4.9 and 30 degrees C, the fusion of influenza A/PR/8/34 virus with ganglioside-bearing liposomes was complete within 6 min. Virus preincubated at pH 4.9 and 30 degrees C in the absence of liposomes for 2 or 10 min retained most of its fusion activity. However, fusion activity was dramatically reduced after 30 min, and virtually abolished after a 60-min preincubation. Cryo-electron microscopy showed that the hemagglutinin spikes of virions exposed to pH 4.9 at 30 degrees C for 10 min underwent no major morphological changes. After 30 min, however, the spike morphology changed dramatically, and further changes occurred for up to 60 min after exposure to low pH. Because the morphological changes occur at a rate corresponding to the loss of fusion activity, and because these changes are much slower than the rate at which fusion occurs, we conclude that the morphologically altered HA is inactive with respect to fusion-promoting activity. Molecular modeling studies indicate that the formation of an extended coiled coil within the HA trimer, as proposed for HA at low pH, requires a major conformational change in HA, and that the morphological changes we observe are consistent with the formation of an extended coiled coil. These results imply that the crystallographically determined low-pH form of HA does occur in the intact virus, but that this form is not a precursor of viral fusion. It is speculated that the motion to the low-pH form may be responsible for the membrane destabilization leading to fusion.

Influenza hemagglutinin is spring-loaded by a metastable native conformation

Enveloped viruses enter cells by protein-mediated membrane fusion. For influenza virus, membrane fusion is regulated by the conformational state of the hemagglutinin (HA) protein, which switches from a native (nonfusogenic) structure to a fusion-active (fusogenic) conformation when exposed to the acidic environment of the cellular endosome. Here we demonstrate that destabilization of HA at neutral pH, with either heat or the denaturant urea, triggers a conformational change that is biochemically indistinguishable from the change triggered by low pH. In each case, the conformational change is coincident with induction of membrane-fusion activity, providing strong evidence that the fusogenic structure is formed. These results indicate that the native structure of HA is trapped in a metastable state and that the fusogenic conformation is released by destabilization of native structure. This strategy may be shared by other enveloped viruses, including those that enter the cell at neutral pH, and could have implications for understanding the membrane-fusion step of HIV infection.

Structural features of membrane fusion between influenza virus and liposome as revealed by quick-freezing electron microscopy

The structure of membrane fusion intermediates between the A/PR/8(H1N1) strain of influenza virus and a liposome composed of egg phosphatidylcholine, cholesterol, and glycophorin was studied using quick-freezing electron microscopy. Fusion by viral hemagglutinin protein was induced at pH 5.0 and 23 degrees C. After a 19-s incubation under these conditions, small protrusions with a diameter of 10-20 nm were found on the fractured convex faces of the liposomal membranes, and small pits complementary to the protrusions were found on the concave faces. The protrusions and pits corresponded to fractured parts of outward bendings of the lipid bilayer or "microprotrusions of the lipid bilayer." At the loci of the protrusions and pits, liposomal membranes had local contacts with viral membranes. In many cases both the protrusions and the pits were aligned in regular polygonal arrangements, which were thought to reflect the array of hemagglutinin spikes on the viral surface. These structures were induced only when the medium was acidic with the virus present. Based on these observations, it was concluded that the microprotrusions of the lipid bilayer are induced by hemagglutinin protein. Furthermore, morphological evidence for the formation of the "initial fusion pore" at the microprotrusion was obtained. The protrusion on the convex face sometimes had a tiny hole with a diameter of <4 nm in the center. The pits transformed into narrow membrane connections <10 nm in width, bridging viruses and liposomes. The structures of the fusion pore and fusion neck with larger sizes were also observed, indicating growth of the protrusions and pits to distinct fusion sites. We propose that the microprotrusion of the lipid bilayer is a fusion intermediate induced by hemagglutinin protein, and suggest that the extraordinarily high curvature of this membrane structure is a clue to the onset of fusion. The possible architecture of the fusion intermediate is discussed with regard to the localization of intramembrane particles at the microprotrusion.

Fusion peptides derived from the HIV type 1 glycoprotein 41 associate within phospholipid membranes and inhibit cell-cell Fusion. Structure-function study

The fusion domain of human immunodeficiency virus (HIV-1) envelope glycoprotein (gp120-gp41) is a conserved hydrophobic region located at the N terminus of the transmembrane glycoprotein (gp41). A V2E mutant has been shown to dominantly interfere with wild-type envelope-mediated syncytium formation and virus infectivity. To understand this phenomenon, a 33-residue peptide (wild type, WT) identical to the N-terminal segment of gp41 and its V2E mutant were synthesized, fluorescently labeled, and characterized. Both peptides inhibited HIV-1 envelope-mediated cell-cell fusion and had similar alpha-helical content in membrane mimetic environments. Studies with fluorescently labeled peptide analogues revealed that both peptides have high affinity for phospholipid membranes, are susceptible to digestion by proteinase-K in their membrane-bound state, and tend to self- and coassemble in the membranes. In SDS-polyacrylamide gel electrophoresis the WT peptide formed dimers as well as higher order oligomers, whereas the V2E mutant only formed dimers. The WT, but not the V2E mutant, induced liposome aggregation, destabilization, and fusion. Moreover, the V2E mutant inhibited vesicle fusion induced by the WT peptide, probably by forming inactive heteroaggregates. These data form the basis for an explanation of the mechanism by which the gp41 V2E mutant inhibits HIV-1 infectivity in cells when co-expressed with WT gp41.

Molecular mechanism underlying the action of a novel fusion inhibitor of influenza A virus

In the initial stages of influenza virus infection, the hemagglutinin (HA) protein of influenza virus mediates both adsorption and penetration of the virus into the host cell. Recently, we identified and characterized BMY-27709 as an inhibitor of the H1 and H2 subtypes of influenza A virus that specifically inhibits the HA function necessary for virus-cell membrane fusion (G.-X. Luo, R. Colonno, and M. Krystal, Virology 226:66-76, 1996). Studies presented herein show that the inhibition is mediated through specific interaction with the HA protein. This binding represses the low-pH-induced conformational change of the HA protein which is a prerequisite for membrane fusion. In an attempt to define the binding pocket within the HA molecule, a number of drug-resistant viruses have been isolated and characterized. Sequence analyses of the HA gene of these drug-resistant viruses mapped amino acid changes responsible for drug resistance to a region located near the amino terminus of HA2. In addition, we have identified inactive analogs of BMY-27709 which are able to compete out the inhibitory activity of BMY-27709. This finding suggests that inhibition of the HA-mediated membrane fusion by this class of compounds is not solely the result of binding within the HA molecule but requires specific interactions.

ADM-1, a protein with metalloprotease- and disintegrin-like domains, is expressed in syncytial organs, sperm, and sheath cells of sensory organs in Caenorhabditis elegans

A search was carried out for homologues of possible fusogenic proteins to study their function in a genetically tractable animal. The isolation, molecular, and cellular characterization of the Caenorhabditis elegans adm-1 gene (a disintegrin and metalloprotease domain) are described. A glycoprotein analogous to viral fusion proteins has been identified on the surface of guinea pig sperm (PH-30/fertilin) and is implicated in sperm-egg fusion. adm-1 is the first reported invertebrate gene related to PH-30 and a family of proteins containing snake venom disintegrin- and metalloprotease-like domains. ADM-1 shows a domain organization identical to PH-30. It contains prepro, metalloprotease, disintegrin, cysteine rich with putative fusion peptide, epidermal growth factor-like repeat, transmembrane, and cytoplasmic domains. Antibodies which recognize ADM-1 protein in immunoblots were generated. Using immunofluorescence and in situ hybridization, the products of adm-1 have been detected in specific cells during different stages of development. The localization of ADM-1 to the plasma membrane of embryonic cells and to the sheath cells of sensory organs suggests a function in cell adhesion. ADM-1 expression in the hypodermis, pharynx, vulva, and mature sperm is consistent with a putative role in somatic and gamete cell fusions.

Characterization of chimeras between the ecotropic Moloney murine leukemia virus and the amphotropic 4070A envelope proteins

A series of 22 chimeric envelope (env) genes were generated between the ecotropic Moloney murine leukemia virus and the amphotropic 4070A isolate. The chimeric envelopes were expressed within the complete, replication-competent provirus and tested for virus viability by transient expression assays. Eleven of the 22 viruses were viable. Five of these chimeric viruses showed an ecotropic host range, and six exhibited an amphotropic host range and viral interference. The host range determinants map to the first half of the surface (SU) protein. The N-terminal 72 amino acids of 4070A (42 of processed SU) are not required for amphotropic receptor usage. Ecotropic and amphotropic viruses differ in their ability to form large, multinucleated syncytia when cocultured with the rat XC cell line. Ecotropic murine leukemia virus forms large syncytia with XC cells, whereas no syncytia are reported for amphotropic virus. All chimeras which contained the N-terminal half of the ecotropic SU protein, encoding the receptor binding domain, formed the large multinucleated syncytia with XC cells.

A trimeric structural domain of the HIV-1 transmembrane glycoprotein

Infection with HIV-1 is initiated by fusion of cellular and viral membranes. The gp41 subunit of the HIV-1 envelope plays a major role in this process, but the structure of gp41 is unknown. We have identified a stable, proteinase-resistant structure comprising two peptides, N-51 and C-43, derived from a recombinant protein fragment of the gp41 ectodomain. In isolation, N-51 is predominantly aggregated and C-43 is unfolded. When mixed, however, these peptides associate to form a stable, alpha-helical, discrete trimer of heterodimers. Proteolysis experiments indicate that the relative orientation of the N-51 and C-43 helices in the complex is antiparallel. We propose that N-51 forms an interior, parallel, homotrimeric, coiled-coil core, against which three C-43 helices pack in an antiparallel fashion. We suggest that this alpha-helical, trimeric complex is the core of the fusion-competent state of the HIV-1 envelope.

The effects of interhelical electrostatic repulsions between glutamic acid residues in controlling the dimerization and stability of two-stranded alpha-helical coiled-coils

The effects of interhelical electrostatic repulsions in controlling the dimerization and stability of two-stranded alpha-helical coiled-coils have been studied using de novo designed synthetic coiled-coils. A native coiled-coil was snythesized, which consisted of two identical 35-residue polypeptide chains with a heptad repeat QgVaGbAcLdQeKf and a Cys residue at position 2 to allow formation of an interchain 2-2' disulfide bridge. This peptide, designed to contain no intrachain or interchain electrostatic interactions, forms a stable coiled-coil structure at 20 degrees C in benign medium (50 mM KCl, 25 mM PO4, pH 7) with a [urea]1/2 value of 6.1 M. Five mutant coiled-coils were designed in which Gln residues at the e and g positions of the heptad repeat were substituted with Glu systematically from the N terminus toward the C terminus, resulting in each polypeptide chain having 2, 4, 6, 8, or 10 Glu residues. These substituted Glu residues are able to form interchain i to i' +5 electrostatic repulsions across the dimer interface. As the number of interchain repulsions increases, a steady loss of helical content is observed by circular dichroism spectroscopy. The effects of the interchain Glu-Glu repulsions on the coiled-coil structure are partly overcome by the presence of an interchain disulfide bridge; the peptide with six Glu substitutions is only 15% helical in the reduced form but 85% helical in the oxidized form. The stabilities of the coiled-coils were determined by urea and guanidine hydrochloride (GdnHCl) denaturation studies at 20 degrees C. The stabilities of the coiled-coils determined by urea denaturation indicate a decrease in stability, which correlates with an increasing number of interchain repulsions ([urea]1/2 values ranging from 8.4 to 3.7 M in the presence of M KCl). In contrast, all coiled-coils had similar stabilities when determined by GdnHCl denaturation (approximately 2.9 M). KCl could not effectively screen the effects of interchain repulsions on coiled-coil stability as compared to GdnHCl.

Identification of a membrane fusion domain and an oligomerization domain in the baculovirus GP64 envelope fusion protein

The baculovirus GP64 envelope fusion protein (GP64 EFP) is the major envelope glycoprotein of the budded virion and has been shown to mediate acid-triggered membrane fusion both in virions and when expressed alone in transfected cells. Using site-directed mutagenesis and functional assays for oligomerization, transport, and membrane fusion, we localized two functional domains of GP64 EFP. To identify a fusion domain in the GP64 EFP of the Orgyia pseudotsugata multiple nuclear polyhedrosis virus (OpMNPV), we examined two hydrophobic regions in the GP64 EFP ectodomain. Hydrophobic region I (amino acids 223 to 228) is a cluster of 6 hydrophobic amino acids exhibiting the highest local hydrophobicity in the ectodomain. Hydrophobic region II (amino acids 330 to 338) lies within a conserved region of GP64 EFP that contains a heptad repeat of leucine residues and is predicted to form an amphipathic alpha-helix. In region I, nonconservative amino acid substitutions at Leu-226 and Leu-227 (at the center of the hydrophobic cluster) completely abolished fusion activity but did not prevent GP64 EFP oligomerization or surface localization. To confirm the role of region I in membrane fusion activity, we used a synthetic 21-amino-acid peptide to generate polyclonal antibodies against region I and demonstrated that antipeptide antibodies were capable of both neutralizing membrane fusion activity and reducing infectivity of the virus. In hydrophobic region II, mutations were designed to disrupt several structural characteristics: a heptad repeat of leucine, a predicted alpha-helix, or the local hydrophobicity along one face of the helix. Single alanine substitutions for heptad leucines did not prevent oligomerization, transport, or fusion activity. However, multiple alanine substitutions or proline (helix-destabilizing) substitutions disrupted both oligomerization and transport of GP64 EFP. In addition, a deletion that removed region II and the predicted alpha-helix was defective for oligomerization, whereas a larger deletion that retained region II and the predicted helix was oligomerized. These results indicate that region II is required for oligomerization and transport and suggest that the predicted helical structure of this region may be important for this function. Thus, by using mutagenesis, functional assays, and antibody inhibition, two functional domains were localized within the baculovirus GP64 EFP: a fusion domain located at amino acids 223 to 228 and an oligomerization domain located at amino acids 327 to 335 within a predicted amphipathic alpha-helix.