Influenza B virus BM2 protein is an oligomeric integral membrane protein expressed at the cell surface

The Potential of Cyclodextrins as Inhibitors for the BM2 Protein: An In Silico Investigation

The influenza BM2 transmembrane domain (BM2TM), an acid-activated proton channel, is an attractive antiviral target due to its essential roles during influenza virus replication, whereas no effective inhibitors have been reported for BM2. In this study, we draw inspiration from the properties of cyclodextrins (CDs) and hypothesize that CDs of appropriate sizes may possess the potential to act as inhibitors of the BM2TM proton channel. To explore this possibility, molecular dynamics simulations were employed to assess their inhibitory capabilities. Our findings reveal that CD4, CD5, and CD6 are capable of binding to the BM2TM proton channel, resulting in disrupted water networks and reduced hydrogen bond occupancy between H19 and the solvent within the BM2TM channel necessary for proton conduction. Notably, CD4 completely obstructs the BM2TM water channel. Based on these observations, we propose that CD4, CD5, and CD6 individually contribute to diminishing the proton transfer efficiency of the BM2 protein, and CD4 demonstrates promising potential as an inhibitor for the BM2 proton channel.

Epitope-targeting platform for broadly protective influenza vaccines

Seasonal influenza vaccines are often ineffective because they elicit strain-specific antibody responses to mutation-prone sites on the hemagglutinin (HA) head. Vaccines that provide long-lasting immunity to conserved epitopes are needed. Recently, we reported a nanoparticle-based vaccine platform produced by solid-phase peptide synthesis (SPPS) for targeting linear and helical protein-based epitopes. Here, we illustrate its potential for building broadly protective influenza vaccines. Targeting known epitopes in the HA stem, neuraminidase (NA) active site, and M2 ectodomain (M2e) conferred 50-75% survival against 5LD50 influenza B and H1N1 challenge; combining stem and M2e antigens increased survival to 90%. Additionally, protein sequence and structural information were employed in tandem to identify alternative epitopes that stimulate greater protection; we report three novel HA and NA sites that are highly conserved in type B viruses. One new target in the HA stem stimulated 100% survival, highlighting the value of this simple epitope discovery strategy. A candidate influenza B vaccine targeting two adjacent HA stem sites led to >104-fold reduction in pulmonary viral load. These studies describe a compelling platform for building vaccines that target conserved influenza epitopes.

The Structure, Function, and Pathobiology of the Influenza A and B Virus Ion Channels

Influenza A virus AM2 protein is an integral membrane protein that is an ion channel (also known as a viroporin). The channel has 24 extracellular residues, 19 residues that span the membrane once and acts as both the channel pore and also the membrane anchoring domain, and a 54-residue cytoplasmic tail. The M2 protein has four identical chains linked via two disulfide bonds that form a four-helix bundle that is 10⁷-10⁸ more permeable to protons than Na⁺ ions. The M2 channel is activated by low pH, His residue 37 is the pH sensor, and Trp residue 41 is the channel gate. The channel is blocked by the antiviral drug amantadine hydrochloride. The influenza B virus BM2 protein does not have homology with the AM2 channel, but BM2 does have the His proton sensor, Trp gate, and is activated by low pH. It is thought that the AM2 and BM2 proteins have common functions in the influenza A and B virus life cycles. Both BM2 and AM2 also facilitate virus budding. The amphipathic helix in the AM2 cytoplasmic tail has an important role in the assembly of the virus, and functional AM2 protein makes the virus independent of the "endosomal sorting complex required for transport" (ESCRT) complex scission.

Comparing Interfacial Trp, Interfacial His and pH Dependence for the Anchoring of Tilted Transmembrane Helical Peptides

Charged and aromatic amino acid residues, being enriched toward the terminals of membrane-spanning helices in membrane proteins, help to stabilize particular transmembrane orientations. Among them, histidine is aromatic and can be positively charge at low pH. To enable investigations of the underlying protein-lipid interactions, we have examined the effects of single or pairs of interfacial histidine residues using the constructive low-dynamic GWALP23 (acetyl-GG²ALW⁵LALALALALALALW¹⁹LAG²²A-amide) peptide framework by incorporating individual or paired histidines at locations 2, 5, 19 or 22. Analysis of helix orientation by means of solid-state ²H NMR spectra of labeled alanine residues reveals marked differences with H^2,22 compared to W^2,22. Nevertheless, the properties of membrane-spanning H^2,22WALP23 helices show little pH dependence and are similar to those having Gly, Arg or Lys at positions 2 and 22. The presence of H5 or H19 influences the helix rotational preference but not the tilt magnitude. H5 affects the helical integrity, as residue 7 unwinds from the core helix; yet once again the helix orientation and dynamic properties show little sensitivity to pH. The overall results reveal that the detailed properties of transmembrane helices depend upon the precise locations of interfacial histidine residues.

M2e-based universal influenza vaccines: a historical overview and new approaches to development

The influenza A virus was isolated for the first time in 1931, and the first attempts to develop a vaccine against the virus began soon afterwards. In addition to causing seasonal epidemics, influenza viruses can cause pandemics at random intervals, which are very hard to predict. Vaccination is the most effective way of preventing the spread of influenza infection. However, seasonal vaccination is ineffective against pandemic influenza viruses because of antigenic differences, and it takes approximately six months from isolation of a new virus to develop an effective vaccine. One of the possible ways to fight the emergence of pandemics may be by using a new type of vaccine, with a long and broad spectrum of action. The extracellular domain of the M2 protein (M2e) of influenza A virus is a conservative region, and an attractive target for a universal influenza vaccine. This review gives a historical overview of the study of M2 protein, and summarizes the latest developments in the preparation of M2e-based universal influenza vaccines.

Potential Viroporin Candidates From Pathogenic Viruses Using Bacteria-Based Bioassays

Viroporins are a family of small hydrophobic proteins found in many enveloped viruses that are capable of ion transport. Building upon the ability to inhibit influenza by blocking its archetypical M2 H⁺ channel, as a family, viroporins may represent a viable target to curb viral infectivity. To this end, using three bacterial assays we analyzed six small hydrophobic proteins from biomedically important viruses as potential viroporin candidates. Our results indicate that Eastern equine encephalitis virus 6k, West Nile virus MgM, Dengue virus 2k, Dengue virus P1, Variola virus gp170, and Variola virus gp151 proteins all exhibit channel activity in the bacterial assays, and as such may be considered viroporin candidates. It is clear that more studies, such as patch clamping, will be needed to characterize the ionic conductivities of these proteins. However, our approach presents a rapid procedure to analyze open reading frames in other viruses, yielding new viroporin candidates for future detailed investigation. Finally, if conductivity is proven vital to their cognate viruses, the bio-assays presented herein afford a simple approach to screen for new channel blockers.

Viroporins in the Influenza Virus

Influenza is a highly contagious virus that causes seasonal epidemics and unpredictable pandemics. Four influenza virus types have been identified to date: A, B, C and D, with only A–C known to infect humans. Influenza A and B viruses are responsible for seasonal influenza epidemics in humans and are responsible for up to a billion flu infections annually. The M2 protein is present in all influenza types and belongs to the class of viroporins, i.e., small proteins that form ion channels that increase membrane permeability in virus-infected cells. In influenza A and B, AM2 and BM2 are predominantly proton channels, although they also show some permeability to monovalent cations. By contrast, M2 proteins in influenza C and D, CM2 and DM2, appear to be especially selective for chloride ions, with possibly some permeability to protons. These differences point to different biological roles for M2 in types A and B versus C and D, which is also reflected in their sequences. AM2 is by far the best characterized viroporin, where mechanistic details and rationale of its acid activation, proton selectivity, unidirectionality, and relative low conductance are beginning to be understood. The present review summarizes the biochemical and structural aspects of influenza viroporins and discusses the most relevant aspects of function, inhibition, and interaction with the host.

Elucidating Relayed Proton Transfer through a His-Trp-His Triad of a Transmembrane Proton Channel by Solid-State NMR

Proton transfer through membrane-bound ion channels is mediated by both water and polar residues of proteins, but the detailed molecular mechanism is challenging to determine. The tetrameric influenza A and B virus M2 proteins form canonical proton channels that use an HxxxW motif for proton selectivity and gating. The BM2 channel also contains a second histidine (His), H27, equidistant from the gating tryptophan, which leads to a symmetric H¹⁹xxxW²³xxxH²⁷ motif. The proton-dissociation constants (pK_a's) of H19 in BM2 were found to be much lower than the pK_a's of H37 in AM2. To determine if the lower pK_a's result from H27-facilitated proton dissociation of H19, we have now investigated a H27A mutant of BM2 using solid-state NMR. ¹⁵N NMR spectra indicate that removal of the second histidine converted the protonation and tautomeric equilibria of H19 to be similar to the H37 behavior in AM2, indicating that the peripheral H27 is indeed the origin of the low pK_a's of H19 in wild-type BM2. Measured interhelical distances between W23 sidechains indicate that the pore constriction at W23 increases with the H19 tetrad charge but is independent of the H27A mutation. These results indicate that H27 both accelerates proton dissociation from H19 to increase the inward proton conductance and causes the small reverse conductance of BM2. The proton relay between H19 and H27 is likely mediated by the intervening gating tryptophan through cation-π interactions. This relayed proton transfer may exist in other ion channels and has implications for the design of imidazole-based synthetic proton channels.

The Transmembrane Conformation of the Influenza B Virus M2 Protein in Lipid Bilayers

Influenza A and B viruses cause seasonal flu epidemics. The M2 protein of influenza B (BM2) is a membrane-embedded tetrameric proton channel that is essential for the viral lifecycle. BM2 is a functional analog of AM2 but shares only 24% sequence identity for the transmembrane (TM) domain. The structure and function of AM2, which is targeted by two antiviral drugs, have been well characterized. In comparison, much less is known about the structure of BM2 and no drug is so far available to inhibit this protein. Here we use solid-state NMR spectroscopy to investigate the conformation of BM2(1–51) in phospholipid bilayers at high pH, which corresponds to the closed state of the channel. Using 2D and 3D correlation NMR experiments, we resolved and assigned the ¹³C and ¹⁵N chemical shifts of 29 residues of the TM domain, which yielded backbone (φ, ψ) torsion angles. Residues 6–28 form a well-ordered α-helix, whereas residues 1–5 and 29–35 display chemical shifts that are indicative of random coil or β-sheet conformations. The length of the BM2-TM helix resembles that of AM2-TM, despite their markedly different amino acid sequences. In comparison, large ¹⁵N chemical shift differences are observed between bilayer-bound BM2 and micelle-bound BM2, indicating that the TM helix conformation and the backbone hydrogen bonding in lipid bilayers differ from the micelle-bound conformation. Moreover, H^N chemical shifts of micelle-bound BM2 lack the periodic trend expected for coiled coil helices, which disagree with the presence of a coiled coil structure in micelles. These results establish the basis for determining the full three-dimensional structure of the tetrameric BM2 to elucidate its proton-conduction mechanism.

Structural and Functional Properties of Viral Membrane Proteins

Viruses have developed a large variety of transmembrane proteins to carry out their infectious cycles. Some of these proteins are simply anchored to membrane via transmembrane helices. Others, however, adopt more interesting structures to perform tasks such as mediating membrane fusion and forming ion-permeating channels. Due to the dynamic or plastic nature shown by many of the viral membrane proteins, structural and mechanistic understanding of these proteins has lagged behind their counterparts in prokaryotes and eukaryotes. This chapter provides an overview of the use of NMR spectroscopy to unveil the transmembrane and membrane-proximal regions of viral membrane proteins, as well as their interactions with potential therapeutics.

Functional studies reveal the similarities and differences between AM2 and BM2 proton channels from influenza viruses

AM2 and BM2 proton channels are attractive antiviral drug targets due to their essential roles during influenza virus replication. Although both AM2 and BM2 are proton-selective ion channels, they share little sequence similarity except for the HXXXW sequence, which suggests that their proton conductance properties might differ. To test this hypothesis, we applied two-electrode voltage clamp electrophysiological assays to study the specific conductance, leakage current, channel activation, and inhibition of AM2 and BM2 proton channels. It was found that BM2 channel has a higher specific conductance than AM2 channel at pH5.5. Unlike AM2 channel, whose proton conductance is asymmetric (from viral exterior to interior), BM2 channel is capable of conducting proton in both directions. Moreover, BM2 requires a more acidic pH for channel activation than AM2, as revealed by its lower pK_a values. Finally, both AM2 and BM2 can be inhibited by Cu(II) and Cu(I). Overall, the results from this side-by-side comparison of AM2 and BM2 channels reveal the structure-function relationships of these two viroporins, and such information might be important for the designing of novel ion channels.

Expression and purification of native and functional influenza A virus matrix 2 proton selective ion channel

Influenza A virus displays one of the highest infection rates of all human viruses and therefore represents a severe human health threat associated with an important economical challenge. Influenza matrix protein 2 (M2) is a membrane protein of the viral envelope that forms a proton selective ion channel. Here we report the expression and native isolation of full length active M2 without mutations or fusions. The ability of the influenza virus to efficiently infect MDCK cells was used to express native M2 protein. Using a Calixarene detergents/surfactants based approach; we were able to solubilize most of M2 from the plasma membrane and purify it. The tetrameric form of native M2 was maintained during the protein preparation. Mass spectrometry shows that M2 was phosphorylated in its cytoplasmic tail (serine 64) and newly identifies an acetylation of the highly conserved Lysine 60. ELISA shows that solubilized and purified M2 was specifically recognized by M2 antibody MAB65 and was able to displace the antibody from M2 MDCK membranes. Using a bilayer voltage clamp measurement assay, we demonstrate a pH dependent proton selective ion channel activity. The addition of the M2 ion channel blocker amantadine allows a total inhibition of the channel activity, illustrating therefore the specificity of purified M2 activity. Taken together, this work shows the production and isolation of a tetrameric and functional native M2 ion channel that will pave the way to structural and functional characterization of native M2, conformational antibody development, small molecules compounds screening towards vaccine treatment.

Targeting the Channel Activity of Viroporins

Since the discovery that certain small viral membrane proteins, collectively termed as viroporins, can permeabilize host cellular membranes and also behave as ion channels, attempts have been made to link this feature to specific biological roles. In parallel, most viroporins identified so far are virulence factors, and interest has focused toward the discovery of channel inhibitors that would have a therapeutic effect, or be used as research tools to understand the biological roles of viroporin ion channel activity. However, this paradigm is being shifted by the difficulties inherent to small viral membrane proteins, and by the realization that protein-protein interactions and other diverse roles in the virus life cycle may represent an equal, if not, more important target. Therefore, although targeting the channel activity of viroporins can probably be therapeutically useful in some cases, the focus may shift to their other functions in following years. Small-molecule inhibitors have been mostly developed against the influenza A M2 (IAV M2 or AM2). This is not surprising since AM2 is the best characterized viroporin to date, with a well-established biological role in viral pathogenesis combined the most extensive structural investigations conducted, and has emerged as a validated drug target. For other viroporins, these studies are still mostly in their infancy, and together with those for AM2, are the subject of the present review.

Knowns and unknowns of influenza B viruses

Influenza B viruses (IBVs) circulate annually along with influenza A (IAV) strains during seasonal epidemics. IBV can dominate influenza seasons and cause severe disease, particularly in children and adolescents. Research has revealed interesting aspects of IBV and highlighted the importance of these viruses in clinical settings. Yet, many important questions remain unanswered. In this review, the clinical relevance of IBV is emphasized, unique features in epidemiology, host range and virology are highlighted and gaps in knowledge pinpointed. Multiple aspects of IBV epidemiology, evolution, virology and immunology are discussed. Future research into IBV is needed to understand how we can prevent severe disease in high-risk groups, especially children and elderly.

Type II integral membrane protein, TM of J paramyxovirus promotes cell-to-cell fusion

Paramyxoviruses include many important animal and human pathogens. Most paramyxoviruses have two integral membrane proteins: fusion protein (F) and attachment proteins hemagglutinin, hemagglutinin-neuraminidase, or glycoprotein (G), which are critical for viral entry into cells. J paramyxovirus (JPV) encodes four integral membrane proteins: F, G, SH, and transmembrane (TM). The function of TM is not known. In this work, we have generated a viable JPV lacking TM (JPV∆TM). JPV∆TM formed opaque plaques compared with JPV. Quantitative syncytia assays showed that JPV∆TM was defective in promoting cell-to-cell fusion (i.e., syncytia formation) compared with JPV. Furthermore, cells separately expressing F, G, TM, or F plus G did not form syncytia whereas cells expressing F plus TM formed some syncytia. However, syncytia formation was much greater with coexpression of F, G, and TM. Biochemical analysis indicates that F, G, and TM interact with each other. A small hydrophobic region in the TM ectodomain from amino acid residues 118 to 132, the hydrophobic loop (HL), was important for syncytial promotion, suggesting that the TM HL region plays a critical role in cell-to-cell fusion.

Interactions of lipids and detergents with a viral ion channel protein: molecular dynamics simulation studies

Structural studies of membrane proteins have highlighted the likely influence of membrane mimetic environments (i.e., lipid bilayers versus detergent micelles) on the conformation and dynamics of small α-helical membrane proteins. We have used molecular dynamics simulations to compare the conformational dynamics of BM2 (a small α-helical protein from the membrane of influenza B) in a model phospholipid bilayer environment with its behavior in protein–detergent complexes with either the zwitterionic detergent dihexanoylphosphatidylcholine (DHPC) or the nonionic detergent dodecylmaltoside (DDM). We find that DDM more closely resembles the lipid bilayer in terms of its interaction with the protein, while the short-tailed DHPC molecule forms “nonphysiological” interactions with the protein termini. We find that the intrinsic micelle properties of each detergent are conserved upon formation of the protein–detergent complex. This implies that simulations of detergent micelles may be used to help select optimal conditions for experimental studies of membrane proteins.

Characterization of the nucleocytoplasmic shuttle of the matrix protein of influenza B virus

Influenza B virus is an enveloped negative-strand RNA virus that contributes considerably to annual influenza illnesses in human. The matrix protein of influenza B virus (BM1) acts as a cytoplasmic-nuclear shuttling protein during the early and late stages of infection. The mechanism of this intracellular transport of BM1 was revealed through the identification of two leucine-rich CRM1-dependent nuclear export signals (NESs) (3 to 14 amino acids [aa] and 124 to 133 aa), one bipartite nuclear localization signal (NLS) (76 to 94 aa), and two phosphorylation sites (80T and 84S) in BM1. The biological function of the NLS and NES regions were determined through the observation of the intracellular distribution of enhanced green fluorescent protein (EGFP)-tagged signal peptides, and wild-type, NES-mutant, and NLS-mutant EGFP-BM1. Furthermore, the NLS phosphorylation sites 80T and 84S, were found to be required for the nuclear accumulation of EGFP-NLS and for the efficient binding of EGFP-BM1 to human importin-α1. Moreover, all of these regions/sites were required for the generation of viable influenza B virus in a 12-plasmid virus rescue system.

IMPORTANCE

This study expands our understanding of the life cycle of influenza B virus by defining the dynamic mechanism of the nucleocytoplasmic shuttle of BM1 and could provide a scientific basis for the development of antiviral medication.

The minimalist architectures of viroporins and their therapeutic implications

Many viral genomes encode small, integral membrane proteins that form homo-oligomeric channels in membrane, and they transport protons, cations, and other molecules across the membrane barrier to aid various steps of viral entry and maturation. These viral proteins, collectively named viroporins, are crucial for viral pathogenicity. In the past five years, structures obtained by nuclear magnetic resonance (NMR), X-ray crystallography, and electron microscopy (EM) showed that viroporins often adopt minimalist architectures to achieve their functions. A number of small molecules have been identified to interfere with their channel activities and thereby inhibit viral infection, making viroporins potential drug targets for therapeutic intervention. The known architectures and inhibition mechanisms of viroporins differ significantly from each other, but some common principles are shared between them. This review article summarizes the recent developments in the structural investigation of viroporins and their inhibition by antiviral compounds. This article is part of a Special Issue entitled: Viral Membrane Proteins-Channels for Cellular Networking.

Exploring the proton conductance and drug resistance of BM2 channel through molecular dynamics simulations and free energy calculations at different pH conditions

BM2 channel plays an important role in the replication of influenza virus B. However, few studies attempt to investigate the mechanism of the proton conductance in BM2 channel, as well as the drug resistance of the BM2 channel. The first experimental structure of the BM2 protein channel has recently been solved, enabling us to theoretically study BM2 systems with different protonation states of histidine. By performing molecular dynamics simulations on the BM2 systems with different protonation states of four His19 residues, we provided our understanding of the structure, dynamics, and drug resistance of the BM2 channel. In general, the results of our study and other investigations both have demonstrated that whether the BM2 channel adopts an open or a closed form depends on the protonation state of His19. Meanwhile, we discovered that a drug (amantadine) was unable to enter into the center of the BM2 channel even at a low pH condition probably due to the number of hydrophilic residues of the BM2 channel. Finally, potentials of mean force (PMF) calculations were performed for the drug binding BM2 channel, energetically explaining why the BM2 channel exhibited drug resistance to two inhibitors of the AM2 channel, amantadine and rimantadine.

Structure of the ulster strain newcastle disease virus hemagglutinin-neuraminidase reveals auto-inhibitory interactions associated with low virulence

Paramyxovirus hemagglutinin-neuraminidase (HN) plays roles in viral entry and maturation, including binding to sialic acid receptors, activation of the F protein to drive membrane fusion, and enabling virion release during virus budding. HN can thereby directly influence virulence and in a subset of avirulent Newcastle disease virus (NDV) strains, such as NDV Ulster, HN must be proteolytically activated to remove a C-terminal extension not found in other NDV HN proteins. Ulster HN is 616 amino acids long and the 45 amino acid C-terminal extension present in its precursor (HN₀) form has to be cleaved to render HN biologically active. Here we show that Ulster HN contains an inter-subunit disulfide bond within the C-terminal extension at residue 596, which regulates HN activities and neuraminidase (NA) domain dimerization. We determined the crystal structure of the dimerized NA domain containing the C-terminal extension, which extends along the outside of the sialidase β-propeller domain and inserts C-terminal residues into the NA domain active site. The C-terminal extension also engages a secondary sialic acid binding site present in NDV HN proteins, which is located at the NA domain dimer interface, that most likely blocks its attachment function. These results clarify how the Ulster HN C-terminal residues lead to an auto-inhibited state of HN, the requirement for proteolytic activation of HN₀ and associated reduced virulence.

Structural investigation of rimantadine inhibition of the AM2-BM2 chimera channel of influenza viruses

The M2 channel of influenza A is a target of the adamantane family antiviral drugs. Two different drug-binding sites have been reported: one inside the pore, and the other is a lipid-facing pocket. A previous study showed that a chimera of M2 variants from influenza A and B that contains only the pore-binding site is sensitive to amantadine inhibition, suggesting that the primary site of inhibition is inside the pore. To obtain atomic details of channel-drug interaction, we determined the structures of the chimeric channel with and without rimantadine. Inside the channel and near the N-terminal end, methyl groups of Val27 and Ala30 from four subunits form a hydrophobic pocket around the adamantane, and the drug amino group appears to be in polar contact with the backbone oxygen of Ala30. The structures also reveal differences between the drug-bound and -unbound states of the channel that can explain drug resistance.

Mechanism of function of viral channel proteins and implications for drug development

Viral channel-forming proteins comprise a class of viral proteins which, similar to their host companions, are made to alter electrochemical or substrate gradients across lipid membranes. These proteins are active during all stages of the cellular life cycle of viruses. An increasing number of proteins are identified as channel proteins, but the precise role in the viral life cycle is yet unknown for the majority of them. This review presents an overview about these proteins with an emphasis on those with available structural information. A concept is introduced which aligns the transmembrane domains of viral channel proteins with those of host channels and toxins to give insights into the mechanism of function of the viral proteins from potential sequence identities. A summary of to date investigations on drugs targeting these proteins is given and discussed in respect of their mode of action in vivo.

Human influenza vaccines and assessment of immunogenicity

Influenza is responsible for the infection of approximately 20% of the population every season and for an annual death toll of approximately half a million people. The most effective means for controlling infection and thereby reducing morbidity and mortality is vaccination by injection with an inactivated vaccine, or by intranasal administration of a live-attenuated vaccine. Protection is not always optimal and there is a need for the development of new vaccines with improved efficacy and for the expansion of enrollment into vaccination programs. An overview of old and new vaccines is presented. Methods of monitoring immune responses such as hemagglutination-inhibition, ELISA and neutralization tests are evaluated for their accuracy in the assessment of current and new-generation vaccines.

Na(+)/K (+)-ATPase β1 subunit interacts with M2 proteins of influenza A and B viruses and affects the virus replication

Interplay between the host and influenza virus has a pivotal role for the outcome of infection. The matrix proteins M2/BM2 from influenza (A and B) viruses are small type III integral membrane proteins with a single transmembrane domain, a short amino-terminal ectodomain and a long carboxy-terminal cytoplasmic domain. They function as proton channels, mainly forming a membrane-spanning pore through the transmembrane domain tetramer, and are essential for virus assembly and release of the viral genetic materials in the endosomal fusion process. However, little is known about the host factors which interact with M2/BM2 proteins and the functions of the long cytoplasmic domain are currently unknown. Starting with yeast two-hybrid screening and applying a series of experiments we identified that the β1 subunit of the host Na(+)/K(+)-ATPase β1 subunit (ATP1B1) interacts with the cytoplasmic domain of both the M2 and BM2 proteins. A stable ATP1B1 knockdown MDCK cell line was established and we showed that the ATP1B1 knockdown suppressed influenza virus A/WSN/33 replication, implying that the interaction is crucial for influenza virus replication in the host cell. We propose that influenza virus M2/BM2 cytoplasmic domain has an important role in the virus-host interplay and facilitates virus replication.

Molecular studies of influenza B virus in the reverse genetics era

Recovery of an infectious virus of defined genetic structure entirely from cDNA and the deduction of information about the virus resulting from phenotypic characterization of the mutant is the process of reverse genetics. This approach has been possible for a number of negative-strand RNA viruses since the recovery of rabies virus in 1994. However, the recovery of recombinant orthomyxoviruses posed a greater challenge due to the segmented nature of the genome. It was not until 1999 that such a system was reported for influenza A viruses, but since that time our knowledge of influenza A virus biology has grown dramatically. Annual influenza epidemics are caused not only by influenza A viruses but also by influenza B viruses. In 2002, two groups reported the successful recovery of influenza B virus entirely from cDNA. This has allowed greater depth of study into the biology of these viruses. This review will highlight the advances made in various areas of influenza B virus biology as a result of the development of reverse genetics techniques for these viruses, including (i) the importance of the non-coding regions of the influenza B virus genome; (ii) the generation of novel vaccine strains; (iii) studies into the mechanisms of drug resistance; (iv) the function(s) of viral proteins, both those analogous to influenza A virus proteins and those unique to influenza B viruses. The information generated by the application of influenza B virus reverse genetics systems will continue to contribute to our improved surveillance and control of human influenza.

Influenza M2 proton channels

M2 of the influenza virus is an intriguing transmembrane protein that forms a minuscule proton channel in the viral envelope. Its recognized function is to equilibrate pH across the viral membrane during cell entry and across the trans-Golgi membrane of infected cells during viral maturation. It is vital for viral replication and it is a target for the anti-influenza drugs, amantadine and rimantadine. Recently, high resolution structures of M2 channels of both flu A and B have been obtained, providing the desperately needed structural details for understanding the mechanism of proton conductance. In particular, the establishment of the functional solution NMR system of the proton channels enabled simultaneous high resolution structure characterization and measurement of channel dynamics coupled to channel activity. This review summarizes our current understanding of how protons are conducted through the M2 channel from a structural point of view, as well as the modes by which important channel gating elements function during proton conduction.

Flu channel drug resistance: a tale of two sites

The M2 proteins of influenza A and B virus, AM2 and BM2, respectively, are transmembrane proteins that oligomerize in the viral membrane to form proton-selective channels. Proton conductance of the M2 proteins is required for viral replication; it is believed to equilibrate pH across the viral membrane during cell entry and across the trans-Golgi membrane of infected cells during viral maturation. In addition to the role of M2 in proton conductance, recent mutagenesis and structural studies suggest that the cytoplasmic domains of the M2 proteins also play a role in recruiting the matrix proteins to the cell surface during virus budding. As viral ion channels of minimalist architecture, the membrane-embedded channel domain of M2 has been a model system for investigating the mechanism of proton conduction. Moreover, as a proven drug target for the treatment of influenza A infection, M2 has been the subject of intense research for developing new anti-flu therapeutics. AM2 is the target of two anti-influenza A drugs, amantadine and rimantadine, both belonging to the adamantane class of compounds. However, resistance of influenza A to adamantane is now widespread due to mutations in the channel domain of AM2. This review summarizes the structure and function of both AM2 and BM2 channels, the mechanism of drug inhibition and drug resistance of AM2, as well as the development of new M2 inhibitors as potential anti-flu drugs.

Simulations of the BM2 proton channel transmembrane domain from influenza virus B

BM2 is a small integral membrane protein from influenza B virus which forms proton-permeable channels. Coarse-grained (CG) molecular dynamics simulations have been used to produce a model of the BM2 channel by self-assembly of a tetrameric bundle of BM2 transmembrane helices in a lipid bilayer. The BM2 channel model is conformationally stable on a 5 mus time scale. This CG model was converted to atomistic resolution to refine interhelix and channel-water interactions. Atomistic molecular dynamics simulations indicate that the BM2 channel is closed when no more than two of the four His19 residues are protonated. Protonating a third His19 side chain initiates a conformational change that opens the channel. In summary, our simulations suggest a common mechanism for BM2 and A/M2, whereby changes in helix packing play a functional role in channel gating.

Solution structure and functional analysis of the influenza B proton channel

Influenza B virus contains an integral membrane protein, BM2, that oligomerizes in the viral membrane to form pH-activated proton channel. Here we report the solution structures of both the membrane-embedded channel domain and the cytoplasmic domain of BM2. The channel domain forms a left-handed coiled-coil tetramer with a helical packing angle of -37{degree sign} to form a polar pore in the membrane for conducting ions. Mutagenesis and proton flux experiments identified residues involved in proton relay and suggest a mechanism of proton conductance. The cytoplasmic domain of BM2 also forms a coiled-coil tetramer. It has a bipolar charge distribution, in which a negatively charged region interacts specifically with the M1 matrix protein that is involved in packaging the genome in the virion. This interaction suggests another role of BM2 in recruiting the matrix proteins to the cell surface during virus budding. Therefore BM2 is an unusual membrane protein which has the dual functionality of conducting ions and recruiting proteins to the membrane.

An amantadine-sensitive chimeric BM2 ion channel of influenza B virus has implications for the mechanism of drug inhibition

Influenza A virus M2 (A/M2) and the influenza B virus BM2 are both small integral membrane proteins that form proton-selective ion channels. Influenza A virus A/M2 channel is the target of the antiviral drug amantadine (and its methyl derivative rimantadine), whereas BM2 channel activity is not affected by the drug. The atomic structure of the pore-transmembrane (TM) domain peptide has been determined by x-ray crystallography [Stouffer et al. (2008) Nature 451:596-599] and of a larger M2 peptide by NMR methods [Schnell and Chou (2008) Nature 451:591-595]. The crystallographic data show electron density (at 3.5 A resolution) in the channel pore, consistent with amantadine blocking the pore of the channel. In contrast, the NMR data show 4 rimantadine molecules bound on the outside of the helices toward the cytoplasmic side of the membrane. Drug binding includes interactions with residues 40-45 and a polar hydrogen bond between rimantadine and aspartic acid residue 44 (D44). These 2 distinct drug-binding sites led to 2 incompatible drug inhibition mechanisms. We have generated chimeric channels between amantadine-sensitive A/M2 and amantadine-insensitive BM2 designed to define the drug-binding site. Two chimeras containing 5 residues of the A/M2 ectodomain and residues 24-36 of the A/M2 TM domain show 85% amantadine/rimantadine sensitivity and specific activity comparable to that of WT BM2. These functional data suggest that the amantadine/rimantadine binding site identified on the outside of the 4 helices is not the primary site associated with the pharmacologic inhibition of the A/M2 ion channel.

Making the most of fusion tags technology in structural characterization of membrane proteins

Membrane proteins can be investigated at various structural levels, including the topological structure, the high-resolution three-dimensional structure, and the organization and assembly of membrane protein complexes. Gene fusion technology makes it possible to insert a polynucleotide encoding a protein or polypeptide tag into the gene encoding a membrane protein of interest. Resultant recombinant proteins may possess the functions of the original membrane proteins, together with the biochemical properties of the imported fusion tag, greatly enhancing functional and structural studies of membrane proteins. In this article, the latest literature is reviewed in relation to types, applications, strategies, and approaches to fusion tag technology for structural investigations of membrane proteins.

Interactions between histidine and tryptophan residues in the BM2 proton channel from influenza B virus

The BM2 protein of influenza B virus forms a transmembrane proton channel essential for the virus infection. We investigated the structure and mechanism of the BM2 proton channel by using a 31-mer peptide (BM2-TMP) representing the putative transmembrane domain of BM2, with special focus on His19, Trp23 and His27. Like the full-length protein, BM2-TMP formed a transmembrane proton channel activated at acidic pH with a midpoint of transition at pH 6.4 +/- 0.1. Mutation of His19 to Ala almost abolished the channel activity, whereas the His27-to-Ala mutant retained partial activity. The proton selectivity of the channel was lost upon substitution of Phe for Trp23. Comparison of CD, fluorescence and Raman spectra measured for wild-type and mutated BM2-TMP at varied pH showed the pK(a) of the imidazole ring to be approximately 6.5 for His19 and approximately 7.6 for His27. Analysis of the pH-dependent fluorescence and Raman intensities suggested the occurrence of cation-pi interaction between the protonated imidazole ring of His and the indole ring of Trp. The His19-Trp23 cation-pi interaction below pH 6.5 is likely to trigger the opening of the proton channel, whereas His27 is not essential but enhances the channel activity through interaction with Trp23, which constitutes the proton-selective gate.

Viral channel-forming proteins

Channel-forming proteins are found in a number of viral genomes. In some cases, their role in the viral life cycle is well understood, in some cases it needs still to be elucidated. A common theme is that their mode of action involves a change of electrochemical or proton gradient across the lipid membrane which modulates the viral or cellular activity. Blocking these proteins can be a suitable therapeutic strategy as for some viruses this may be "lethal." Besides the many biological relevant questions still to be answered, there are also many open questions concerning the biophysical side as well as structural information and the mechanism of function on a molecular level. The immanent biophysical issues are addressed and the work in the field is summarized.

Functional studies indicate amantadine binds to the pore of the influenza A virus M2 proton-selective ion channel

Influenza A and B viruses contain proton-selective ion channels, A/M2 and BM2, respectively, and the A/M2 channel activity is inhibited by the drugs amantadine and its methyl derivative rimantadine. The structure of the pore-transmembrane domain has been determined by both x-ray crystallography [Stouffer et al. (2008) Nature 451:596-599] and by NMR methods [Schnell and Chou (2008) Nature 451:591-595]. Whereas the crystal structure indicates a single amantadine molecule in the pore of the channel, the NMR data show four rimantadine molecules bound on the outside of the helices toward the cytoplasmic side of the membrane. Drug binding includes interactions with residues 40-45 with a polar hydrogen bond between rimantadine and aspartic acid residue 44 (D44) that appears to be important. These two distinct drug-binding sites led to two incompatible drug inhibition mechanisms. We mutagenized D44 and R45 to alanine as these mutations are likely to interfere with rimantadine binding and lead to a drug insensitive channel. However, the D44A channel was found to be sensitive to amantadine when measured by electrophysiological recordings in oocytes of Xenopus laevis and in mammalian cells, and when the D44 and R45 mutations were introduced into the influenza virus genome. Furthermore, transplanting A/M2 pore residues 24-36 into BM2, yielded a pH-activated chimeric ion channel that was partially inhibited by amantadine. Thus, taken together our functional data suggest that amantadine/rimantadine binding outside of the channel pore is not the primary site associated with the pharmacological inhibition of the A/M2 ion channel.

Identification of the pore-lining residues of the BM2 ion channel protein of influenza B virus

The influenza B virus BM2 proton-selective ion channel is essential for virus uncoating, a process that occurs in the acidic environment of the endosome. The BM2 channel causes acidification of the interior of the virus particle, which results in dissociation of the viral membrane protein from the ribonucleo-protein core. The BM2 protein is similar to the A/M2 protein ion channel of influenza A virus (A/M2) in that it contains an HXXXW motif. Unlike the A/M2 protein, the BM2 protein is not inhibited by the antiviral drug amantadine. We used mutagenesis to ascertain the pore-lining residues of the BM2 ion channel. The specific activity (relative to wild type), reversal voltage, and susceptibility to modification by (2-aminoethyl)-methane thiosulfonate and N-ethylmaleimide of cysteine mutant proteins were measured in oocytes. It was found that mutation of transmembrane domain residues Ser(9), Ser(12), Phe(13), Ser(16), His(19), and Trp(23) to cysteine were most disruptive for ion channel function. These cysteine mutants were also most susceptible to (2-aminoethyl)-methane thiosulfonate and N-ethylmaleimide modification. Furthermore, considerable amounts of dimer were formed in the absence of oxidative reagents when cysteine was introduced at positions Ser(9), Ser(12), Ser(16), or Trp(23). Based on these experimental data, a BM2 transmembrane domain model is proposed. The presence of polar residues in the pore is a probable explanation for the amantadine insensitivity of the BM2 protein and suggests that related but more polar compounds might serve as useful inhibitors of the protein.

The oligomeric state of the active BM2 ion channel protein of influenza B virus

Influenza A virus and influenza B virus particles both contain small integral membrane proteins (A/M2 and BM2, respectively) that function as a pH-sensitive proton channel and are essential for virus replication. The mechanism of action of the M2 channels is a subject of scientific interest particularly as A/M2 channel was shown to be a target for the action of the antiviral drug amantadine. Unfortunately, an inhibitor of the BM2 channel activity is not known. Thus, knowledge of the structural and functional properties of the BM2 channel is essential for the development of potent antiviral drugs. The characterization of the oligomeric state of the BM2 channel is an essential first step in the understanding of channel function. Here we describe determination of the stoichiometry of the BM2 proton channel by utilizing three different approaches. 1) We demonstrated that BM2 monomers can be chemically cross-linked to yield species consistent with dimers, trimers, and tetramers. 2) We studied electrophysiological and biochemical properties of mixed oligomers consisting of wild-type and mutated BM2 subunits and related these data to predicted binomial distribution models. 3) We used fluorescence resonance energy transfer (FRET) in combination with biochemical measurements to estimate the relationships between BM2 channel subunits expressed in the plasma membrane. Our experimental data are consistent with a tetrameric structure of the BM2 channel. Finally, we demonstrated that BM2 transmembrane domain is responsible for the channel oligomerization.

Cytoplasmic domain of influenza B virus BM2 protein plays critical roles in production of infectious virus

Influenza B virus BM2 is a type III integral membrane protein that displays H(+) ion channel activity. Analysis of BM2 knockout mutants has suggested that this protein is a necessary component for the capture of M1-viral ribonucleoprotein (vRNP) complex at the plasma membrane and for incorporation of vRNP complex into the virion during the assembly process. BM2 comprises 109 amino acid residues and possesses a longer cytoplasmic domain than the other 3 integral membrane proteins (hemagglutinin, neuraminidase, and NB). To explore whether the cytoplasmic domain of BM2 is important for infectious virus production, a series of BM2 deletion mutants lacking three to nine amino acid residues at the carboxyl terminus, BM2Delta107-109, BM2Delta104-109, and BM2Delta101-109, was generated by reverse genetics. Intracellular transport and incorporation into virions were indistinguishable between truncated BM2 proteins and wild-type BM2. The BM2Delta107-109 mutant produced levels of infectious virus similar to those of wild-type virus and displayed a spherical shape. However, the BM2Delta104-109 and BM2Delta101-109 mutants produced viruses containing dramatically reduced vRNP complex, as with BM2 knockout mutants, and formed enlarged, irregularly shaped virions. Moreover, gradient separation of membranes indicated that membrane association of M1 from mutants was greatly affected by carboxyl-terminal truncations of BM2. Studies of alanine substitution mutants further suggested that amino acid sequences in the 98-109 region are variable while those in the 86-97 region are a prerequisite for innate BM2 function. These results indicate that the cytoplasmic domain of the BM2 protein is required for firm association of the M1 protein with lipid membranes, vRNP complex incorporation into virions, and virion morphology.

The Alphavirus 6K Protein Activates Endogenous Ionic Conductances when Expressed in Xenopus Oocytes

The Alphavirus Sindbis 6K protein is involved in several functions. It contributes to the processing and membrane insertion of E1 and PE2 viral envelope glycoproteins and to virus budding. It also permeabilizes Escherichia coli and mammalian cells. These viroporin-like properties have been proposed to help virus budding by modifying membrane permeabilities. We expressed Sindbis virus 6K cRNA in Xenopus oocytes to further characterize the effect of 6K on membrane conductances and permeabilization. Although no intrinsic channel properties were seen, cell shrinkage was observed within 24 h. Voltage-clamp experiments showed that 6K upregulated endogenous currents: a hyperpolarization-activated inward current (I (in)) and a calcium-dependent chloride current (I (Cl)). 6K was located at both the plasma and the endoplasmic reticulum membranes. The plasma membrane current upregulation likely results from disruption of the calcium homeostasis of the cell at the endoplasmic reticulum level. Indeed, 6K cRNA expression induced reticular calcium store depletion and capacitative calcium entry activation. By experimental modifications of the incubation medium, we showed that downstream of these events cell shrinkage resulted from a 6K -induced KCl efflux (I (Cl) upregulation leads to chloride efflux, which itself electrically drives potassium efflux), which was responsible for an osmotic water efflux. Our data confirm that 6K specifically triggers a sequential cascade of events that leads to cytoplasmic calcium elevation and cell permeabilization, which likely play a role in the Sindbis virus life cycle.

DNA-nanotube-induced alignment of membrane proteins for NMR structure determination

Membrane proteins are encoded by 20-35% of genes but represent <1% of known protein structures to date. Thus, improved methods for membrane-protein structure determination are of critical importance. Residual dipolar couplings (RDCs), commonly measured for biological macromolecules weakly aligned by liquid-crystalline media, are important global angular restraints for NMR structure determination. For alpha-helical membrane proteins >15 kDa in size, Nuclear-Overhauser effect-derived distance restraints are difficult to obtain, and RDCs could serve as the main reliable source of NMR structural information. In many of these cases, RDCs would enable full structure determination that otherwise would be impossible. However, none of the existing liquid-crystalline media used to align water-soluble proteins are compatible with the detergents required to solubilize membrane proteins. We report the design and construction of a detergent-resistant liquid crystal of 0.8-microm-long DNA-nanotubes that can be used to induce weak alignment of membrane proteins. The nanotubes are heterodimers of 0.4-microm-long six-helix bundles each self-assembled from a 7.3-kb scaffold strand and >170 short oligonucleotide staple strands. We show that the DNA-nanotube liquid crystal enables the accurate measurement of backbone N(H) and C(alpha)H(alpha) RDCs for the detergent-reconstituted zeta-zeta transmembrane domain of the T cell receptor. The measured RDCs validate the high-resolution structure of this transmembrane dimer. We anticipate that this medium will extend the advantages of weak alignment to NMR structure determination of a broad range of detergent-solubilized membrane proteins.

Controlling influenza virus replication by inhibiting its proton channel

Influenza A and viruses encode minimalistic proton-selective ion channels known as A/M2 and BM2, respectively. The A/M2 channel is the target of the antiviral drug, amantadine. The structural and mechanistic aspects of proton conductance of the viral ion channels are described and the review makes a case for the development of more effective antivirals.

The small envelope protein of porcine reproductive and respiratory syndrome virus possesses ion channel protein-like properties

The small envelope (E) protein of porcine reproductive and respiratory syndrome virus (PRRSV) is a hydrophobic 73 amino acid protein encoded in the internal open reading frame (ORF) of the bicistronic mRNA2. As a first step towards understanding the biological role of E protein during PRRSV replication, E gene expression was blocked in a full-length infectious clone by mutating the ATG translational initiation to GTG, such that the full-length mutant genomic clone was unable to synthesize the E protein. DNA transfection of PRRSV-susceptible cells with the E gene knocked-out genomic clone showed the absence of virus infectivity. P129-ΔE-transfected cells however produced virion particles in the culture supernatant, and these particles contained viral genomic RNA, demonstrating that the E protein is essential for PRRSV infection but dispensable for virion assembly. Electron microscopy suggests that the P129-ΔE virions assembled in the absence of E had a similar appearance to the wild-type particles. Strand-specific RT-PCR demonstrated that the E protein-negative, non-infectious P129-ΔE virus particles were able to enter cells but further steps of replication were interrupted. The entry of PRRSV has been suggested to be via receptor-mediated endocytosis, and lysomotropic basic compounds and known ion-channel blocking agents both inhibited PRRSV replication effectively during the uncoating process. The expression of E protein in Escherichia coli-mediated cell growth arrests and increased the membrane permeability. Cross-linking experiments in cells infected with PRRSV or transfected with E gene showed that the E protein was able to form homo-oligomers. Taken together, our data suggest that the PRRSV E protein is likely an ion-channel protein embedded in the viral envelope and facilitates uncoating of virus and release of the genome in the cytoplasm.

Prospects for universal influenza virus vaccine

The current vaccination strategy against influenza A and B viruses is vulnerable to the unanticipated emergence of epidemic strains that are poorly matched by the vaccine. A vaccine that is less sensitive to the antigenic evolution of the virus would be a major improvement. The general feasibility of this goal is supported by studies in animal models that show that immunologic activities directed against relatively invariant viral determinants can reduce illness and death. The most promising approaches are based on antibodies specific for the relatively conserved ectodomain of matrix protein 2 and the intersubunit region of hemagglutinin. However, additional conserved determinants for protective antibodies are likely to exist, and their identification should be encouraged. Most importantly, infection and current vaccines do not appear to effectively induce these antibodies in humans. This finding provides a powerful rationale for testing the protective activity of these relatively conserved viral components in humans.

Influenza virus proton channels

The M2 ion channel proteins of influenza A and B viruses are essential to viral replication. The two ion channels share a common motif, HXXXW, that is responsible for proton selectivity and activation. The ion channel for the influenza A virus, but not influenza B virus, is inhibited by the antiviral drug amantadine and amantadine-resistant escape mutants form in treated influenza A patients. The studies reviewed suggest that an antiviral compound directed against the conserved motif would be more useful than amantadine in inhibiting viral replication.

Universal influenza B vaccine based on the maturational cleavage site of the hemagglutinin precursor

Conventional influenza vaccines can prevent infection, but their efficacy depends on the degree of antigenic "match" between the strains used for vaccine preparation and those circulating in the population. A universal influenza vaccine based on invariant regions of the virus, able to provide broadly cross-reactive protection, without requiring continuous manufacturing update, would solve a major medical need. Since the temporal and geographical dominance of the influenza virus type and/or subtype (A/H3, A/H1, or B) cannot yet be predicted, a universal vaccine, like the vaccines currently in use, should include both type A and type B influenza virus components. However, while encouraging preclinical data are available for influenza A virus, no candidate universal vaccine is available for influenza B virus. We show here that a peptide conjugate vaccine, based on the highly conserved maturational cleavage site of the HA(0) precursor of the influenza B virus hemagglutinin, can elicit a protective immune response against lethal challenge with viruses belonging to either one of the representative, non-antigenically cross-reactive influenza B virus lineages. We demonstrate that protection by the HA(0) vaccine is mediated by antibodies, probably through effector mechanisms, and that a major part of the protective response targets the most conserved region of HA(0), the P1 residue of the scissile bond and the fusion peptide domain. In addition, we present preliminary evidence that the approach can be extended to influenza A virus, although the equivalent HA(0) conjugate is not as efficacious as for influenza B virus.

The influenza A virus M2 cytoplasmic tail is required for infectious virus production and efficient genome packaging

The M2 integral membrane protein encoded by influenza A virus possesses an ion channel activity that is required for efficient virus entry into host cells. The role of the M2 protein cytoplasmic tail in virus replication was examined by generating influenza A viruses encoding M2 proteins with truncated C termini. Deletion of 28 amino acids (M2Stop70) resulted in a virus that produced fourfold-fewer particles but >1,000-fold-fewer infectious particles than wild-type virus. Expression of the full-length M2 protein in trans restored the replication of the M2 truncated virus. Although the M2Stop70 virus particles were similar to wild-type virus in morphology, the M2Stop70 virions contained reduced amounts of viral nucleoprotein and genomic RNA, indicating a defect in vRNP packaging. The data presented indicate the M2 cytoplasmic tail plays a role in infectious virus production by coordinating the efficient packaging of genome segments into influenza virus particles.

Influenza B virus BM2 protein is a crucial component for incorporation of viral ribonucleoprotein complex into virions during virus assembly

Influenza B virus contains four integral membrane proteins in its envelope. Of these, BM2 has recently been found to have ion channel activity and is considered to be a functional counterpart to influenza A virus M2, but the role of BM2 in the life cycle of influenza B virus remains unclear. In an effort to explore its function, a number of BM2 mutant viruses were generated by using a reverse genetics technique. The BM2DeltaATG mutant virus synthesized BM2 at markedly lower levels but exhibited similar growth to wild-type (wt) virus. In contrast, the BM2 knockout virus, which did not produce BM2, did not grow substantially but was able to grow normally when BM2 was supplemented in trans by host cells expressing BM2. These results indicate that BM2 is a required component for the production of infectious viruses. In the one-step growth cycle, the BM2 knockout virus produced progeny viruses lacking viral ribonucleoprotein complex (vRNP). The inhibited incorporation of vRNP was regained by trans-supplementation of BM2. An immunofluorescence study of virus-infected cells revealed that distribution of hemagglutinin, nucleoprotein, and matrix (M1) protein of the BM2 knockout virus at the apical membrane did not differ from that of wt virus, whereas the sucrose gradient flotation assay revealed that the membrane association of M1 was greatly affected in the absence of BM2, resulting in a decrease of vRNP in membrane fractions. These results strongly suggest that BM2 functions to capture the M1-vRNP complex at the virion budding site during virus assembly.

Influenza B virus requires BM2 protein for replication

The BM2 protein of influenza B virus functions as an ion channel, which is suggested to be important for virus uncoating in endosomes of virus-infected cells. Because direct support for this function is lacking, whether BM2 plays an essential role in the viral life cycle remains unknown. We therefore attempted to generate BM2 knockout viruses by reverse genetics. Mutant viruses possessing M segments with the mutated initiation codon of BM2 protein at the stop-start pentanucleotide were viable and still expressed BM2. The introduction of multiple stop codons and a one-nucleotide deletion downstream of the stop-start pentanucleotide, in addition to disablement of the BM2 initiation codon, failed to generate viable mutant viruses, but the mutant M segments still expressed proteins that reacted with the BM2 peptide antiserum. To completely abolish BM2 expression, we generated a mutant M gene whose BM2 open reading frame was deleted. Although this mutant was not able to replicate in normal MDCK cells, it did replicate in a cell line that we established which constitutively expresses BM2. Furthermore, a virus possessing the mutant M gene lacking the BM2 open reading frame and a mutant NA gene containing the BM2 open reading frame instead of the NA open reading frame underwent multiple cycles of replication in MDCK cells, with exogenous sialidase used to supplement the deleted viral sialidase activity. These findings demonstrate that the BM2 protein is essential for influenza B virus replication.

Reduced incorporation of the influenza B virus BM2 protein in virus particles decreases infectivity

BM2 is the fourth integral membrane protein encoded by the influenza B virus genome. It is synthesized late in infection and transported to the plasma membrane from where it is subsequently incorporated into progeny virus particles. It has recently been reported that BM2 has ion channel activity and may be the functional homologue of the influenza A virus M2 protein acting as an ion channel involved in viral entry. Using a reverse genetic approach it was not possible to recover virus which lacked BM2. A recombinant influenza B virus was generated in which the BM2 AUG initiation codon was mutated to GUG. This decreased the efficiency of translation of BM2 protein such that progeny virions contained only 1/8 the amount of BM2 seen in wild-type virus. The reduction in BM2 incorporation resulted in a reduction in infectivity although there was no concomitant decrease in the numbers of virions released from the infected cells. These data imply that the incorporation of sufficient BM2 protein into influenza B virions is required for infectivity of the virus particles.