Ion selectivity and activation of the M2 ion channel of influenza virus

Mechanism for proton conduction of the M(2) ion channel of influenza A virus

The M(2) integral membrane protein of influenza A virus forms a proton-selective ion channel. We investigated the mechanism for proton transport of the M(2) protein in Xenopus oocytes using a two-electrode voltage clamp and in CV-1 cells using the whole cell patch clamp technique. Membrane currents were recorded while manipulating the external solution to alter either the total or free proton concentration or the solvent itself. Membrane conductance decreased by approximately 50% when D(2)O replaced H(2)O as the solvent. From this, we conclude that hydrogen ions do not pass through M(2) as hydronium ions, but instead must interact with titratable groups that line the pore of the channel. M(2) currents measured in solutions of low buffer concentration (<15 mM in oocytes and <0.15 mM in CV-1 cells) were smaller than those studied in solutions of high buffer concentration. Furthermore, the reversal voltage measured in low buffer was shifted to a more negative voltage than in high buffer. Also, at a given pH, M(2) current amplitude in 15 mM buffer decreased when pH-pK(a) was increased by changing the buffer pK(a). Collectively, these results demonstrate that M(2) currents can be limited by external buffer capacity. The data presented in this study were also used to estimate the maximum single channel current of the M(2) ion channel, which was calculated to be on the order of 1-10 fA.

Influenza M2 proton channel activity selectively inhibits trans-Golgi network release of apical membrane and secreted proteins in polarized Madin-Darby canine kidney cells

The function of acidification in protein sorting along the biosynthetic pathway has been difficult to elucidate, in part because reagents used to alter organelle pH affect all acidified compartments and are poorly reversible. We have used a novel approach to examine the role of acidification in protein sorting in polarized Madin-Darby canine kidney (MDCK) cells. We expressed the influenza virus M2 protein, an acid-activated ion channel that equilibrates lumenal and cytosolic pH, in polarized MDCK cells and examined the consequences on the targeting and delivery of apical and basolateral proteins. M2 activity affects the pH of only a subset of acidified organelles, and its activity can be rapidly reversed using ion channel blockers (Henkel, J.R., G. Apodaca, Y. Altschuler, S. Hardy, and O.A. Weisz. 1998. Mol. Biol. Cell. 8:2477-2490; Henkel, J.R., J.L. Popovich, G.A. Gibson, S.C. Watkins, and O.A. Weisz. 1999. J. Biol. Chem. 274:9854-9860). M2 expression significantly decreased the kinetics of cell surface delivery of the apical membrane protein influenza hemagglutinin, but not of the basolaterally delivered polymeric immunoglobulin receptor. Similarly, the kinetics of apical secretion of a soluble form of gamma-glutamyltranspeptidase were reduced with no effect on the basolaterally secreted fraction. Interestingly, M2 activity had no effect on the rate of secretion of a nonglycosylated protein (human growth hormone [hGH]) that was secreted equally from both surfaces. However, M2 slowed apical secretion of a glycosylated mutant of hGH that was secreted predominantly apically. Our results suggest a role for acidic trans-Golgi network pH in signal-mediated loading of apical cargo into forming vesicles.

Helix tilt of the M2 transmembrane peptide from influenza A virus: an intrinsic property

Solid-state NMR has been used to study the influence of lipid bilayer hydrophobic thickness on the tilt of a peptide (M2-TMP) representing the transmembrane portion of the M2 protein from influenza A. Using anisotropic (15)N chemical shifts as orientational constraints, single-site isotopically labeled M2-TMPs were studied in hydrated dioleoylphosphatidylcholine (DOPC) and dimyristoylphosphatidylcholine (DMPC) lipid bilayers oriented between thin glass plates. These chemical shifts provide orientational information for the molecular frame with respect to the magnetic field in the laboratory frame. When modeled as a uniform ideal alpha-helix, M2-TMP has a tilt of 37(+/-3) degrees in DMPC and 33(+/-3) degrees in DOPC with respect to the bilayer normal in these lipid environments. The difference in helix tilt between the two environments appears to be small. This lack of a substantial change in tilt further suggests that significant interactions occur between the helices, as in an oligomeric state, to prevent a change in tilt in thicker lipid bilayers.

Computer simulation of ion channel gating: the M(2) channel of influenza A virus in a lipid bilayer

The transmembrane fragment of the influenza virus M(2) protein forms a homotetrameric channel that transports protons. In this paper, we use molecular dynamics simulations to help elucidate the mechanism of channel gating by four histidines that occlude the channel lumen in the closed state. We test two competing hypotheses. In the "shuttle" mechanism, the delta nitrogen atom on the extracellular side of one histidine is protonated by the incoming proton, and, subsequently, the proton on the epsilon nitrogen atom is released on the opposite side. In the "water-wire" mechanism, the gate opens because of electrostatic repulsion between four simultaneously biprotonated histidines. This allows for proton transport along the water wire that penetrates the gate. For each system, composed of the channel embedded in a hydrated phospholipid bilayer, a 1.3-ns trajectory was obtained. It is found that the states involved in the shuttle mechanism, which contain either single-protonated histidines or a mixture of single-protonated histidines plus one biprotonated residue, are stable during the simulations. Furthermore, the orientations and dynamics of water molecules near the gate are conducive to proton transfer. In contrast, the fully biprotonated state is not stable. Additional simulations show that if only two histidines are biprotonated, the channel deforms but the gate remains closed. These results support the shuttle mechanism but not the gate-opening mechanism of proton gating in M(2).

Effect of cytoplasmic tail truncations on the activity of the M(2) ion channel of influenza A virus

The M(2) protein of influenza A virus forms a proton channel that is required for viral replication. The M(2) ion channel is a homotetramer and has a 24-residue N-terminal extracellular domain, a 19-residue transmembrane domain, and a 54-residue cytoplasmic tail. We show here that the N-terminal methionine residue is cleaved from the mature protein. Translational stop codons were introduced into the M(2) cDNA at residues 46, 52, 62, 72, 77, 82, 87, and 92. The deletion mutants were designated truncx, according to the amino acid position that was changed to a stop codon. We studied the role of the cytoplasmic tail by measuring the ion channel activity (the current sensitive to the M(2)-specific inhibitor amantadine) of the cytoplasmic tail truncation mutants expressed in oocytes of Xenopus laevis. When their conductance was measured over time, mutants trunc72, trunc77, and trunc92 behaved comparably to wild-type M(2) protein (a decrease of only 4% over 30 min). In contrast, conductance decreased by 28% for trunc82, 27% for trunc62, and 81% for trunc52 channels. Complete closure of the channel could be observed in some cells for trunc62 and trunc52 within 30 min. These data suggest that a role of the cytoplasmic tail region of the M(2) ion channel is to stabilize the pore against premature closure while the ectodomain is exposed to low pH.

Total chemical synthesis of the integral membrane protein influenza A virus M2: role of its C-terminal domain in tetramer assembly

The M2 protein from influenza A virus is a 97-residue homotetrameric membrane protein that functions as a proton channel. To determine the features required for the assembly of this protein into its native tetrameric state, the protein was prepared by total synthesis using native chemical ligation of unprotected peptide segments. Circular dichroism spectroscopy of synthetic M2 protein in dodecylphosphocholine (DPC) micelles indicated that approximately 40 residues were in an alpha-helical secondary structure. The tetramerization of the full-length protein was compared to that of a 25-residue transmembrane (TM) fragment. Analytical ultracentrifugation demonstrated that both the peptide and the full-length protein in DPC micelles existed in a monomer-tetramer equilibrium. Comparison of the association constants for the two sequences showed the free energy of tetramerization of the full-length protein was more favorable by approximately 7 kcal/mol. Partial proteolysis of DPC-solubilized M2 was used as a further probe of the structure of the full-length protein. A 15-20-residue segment C-terminal to the membrane-spanning region was found to be highly resistant to digestion by chymotrypsin and trypsin. This region, which we have modeled as an extension of the TM helices, may help to stabilize the tetrameric assembly.

Chemotherapy of respiratory viruses: prospects and challenges

Billions of people are infected with respiratory viruses annually. Infants and young children, the elderly, immunocompromised individuals and those debilitated by other diseases or nutritional deficiencies are most at risk for serious disease. There are few vaccines available for use against these viruses, and even where there are (influenza, measles and adenovirus), infections remain common. The continued prevalence of respiratory virus infections has lead to renewed efforts to find safe agents effective against the most medically important respiratory viruses: influenza, respiratory syncytial, parainfluenza, measles, rhino- and adenovirus. Copyright 1999 Harcourt Publishers Ltd.

Modified M2 proteins produce heterotypic immunity against influenza A virus

Vaccination with the influenza A transmembrane protein M2 provides enhanced viral clearance and recovery from influenza A virus infection in mice. However, the high degree of hydrophobicity of the protein limits its purification for vaccine purposes. We have attempted to alter the structure of the M2 protein to allow high level recombinant expression in Escherichia coli, to reduce its hydrophobicity and improve protein solubility, thus improving its properties as a vaccine subunit candidate. Constructs investigated include deletion of the transmembrane domain of M2 (residues 26-43) and an extended deletion (residues 26-55). A full-length M2 protein was not pursued because of poor expression, even in the presence of amantadine. Expressed as glutathione S-transferase fusion proteins and used to vaccinate mice, either deletion construct was found to raise M2-specific serum antibodies and enhance viral clearance in mice challenged with homologous and heterologous influenza A viruses. Enzymatic cleavage from the GST fusion domain produces soluble protein giving similar results. The results demonstrate that large alterations of M2 protein structure can improve its isolation and purification characteristics without detracting from its immunogenic properties.

Cu(II) inhibition of the proton translocation machinery of the influenza A virus M2 protein

The homotetrameric M2 integral membrane protein of influenza virus forms a proton-selective ion channel. An essential histidine residue (His-37) in the M2 transmembrane domain is believed to play an important role in the conduction mechanism of this channel. Also, this residue is believed to form hydrogen-bonded interactions with the ammonium group of the anti-viral compound, amantadine. A molecular model of this channel suggests that the imidazole side chains of His-37 from symmetry-related monomers of the homotetrameric pore converge to form a coordination site for transition metals. Thus, membrane currents of oocytes of Xenopus laevis expressing the M2 protein were recorded when the solution bathing the oocytes contained various transition metals. Membrane currents were strongly and reversibly inhibited by Cu2+ with biphasic reaction kinetics. The biphasic inhibition curves may be explained by a two-site model involving a fast-binding peripheral site with low specificity for divalent metal ions, as well as a high affinity site (Kdiss approximately 2 microM) that lies deep within the pore and shows rather slow-binding kinetics (kon = 18.6 +/- 0.9 M-1 s-1). The pH dependence of the interaction with the high affinity Cu2+-binding site parallels the pH dependence of inhibition by amantadine, which has previously been ascribed to protonation of His-37. The voltage dependence of the inhibition at the high affinity site indicates that the binding site lies within the transmembrane region of the pore. Furthermore, the inhibition by Cu2+ could be prevented by prior application of the reversible blocker of M2 channel activity, BL-1743, providing further support for the location of the site within the pore region of M2. Finally, substitutions of His-37 by alanine or glycine eliminated the high affinity site and resulted in membrane currents that were only partially inhibited at millimolar concentrations of Cu2+. Binding of Cu2+ to the high affinity site resulted in an approximately equal inhibition of both inward and outward currents. The wild-type protein showed very high specificity for Cu2+ and was only partially inhibited by 1 mM Ni2+, Pt2+, and Zn2+. These data are discussed in terms of the functional role of His-37 in the mechanism of proton translocation through the channel.

Experimentally based orientational refinement of membrane protein models: A structure for the Influenza A M2 H+ channel

The 97-residue M2 protein from Influenza A virus forms H+-selective ion channels which can be attributed solely to the homo-tetrameric alpha-helical transmembrane domain. Site-directed infrared dichroism spectra were obtained for the transmembrane domain of M2, reconstituted in lipid vesicles. Data analysis yielded the helix tilt angle beta=31.6(+/-6.2) degrees and the rotational pitch angle about the helix axis for residue Ala29 omegaAla29=-59.8(+/-9.9) degrees, whereby omega is defined as zero for a residue located in the direction of the helix tilt. A structure was obtained from an exhaustive molecular dynamics global search protocol in which the orientational data are utilised directly as an unbiased refinement energy term. Orientational refinement not only allowed selection of a unique structure but could also be shown to increase the convergence towards that structure during the molecular dynamics procedure. Encouragingly, the structure obtained is highly consistent with all available mutagenesis and conductivity data and offers a direct chemical insight that relates the altered functionality of the channel to its structure.

Two models of the influenza A M2 channel domain: verification by comparison

BACKGROUND

The influenza M2 protein is a simple membrane protein, containing a single transmembrane helix. It is representative of a very large family of single-transmembrane helix proteins. The functional protein is a tetramer, with the four transmembrane helices forming a proton-permeable channel across the bilayer. Two independently derived models of the M2 channel domain are compared, in order to assess the success of applying molecular modelling approaches to simple membrane proteins.

RESULTS

The Calpha RSMD between the two models is 1.7 A. Both models are composed of a left-handed bundle of helices, with the helices tilted roughly 15 degrees relative to the (presumed) bilayer normal. The two models have similar pore radius profiles, with a pore cavity lined by the Ser31 and Gly34 residues and a pore constriction formed by the ring of His37 residues.

CONCLUSIONS

Independent studies of M2 have converged on the same structural model for the channel domain. This model is in agreement with solid state NMR data. In particular, both model and NMR data indicate that the M2 helices are tilted relative to the bilayer normal and form a left-handed bundle. Such convergence suggests that, at least for simple membrane proteins, restraints-directed modelling might yield plausible models worthy of further computational and experimental investigation.

Viral ion channels: molecular modeling and simulation

In a number of membrane-bound viruses, ion channels are formed by integral membrane proteins. These channel proteins include M2 from influenza A, NB from influenza B, and, possibly, Vpu from HIV-1. M2 is important in facilitating uncoating of the influenza A viral genome and is the target of amantadine, an anti-influenza drug. The biological roles of NB and Vpu are less certain. In all cases, the protein contains a single transmembrane alpha-helix close to its N-terminus. Channels can be formed by homo-oligomerization of these proteins, yielding bundles of transmembrane helices that span the membrane and surround a central ion-permeable pore. Molecular modeling may be used to integrate and interpret available experimental data concerning the structure of such transmembrane pores. This has proved successful for the M2 channel domain, where two independently derived models are in agreement with one another, and with solid-state nuclear magnetic resonance (NMR) data. Simulations based on channel models may yield insights into possible ion conduction and selectivity mechanisms.

A functionally defined model for the M2 proton channel of influenza A virus suggests a mechanism for its ion selectivity

The M2 protein from influenza A virus forms proton-selective channels that are essential to viral function and are the target of the drug amantadine. Cys scanning was used to generate a series of mutants with successive substitutions in the transmembrane segment of the protein, and the mutants were expressed in Xenopus laevis oocytes. The effect of the mutations on reversal potential, ion currents, and amantadine resistance were measured. Fourier analysis revealed a periodicity consistent with a four-stranded coiled coil or helical bundle. A three-dimensional model of this structure suggests a possible mechanism for the proton selectivity of the M2 channel of influenza virus.

Modeling transmembrane helical oligomers

Recently, methods for the analysis and design of water-soluble, oligomeric bundles of alpha helices, including coiled coils, have reached a high level of sophistication. These same methods may now be applied to transmembrane helical bundles. Studies of the transmembrane domains of glycophorin, phospholamban, and the M2 protein from influenza A virus exemplify this general approach.

The influenza A virus M2 channel: a molecular modeling and simulation study

The M2 protein of influenza virus forms ion channels activated by low pH which are proton permeable and play a key role in the life cycle of the virus. M2 is a 97-residue integral membrane protein containing a single transmembrane (TM) helix. M2 is present as disulfide-linked homotetramers. The TM domain of M2 has been modeled as a bundle of four parallel M2 helices. The helix bundle forms a left-handed supercoil surrounding a central pore. Residue H37 has been implicated in the mechanism of low-pH activation of the channel. Models generated with H37 in a fully deprotonated state exhibit a pore occluded by a ring of H37 side chains oriented toward the lumen of the pore. Models with H37 in a fully protonated state no longer exhibit such occlusion of the pore, as the H37 side chains adopt a more interfacial location. Extended molecular dynamics simulations with water molecules within and at the mouths of the pores support this distinction between the H37-deprotonated and H37-protonated models. These simulations suggest that only in the H37-protonated model is there a continuous column of water extending the entire length of the central pore. A mechanism for activation of M2 by low pH is presented in which the H37-deprotonated model corresponds to the "closed" form of the channel, while the H37-protonated model corresponds to the "open" form. A switch from the closed to the open form of the channel occurs if H37 is protonated midway through a simulation. The open channel is suggested to contain a wire of H-bonded water molecules which enables proton permeability.

The active oligomeric state of the minimalistic influenza virus M2 ion channel is a tetramer

The influenza A virus M2 integral membrane protein is an ion channel that permits protons to enter virus particles during uncoating of virions in endosomes and also modulates the pH of the trans-Golgi network in virus-infected cells. The M2 protein is a homo-oligomer of 97 residues, and analysis by chemical cross-linking and SDS/PAGE indicates M2 forms a tetramer. However, a higher order molecular form is sometimes observed and, thus, it is necessary to determine the active form of the molecule. This was done by studying the currents of oocytes that expressed mixtures of the wild-type M2 protein (epitope tagged) and the mutant protein M2-V27S, which is resistant to the inhibitor amantadine. The composition of mixed oligomers of the two proteins expressed at the plasma membrane of individual oocytes was quantified after antibody capture of the cell surface expressed molecules and it was found that the subunits mixed freely. When the ratio of wild-type to mutant protein subunits was 0. 85:0.15, the amantadine sensitivity was reduced to 50% and for a ratio of 0.71:0.29 to 20%. These results are consistent with the amantadine-resistant mutant being dominant and the oligomeric state being a tetramer.

Effect of M1 protein and low pH on nuclear transport of influenza virus ribonucleoproteins

Influenza virus enters its host cell by receptor-mediated endocytosis followed by acid-activated membrane fusion in endosomes. The viral ribonucleoprotein particles (vRNPs) delivered into the cytosol then dissociate from the matrix protein, M1, and from each other, after which they are individually imported into the nucleus via the nuclear pores. For some time, it has been believed that the low pH in endosomes may, in some way, trigger the capsid disassembly events necessary for nuclear transport. This report provides direct evidence that the association of M1 with vRNPs is sensitive to mildly acidic pH within the infected cell. Recombinant M1, expressed in cultured cells, was found to associate with vRNPs and inhibit their nuclear import. Brief acidification of the cytosolic compartment eliminated the interfering activity and allowed the incoming vRNPs to enter the nucleus. Newly assembled progeny M1-vRNP complexes in the cytosol of infected cells were also dissociated by brief acidification. Acidic pH was thus found to serve as a switch that allowed M1 to carry out its multiple functions in the uncoating, nuclear transport, and assembly of vRNPs.

Selective proton permeability and pH regulation of the influenza virus M2 channel expressed in mouse erythroleukaemia cells

1. The M2 protein of influenza A virus is implicated in transmembrane pH regulation during infection. Whole-cell patch clamp of mouse erythroleukaemia cells expressing the M2 protein in the surface membrane showed a conductance due to M2 which was specifically blocked by the anti-influenza drug rimantadine. 2. The ion selectivity of the rimantadine-sensitive current through M2 was determined. Reversal potentials were close to equilibrium potentials for transmembrane pH gradients and not to those for Na+, K+ or Cl- concentration gradients. M2 permeability to Na+ relative to H+ was estimated to be less than 6 x 10(-7). 3. The M2 conductance increased as external pH decreased below 8.5 and approached saturation at an external pH of 4, effects attributable to increased permeability due to increased driving potential and to activation by low external pH. Both activation and permeation could be described by interaction of protons with sites on M2, with apparent dissociation constants of approximately 0.1 microM and 1 microM, respectively, under physiological conditions. 4. The M2 protein can transfer protons selectively across membranes with the H+ electrochemical gradient, properties consistent with its role in modifying virion and trans-Golgi pH during virus infection.

Characterization of inhibition of M2 ion channel activity by BL-1743, an inhibitor of influenza A virus

The influenza A virus M2 integral membrane protein has ion channel activity that can be inhibited by the antiviral drug amantadine. Recently, a spirene-containing compound, BL-1743 (2-[3-azaspiro (5,5)undecanol]-2-imidazoline), that inhibits influenza virus growth was identified (S. Kurtz, G. Lao, K. M. Hahnenberger, C. Brooks, O. Gecha, K. Ingalls, K.-I. Numata, and M. Krystal, Antimicrob. Agents Chemother. 39:2204-2209, 1995). We have examined the ability of BL-1743 to inhibit the M2 ion channel when expressed in oocytes of Xenopus laevis. BL-1743 inhibition is complete as far as can be measured by electrophysiological methods and is reversible, with a reverse reaction rate constant of 4.0 x 10(-3) s(-1). In contrast, amantadine inhibition is irreversible within the time frame of the experiment. However, BL-1743 inhibition and amantadine inhibition have similar properties. The majority of isolated influenza viruses resistant to BL-1743 are also amantadine resistant. In addition, all known amino acid changes which result in amantadine resistance also confer BL-1743 resistance. However, one BL-1743-resistant virus isolated, designated M2-I35T, contained the change Ile-35-->Thr. This virus is >70-fold more resistant to BL-1743 and only 10-fold more resistant to amantadine than the wild-type virus. When the ion channel activity of M2-I35T was examined in oocytes, it was found that M2-I35T is BL-1743 resistant but is reversibly inhibited by amantadine. These findings suggest that these two drugs interact differently with the M2 protein transmembrane pore region.