M2 Proton Channel Of Influenza Research Articles

The Interplay of Functional Tuning, Drug Resistance, and Thermodynamic Stability in the Evolution of the M2 Proton Channel from the Influenza A Virus

We explore the interplay between amino acid sequence, thermodynamic stability, and functional fitness in the M2 proton channel of influenza A virus. Electrophysiological measurements show that drug-resistant mutations have minimal effects on M2's specific activity, and suggest that resistance is achieved by altering a binding site within the pore rather than a less direct allosteric mechanism. In parallel, we measure the effects of these mutations on the free energy of assembling the homotetrameric transmembrane pore from monomeric helices in micelles and bilayers. Although there is no simple correlation between the evolutionary fitness of the mutants and their stability, all variants formed more stable tetramers in bilayers, and the least-fit mutants showed the smallest increase in stability upon moving from a micelle to a bilayer environment. We speculate that the folding landscape of a micelle is rougher than that of a bilayer, and more accommodating of conformational variations in nonoptimized mutants.

Structure and mechanism of the M2 proton channel of influenza A virus

The integral membrane protein M2 of influenza virus forms pH-gated proton channels in the viral lipid envelope. The low pH of an endosome activates the M2 channel before haemagglutinin-mediated fusion. Conductance of protons acidifies the viral interior and thereby facilitates dissociation of the matrix protein from the viral nucleoproteins--a required process for unpacking of the viral genome. In addition to its role in release of viral nucleoproteins, M2 in the trans-Golgi network (TGN) membrane prevents premature conformational rearrangement of newly synthesized haemagglutinin during transport to the cell surface by equilibrating the pH of the TGN with that of the host cell cytoplasm. Inhibiting the proton conductance of M2 using the anti-viral drug amantadine or rimantadine inhibits viral replication. Here we present the structure of the tetrameric M2 channel in complex with rimantadine, determined by NMR. In the closed state, four tightly packed transmembrane helices define a narrow channel, in which a 'tryptophan gate' is locked by intermolecular interactions with aspartic acid. A carboxy-terminal, amphipathic helix oriented nearly perpendicular to the transmembrane helix forms an inward-facing base. Lowering the pH destabilizes the transmembrane helical packing and unlocks the gate, admitting water to conduct protons, whereas the C-terminal base remains intact, preventing dissociation of the tetramer. Rimantadine binds at four equivalent sites near the gate on the lipid-facing side of the channel and stabilizes the closed conformation of the pore. Drug-resistance mutations are predicted to counter the effect of drug binding by either increasing the hydrophilicity of the pore or weakening helix-helix packing, thus facilitating channel opening.

Solid-State 19F NMR Spectroscopy Reveals That Trp41 Participates in the Gating Mechanism of the M2 Proton Channel of Influenza A Virus

The integral membrane protein M2 of influenza A virus assembles as a tetrameric bundle to form a proton-conducting channel that is activated by low pH. The side chain of His37 in the transmembrane alpha-helix is known to play an important role in the pH activation of the proton channel. It has also been suggested that Trp41, which is located in an adjacent turn of the helix, forms part of the gating mechanism. Here, a synthetic 25-residue peptide containing the M2 transmembrane domain was labeled with 6F-Trp41 and studied in lipid membranes by solid-state 19F NMR. We monitored the pH-dependent differences in the 19F dipolar couplings and motionally narrowed chemical shift anisotropies of this 6F-Trp41 residue, and we discuss the pH activation mechanism of the H+ channel. At pH 8.0, the structural parameters implicate an inactivated state, while at pH 5.3 the tryptophan conformation represents the activated state. With the aid of COSMOS force field simulations, we have obtained new side-chain torsion angles for Trp41 in the inactivated state (chi1 = -100 degrees +/- 10 degrees , chi2 = +110 degrees +/- 10 degrees ), and we predict a most probable activated state with chi1 = -50 degrees +/- 10 degrees and chi2 = +115 degrees +/- 10 degrees . We have also validated the torsion angles of His37 in the inactivated state as chi1 = -175 degrees +/- 10 degrees and chi2 = -170 degrees +/- 10 degrees .

Influence of residue 44 on the activity of the M2 proton channel of influenza A virus

The influenza A virus M2 proton channel plays a role in two stages of virus replication. The proteins of two closely related strains of the avian H7 subtype of influenza A virus, Rostock and Weybridge, were found to differ in their pH-modulating activities and activation characteristics. Of three amino acid differences at residues 27, 38 and 44 within the membrane-spanning domain, substitution at residue 44 was necessary and sufficient to account for differences in trans-Golgi pH-modulating activity, whereas changes in all three were required to switch the activation characteristics of the Weybridge M2 to those of the Rostock M2. These results not only separate the two phenomena genetically, but also indicate that the 'unique' activation characteristics of the Rostock M2 channel were selected specifically. In addition, they point to the importance of functional complementarity between the activation characteristics of the M2 channel and the pH of membrane fusion by haemagglutinin during virus entry.

Site‒specific IR spectroscopy and molecular modelling combined towards solving transmembrane protein structure

Membrane protein structures are underrepresented in structural databases despite their abundance and biomedical importance. This review focuses on the novel method of site-specific infrared dichroism (SSID) combined with constraint molecular dynamics simulation, which has recently emerged as a powerful method to obtain structures of transmembrane α-helical bundles. The theory of SSID including its latest developments is reviewed with the aim to encourage widespread application of this method. This is followed by an outline of the conformational search using experimentally constraint molecular dynamics simulations. Finally a critical evaluation of recent applications, namely the Influenza M2 proton channel, the vpu ion channel of HIV-1 and the MHC-class II associated invariant chain, is conducted.

The tryptophan 41 is not essential for the function of the M2 proton channel of influenza A virus

The influenza A virus M2 protein as a proton-selective ion channel plays an important role in two steps of virus replication. Co-expression of the M2 protein with pH-sensitive HA reduces the conversion of HA to its low pH conformation during transport to the cell surface. Mutational analyses showed that replacement of Histidine 37 (His37) by Trp, Phe and Gln resulted in loss of activity. M2, with the mutation His37Ala, remained active but its severe toxicity is intolerable for cells. Whereas substitution of Ala for Trp 41 abolished detectable activity of the M2 channel, substitution of Leu resulted in a functional ion channel but with reduced activity. The activity of the Trp41Leu mutant M2, initially identified in a mutant virus, shows that the proposed role of tryptophan 41 in activation is not essential for functional activity.

Studies of structural changes in the M2 proton channel of influenza A virus by tryptophan fluorescence

Studies of tryptophan fluorescence of purified influenza A virus M2 protein have identified two pH-dependent structural changes. (1) An increase in fluorescence on reduction of pH from 8 to 6 that involves tryptophan 15 within the N-terminal domain of the protein and may be associated with proton activation of the channel. (2) Quenching of the fluorescence of tryptophan 41 within the transmembrane domain of the channel by histidine 37 below pH 6 which is specifically reversed by drugs that block the M2 channel. The pH dependence of the latter effect, which monitors changes in the protonation of histidine 37, corresponds to that of proton current through the channel and provides evidence for the involvement of histidine 37 in proton permeation.

A model for the cytoplasmic domain of the influenza A virus M2 channel by analogy to the HIV-1 Vpu protein.

A model for the cytoplasmic domain of the M2 proton channel of influenza A virus was formulated based primarily on the cytoplasmic domain of the Vpu protein of HIV-1 using sequence similarity and structure prediction techniques. The model consists of a pair of antiparallel helices followed by a strand parallel to the first helix. Structural analogies with other proteins contribute support for features of the model and suggest ways in which the M2 cytoplasmic domain can interact with other viral and cellular factors.

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.

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.