Solid-state 19F NMR spectroscopy reveals that Trp41 participates in the gating mechanism of the M2 proton channel of influenza A virus

Permeating disciplines: Overcoming barriers between molecular simulations and classical structure-function approaches in biological ion transport

Ion translocation across biological barriers is a fundamental requirement for life. In many cases, controlling this process-for example with neuroactive drugs-demands an understanding of rapid and reversible structural changes in membrane-embedded proteins, including ion channels and transporters. Classical approaches to electrophysiology and structural biology have provided valuable insights into several such proteins over macroscopic, often discontinuous scales of space and time. Integrating these observations into meaningful mechanistic models now relies increasingly on computational methods, particularly molecular dynamics simulations, while surfacing important challenges in data management and conceptual alignment. Here, we seek to provide contemporary context, concrete examples, and a look to the future for bridging disciplinary gaps in biological ion transport. This article is part of a Special Issue entitled: Beyond the Structure-Function Horizon of Membrane Proteins edited by Ute Hellmich, Rupak Doshi and Benjamin McIlwain.

Decoupled side chain and backbone dynamics for proton translocation - M2 of influenza A

The 97 amino acid bitopic membrane protein M2 of influenza A forms a tetrameric bundle in which two of the monomers are covalently linked via a cysteine bridge. In its tetrameric assembly the protein conducts protons across the viral envelope and within intracellular compartments during the infectivity cycle of the virus. A key residue in the translocation of the protons is His-37 which forms a planar tetrad in the configuration of the bundle accepting and translocating the incoming protons from the N terminal side, exterior of the virus, to the C terminal side, inside the virus. With experimentally available data from NMR spectroscopy of the transmembrane domains of the tetrameric M2 bundle classical MD simulations are conducted with the protein bundle in different protonation stages in respect to His-37. A full correlation analysis (FCA) of the data sets with the His-37 tetrad either in a fully four times unprotonated or protonated state, assumed to mimic high and low pH in vivo, respectively, in both cases reveal asymmetric backbone dynamics. His-37 side chain rotation dynamics is increased at full protonation of the tetrad compared to the dynamics in the fully unprotonated state. The data suggest that proton translocation can be achieved by decoupled side chain or backbone dynamics. Graphical abstract Visualization of the tetrameric bundle of the transmembrane domains of M2 of influenza A after 200 ns of MD simulations (upper left). The four histidine residues 37 are either not protonated as in M2⁰ or fully protonated is in M2⁴⁺. The asymmetric dynamics of the backbones are shown after a full correlation analysis (FCA) in blue (lower left). The rotational dynamics of the χ2 dihedral angles of the histidines in M2⁰ (upper right) are less than those in M2⁴⁺ (lower right).

Beyond Structural Biology to Functional Biology: Solid-State NMR Experiments and Strategies for Understanding the M2 Proton Channel Conductance

In terms of structural biology, solid-state NMR experiments and strategies have been well established for resonance assignments, leading to the determination of three-dimensional structures of insoluble membrane proteins in their native-like environment. It is also known that NMR has the unique capabilities to characterize structure-function relationships of membrane-bound biological systems beyond structural biology. Here, we report on solid-state NMR experiments and strategies for extracting functional activities on a sub-millisecond time scale. Specifically, we use the His37-labeled full length M2 (M2FL) protein of the Influenza A virus embedded in synthetic lipid bilayers as an example to characterize the proton conduction mechanism and kinetics. The integral membrane M2 protein assembles as a tetrameric bundle to form a proton-conducting channel that is activated by low pH and is essential for the viral lifecycle. Our results present convincing evidence for the formation of imidazolium-imidazole hydrogen bonds in the His37 tetrad at low pH and that these hydrogen bonds have a low barrier that facilitates the proton conduction mechanism in the M2FL protein. Moreover, it has been possible to measure hydronium ion exchange between water and the protons in the His37 NH bonds based on chemical exchange spectroscopy with minimized spin diffusion. The results identify an exchange rate constant of ∼4000 s^-1 for pH 5.8 at -10 °C.

Structural Influences: Cholesterol, Drug, and Proton Binding to Full-Length Influenza A M2 Protein

The structure and functions of the M2 protein from Influenza A are sensitive to pH, cholesterol, and the antiinfluenza drug Amantadine. This is a tetrameric membrane protein of 97 amino-acid residues that has multiple functions, among them as a proton-selective channel and facilitator of viral budding, replacing the need for the ESCRT proteins that other viruses utilize. Here, various amino-acid-specific-labeled samples of the full-length protein were prepared and mixed, so that only interresidue (13)C-(13)C cross peaks between two differently labeled proteins representing interhelical interactions are observed. This channel is activated at slightly acidic pH values in the endosome when the His(37) residues in the middle of the transmembrane domain take on a +2 or +3 charged state. Changes observed here in interhelical distances in the N-terminus can be accounted for by modest structural changes, and no significant changes in structure were detected in the C-terminal portion of the channel upon activation of the channel. Amantadine, which blocks proton conductance by binding in the aqueous pore near the N-terminus, however, significantly modifies the tetrameric structure on the opposite side of the membrane. The interactions between the juxtamembrane amphipathic helix of one monomer and its neighboring monomer observed in the absence of drug are disrupted in its presence. However, the addition of cholesterol prevents this structural disruption. In fact, strong interactions are observed between cholesterol and residues in the amphipathic helix, accounting for cholesterol binding adjacent to a native palmitoylation site and near to an interhelix crevice that is typical of cholesterol binding sites. The resultant stabilization of the amphipathic helix deep in the bilayer interface facilitates the bilayer curvature that is essential for viral budding.

Infrared and fluorescence assessment of the hydration status of the tryptophan gate in the influenza A M2 proton channel

The M2 proton channel of the influenza A virus has been the subject of extensive studies because of its critical role in viral replication. As such, we now know a great deal about its mechanism of action, especially how it selects and conducts protons in an asymmetric fashion. The conductance of this channel is tuned to conduct protons at a relatively low biologically useful rate, which allows acidification of the viral interior of a virus entrapped within an endosome, but not so great as to cause toxicity to the infected host cell prior to packaging of the virus. The dynamic, structural and chemical features that give rise to this tuning are not fully understood. Herein, we use a tryptophan (Trp) analog, 5-cyanotryptophan, and various methods, including linear and nonlinear infrared spectroscopies, static and time-resolved fluorescence techniques, and molecular dynamics simulations, to site-specifically interrogate the structure and hydration dynamics of the Trp41 gate in the transmembrane domain of the M2 proton channel. Our results suggest that the Trp41 sidechain adopts the t90 rotamer, the χ2 dihedral angle of which undergoes an increase of approximately 35° upon changing the pH from 7.4 to 5.0. Furthermore, we find that Trp41 is situated in an environment lacking bulk-like water, and somewhat surprisingly, the water density and dynamics do not show a measurable difference between the high (7.4) and low (5.0) pH states. Since previous studies have shown that upon channel opening water flows into the cavity above the histidine tetrad (His37), the present finding thus provides evidence indicating that the lack of sufficient water molecules near Trp41 needed to establish a continuous hydrogen bonding network poses an additional energetic bottleneck for proton conduction.

Solid-state NMR methods for oriented membrane proteins

Oriented-sample solid-state NMR represents one of few experimental methods capable of characterising the membrane-bound conformation of proteins in the cell membrane. Since the technique was developed 25 years ago, the technique has been applied to study the structure of helix bundle membrane proteins and antimicrobial peptides, characterise protein-lipid interactions, and derive information on dynamics of the membrane anchoring of membrane proteins. We will review the major developments in various aspects of oriented-sample solid-state NMR, including sample-preparation methods, pulse sequences, theory required to interpret the experiments, perspectives for and guidelines to new experiments, and a number of representative applications.

Magic angle spinning NMR of viruses

Viruses, relatively simple pathogens, are able to replicate in many living organisms and to adapt to various environments. Conventional atomic-resolution structural biology techniques, X-ray crystallography and solution NMR spectroscopy provided abundant information on the structures of individual proteins and nucleic acids comprising viruses; however, viral assemblies are not amenable to analysis by these techniques because of their large size, insolubility, and inherent lack of long-range order. In this article, we review the recent advances in magic angle spinning NMR spectroscopy that enabled atomic-resolution analysis of structure and dynamics of large viral systems and give examples of several exciting case studies.

3D hydrophobic moment vectors as a tool to characterize the surface polarity of amphiphilic peptides

The interaction of membranes with peptides and proteins is largely determined by their amphiphilic character. Hydrophobic moments of helical segments are commonly derived from their two-dimensional helical wheel projections, and the same is true for β-sheets. However, to the best of our knowledge, there exists no method to describe structures in three dimensions or molecules with irregular shape. Here, we define the hydrophobic moment of a molecule as a vector in three dimensions by evaluating the surface distribution of all hydrophilic and lipophilic regions over any given shape. The electrostatic potential on the molecular surface is calculated based on the atomic point charges. The resulting hydrophobic moment vector is specific for the instantaneous conformation, and it takes into account all structural characteristics of the molecule, e.g., partial unfolding, bending, and side-chain torsion angles. Extended all-atom molecular dynamics simulations are then used to calculate the equilibrium hydrophobic moments for two antimicrobial peptides, gramicidin S and PGLa, under different conditions. We show that their effective hydrophobic moment vectors reflect the distribution of polar and nonpolar patches on the molecular surface and the calculated electrostatic surface potential. A comparison of simulations in solution and in lipid membranes shows how the peptides undergo internal conformational rearrangement upon binding to the bilayer surface. A good correlation with solid-state NMR data indicates that the hydrophobic moment vector can be used to predict the membrane binding geometry of peptides. This method is available as a web application on http://www.ibg.kit.edu/HM/.

Fast Atomic Charge Calculation for Implementation into a Polarizable Force Field and Application to an Ion Channel Protein

Polarization of atoms plays a substantial role in molecular interactions. Class I and II force fields mostly calculate with fixed atomic charges which can cause inadequate descriptions for highly charged molecules, for example, ion channels or metalloproteins. Changes in charge distributions can be included into molecular mechanics calculations by various methods. Here, we present a very fast computational quantum mechanical method, the Bond Polarization Theory (BPT). Atomic charges are obtained via a charge calculation method that depend on the 3D structure of the system in a similar way as atomic charges ofab initiocalculations. Different methods of population analysis and charge calculation methods and their dependence on the basis set were investigated. A refined parameterization yielded excellent correlation ofR=0.9967. The method was implemented in the force field COSMOS-NMR and applied to the histidine-tryptophan-complex of the transmembrane domain of the M2 protein channel of influenza A virus. Our calculations show that moderate changes of side chain torsion angleχ1and small variations ofχ2of Trp-41 are necessary to switch from the inactivated into the activated state; and a rough two-side jump model of His-37 is supported for proton gating in accordance with a flipping mechanism.

Proton release from the histidine-tetrad in the M2 channel of the influenza A virus

The activity of the M2 proton channel of the influenza A virus is controlled by pH. The tautomeric state and conformation of His37, a key residue in the M2 transmembrane four-helix bundle, controls the gating of the channel. Previously, we compared the energetics and dynamics of two alternative conformations of the doubly protonated state at neutral pH, namely, a 4-fold symmetric “histidine-box” and a 2-fold symmetric “dimer-of-dimers”, and proposed a multiconfiguration model for this charge state. Here, we elaborate this model by further studying configurations of the His37 tetrad in the triply protonated state and its subsequent deprotonation via quantum mechanics/molecular mechanics (QM/MM) molecular dynamics (MD) simulations, starting with the aforementioned configurations, to gain information about proton release in a viral membrane environment. Interestingly, the two configurations converge under acidic pH conditions. Protons can be transferred from one charged His37 to a neighboring water cluster at the C-terminal side of the channel when the Trp41 gate is open transiently. With limited backbone expansion, the free energy barrier for proton release to the viral interior at low pH is ∼6.5 kcal/mol in both models, which is much lower than at either neutral pH or for an isolated His37 cluster without a membrane environment. Our calculations also suggest that the M2 protein would seem to exclude the entrance of anions into the central channel through a special mechanism, due to the latter’s potential inhibitory effect on proton conduction.

Activation and proton transport mechanism in influenza A M2 channel

Molecular dynamics trajectories 2 μs in length have been generated for the pH-activated, tetrameric M2 proton channel of the influenza A virus in all protonation states of the pH sensor located at the His(37) tetrad. All simulated structures are in very good agreement with high-resolution structures. Changes in the channel caused by progressive protonation of His(37) provide insight into the mechanism of proton transport. The channel is closed at both His(37) and Trp(41) sites in the singly and doubly protonated states, but it opens at Trp(41) upon further protonation. Anions access the charged His(37) and by doing so stabilize the protonated states of the channel. The narrow opening at the His(37) site, further blocked by anions, is inconsistent with the water-wire mechanism of proton transport. Instead, conformational interconversions of His(37) correlated with hydrogen bonding to water molecules indicate that these residues shuttle protons in high-protonation states. Hydrogen bonds between charged and uncharged histidines are rare. The valve at Val(27) remains on average quite narrow in all protonation states but fluctuates sufficiently to support water and proton transport. A proton transport mechanism in which the channel, depending on pH, opens at either the histidine or valine gate is only partially supported by the simulations.

Proton affinity of the histidine-tryptophan cluster motif from the influenza A virus from ab initio molecular dynamics

Ab initio molecular dynamics calculations have been used to compare and contrast the deprotonation reaction of a histidine residue in aqueous solution with the situation arising in a histidine-tryptophan cluster. The latter is used as a model of the proton storage unit present in the pore of the M2 proton conducting ion channel. We compute potentials of mean force for the dissociation of a proton from the Nδ and Nε positions of the imidazole group to estimate the pKa's. Anticipating our results, we will see that the estimated pKa for the first protonation event of the M2 channel is in good agreement with experimental estimates. Surprisingly, despite the fact that the histidine is partially desolvated in the M2 channel, the affinity for protons is similar to that of a histidine in aqueous solution. Importantly, the electrostatic environment provided by the indoles is responsible for the stabilization of the charged imidazolium.

pH-dependent conformation, dynamics, and aromatic interaction of the gating tryptophan residue of the influenza M2 proton channel from solid-state NMR

The M2 protein of the influenza virus conducts protons into the virion under external acidic pH. The proton selectivity of the tetrameric channel is controlled by a single histidine (His(37)), whereas channel gating is accomplished by a single tryptophan (Trp(41)) in the transmembrane domain of the protein. Aromatic interaction between these two functional residues has been previously observed in Raman spectra, but atomic-resolution evidence for this interaction remains scarce. Here we use high-resolution solid-state NMR spectroscopy to determine the side-chain conformation and dynamics of Trp(41) in the M2 transmembrane peptide by measuring the Trp chemical shifts, His(37)-Trp(41) distances, and indole dynamics at high and low pH. The interatomic distances constrain the Trp41 side-chain conformation to trans for χ1 and 120-135° for χ2. This t90 rotamer points the Nε1-Cε2-Cζ2 side of the indole toward the aqueous pore. The precise χ1 and χ2 angles differ by ∼20° between high and low pH. These differences, together with the known changes in the helix tilt angle between high and low pH, push the imidazole and indole rings closer together at low pH. Moreover, the measured order parameters indicate that the indole rings undergo simultaneous χ1 and χ2 torsional fluctuations at acidic pH, but only restricted χ1 fluctuations at high pH. As a result, the Trp(41) side chain periodically experiences strong cation-π interactions with His(37) at low pH as the indole sweeps through its trajectory, whereas at high pH the indole ring is further away from the imidazole. These results provide the structural basis for understanding how the His(37)-water proton exchange rate measured by NMR is reduced to the small proton flux measured in biochemical experiments. The indole dynamics, together with the known motion of the imidazolium, indicate that this compact ion channel uses economical side-chain dynamics to regulate proton conduction and gating.

Simple and inexpensive incorporation of 19F-tryptophan for protein NMR spectroscopy

Fluorine-containing amino acids are valuable probes for the biophysical characterization of proteins. Current methods for (19)F-labeled protein production involve time-consuming genetic manipulation, compromised expression systems and expensive reagents. We show that Escherichia coli BL21, the workhorse of protein production, can utilise fluoroindole for the biosynthesis of proteins containing (19)F-tryptophan.

Design and synthesis of pinanamine derivatives as anti-influenza A M2 ion channel inhibitors

The adamantanes are a class of anti-influenza drugs that inhibit the M2 ion channel of the influenza A virus. However recently, the clinical effectiveness of these drugs has been called into question due to the emergence of adamantane-insensitive A/M2 mutants. Although we previously reported (1R,2R,3R,5S)-3-pinanamine 3 as a novel inhibitor of the wild type influenza A virus M2 protein (WT A/M2), limited inhibition was found for adamantane-resistant M2 mutants. In this study, we explored whether newly synthesized pinanamine derivatives were capable of inhibiting WT A/M2 and selected adamantane-resistant M2 mutants. Several imidazole and guanazole derivatives of pinanamine were found to inhibit WT A/M2 to a comparable degree as amantadine and one of these compounds 12 exhibits weak inhibition of A/M2-S31N mutant and it is marginally more effective in inhibiting S31NM2 than amantadine. This study provides a new insight into the structural nature of drugs required to inhibit WT A/M2 and its mutants.

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.

Voltage-dependent gating underlies loss of ENaC function in Pseudohypoaldosteronism type 1

Here we explore the mechanism and associated structure-function implications of loss of function for epithelial Na(+) channel (ENaC) containing a pseudohypoaldosteronism type 1 (PHA-1)-causing missense point mutation. As expected, human ENaC that contained subunits harboring PHA-1-causing substitutions within an absolutely conserved, cytosolic Gly residue (e.g., βG37S) had significantly less activity. Unexpectedly, though, such substitution also results in voltage sensitivity with greater activity at hyperpolarizing potentials. This is a consequence of voltage-dependent changes in the single-channel open probability and is not species- or subunit-dependent. Voltage sensitivity in PHA-1 mutants stems from the disruption of critical structure, rather than the development of new properties resulting from the introduction of novel side chains. Residues near the conserved His-Gly sequence of G95 in α-mENaC are particularly important for voltage sensing. Although substitution of I93 in α-mENaC results in voltage sensing, it also slows the activation and deactivation kinetics enough to enable capture of the dynamic changes in single-channel open probability that account for changes in macroscopic activity. This provides definitive proof of the mechanism that underlies loss of function. In addition, the voltage dependence of ENaC with PHA-1 substitutions is akin to that which results from substitution of a critical, interfacial Trp residue conserved at the intracellular base of TM1 (e.g., W112 in α-mENaC). Dynamic interactions between similarly positioned His and Trp residues are essential for gating and the girdle-like structure that lines the intracellular mouth of the M2 proton channel. The similar residues in ENaC may serve a shared function, suggesting the possibility of an intracellular girdle just below the mouth of the ENaC pore.

Asymmetric synthesis of sterically and electronically demanding linear ω-trifluoromethyl containing amino acids via alkylation of chiral equivalents of nucleophilic glycine and alanine

An operationally convenient, scalable asymmetric synthesis of linear, ω-trifluoromethyl-containing amino acids, which were not previously produced in their enantiomerically pure form, has been developed via alkylation of chiral equivalents of nucleophilic glycine and alanine. The simplicity of the experimental procedures and high stereochemical outcome (yields up to 90% and diastereoselectivity up to 99%) of the presented method render these fluorinated amino acids readily available for systematic medicinal chemistry studies and de novo peptide design.

INEPT-based separated-local-field NMR spectroscopy: a unique approach to elucidate side-chain dynamics of membrane-associated proteins

Despite recent advances in NMR approaches for structural biology, determination of membrane protein dynamics in its native environment continues to be a monumental challenge, as most NMR structural studies of membrane proteins are commonly carried out either in micelles or in vesicle systems under frozen conditions. To overcome this difficulty, we propose a solid-state NMR technique that allows for the determination of side-chain dynamics from membrane proteins in lipid bilayers. This new technique, namely dipolar enhanced polarization transfer (DREPT), allows for a wide range of dipolar couplings to be encoded, providing high resolution and sensitivity for systems that undergo motional averaging such as that of amino acid side chains. NMR observables such as dipolar couplings and chemical shift anisotropy, which are highly sensitive to molecular motions, provide a direct way of probing protein dynamics over a wide range of time scales. Therefore, using an appropriate model, it is possible to determine side-chain dynamics and provide additional information on the topology and function of a membrane protein in its native environment.

Structure and mechanism of proton transport through the transmembrane tetrameric M2 protein bundle of the influenza A virus

The M2 proton channel from influenza A virus is an essential protein that mediates transport of protons across the viral envelope. This protein has a single transmembrane helix, which tetramerizes into the active channel. At the heart of the conduction mechanism is the exchange of protons between the His37 imidazole moieties of M2 and waters confined to the M2 bundle interior. Protons are conducted as the total charge of the four His37 side chains passes through 2(+) and 3(+) with a pK(a) near 6. A 1.65 A resolution X-ray structure of the transmembrane protein (residues 25-46), crystallized at pH 6.5, reveals a pore that is lined by alternating layers of sidechains and well-ordered water clusters, which offer a pathway for proton conduction. The His37 residues form a box-like structure, bounded on either side by water clusters with well-ordered oxygen atoms at close distance. The conformation of the protein, which is intermediate between structures previously solved at higher and lower pH, suggests a mechanism by which conformational changes might facilitate asymmetric diffusion through the channel in the presence of a proton gradient. Moreover, protons diffusing through the channel need not be localized to a single His37 imidazole, but instead may be delocalized over the entire His-box and associated water clusters. Thus, the new crystal structure provides a possible unification of the discrete site versus continuum conduction models.

Computational study of drug binding to the membrane-bound tetrameric M2 peptide bundle from influenza A virus

The M2 protein of influenza A virus performs the crucial function of transporting protons to the interior of virions enclosed in the endosome. Adamantane drugs, amantadine (AMN) and rimantidine (RMN), block the proton conduction in some strains, and have been used for the treatment and prophylaxis of influenza A infections. The structures of the transmembrane (TM) region of M2 that have been solved in micelles using NMR (residues 23-60) (Schnell and Chou, 2008) and by X-ray crystallography (residues 22-46) (Stouffer et al., 2008) suggest different drug binding sites: external and internal for RMN and AMN, respectively. We have used molecular dynamics (MD) simulations to investigate the nature of the binding site and binding mode of adamantane drugs on the membrane-bound tetrameric M2-TM peptide bundles using as initial conformations the low-pH AMN-bound crystal structure, a high-pH model derived from the drug-free crystal structure, and the high-pH NMR structure. The MD simulations indicate that under both low- and high-pH conditions, AMN is stable inside the tetrameric bundle, spanning the region between residues Val27 to Gly34. At low pH the polar group of AMN is oriented toward the His37 gate, while under high-pH conditions its orientation exhibits large fluctuations. The present MD simulations also suggest that AMN and RMN molecules do not show strong affinity to the external binding sites.

Comparative dissociation of peptide polyanions by electron impact and photo-induced electron detachment

We compare product-ion mass spectra produced by electron detachment dissociation (EDD) and electron photodetachment dissociation (EPD) of multi-deprotonated peptides on a Fourier transform and a linear ion trap mass spectrometer, respectively. Both methods, EDD and EPD, involve the electron emission-induced formation of a radical oxidized species from a multi-deprotonated precursor peptide. Product-ion mass spectra display mainly fragment ions resulting from backbone cleavages of C(alpha)-C bond ruptures yielding a and x ions. Fragment ions originating from N-C(alpha) backbone bond cleavages are also observed, in particular by EPD. Although EDD and EPD methods involve the generation of a charge-reduced radical anion intermediate by electron emission, the product ion abundance distributions are drastically different. Both processes seem to be triggered by the location and the recombination of radicals (both neutral and cation radicals). Therefore, EPD product ions are predominantly formed near tryptophan and histidine residues, whereas in EDD the negative charge solvation sites on the backbone seem to be the most favorable for the nearby bond dissociation.

Structure and function of the influenza A M2 proton channel

The M2 protein of influenza A viruses forms a tetrameric pH-activated proton-selective channel that is targeted by the amantadine class of antiviral drugs. Its ion channel function has been extensively studied by electrophysiology and mutagenesis; however, the molecular mechanism of proton transport is still elusive, and the mechanism of inhibition by amantadine is controversial. We review the functional data on proton channel activity, molecular dynamics simulations of the proton conduction mechanism, and high-resolution structural and dynamical information of this membrane protein in lipid bilayers and lipid-mimetic detergents. These studies indicate that elucidation of the structural basis of M2 channel activity and inhibition requires thorough examination of the complex dynamics and conformational plasticity of the protein in different lipid bilayers and lipid-mimetic environments.

Substrate-driven conformational changes in ClC-ec1 observed by fluorine NMR

The CLC 'Cl(-) channel' family consists of both Cl(-)/H(+) antiporters and Cl(-) channels. Although CLC channels can undergo large, conformational changes involving cooperativity between the two protein subunits, it has been hypothesized that conformational changes in the antiporters may be limited to small movements localized near the Cl(-) permeation pathway. However, to date few studies have directly addressed this issue, and therefore little is known about the molecular movements that underlie CLC-mediated antiport. The crystal structure of the Escherichia coli antiporter ClC-ec1 provides an invaluable molecular framework, but this static picture alone cannot depict the protein movements that must occur during ion transport. In this study we use fluorine nuclear magnetic resonance (NMR) to monitor substrate-induced conformational changes in ClC-ec1. Using mutational analysis, we show that substrate-dependent (19)F spectral changes reflect functionally relevant protein movement occurring at the ClC-ec1 dimer interface. Our results show that conformational change in CLC antiporters is not restricted to the Cl(-) permeation pathway and show the usefulness of (19)F NMR for studying conformational changes in membrane proteins of known structure.

The effect of the cosolvent trifluoroethanol on a tryptophan side chain orientation in the hydrophobic core of troponin C

The unique biophysical properties of tryptophan residues have been exploited for decades to monitor protein structure and dynamics using a variety of spectroscopic techniques, such as fluorescence and nuclear magnetic resonance (NMR). We recently designed a tryptophan mutant in the regulatory N-domain of cardiac troponin C (F77W-cNTnC) to study the domain orientation of troponin C in muscle fibers using solid-state NMR. In our previous study, we determined the NMR structure of calcium-saturated mutant F77W-V82A-cNTnC in the presence of 19% 2,2,2-trifluoroethanol (TFE). TFE is a widely used cosolvent in the biophysical characterization of the solution structures of peptides and proteins. It is generally assumed that the structures are unchanged in the presence of cosolvents at relatively low concentrations, and this has been verified for TFE at the level of the overall secondary and tertiary structure for several calcium regulatory proteins. Here, we present the NMR solution structure of the calcium saturated F77W-cNTnC in presence of its biological binding partner troponin I peptide (cTnI(144-163)) and in the absence of TFE. We have also characterized a panel of six F77W-cNTnC structures in the presence and absence TFE, cTnI(144-163), and the extra mutation V82A, and used (19)F NMR to characterize the effect of TFE on the F77(5fW) analog. Our results show that although TFE did not perturb the overall protein structure, TFE did induce a change in the orientation of the indole ring of the buried tryptophan side chain from the anticipated position based upon homology with other proteins, highlighting the potential dangers of the use of cosolvents.

Conformational heterogeneity of the M2 proton channel and a structural model for channel activation

The M2 protein of influenza virus A is a proton-selective ion channel activated by pH. Structure determination by solid-state and solution NMR and X-ray crystallography has contributed significantly to our understanding, but channel activation may involve conformations not captured by these studies. Indeed, solid-state NMR data demonstrate that the M2 protein possesses significant conformational heterogeneity. Here, we report molecular dynamics (MD) simulations of the M2 transmembrane domain (TMD) in the absence and presence of the antiviral drug amantadine. The ensembles of MD conformations for both apo and bound forms reproduced the NMR data well. The TMD helix was found to kink around Gly-34, where water molecules penetrated deeply into the backbone. The amantadine-bound form exhibited a single peak approximately 10 degrees in the distribution of helix-kink angle, but the apo form exhibited 2 peaks, approximately 0 degrees and 40 degrees . Conformations of the apo form with small and large kink angles had narrow and wide pores, respectively, around the primary gate formed by His-37 and Trp-41. We propose a structural model for channel activation, in which the small-kink conformations dominate before proton uptake by His-37 from the exterior, and proton uptake makes the large-kink conformations more favorable, thereby priming His-37 for proton release to the interior.

Intrinsic voltage dependence of the epithelial Na+ channel is masked by a conserved transmembrane domain tryptophan

Tryptophan residues critical to function are frequently located at the lipid-water interface of transmembrane domains. All members of the epithelial Na+ channel (ENaC)/Degenerin (Deg) channel superfamily contain an absolutely conserved Trp at the base of their first transmembrane domain. Here, we test the importance of this conserved Trp to ENaC/Deg function. Targeted substitution of this Trp in mouse ENaC and rat ASIC subunits decrease channel activity. Differential substitution with distinct amino acids in alpha-mENaC shows that it is loss of this critical Trp rather than introduction of residues having novel properties that changes channel activity. Surprisingly, Trp substitution unmasks voltage sensitivity. Mutant ENaC has increased steady-state activity at hyperpolarizing compared with depolarizing potentials associated with transient activation and deactivation times, respectively. The times of activation and deactivation change 1 ms/mV in a linear manner with rising and decreasing slopes, respectively. Increases in macroscopic currents at hyperpolarizing potentials results from a voltage-dependent increase in open probability. Voltage sensitivity is not influenced by divalent cations; however, it is Na+-dependent with a 63-mV decrease in voltage required to reach half-maximal activity per log increase in [Na+]. Mutant channels are particularly sensitive to intracellular [Na+] for removing this sodium abolishes voltage dependence. We conclude that the conserved Trp at the base of TM1 in ENaC/Deg channels protects against voltage by masking an inhibitory allosteric or pore block mechanism, which decreases activity in response to intracellular Na+.

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.

Molecular dynamics calculations suggest a conduction mechanism for the M2 proton channel from influenza A virus

The M2 protein of the influenza A virus is activated by low endosomal pH and performs the essential function of proton transfer into the viral interior. The resulting decrease in pH within the virion is essential for the uncoating and further replication of the viral genetic material. The x-ray crystal [Stouffer AL, et al. (2008) Nature 451:596-599] and solution NMR [Schnell JR, Chou JJ (2008) Nature 451:591-595] structures of the transmembrane region of the M2 homo-tetrameric bundle both revealed pores with narrow constrictions at one end, leaving a question as to how protons enter the channel. His-37, which is essential for proton-gating and selective conduction of protons, lies in the pore of the crystallographic and NMR structures. Here, we explore the different protonation states of the His-37 residues of the M2 bundle in a bilayer using molecular dynamics (MD) simulations. When the His-37 residues are neutral, the protein prefers an Open(out)-Closed(in) conformation in which the channel is open to the environment on the outside of the virus but closed to the interior environment of the virus. Diffusion of protons into the channel from the outside of the virus and protonation of His-37 residues in the tetramer stabilizes an oppositely gated Closed(out)-Open(in) conformation. Thus, protons might be conducted through a transporter-like mechanism, in which the protein alternates between Open(out)-Closed(in) and Closed(out)-Open(in) conformations, and His-37 is protonated/deprotonated during each turnover. The transporter-like mechanism is consistent with the known properties of the M2 bundle, including its relatively low rate of proton flux and its strong rectifying behavior.