Mechanistic insight into the h(2)o/d (2)o isotope effect in the proton transport of the influenza virus m2 protein

Proton Transport Mechanism of M2 Proton Channel Studied by Laser-Induced pH Jump

The M2 proton transport channel of the influenza virus A is an important model system because it conducts protons with high selectivity and unidirectionally when activated at low pH, despite the relative simplicity of its structure. Although it has been studied extensively, the molecular details of the pH-dependent gating and proton conductance mechanisms are incompletely understood. We report direct observation of the M2 proton channel activation process using a laser-induced pH jump coupled with tryptophan fluorescence as a probe. Biphasic kinetics is observed, with the fast phase corresponding to the His37 protonation, and the slow phase associated with the subsequent conformation change. Unusually fast His37 protonation was observed (2.0 × 10¹⁰ M^-1 s^-1), implying the existence of proton collecting antennae for expedited proton transport. The conformation change (4 × 10³ s^-1) was about 2 orders of magnitude slower than protonation at endosomal pH, suggesting that a transporter model is likely not feasible.

Effect of cytosolic pH on inward currents reveals structural characteristics of the proton transport cycle in the influenza A protein M2 in cell-free membrane patches of Xenopus oocytes

Transport activity through the mutant D44A of the M2 proton channel from influenza virus A was measured in excised inside-out macro-patches of Xenopus laevis oocytes at cytosolic pH values of 5.5, 7.5 and 8.2. The current-voltage relationships reveal some peculiarities: 1. "Transinhibition", i.e., instead of an increase of unidirectional outward current with increasing cytosolic H(+) concentration, a decrease of unidirectional inward current was found. 2. Strong inward rectification. 3. Exponential rise of current with negative potentials. In order to interpret these findings in molecular terms, different kinetic models have been tested. The transinhibition basically results from a strong binding of H(+) to a site in the pore, presumably His37. This assumption alone already provides inward rectification and exponential rise of the IV curves. However, it results in poor global fits of the IV curves, i.e., good fits were only obtained for cytosolic pH of 8.2, but not for 7.5. Assuming an additional transport step as e.g. caused by a constriction zone at Val27 resulted in a negligible improvement. In contrast, good global fits for cytosolic pH of 7.5 and 8.2 were immediately obtained with a cyclic model. A "recycling step" implies that the protein undergoes conformational changes (assigned to Trp41 and Val27) during transport which have to be reset before the next proton can be transported. The global fit failed at the low currents at pHcyt = 5.5, as expected from the interference of putative transport of other ions besides H(+). Alternatively, a regulatory effect of acidic cytosolic pH may be assumed which strongly modifies the rate constants of the transport cycle.

Philosophy of voltage-gated proton channels

In this review, voltage-gated proton channels are considered from a mainly teleological perspective. Why do proton channels exist? What good are they? Why did they go to such lengths to develop several unique hallmark properties such as extreme selectivity and ΔpH-dependent gating? Why is their current so minuscule? How do they manage to be so selective? What is the basis for our belief that they conduct H(+) and not OH(-)? Why do they exist in many species as dimers when the monomeric form seems to work quite well? It is hoped that pondering these questions will provide an introduction to these channels and a way to logically organize their peculiar properties as well as to understand how they are able to carry out some of their better-established biological functions.

Theoretical frameworks for multiscale modeling and simulation

Biomolecular systems have been modeled at a variety of scales, ranging from explicit treatment of electrons and nuclei to continuum description of bulk deformation or velocity. Many challenges of interfacing between scales have been overcome. Multiple models at different scales have been used to study the same system or calculate the same property (e.g., channel conductance). Accurate modeling of biochemical processes under in vivo conditions and the bridging of molecular and subcellular scales will likely soon become reality.

Ab initio calculations and validation of the pH-dependent structures of the His37-Trp41 quartet, the heart of acid activation and proton conductance in the M2 protein of Influenza A virus

The M2 protein of Influenza A virus forms a homotetrameric proton channel activated by low pH. The His37-Trp41 quartet is the heart of acid activation and proton conductance, but the functional mechanism is still controversial. We carried out ab initio calculations to model the pH-dependent structures of the His37-Trp41 quartet. In our model at neutral pH, the four His37 residues are configured into a pair of dimers; in each dimer, a proton is shared between Nδ1 on one residue and Nε2 on the other, and, under the restraint of the backbone, the two imidazole rings are nearly parallel, in contrast to a perpendicular arrangement for a free imidazole-imidazolium dimer. Within each dimer the +1 charge is highly delocalized, contributing to its stabilization in a low dielectric environment. The Nδ1-H-Nε2 strong hydrogen bonds result in significantly downfield shifted Nδ1 and Nε2 chemical shifts (at 169.7 and 167.6 ppm, respectively), in good agreement with experiments. In our model at acidic pH (where the channel becomes activated), a third proton binds to an imidazole-imidazolium dimer; the imidazole rings rotate away (each by ~55°) from each other, destroying the dimer structure. The two imidazoliums are stabilized by hydrogen bonds with water molecules and a cation-π interaction with Trp41. The Raman spectra calculated for the His37-Trp41 quartet at neutral and acidic pH are in agreement with experiments. Our calculations support an activation and conductance mechanism in which a hydronium ion from the N-terminal side passes a proton to an imidazole-imidazolium dimer; when the Trp41 gate is open, relaying of a proton onto a water molecule from the C-terminal side then allows the imidazole-imidazolium dimer to reform and be ready for the next round of proton conductance.

Modeling the membrane environment has implications for membrane protein structure and function: influenza A M2 protein

The M2 protein, a proton channel, from Influenza A has been structurally characterized by X-ray diffraction and by solution and solid-state NMR spectroscopy in a variety of membrane mimetic environments. These structures show substantial backbone differences even though they all present a left-handed tetrameric helical bundle for the transmembrane domain. Variations in the helix tilt influence drug binding and the chemistry of the histidine tetrad responsible for acid activation, proton selectivity and transport. Some of the major structural differences do not arise from the lack of precision, but instead can be traced to the influences of the membrane mimetic environments. The structure in lipid bilayers displays unique chemistry for the histidine tetrad, which binds two protons cooperatively to form a pair of imidazole-imidazolium dimers. The resulting interhistidine hydrogen bonds contribute to a three orders of magnitude enhancement in tetramer stability. Integration with computation has provided detailed understanding of the functional mechanism for proton selectivity, conductance and gating of this important drug target.

M2 protein from influenza A: from multiple structures to biophysical and functional insights

The M2 protein from influenza A is a proton channel as a tetramer, with a single transmembrane helix from each monomer lining the pore. Val27 and Trp41 form gates at either end of the pore and His37 mediates the shuttling of protons across a central barrier between the N-terminal and C-terminal aqueous pore regions. Numerous structures of this transmembrane domain and of a longer construct that includes an amphipathic helix are now in the Protein Data Bank. Many structural differences are apparent from samples obtained in a variety of membrane mimetic environments. High-resolution structural results in lipid bilayers have provided novel insights into the functional mechanism of the unique HxxxW cluster in the M2 proton channel.

Equivalence of two approaches for modeling ion permeation through a transmembrane channel with an internal binding site

Ion permeation through transmembrane channels has traditionally been modeled using two different approaches. In one approach, the translocation of the permeant ion through the channel pore is modeled as continuous diffusion and the rate of ion transport is obtained from solving the steady-state diffusion equation. In the other approach, the translocation of the permeant ion through the pore is modeled as hopping along a discrete set of internal binding sites and the rate of ion transport is obtained from solving a set of steady-state rate equations. In a recent work [Zhou, J. Phys. Chem. Lett. 1, 1973 (2010)], the rate constants for binding to an internal site were further calculated by modeling binding as diffusion-influenced reactions. That work provided the foundation for bridging the two approaches. Here we show that, by representing a binding site as an energy well, the two approaches indeed give the same result for the rate of ion transport.