The M2 protein from the influenza A virus, an acid-activated proton-selective channel, has been the subject of numerous conductance, structural, and computational studies. However, little is known at the atomic level about the heart of the functional mechanism for this tetrameric protein, a His37-Trp41 cluster. We report the structure of the M2 conductance domain (residues 22 to 62) in a lipid bilayer, which displays the defining features of the native protein that have not been attainable from structures solubilized by detergents. We propose that the tetrameric His37-Trp41 cluster guides protons through the channel by forming and breaking hydrogen bonds between adjacent pairs of histidines and through specific interactions of the histidines with the tryptophan gate. This mechanism explains the main observations on M2 proton conductance.
Solid-state nuclear magnetic resonance spectroscopy of uniformly aligned preparations of gramicidin A in lipid bilayers has been used to elucidate a high-resolution dimeric structure of the cation channel conformation solely on the basis of the amino acid sequence and 144 orientational constraints. This initial structure defines the helical pitch as single-stranded, fixes the number of residues per turn at six to seven, specifies the helix sense as right-handed, and identifies the hydrogen bonds. Refinement of this initial structure yields reasonable hydrogen-bonding distances with only minimal changes in the torsion angles.
The heart of the H ؉ conductance mechanism in the homotetrameric M2 H ؉ channel from influenza A is a set of four histidine side chains. Here, we show that protonation of the third of these imidazoles coincides with acid activation of this transmembrane channel and that, at physiological pH, the channel is closed by two imidazole-imidazolium dimers, each sharing a low-barrier hydrogen bond. This unique construct succeeds in distributing a pair of charges over four rings and many atoms in a low dielectric environment to minimize charge repulsion. These dimers form with identical pK as of 8.2 ؎ 0.2, suggesting cooperative H ؉ binding and clearly illustrating high H ؉ affinity for this channel. The protonation behavior of the histidine side chains has been characterized by using solid-state NMR spectroscopy on the M2 transmembrane domain in fully hydrated lipid bilayers where the tetrameric backbone structure is known. Furthermore, electrophysiological measurements of multichannel and single-channel experiments confirm that these protein constructs are functional.M2 channel ͉ proton channel ͉ solid-state NMR ͉ low-barrier hydrogen bond ͉ histidine ionization constants A histidine tetrad in the pore of the tetrameric M2 protein has long been associated with key channel features of H ϩ selectivity, pH activation, gating, inhibition, and the specific conductance mechanism. M2 protein from influenza A virus conducts protons into the viral core after endocytosis, which leads to the uncoating and release of genetic material into the cytoplasm after fusion of the viral coat with the endosomal wall (1, 2). Much is known about this system from its tetrameric state (2-4), the backbone structure of the transmembrane (TM) domain (5), and numerous electrophysiological (6, 7), biophysical (8-10), and modeling (11) studies that have cast a fascinating tale for this important influenza drug target and the only proton channel of its kind to be characterized in such detail. However, the specific role of His-37 in the tetrameric protein has not been elucidated. Here, we have characterized the pK a s associated with this cluster of four histidine residues in the hydrophobic interstices of the membrane. These pK a values have led us to substantial mechanistic conclusions.There are many lines of evidence, reviewed by Kelly et al. (6), that support the conclusion that M2 is responsible for viral acidification. In vivo ion conductance recordings have shown pH sensitive conductance resulting in rapid acidification of the Xenopus oocytes (12, 13) and mammalian cells (13-15) containing M2 protein.Preparations of purified M2 protein have also been used to show proton conductance in synthetic lipid bilayers (16,17). Singlechannel conductance measurements with membranes containing M2 protein give clear evidence that it is H ϩ conductance, not counterion conductance, that is observed. Furthermore, the channel conductance is unchanged by addition of an excess of NaCl (18). Conductance measurements for the isolated TM domain of M2 protein have als...
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