Exploring the Conformational and Hydration Changes That Enable Acid Activation and Proton Transport in Influenza A M2 Channel

Laura Watkins, University of Chicago

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The influenza A M2 proton channel is an acid-activated, homo-tetrameric protein responsible for the acidification of the viral interior, a critical step in the viral life cycle. This channel has four central histidine residues forming an acid-activated gate, binding protons from the exterior until reaching an activated state that allows proton transport (PT) to the interior. Activation from the interior, however, is not allowed, a feature known as rectification. While much is known about the M2 channel in fixed protonation states, the changes that occur while an excess proton moves through the channel are not well understood. Similarly, previous work has resolved the free-energy profiles for PT through the channel, but the dynamic interactions between the proton, channel and internal water have not been resolved. In this study, extensive Multiscale Reactive Molecular Dynamics (MS-RMD) simulations with an explicit, reactive proton are used to explore detailed molecular-level interactions between the proton, the protein and the internal water as a function of the excess proton position as it moves through the channel. The results demonstrate how the excess proton strongly influences the local water structure and hydrogen-bonding network throughout the channel, and can dynamically, as a function of location, shift the protein structure away from its equilibrium distribution in fixed protonation states. We also find that the proton distribution is not symmetric about the membrane normal due to the channel's tilted axis, which has great implications for future drug design efforts. Finally, a detailed examination of the channel and water as the proton approaches and leaves the central histidine gate reveals how the water structure is optimally arranged for PT to the interior only after it reaches the +2 state and how the channel's rectification behavior can be understood through these water arrangements.

Abstract Author(s): Laura C. Watkins, Ruibin Liang, Jessica M.J. Swanson, Gregory A. Voth