Abstract
The influenza M2 protein forms a tetrameric proton channel that conducts protons from the acidic endosome into the virion by shuttling protons between water and a transmembrane histidine. Previous NMR studies have shown that this histidine protonates and deprotonates on the microsecond time scale. However, M2's proton conduction rate is 10-1000 s(-1), more than 2 orders of magnitude slower than the histidine-water proton-exchange rate. M2 is also known to be conformationally plastic. To address the disparity between the functional time scale and the time scales of protein conformational dynamics and water dynamics, we have now investigated a W41F mutant of the M2 transmembrane domain using solid-state NMR. (13)C chemical shifts of the membrane-bound peptide indicate the presence of two distinct tetramer conformations, whose concentrations depend exclusively on pH and hence the charge-state distribution of the tetramers. High-temperature 2D correlation spectra indicate that these two conformations interconvert at a rate of ∼400 s(-1) when the +2 and +3 charge states dominate, which gives the first experimental evidence of protein conformational motion on the transport time scale. Protein (13)C-detected water (1)H T(2) relaxation measurements show that channel water relaxes an order of magnitude faster than bulk water and membrane-associated water, indicating that channel water undergoes nanosecond motion in a pH-independent fashion. These results connect motions on three time scales to explain M2's proton-conduction mechanism: picosecond-to-nanosecond motions of water molecules facilitate proton Grotthuss hopping, microsecond motions of the histidine side chain allow water-histidine proton transfer, while millisecond motions of the entire four-helix bundle constitute the rate-limiting step, dictating the number of protons released into the virion.