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.