Solid-State NMR Investigation of the Conformation, Proton Conduction, and Hydration of the Influenza B Virus M2 Transmembrane Proton Channel

Together with the influenza A virus, influenza B virus causes seasonal flu epidemics. The M2 protein of influenza B (BM2) forms a tetrameric proton-conducting channel that is important for the virus lifecycle. BM2 shares little sequence homology with AM2, except for a conserved HxxxW motif in the transmembrane (TM) domain. Unlike AM2, no antiviral drugs have been developed to block the BM2 channel. To elucidate the proton-conduction mechanism of BM2 and to facilitate the development of BM2 inhibitors, we have employed solid-state NMR spectroscopy to investigate the conformation, dynamics, and hydration of the BM2 TM domain in lipid bilayers. BM2 adopts an a-helical conformation in lipid membranes. At physiological temperature and low pH, the proton-selective residue, His19, shows relatively narrow N-15 chemical exchange peaks for the imidazole nitrogens, indicating fast proton shuttling that interconverts cationic and neutral histidines. Importantly, pH-dependent 15N chemical shifts indicate that His19 retains the neutral population to much lower pH than His37 in AM2, indicating larger acid-dissociation constants or lower pK(a)s. We attribute these dynamical and equilibrium differences to the presence of a second titratable histidine, His27, which may increase the proton-dissociation rate of His19. Two-dimensional H-1-C-13 correlation spectra probing water H-1 polarization transfer to the peptide indicates that the BM2 channel becomes much more hydrated at low pH than at high pH, particularly at Ser12, indicating that the pore-facing serine residues in BM2 mediate proton relay to the proton-selective histidine.