Ion Conduction Mechanism of a Viral Proton Channel from Solid-State NMR

Abstract

Mei Hong, Department of Chemistry, Iowa State University, Ames, IAThe M2 protein of the influenza virus forms a proton-selective channel that is important for the lifecycle of the virus. Using high-resolution magic-angle-spinning (MAS) solid-state NMR spectroscopy, we have obtained rich information on the proton-conduction mechanism in this prototypical ion channel. The proton-selective residue, a histidine in the transmembrane (TM) domain of M2, undergoes microsecond reorientations, at the same time exchanging protons with water molecules. The rate and energy barrier of the reorientation motion and the kinetics of proton exchange are quantified through measurements of 15N exchange NMR spectra, motionally averaged nuclear-spin couplings, and 1H chemical shifts. The pH-dependent charged state and the rotameric structure of the histidine rings were determined from chemical shifts and internuclear distances. Aromatic interactions between the histidine and an adjacent tryptophan residue provide novel insight into the two-orders-of-magnitude difference between the histidine-water proton exchange rate and the actual proton flux into the virion. Comparison of the 15N spectra of wild-type M2 with a mutant with higher proton conductance revealed that histidine - water proton exchange not only involved transformation of the neutral imidazole to the cationic imidazolium, but also involved transitions between the two neutral tautomeric states of histidine, indicating that quantum-mechanical tunneling is a second mechanism for proton relay. Mechanistic insights obtained from the TM domain of M2 are extended to influenza B virus M2 and to a longer construct of the protein to investigate the influence the extra-membrane domains on proton conduction.Acknowledgements: NIH grant GM088204.