Molecular Dynamics Studies of Ion Permeation in Human Voltage-Gated Proton Channel

Abstract

The human voltage-gated proton channel (hHv1) is a transmembrane protein responsible for selective proton permeation across cell membranes in nasal mucosa, sperm, and white blood cells. Its pathological states include male infertility, allergies, and diseases such as cystic fibrosis, asthma, and lupus. Its involvement in ischemic stroke and invasiveness of breast cancer cells has substantiated hHv1 as a therapeutic target for drug designs for which require the understanding of hHv1 structure and proton conduction mechanism. We recently constructed and validated a homology model (Kulleperuma et al., 2013) of hHv1 characterized by the presence of a salt bridge between anionic D112 and cationic R208 side chains in the narrow region of the hydrated pore. Thanks to the pairing of these and other charged residues in ionic networks, the distribution of charged and polar residues in the wild-type channel is a priori compatible with permeation of either cations or anions. However, a static-field electrostatic barrier opposing cation movement arises in neutral mutants of residue D112, consistent with the observation that these mutants are selective to anions (Musset et al., 2011). Our recent experiments show that proton selectivity is restored in double mutant D112V/V116D, while D112S and D112V/D116S are anion-selective and D112V does not conduct ions. Atomistic molecular dynamics simulations of these mutants in lipid bilayers show that their structures differ in the organization of ionic side chains in the external vestibule and suggest that, consistent with the above analysis, the distribution of charged groups plays a role in modulating the selectivity of the pore for either anions or protons. To elucidate the molecular mechanism of permeation and selectivity, we are conducting free energy simulations for the translocation of protons and other ions in both wild-type and mutant forms of hHv1.