Effect of Ionization State on Voltage-Sensor Structure in Resting State of the Hv1 Channel

Voltage-gated
proton-selective channels (Hv1) mediate proton extrusion
during intracellular acidification. Hv1 is gated by the proton electrochemical
gradient. Intracellular ionizable residues in Hv1 have been proposed
to serve as proton-binding sites for pH-dependent gating, but detailed
descriptions remain unclear. Here, molecular dynamics (MD) simulations
were performed to investigate the effect of ionization states of charged
residues on the X-ray structure of Hv1. Modification of the protonation
state of acidic residues affected the resting conformation of Hv1
by disrupting salt bridges between S4 and the other segments. Upon
protonation, conformational changes enabled the displacement of the
S4 arginines toward the extracellular side and increased the mobility
of hydrophobic residues at the gate. The aqueous crevice was considerably
wider with increased hydration in the pore. Solvation free energies
of the pore residues were low at the extra- and intracellular entrances,
whereas the narrowest region exhibited the energy barrier for water
translocation. Our MD data showed that water molecules in the upper
and lower pore oriented differently. In neutral pH, the pore water
oriented its dipole pointing away from the voltage-sensing domain
center, whereas the opposite direction of the water dipole was observed
in acidic pH.