Water Transport across the Voltage Sensor of Shaker K-Channels during Activation

Abstract

Voltage gated K-channels are formed by two well defined domains: the Pore Domain (PD), which is responsible for the K+ ions conduction process, and the Voltage Sensor Domain (VSD), which sense the transmembrane potential due to the presence of several charged moieties. VSD moves upon activation such that ∼4 net positive charges translocate across the membrane. The molecular details of such movement have been subject of intense controversy. We asked if part of the charges is hydrated at the resting and activated states, and if they change their hydration status during voltage activation. We measured the gating currents of a constitutively closed Shaker-V478W in macro patches of Xenopus oocytes in the presence of internal, external, or symmetric 2M Sucrose to reduce the water availability for eventual VSD hydration. We also determined the number of gating charges coupled to pore opening by measuring the channel open probability at negative potentials in the presence of 2M sucrose. Our results are consistent with the idea that some charged residues are hydrated when exposed to the cytosol at resting, dehydrate before translocation and rehydrate externally in the activated conformation. Sucrose also decreases the effective valence to ∼8e0. These suggest that water plays important roles in channel gating; i) by shaping the energetic landscape for VSD activation and ii) tuning the VSD/PD electromechanical coupling in voltage gated K-channels.ID-F is a MECESUP Fellow. We thank to Iniciativa Cientifica Milenio (P09-022-F) and Fondecyt 1120819.