Water wires in atomistic models of the Hv1 proton channel

Abstract

The voltage-gated proton channel (Hv1) is homologous to the voltage-sensing domain (VSD) of voltage-gated potassium (Kv) channels but lacks a separate pore domain. The Hv1 monomer has dual functions: it gates the proton current and also serves as the proton conduction pathway. To gain insight into the structure and dynamics of the yet unresolved proton permeation pathway, we performed all-atom molecular dynamics simulations of two different Hv1 homology models in a lipid bilayer in excess water. The structure of the Kv1.2-Kv2.1 paddle-chimera VSD was used as template to generate both models, but they differ in the sequence alignment of the S4 segment. In both models, we observe a water wire that extends through the membrane, whereas the corresponding region is dry in simulations of the Kv1.2-Kv2.1 paddle-chimera. We find that the kinetic stability of the water wire is dependent upon the identity and location of the residues lining the permeation pathway, in particular, the S4 arginines. A measurement of water transport kinetics indicates that the water wire is a relatively static feature of the permeation pathway. Taken together, our results suggest that proton conduction in Hv1 may occur via Grotthuss hopping along a robust water wire, with exchange of water molecules between inner and outer ends of the permeation pathway minimized by specific water-protein interactions.

Hydrophobic plug functions as a gate in voltage-gated proton channels

“…The sequence similarity between Hv1 and the VSD of Kv1.2 is very limited, which makes it difficult to generate a homology model based only on sequence alignment, especially for the positioning of the three arginines in the S4 segment of Hv1 (10,11,22). Three other groups have proposed structural models of the open state of Hv1 channel.…”

“…Therefore, the continuous aqueous pathway in this model might be caused by the farther outward positioning of the S4 segment. Wood et al (11) suggested two different open-state models (R1-Hv1 and R2-Hv1) with different S4 alignments (Fig. S8 C and D).…”

“…This difference is defined as the coupling energy, indicating the strength of interactions between these two residues. Usually, a coupling energy of greater than 1.5 kT (∼0.89 kcal/mol) represents a significant interaction between two residues (19 (10,11,22). However, unambiguous alignment of the VSD of Kv channels and Hv1 channels remains challenging, especially for the S4 helix (23).…”

“…The removal of the C terminus generates monomeric Hv1 channels that still function as voltage-gated proton channels, indicating that the proton permeation pathway is intrinsic to a single VSD of the Hv1 channel. Ramsey et al (10) and Wood et al (11) constructed a homology model of the Hv1 channel in the open state. Molecular dynamics (MD) simulations of this model suggested a Grotthuss mechanism of the proton hopping along a water wire for proton conduction in Hv1 (10,11).…”

“…Ramsey et al (10) and Wood et al (11) constructed a homology model of the Hv1 channel in the open state. Molecular dynamics (MD) simulations of this model suggested a Grotthuss mechanism of the proton hopping along a water wire for proton conduction in Hv1 (10,11). Subsequently, a conserved and unique aspartate (Asp112 in hHv1) was identified as the selectivity filter for the permeation of protons in Hv1 channels (12).…”