Molecular mechanism of anion permeation through aquaporin 6

Aquaporins (AQPs) are recognized as transmembrane water channels that facilitate selective water permeation through their monomeric pores. Among the AQP family, AQP6 has an intriguing characteristic as an anion channel, which is allosterically controlled by pH conditions and is eliminated by a single amino acid mutation. However, the molecular mechanism of anion permeation through AQP6 remains unclear. Using molecular dynamics simulations in the presence of a transmembrane voltage utilizing an ion concentration gradient, we show that chloride ions permeate through the pore corresponding to the central axis of the AQP6 homotetramer. Under low pH conditions, a subtle opening of the hydrophobic selectivity filter (SF), located near the extracellular part of the central pore, becomes wetted and enables anion permeation. Our simulations also indicate that a single mutation (N63G) in human AQP6, located at the central pore, significantly reduces anion conduction, consistent with experimental data. Moreover, we demonstrate that the pH-sensing mechanism in which the protonation of H184 and H189 under low pH conditions allosterically triggers the gating of the SF region. These results suggest a unique pH-dependent allosteric anion permeation mechanism in AQP6 and could clarify the role of the central pore in some of the AQP tetramers. SIGNIFICANCE AQPs are generally known as tetrameric water channels, while some AQPs also facilitate the permeation of other solutes. Within the AQP family, AQP6 has been identified as an anion channel. However, the molecular mechanism for anion permeation remains elusive. Using molecular dynamics simulations, we demonstrate that chloride ions permeate through the central pore formed by the homotetrameric formation. Under low pH conditions, the protein conformation is allosterically modulated, resulting in a subtle opening and wetting of the hydrophobic selectivity filter within the central pore, thereby allowing anion permeation. These results provide insights into the regulation of allosteric ion permeation, a phenomenon common in multisubunit assembly structures in various ion channels.