| Aquaporin-1 belongs to the aquaporin family of integral membrane water channels and assembles into a homotetramer, with each of the four monomers independently forming its own functional water pore rather than the four subunits jointly constructing a single shared channel. Each monomer folds into a right-handed barrel built from six full-length transmembrane helices surrounding two additional half-helices, contributed by loops B and E, which insert into the channel from opposite sides of the membrane and meet at the pore's center; each half-helix carries a conserved asparagine-proline-alanine, or NPA, motif, and the two NPA motifs interlock directly opposite one another at the channel midpoint, together forming what is effectively a seventh, pseudo-transmembrane element. This central NPA region generates the channel's proton-exclusion mechanism: the two half-helices orient their positive macroscopic dipole moments toward the pore center, and non-equilibrium molecular dynamics simulations that explicitly model proton transfer reactions show that the resulting electrostatic field, rather than a physical break in the single-file chain of hydrogen-bonded water molecules as previously proposed, constitutes the dominant energy barrier blocking proton conduction through the channel, even though the same pore conducts water at high efficiency. A second constriction, formed by aromatic and arginine residues positioned toward the extracellular end of the pore, defines the narrowest point of the channel and functions as the primary size- and polarity-based selectivity filter, working together with the NPA-based electrostatic barrier to restrict permeation to water while excluding ions, protons, and larger polar solutes. Free energy calculations along the permeation pathway show that water crosses this combined NPA and aromatic/arginine constriction region as a single file, with the pore lined by hydrophobic residues elsewhere along its length to minimize competing water-protein hydrogen bonding and thereby maintain a high single-channel water permeability. Because this same pore architecture, built from a core structural fold conserved across the aquaporin family, distinguishes strictly water-selective channels like AQP1 from related aquaglyceroporins that additionally conduct glycerol and other small solutes, the specific residues lining the aromatic/arginine constriction directly determine each aquaporin family member's substrate range. AQP1 is expressed in red blood cells, renal proximal tubule and descending limb epithelium, and pulmonary microvascular endothelium, where its NPA- and aromatic/arginine-defined selectivity supports rapid, ion-impermeable water flux essential to renal urine concentration and pulmonary fluid handling, and disruption of this water-selective permeation pathway is implicated in conditions of impaired renal water reabsorption and altered pulmonary fluid balance. |
