Characterization of aquaporin-6 as a nitrate channel in mammalian cells. Requirement of pore-lining residue threonine 63

Aquaporins (AQP) were originally regarded as plasma membrane channels that are freely permeated by water or small uncharged solutes but not by ions. Unlike other aquaporins, AQP6 overexpressed in Xenopus laevis oocytes was previously found to exhibit Hg²⁺ or pH-activated ion conductance. AQP6 could not be analyzed electrophysiologically in mammalian cells, however, because the protein is restricted to intracellular vesicles. Here we report that addition of a green fluorescence protein (GFP) tag to the N terminus of rat AQP6 (GFP-AQP6) redirects the protein to the plasma membranes of transfected mammalian cells. This permitted measurement of rapid, reversible, pH.induced anion currents by GFP-AQP6 in human embryonic kidney 293 cells. Surprisingly, anion selectivity relative to Cl^- revealed high nitrate permeability even at pH 7.4; P_NO3/P_Cl >9.8. Site-directed mutation of a pore-lining threonine to isoleucine at position 63 at the midpoint of the channel reduced NO₃^-/Cl^- selectivity. Moreover, no anomalous mole-fraction behavior was observed with NO₃^-/Cl^- mixtures, suggesting a single ion-binding pore in each subunit. Our studies indicate that AQP6 exhibits a new form of anion permeation with marked specificity for nitrate conferred by a specific pore-lining residue, observations that imply that the primary role of AQP6 may be in cellular regulation rather than simple fluid transport.