Rapid gating and anion permeability of an intracellular aquaporin

Aquaporin (AQP) water-channel proteins are freely permeated by water but no t bg ions or charged solutes(1), Although mammalian aquaporins were believe d to be located in plasma membranes, rat AQP6 is restricted to intracellula r vesicles in renal epithelia(2). Here we show that AQP6 is functionally di stinct from other known aquaporins. When expressed in Xenopus laevis oocyte s, AQP6 exhibits low basal water permeability; however, when treated with t he known water channel inhibitor, Hg2+, the water permeability of AQP6 oocy tes rapidly rises up to tenfold and is accompanied by ion conductance. AQP6 colocalizes with H+-ATPase in intracellular vesicles of acid-secreting alp ha-intercalated cells in renal collecting duct. At pH less than 5.5, anion conductance is rapidly and reversibly activated in AQP6 oocytes. Site-direc ted mutation of lysine to glutamate at position 72 in the cytoplasmic mouth of the pore changes the cation/anion selectivity: but leaves low pH activa tion intact, Our results demonstrate unusual biophysical properties of an a quaporin, and indicate that anion-channel function may now be explored in a protein with known structure.