Yasui M, Hazama A, Kwon T H, Nielsen S, Guggino W B, Agre P
Department of Biological Chemistry, Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA.
Nature. 1999 Nov 11;402(6758):184-7. doi: 10.1038/46045.
Aquaporin (AQP) water-channel proteins are freely permeated by water but not by ions or charged solutes. Although mammalian aquaporins were believed to be located in plasma membranes, rat AQP6 is restricted to intracellular vesicles in renal epithelia. Here we show that AQP6 is functionally distinct from other known aquaporins. When expressed in Xenopus laevis oocytes, AQP6 exhibits low basal water permeability; however, when treated with the known water channel inhibitor, Hg2+, the water permeability of AQP6 oocytes rapidly rises up to tenfold and is accompanied by ion conductance. AQP6 colocalizes with H+-ATPase in intracellular vesicles of acid-secreting alpha-intercalated cells in renal collecting duct. At pH less than 5.5, anion conductance is rapidly and reversibly activated in AQP6 oocytes. Site-directed mutation of lysine to glutamate at position 72 in the cytoplasmic mouth of the pore changes the cation/anion selectivity, but leaves low pH activation intact. Our results demonstrate unusual biophysical properties of an aquaporin, and indicate that anion-channel function may now be explored in a protein with known structure.
水通道蛋白（AQP）是一类可让水自由通透但不让离子或带电荷溶质通透的蛋白。尽管人们曾认为哺乳动物水通道蛋白位于质膜中，但大鼠AQP6局限于肾上皮细胞的细胞内囊泡中。在此我们表明，AQP6在功能上不同于其他已知的水通道蛋白。当在非洲爪蟾卵母细胞中表达时，AQP6表现出较低的基础水通透性；然而，当用已知的水通道抑制剂Hg2+处理时，AQP6卵母细胞的水通透性会迅速提高至原来的十倍，并伴有离子电导。AQP6与H+-ATP酶在肾集合管分泌酸的α-闰细胞的细胞内囊泡中共定位。在pH小于5.5时，AQP6卵母细胞中的阴离子电导会迅速且可逆地被激活。孔道胞质口处第72位赖氨酸定点突变为谷氨酸会改变阳离子/阴离子选择性，但低pH激活作用仍保持完整。我们的结果证明了一种水通道蛋白具有不同寻常的生物物理特性，并表明现在可以在具有已知结构的蛋白中探索阴离子通道功能。
