Ishibashi K, Sasaki S, Fushimi K, Uchida S, Kuwahara M, Saito H, Furukawa T, Nakajima K, Yamaguchi Y, Gojobori T
School of Medicine, Tokyo Medical and Dental University, Japan.
Proc Natl Acad Sci U S A. 1994 Jul 5;91(14):6269-73. doi: 10.1073/pnas.91.14.6269.
Water transport in highly water-permeable membranes is conducted by water-selective pores--namely, water channels. The recent cloning of water channels revealed the water-selective characteristics of these proteins when expressed in Xenopus oocytes or reconstituted in liposomes. Currently, it is assumed that the function of water channels is to transport only water. We now report the cloning of a member of the water channel that also transports nonionic small molecules such as urea and glycerol. We named this channel aquaporin 3 (AQP3) for its predominant water permeability. AQP3 has amino acid sequence identity with major intrinsic protein (MIP) family proteins including AQP-channel-forming integral membrane protein, AQP-collecting duct, MIP, AQP-gamma tonoplast intrinsic protein, nodulin 26, and glycerol facilitator (33-42%). Thus, AQP3 is an additional member of the MIP family. Osmotic water permeability of Xenopus oocytes measured by videomicroscopy was 10-fold higher in oocytes injected with AQP3 transcript than with water-injected oocytes. The increase in osmotic water permeability was inhibited by HgCl2, and this effect was reversed by a reducing agent, 2-mercaptoethanol. Although to a smaller degree, AQP3 also facilitated the transport of nonionic small solutes such as urea and glycerol, while the previously cloned water channels are permeable only to water when expressed in Xenopus oocytes. AQP3 mRNA was expressed abundantly in kidney medulla and colon. In kidney, it was exclusively immunolocalized at the basolateral membrane of collecting duct cells. AQP3 may function as a water and urea exit mechanism in antidiuresis in collecting duct cells.
在高水渗透性膜中，水的运输是通过水选择性孔道——即水通道进行的。最近水通道的克隆揭示了这些蛋白质在非洲爪蟾卵母细胞中表达或在脂质体中重构时的水选择性特征。目前，人们认为水通道的功能仅仅是运输水。我们现在报告克隆了一种水通道成员，它还能运输尿素和甘油等非离子小分子。因其主要的水渗透性，我们将这个通道命名为水通道蛋白3（AQP3）。AQP3与主要内在蛋白（MIP）家族蛋白具有氨基酸序列同源性，这些蛋白包括形成水通道的整合膜蛋白、集合管水通道蛋白、MIP、液泡膜内在蛋白γ、根瘤菌素26和甘油转运蛋白（同源性为33 - 42%）。因此，AQP3是MIP家族的又一个成员。通过视频显微镜测量，注射AQP3转录本的非洲爪蟾卵母细胞的渗透水渗透性比注射水的卵母细胞高10倍。HgCl2抑制了渗透水渗透性的增加，而还原剂2 - 巯基乙醇可逆转这种效应。虽然程度较小，但AQP3也促进了尿素和甘油等非离子小溶质的运输，而之前克隆的水通道在非洲爪蟾卵母细胞中表达时仅对水有通透性。AQP3 mRNA在肾髓质和结肠中大量表达。在肾脏中，它仅免疫定位在集合管细胞的基底外侧膜上。AQP3可能在集合管细胞抗利尿过程中作为水和尿素的排出机制发挥作用。
