Department of Cell Physiology, University of Nijmegen, PO Box 9101, 6500 HB Nijmegen, The Netherlands.
Pflugers Arch. 1998 Jul;436(4):599-607. doi: 10.1007/s004240050677.
Aquaporins are transmembrane proteins that contain six bilayer-spanning domains, connected by loops A to E. The hourglass model predicts that the conserved loops B and E are essential for the formation of the water pore. To test the importance of loops B and E in the determination of the transport characteristics, we exchanged loops B and/or E between AQP0, AQP2, and AQP3. Detailed functional, immunoblot and immunocytochemical analyses of expression in Xenopus oocytes revealed that six out of the nine chimeric aquaporin proteins were not functional, because of misrouting. AQP0 with loop E of AQP2 was not impaired in its routing and revealed a low water permeability equal to that of wild-type AQP0. AQP2 with loop B of AQP0 was also routed normally and gave a high water permeability, similar to that of wild-type AQP2. AQP0 with loops B and E of AQP2 (AQP0–2BE) did not result in an increase in water permeability and was partly misrouted. However, the plasma membrane expression was high enough to expect an increase in water permeability, as loops B and E of AQP2 confer AQP2’s water permeability to AQP0. Although it is unclear for the dual chimera (AQP0–2BE), the parental water permeabilities obtained in oocytes expressing AQP0 with loop E of AQP2 or AQP2 with loop B of AQP0 indicate that, besides loops B and E, other parts of the AQP protein are important in the determination of the characteristics of the channel.
水通道蛋白是含有六个双层跨膜结构域的跨膜蛋白，由 A 至 E 环连接。沙漏模型预测，保守的 B 环和 E 环对于水孔的形成是必不可少的。为了测试 B 环和 E 环在决定转运特性中的重要性，我们在 AQP0、AQP2 和 AQP3 之间交换了 B 环和/或 E 环。在非洲爪蟾卵母细胞中的详细功能、免疫印迹和免疫细胞化学分析表明，由于错误途径，九种嵌合水通道蛋白中有六种没有功能。AQP0 的 E 环来自 AQP2，其途径没有受损，表现出与野生型 AQP0 相同的低水通透性。AQP0 的 B 环来自 AQP0，其途径也正常，水通透性高，与野生型 AQP2 相似。AQP0 的 B 环和 E 环来自 AQP2（AQP0–2BE）并没有增加水通透性，并且部分途径错误。然而，由于 AQP2 的 B 环和 E 环赋予 AQP0 水通透性，其质膜表达足够高，预计水通透性会增加。尽管对于双重嵌合体（AQP0–2BE）而言，在表达 AQP0 的 E 环来自 AQP2 或 AQP2 的 B 环来自 AQP0 的卵母细胞中获得的亲本水通透性尚不清楚，但表明除了 B 环和 E 环外，AQP 蛋白的其他部分对于决定通道的特性也很重要。
