The exchange of functional domains among aquaporins with different transport characteristics.

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.