THE EXCHANGE OF FUNCTIONAL DOMAINS AMONG AQUAPORINS WITH DIFFERENT TRANSPORT CHARACTERISTICS

Aquaporins are transmembrane proteins that contain six bilayer-spannin g domains, connected by loops A to E. The hourglass model predicts tha t the conserved loops B and E are essential for the formation of the w ater pore. To test the importance of loops B and E in the determinatio n of the transport characteristics, we exchanged loops B and/or E betw een AQP0, AQP2, and AQP3. Detailed functional, immunoblot and immuno-c ytochemical analyses of expression in Xenopus oocytes revealed that si x out of the nine chimeric aquaporin proteins were not functional, bec ause of misrouting. AQP0 with loop E of AQP2 was not impaired in its r outing and revealed a low water permeability equal to that of wildtype AQP0. AQP2 with loop B of AQP0 was also routed normally and gave a hi gh water permeability, similar to that of wild-type AQP2. AQP0 with lo ops B and E of AQP2 (AQP0-2BE) did not result in an increase in water permeability and was partly misrouted. However, the plasma membrane ex pression was high enough to expect an increase in water permeability, as loops B and E of AQP2 confer AQP2's water permeability to AQP0. Alt hough it is unclear for the dual chimera (AQP0-2BE), the parental wate r permeabilities obtained in oocytes expressing AQP0 with loop E of AQ P2 or AQP2 with loop B of AQP0 indicate that, besides loops B and E, o ther parts of die AQP protein are important in the determination of th e characteristics of the channel.