MOLECULAR-CLONING AND CHARACTERIZATION OF AN INSECT AQUAPORIN - FUNCTIONAL COMPARISON WITH AQUAPORIN-1

We previously described the structural organization of P25, a member o f the major-intrinsic-protein family found in the digestive tract of h omopteran sap-sucking insects [Beuron, F., Le Caherec, F., Guillam, M. T., Cavalier, A., Garret, A., Tassan, J. P., Delamarche, C., Schultz, P., Mallouh, V., Rolland, J. P., Hubert, J.-F., Gouranton, J. & Thoma s, D. (1995) J. Biol. Chem. 270, 17414-17422]. We demonstrated, by mea ns of introducing P25 tetramers into the membranes of Xenopus oocytes, that this protein exhibits functional properties similar to those of aquaporin 1, the archetypal water channel [Le Caherec, F., Bron, P., V erbavatz, J. M., Garret, A., Morel, G., Cavalier, A., Bonnec, G., Thom as, D., Gouranton, J. & Hubert, J.-F. (1996) J. Cell Sci. 109, 1285-12 95]. In the present work, we cloned a full-length cDNA from a Cicadell a viridis library with an open reading frame of 765 bp that encoded a 26-kDa protein whose sequence was 43, 40, 36 and 36% identical to aqua porins 1, 2, z and tonoplast intrinsic protein gamma, respectively. Tr anslation of the corresponding RNA in Xenopus oocytes generated a poly peptide that was specifically recognized by polyclonal antibodies rais ed against native P25. Expression of the protein in Xenopus oocyte mem branes was assessed by immunocytochemistry and led to a 15-fold increa se of osmotic membrane water permeability. This increase was inhibited by HgCl2. The permeability had an Arrhenius activation energy of 11.7 1 kJ/mol. We called this protein Cicadella aquaporin (AQPcic). The ooc ytes expressing Cicadella aquaporin were less sensitive to HgCl2 than oocytes expressing aquaporin 1. Tn the Xenopus oocyte system, Cicadell a aquaporin failed to transport glycerol, urea and ions. It exhibited permeabilities to ethylene glycol and formamide similar to those measu red for aquaporin 1 under the same conditions.