Molecular Cloning and Characterization of an Insect Aquaporin

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Abstract
We previously described the structural organization of P25, a member of the major‐intrinsic‐protein family found in the digestive tract of homopteran sap‐sucking insects [Beuron, F., Le Cahérec, 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. & Thomas, D. (1995)J. Biol. Chem. 270, 17414–17422]. We demonstrated, by means 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 Cahérec, F., Bron, P., Verbavatz, J. M., Garret, A., Morel, G., Cavalier, A., Bonnec, G., Thomas, D., Gouranton, J. & Hubert, J.‐F. (1996)J. Cell Sci. 109, 1285–1295]. In the present work, we cloned a full‐length cDNA from a Cicadella 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 aquaporins 1, 2, z and tonoplast intrinsic protein γ, respectively. Translation of the corresponding RNA in Xenopus oocytes generated a polypeptide that was specifically recognized by polyclonal antibodies raised against native P25. Expression of the protein in Xenopus oocyte membranes was assessed by immunocytochemistry and led to a 15‐fold increase of osmotic membrane water permeability. This increase was inhibited by HgCl2. The permeability had an Arrhenius activation energy of 11.7 kJ/mol. We called this protein Cicadella aquaporin (AQPcic). The oocytes expressing Cicadella aquaporin were less sensitive to HgCl2 than oocytes expressing aquaporin 1. In the Xenopus oocyte system, Cicadella aquaporin failed to transport glycerol, urea and ions. It exhibited permeabilities to ethylene glycol and formamide similar to those measured for aquaporin 1 under the same conditions.