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First published online August 3, 2006
Journal of Experimental Biology 209, 3199-3208 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02351

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Molecular and cellular characterization of a new aquaporin, AQP-x5, specifically expressed in the small granular glands of Xenopus skin

Makoto Kubota, Takahiro Hasegawa, Takashi Nakakura, Haruna Tanii, Masakazu Suzuki and Shigeyasu Tanaka^*

Department of Biology, Faculty of Science, Shizuoka University, Ohya 836, Shizuoka 422-8529, Japan

^* Author for correspondence (e-mail: sbstana{at}ipc.shizuoka.ac.jp)

Accepted 30 May 2006

A new toad aquaporin (AQP) cDNA was cloned from a cDNA library constructed from the ventral skin of Xenopus laevis. This AQP (Xenopus AQP-x5) consisted of 273 amino acid residues with a high sequence homology to mammalian AQP5. The predicted amino acid sequence contained the two conserved Asn-Pro-Ala motifs found in all major intrinsic protein (MIP) family members and six putative transmembrane domains. The sequence also contained a mercurial-sensitive cysteine and a putative phosphorylation motif site for protein kinase A at Ser-257. The swelling assay using Xenopus oocytes revealed that AQP-x5 facilitated water permeability. Expression of AQP-x5 mRNA was restricted to the skin, brain, lungs and testes. Immunofluorescence and immunoelectron microscopical studies using an anti-peptide antibody (ST-156) against the C-terminal region of the AQP-x5 protein revealed the presence of immunopositive cells in the skin, with the label predominately localized in the apical plasma membrane of the secretory cells of the small granular glands. These glands are unique both in being close to the epidermal layer of the skin and in containing mitochondria-rich cells with vacuolar H⁺-ATPase dispersed among its secretory cells. Results from immunohistochemical experiments on the mucous or seromucous glands of several other anurans verified this result. We conclude that the presence of AQP-x5 in the apical plasma membrane of the small granular glands suggests its involvement in water secretion from the skins. The physiological roles of the AQP-x5 protein in the small or mucous glands are discussed.

Key words: aquaporin, water channel, cDNA cloning, exocrine gland, skin, immunocytochemistry, Xenopus laevis

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