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J. Biol. Chem., Vol. 279, Issue 12, 11364-11367, March 19, 2004

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Aquaporin-1, Nothing but a Water Channel^*

Satoshi P. Tsunoda, Burkhard Wiesner, Dorothea Lorenz, Walter Rosenthal, and Peter Pohl¶||

From the Forschungsinstitut fuer Molekulare Pharmakologie, Robert-Roessle-Strasse 10, D-13125 Berlin, Institut fuer Pharmakologie, Freie Universitaet, Thielallee 67-73, D-14195 Berlin, and ^¶Experimentelle Biophysik, Institut fuer Biologie, Humboldt-Universitaet, Invaliden Strasse 43, 10115 Berlin, Germany

Aquaporin-1 (AQP1) is a membrane channel that allows rapid water movement driven by a transmembrane osmotic gradient. It was claimed to have a secondary function as a cyclic nucleotide-gated ion channel. However, upon reconstitution into planar bilayers, the ion channel exhibited a 10-fold lower single channel conductance than in Xenopus oocytes and a 100-fold lower open probability (<10^-6) of doubtful physiological significance (Saparov, S. M., Kozono, D., Rothe, U., Agre, P., and Pohl, P. (2001) J. Biol. Chem. 276, 31515–31520). Investigating AQP1 expressed in human embryonic kidney cells, we now have shown that the discrepancy is not due to alterations of AQP1 properties upon reconstitution into bilayers but rather to regulatory processes of the oocyte expression system that may have been misinterpreted as AQP1 ion channel activity. As confirmed by laser scanning reflection microscopy, from 0.8 to 1.4 x 10⁶ AQP1 copies/cell contributed to osmotic cell swelling. The proper plasma membrane localization was confirmed by observing the fluorescence of the N-terminal yellow fluorescent protein tag. Whole-cell patch clamp experiments of wild type or tagged AQP1-expressing cells revealed that neither cGMP nor cAMP mediated ion channel activity. The lack of significant CNG ion channel activity rules out a secondary role of AQP1 water channels in cellular signal transduction.

Received for publication, October 2, 2003 , and in revised form, December 24, 2003.

^* The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

^|| Supported by the Deutsche Forschungsgemeinschaft (Po 533/7-1). To whom correspondence should be addressed. Tel.: 49-30-94793283; Fax: 49-30-94793291; E-mail: pohl{at}fmp-berlin.de.

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