The Journal of General Physiology
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Published online October 15, 2007
doi:10.1085/jgp.200709826
The Journal of General Physiology, Vol. 130, No. 5, 457-464
The Rockefeller University Press, 0022-1295 $30.00
© 2007 Németh-Cahalan et al.
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ARTICLE

 Zinc Modulation of Water Permeability Reveals that Aquaporin 0 Functions as a Cooperative Tetramer



Karin L. Németh-Cahalan, Katalin Kalman, Alexandrine Froger, and James E. Hall

Department of Physiology and Biophysics, University of California, Irvine, CA 92697

Correspondence to James E. Hall: jhall{at}uci.edu

We previously showed that the water permeability of AQP0, the water channel of the lens, increases with acid pH and that His40 is required (Németh-Cahalan, K.L., and J.E. Hall. 2000. J. Biol. Chem. 275:6777–6782; Németh-Cahalan, K.L., K. Kalman, and J.E. Hall. 2004. J. Gen. Physiol. 123:573–580). We have now investigated the effect of zinc (and other transition metals) on the water permeability of AQP0 expressed in Xenopus oocytes and determined the amino acid residues that facilitate zinc modulation. Zinc (1 mM) increased AQP0 water permeability by a factor of two and prevented any additional increase induced by acid pH. Zinc had no effect on water permeability of AQP1, AQP4 or MIPfun (AQP0 from killifish), or on mutants of AQP1 and MIPfun with added external histidines. Nickel, but not copper, had the same effect on AQP0 water permeability as zinc. A fit of the concentration dependence of the zinc effect to the Hill equation gives a coefficient greater than three, suggesting that binding of more than one zinc ion is necessary to enhance water permeability. His40 and His122 are necessary for zinc modulation of AQP0 water permeability, implying structural constraints for zinc binding and functional modulation. The change in water permeability was highly sensitive to a coinjected zinc-insensitive mutant and a single insensitive monomer completely abolished zinc modulation. Our results suggest a model in which positive cooperativity among subunits of the AQP0 tetramer is required for zinc modulation, implying that the tetramer is the functional unit. The results also offer the possibility of a pharmacological approach to manipulate the water permeability and transparency of the lens.

Abbreviations used in this paper: AQP, aquaporin; MIP, major intrinsic protein.


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Related Article

Molecular Basis of pH and Ca2+ Regulation of Aquaporin Water Permeability
Karin L. Németh-Cahalan, Katalin Kalman, and James E. Hall
J. Gen. Physiol. 2004 123: 573-580. [Abstract] [Full Text] [PDF]



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K. Kalman, K. L. Nemeth-Cahalan, A. Froger, and J. E. Hall
Phosphorylation Determines the Calmodulin-mediated Ca2+ Response and Water Permeability of AQP0
J. Biol. Chem., July 25, 2008; 283(30): 21278 - 21283.
[Abstract] [Full Text] [PDF]


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