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J. Biol. Chem., Vol. 277, Issue 39, 36782-36786, September 27, 2002
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From the Departments of Medicine and Physiology, Cardiovascular
Research Institute, University of California,
San Francisco, California 94143-0521
We reported increased water permeability and a
low urea reflection coefficient in Xenopus oocytes
expressing urea transporter UT-B (former name UT3), suggesting that
water and urea share a common aqueous pathway (Yang, B., and Verkman,
A. S. (1998) J. Biol. Chem. 273, 9369-9372).
Although increased water permeability was confirmed in the
Xenopus oocyte expression system, it has been argued
(Sidoux-Walter, F., Lucien, N., Olives, B., Gobin, R., Rousselet, G.,
Kamsteeg, E. J., Ripoche, P., Deen, P. M., Cartron, J. P., and Bailly, P. (1999) J. Biol. Chem. 274, 30228-30235) that UT-B does not transport water when expressed at
normal levels in mammalian cells such as erythrocytes. To quantify
UT-B-mediated water transport, we generated double knockout mice
lacking UT-B and the major erythrocyte water channel, aquaporin-1
(AQP1). The mice had reduced survival, retarded growth, and defective
urinary concentrating ability. However, erythrocyte size and morphology were not affected. Stopped-flow light scattering measurements indicated
erythrocyte osmotic water permeabilities (in cm/s × 0.01, 10 °C): 2.1 ± 0.2 (wild-type mice), 2.1 ± 0.05 (UT-B
null), 0.19 ± 0.02 (AQP1 null), and 0.045 ± 0.009 (AQP1/UT-B null). The low water permeability found in AQP1/UT-B null
erythrocytes was also seen after HgCl2 treatment of UT-B
null erythrocytes or phloretin treatment of AQP1 null erythrocytes. The
apparent activation energy for UT-B-mediated water transport was low,
<2 kcal/mol. Estimating 14,000 UT-B molecules per mouse erythrocyte,
the UT-B-dependent Pf of 0.15 × 10
Analysis of Double Knockout Mice Lacking Aquaporin-1
and Urea Transporter UT-B
EVIDENCE FOR UT-B-FACILITATED WATER TRANSPORT IN
ERYTHROCYTES*
and
4 cm/s indicated a substantial single channel water
permeability of UT-B of 7.5 × 1014
cm3/s, similar to that of AQP1. These results
provide direct functional evidence for UT-B-facilitated water transport
in erythrocytes and suggest that urea traverses an aqueous pore in the
UT-B protein.
*
This work was supported by National Institutes of Health
Grants DK35124, HL58198, HL60288, EB00415, and EY13574 and Grant R613
from the National Cystic Fibrosis Foundation.The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement" in
accordance with 18 U.S.C. Section
1734 solely to indicate this fact.
To whom correspondence should be addressed: 1246 Health Sciences
East Tower, Cardiovascular Research Inst., University of California,
San Francisco, CA 94143-0521. Tel.: 415-476-8530; Fax: 415-665-3847;
E-mail: byang@itsa.ucsf.edu; www.ucsf.edu/verklab.
Copyright © 2002 by The American Society for Biochemistry and Molecular Biology, Inc.
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