Vol. 188, No. 3, 1992 BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS November 16, 1992 Pages 1267-1273 ISOLATION OF A cDNA FOR RAT CHIP28 WATER CHANNEL: HIGH mRNA EXPRESSION IN KIDNEY CORTEX AND INNER MEDULLA1 Peter M.T. Deen*#*, Jacqueline A. Dempster*. Be Wieringa# and Care1 H. Van OS* *Department of Physiology and #Department of Cellular Biology, University of Nijmegen, P.O. Box 9101,650O HB Nijmegen, The Netherlands Received September 28, 1992 The cDNA coding for the rat CHIP28 water channel was isolated from a kidney library. At the amino acid level, rat CHIP28 is 93 % identical to the recently published human protein (1). Expression of rat CHIP28 mRNA was highest in the renal inner medulla, unchanged during antidiuresis and twice the level expressed in outer cortex, with lower expression levels also apparent in parotid gland, urinary bladder and prostate. The evidence suggests that CHIP28 water channels in the ADH-sensitive collecting tubules are identical to those of the ADH-insensitive proximal convoluted tubules and possibly other tissues special&d in fluid transport. 0 1992 Academic Press,MC. In most cells, water exchange occurs via diffusion across the lipid matrix. In contrast, the route of transmembrane water permeation in erythrocytes and the renal cells of proximal tubules and ADH-treated collecting ducts is mediated via special&d water channel proteins. Functional evidence stems from high osmotic water permeabilities, ratios of osmotic-to-diffusional water permeabilities greater than unity, low activation energies, reversible inhibition by mercurial sulphydryl reagents, and successful expression in Xenopus oocytes (reviews: 2,3,4). Furthermore, radiation target analysis revealed a 30-kDa functional size for water channels in both renal cortical brush border membranes (5) and erythrocyte membranes (6). The recent isolation of the human erythrocyte cDNA sequence encoding a 28kDa protein (CHIP28; l), which was abundant in both human erythrocytes and rat kidney (7), concluded the long quest for the molecular identification of a water channel. The deduced amino acid sequence predicts that CHIP28 composes six transmembmne domains and reveals strong similarity with the major intrinsic protein of bovine lens (MIP26; l), the prototype of an ancient family of channel proteins, recently identified in a wide variety of species (8). In contrast to other MIP-related 1Sequence data from this article have been deposited with the EMBL/GenBank Data Libraries under Accession No. X67948. *To whom correspondence and reprint requests should be addressed. Abbreviutiom. ADH, anti-diuretic hormone (vasopressin); GAPDH, glyceraldehyde-3-phosphate dehydrogenadse. 1267 0006-29 I X/92 $4.00 Copyright 0 1992 by Academic Prus.~, Inc. All rights of reproduction in an! f~wrn rrsrrveed.