Cloning, functional analysis and cell localization of a kidney proximal tubule water transporter homologous to CHIP28

The localization and transporting properties of a kidney protein homologous to human erythrocyte protein CHIP28 was evaluated. The cDNA encoding rat kidney protein CHIP28k was isolated from a rat renal cortex cDNA library. A 2.8-kb cDNA was identified which contained an 807 bp open reading frame encoding a 28.8 kD protein with 94% amino acid identity to CHIP28. in vitro translation of CHIP28k cDNA in rabbit reticulocyte lysate generated a 28-kD protein; addition of ER-derived microsomes gave a 32-kD transmembrane glycoprotein. Translation of truncated RNA demonstrated glycosylation of residue Asn42 which is predicted to lie between the first and second transmembrane domains. Expression of in vitro transcribed mRNA encoding CHIP28k in Xenopus oocytes increased oocyte osmotic water permeability (P(f)) from (4 ± 1) x 10^-4 to (33 ± 4) x 10^-4 cm/s at 10°C; the increase in oocyte P(f) was weakly temperature dependent and inhibited by HgCl₂. Two- electrode voltage clamp measurements indicated that CHIP28k was not permeable to ions. Oocyte P(f) also increased with expression of total mRNA from kidney cortex and papilla; the increase in P(f) with mRNA from cortex, but not kidney papilla, was blocked by coinjection with excess antisense CHIP28k cRNA. In situ hybridization of a 150 base cRNA antisense probe to tissue sections from rat kidney showed selective CHIP28k localization to epithelial cells in proximal tubule and thin descending limb of Henle. P(f) in purified apical membrane vesicles from rat and human proximal tubule, and in proteoliposomes reconstituted with purified protein, was very high and inhibited by HgCl₂; stripping of apical vesicles with N-lauroylsarcosine enriched a 28-kD protein by 25-fold and yielded a vesicle population with high water, but low urea and proton permeabilities. CHIP28k identity was confirmed by NH₂-terminus sequence analysis. These results indicate that CHIP28k is a major and highly selective water transporting protein in the kidney proximal tubule and thin descending limb of Henle, but not collecting duct.