Localization of the CHIP28 water channel in rat kidney

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Abstract
CHIP28 is an integral membrane protein that has been identified as the erythrocyte water channel and that is also expressed in the kidney. Antibodies against erythrocyte CHIP28 were used to localize this protein along the rat urinary tubule. By Western blotting, CHIP28 was detected in kidney plasma membrane and endosome fractions. With the use of immunocytochemistry, CHIP28 was located in brush-border and basolateral plasma membranes of the proximal tubule. The initial S1 segment was weakly stained, but the S2 and S3 segments were heavily labeled. Subapical vesicles were also positive. Apical and basolateral membranes of the long thin descending limb were strongly labeled, but ascending thin and thick limbs of Henle and distal convoluted tubules were negative. Some vasa recta profiles in the medulla were positive. CHIP28 is, therefore, present in membranes with a high constitutive water permeability, where it probably acts as a transmembrane water-conducting channel. Finally, a weak staining of apical and basolateral membranes of cortical collecting duct principal cells was detectable, suggesting a potential relationship of CHIP28 to the vasopressin-sensitive water channel.