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FAPP2 is an adaptor protein of phosphatidylinositol-4-phosphate and is involved in the transport of some apical cargos from the trans-Golgi network (TGN). To inves- tigate whether the regulated apical transport of aquaporin-2 (AQP2) is involved in the FAPP2-dependent apical protein-sorting machinery, we measured apical sorting of AQP2 in Madin-Darby canine kidney (MDCK) cells with or without FAPP2 knockdown. We established MDCK cell lines that stably express rat AQP2 without any tag sequence. Then, FAPP2-deficient stable cell lines were established from the AQP2-expressing cell lines by a retrovirus-mediated RNA interference system. In the established cell lines, AQP2 was detected in both apical and basolateral membranes. Forskolin increased only the apical localization of AQP2, which was not affected by basolateral treatment with 0.5% tannic acid, indicating that the forskolin-induced apical transport of AQP2 did not include the transcytotic pathway from basolateral to apical membranes but is a direct transport from TON to the apical membranes. Using these cell lines, we tested the effect of FAPP2 knockdown on the polarized AQP2 transport to plasma membranes and found that the forskolin-induced apical transport of AQP2 was completely abolished by FAPP2 knockdown. By contrast, the basolateral localization of AQP2 was not affected by FAPP2 knockdown. AQP2 phosphorylation by forskolin was also impaired in FAPP2 knockdown MDCK cells. These results suggest that FAPP2 is necessary to generate AQP2-bearing vesicles at trans-Golgi that will undergo phosphorylation by PKA in subapical regions. [ABSTRACT FROM AUTHOR]