Mutations in WNK4 have been linked to hypertension in PHAII. Paracellular ion transport has been reported to be involved in this disease process; however, the specific molecular target has not been identified. In this study, we found that TJ protein claudin-7 and WNK4 were partially co-localized in renal tubules of rat kidney and co-immunoprecipitated in kidney epithelial cells. The wild-type and PHAII-causing mutant, but not the kinase-dead mutant, phosphorylated claudin-7. We have identified ser(206) in the COOH-terminus of claudin-7 as a putative phosphorylation site for WNK4. More importantly, disease-causing mutant enhanced claudin-7 phosphorylation and significantly increased paracellular permeability to Cl(-).