Human Nedd4 interacts with the human epithelial Na+ channel: WW3 but not WW1 binds to Na+-channel subunits

The epithelial Na+ channel (ENaC) regulates Na+ absorption in epithelial ti ssues including the lung, colon and sweat gland, and in the distal nephrons of the kidney. When Na+-channel function is disrupted, salt and water homo eostasis is affected. The cytoplasmic regions of the Na+-channel subunits p rovide binding sites for other proteins to interact with and potentially re gulate Na+-channel activity. Previously we showed that a proline-rich regio n of the alpha subunit of the Na+ channel bound to a protein of 116 kDa fro m human lung cells. Here we report the identification of this protein as hu man Nedd4, a ubiquitin-protein ligase that binds to the Na+-channel subunit s via its WW domains. Further, we show that WW domains 2, 3 and 4 of human Nedd4 bind to the alpha, beta and gamma Na+-channel subunits but not to a m utated beta subunit. In addition, when co-expressed in Xenopus oocytes, hum an Nedd4 down-regulates Na+-channel activity.