All three WW domains of murine Nedd4 are involved in the regulation of epithelial sodium channels by intracellular Na⁺

The amiloride-sensitive epithelial sodium channel (ENaC) plays a critical role in fluid and electrolyte homeostasis and consists of α, β, and γ subunits. The carboxyl terminus of each ENaC subunit contains a PPxY motif which is necessary for interaction with the WW domains of the ubiquitin-protein ligase, Nedd4. Disruption of this interaction, as in Liddle's syndrome where mutations delete or alter the PY motif of either the β or γ subunits, results in increased ENaC activity. We have recently shown using the whole-cell patch clamp technique that Nedd4 mediates the ubiquitin- dependent down-regulation of Na⁺ channel activity in response to increased intracellular Na⁺. In this paper, we demonstrate that WW domains 2 and 3 bind α-, β-, and γ-ENaC with varying degrees of affinity, whereas WW domain 1 does not bind to any of the subunits. We further show using whole- cell patch clamp techniques that Nedd4-mediated down-regulation of ENaC in mouse mandibular duct cells involves binding of the WW domains of Nedd4 to three distinct sites. We propose that Nedd4-mediated down-regulation of Na⁺ channels involves the binding of WW domains 2 and 3 to the Na⁺ channel and of WW domain 1 to an unknown associated protein.