Epithelial sodium channel (ENaC) is multi-ubiquitinated at the cell surface

The human ENaC (epithelial sodium channel), a complex of three subunits, provides the rate-limiting step for sodium uptake in the distal nephron, and therefore plays a key role in salt homoeostasis and in regulating blood pressure. The number of active sodium channel complexes present at the plasma membrane appears to be tightly controlled. In Liddle's syndrome, a form of hypertension caused by an increase in the number of active sodium channels at the cell membrane, the betaENaC or gammaENaC subunit gene contains a mutation that disrupts the binding site for the Nedd4 (neuronal precursor cell expressed developmentally down-regulated gene 4) family of ubiquitin-protein ligases. Therefore ubiquitination of channel subunits may be involved in altering cell surface ENaC. Here, we provide evidence that the ENaC subunits located at the cell surface are modified with multiple mono-ubiquitins (multi-ubiquitination) and that Nedd4-2 modulates this ubiquitination. We confirm that ENaC is associated with the mu2 subunit of the AP-2 (adaptor protein 2) clathrin adaptor. Since mono- or multi-ubiquitination of other membrane proteins is a signal for their internalization by clathrin-mediated endocytosis and subsequent trafficking, our results support a model whereby ubiquitin and clathrin adaptor binding sites act in concert to remove ENaC from the cell surface.