Wichmann_2018_J.Biol.Chem_293_6647

The epithelial sodium channel (ENaC) is a critical regulator of vertebrate electrolyte homeostasis. ENaC is the only constitutively open ion channel in the degenerin/ENaC protein family, and its expression, membrane abundance, and open probability therefore are tightly controlled. The canonical ENaC is composed of three subunits (alpha, beta, and gamma), but a fourth delta-subunit may replace alpha and form atypical deltabetagamma-ENaCs. Using Xenopus laevis as a model, here we found that mRNAs of the alpha- and delta-subunits are differentially expressed in different tissues and that delta-ENaC predominantly is present in the urogenital tract. Using whole-cell and single-channel electrophysiology of oocytes expressing Xenopus alphabetagamma- or deltabetagamma-ENaC, we demonstrate that the presence of the delta-subunit enhances the amount of current generated by ENaC due to an increased open probability, but also changes current into a transient form. Activity of canonical ENaCs is critically dependent on proteolytic processing of the alpha- and gamma-subunits, and immunoblotting with epitope-tagged ENaC subunits indicated that, unlike alpha-ENaC, the delta-subunit does not undergo proteolytic maturation by the endogenous protease furin. Furthermore, currents generated by deltabetagamma-ENaC were insensitive to activation by extracellular chymotrypsin, and presence of the delta-subunit prevented cleavage of gamma-ENaC at the cell surface. Our findings suggest that subunit composition constitutes an additional level of ENaC regulation, and we propose that the Xenopus delta-ENaC subunit represents a functional example that demonstrates the importance of proteolytic maturation during ENaC evolution.