The epithelial sodium channel (ENaC) is intracellularly located as a tetramer

Abstract.

The epithelial sodium channel (ENaC) plays an important role in Na⁺ homeostasis by determining the Na⁺ transport rate in so-called end-organs such as the renal collecting duct, distal colon, salivary and sweat gland ducts. ENaC is formed by heteromultimerization of three homologous subunits, termed α, β, and γ ENaC. The number of subunits and stoichiometry remain a matter of debate. In this study, sucrose gradient analysis of Xenopus laevis oocytes expressing rENaC revealed that ENaC forms heterotetramers, when the membrane fraction was solubilized in 0.1% (wt/vol) Na-deoxycholate. However, solubilization of the membrane proteins in higher concentrations of detergents dissociated the ENaC subunits of the tetramers in dimers. Co-immunoprecipitation studies with FLAG-tagged ENaC subunits suggest that during dissociation of ENaC tetramers the composition of dimers is completely random. Glycosidase digestion studies show that the ENaC subunits are retarded in the endoplasmic reticulum (ER) and pre-Golgi, whereas only a small fraction is inserted into the plasma membrane. Immunocytochemical analysis confirmed that ENaC is primarily located intracellularly. In addition, these findings are not restricted to the oocyte expression system, since identical results were found in rabbit connecting tubule and cortical collecting duct cells in primary culture and in rabbit colon.