Structural Insights into Subunit-Dependent Functional Regulation in Epithelial Sodium Channels

Epithelial sodium channels (ENaC) play a crucial role in Na+ reabsorption in mammals. To date, four subunits have been identified--, {beta}, {gamma}, and {delta}--believed to form different heteromeric complexes. Currently, only the structure of the {beta}{gamma} complex is known. To understand how these channels form with varying subunit compositions and define the contribution of each subunit to distinct properties, we co-expressed human {delta}, {beta}, and {gamma}. Using single-particle cryo-electron microscopy, we observed three distinct ENaC complexes. The structures unveil a pattern in which {beta} and {gamma} positions are conserved among the different complexes while the position in {beta}{gamma} trimer is occupied by either {delta} or another {beta}. The presence of {delta} induces structural rearrangements in the {gamma} subunit explaining the differences in channel activity observed between {beta}{gamma} and {delta}{beta}{gamma} channels. These structures define the mechanism by which ENaC subunit composition tunes ENaC function.