Atomic Force Microscopy Reveals the Architecture of the Epithelial Sodium Channel (ENaC)

The epithelial sodium channel (ENaC) is a member of the ENaC/degenerin superfamily. ENaC is a heteromultimer containing three homologous subunits (alpha, beta, and gamma); however, the subunit stoichiometry is still controversial. Here, we addressed this issue using atomic force microscopy imaging of complexes between isolated ENaC and antibodies/Fab fragments directed against specific epitope tags on the alpha-, beta- and gamma-subunits. We show that for alpha-, beta- and gamma-ENaC alone, pairs of antibodies decorate the channel at an angle of 120 degrees, indicating that the individual subunits assemble as homotrimers. A similar approach demonstrates that alpha beta gamma-ENaC assembles as a heterotrimer containing one copy of each subunit. Intriguingly, all four subunit combinations also produce higher-order structures containing two or three individual trimers. The trimer-of-trimers organization would account for earlier reports that ENaC contains eight to nine subunits.