Gating induces a conformational change in the outer vestibule of ENaC

The epithelial Na+ channel (ENaC) is comprised of three homologous subunits (alpha, beta, and gamma). The channel forms the pathway for Na+ absorption in the kidney and mutations cause disorders of Na+ homeostasis. However little is known about the mechanisms that control the gating of ENaC. We investigated the gating mechanism by introducing bulky side chains at a position adjacent to the extracellular end of the second membrane spanning segment (549, 520, and 529 in alpha, beta, and gamma ENaC, respectively). Equivalent "DEG" mutations in related DEG/ENaC channels in Caenorhabditis elegans cause swelling neurodegeneration, presumably by increasing channel activity. We found that the Na+ current was increased by mutagenesis or chemical modification of this residue and adjacent residues in alpha, beta, and gamma ENaC. This resulted from a change in the gating of ENaC; modification of a cysteine at position 520 in beta ENaC increased the open state probability from 0.12 to 0.96. Accessibility to this side chain from the extracellular side was state-dependent; modification occurred only when the channel was in the open conformation. Single-channel conductance decreased when the side chain contained a positive, but not a negative charge. However, alterations in the side chain did not alter the selectivity of ENaC. This is consistent with a location for the DEG residue in the outer vestibule. The results suggest that channel gating involves a conformational change in the outer vestibule of ENaC. Disruption of this mechanism could be important clinically since one of the mutations that increased Na+ current (gamma (N530K)) was identified in a patient with renal disease.