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Original Article |
psnyder{at}blue.weeg.uiowa.edu
The epithelial Na+ channel (ENaC) is comprised of three homologous subunits (
, β, and 
). 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 , β, and 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 , β, and ENaC. This resulted from a change in the gating of ENaC; modification of a cysteine at position 520 in βENaC 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 (N530K) was identified in a patient with renal disease.
Key Words: hypertension • amiloride • sodium channel • epithelia • degenerin
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