Palm domains of the epithelial sodium channel and sodium self‐inhibition

Epithelial Na + channels (ENaC) play an essential role in body fluid volume homeostasis by mediating apical Na + entry in epithelial cells. The structure of chicken acid sensing ion channel 1 (cASIC1) homologous to ENaCs reveals that the palm domain offers the sole link between the extracellular domain and the channel pore within a subunit. We investigated the functional roles of the two charged residues within a palm domain β‐sheet (β12) in the mechanism of Na + self‐inhibition by site‐directed mutagenesis. The Na + self‐inhibition responses for wild type and mutant channels expressed in Xenopus oocytes were examined by two electrode voltage clamp. Charge reversal mutations at a homologous site (αK561E and γR520E) of β12 significantly reduced the magnitude and speed of Na + self‐inhibition response. The Na + inhibition was absent in oocytes expressing the double mutant channel (αK561E βγR520E). Mutation of the other charged residue in γ subunit (γE524R) also significantly suppressed Na + self‐inhibition whereas the homologous mutation in α(α E565R) did not alter the inhibition. Our results suggest that certain charged residues in αand γpalm domains are critical determinants for Na + self‐inhibition. Based on our observations and the insights from the cASIC1 structure, we propose that the palm domains play a pivotal role in Na + self‐inhibition of ENaC. (Supported by NIH ES14701, DK54354 and DK079307)