Accessibility of ENaC extracellular domain central core residues

The epithelial Na+ channel (ENaC)/degenerin family has a similar extracellular architecture, where specific regulatory factors interact and alter channel gating behavior. The extra cellular palm domain serves as a key link to the channel pore. In this study, we used cysteine-scanning mutagenesis to assess the functional effects of Cys-modifying reagents on palm domain beta 10 strand residues in mouse ENaC. Of the 13 ENaC alpha subunit mutants with Cys substitutions examined, only mutants at sites in the proximal region of beta 10 exhibited changes in channel activity in response to methanethiosulfonate reagents. Additionally, Cys substitutions at three proximal sites of beta and gamma subunit beta 10 strands also rendered mutant channels methanethiosulfonate-responsive. Moreover, multiple Cys mutants were activated by low concentrations of thiophilic Cd2+. Using the Na+ self-inhibition response to assess ENaC gating behavior, we identified four alpha, two beta, and two gamma subunit beta 10 strand mutations that changed the Na+ self-inhibition response. Our results suggest that the proximal regions of beta 10 strands in all three subunits are accessible to small aqueous compounds and Cd(2+ )and have a role in modulating ENaC gating. These results are consistent with a structural model of mouse ENaC that predicts the presence of aqueous tunnels adjacent to the proximal part of beta 10 and with previously resolved structures of a related family member where palm domain structural transitions were observed with channels in an open or closed state.

Automated summary

This study used cysteine-scanning mutagenesis to assess the functional effects of Cys-modifying reagents on the palm domain beta 10 strand residues in mouse ENaC. The results suggest that the proximal regions of beta 10 strands in all three subunits are accessible to small aqueous compounds and Cd(2+) and have a role in modulating ENaC gating.