The second hydrophobic domain contributes to the kinetic properties of epithelial sodium channels

The epithelial sodium channel (ENaC) is the prototype of a new class of ion channels known as the ENaC/Deg family. The hallmarks of ENaC are a high se lectivity for Na+, block by amiloride, small conductance, and slow kinetics that are voltage-independent. We have investigated the contribution of the second hydrophobic domain of each of the homologous subunits alpha, beta, and gamma to the kinetic properties of ENaC. Chimeric subunits were constru cted between alpha and beta subunits (alpha-beta) and between gamma and bet a subunits (gamma-beta), Chimeric and wild-type subunits were expressed in various combinations in Xenopus oocytes. Analysis of whole-cell and unitary currents made it possible to correlate functional properties with specific sequences in the subunits. Functional channels were generated without the second transmembrane domain from a subunits, indicating that it is not esse ntial to form functional pores. The open probability and kinetics varied wi th the different channels and were influenced by the second hydrophobic dom ains. Amiloride affinity, Li+/Na+ selectivity, and single channel conductan ce were also affected by this segment.