A Single Point Mutation in the Pore Region of the Epithelial Na+Channel Changes Ion Selectivity by Modifying Molecular Sieving

The epithelial Na+channel (ENaC) belongs to a new class of channel proteins called the ENaC/DEG superfamily involved in epithelial Na+transport, mechano-transduction, and neurotransmission. The role of ENaC in Na+homeostasis and in the control of blood pressure has been demonstrated recently by the identification of mutations in ENac β and γ subunits causing hypertension. The function of ENaC in Na+reabsorption depends critically on its ability to discriminate between Na+and other ions like K+or Ca2+. ENaC is virtually impermeant to K+ions, and the molecular basis for its high ionic selectivity is largely unknown. We have identified a conserved Ser residue in the second transmembrane domain of the ENaC α subunit (α S589), which when mutated allows larger ions such as K+, Rb+, Cs+and divalent cations to pass through the channel. The relative ion permeability of each of the α S589 mutants is related inversely to the ionic radius of the permeant ion, indicating that α S589 mutations increase the molecular cutoff of the channel by modifying the pore geometry at the selectivity filter. Proper geometry of the pore is required to tightly accommodate Na+and Li+ions and to exclude larger cations. We provide evidence that ENaC discriminates between cations mainly on the basis of their size and the energy of dehydration.