FUNCTIONAL DEGENERIN-CONTAINING CHIMERAS IDENTIFY RESIDUES ESSENTIAL FOR AMILORIDE-SENSITIVE NA+ CHANNEL FUNCTION

The highly selective, amiloride-sensitive Na+ channel is formed of three homologous subunits termed alpha, beta, and gamma. The three subunits exhibit similarities with Caenorhabditis elegans proteins called degenerins involved in sensory touch transduction and, when mutated, in neurodegeneration. Swelling of neurons observed in neurodegeneration suggests an involvement of ion transport, but the channel function of degenerins has not yet been demonstrated. We used chimeras to study the functional relationship between the epithelial sodium channel and the degenerin Mec-4. Exchange of the hydrophobic domains of the Na+ channel alpha subunit by those of Mec-4 results in a functional ion channel with changed pharmacology for amiloride and benzamil and changed selectivity, conductance, gating, and voltage dependence. All of these differences were also obtained by exchanging Ser-589 and Ser-593 in the second transmembrane region by the corresponding residues of Mec-4, suggesting that these two residues are essential for the ionic pore function of the channel.