Gating of amiloride-sensitive Na+ channels: Subunit-subunit interactions and inhibition by the cystic fibrosis transmembrane conductance regulator

In search of the structural basis for gating of amiloride-sensitive Na+ cha nnels, kinetic properties of single homo and heterooligomeric ENaCs formed by the subunits with individual truncated cytoplasmic domains were studied in a cell-free planar lipid bilayer reconstitution system. Our results iden tify the N-terminus of the alpha-subunit as a major determinant of kinetic behavior of both homooligomeric and heterooligomeric ENaCs, although the ca rboxy-terminal domains of beta- and gamma-ENaC subunits play important role (s) in modulation of the kinetics of heterooligomeric channels. We also fou nd that the cystic fibrosis transmembrane conductance regulator (CFTR) inhi bits amiloride-sensitive channels, at least in part, by modulating their ga ting. Comparison of these data suggests that the modulatory effects of the beta- and gamma-ENaC subunits, and of the CFTR, may involve the same, or cl osely related, mechanism(s); namely, "locking" the heterooligomeric channel s in their closed state. These mechanisms, however, do not completely overr ide the gating mechanism of the alpha-channel.