Kinetic interconversion of rat and bovine homologs of the alpha subunit of an amiloride-sensitive Na+ channel by C-terminal truncation of the bovine subunit

We have recently cloned the a subunit of a bovine amiloride-sensitive Na+ channel (alpha bENaC). This subunit shares extensive homology with both rat and human alpha ENaC subunits but shows marked divergence at the C terminus beginning at amino acid 584 of the 697-residue sequence, When incorporated into planar lipid bilayers, alpha bENaC almost exclusively exhibits a main transition to 39 picosiemens (pS) with very rare 13 pS step transitions to one of two subconductance states (26 and 13 pS), In contrast, the alpha subunit of the rat renal homolog of ENaC (alpha rENaC) has a main transition step to 13 pS that is almost constituitively open, with a second stepwise transition of 26 to 39 pS, A deletion mutant of alpha bENaC, encompassing the entire C-terminal region (R567X), converts the kinetic behavior of alpha bENaC to that of alpha rENaC, i.e. a transition to 13 pS followed by a second 26 pS transition to 39 pS, Chemical cross-linking of R567X restores the wild-type alpha bENaC gating pattern, whereas treatment with the reducing agent dithiothreitol produced only 13 pS transitions, In contrast, an equivalent C-terminal truncation of alpha rENaC (R613X) had no effect on the gating pattern of alpha rENaC. These results are consistent with the hypothesis that interactions between the C termini of alpha bENaC account for the different kinetic behavior of this member of the ENaC family of Na+ channels.