KINETIC INTERCONVERSION OF RAT AND BOVINE HOMOLOGS OF THE ALPHA-SUBUNIT OF AN AMILORIDE-SENSITIVE NA-TERMINAL TRUNCATION OF THE BOVINE SUBUNIT( CHANNEL BY C)

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 sequenc e, When incorporated into planar lipid bilayers, alpha bENaC almost ex clusively exhibits a main transition to 39 picosiemens (pS) with very rare 13 pS step transitions to one of two subconductance states (26 an d 13 pS), In contrast, the alpha subunit of the rat renal homolog of E NaC (alpha rENaC) has a main transition step to 13 pS that is almost c onstituitively 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 o f alpha rENaC, i.e. a transition to 13 pS followed by a second 26 pS t ransition to 39 pS, Chemical cross-linking of R567X restores the wild- type alpha bENaC gating pattern, whereas treatment with the reducing a gent dithiothreitol produced only 13 pS transitions, In contrast, an e quivalent C-terminal truncation of alpha rENaC (R613X) had no effect o n 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.