The molecular composition of a core conduction element formed by the α- subunit of cloned epithelial Na+ channels (ENaC) was studied in Planar lipid bilayers. Two pairs of in vitro translated proteins were employed in combinatorial experiments: 1) wild-type (WT) and an N-terminally truncated α(ΔN)-rENaC that displays accelerated kinetics (τ(o) = 32 ± 13 ms, τ(o) = 42 ± 11 ms), as compared with the WT channel (τ(c1) = 18 ± 8 ms, τ(c2) = 252 ± 31 ms, and τ(o) = 157 ± 43 ms); and 2) WT and an amiloride binding mutant, α(Δ278-283)-rENaC. The channels that formed in a α(WT):α(AN) mixture fell into two groups: one with τ(o) and τ(c) that corresponded to those exhibited by the α(ΔN)-rENaC alone, and another with a double- exponentially distributed closed time and a single-exponentially distributed open time that corresponded to the α(WT)-rENaC alone. Five channel subtypes with distinct sensitivities to amiloride were found in a 1α(WT):1α(Δ278- 283) protein mixture. Statistical analyses of the distributions of channel phenotypes observed for either set of the WT:mutant combinations suggest a tetrameric organization of α-subunits as a minimal model for the core conduction element in ENaCs.
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