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Correspondence to Lawrence G. Palmer: lgpalm{at}med.cornell.edu
The epithelial Na+ channel (ENaC) is a multimeric membrane protein consisting of three subunits, 
, ß, and 
. The total number of subunits per functional channel complex has been described variously to follow either a tetrameric arrangement of 2:1ß:1 or a higher-ordered stoichiometry of 3:3ß:3. Therefore, while it is clear that all three ENaC subunits are required for full channel activity, the number of the subunits required remains controversial. We used a new approach, based on single-channel measurements in Xenopus oocytes to address this issue. Individual mutations that alter single-channel conductance were made in pore-lining residues of ENaC , ß, or subunits. Recordings from patches in oocytes expressing a single species, wild type or mutant, of , ß, and showed a well-defined current transition amplitude with a single Gaussian distribution. When cRNAs for all three wild-type subunits were mixed with an equimolar amount of a mutant -subunit (either S589D or S592T), amplitudes corresponding to pure wild-type or mutant conductances could be observed in the same patch, along with a third intermediate amplitude most likely arising from channels with at least one wild-type and at least 1 mutant -subunit. However, intermediate or hybrid conductances were not observed with coexpression of wild-type and mutant ßG529A or G534E subunits. Our results support a tetrameric arrangement of ENaC subunits where 2, 1ß, and 1 come together around central pore.
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  A. Anantharam and L. G. Palmer Determination of Epithelial Na+ Channel Subunit Stoichiometry from Single-channel Conductances J. Cell Biol., October 3, 2007; 178(1): i4 - i4. [Full Text]
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