5' Heterogeneity in epithelial sodium channel α-subunit mRNA leads to distinct NH2-terminal variant proteins

Abstract

The amiloride-sensitive epithelial sodium channel (ENaC) is composed of three subunits: α, β, and γ. The human α-ENaC subunit is expressed as at least two transcripts (N. Voilley, E. Lingueglia, G. Champigny, M. G. Mattei, R. Waldmann, M. Lazdunski, and P. Barbry. Proc. Natl. Acad. Sci. USA 91: 247- 251, 1994). To determine the origin of these transcripts, we characterized the 5' end of the α-ENaC gene. Four transcripts that differ at their first exon were identified. Exon 1A splices to exon 2 to form the 5' end of α- ENaC1, whereas exon lB arises separately and continues into exon 2 to form α-ENaC2. Other variant mRNAs, α-ENaC3 and α-ENaC4, are formed by activating 5' splice sites within exon 1B. Although α-ENaC3 and -4 did not change the open reading frame for α-ENaC, α-ENaC2 contains upstream ATGs that add 59 amino acids to the previous (a-ENaC1) protein. To address the significance of these isoforms, both proteins were expressed in Xenopus oocytes. The cRNA for each α-ENaC transcript when combined with β- and γ- ENaC cRNA reconstituted a low-conductance ion channel with amiloride- sensitive currents of similar characteristics. We have thus identified variant α-ENaC mRNAs that lead to functional ENaC peptides.