5 8 Heterogeneity in epithelial sodium channel a - subunit mRNA leads to distinct NH 2 - terminal variant proteins CHRISTIE

Thomas, Christie P., Scott Auerbach, John B. Stokes, and Kenneth A. Volk. 58 Heterogeneity in epithelial sodium channel a-subunit mRNA leads to distinct NH2-terminal variant proteins. Am. J. Physiol. 274 (Cell Physiol. 43): C1312–C1323, 1998.—The amiloride-sensitive epithelial sodium channel (ENaC) is composed of three subunits: a, b, and g. The human a-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 58 end of the a-ENaC gene. Four transcripts that differ at their first exon were identified. Exon 1A splices to exon 2 to form the 58 end of a-ENaC1, whereas exon 1B arises separately and continues into exon 2 to form a-ENaC2. Other variant mRNAs, aENaC3 and a-ENaC4, are formed by activating 58 splice sites within exon 1B. Although a-ENaC3 and -4 did not change the open reading frame for a-ENaC, a-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 a-ENaC transcript when combined with band g-ENaC cRNA reconstituted a low-conductance ion channel with amiloride-sensitive currents of similar characteristics. We have thus identified variant a-ENaC mRNAs that lead to functional ENaC peptides.