Characterization of gene organization and generation of heterogeneous mRNA species of rat I^SK protein

The I_SK protein is a novel, probably epithelial potassium channel which differs from conventional potassium channels in its structure, electrophysiology, and tissue distribu tion. In this investigation, we isolated and analyzed genomic and cDNA clones coding for the rat I_SK protein to characterize the structural organization and expression pattern of the ISK protein gene. This analysis, together with primer extension and RNase protection experiments, indicated that the I_SK protein mRNA is initiated from two different upstream exons and then encoded by an uninterrupted downstream exon covering the protein-coding and the 3′-untranslated regions of the mRNA. RNA blot hybridization analysis showed additional generation of several large species of mRNAs which result from Inclusion of a part of the intron sequence and the 3′-flanking region of the I_SK protein gene. Thus, the single I_SK protein gene is involved in the production of multiple species of mRNAs through a variety of cellular mechanisms including transcription initiation at different sites, alterna tive RNA splicing, and polyadenylation at different sites. The heterogeneity of the I_SK protein mRNM may be associated with the emergence of the functional and regulatory diversity observed for potassium ion permeation in epithelial cells.