Splicing of a retained intron within ROMK K+ channel RNA generates a novel set of isoforms in rat kidney

The renal outermedulla K⁺ channel (ROMK) familyof K⁺ channels may constitute amajor pathway for K⁺ secretion inthe distal nephron. To date, four main isoforms of this gene have beenidentified in the rat that differ only in theirNH₂-terminal amino acids and thatshare a common "core exon" that determines the remaining proteinsequence. Using RT-PCR, we have identified a new set of ROMK isoformsin rat kidney that are generated by the deletion of a region within theROMK core sequence that is identifiable as a typical mammalian intron.This splicing event was shown to be reproducible in vitro by detection of deleted ROMK mRNA in Madin-Darby canine kidney (MDCK) cells stablytransfected with the gene for ROMK2. Translation of the deletionvariant of ROMK2 was confirmed in vitro and visualized in MDCK cellsfollowing transient transfection with an enhanced green fluorescentprotein tag. The deletion in this core region is predicted to generatehydrophilic proteins that are approximately one-third of the size ofnative ROMK and lack membrane-spanning domains.