ALTERNATIVE SPLICING OF HUMAN INWARDLY RECTIFYING K-RNA( CHANNEL ROMK1 MESSENGER)

Recent studies have identified a new family of inwardly rectifying Kchannels, members of which are known by the acronyms ROMK1, IRK1, and GIRK1. We have isolated cDNAs encoding the human homologue of ROMK1 fr om an adult kidney cDNA library. The sequences of the human kidney ROM K1 cDNA clones indicated that they were derived from at least two type s of mRNAs, human ROMK1A and human ROMK1B, differing in sequence at th eir 5' ends. The isolation of the human ROMK1 gene, localized to chrom osome band 11q24 by fluorescence in situ hybridization, indicated that the different ROMK1 transcripts were generated by alternative splicin g. Human ROMK1A mRNA was predicted to encode a protein of 389 amino ac ids, having 93% identity with the 391-residue rat ROMK1 protein, and e xpression studies in Xenopus oocytes indicated that it encoded a Ba2+- sensitive inwardly rectifying K+ channel with properties similar to th ose reported for cloned rat ROMK1. Human ROMK1B mRNA was predicted to encode a protein of 372 amino acids whose sequence was truncated at th e amino terminus but otherwise identical to that of the human ROMK1A p rotein. Translation of human ROMK1B mRNA was predicted to initiate at a codon corresponding to Met-18 of human ROMK1A mRNA. Reverse transcri ptase-polymerase chain reaction amplification of human kidney mRNA rev ealed human ROMK1A and -B transcripts as well as a third type of trans cript, human ROMK1C mRNA, which was predicted to encode a protein iden tical to human ROMK1B. Human ROMK1A, -B, and -C transcripts were ident ified in kidney, whereas only human ROMK1A mRNA could be detected in p ancreatic islets and other tissues in which human ROMK1 was expressed at low levels. Thus, tissue-specific alternative splicing of human ROM K1 mRNA may result in the expression of a family of ROMK1 proteins.