ALTERNATIVE SPLICING OF GLUCOKINASE MESSENGER-RNA IN RAT-LIVER

The sequences of two near full-length cDNAs encoding rat liver glucokinase are reported. One of the cDNAs is essentially identical to the cDNA cloned by Andreone, Printz, Pilkis, Magnuson and Granner [(1989) J. Biol. Chem. 264, 363-369]. The other cDNA contains a 151 bp insertion and a downstream 52 bp deletion. The inserted block of bases has been shown to originate from an optional cassette exon, termed 2A, between the previously described exons 1 and 2. The conceptual translation product from the variant mRNA is identical to the original glucokinase protein for the first 15 amino acids. Next there is a novel polypeptide sequence of 87 residues, comprising 50 residues encoded by the cassette exon and 37 residues specified by an altered reading frame in exon 2. Due to the 52 bp deletion, 17 amino acids of the reference sequence are then missing, after which the sequence reverts to the original. Northern blot analysis with oligonucleotide probes has shown that alternatively spliced mRNA represents about 5% of total glucokinase mRNA. Alternative splicing of glucokinase mRNA in liver may explain earlier findings of minor isoforms of hepatic glucokinase.