Rat inositol 1,4,5-trisphosphate 3-kinase C is enzymatically specialized for basal cellular inositol trisphosphate phosphorylation and shuttles actively between nucleus and cytoplasm

The calcium-liberating second messenger inositol 1,4,5-trisphosphate (Ins(1,4,5) P-3) is converted to inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5) P-4) by Ins(1,4,5) P-3 3-kinases (IP3Ks) that add a fourth phosphate group to the 3-position of the inositol ring. Two isoforms of IP3Ks ( named A and B) from different vertebrate species have been well studied. Recently the cloning and examination of a human full-length cDNA encoding a novel isoform, termed human IP3K-C (HsIP3K-C), has been reported. In the present study we report the cloning of a full-length cDNA encoding a rat homologue of HsIP3K-C with a unique mRNA expression pattern, which differs remarkably from the tissue distribution of HsIP3K-C. Of the rat tissues examined, rat IP3K-C ( RnIP3K-C) is mainly present in heart, brain, and testis and shows the strongest expression in an epidermal tissue, namely tongue epithelium. RnIP3K-C has a calculated molecular mass of similar to74.5 kDa and shows an overall identity of similar to75% with HsIP3K-C. A bacterially expressed, enzymatically active and Ca2+-calmodulin-regulated fragment of this isoform displays remarkable enzymatic properties like a very low K-m for Ins(1,4,5) P-3 (similar to0.2 muM), substrate inhibition by high concentrations of Ins( 1,4,5) P-3, allosteric product activation by Ins( 1,3,4,5) P-4 in absence of Ca2+-calmodulin (K-a(app) 0.52 muM), and the ability to efficiently phosphorylate a second InsP(3) substrate, inositol 2,4,5-trisphosphate, to inositol 2,4,5,6-tetrakisphosphate in the presence of Ins( 1,3,4,5) P-4. Furthermore, the RnIP3K-C fused with a fluorescent protein tag is actively transported into and out of the nucleus when transiently expressed in mammalian cells. A leucine-rich nuclear export signal and an uncharacterized nuclear import activity are localized in the N-terminal domain of the protein and determine its nucleocytoplasmic shuttling. These findings point to a particular role of RnIP3K-C in nuclear inositol trisphosphate phosphorylation and cellular growth.