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)P3) is converted to inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P4) by Ins(1,4,5)P3 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 approximately 74.5 kDa and shows an overall identity of approximately 75% with HsIP3K-C. A bacterially expressed, enzymatically active and Ca2+-calmodulin-regulated fragment of this isoform displays remarkable enzymatic properties like a very low Km for Ins(1,4,5)P3 ( approximately 0.2 microm), substrate inhibition by high concentrations of Ins(1,4,5)P3, allosteric product activation by Ins(1,3,4,5)P4 in absence of Ca2+-calmodulin (Ka(app) 0.52 microm), and the ability to efficiently phosphorylate a second InsP3 substrate, inositol 2,4,5-trisphosphate, to inositol 2,4,5,6-tetrakisphosphate in the presence of Ins(1,3,4,5)P4. 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.
Nalaskowski
Marcus M
MM
Institute for Cellular Signal Transduction, University Hospital Hamburg-Eppendorf, Martinistrasse 52, Germany.
Bertsch
Uwe
U
Fanick
Werner
W
Stockebrand
Malte C
MC
Schmale
Hartwig
H
Mayr
Georg W
GW
D000818:Animals; D001483:Base Sequence; D002467:Cell Nucleus / Q000378:metabolism*; D003001:Cloning, Molecular; D003593:Cytoplasm / Q000378:metabolism*; D017931:DNA Primers; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism*; D010766:Phosphorylation; D017853:Phosphotransferases (Alcohol Group Acceptor) / Q000235:genetics / Q000378:metabolism*; D021381:Protein Transport; D051381:Rats
D017931:DNA Primers; D015544:Inositol 1,4,5-Trisphosphate; D017853:Phosphotransferases (Alcohol Group Acceptor); C051038:Inositol 1,4,5-trisphosphate 3-kinase