Inositol 1,4,5-trisphosphate 3-kinase (IP3 3-kinase/IP3K) plays an important role in signal transduction in animal cells by phosphorylating inositol 1,4,5-trisphosphate (IP3) to inositol 1,3,4,5-tetrakisphosphate (IP4). Both IP3 and IP4 are critical second messengers which regulate calcium (Ca2+) homeostasis. Mammalian IP3Ks are involved in many biological processes, including brain development, memory, learning and so on. It is widely reported that Ca2+is a canonical second messenger in higher plants. Therefore, plant IP3K should also play a crucial role in plant development. Recently, we reported the identification of plant IP3K gene (AtIpk2β/AtIP3K) from Arabidopsis thaliana and its characterization. Here, we summarize the molecular cloning, biochemical properties and biological functions of IP3Ks from animal, yeast and plant. This review also discusses potential functions of IP3Ks in signaling crosstalk, inositol phosphate metabolism, gene transcriptional control and so on.

Inositol 1,4,5-trisphosphate 3-kinase, and more generally inositol polyphosphate kinases (Ipk), play important roles in signal transduction in animal cells; however, their functions in plant cells remain to be elucidated. Here, we report the molecular cloning of a cDNA (AtIpk2β) from a higher plant, Arabidopsis. Arabidopsis AtIpk2βis a 33-kD protein that exhibits weak homology (～25% identical amino acids) with Ipk proteins from animals and yeast and lacks a calmodulin binding site, as revealed by sequence analysis and calmodulin binding assays. However, recombinant AtIpk2βphosphorylates inositol 1,4,5-trisphosphate to inositol 1,4,5,6-tetrakisphosphate and also converts it to inositol 1,3,4,5,6-pentakisphosphate [Ins (1,3,4,5,6)P 5]. AtIpk2βalso phosphorylates inositol 1,3,4,5-tetrakisphosphate to Ins (1,3,4,5,6)P 5. Thus, the enzyme is a D3/D6 dual-specificity inositol phosphate kinase. AtIpk2 β complements a yeast ARG82/IPK2 mutant lacking a functional ArgR-Mcm1 transcription complex. This complex is involved in regulating Arg metabolism–related gene expression and requires inositol polyphosphate kinase activity to function. AtIpk2βwas found to be located predominantly in the nucleus of plant cells, as demonstrated by immunolocalization and fusion to green fluorescent protein. RNA gel blot analysis and promoter-β-glucuronidase reporter gene studies demonstrated AtIpk2β gene expression in various organs tested. These data suggest a role for AtIpk2β as a transcriptional control mediator in plants.

We have cloned a phosphatidylinositol-4-phosphate 5-kinase (PIP5K) cDNA (AtP5K1) from Arabidopsis thaliana. By the application of cell permeabilization and short-term nonequilibrium labelling we show that expression of AtP5K1 in Baculovirus-infected insect (Spodoptera frugiperda) cells directs synthesis of PtdIns (4,5) P2 and PtdIns (3,4,5) P3. The same phosphoinositides were produced by isolated whole-cell membrane fractions of AtP5K1-expressing insect cells. Their synthesis was not affected by adding defined precursor lipids, that is PtdIns (3) P, PtdIns (4) P, PtdIns (3,4) P2, or PtdIns (4,5) P2, in excess, indicating that substrates for the plant enzyme were not limiting in vivo. Enzymatic dissection of lipid headgroups revealed that AtP5K1-directed synthesis of PtdIns (4,5)P2 and PtdIns (3,4,5) P3 proceeds via 5-phosphorylation of precursors. Analysis of promoter-reporter gene (b-glucuronidase) fusions in transgenic plants revealed that expression of the AtP5K1 gene is strongest in vascular tissues of leaves, flowers, and roots, namely in cells of the lateral meristem, that is the procambium. Single-cell sampling of sap from flower stem meristem tissue and neighbouring phloem cells, when coupled to reverse transcriptase-polymerase chain reaction, con