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Arabidopsis Inositol Polyphosphate 6-/3-Kinase Is a Nuclear Protein That Complements a Yeast Mutant Lacking a Functional ArgR-Mcm1 Transcription Complex

^a Max-Planck-Institute of Molecular Plant Physiology, D-14424 Potsdam, Germany
^b School of Biological Sciences, University of East Anglia, Norwich NR4 7TJ, United Kingdom
^c Department of Plant Sciences, The Weizmann Institute of Science, Rehovot 76100, Israel
^d Centre for Plant Sciences, Leeds Institute for Biotechnology and Agriculture, School of Biology, University of Leeds, Leeds LS2 9JT, United Kingdom

² To whom correspondence should be addressed. E-mail bmr{at}rz.uni-potsdam.de; fax 49-331-977-2512

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 (AtIpk2) from a higher plant, Arabidopsis. Arabidopsis AtIpk2 is 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 AtIpk2 phosphorylates 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)P₅]. AtIpk2 also phosphorylates inositol 1,3,4,5-tetrakisphosphate to Ins(1,3,4,5,6)P₅. Thus, the enzyme is a D3/D6 dual-specificity inositol phosphate kinase. AtIpk2 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. AtIpk2 was 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–-glucuronidase reporter gene studies demonstrated AtIpk2 gene expression in various organs tested. These data suggest a role for AtIpk2 as a transcriptional control mediator in plants.

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