Arabidopsis ITPK1 and ITPK2 Have an Evolutionarily Conserved Phytic Acid Kinase Activity

Diphospho- myo -inositol polyphosphates, also termed inositol pyrophosphates, are molecular messengers containing at least one high-energy phosphoanhydride bond and regulate a wide range of cellular processes in eukaryotes. While inositol pyrophosphates InsP 7 and InsP 8 are present in different plant species, both the identity of enzymes responsible for InsP 7 synthesis and the isomer identity of plant InsP 7 remain unknown. This study demonstrates tha Arabidopsis ITPK1 and ITPK2 catalyze the phosphorylation of phytic acid (InsP 6 ) to the symmetric InsP 7 isomer 5-InsP 7 and that the InsP 6 kinase activity of ITPK enzymes is evolutionarily conserved from humans to plants. We also show by 31 P nuclear magnetic resonance that plant InsP 7 is structurally identical to the in vitro InsP 6 kinase products of ITPK1 and ITPK2. Our findings lay the biochemical and genetic basis for uncovering physiological processes regulated by 5-InsP 7 in plants.