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 InsP₇ and InsP₈ are present in different plant species, both the identity of enzymes responsible for InsP₇ synthesis and the isomer identity of plant InsP₇ remain unknown. This study demonstrates that Arabidopsis ITPK1 and ITPK2 catalyze the phosphorylation of phytic acid (InsP₆) to the symmetric InsP₇ isomer 5-InsP₇ and that the InsP₆ kinase activity of ITPK enzymes is evolutionarily conserved from humans to plants. We also show by ³¹P nuclear magnetic resonance that plant InsP₇ is structurally identical to the in vitro InsP₆ kinase products of ITPK1 and ITPK2. Our findings lay the biochemical and genetic basis for uncovering physiological processes regulated by 5-InsP₇ in plants.