An ATP-responsive metabolic cassette comprised of inositol tris/tetrakisphosphate kinase 1 (ITPK1) and inositol pentakisphosphate 2-kinase (IPK1) buffers diphosphosphoinositol phosphate levels

摘要

Inositol polyphosphates are ubiquitous molecular signals in metazoans, as are their pyro-phosphorylated derivatives that bear a so-called ‘high-energy’ phosphoanhydride bond.A structural rationale is provided for the ability of Arabidopsis inositol tris/tetrakispho-sphate kinase 1 to discriminate between symmetric and enantiomeric substrates in theproduction of diverse symmetric and asymmetric myo-inositol phosphate anddiphospho-myo-inositol phosphate (inositol pyrophosphate) products. Simple tools areapplied to chromatographic resolution and detection of known and novel diphosphoinosi-tol phosphates without resort to radiolabeling approaches. It is shown that inositol tris/tetrakisphosphate kinase 1 and inositol pentakisphosphate 2-kinase comprise areversible metabolic cassette converting Ins(3,4,5,6)P 4 into 5-InsP 7 and back in a nucleo-tide-dependent manner. Thus, inositol tris/tetrakisphosphate kinase 1 is a nexus ofbioenergetics status and inositol polyphosphate/diphosphoinositol phosphate meta-bolism. As such, it commands a role in plants that evolution has assigned to a differentclass of enzyme in mammalian cells. The findings and the methods described will enablea full appraisal of the role of diphosphoinositol phosphates in plants and particularly therelative contribution of reversible inositol phosphate hydroxykinase and inositol phos-phate phosphokinase activities to plant physiology.