Inositol hexakisphosphate in Schizosaccharomyces pombe: synthesis from Ins(1,4,5)P₃ and osmotic regulation

Schizosaccharomyces pombe extracts synthesize InsP₆ (myo-inositol hexaphosphate) from Ins(1,4,5)P₃ plus ATP. An S. pombe soluble fraction converts Ins(1,4,5)P₃ into Ins(1,4,5,6)P₄ and Ins(1,3,4,5)P₄, in a constant ratio of ≃5:1, and thence to Ins(1,3,4,5,6)P₅ and InsP₆. We have purified a soluble Mg²⁺-dependent kinase of molecular mass ≃41 kDa that makes Ins(1,4,5,6)P₄ and Ins(1,3,4,5)P, in the same ratio and also converts Ins(1,4,5,6)P, or Ins(1,3,4,5)P₄ into Ins(1,3,4,5,6)P₅ and InsP₆. Of InsP₃ isomers other than Ins(1,4,5)P₃, only the nonbiological molecule Ins(1,4,6)P₃ potently 'competed' with all steps in conversion of Ins(1,4,5)P₃ into InsP₆. Examination of molecular graphics representations allowed us to draw tentative conclusions about the environment needed for an hydroxyl group to be phosphorylated by this kinase and to predict successfully that the purified kinase would phosphorylate the 5-hydroxyl of Ins(1,4,6)P₃. S. pombe that have been cultured with [³H]inositol contains a variety of ³H-labelled inositol polyphosphates, with Ins(1,4,5)P₃ and InsP₆ the most prominent, and the InsP₆ concentration quickly increases in hyper-osmotically stressed S. pombe. This yeast therefore contains InsP₆ and Ins(1,4,5)P₃ as normal constituents, makes more InsP₆ when hyper-osmotically stressed and contains a versatile inositol polyphosphate kinase that synthesizes InsP₆ from Ins(1,4,5)P₃.