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

Schizosaccharomyces pombe extracts synthesize InsP(6) (myo-inositol hexaphosphate) from Ins(1,4,5)P-3 plus ATP. An S. pombe soluble fraction converts Ins(1,4,5)P-3 into Ins(1,4,5,6)P-4 and Ins(1,3,4,5)P-4, in a constant ratio of approximate to 5:1, and thence to Ins(1,3,4,5,6)P-5 and InsP(6). We have purified a soluble Mg2+-dependent kinase of molecular mass approximate to 41 kDa that makes Ins(1,4,5,6)P-4 and Ins(1,3,4,5)P-4 in the same ratio and also converts Ins(1,4,5,6)P, or Ins(1,3,4,5)P, into Ins(1,3,4,5,6)P, and InsP(6). Of InsP(3) isomers other than Ins(1,4,5)P-3, only the nonbiological molecule Ins(1,4,6)P-3 potently 'competed' with all steps in conversion of Ins(1,4,5)P-3 into InsP(6). 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)P-3. S. pombe that have been cultured with [H-3]inositol contains a variety of H-3-labelled inositol polyphosphates, with Ins(1,4,5)P-3 and InsP(6) the most prominent, and the InsP(6) concentration quickly increases in hyper-osmotically stressed S. pombe. This yeast therefore contains InsP(6) and Ins(1,4,5)P-3 as normal constituents, makes more InsP(6) when hyper-osmotically stressed and contains a versatile inositol polyphosphate kinase that synthesizes InsP(6) from Ins(1,4,5)P-3.