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

Abstract

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 approximately 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 approximately 41 kDa that makes Ins(1,4,5, 6)P4 and Ins(1,3,4,5)P4 in the same ratio and also converts Ins(1,4, 5,6)P4 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 non-biological 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.

Inositol polyphosphate multikinase (ArgRIII) determines nuclear mRNA export inSaccharomyces cerevisiae

“…1). Ipmk is also probably very similar to a 41 kDa kinase puri¢ed from Schizosaccharomyces pombe that phosphorylates Ins(1,4,5)P 3 to Ins-(1,3,4,5,6)P 5 , via Ins(1,4,5,6)P 4 and Ins(1,3,4,5)P 4 [16].…”

“…2, Table 1), presumably re£ecting some dephosphorylation of the excess Ins(1,4,5)P 3 . In yeasts, Ins(1,4,5)P 3 has been proposed to be a precursor for the synthesis of InsP 5 and InsP 6 [5,16]. This hypothesis is supported by the observations that, relative to the wild-type yeast, the ipmkv strain contained 100-fold lower levels of InsP 6 , and at least 10-fold lower levels of D/L-Ins(1,2,4,5,6)P 5 and Ins(1,3,4,5,6)P 5 (Fig.…”

Inositol hexakisphosphate is a physiological signal regulating the K + -inward rectifying conductance in guard cells

“…This is in part a consequence of an apparent lack of response of InsP 6 to extracellular signals, with a notable exception in the response of Schizosaccharomyces pombe to hypertonic osmotic shock, which leads to rapid increases in the levels of InsP 6 (2). A role for InsP 6 in cell signaling is suggested by recent work in two other systems.…”

Inositol hexakisphosphate mobilizes an endomembrane store of calcium in guard cells

“…The calcium reservoir, claimed initially to be the vacuole (6), could also include nonvacuolar stores, because most of the InsP 3 -binding capacity in the cell is in nonvacuolar membranes (7). Our ignorance of the role on InsP 6 in signaling chains stems in part from a general lack of responsiveness of InsP 6 to extracellular stimuli in systems other than guard cells (1), fission yeast (8), and hippocampal neurons (9). It is striking that the levels of InsP 6 quickly increase in hyperosmotically stressed Schizosaccharomyces pombe (8), and that in plants, treatment with ABA also quickly increases InsP 6 levels in guard cells (1).…”

“…Our ignorance of the role on InsP 6 in signaling chains stems in part from a general lack of responsiveness of InsP 6 to extracellular stimuli in systems other than guard cells (1), fission yeast (8), and hippocampal neurons (9). It is striking that the levels of InsP 6 quickly increase in hyperosmotically stressed Schizosaccharomyces pombe (8), and that in plants, treatment with ABA also quickly increases InsP 6 levels in guard cells (1). It is clearly important to establish the role of InsP 6 in ABA-induced signaling chains.…”