Heilmann I, Perera IY, Gross W, Boss WF (2001) Plant Physiol 126: 1507-1518

Plasma membrane phosphatidylinositol 4,5-bisphosphate levels decrease with time in culture.

During the stationary phase of growth, after 7 to12 days in culture, the levels of phos-phatidyl-ino-sitol 4,5-bis-phos-phate (PtdInsP2) decreased by 75% in plasma membranes of the red alga Galdieria sulphuraria. Concomitant with the decrease in PtdInsP2 levels in plasma membranes, there was an increase in PtdInsP2 in microsomes, suggesting that the levels of plasma membrane PtdInsP2 are regulated differentially. The decline of PtdInsP2 in plasma membranes was accompanied by a 70%-decrease in the specific activity of PtdInsP kinase and by reduced levels of protein cross-reacting with antisera against a conserved PtdInsP kinase domain. Upon osmotic stimulation, the loss of PtdInsP2 from the plasma membrane increased from 10% in 7-day-old cells to 60% in 12-day-old cells, although the levels of inositol 1,4,5-trisphosphate (InsP3) produced in whole cells were roughly equal at both times. When cells from different time points during the stationary phase were osmotically stimulated, a mild osmotic stress (12.5 mM KCl) activated PtdInsP kinase prior to InsP3 production in cells with low plasma membrane PtdInsP2 levels, whereas in cells with high plasma membrane PtdInsP2 more severe stress (250 mM KCl) was required to induce an increase in PtdInsP kinase activity. The differential regulation of a plasma membrane signaling pool of PtdInsP2 is discussed with regard to the implications for understanding the responsive state of cells.