Plasma membrane phosphatidylinositol 4,5-bisphosphate levels decrease withtime in culture

During the stationary phase of growth, after 7 to 12 d in culture, the leve ls of phosphatidylinositol 4,5-bisphosphate (PtdInsP(2)) decreased by 75% i n plasma membranes of the red alga Galdieria sulphuraria. Concomitant with the decrease in PtdInsP(2). levels in plasma membranes, there was an increa se in PtdInsP(2) in microsomes, suggesting that the levels of plasma membra ne PtdInsP(2) are regulated differentially. The decline of PtdInsP(2) in pl asma membranes was accompanied by a 70% decrease in the specific activity o f PtdInsP kinase and by reduced levels of protein cross-reacting with antis era against a conserved PtdInsP kinase domain. Upon osmotic stimulation, th e loss of PtdInsP(2) from the plasma membrane increased from 10% in 7-d-old cells to 60% in 12-d-old cells, although the levels of inositol 1,4,5-tris phosphate (InsP(3)) produced in whole cells were roughly equal at both time s. When cells with low plasma membrane PtdInsP(2). levels were osmotically stimulated, a mild osmotic stress (12.5 mm KCl) activated PtdInsP kinase pr ior to InsP(3) production, whereas in cells with high plasma membrane PtdIn sP(2), more severe stress (250 mm KCI) was required to induce an increase i n PtdInsP kinase activity. The differential regulation of a plasma membrane signaling pool of PtdInsP(2) is discussed with regard to the implications for understanding the responsive state of cells.