I. Heilmann et al., Plasma membrane phosphatidylinositol 4,5-bisphosphate levels decrease withtime in culture, PLANT PHYSL, 126(4), 2001, pp. 1507-1518
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.