Changes in Phosphoinositide Metabolism with Days in Culture Affect Signal Transduction Pathways in Galdieria sulphuraria

The metabolism of phosphatidylinositol-4,5-bisphosphate (PIP(2)) changed during the culture period of the thermoacidophilic red alga Galdieria sulphuraria. Seven days after inoculation, the amount of PIP(2) in the cells was 910 ± 100 pmol g(−1) fresh weight; by 12 d, PIP(2) levels increased to 1200 ± 150 pmol g(−1) fresh weight. In vitro assays indicated that phosphatidylinositol monophosphate (PIP) kinase specific activity increased from 75 to 230 pmol min(−1) mg(−1) protein between d 7 and 12. When G. sulphuraria cells were osmostimulated, transient increases of up to 4-fold could be observed in inositol-1,4,5-trisphosphate (IP(3)) levels within 90 s, regardless of the age of the cells. In d-12 cells, the increase in IP(3) was preceded by a transient increase of up to 5-fold in specific PIP kinase activity, whereas no such increase was detected after osmostimulation of d-7 cells. The increase in PIP kinase activity before IP(3) signaling in d-12 cells indicates that there is an additional pathway for regulation of phosphoinositide metabolism after stimulation other than an initial activation of phospholipase C. Also, the rapid activation of PIP(2) biosynthesis in cells with already-high PIP(2) levels suggests that the PIP(2) present was not available for signal transduction. By comparing the response of the cells at d 7 and 12, we have identified two potentially distinct pools of PIP(2).