Phosphatidylinositol 4,5-Bisphosphate Phospholipase C and Phosphomonoesterase in Dunaliella salina Membranes

Abstract

In comparison with other cell organelles, the Dunaliella salina plasma membrane was found to be highly enriched in phospholipase C activity toward exogenous [(3)H]phosphatidylinositol 4,5-bisphosphate (PIP(2)). Based on release of [(3)H]inositol phosphates, the plasma membrane exhibited a PIP(2)-phospholipase C activity nearly tenfold higher than the nonplasmalemmal, nonchloroplast ;bottom phase' (BP) membrane fraction and 47 times higher than the chloroplast membrane fraction. The majority of phospholipase activity was clearly of a phospholipase C nature since over 80% of [(3)H]inositol phosphates released were recovered as [(3)H]inositol trisphosphate (IP(3)). These results suggest a plausible mechanism for the rapid breakdown of PIP(2) and phosphatidylinositol 4-phosphate (PIP) following hypoosmotic shock. Quantitative analysis of major [(3)H]inositol phospholipids during these assays revealed that some of the [(3)H]-PIP(2) was converted to [(3)H]phosphatidylinositol 4-monophosphate (PIP) and to [(3)H]phosphatidyl-inositol (PI) in the BP fraction of membrane remaining after removal of plasmalemma and chloroplasts. This latter fraction is enriched more than fivefold in PIP(2)/PIP phosphomonoesterase activity when compared to the plasmalemma or chloroplast membrane fractions. We have also examined some of the in vitro characteristics of the plasma membrane phospholipase C activity and have found it to be calcium sensitive, reaching maximal activity at 10 micromolar free [Ca(2+)]. We also report here that 100 micromolar GTPgammaS stimulates phosphospholipase C activity over a range of free [Ca(2+)]. Together, these results provide evidence that the plasma membrane PIP(2)-phospholipase C of D. salina may be subject to Ca(2+) and G-protein regulation.