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

In comparison with other cell organelles, the Dunaliella salina plasma membrane was found to be highly enriched in phospholipase C activity toward exogenous ³H]phosphatidylinositol 4,5-bisphosphate (PIP₂). Based on release of ³H]inositol phosphates, the plasma membrane exhibited a PIP₂-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 ³H]inositol phosphates released were recovered as ³H]inositol trisphosphate (IP₃). These results suggest a plausible mechanism for the rapid breakdown of PIP₂ and phosphatidylinositol 4-phosphate (PIP) following hypoosmotic shock. Quantitative analysis of major ³H]inositol phospholipids during these assays revealed that some of the ³H]-PIP₂ was converted to ³H]phosphatidylinositol 4-monophosphate (PIP) and to ³H]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 PIP₂/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 Ca²⁺]. We also report here that 100 micromolar GTPγS stimulates phosphospholipase C activity over a range of free Ca²⁺]. Together, these results provide evidence that the plasma membrane PIP₂-phospholipase C of D. salina may be subject to Ca²⁺ and G-protein regulation.