| The inositol phospholipids are important membrane components which function in biochemically transducing many extracellular stiumuli and which thereby alter cellular physiology. Very little is known about their function in sensing environmental conditions and virtually nothing is known regarding their role in photosynthetic organisms. In the unicellular alga, Dunaliella salina, phosphatidylinositol, phosphatidylinositol 4-phosphate (PIP), and phosphatidylinositol 4,5-bisphosphate (PIP{dollar}sb2{dollar}) comprise 14.8, 1.2, and 0.3 mol %, respectively, of cellular phospholipids. In isolated plasma membrane fractions, PIP and PIP{dollar}sb2{dollar} are highly concentrated, together comprising 9.5 mol % of plasmalemma phospholipids. The metabolism of these inositol phospholipids and phosphatidic acid (PA) is very rapid under normal growth conditions, as illustrated by the fact that within 5 minutes after introduction of {dollar}sp{lcub}32{rcub}{dollar}P{dollar}sb{lcub}rm i{rcub}{dollar} into the growth medium, over 75% of lipid-bound label was found in these quantitatively minor phospholipids. Within 2 minutes after a sudden hypoosmotic shock, the levels of PIP{dollar}sb2{dollar} and PIP dropped to 65 and 79%, respectively, of controls. Within the same time frame PA rose to 141% of control values. These data suggest that a rapid breakdown of the polyphosphoinositides may mediate the profound morphological and physiological changes which allow this organism to survive drastic hypoosmotic stress. The demonstration of a plasmalemmal phospholipase C in Dunaliella salina further supports such a hypothesis. In contrast to hypoosmotic shock, hyperosmotic shock induced a rise in PIP{dollar}sb2{dollar} levels to 131% of control values, whereas the level of PA dropped to 56% of controls after 4 minutes. Further analysis demonstrated that within 10 minutes, there was a 2.5-fold increase in phosphatidylcholine turnover, a 2-fold increase in lysophosphatidylcholine mass, a 4-fold increase in lysophosphatidate mass, and an elevation in free fatty acids to 124% of control, all observations suggesting activation of phospholipase A. These two different types of osmotic stress affect inositol phospholipid metabolism in a fundamentally opposite manner, with only hypoosmotic shock inducing a net decrease in polyphosphoinositides. These studies provide one of the first correlations between inositol phospholipid hydrolysis and a cellular response to hypoosmotic shock. These studies also provide one of the most thorough characterizations, to date, of an inositol phospholipid signal cascade in a photosynthetic organism. |
