Osmotic shock induced regulation of the signaling lipid PI3,5P2.
Jason E. Duex, Johnathan J. Nau, Lois S. Weisman
Biochemistry, University of Iowa, 2117 MERF, Iowa City, IA, 52246, United States
Phosphatidylinositol 3,5-bisphosphate (PI3,5P2) is one of four phosphorylated PI lipids found in yeast. The steady state levels of PI3,5P2 under basal conditions are 20-fold lower than the other phosphorylated PI lipids. When yeast cells are stimulated with hyperosmotic media the levels of PI3,5P2 rise dramatically. We find that by 5 minutes PI3,5P2 levels match the levels of the other phosphorylated PIs. Furthermore, the turnover of PI3,5P2 is equally rapid. After 10 minutes at elevated levels, PI3,5P2 is rapidly turned over and by 20 minutes PI3,5P2 levels have returned close to basal levels. A dramatic reduction in vacuolar volume occurs concomitant with the rise of PI3,5P2 levels. With the reduction in PI3,5P2 levels, vacuole volume returns to normal. These observations suggest PI3,5P2 plays an acute protective role. Fab1p is the lipid kinase that generates PI3,5P2 from PI3P. Our lab has identified two activators of Fab1p, Vac7p and Vac14p. Fig4p, a phosphatidylinositol phosphatase, dephosphorylates PI3,5P2 in vitro and is likely one of the enzymes important for PI3,5P2 turnover (Rudge, et al. Mol. Biol. Cell, 2003). In further support of this role we observe a delay in PI3,5P2 turnover in a fig4delta strain. Surprisingly, the Fab1p activator, Vac14p, interacts with Fig4p. Moreover, vac14delta and fig4delta cells display a similar delay in PI3,5P2 turnover. These data suggest that following hyperosmotic stimulation, synthesis and turnover of PI3,5P2 are acutely coupled.
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