Osmotic stress-induced increase of phosphatidylinositol 3,5-bisphosphate requires Vac14p, an activator of the lipid kinase Fab1p.
Phosphatidylinositol 3,5-bisphosphate (PtdIns[3,5]P(2)) was first identified as a non-abundant phospholipid whose levels increase in response to osmotic stress. In yeast, Fab1p catalyzes formation of PtdIns(3,5)P(2) via phosphorylation of PtdIns(3)P. We have identified Vac14p, a novel vacuolar protein that regulates PtdIns(3,5)P(2) synthesis by modulating Fab1p activity in both the absence and presence of osmotic stress. We find that PtdIns(3)P levels are also elevated in response to osmotic stress, yet, only the elevation of PtdIns(3,5)P(2) levels are regulated by Vac14p. Under basal conditions the levels of PtdIns(3,5)P(2) are 18-28-fold lower than the levels of PtdIns(3)P, PtdIns(4)P, and PtdIns(4,5)P(2). After a 10 min exposure to hyperosmotic stress the levels of PtdIns(3,5)P(2) rise 20-fold, bringing it to a cellular concentration that is similar to the other phosphoinositides. This suggests that PtdIns(3,5)P(2) plays a major role in osmotic stress, perhaps via regulation of vacuolar volume. In fact, during hyperosmotic stress the vacuole morphology of wild-type cells changes dramatically, to smaller, more highly fragmented vacuoles, whereas mutants unable to synthesize PtdIns(3,5)P(2) continue to maintain a single large vacuole. These findings demonstrate that Vac14p regulates the levels of PtdIns(3,5)P(2) and provide insight into why PtdIns(3,5)P(2) levels rise in response to osmotic stress.
Bonangelino
Cecilia J
CJ
Department of Biochemistry, University of Iowa, Iowa City, IA 52242, USA.
Nau
Johnathan J
JJ
Duex
Jason E
JE
Brinkman
Mikala
M
Wurmser
Andrew E
AE
Gary
Jonathan D
JD
Emr
Scott D
SD
Weisman
Lois S
LS
D001483:Base Sequence / Q000235:genetics; D002872:Chromosome Deletion; D015966:Gene Expression Regulation, Fungal / Q000502:physiology; D007425:Intracellular Membranes / Q000378:metabolism* / Q000648:ultrastructure; D008565:Membrane Proteins / Q000235:genetics* / Q000378:metabolism*; D008969:Molecular Sequence Data; D009154:Mutation / Q000235:genetics; D009997:Osmotic Pressure; D019869:Phosphatidylinositol 3-Kinases / Q000235:genetics / Q000378:metabolism; D018129:Phosphatidylinositol Phosphates / Q000378:metabolism*; D017853:Phosphotransferases (Alcohol Group Acceptor) / Q000378:metabolism*; D021381:Protein Transport / Q000502:physiology*; D029701:Saccharomyces cerevisiae Proteins / Q000235:genetics* / Q000378:metabolism*; D017386:Sequence Homology, Amino Acid; D015854:Up-Regulation / Q000502:physiology*; D014617:Vacuoles / Q000378:metabolism* / Q000648:ultrastructure; D015003:Yeasts / Q000166:cytology / Q000378:metabolism*
D008565:Membrane Proteins; D018129:Phosphatidylinositol Phosphates; D029701:Saccharomyces cerevisiae Proteins; C452435:VAC14 protein, S cerevisiae; C452436:VAC7 protein, S cerevisiae; C106336:phosphatidylinositol 3,5-diphosphate; C095504:FAB1 protein, S cerevisiae; D019869:Phosphatidylinositol 3-Kinases; D017853:Phosphotransferases (Alcohol Group Acceptor)