STT4 IS AN ESSENTIAL PHOSPHATIDYLINOSITOL 4-KINASE THAT IS A TARGET OF WORTMANNIN IN SACCHAROMYCES-CEREVISIAE

Wortmannin is a natural product that inhibits signal transduction. One target of wortmannin in mammalian cells is the 110-kDa catalytic subu nit of phosphatidylinositol S-kinase (PI 3-kinase). We show that wortm annin is toxic to the yeast Saccharomyces cerevisiae and present genet ic and biochemical evidence that a phosphatidylinositol 4-kinase (PI 4 -kinase), STT4, is a target of wortmannin in yeast. In a strain backgr ound in which stt4 mutants are rescued by osmotic support with sorbito l, the toxic effects of wortmannin are similarly prevented by sorbitol . In contrast, in a different strain background, STT4 is essential und er all conditions and wortmannin toxicity is not mitigated by sorbitol . Overexpression of STT4 confers wortmannin resistance, but overexpres sion of PIK1, a related PI 4-kinase, does not. In vitro, the PI 4-kina se activity of STT4, but not of PIK1, was potently inhibited by wortma nnin. Overexpression of the phosphatidylinositol 4-phosphate 5-kinase homolog MSS4 conferred wortmannin resistance, as did deletion of phosp holipase C-1. These observations support a model for a phosphatidylino sitol metabolic cascade involving STT4, MSS4, and phospholipase C-1 an d provide evidence that an essential product of this pathway is the li pid phosphatidylinositol 4,5-bisphosphate.