The Yeast BSD2-1 Mutation Influences Both the Requirement for Phosphatidylinositol Transfer Protein Function and Derepression of Phospholipid Biosynthetic Gene Expression in Yeast

摘要

The BSD2-1 allele renders Saccharomyces cermisiae independent of its normally essential requirement for phosphatidylinositol transfer protein (Secl4p) in the stimulation of Golgi secretory function and cell viability. We now report that BSD2-1 yeast mutants also exhibit yet another phenotype, an inositol auxotrophy. We demonstrate that the basis for this Ino? phenotype is the inability of BSD2-1 strains to derepress transcription of INO1 , the structural gene for the enzyme that catalyzes the committed step in de novo inositol biosynthesis in yeast. This constitutive repression of INO1 expression is mediated through specific inactivation of InoSp, a factor required for transactivation of INOl transcription, and we show that these transcriptional regulatory defects can be uncoupled from the “bypass Secl4p” phenotype of BSD2-1 strains. Finally, we present evidence that newly synthesized phosphatidylinositol is subject to accelerated turnover in BSD2-1 mutants and that prevention of this accelerated phosphatidylinositol turnover in turn negates suppression of Secl4p defects by BSD2-1 . We propose that, in BSD2-1 strains, a product(s) generated by phosphatidylinositol turnover coordinately modulates the activities of both the Secl4p/Golgi pathway and the pathway through which transcription of phospholipid biosynthetic genes is derepressed.