Posttranscriptional regulation of Git1p, the glycerophosphoinositol/glycerophosphocholine transporter of Saccharomyces cerevisiae

Glycerophosphoinositol (GroPIns) and glycerophosphocholine (GroPCho) are the products of phospholipase-B mediated deacylation of phosphatidylinositol and phosphatidylcholine, respectively. GroPIns and GroPCho are transported across the Saccharomyces cerevisiae plasma membrane into the cell via the transporter encoded by GIT1. Previous studies have shown that GIT1 expression is regulated by inorganic phosphate (Pi) availability through the transcription factors Pho2p and Pho4p. We now report that posttranscriptional mechanisms also regulate Git1p activity in response to Pi availability. Mutations that inhibit endocytosis and vacuolar proteolysis inhibit Git1p degradation, indicating that Git1p downregulation involves internalization and subsequent degradation in the vacuole. Similar to the effect seen with Pi, provision of cells with high levels of the Git1p substrates, GroPIns and GroPCho, posttranscriptionally downregulates Git1p activity. Unlike Pi, high levels of GroPCho and GroPIns do not repress GIT1 promoter-driven reporter gene activity. These results indicate that Git1p transport activity is regulated at multiple levels by Pi availability. In addition, the results indicate that the Git1p substrates (and alternate phosphate sources) GroPIns and GroPCho behave distinctly from Pi in their ability to affect GIT1 expression.