The Role of the B-Type Phospholipases in S. cerevisiae: Function, Regulation, and Physiological Relevance in Lipid Homeostasis

Membrane phospholipid synthesis and turnover is a continual process during normal cell growth. The turnover of the glycerophospholipids by B-type phospholipases (PLBs) in Saccharomyces cerevisiae results in the formation glycerophosphodiesters through a deacylation reaction. Here, I address several aspects of the glycerophospholipid deacylation, transport, and reutilization pathway in S. cerevisiae. First, I show the RAS GTPase-activating proteins, Ira1 and Ira2, are required for utilization of the glycerophosphodiester - glycerophosphoinositol (GroPIns) - as a phosphate source. Second, I demonstrate loss of the cell surface associated PLBs, Plb1-3, and/or utilization of GroPIns causes actin cytoskeleton defects and an increased cell size. For the third and major part of my dissertation, I identified a novel interaction between Ypk1 and Plb1. Ypk1, the yeast homolog of the human serum- and glucocorticoid-induced kinase (Sgk1), affects diverse cellular activities, including sphingolipid homeostasis. Here, I report that Ypk1 also impacts the turnover of the major phospholipid, phosphatidylcholine (PC). Pulse-chase radiolabeling reveals that a ypk1∆ mutant exhibits increased Plb1-mediated PC deacylation and glycerophosphocholine (GroPCho) production compared to wild type. Consistent with a link between Ypk1 and Plb1, the levels of both Plb1 protein and PLB1 message are elevated in a ypk1∆ strain compared to WT yeast. Furthermore, I discovered that an increase in PLB1 expression also occurs upon disruption to sphingolipid synthesis and is mediated by the Crz1 transcription factor. Taken together, these findings suggest that sphingolipid synthesis is coordinated with PC turnover to maintain optimal lipid homeostasis