Membrane homeostasis is a fundamental aspect of cell function and requires the coordinated control of multiple metabolic activities, including biosynthetic enzymes involved in lipid synthesis and phospholipases involved in lipid turnover. Defects in lipid homeostasis can contribute to the development of many diseases, including obesity, diabetes, and cancer. In preliminary results presented here, we have identified Ypk1, the yeast homolog of the human serum and glucocorticoid-induced kinase (SGK1), as a novel regulator of phosphatidylcholine (PC) turnover at the plasma membrane (PM). Importantly, human SGK1 rescues growth defects associated with a ypk1 mutant, indicating functional conservation between the human and yeast proteins. A ypk1 yeast mutant exhibits increased Plb1-mediated PC deacylation at the PM. Furthermore, the accelerated Plb1 activity that occurs in a ypk1 mutant is a compensatory mechanism for cell survival, as a ypk1plb1 double mutant exhibits an increased growth defect and increased sensitivity to the cell wall perturbing agent, calcofluor, as compared to the ypk1 mutant. Because PC is an abundant phospholipid, alterations in its metabolism might be expected to affect the composition and physical properties of the membrane, and, therefore, membrane proteins that reside within. Indeed, we have identified two PM proteins whose activities are compromised by loss of Ypk1 or Plb1. Our central hypothesis is that deletion of YPK1 or PLB1 alters PM lipid composition, which, in turn, impinges upon the activity of integral PM proteins and results in growth defects. In the studies proposed here, we will delineate the process by which Ypk1 and Plb1 modulate integral PM protein activity, including examining the role that Plb1 and Ypk1 play in controlling PM lipid composition. In addition, the lipid requirements of a PM permease whose activity is reduced in response to loss of Ypk1 or Plb1 will be examined using unilamellar lipid vesicles.
Membrane homeostasis is a fundamental aspect of cell function and requires the coordinated control of multiple metabolic activities, including phospholipases involved in lipid turnover. Defects in membrane lipid homeostasis can contribute to the development of many diseases, including obesity, diabetes, and cancer. The proposed studies characterize a novel regulator of lipid turnover and examine its affect on membrane protein function and cell survival.
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