UBC Theses and Dissertations
Lipids as pH biosensors Shin, John J. H.
Recognition of membrane lipids by soluble proteins is important for the recruitment of these proteins onto membranes. Hence, changes in the concentration of these lipids affect the activity of these proteins, which alters downstream signal transduction pathways. Therefore, these lipids play significant roles as signaling molecules. Phosphatidic acid (PA) and phosphoinositides are signaling lipids that are present in all eukaryotes and are involved in the regulation of numerous critical cellular processes. The objective of this thesis was to identify new regulators and mechanisms of PA and phosphoinositide signaling through the utilization of the model eukaryote Saccharomyces cerevisiae. Based on a genome-wide screen to identify novel factors affecting PA signaling, the binding of proteins to PA was found to be dependent on intracellular pH (pHi) and the protonation state of its phosphomonoester headgroup. In yeast, a rapid decrease in pHi in response to glucose starvation regulated binding of PA to a transcription factor, Opi1, that coordinately repressed phospholipid metabolic genes. Hence, PA is a pH biosensor that enabled coupling of membrane biogenesis to nutrient availability (Chapter 2). Many phosphoinositides also possess phosphomonoesters on their headgroup that are sensitive to protonation within the physiological pHi range; including phosphatidylinositol 4-phosphate (PI(4)P) that is enriched in the trans-Golgi. Binding of Osh1, a member of the oxysterol-binding protein (OSBP)-related protein family (ORP), to late-Golgi PI(4)P was also found to be dependent on pHi and the protonation state of its phosphomonoester. Osh1 binding to Golgi PI(4)P regulated TORC1 (target of rapamycin complex 1) signaling and facilitated the expression of downstream genes involved in amino acid metabolism, which was inhibited by the release of Osh1 from Golgi PI(4)P due to pHi acidification. Hence, PI(4)P is a pH biosensor that regulates amino acid metabolism (Chapter 3). Together, these findings indicate that pHi is a signal that utilizes pH-sensing by lipids to regulate anabolism in yeast. A number of other potent signaling lipids also contain headgroups with phosphomonoesters, implying that pH sensing by lipids may be widespread in biology.
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