Spatial organization of lipid droplet catabolism in yeast

Eukaryotic cells store lipids in specialized organelles termed lipid droplets (LDs), composed of a core of neutral lipids surrounded by a phospholipid monolayer. The biosynthesis and consumption of LDs is under the control of the nutritional state of the cell, and conserved molecular pathways allow cells to sense and respond to fluctuating nutrient availability, facilitating metabolic adaptation and subcellular reorganization. This thesis explores distinct alterations in lipid metabolism as cells age, adapt to nutrient scarcity and are challenged with fresh nutrients to re-enter the cell cycle, using the budding yeast Saccharomyces cerevisiae as unicellular eukaryotic model organism. A special emphasis is placed on the dynamic balance between LD formation and catabolism, in particular the two main cellular routes to consume LDs, namely lipolysis and lipophagy. In Paper I, we unravel how the exhaustion of either glucose or phosphate affects LD biosynthesis and consumption, showing that these processes differently contribute to the maintenance and exit of quiescence depending on the respective nutrient limitation. In Paper II, we identify the tethering machinery that enables LDs to dock to the vacuolar membrane during lipophagy for subsequent microautophagic degradation of LDs within the vacuolar lumen. We demonstrate that this novel organelle contact site, termed vCLIP for vacuole-lipid droplet contact site, is critical for efficient lipophagy and long-term survival during glucose restriction. In Paper III, we show that cytosolic lipases assemble into distinct foci on the LD surface to drive lipolysis in response to starvation. Finally, in paper IV, we demonstrate that the lipolysis machinery, so far only connected to the cytosolic turnover of LDs, is essential for LD consumption in the vacuole during lipophagy. Collectively, our results provide mechanistic insights into distinct aspects of LD catabolism during nutritional stress and introduce a concept of LD consumption where the coordinated activity of lipophagy and lipolysis allows cells to adapt to changing metabolic demands.