TORC1 Inhibition Induces Lipid Droplet Replenishment in Yeast

Abstract

Lipid droplets (LDs) are intracellular structures that regulate neutral lipid homeostasis. In mammals, LD synthesis is inhibited by rapamycin, a known inhibitor of the mTORC1 pathway. In S. cerevisiae, LD dynamics is modulated by the growth phase, however the regulatory pathways involved are unknown. Therefore, we decided to study the role of the TORC1 pathway on LD metabolism in S. cerevisiae. Interestingly, rapamycin treatment resulted in a fast LDs replenishment and growth inhibition. The discovery that osmotic stress (1M sorbitol) also induced LD synthesis but not growth inhibition, suggested that the induction of LDs in yeast is not a secondary response to reduced growth. Induction of LDs by rapamycin was due to increased triacylglycerol but not sterol esters synthesis. Induction was dependent on the TOR downstream effectors, the PP2A-related phosphatase Sit4p and the regulatory protein Tap42p. The TORC1-controlled transcriptional activators GLN3, GAT1, RTG1 and RTG3 but not MSN2 and MSN4 were required for full induction of LDs by rapamycin. Furthermore, we show that deletion of GLN3 and GAT1 transcription factors, which are activated in response to nitrogen availability, led to abnormal LD dynamics. These results reveal that the TORC1 pathway is involved in neutral lipid homeostasis in yeast.