Mdm1 maintains endoplasmic reticulum homeostasis by spatially regulating lipid droplet biogenesis

Excess fatty acids are toxic to cells but can be sequestered as triacylglycerides in lipid droplets. Hariri et al. show that the tethering protein Mdm1 spatially regulates this process at the junction between the endoplasmic reticulum and the yeast vacuole. These findings suggest that Mdm1 can drive spatially defined lipid droplet production to maintain cell homeostasis and protect against lipotoxicity.
Lipid droplets (LDs) serve as cytoplasmic reservoirs for energy-rich fatty acids (FAs) stored in the form of triacylglycerides (TAGs). During nutrient stress, yeast LDs cluster adjacent to the vacuole/lysosome, but how this LD accumulation is coordinated remains poorly understood. The ER protein Mdm1 is a molecular tether that plays a role in clustering LDs during nutrient depletion, but its mechanism of function remains unknown. Here, we show that Mdm1 associates with LDs through its hydrophobic N-terminal region, which is sufficient to demarcate sites for LD budding. Mdm1 binds FAs via its Phox-associated domain and coenriches with fatty acyl–coenzyme A ligase Faa1 at LD bud sites. Consistent with this, loss of MDM1 perturbs free FA activation and Dga1-dependent synthesis of TAGs, elevating the cellular FA level, which perturbs ER morphology and sensitizes yeast to FA-induced lipotoxicity. We propose that Mdm1 coordinates FA activation adjacent to the vacuole to promote LD production in response to stress, thus maintaining ER homeostasis.