Glucose Starvation Sensing through Membrane Remodeling at theNucleus-Vacuole Junction Coordinates Ergosterol Biosynthesis

Membrane contact sites (MCSs), where organelle membranes come into close proximity, function as dynamic hubs for lipid metabolism in response to metabolic and stress signals. In yeast, the nucleus-vacuole junction (NVJ) expands during glucose starvation through the recruitment of stress-specific proteins, but the underlying mechanisms and physiological significance remained unclear. Here, we identify the yeast INSIG homologs Nsg1, Nsg2, along with the aspartyl protease Ypf1 as glucose starvation-specific NVJ-localized factors. Ypf1 promotes the recruitment of INSIG proteins and the HMG-CoA reductases Hmg1 and Hmg2 to the NVJ, likely in response to nuclear membrane remodeling induced by changes in very long-chain fatty acid (VLCFA) metabolism. We further demonstrate that this membrane remodeling destabilizes Nsg1, promoting the NVJ localization and activation of Hmg1, while stabilizing Nsg2, which counteracts Hmg1 activation. These findings reveal that Nsg1 and Nsg2, previously known as regulators of Hmg2 stability, also act as negative regulators of Hmg1. Based on these results, we propose a model in which VLCFAs modulate the physical properties of the nuclear membrane, enabling cells to sense glucose starvation and regulate sterol metabolism during glucose starvation. Overall, our study provides new insights into MCS-mediated metabolic stress responses and highlights the role of membrane properties as active regulators of lipid homeostasis.