Roles for L (o) microdomains and ESCRT in ER stress-induced lipid droplet microautophagy in budding yeast

Microlipophagy (µLP), degradation of lipid droplets (LDs) by microautophagy, occurs by autophagosome-independent direct uptake of LDs at lysosomes/vacuoles in response to nutrient limitations and ER stressors in Saccharomyces cerevisiae. In nutrient-limited yeast, liquid-ordered (L(o)) microdomains, sterol-rich raftlike regions in vacuolar membranes, are sites of membrane invagination during LD uptake. The endosome sorting complex required for transport (ESCRT) is required for sterol transport during L(o) formation under these conditions. However, ESCRT has been implicated in mediating membrane invagination during µLP induced by ER stressors or the diauxic shift from glycolysis- to respiration-driven growth. Here we report that ER stress induced by lipid imbalance and other stressors induces L(o) microdomain formation. This process is ESCRT independent and dependent on Niemann-Pick type C sterol transfer proteins. Inhibition of ESCRT or L(o) microdomain formation partially inhibits lipid imbalance-induced µLP, while inhibition of both blocks this µLP. Finally, although the ER stressors dithiothreitol or tunicamycin induce L(o) microdomains, µLP in response to these stressors is ESCRT dependent and L(o) microdomain independent. Our findings reveal that L(o) microdomain formation is a yeast stress response, and stress-induced L(o) microdomain formation occurs by stressor-specific mechanisms. Moreover, ESCRT and L(o) microdomains play functionally distinct roles in LD uptake during stress-induced µLP.