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

Microlipophagy (mu LP), 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 mu LP 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 mu LP, while inhibition of both blocks this mu LP. Finally, although the ER stressors dithiothreitol or tunicamycin induce L-o microdomains, mu LP 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 mu LP.