The mitochondrial PE synthase Psd1 moonlights at ER subdomains to promote LD biogenesis

Abstract

The asymmetric distribution of phospholipids in membranes is a fundamental principle of cellular compartmentalization and organization and is a crucial factor regulating organelle shape and biogenesis. Phosphatidylethanolamine (PE), a nonbilayer phospholipid that generates negative membrane curvature, has many cellular roles, including regulating mitochondrial architecture and lipidation of the autophagy factor LC3. Previously, we demonstrated in budding yeast that the PE synthase Psd1, which canonically operates on the mitochondrial inner membrane, unexpectedly also localizes to the ER. However, it has been unclear what the role of ER‐localized Psd1 is. We now resolve this mystery and demonstrate that ER‐localized Psd1 is a critical regulator of lipid droplet (LD) biogenesis at ER subdomains. While it is constitutively targeted to the ER membrane, Psd1 transiently concentrates on the ER to LD attachment sites specifically during stimulated LD biogenesis. Using an inducible LD biogenesis assay, we demonstrate that ER‐localized Psd1 is required for normal LD formation. Further, we identify a LD binding motif on Psd1 and show that this motif is required for Psd1 to influence LD morphogenesis. We also find that the role of phosphatidylserine decarboxylase (PSD) enzymes in LD formation is conserved in fission yeast, though occurs through a distinct targeting mechanism than in budding yeast. Thus, we have identified PSD enzymes as novel regulators of LDs and demonstrate that both mitochondria and LDs are organized and shaped by the spatial positioning of a single PE synthesis enzyme.