ORP5 AND ORP8 ORCHESTRATE LIPID DROPLET BIOGENESIS AND MAINTENANCE AT ER-MITOCHONDRIA CONTACT SITES

SUMMARY Lipid droplets (LDs) are the primary organelles of lipid storage, buffering energy fluctuations of the cell. They store neutral lipids in their core that is surrounded by a protein-decorated phospholipid monolayer. LDs arise from the Endoplasmic Reticulum (ER). The ER-protein seipin, localizing at ER-LD junctions, controls LD nucleation and growth. However, how LD biogenesis is spatially and temporally coordinated remains elusive. Here, we show that the lipid transfer proteins ORP5 and ORP8 control LD biogenesis at Mitochondria-Associated ER Membrane (MAM) subdomains, enriched in phosphatidic acid. We found that ORP5/8 regulate seipin recruitment to these MAM-LD contacts, and their loss impairs LD biogenesis. Importantly, the integrity of ER-mitochondria contact sites is crucial for the ORP5/8 function in regulating seipin-mediated LD biogenesis. Our study uncovers an unprecedented ORP5/8 role in orchestrating LD biogenesis at MAMs and brings novel insights into the metabolic crosstalk between mitochondria, ER, and LDs at membrane contact sites. HIGHLIGHTS ORP5 and ORP8 localize at MAM subdomains where LDs originate. Phosphatidic acid is enriched in MAM subdomains that are the birthplace of LDs. ORP5 and ORP8 knockdown impairs LD biogenesis. ORP5 and ORP8 regulate seipin recruitment to MAM-LD contact sites.