Abstract

Sui et al. report the cryo-EM structure of the conserved luminal domain of the lipid droplet (LD)-formation protein seipin. The structure reveals key features of this domain and suggest a new model for seipins role in LD formation.\n\nMetabolic energy is stored in cells primarily as triacylglycerols in lipid droplets (LDs), and LD dysregulation leads to metabolic diseases. The formation of monolayer-bound LDs from the endoplasmic reticulum (ER) bilayer is poorly understood, but the ER protein seipin is essential to this process. Here, we report a cryo-electron microscopy structure and functional characterization of D. melanogaster seipin. The structure reveals a ring-shaped dodecamer, with the luminal domain of each monomer resolved at [~]4.0 [A]. Each luminal domain monomer exhibits two distinctive features: a hydrophobic helix positioned towards the ER bilayer, and a {beta}-sandwich domain that has structural similarity with lipid-binding proteins. This structure, and our functional testing in cells, suggest a model in which seipin oligomers initially detect forming LDs in the ER via hydrophobic helices and subsequently act as membrane anchors to enable lipid transfer and LD growth.