Regulated conformational transitions in seipin define a functional ER–lipid droplet interface
Lipid droplets (LDs) are key organelles in cellular lipid homeostasis that form at the endoplasmic reticulum (ER) through a sequence of membrane rearrangements. While seipin emerged as an essential protein complex for LD biogenesis, how seipin-mediated LD formation proceeds beyond the initial step of neutral lipid nucleation remains unknown. Using a combination of in vitro and in-cell cryogenic electron microscopy (cryo-EM), ultrastructural expansion microscopy, molecular simulations and tailored genetic perturbations, we show that the seipin transmembrane domains undergo large-scale conformational rearrangements that define the architecture of the ER-LD interface and enable LD growth. Cryo-EM of purified Xenopus seipin revealed coexistence of two states: a compact "closed" conformation, consistent with early LD biogenesis, and an "open" conformation in which the transmembrane helices splay out laterally. Molecular dynamics simulations indicate that this open state induces local membrane curvature and promotes triacylglycerol accumulation. We identify conserved flexible linkers between the seipin luminal and transmembrane regions that act as mechanical hinges, enabling this conformational transition. We demonstrate that mutations in these hinge regions hinder seipin opening and affect LD formation in yeast and human cells. Analysis of native ER-LD contacts in human cells using light microscopy and cryo-electron tomography confirms that the seipin complex opens to establish stereotypical ~21-nm necks connecting the ER bilayer and LD monolayer. Moreover, we identify the liver-enriched microprotein SMLR1 as an inhibitor of this seipin conformational transition, providing a regulatory mechanism for seipin-dependent lipid storage in a tissue-specific manner. Together, these data establish seipin opening as a key structural rearrangement at the ER-LD interface that is essential for LD biogenesis and growth.