Seipin traps triacylglycerols to facilitate their nanoscale clustering in the endoplasmic reticulum membrane

Seipin is a disk-like oligomeric endoplasmic reticulum (ER) protein important for lipid droplet (LD) biogenesis and triacylglycerol (TAG) delivery to growing LDs. Here we show through biomolecular simulations bridged experiments that seipin can trap TAGs in the ER bilayer via luminal hydrophobic helices of protomers delineating inner opening disk. This promotes nanoscale sequestration at concentration by itself insufficient induce TAG clustering membrane. We identify Ser166 α3 helix as favored occupancy site mutating it compromises ability complexes sequester silico promote transfer LDs cells. While S166D-seipin mutant colocalizes poorly with promethin, association nascent wild-type promethin promoted TAGs. Together, these results suggest traps its helices, serving catalyst seeding cluster from dissolved monomers inside ring, thereby generating favorable binding interface.