TY - JOUR T1 - Seipin traps triacylglycerols to facilitate their nanoscale clustering in the ER membrane JF - bioRxiv DO - 10.1101/2020.10.26.355065 SP - 2020.10.26.355065 AU - Xavier Prasanna AU - Veijo T. Salo AU - Shiqian Li AU - Katharina Ven AU - Helena Vihinen AU - Eija Jokitalo AU - Ilpo Vattulainen AU - Elina Ikonen Y1 - 2020/01/01 UR - http://biorxiv.org/content/early/2020/10/26/2020.10.26.355065.abstract N2 - Seipin is a disk-like oligomeric ER protein important for lipid droplet (LD) biogenesis and triacylglycerol (TAG) delivery to growing LDs. Here we show through biomolecular simulations bridged to experiments that seipin can trap TAGs in the ER bilayer via the luminal hydrophobic helices of the protomers delineating the inner opening of the seipin disk. This promotes the nanoscale sequestration of TAGs at a concentration that by itself is insufficient to induce TAG clustering in a lipid membrane. We identify Ser166 in the α3 helix as a favored TAG occupancy site and show that mutating it compromises the ability of seipin complexes to sequester TAG in silico and to promote TAG transfer to LDs in cells. While seipin-S166D mutant colocalizes poorly with promethin, the association of nascent wild-type seipin complexes with promethin is promoted by TAGs. Together, these results suggest that seipin traps TAGs via its luminal hydrophobic helices, serving as a catalyst for seeding the TAG cluster from dissolved monomers inside the seipin ring, thereby generating a favorable promethin binding interface.Competing Interest StatementThe authors have declared no competing interest. ER -