Department of Molecular Metabolism, Harvard T H Chan School of Public Health, Boston, United States; Department of Cell Biology, Harvard Medical School, Boston, United States
Henning Arlt
Department of Molecular Metabolism, Harvard T H Chan School of Public Health, Boston, United States; Department of Cell Biology, Harvard Medical School, Boston, United States; Howard Hughes Medical Institute, Harvard T H Chan School of Public Health, Boston, United States
Alexander J Pak
Department of Chemical and Biological Engineering, Colorado School of Mines, Golden, United States
Department of Molecular Metabolism, Harvard T H Chan School of Public Health, Boston, United States; Department of Cell Biology, Harvard Medical School, Boston, United States; Broad Institute of Harvard and MIT, Cambridge, United States
Tobias C Walther
Department of Molecular Metabolism, Harvard T H Chan School of Public Health, Boston, United States; Department of Cell Biology, Harvard Medical School, Boston, United States; Howard Hughes Medical Institute, Harvard T H Chan School of Public Health, Boston, United States; Broad Institute of Harvard and MIT, Cambridge, United States
Department of Chemistry, Chicago Center for Theoretical Chemistry, James Franck Institute, and Institute for Biophysical Dynamics, The University of Chicago, Chicago, United States
Lipid droplets (LDs) are organelles formed in the endoplasmic reticulum (ER) to store triacylglycerol (TG) and sterol esters. The ER protein seipin is key for LD biogenesis. Seipin forms a cage-like structure, with each seipin monomer containing a conserved hydrophobic helix and two transmembrane (TM) segments. How the different parts of seipin function in TG nucleation and LD budding is poorly understood. Here, we utilized molecular dynamics simulations of human seipin, along with cell-based experiments, to study seipin’s functions in protein–lipid interactions, lipid diffusion, and LD maturation. An all-atom simulation indicates that seipin TM segment residues and hydrophobic helices residues located in the phospholipid tail region of the bilayer attract TG. Simulating larger, growing LDs with coarse-grained models, we find that the seipin TM segments form a constricted neck structure to facilitate conversion of a flat oil lens into a budding LD. Using cell experiments and simulations, we also show that conserved, positively charged residues at the end of seipin’s TM segments affect LD maturation. We propose a model in which seipin TM segments critically function in TG nucleation and LD growth.
