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

Xavier Prasanna,Shiqian Li,Veijo T Salo,Ilpo Vattulainen,Katharina Ven,Elina Ikonen,Helena Vihinen,Eija Jokitalo,Ana J Garcia-Saez

doi:10.1371/journal.pbio.3000998

Xavier Prasanna, Shiqian Li + Show 7 more

Open Access

https://doi.org/10.1371/journal.pbio.3000998

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Journal: PLoS Biology	Publication Date: Jan 22, 2021
Citations: 62	License type: CC BY 4.0

Affiliation: University of Helsinki, Minerva Foundation

Abstract

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 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 the S166D-seipin 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.

Highlights

Lipid droplets (LDs) are intracellular storage organelles critical for energy metabolism [1,2]
We recently reported that seipin facilitates TAG delivery to the LD and counteracts ripening-induced shrinkage of small LDs [35]
Our results suggest that the seipin complex, via its hydrophobic membrane intercalated helices, can trap TAGs in the bilayer, thereby serving as a seed for TAG clustering inside the lumen of the seipin ring

Summary

Introduction

Lipid droplets (LDs) are intracellular storage organelles critical for energy metabolism [1,2]. As the 165A/166A double mutant has been reported to impair seipin function [44], we switched both S165 and S166 to Ala and found that this switch impaired TAG– S166 interaction, with the short-time diffusion coefficient of TAG being approximately 100% larger than the D of TAG bound to S166, and a lifetime of approximately 0.1 μs for the S166A– TAG complex These data suggest that S166 serves as a nucleation site in the formation of TAG clusters. These data suggest that neutral lipids can facilitate the colocalization of seipin and promethin We speculate that this may arise from local membrane alterations induced by TAG clustering within the seipin oligomer, providing a favorable environment for promethin association. Such a TAG-harvesting molecular function of seipin likely contributes to its role in preventing ripening and sustaining LD growth [35]

Materials and methods

Findings

Computational methods