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

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.

Retinyl esters form lipid droplets independently of triacylglycerol and seipin

“…Our data suggest that the role of seipin in LD formation is determined in part by the neutral lipids being stored in LDs. Seipin enhances TAG sequestering inside ringlike oligomeric structures ( Chung et al, 2019 ; Salo et al, 2019 ; Prasanna et al, 2021 ; Zoni et al, 2021 ). Seipin oligomers can trap TAGs in the ER bilayer via luminal hydrophobic helices and promote TAG clustering at low concentrations ( Prasanna et al, 2021 ).…”

“…Seipin enhances TAG sequestering inside ringlike oligomeric structures ( Chung et al, 2019 ; Salo et al, 2019 ; Prasanna et al, 2021 ; Zoni et al, 2021 ). Seipin oligomers can trap TAGs in the ER bilayer via luminal hydrophobic helices and promote TAG clustering at low concentrations ( Prasanna et al, 2021 ). It remains to be established whether a similar mechanism exists for the efficient sequestering of REs in the ER membrane before the formation of RE-filled LDs.…”

“…Seipin has also been suggested to determine the site of LD biogenesis in the ER ( Chung et al, 2019 ; Salo et al, 2019 ), to regulate the flow of neutral lipids and proteins from the ER to nascent LDs at ER–LD contacts ( Grippa et al, 2015 ; Salo et al, 2019 ; Wang et al, 2014 ), and to regulate lipid metabolism at LD biogenesis sites ( Renne et al, 2020 ). Recently, seipin was shown to trap TAGs in the ER bilayer via luminal hydrophobic helices ( Prasanna et al, 2021 ; Zoni et al, 2021 ).…”

Interactions of Lipid Droplets with the Intracellular Transport Machinery

“…This contrasts with an alternate view in which seipin puncta capture pre-existing triglyceride lenses [36]. Recent molecular dynamic simulations have supported the idea that the tightly clustered hydrophobic domains in seipin clusters can nucleate triglyceride lenses [37]. fat storage inducting transmembrane protein 2 (FIT2)/Scs3 is another protein which plays a key role in LD formation [38].…”

Physical Characterization of Triolein and Implications for Its Role in Lipid Droplet Biogenesis

“…This approach has been used in all-atom (AA) MD [20][21][22][23] and coarse-grained (CG) MD simulations, the latter run with the MARTINI CG force field. [24][25][26][27] Recent 10 µs-long AA simulations of LDs report surface-oriented TG (called SURF-TG in this work), 22 which was also implicated in NMR experiments. 28 The relative amount of SURF-TG to PLs, which ranges from 5-8% in the unstressed LD 22 to ~20% in the stressed LD, 23 has been shown to modulate the area per lipid (APL) and interdigitation with PLs.…”