Seipin traps triacylglycerols to facilitate their nanoscale clustering in the ER membrane

Abstract

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.

Pre-existing bilayer stresses modulate triglyceride accumulation in the ER versus lipid droplets

“…Recent experimental and computational results suggest that nucleation is largely driven by protein activity in vivo ( Zoni et al, 2020 Prasanna et al, 2021 ). On the other hand, it remains unclear whether variations in PL content could alter TG concentration in the ER.…”

A Unique Junctional Interface at Contact Sites Between the Endoplasmic Reticulum and Lipid Droplets

“…Furthermore, the seipin complex at the base of ER-LD junctions might create a selectivity barrier and thereby uncouples the lipid composition of these ER subdomains from the bulk of the ER. Based on molecular dynamic simulations, seipin has recently been proposed to directly interact with, and thereby enrich, TAG and DAG within its ring-shaped structure (Prasanna et al, 2020;Zoni et al, 2020). Hence, seipin might function as a lipid transporter and/or gatekeeper to locally regulate lipid levels at ER-LD junctions.…”

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 ].…”