Seipin forms a flexible cage at lipid droplet formation sites

Abstract

Lipid droplets (LDs) form in the endoplasmic reticulum by phase separation of neutral lipids. This process is facilitated by the seipin protein complex, which consists of a ring of seipin monomers, with a yet unclear function. Here, we report a structure of S. cerevisiae seipin based on cryogenic-electron microscopy and structural modeling data. Seipin forms a decameric, cage-like structure with the lumenal domains forming a stable ring at the cage floor and transmembrane segments forming the cage sides and top. The transmembrane segments interact with adjacent monomers in two distinct, alternating conformations. These conformations result from changes in switch regions, located between the lumenal domains and the transmembrane segments, that are required for seipin function. Our data indicate a model for LD formation in which a closed seipin cage enables triacylglycerol phase separation and subsequently switches to an open conformation to allow LD growth and budding.

Identification of two pathways mediating protein targeting from ER to lipid droplets

“…How these ER proteins, but not others, access forming LDs is unclear. Current data suggest that the oligomeric seipin ring at LDACs restricts some proteins from accessing nascent LDs 55 – 57 while permitting others (Fig. 7d ).…”

Insights Into the Biogenesis and Emerging Functions of Lipid Droplets From Unbiased Molecular Profiling Approaches

“…It is difficult to overstate the contribution of yeast systematic genetics approaches to our current understanding of eukaryotic lipid metabolism and LD biology ( Radulovic et al, 2013 ). A few recent examples include: insights onto the regulated dynamics and spatial positioning of LDs during nutrient stress conditions to supply free fatty acids ( Henne et al, 2018 ; Hariri et al, 2019 ); the first mechanistic and structural descriptions of the seipin complex driving LD budding from the ER ( Klug et al, 2021 ; Arlt et al, 2022 ), and systematic screens for its regulatory partners ( Eisenberg-Bord et al, 2018 ); the description of specific mechanisms by which LDs contribute to manage protein aggregates ( see also below ) ( Moldavski et al, 2015 ); or the discovery of a new class of LDs stemming from nuclear membranes ( Romanauska and Köhler, 2018 ). Innovative applications of imaging technologies for automated screening, such as 3D imaging ( Lv et al, 2019 ), have also been reported in this system.…”

Role of Seipin in Human Diseases and Experimental Animal Models

“…On the ER, the accumulation of PA increases surface tension and decreases line tension of local ER, impeding unidirectional LD budding and normal ER-LD contact [ 32 ]. (2) The structural role of seipin in TAG and diacylglycerol (DAG) facilitates TAG phase separation, LD budding, and growth [ 33 , 34 , 35 ]. Seipin protein complex not only captures TAG and DAG inside its ring, but also provides TAG clusters for multiple nucleation sites [ 33 , 34 ].…”

“…Seipin protein complex not only captures TAG and DAG inside its ring, but also provides TAG clusters for multiple nucleation sites [ 33 , 34 ]. Recently, the seipin protein complex has also been found to form a flexible cage-like structure, and the dynamic conformation changes in this structure contribute to the TAG phase separation, as well as LD growth and budding [ 35 ]. (3) Compelling evidence indicates that seipin plays a key role in the structure of ER-LD contact to promote the biogenesis or maintenance of LDs [ 4 , 36 , 37 , 38 , 39 ].…”

“…Then, seipin promotes LD growth by transferring the cargo of ER proteins and lipids into nascent LDs [ 38 ]. Thereafter, seipin further prevents the shrinkage of LDs during TAG flow from smaller to larger LDs, a process called ripening [ 35 ], through mobilising TAGs to small LDs [ 4 , 37 ]. Moreover, Fld1 (yeast seipin) cooperates with its partner Pex30 to promote LDs budding and pre-peroxisomal vesicle budding by stabilising ER domains [ 39 ]; the Fld1/Ldb16 complex establishes a diffusion barrier, which is necessary for LD surface tension and identity via stabilising ER-LD contacts to avoid phospholipid packing defects and abnormal LDs formation [ 40 ].…”