Lipid droplets (LDs) are intracellular organelles responsible for lipid storage, and they emerge from the endoplasmic reticulum (ER) upon the accumulation of neutral lipids, mostly triglycerides (TG), between the two leaflets of the ER membrane. LD biogenesis takes place at ER sites that are marked by the protein seipin, which subsequently recruits additional proteins to catalyze LD formation. Deletion of seipin, however, does not abolish LD biogenesis, and its precise role in controlling LD assembly remains unclear. Here, we use molecular dynamics simulations to investigate the molecular mechanism through which seipin promotes LD formation. We find that seipin clusters TG, as well as its precursor diacylglycerol, inside its unconventional ring-like oligomeric structure and that both its luminal and transmembrane regions contribute to this process. This mechanism is abolished upon mutations of polar residues involved in protein–TG interactions into hydrophobic residues. Our results suggest that seipin remodels the membrane of specific ER sites to prime them for LD biogenesis.
Cells store energy in the form of neutral lipids packaged into micrometer-sized organelles named lipid droplets (LD). These structures emerge from the endoplasmic reticulum (ER) at sites marked by the protein seipin, but the mechanisms regulating their biogenesis remain poorly understood. Using a combination of molecular simulations, yeast genetics and fluorescence microscopy, we show that interactions between lipids' acyl-chains modulate the propensity of neutral lipids to be stored in LD, in turn preventing or promoting their accumulation in the ER membrane. Our data suggest that diacylglycerol, that is enriched at sites of LD formation, promotes the packaging of neutral lipids into LDs, together with ER-abundant lipids, such as phosphatidylethanolamine. On the opposite end, short and saturated acyl-chains antagonize fat storage in LD and promote accumulation of neutral lipids in the ER. Our results provide a new conceptual understanding of LD biogenesis in the context of ER homeostasis and function.
Sphingolipids have been shown to play important roles in physiology and cell biology, but a systematic examination of their functions is lacking. We performed a genome-wide CRISPRi screen in sphingolipid-depleted cells and identified hypersensitive mutants in genes of membrane trafficking and lipid biosynthesis, including ether lipid synthesis. Systematic lipidomic analysis showed a coordinate regulation of ether lipids with sphingolipids, where depletion of one of these lipid types resulted in increases in the other, suggesting an adaptation and functional compensation. Biophysical experiments on model membranes show common properties of these structurally diverse lipids that also share a known function as GPI anchors in different kingdoms of life. Molecular dynamics simulations show a selective enrichment of ether phosphatidylcholine around p24 proteins, which are receptors for the export of GPI-anchored proteins and have been shown to bind a specific sphingomyelin species. Our results support a model of convergent evolution of proteins and lipids, based on their physico-chemical properties, to regulate GPI-anchored protein transport and maintain homeostasis in the early secretory pathway. INTRODUCTIONThe maintenance of membrane lipid homeostasis is an energetically expensive yet necessary process in cells. Lipid diversity has evolved together with cell complexity to give rise to thousands of different lipids species with specific functions, many of which are still unexplored 1, 2 . Moreover, different lipid metabolic pathways are interconnected, and cells show a high phenotypic plasticity when adapting to changes in membrane lipid composition, which makes it difficult to disentangle the function of individual lipid species 3 . A systematic analysis of the cellular responses to perturbation of specific synthetic pathways is thus needed to reveal co-regulated lipid networks and uncover new lipid functions. Sphingolipids (SL) are a class of lipids that contain a sphingoid-base backbone, in contrast to the more commonly found glycerol backbone in glycerophospholipids (GPL). These bioactive lipids have been extensively studied in the last decades, revealing distinctive physico-chemical properties and connections to diseases 4,5 . SL have been implicated in diabetes 6 , cancer 7 and inflammation 8 , and mutations in SL synthetic or metabolic enzymes are associated with severe genetic disorders [9][10][11] . SL species sphingosine (So) and ceramide (Cer) can permeabilize membranes 12,13 ; Cer induces the flip-flop of neighbouring lipids 14 and can phase-separate to form membrane platforms important for signalling 15 . The most abundant SL species, sphingomyelin (SM), has been shown to modulate membrane properties and regulate signalling pathways 16,17 . Besides direct phosphorylation of ceramide synthases [18][19][20] , the only direct regulators of sphingolipid synthesis identified are Orm proteins (ORMDL in mammalian cells), that associate with serine palmitoyl transferase (SPT), the first enzyme of the sphingolipi...
Protein sorting in the secretory pathway is crucial to maintain cellular compartmentalization and homeostasis. In addition to coat-mediated sorting, the role of lipids in driving protein sorting during secretory transport is a longstanding fundamental question that still remains unanswered. Here, we conduct 3D simultaneous multicolor high-resolution live imaging to demonstrate in vivo that newly synthesized glycosylphosphatidylinositol-anchored proteins having a very long chain ceramide lipid moiety are clustered and sorted into specialized endoplasmic reticulum exit sites that are distinct from those used by transmembrane proteins. Furthermore, we show that the chain length of ceramide in the endoplasmic reticulum membrane is critical for this sorting selectivity. Our study provides the first direct in vivo evidence for lipid chain length–based protein cargo sorting into selective export sites of the secretory pathway.
Diacylglycerols (DAGs) are bioactive lipids that are ubiquitously present at low concentrations in cellular membranes. Upon the activation of lipid remodeling enzymes such as phospholipase C and phosphatidic acid phosphatase, DAG concentration increases, leading to a disruption of the lamellar phase of lipid membranes. To investigate the structural origin of these phenomena, here we develop a coarse-grained model for DAGs that is able to correctly reproduce its physicochemical properties, including interfacial tension and flip-flop rate. We find that even at low concentrations a nonnegligible percentage of DAG molecules occupies the interleaflet space. At high concentrations, DAG molecules undergo a phaseseparation process from lamellar lipids, segregating in DAG-only blisters and effectively reducing the DAG surface pool available to peripheral enzymes. Our results allow for a better understanding of the role of DAGs in cellular membranes and provide a new tool for the quantitative estimation of low-abundance lipids on membrane properties.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.