![]() These membrane contacts are maintained by ER-resident integral membrane proteins, including seipin and fat storage-inducing transmembrane (FIT) proteins, which are both required for proper LD biogenesis and modulate LD size and abundance ( Kadereit et al., 2008 Szymanski et al., 2007). The transfer of proteins and lipids between the ER and LDs is likely to occur through membrane contact sites between the two compartments. Hence, LD homeostasis is associated with multiple prevalent human diseases, including obesity and atherosclerosis ( Olzmann and Carvalho, 2018 Thiam et al., 2013 Thiam and Dugail, 2019 Walther et al., 2017). Thus, LDs can buffer both an excess and a lack of fatty acids, and thereby provide a protective role in lipotoxicity. In addition, lipids stored in LDs can also serve as readily available building blocks for rapid membrane proliferation. LDs serve to store metabolic energy, which is released upon β-oxidation of fatty acids esterified to these neutral lipids. The morphology of LDs is somewhat reminiscent of that of apolipoproteins and milk globules, but unlike these structures, LDs are not secreted. This hydrophobic core of neutral lipids is surrounded by a phospholipid monolayer that harbors a specific set of proteins, many of which function in neutral lipid metabolism, such as lipases or acyltransferases. LDs are mostly composed of neutral lipids, particularly triacylglycerols (TAGs) and steryl esters (STEs). Adipocytes typically contain one large unilocular LD, whereas other cells, such as yeast, may contain up to a dozen distinct LDs that are relatively small. Lipid droplets (LDs) are present in most cells, where they are discernible as globular structures. This article has an associated First Person interview with the first author of the paper. These data indicate that the ER–LD junction constitutes a barrier for ER-resident integral membrane proteins. Expression of OM14–PLIN3 induced a close apposition between LDs and mitochondria. A LD-tethering function of PLIN3-containing membrane proteins was confirmed by fusing PLIN3 to the cytoplasmic domain of an outer mitochondrial membrane protein, OM14. This peripheral LD localization of the fusion proteins, however, was due to a redistribution of the ER around LDs, as revealed by bimolecular fluorescence complementation between ER- and LD-localized partners. The resulting fusion proteins localized to the periphery of LDs in both yeast and mammalian cells. To test this, we fused perilipin 3 (PLIN3), a mammalian LD-targeted protein, to ER-resident proteins. Here, we address the question of whether integral membrane proteins can localize to the surface of LDs. Most of these proteins associate with LDs through either an amphipathic helix or a membrane-embedded hairpin motif. LDs are closely associated with the endoplasmic reticulum (ER) and are limited by a phospholipid monolayer harboring a specific set of proteins. Lipid droplets (LDs) are globular subcellular structures that store neutral lipids. ![]()
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