Arf1/COPI machinery acts directly on lipid droplets and enables their connection to the ER for protein targeting

Abstract

Lipid droplets (LDs) are ubiquitous organelles that store neutral lipids, such as triacylglycerol (TG), as reservoirs of metabolic energy and membrane precursors. The Arf1/COPI protein machinery, known for its role in vesicle trafficking, regulates LD morphology, targeting of specific proteins to LDs and lipolysis through unclear mechanisms. Recent evidence shows that Arf1/COPI can bud nano-LDs (∼60 nm diameter) from phospholipid-covered oil/water interfaces in vitro. We show that Arf1/COPI proteins localize to cellular LDs, are sufficient to bud nano-LDs from cellular LDs, and are required for targeting specific TG-synthesis enzymes to LD surfaces. Cells lacking Arf1/COPI function have increased amounts of phospholipids on LDs, resulting in decreased LD surface tension and impairment to form bridges to the ER. Our findings uncover a function for Arf1/COPI proteins at LDs and suggest a model in which Arf1/COPI machinery acts to control ER-LD connections for localization of key enzymes of TG storage and catabolism.DOI: http://dx.doi.org/10.7554/eLife.01607.001

ER Membrane Phospholipids and Surface Tension Control Cellular Lipid Droplet Formation

“…These conditions correspond, for example, to the accumulation of negatively curved lipids such as diacylglycerols, PA, or cholesterol ( Figure 1G tensions ( Figure 4G). Our prediction is consistent with reported regulations of GPAT4 trafficking from ER to LDs: in increased PA levels in the ER membrane of various cell types lacking Seipin protein, a massive transfer of GPAT4 from the ER to almost all LDs is observed in comparison with normal cells (Fei et al, 2011;Pagac et al, 2016;Szymanski et al, 2007;Wang et al, 2016); likewise, under conditions where GPAT4 should exclusively remain in the ER membrane of fly cells (Wilfling et al, 2014b), the induction of cholesterol accumulation in the membrane restored GPAT4 targeting from ER to LDs (Wilfling et al, 2014b). Consequently, ER tension and phospholipid composition seem to be critical for determining the protein composition of LD during formation.…”

“…Finally, many proteins important for LD functionality target LDs from the ER, either during LD formation or via ER-LD connection bridges (Jacquier et al, 2011;Wilfling et al, 2013Wilfling et al, , 2014aWilfling et al, , 2014b. These proteins encompass the glycerol-3-phosphate acyltransferase enzyme (GPAT4) (Wilfling et al, 2013) that localizes to LD surface to mediate the local synthesis of neutral lipids.…”

Lipid droplet growth: regulation of a dynamic organelle

“…In addition, ADP-ribosylation factor 1 (Arf1)/COPI complex has been found to have a key role in the triggering the formation of membrane bridges linking the ER and LDs [28,29]. However, freeze-fracture electron microscopy showed adjacent LDs and ER with independent membranes [30], and ER proteins such as GPAT4 are only observed in a subset of LDs, indicating that such connections could be transient or not present in some LDs.…”

Arfs at a Glance

“…Arf1-GTP and COPI can function directly on the lipid droplet surface and have been shown to mediate the budding of 60-100-nm diameter droplets from an artificial droplet surface in vitro (Thiam et al, 2013). This mechanism is important for the regulation of the surface properties of lipid droplets during different stages of their metabolism in cells (Wilfling et al, 2014).…”