Protein Crowding Is a Determinant of Lipid Droplet Protein Composition - PubMed (original) (raw)

Protein Crowding Is a Determinant of Lipid Droplet Protein Composition

Nora Kory et al. Dev Cell. 2015.

Abstract

Lipid droplets (LDs) are lipid storage organelles that grow or shrink, depending on the availability of metabolic energy. Proteins recruited to LDs mediate many metabolic functions, including phosphatidylcholine and triglyceride synthesis. How the LD protein composition is tuned to the supply and demand for lipids remains unclear. We show that LDs, in contrast to other organelles, have limited capacity for protein binding. Consequently, macromolecular crowding plays a major role in determining LD protein composition. During lipolysis, when LDs and their surfaces shrink, some, but not all, proteins become displaced. In vitro studies show that macromolecular crowding, rather than changes in monolayer lipid composition, causes proteins to fall off the LD surface. As predicted by a crowding model, proteins compete for binding to the surfaces of LDs. Moreover, the LD binding affinity determines protein localization during lipolysis. Our findings identify protein crowding as an important principle in determining LD protein composition.

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Figures

Figure 1. During Lipolysis, LDs Shrink and LD Protein Composition Changes

(A) LDs are consumed during lipid starvation. After 48 hr in medium withoutlipids, LDs shrink and are removed from cells. LDs are stained with BODIPY.Representative images are shown. Scale bar, 5 μm. (B) LD size decreases during lipid starvation. Mean LD area per cell, median LDdiameter of the small LD population, and compression factor (r2(time 0)/r2(respective time point); r = radius) during lipolysis is shown. Values are means± SD or medians as indicated (n > 20). (C) Endogenous CCT1 detected by immunofluorescence, but not GPAT4, isdisplaced from LDs during shrinkage. Representative images are shown. Scalebar, 5 μm. Inlay, 3× magnification. (D,E) Endogenous CCT1 is displaced from LDs during lipid starvation, whereas CG9186 concentrates on LDs. (D) LDs were stained with AUTOdot. Representative images are shown. Scale bar, 5 μm. Inlay, 3× magnification. (E) Mean fluorescence on LDs ± SD (n > 20). A.U. = arbitrary units. Note, CG9186 remained targeted to their surfaces throughout lipolysis, suggesting that these structures are cytosolic LDs.

Figure 2. Differential Binding of LD Proteins during Lipolysis

During lipolysis, some proteins are reduced on LDs whereas others remain bound. Cells were imaged after oleate loading (+OA) or after 24 hr (-OA 24 hr) of lipid starvation. LDs were stained with BODIPY. (A) Representative images are shown. Scale bar, 5 μm. Inlay, 3× magnification. (B) Percent protein displacement (% protein initially on LDs – % protein on LDs after starvation)/(% protein initially on LDs) is reported. Values are means (n > 12).

Figure 3. CCT1 Falls Off Shrinking LDs

(A, B) CCT1 is not degraded but falls off LDs when cells are starved for lipids. Photoactivatable GFP (PAGFP)-CCT1 was activated on LDs before starvation. Cells were imaged before, immediately after photoactivation, and after 10 hr and 20 hr in medium containing delipidated serum (–OA 10 hr, –OA 20 hr, respectively). LDs were stained with LipidTOX. (A) Representative images are shown. Scale bar, 10 μm. (B) Percent mean fluorescence of PAGFP-CCT1 on LDs and the nucleus ± SD (n = 10). (C) Total CCT1 levels increase during the first 20 hr of starvation. A representative Western blot using an antibody against endogenous CCT1 in cell lysates is shown. Tubulin was used as a loading control. (D,E,F) Lipase inhibition blocks CCT1 displacement. Cells expressing mCherry-CCT1 were oleate loaded, imaged (+OA), or oleate loaded, starved of lipids for 24 hr in the presence of 0–150 μM Orlistat in DMSO and imaged (–OA). LDs were stained with BODIPY. (D) Representative images are shown. Scale bar = 5 μm. Inlay, 3× magnification. (E) Percent mean fluorescence of mCherry-CCT1 on LDs ± SD. Values are means (n > 12). (F) Lipase inhibition prevents LD shrinkage and clearance. A box plot is shown. Mean values of the LD area in one plane of the cell are reported. Whiskers indicate Min and Max values. See also Figure S1.

Figure 4. CCT1, but Not GPAT4, Falls Off a Shrinking Oil-Water Interface In Vitro

(A) Schematic of the in vitro system. LDs in buffer are mixed with TG oil to generate a water-in-oil emulsion. LD proteins then bind to the resulting oil-water interface. (B,C) During shrinkage of drops in vitro, CCT1 falls off the oil-water interface, whereas GPAT4 remains bound. (B) Representative images are shown. Scale bar, 10 μm. (C) Surface mean concentration and mean surface-bound fraction for _mCherry_-CCT1 and GFP-GPAT4 are reported. Lines represent trends. A.U. = arbitrary units. See also Figure S2.

Figure 5. Macromolecular Crowding, Not Changes in PC Concentration, Causes CCT1 Displacement In Vitro

(A,B) PC addition does not affect CCT1 binding to the oil-water interface. Excess PC (2% w/w to TG, 25mM) was added to the TG oil phase of the inverse emulsion after mGFP-CCT1 was bound at the oil-water interface. (A) Representative images are shown. Scale bar, 10 μm. (B) Mean fluorescence on LDs ± SD (n = 11). A.U. = arbitrary units. (C) Protein diffusion at the oil-water interface of an in vitro drop is gradually decreased upon interface shrinkage according to FRAP analysis. Representative images are shown. Scale bar, 10 μm. (D) The diffusion of CCT at the oil-water interface is inversely correlated with the concentration of CCT at the drop surface according to FRAP analysis (C). D ∞ 1/C, assuming a Stoke-Einstein-like law, is used to fit the data. Note, shrunken drops have a high concentration of CCT1 at their surface and volume and a low diffusion rate along the surface. A.U. = arbitrary units. (E) CCT1 displacement occurs before its diffusion is limited. Mean diffusion (±SD, n = 4) and fraction of surface-bound CCT1 were measured on drops andplotted against the compression factor of the drop. Lines indicate trends. (F,G) High-, but not low-, molecular-weight PEGs crowd out CCT1 from the oil-water interface. PEGs were added at room temperature (2% w/w of the oil) to drops whose interface was bound by mGFP-CCT1. (F) Representative images are shown. Scale bar, 50 μm. (G) Mean mGFP-CCT1 fluorescence ± SD (n = 7) on the drop surface for indicated times over time is shown. The value at time 0 was normalized to 1. Lines are trend lines. A.U. = arbitrary units. See also Figure S3.

Figure 6. Proteins Compete for Binding at the Lipid Droplet Surface

(A) High levels of LSD1 compete off some, but not all, LD proteins. _mCherry_-LSD1 was co-expressed with GFP-CCT1 in LD-containing Drosophila S2 cells. One representative cell with low expression (upper panel) and one with high expression of LSD1 (lower panel) are shown. LDs were stained with AUTOdot. Scale bar is 5 μm. Inlay, 3× magnification. (B) Some proteins compete more strongly than others against LSD1 at the LD binding surface. Mean fluorescence on LDs ± SD (n > 15). A.U. = arbitrary units. (C,D) High levels of CG9186 outcompete CCT at the surface of LDs. _mCherry_-CG9186 was co-expressed with GFP-CCT1 in LD-containing Drosophila S2 cells. (C) One representative cell with low expression (upper panel) and one with high expression of LSD1 (lower panel) are shown. LDs were stained with AUTOdot. Scale bar is 5 μm. Inlay, (D) Mean fluorescence on LDs ± SD (n > 15). A.U. = arbitrary units. See also Figure S4.

Figure 7. Binding Affinity Determines Protein Lipid Droplet Localization During Lipolysis

(A) LD proteins are displaced from LDs to different degrees during lipid starvation. The localization index is defined as the difference of the fold change in percentage of a protein on LDs versus the rest of the cell from 1. (B) The correlation of localization index and critical LSD1 concentration needed to replace half of the amount of a bound protein from LDs as determined in Figure 6B. Linear regression, GPAT4 data was omitted from modeling. (C) A schematic of GFP-tagged full-length CCT1, the LD binding domain (M-domain) and two copies of the M domain. (D) LSD1 displaces mGFP-CCT1M at a lower concentration than mGFP-CCT1M2. A.U. = arbitrary units. (E,F) mGFP-CCT1M2 falls off LDs less than mGFP-CCT1M. LDs were stained with LipidTOX. (E) Representative images are shown. Scale bar, 5 μm. Inlay, 3× magnification. (F) Mean fluorescence on LDs ± SD (n > 12). A.U. = arbitrary units. See also Figure S5.

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Protein Crowding Is a Determinant of Lipid Droplet Protein Composition - PubMed (original) (raw)