Decreasing Phosphatidylcholine on the Lipid Droplet Surface Selectively Recruits Proteins with Amphipathic Alpha Helices

Abstract

Lipid droplets are ubiquitous, intracellular fat‐storage organelles. Each lipid droplet is surrounded by a phospholipid monolayer primarily composed of two phospholipids, phosphatidylcholine (PC) and phosphatidylethanolamine (PE). We have previously shown that lipid droplets accumulating in a rat model of alcoholic fatty liver disease have a decreased PC:PE ratio compared to healthy control animals. Moreover, altering the phospholipid composition of the lipid droplet surface changes the amount of specific lipid droplet proteins, suggesting a mechanism for fat accumulation in alcoholic fatty liver disease. Using subcellular fractionation and immunoblotting, we investigated the abundance and lipid droplet association of multiple proteins in AML12 cells cultured under conditions that alter the PC:PE ratio and mimic the lipid droplet structure in alcoholic fatty liver disease. With this approach, we measured the relative binding of proteins that interact with lipid droplets through a variety of structural motifs, including amphipathic alpha helices, lipid anchors, and hairpin loops. Our results indicate that the amphipathic alpha helix‐containing proteins perilipin 1, perilipin 2, and CIDE‐C are significantly more likely to associate with lipid droplets with decreased PC:PE ratios. This same change in PC:PE levels did not change association of rab 18, a protein that interacts with lipid droplets through a lipid anchor, and decreased lipid droplet association of a protein (DGAT2) that trafficks to lipid droplets as they bud from the ER. Our results support a model where increased binding of the amphipathic alpha helical domains of some lipid droplet proteins contributes to lipid droplet overaccumulation in alcoholic fatty liver disease. This work builds on our understanding of alcoholic fatty liver disease and the mechanisms that facilitate protein binding to lipid droplets.