A role for lipid droplets in inter-membrane lipid traffic

Abstract

All cells have the capacity to accumulate neutral lipids and package them into lipid droplets. Recent proteomic analyses indicate that lipid droplets are not simple lipid storage depots, but rather complex organelles that have multiple cellular functions. One of these proposed functions is to distribute neutral lipids as well as phospholipids to various membrane-bound organelles within the cell. Here, we summarize the lipid droplet-associated membrane-trafficking proteins and review the evidence that lipid droplets interact with endoplasmic reticulum, endosomes, peroxisomes, and mitochondria. Based on this evidence, we present a model for how lipid droplets can distribute lipids to specific membrane compartments.

Figure 1

Transient Inter-Compartmental Contact Sites (TICCS) mediate lipid traffic between various membrane compartments and lipid droplets. The figure represents TICCS hypothesis. The top blue area is extracellular compartment. Cargo lipids (yellow line for lipids on membrane and yellow circle for lipids in lipid droplets) move between the cell surface and lipid droplets using endosomal intermediates (blue circular compartment). Migration is bi-directional and depends on the formation endosome-endosome as well as endosome-lipid droplet TICCS. Recognition sites (red square) for each organelle contain the molecular machinery necessary for the formation of TICCS. Compartmental specificity of TICCS formation is regulated by Rab-GTP (from [46] with permission).