Abstract
Historically, studies of intracellular membrane trafficking have focused on the secretory and endocytic pathways and their major organelles. However, these pathways are also directly implicated in the biogenesis and function of other important intracellular organelles, the best studied of which are peroxisomes and lipid droplets. There is a large recent body of work on these organelles, which have resulted in the introduction of new paradigms regarding the roles of membrane trafficking organelles. In this review, we discuss the roles of membrane trafficking in the life cycle of lipid droplets. This includes the complementary roles of lipid phase separation and proteins in the biogenesis of lipid droplets from endoplasmic reticulum (ER) membranes, and the attachment of mature lipid droplets to membranes by lipidic bridges and by more conventional protein tethers. We also discuss the catabolism of neutral lipids, which in part results from the interaction of lipid droplets with cytosolic molecules, but with important roles for both macroautophagy and microautophagy. Finally, we address their eventual demise, which involves interactions with the autophagocytotic machinery. We pay particular attention to the roles of small GTPases, particularly Rab18, in these processes.
Highlights
The study of intracellular membrane trafficking has historically concentrated on the movement of proteins and lipids between the membrane-bound compartments of the secretory and endocytic pathways
A recent study [70] reported that ORP2 binds adipose triglyceride lipase (ATGL) and the beta subunit of coatomer protein I (COPI), which could suggest an alternate role for ORP2 as a scaffold, facilitates the recruitment of other proteins to the lipid droplet (LD) surface
Another family of lipid transfer proteins implicated in contacts between endoplasmic reticulum (ER) and LDs are vacuolar protein sorting 13A (Vps13A) and vacuolar protein sorting 13C (Vps13C), proteins initially shown to be involved in trafficking to the vacuole in yeast with strong structural similarities to Atg2
Summary
The study of intracellular membrane trafficking has historically concentrated on the movement of proteins and lipids between the membrane-bound compartments of the secretory and endocytic pathways. While historically lipid droplets were considered inert storage structures, increasing evidence suggests that they were highly dynamic structures which interacted with numerous intracellular organelles, including endoplasmic reticulum (ER), endosomes, plasma membrane and mitochondria. Some of these functions are regulated by families of proteins known to be important in membrane trafficking pathways, including ADP ribosylation factors (Arfs), Rabs and tethers. LDs were originally considered to be static stores of triglycerides and cholesterol esters They are known to be dynamic organelles which interact extensively with other organelles and numerous regulatory pathways to support the logistics of lipid storage and mobilization. As a number of distinct topics are covered and the total volume of relevant literature is large, this review is not intended to be comprehensive, but instead to give a broad overview, with a focus on recent findings
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