Navigating lipid droplet proteins - part II: molecular mechanisms underlying ER-to-lipid droplet protein partitioning.

Lipid droplets (LDs) originate from the endoplasmic reticulum (ER) and are unique among cellular organelles, as they consist of a hydrophobic core of neutral lipids that is surrounded by a phospholipid monolayer. Proteins and enzymes embedded into this monolayer are essential for regulating dynamic lipid storage and consumption and hence, for the cellular adaptation to metabolic changes. Their activity and abundance on the LD surface must therefore be well-controlled. Many of these proteins are first inserted into the phospholipid bilayer membrane of the ER before they partition to the LD monolayer. While a monotopic membrane topology is required for enabling the targeting of these ERTOLD proteins from the ER to LDs, the molecular mechanisms underlying this partitioning are only beginning to emerge. In this second part of the bipartite review 'Navigating lipid droplet proteins,' we discuss recent conceptual advances regarding ER-to-LD protein partitioning and focus on novel insights into the structural dynamics of LD-destined proteins, how their partitioning to LDs is temporally controlled, and the hierarchies involved in selective and competitive protein recruitment to LDs according to metabolic needs and functions.