Lipid droplet lifecycle

Lipid droplets store neutral lipids. They are born in the endoplasmic reticulum when triacylglycerol and sterol esters made in the membrane coalesce at seipin-marked sites and bud toward the cytosol; they grow by local synthesis, by transfer of lipid from the ER and by fusion; and they are consumed by lipases on their surface or by lipophagy, releasing fatty acids for energy and membrane synthesis. This umbrella module composes the modules for those stages by reference. Triacylglycerol synthesis by a DGAT is required in every eukaryote; the pathway and lipolysis descriptions are optional.

Organelle lifecycle umbrella (see projects/ORGANELLE_LIFECYCLES.md); stage roles follow the shared skeleton. Read as a satisfiability rule, the umbrella is satisfied when the DGAT step is. The glycerolipid pathway (triacylglycerol_biosynthesis), plant seed oil synthesis (seed_triacylglycerol_biosynthesis) and the human ATGL/HSL/MGLL lipolysis cascade (triacylglycerol_lipolysis) are optional references; none is written as a eukaryote-wide satisfiability rule. Not yet modelled: seipin-dependent nucleation (no local PANTHER entry for human BSCL2), droplet growth and fusion, a eukaryote-wide lipolysis step (the local PANTHER family of human ATGL lists only animal members, and where yeast Tgl3/Tgl4 and plant SDP1 are placed has not been checked), and lipophagy.