Lipid Metabolism and Trafficking in the Golgi Apparatus.

In organelle biogenesis pathways, the Golgi apparatus plays a central role in synthesizing and distributing complex sphingolipids, glycerolipids, and cholesterol, which form the membranes of organelles. The concept of 'lipid flow' describes the circulation of lipids throughout the endomembrane system, mediated by vesicle-based carriers and lipid transfer proteins that directly transfer lipids at sites of organelle-organelle contact. The synthesis and trafficking of lipids are closely interconnected, as they undergo successive enzymatic transformations that begin in the endoplasmic reticulum and are completed in the Golgi apparatus before being distributed to other organelles. A phosphatidylinositol 4-phosphate signaling network that converges on the trans cisterna and trans Golgi network compartments regulates lipid flow to, though, and out of the Golgi apparatus, acting as a rheostat that tunes the rates and specificities of lipid trafficking by controlling phosphatidylinositol 4-phosphate levels in Golgi membranes. The integration of glycolipid and glycoprotein processing reactions within the Golgi with lipid sorting and trafficking pathways suggests that the Golgi apparatus serves as a lipid-based sorting station at the heart of organelle biogenesis pathways.