Vesicular trafficking of hepatic apolipoprotein B100 and its maturation to very low-density lipoprotein particles; studies from cells and cell-free systems.

A cell-free system was established to study the process by which apolipoprotein (apo)B100-containing lipoproteins exit the endoplasmic reticulum (ER). ApoB was found in COPII vesicles with physical properties distinct from those containing other secreted proteins. When lipid synthesis in rat hepatoma cells was stimulated by fatty acid addition, fully lipidated apoB-lipoproteins of very low-density lipoprotein density were absent from the vesicles, but instead formed in a post-ER compartment. These data suggest that the COPII machinery in cells of hepatic and intestinal origin has evolved to sequester secreted cargoes with unique properties compared with those in other tissues, and that final lipidation occurs after a protein quality-control checkpoint is passed in the ER.