Apolipoprotein B in the rough endoplasmic reticulum: translation, translocation and the initiation of lipoprotein assembly.

Apolipoprotein (apo) B and the microsomal triglyceride transfer protein are essential for the hepatic assembly and secretion of triglyceride-rich VLDL. To understand how apoB initiates the process of lipoprotein formation, interest has focused on the biogenesis of its amino terminal globular domain (alpha1 domain). When only this domain is expressed in hepatoma cells, no lipoprotein particle will form. However, proper folding of the alpha1 domain is essential for the internal lipophilic regions of apoB to engage in cotranslational lipid recruitment. The essential function of this domain may be related to its capacity to promote a specific physical interaction with the microsomal triglyceride transfer protein, necessary for apoB's proper folding and lipidation. Alternatively, this domain may promote an autonomous lipid recruitment step that nucleates microsomal triglyceride transfer protein-dependent lipid sequestration by apoB. Forms of apoB that fail to initiate particle assembly or forms associated with aberrant underlipidated particles are targeted for intracellular turnover. Two sites of apoB degradation have been identified. In hepatocarcinoma-derived cells, misassembled apoB may undergo progressive reverse translocation from the endoplasmic reticulum lumen to the cytosol, a process that is mechanistically coupled to polyubiquitination and proteasome-mediated degradation on the cytosolic side of the membrane. Alternatively, studies in primary hepatocytes reveal that apoB may undergo sorting to a post-endoplasmic reticulum compartment for presecretory degradation. In either case, the balance between assembly and presecretory degradation of apoB may represent a control point for the production of hepatic VLDL.