Fig. 1.

Schematic illustration of the role of lipid transfer proteins in the assembly of apoB-containing lipoproteins. apoB undergoes cotranslational lipidation during translocation through the ER membrane. Nascent apoB peptide interacts with the inner phospholipid monolayer, resulting in the formation of nucleation sites for the assembly of lipoproteins. MTTP is a critical chaperone for lipoprotein assembly and most likely helps by transferring lipids to nascent apoB and by physically interacting with apoB. PLTP is another ER lumenal lipid transfer protein that may assist the assembly of lipoproteins. As opposed to MTTP and PLTP, CIDEB and AUP1 are ER membrane proteins. CIDEB may facilitate addition of triacylglycerols to nascent apoB, whereas AUP1 appears to antagonize this function. Once a smaller primordial lipoprotein is formed, it undergoes a second step, namely, core expansion, which involves bulk addition of lipids from lumenal droplets to primordial lipoproteins, most likely involving membrane fusion. Several exchangeable apolipoproteins may stabilize the surface of these larger lipoproteins and assist in their assembly. All of these steps and proteins are critical for lipoprotein assembly and secretion in both the intestine and liver, except for PLTP that may not be as critical for intestinal lipoprotein assembly. The role of AUP1 has not been addressed in the intestine.