We previously demonstrated that the N-terminal 1000 amino acid residues of human apolipoprotein (apo) B (designated apoB:1000) are competent to fold into a three-sided lipovitellin-like lipid binding cavity to form the apoB "lipid pocket" without a structural requirement for microsomal triglyceride transfer protein (MTP). Our results established that this primordial apoB-containing particle is phospholipid-rich (Manchekar, M., Richardson, P. E., Forte, T. M., Datta, G., Segrest, J. P., and Dashti, N.
Apolipoprotein B (apoB)2 is synthesized primarily in hepatocytes and enterocytes and has a fundamental role in the transport and metabolism of plasma triacylglycerols (TAG) and cholesterol (1, 2). ApoB is a predominant protein component of very low density lipoproteins (VLDL) and intermediate density lipoproteins and is essentially the only apoprotein component of low density lipoproteins (LDL 2 ) (3, 4). ApoB100 (the fulllength protein) is one of the largest monomeric proteins known with 4536 amino acid residues (2). It is expressed primarily in mammalian liver, is an essential structural component for the formation and secretion of VLDL, and serves as a ligand for the LDL receptor (2). ApoB is present as a single molecule per lipoprotein particle (5); and therefore, its concentration in the plasma approximates the number of potential atherogenic lipoprotein particles.The processes involved in the assembly of apoB-containing lipoproteins in the liver are complex and are regulated at multiple levels throughout the secretory pathway. The assembly of apoB-containing lipoproteins occurs co-translationally (1), i.e. while the C-terminal portion is still being synthesized on the ribosome of the endoplasmic reticulum (ER), the N-terminal portion is translocated across the ER and is assembled as a small lipoprotein particle. The addition of lipids to apoB is widely believed to occur in two steps (2, 6, 7). The first step involves the addition of small amounts of lipids to apoB, as it is translated and translocated into the lumen of ER preventing its degradation and formation of a partially lipidated small pre-VLDL particle in the high density lipoprotein (HDL) density range (2,7,8). In the second step, this pre-VLDL particle is believed to acquire the bulk of its core lipids and is converted to bona fide VLDL (2, 7, 9), presumably by fusing with a large, VLDL-sized, apoB-free TAG particle (9). Biochemical studies of VLDL assembly support the concept that the bulk of neutral lipids are added in the second step after apoB translation is completed (10).* This work was supported by the National Institutes of Health Grants HL084685 and PO1 HL34343. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. 2 The abbreviations used are: apoB, apolipoprotein B; apoB:1000, N-terminal 22.05% (residues 1-1000) of the mature protein; BSA, bovine serum albu-