Defining lipid-interacting domains in the N-terminal region of apolipoprotein B.
Apolipoprotein B (apoB) is a nonexchangeable apolipoprotein that dictates the synthesis of chylomicrons and very low density lipoproteins. ApoB is the major protein in low density lipoprotein, also known as the "bad cholesterol" that is directly implicated in atherosclerosis. It has been suggested that the N-terminal domain of apoB plays a critical role in the formation of apoB-containing lipoproteins through the initial recruitment of phospholipids in the endoplasmic reticulum. However, very little is known about the mechanism of lipoprotein nucleation by apoB. Here we demonstrate that a strong phospholipid remodeling function is associated with the predicted alpha-helical and C-sheet domains in the N-terminal 17% of apoB (B17). Using dimyristoylphosphatidylcholine (DMPC) as a model lipid, these domains can convert multilamellar DMPC vesicles into discoidal-shaped particles. The nascent particles reconstituted from different apoB domains are distinctive and compositionally homogeneous. This phospholipid remodeling activity is also observed with egg phosphatidylcholine (egg PC) and is therefore not DMPC-dependent. Using kinetic analysis of the DMPC clearance assay, we show that the identified phospholipid binding sequences all map to the surface of the lipid binding pocket in the B17 model based on the homologous protein, lipovitellin. Since both B17 and microsomal triglyceride transfer protein (MTP), a critical chaperone during lipoprotein assembly, are homologous with lipovitellin, the identification of these phospholipid remodeling sequences in B17 provides important insights into the potential mechanism that initiates the assembly of apoB-containing lipoproteins.
Jiang
Zhenghui Gordon
ZG
Department of Physiology and Biophysics, Boston University School of Medicine, Boston, Massachusetts 02118, USA.
Gantz
Donald
D
Bullitt
Esther
E
McKnight
C James
CJ
D001055:Apolipoproteins B / Q000737:chemistry* / Q000378:metabolism; D050197:Atherosclerosis / Q000378:metabolism; D002352:Carrier Proteins / Q000737:chemistry / Q000378:metabolism; D002914:Chylomicrons / Q000096:biosynthesis / Q000737:chemistry; D004527:Egg Proteins; D015387:Egg Proteins, Dietary / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000737:chemistry / Q000378:metabolism; D006801:Humans; D007700:Kinetics; D008079:Lipoproteins, VLDL / Q000737:chemistry / Q000378:metabolism; D008958:Models, Molecular; D010713:Phosphatidylcholines / Q000378:metabolism; D011485:Protein Binding; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary
D001055:Apolipoproteins B; D002352:Carrier Proteins; D002914:Chylomicrons; D004527:Egg Proteins; D015387:Egg Proteins, Dietary; D008079:Lipoproteins, VLDL; D010713:Phosphatidylcholines; C106092:microsomal triglyceride transfer protein; C001549:lipovitellin