Specificity of lipid incorporation is determined by sequences in the N-terminal 37 of apoB.
The N-terminal 17% of apolipoprotein B (apoB-17) is secreted lipid-poor while apoB-41 particles are secreted with a triacylglycerol (TAG)-rich core. Thus, the sequence between apoB-17 and apoB-41 is necessary for the assembly of TAG-rich lipoproteins. To delineate this region, C127 cells were permanently transfected to secrete the N-terminal 29, 32.5, or 37% of apoB. Density gradient centrifugation showed that secreted apoB-29, apoB-32.5, and apoB-37 had peak densities of 1.25, 1.22, and 1.16 g/mL and percent lipid of particle weights of 30, 37, and 49%, respectively. Calculated anhydrous particle diameters were: apoB-29 = 81 A, apoB-32.5 = 88 A, and apoB-37 = 101 A. Immunoprecipitated particles labeled with [(3)H]oleate showed that, as apoB length increased from apoB-29 to apoB-32.5 and apoB-37, the number of TAG (core) molecules per apoB particle increased almost 16-fold from 8 to 32 to 124, while phospholipids and diacylglycerols (surface lipids) increased only slightly from 71 to 87 to 97 molecules, respectively. Thus, sequences in the C-terminus of apoB-29 bind phospholipids and diacylglycerols, sequences between apoB-29 and apoB-32.5 augment TAG binding and sequences between apoB-32.5 and apoB-41 account for the marked incorporation of TAG at a rate of approximately 1 TAG per 2 amino acids. Cryoelectron micrographs of isolated apoB-37 particles revealed mostly spherical particles of approximately 110 A (11.0 nm) with an electron lucent center, consistent with these particles having a TAG core. We suggest that the predicted amphipathic beta-sheets beginning at apoB-29, starts to preferentially recruit core lipids into apoB and propose that the consistent presence of DAG in the secreted particles may have a role in fission of the nascent lipoprotein particles from the endoplasmic reticulum membrane.
Department of Biophysics and Department of Medicine, Cardiovascular Institute, Center for Advanced Biomedical Research at Boston University School of Medicine, 715 Albany Street Boston, Massachusetts 02118-2526, USA. carraway@med-biophd.bu.edu
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D001055:Apolipoproteins B / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D001665:Binding Sites; D008074:Lipoproteins / Q000378:metabolism*; D008954:Models, Biological; D010446:Peptide Fragments / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D014176:Protein Biosynthesis; D017433:Protein Structure, Secondary; D014280:Triglycerides / Q000378:metabolism*
D001055:Apolipoproteins B; D008074:Lipoproteins; D010446:Peptide Fragments; D014280:Triglycerides