Complexity in the Secretory Pathway: The Assembly and Secretion of Apolipoprotein B-containing Lipoproteins

Apolipoprotein B100 (apoB100) is expressed primarily in mammalian liver. It has 4536 amino acids, 25 cysteines (16 of which are in intramolecular disulfide bonds), and 20 N-linked glycosylation sites. A smaller form, apoB48, is expressed in mammalian intestine and in the livers of some non-human mammals. ApoB48 results from a post-transcriptional modification of the apoB mRNA at codon 2153 that converts a glutamine codon to a stop codon at ∼48% of the full-length coding sequence. Both forms of apoB have a complex structure that includes a globular amphipathic NH2 domain spanning the first 15–20% (using B100 as a standard length) of the polypeptide followed by an extended hydrophobic β-sheet domain from about 20 to 48%.