The process of assembly of apolipoprotein (apo) B-containing lipoprotein particles occurs co-translationally after disulfide-dependent folding of the N-terminal domain of apoB but the mechanism is not understood. During a recent database search for protein sequences that contained similar amphipathic β strands to apoB-100, four vitellogenins, the precursor form of lipovitellin, an egg yolk lipoprotein, from chicken, frog, lamprey, and C. elegans appeared on the list of candidate proteins. The X-ray crystal structure of lamprey lipovitellin is known to contain a 'lipid pocket' lined by antiparallel amphipathic β sheets. Here we report that the first 1000 residues of human apoB-100 (the α1 domain plus the first 200 residues of the β1 domain) have sequence and amphipathic motif homologies to the lipid- binding pocket of lamprey lipovitellin. We also show that most of the α1 domain of human apoB-100 has sequence and amphipathic motif homologies to human microsomal triglyceride transfer protein (MTP), a protein required for assembly of apoB-containing lipoproteins. Based upon these results, we suggest that an LV-like 'proteolipid' intermediate containing a 'lipid pocket' is formed by the N-terminal portion of apoB alone or, more likely, as a complex with MTP. This intermediate produces a lipid nidus required for assembly of apoB-containing lipoprotein particles; pocket expansion through the addition of amphipathic β strands from the β1 domain of apoB results in the formation of a progressively larger high density lipoprotein (HDL)- like, then very low density lipoprotein (VLDL)like, spheroidal lipoprotein particle.