Relationship between Amphipathic ß Structures in the ß1 Domain of Apolipoprotein B and the Properties of the Secreted Lipoprotein Particles in McA-RH7777 Cells.

ABSTRACT

Our previous studies demonstrated that the first 1000 amino acid residues (the ßα1 domain) of human apolipoprotein (apo) B-100, termed apoB1000, are required for the initiation of lipoprotein assembly and the formation of a monodisperse stable phospholipid (PL)-rich particle. The objectives of this study were (a) to assess the effects on the properties of apoB truncates undergoing sequential inclusion of the amphipathic ß strands in the 700 N-terminal residues of the ß1 domain of apoB-100 and (b) to identify the subdomain in the ß1 domain that is required for the formation of a microsomal triglyceride transfer protein (MTP)-dependent triacylglycerol (TAG)-rich apoB-containing particle. Characterization of particles secreted by stable transformants of McA-RH7777 cells demonstrated the following. (1) The presence of amphipathic ß strands in the 200 N-terminal residues of the ß1 domain resulted in the secretion of apoB truncates (apoB1050 to apoB1200) as both lipidated and lipid-poor particles. (2) Inclusion of residues 300-700 of the ß1 domain led to the secretion of apoB1300, apoB1400, apoB1500, and apoB1700 predominantly as lipidated particles. (3) Particles containing residues 1050-1500 were all rich in PL. (4) There was a marked increase in the lipid loading capacity and TAG content of apoB1700-containing particles. (5) Only the level of secretion of apoB1700 was markedly diminished by MTP inhibitor BMS-197636. These results suggest that apoB1700 marks the threshold for the formation of a TAG-rich particle and support the concept that MTP participates in apoB assembly and secretion at the stage where particles undergo a transition from PL-rich to TAG-rich.