Interfacial properties of apolipoprotein B292-593 (B6.4-13) and B611-782 (B13-17). Insights into the structure of the lipovitellin homology region in apolipoprotein B.

The N-terminal sequence of apolipoprotein B (apoB) is critical in triacylglycerol-rich lipoprotein assembly. The first 17% of apoB (B17) is thought to consist of three domains: B5.9, a beta-barrel, B6.4-13, a series of 17 alpha-helices, and B13-17, a putative beta-sheet. B5.9 does not bind to lipid, while B6.4-13 and B13-17 contain hydrophobic interfaces that can interact with lipids. To understand how B6.4-13 and B13-17 might interact with triacylglycerol during lipoprotein assembly, the interfacial properties of both peptides were studied at the triolein/water interface. Both B6.4-13 and B13-17 are surface active. Once bound, the peptides can be neither exchanged nor pushed off the interface. Some residues of the peptides can be ejected from the interface upon compression but readsorb on expansion. B13-17 binds to the interface more strongly. The maximum pressure the peptide can withstand without being partially ejected (Pi(max)) is 19.2 mN/m for B13-17 compared to 16.7 mN/m for B6.4-13. B13-17 is purely elastic at the interface, while B6.4-13 forms a viscous-elastic film. When they are spread at an air/water interface, the limiting area and the collapse pressures are 16.6 A(2)/amino acid and 31 mN/m for B6.4-13 and 17.8 A(2)/amino acid and 35 mN/m for B13-17, respectively. The alpha-helical B6.4-13 contains some hydrophobic helices that stay bound and prevent the peptide from leaving the surface. The beta-sheets of B13-17 bind irreversibly to the surface. We suggest that during lipoprotein assembly, the N-terminal apoB starts recruiting lipid as early as B6.4, but additional sequences are essential for formation of a lipid pocket that can stabilize lipoprotein emulsion particles for secretion.