Structural and dynamic interfacial properties of the lipoprotein initiating domain of apolipoprotein B.

To better understand the earliest steps in the assembly of triglyceride (TG)-rich lipoproteins, we compared the biophysical and interfacial properties of two closely related apolipoprotein B (apoB) truncation mutants, one of which contains the complete lipoprotein initiating domain (apoB20.1; residues 1-912), and one of which, by virtue of a 50 amino acid C-terminal truncation, is incapable of forming nascent lipoproteins (apoB19; residues 1-862). Spectroscopic studies detected no major differences in secondary structure, and only minor differences in conformation and thermodynamic stability, between the two truncation mutants. Monolayer studies revealed that both apoB19 and apoB20.1 bound to and penetrated egg phosphatidylcholine (EPC) monolayers; however, the interfacial exclusion pressure of apoB20.1 was higher than apoB19 (25.1 mN/m vs. 22.8 mN/m). Oil drop tensiometry revealed that both proteins bound rapidly to the hydrophobic triolein/water interface, reducing interfacial tension by approximately 20 mN/m. However, when triolein drops were first coated with phospholipids (PL), apoB20.1 bound with faster kinetics than apoB19 and also displayed greater interfacial elasticity (26.9 +/- 0.8 mN/m vs. 22.9 +/- 0.8 mN/m). These data establish that the transition of apoB to assembly competence is accompanied by increases in surface activity and elasticity, but not by significant changes in global structure.