The N-terminal 17% of apolipoprotein B (apoB-17) readily associates with dimyristoylphosphatidylcholine (DMPC) multilamellar vesicles (MLV) to form large (240-Å diameter) discoidal particles. Because apoB is normally secreted with triacylglycerol (TAG)-rich lipoproteins, we studied the binding of apoB-17 to triolein-rich emulsions modeling nascent TAG-rich very low density-like lipoproteins. Emulsions with the following composition (by weight) were prepared: 85-89% triolein, 1.1-1.4% cholesterol, and 10-14% phosphatidylcholines (PC) including either egg yolk (EY)-, dimyristoyl (DM)-, or dipalmitoyl (DP)-PC representing (at 25 Њ C), respectively, a fluid surface, a surface at transition, and a mainly solid surface. The respective sizes were 1,260 ؎ 500, 1,070 ؎ 450, and 830 ؎ 300 Å mean diameter. The emulsions were incubated with conditioned medium containing apoB-17, and then reisolated by ultracentrifugation. Analysis of the emulsion-bound proteins by gel electrophoresis showed that all three emulsions bound primarily apoB-17. The DPPC emulsions bound more apoB-17 than EYPC or DMPC emulsions. Immunoaffinitypurified apoB-17 exhibited saturable, high affinity binding to EYPC and DPPC emulsions. The respective K d values were 32 Ϯ 23 and 85 Ϯ 27 nM and capacities (N) were 10 and 58 molecules of apoB-17 per particle. When apoB-17 bound to emulsions was incubated with DMPC MLV at 26 Њ C for 18 h, it remained bound to the emulsions, indicating that once bound to these emulsions it is unable to exchange off and solubilize DMPC into discs. In contrast, apoE-3 bound to emulsions dissociated from the emulsions when incubated with DMPC MLV and formed discs. Thus, apoB-17 binds strongly and irreversibly to emulsions modeling nascent lipoproteins. It therefore may play an important role in the stabilization of nascent VLDL and chylomicrons. -Herscovitz, H.