Large disk intermediate precedes formation of apolipoprotein A-I-dimyristoylphosphatidylcholine small disks

Small approximately 8.5 nm disks formed spontaneously when dimyristoylphosphatidylcholine (DMPC) large unilamellar vesicles (LUVs) were incubated with apolipoprotein A-I (apoA-I) (100:1 molar ratio). However, in a time course study, the transient production of approximately 11 nm large disks was detected and isolated by gel filtration. The intermediate large disks contained three apoA-I molecules and were stable over time; however, when additional apoA-I was added, they formed small disks containing two molecules of apoA-I. The reaction kinetics of apoA-I with DMPC LUVs was monitored by fluorescence resonance energy transfer, and two phases were observed, supporting the presence of the intermediate in the formation of small disks. The lipid dynamics of LUVs and disks were assayed, revealing the presence of sequestered lipid-protein domains upon apoA-I binding to DMPC LUVs. In addition, the lipids in the intermediate large disks were more constrained than those in the small disks. We propose that apoA-I binds with DMPC LUVs to form small lipid-protein domains on the LUV; then the domains are released to form large disks, which can mature in the presence of additional apoA-I to form small disks. Thus, the formation of small apoA-I lipid disks proceeds through the formation of a large disk intermediate.