Apolipoprotein A-I (apoA-I) binds to cholesterol containing liposomes of L-α-dimyristoylphosphatidylcholine (DMPC) to give stable lipid-protein complexes which are readily isolated by gel filtration or by centrifugation in a density gradient of CsCl. The rate and affinity of apoA-I-DMPC association are sensitive functions of sterol content and temperature. ApoA-I spontaneously associates with cholesterol-DMPC liposomes when the cholesterol content of the liposomes is 24 mol % or less. In this range of lipid composition, complex formation is nearly quantitative and the lipid composition of the complex is similar to but not identical with that of the corresponding starting cholesterol-DMPC mixtures. At 33 mol % cholesterol a much lower yield of apoA-I-lipid complexes is obtained. Moreover, in the complexes which are formed, cholesterol composes only 24 mol % of the lipid. The protein-lipid complexes formed from apoA-I, cholesterol, and DMPC are much smaller than the starting liposomes as verified by chromatography on Sepharose CL-4B and by the large decrease in light scattering which accompanies the association of lipid and protein. The changes in light scattering are time dependent, and the rates vary with both temperature and cholesterol content. Between 0 and 33 mol % cholesterol apoA-I binds fastest to 12 mol % cholesterol in DMPC at all temperatures studied, and the rate of apoA-I association with cholesterol-DMPC mixtures was fastest at the transition temperature of DMPC (23.9 °C) regardless of the cholesterol content. The combined effects of cholesterol and temperature can alter the rates of reaction of apoA-I with DMPC by more than three orders of magnitude. We propose that apoA-I preferentially inserts into channel or hole defects at the boundary between coexisting gel and liquid crystalline phases. The greatest number of defects is formed in DMPC containing 12.5 mol % cholesterol at 23.9 °C, the transition temperature of the phospholipid. In our model the initial interaction of apoA-I with cholesterol-DMPC liposomes occurs at the interface of coexisting phases of 1:3 cholesterol-DMPC and pure DMPC.
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