Composition-structure-function correlations in the binding of an apolipoprotein to phosphatidylcholine bilayer mixtures.

We have studied the lipid binding of apoC-III with two types of mixed vesicles of DMPC (dimyristoyl phosphatidylcholine) and DPPC (dipalmitoyl phosphatidyl-choline). DMPC vesicles mixed with those of DPPC produce a macroscopic mixture in which the DMPC and DPPC vesicles remain intact. The circular dichroism and fluorescence spectra of apoC-III in the presence of this macroscopic mixture exhibit major changes near the transition temperature of each of the pure lipids, confirming the independent existence of the two PC's. Combining DMPC:DPPC macroscopic mixtures with apoC-III above the transition temperature, Tc 23 degrees C, of DMPC produces an isolatable complex consisting of 4:1 DMPC:DPPC. If the DMPC and DPPC are within the same vesicle, this microscopic lipid mixture has properties that are functions of the temperature and lipid composition. Spectral analysis of apoC-III in the presence of the micromixtures reveals a single transition, which occurs between the respective thermal transitions of DMPC (23 degrees C) and DPPC (41 degrees C). The lipid: protein complexes isolated from the microscopic mixtures have a DMPC: DPPC ratio identical to that of the vesicle with which the apoprotein was mixed; the formation of these complexes is most efficient above the temperature range where these microscopic mixtures induce major structural changes in the apoprotein. The absence of the preferential binding to DMPC observed in the macroscopic mixtures suggests that apoC-III interacts with large lipid domains within a vesicle and does not selectively bind the low melting DMPC. The temperature dependence of the binding of apoC-III to the microscopic mixtures may be due to phase separation or to cocrystallization; our data support the latter process.