Dissociation of Apolipoprotein E Oligomers to Monomer Is Required for High-Affinity Binding to Phospholipid Vesicles

The apolipoprotein apoE plays a key role in cholesterol and lipid metabolism. There are three isoforms of this protein, one of which, apoE4, is the major risk factor for Alzheimer's disease. At micromolar concentrations all lipid-free apoE isoforms exist primarily as monomers, dimers, and tetramers. However, the molecular weight form of apoE that binds to lipid has not been clearly defined. We have examined the role of self-association of apoE with respect to interactions with phospholipids. Measurements of the time dependence of turbidity clearance of small unilamellar vesicles of dimyristoyl-sn-glycero-3-phosphocholine (DMPC) upon addition of apoE show that higher molecular weight oligomers bind poorly if at all. The kinetic data can be described by a reaction model in which tetramers and dimers of apoE must first dissociate to monomers which then bind to the liposome surface in a fast and reversible manner. A slow but not readily reversible conformational conversion of the monomer then occurs. Prior knowledge of the rate constants for the association dissociation process allows us to determine the rate constant of the conformational conversion. This rate constant is isoform dependent and appears to correlate with the stability of the apoE isoforms with the rate of dissociation of the apoE oligomers to monomers being the rate-limiting process for lipidation. Differences in the lipidation kinetics between the apoE isoforms arise from their differences in the self-association behavior leading to the conclusion that self-association behavior may influence biological functions of apoE in an isoform-dependent manner.