Association-Dissociation Behavior of the Apolipoprotein E Proteins: Implications for Lipid Binding

Abstract

Apolipoprotein-E4 (ApoE4) is a risk factor for Alzheimer's diseases but the structural or functional differences between the isoforms viz, ApoE2, ApoE3 and ApoE4 are unknown. Lipidation of ApoE is important for its functions. However, the molecular mechanism and the isoform specificity of ApoE-lipid interactions are unclear. In vitro lipid free ApoE undergoes self association but which oligomeric form of ApoE interacts with lipids is unknown. Using Fluorescence Correlation Spectroscopy, intermolecular FRET and sedimentation methods we find that association-dissociation reaction of ApoE can be modeled by a monomer-dimer-tetramer process. Dissociation kinetics as measured by changes in FRET show two phases reflecting dissociation of tetramer to dimer and of dimer to monomer. The rate constants are found to be different for the ApoE isoforms. The kinetics of lipidation of ApoE in presence of unilamellar vesicles of DMPC show striking similarity with the kinetics of dissociation of ApoE multimers to monomers. Furthermore, lipidation kinetics are slower at higher ApoE concentrations and kinetic data are consistent with only monomers binding to lipids. The results imply that differences in lipidation properties between apoE isoforms arise due to their differences in association-dissociation behavior.View Large Image | View Hi-Res Image | Download PowerPoint Slide