Although the interaction of apoE isoforms with amyloid-β (Aβ) peptides plays a critical role in the progression of Alzheimer's disease， how they interact with each other remains poorly understood. Here， we investigate the molecular mechanism of apoE-Aβ interactions by comparing the effects of the different domains of apoE on Aβ. The kinetics of aggregation of Aβ1-42 are delayed dramatically in the presence of substoichiometric， nanomolar concentrations of N-terminal fragment (NTF)， C-terminal fragment (CTF) and full-length apoE both in lipid-free and in lipidated forms. However， interactions between apoE and Aβ as measured by intermolecular Förster resonance energy transfer (FRET) analysis were... More
Although the interaction of apoE isoforms with amyloid-β (Aβ) peptides plays a critical role in the progression of Alzheimer's disease， how they interact with each other remains poorly understood. Here， we investigate the molecular mechanism of apoE-Aβ interactions by comparing the effects of the different domains of apoE on Aβ. The kinetics of aggregation of Aβ1-42 are delayed dramatically in the presence of substoichiometric， nanomolar concentrations of N-terminal fragment (NTF)， C-terminal fragment (CTF) and full-length apoE both in lipid-free and in lipidated forms. However， interactions between apoE and Aβ as measured by intermolecular Förster resonance energy transfer (FRET) analysis were found to be minimal at t = 0 but to increase in a time-dependent manner. Thus， apoE must interact with one or more 'intermediates' rather than the monomers of Aβ. Kinetics of FRET between full-length apoE4 labelled with EDANS at position 62 or 139 or 210 or 247 or 276， and tetramethylrhodamine-labelled Aβ (TMR-Aβ)， further support an involvement of all the three domains of apoE in the interactions. However， the above-mentioned residues do not appear to form a single pocket in the 3-dimensional structure of apoE. A competitive binding assay examining the effects of unlabelled fragments or full-length apoE on the FRET between EDANS-apoE and TMR-Aβ show that binding affinity of the full-length apoE to Aβ is much higher than that of the fragments. Furthermore， apoE4 is found to interact more strongly than apoE3. We hypothesize that high affinity of the apoE-Aβ interaction is attained due to multivalent binding mediated by multiple interactions between oligomeric Aβ and full-length apoE.
