¹²⁵I-labeled ApoE binds competitively to fibrils with pathological chaperone proteins

Radiolabeled Apolipoprotein E (Apo E) was used in a competitive binding filtration assay to amyloid fibrils preformed from β(1-40) peptide as a probe of the binding sites for proteins either found in senile plaques in Alzheimer's Disease brain or reported to be associated with the soluble peptide. Apo E, Apo J, Apo A-1, Apo B, laminin, complement components C3 and C4, and α1-antichymotrypsin all displayed sub-micromolar apparent affinities for the Apo E binding site on fibrils. Transthyretin, α2-macroglobulin, amyloid P protein, heparan sulfate proteoglycan, complement component Clq, chondroitin sulfate A, and GM1 ganglioside were much less effective. The ε2, ε3, and ε4 isoforms of Apo E showed different affinities for fibrils and lipidation of these lipoproteins made little difference. Other fibrillar β- peptides also bound Apo E, with Aβ40~ Aβ42 > Aβ(12-28);Aβ(25-35) = 0. A series of soluble β-peptides and fragments failed to effect Apo E binding. Thus, both conformational and quaternary structural features are important in high affinity binding of Apo E to Aβ40 fibrils. Different amyloid plaque- associated molecules apparently associate with alternative primary and secondary structural features on fibrils.