Alpha2-macroglobulin associates with beta-amyloid peptide and prevents fibril formation

We have used the yeast two-hybrid system to isolate cDNAs encoding proteins that specifically interact with the 42-aa beta-amyloid peptide (Abeta), a major constituent of senile plaques in Alzheimer's disease. The carboxy terminus of alpha2-macroglobulin (alpha2M), a proteinase inhibitor released in response to inflammatory stimuli, was identified as a strong and specific interactor of Abeta, utilizing this system. Direct evidence for this interaction was obtained by co-immunoprecipitation of alpha2M with Abeta from the yeast cell, and by formation of SDS-resistant Abeta complexes in polyacrylamide gels by using synthetic Abeta and purified alpha2M. The association of Abeta with alpha2M and various purified amyloid binding proteins was assessed by employing a method measuring protein-protein interactions in liquid phase. The dissociation constant by this technique for the alpha2M-Abeta association using labeled purified proteins was measured (Kd = 350 nM). Electron microscopy showed that a 1:8 ratio of alpha2M to Abeta prevented fibril formation in solution; the same ratio to Abeta of another acute phase protein, alpha1-antichymotrypsin, was not active in preventing fibril formation in vitro. These results were corroborated by data obtained from an in vitro aggregation assay employing Thioflavine T. The interaction of alpha2M with Abeta suggests new pathway(s) for the clearance of the soluble amyloid peptide.