NAC, a 35-residue peptide derived from the neuronal protein alpha-synuclein/NAC precursor, is tightly associated with Abeta fibrils in Alzheimer's disease amyloid, and alpha-synuclein has recently been shown to bind Abeta in vitro. We have studied the interaction between Abeta and synucleins, aiming at determining segments in alpha-synuclein that can account for the binding, as well as identifying a possible interaction between Abeta and the beta-type synuclein. We report that Abeta binds to native and recombinant alpha-synuclein, and to beta-synuclein in an SDS-sensitive interaction (IC50 approx. 20 microM), as determined by chemical cross-linking and solid-phase binding assays. alpha-Synuclein and beta-synuclein were found to stimulate Abeta-aggregation in vitro to the same extent. The synucleins also displayed Abeta-inhibitable binding of NAC and they were capable of forming dimers. Using proteolytic fragmentation of alpha-synuclein and cross-linking to 125I-Abeta, we identified two consecutive binding domains (residues 1-56 and 57-97) by Edman degradation and mass spectrometric analysis, and a synthetic peptide comprising residues 32-57 possessed Abeta-binding activity. To test further the possible significance in pathology, alpha-synuclein was biotinylated and shown to bind specifically to amyloid plaques in a brain with Alzheimer's disease. It is proposed that the multiple Abeta-binding sites in alpha-synuclein are involved in the development of amyloid plaques.