The insulin-degrading enzyme (IDE) plays a key role in type-2 diabetes and typically degrades small peptides such as insulin, amyloid beta and islet amyloid polypeptide. We recently reported a novel non-proteolytical interaction in vitro between IDE and the Parkinson's disease 140-residue protein alpha-synuclein that resulted in dual effects: arrested alpha-synuclein oligomers and, simultaneously, increased IDE proteolysis activity. Here we demonstrate that these outcomes arise due to IDE interactions with the C-terminus of alpha-synuclein. Whereas a peptide containing the first 97 residues of alpha-synuclein did not improve IDE activity and its aggregation was not blocked by IDE, a peptide with the C-terminal 44 residues of alpha-synuclein increased IDE proteolysis to the same degree as full-length alpha-synuclein. Because the alpha-synuclein C-terminus is acidic, the interaction appears to involve electrostatic attraction with IDE's basic exosite, known to be involved in activation.