Coordination of Copper to the Membrane-Bound Form of α-Synuclein

Abstract

Aggregation of the 140-amino acid protein α-synuclein (α-syn) is linked to the development of Parkinson's disease (PD). α-Syn is a copper binding protein with potential function as a regulator of metal-dependent redox activity. Epidemiological studies suggest that human exposure to excess copper increases the incidence of PD. α-Syn exists in both solution and membrane-bound forms. Previous work evaluated the Cu(2+) uptake for α-syn in solution and identified Met1-Asp2 and His50 as primary contributors to the coordination shell, with a dissociation constant of approximately 0.1 nM. When bound to the membrane bilayer, α-syn takes on a predominantly helical conformation, which spatially separates His50 from the N-terminus of the protein and is therefore incompatible with the copper coordination geometry of the solution state. Here we use circular dichroism and electron paramagnetic resonance (continuous wave and pulsed) to evaluate the coordination of copper to the membrane-bound form of α-syn. In this molecular environment, Cu(2+) binds exclusively to the N-terminus of the protein (Met1-Asp2) with no participation from His50. Copper does not alter the membrane-bound α-syn conformation or enhance the release of the protein from the bilayer. The Cu(2+) affinity is similar to that identified for solution α-syn, suggesting that copper coordination is retained in the membrane. Consideration of these results demonstrates that copper exerts its greatest conformational effect on the solution form of α-syn.