Copper(II)-induced self-oligomerization of α-synuclein

α-Synuclein is a component of the abnormal protein depositions in senile plaques and Lewy bodies of Alzheimer's disease (AD) and Parkinson's disease respectively. The protein was suggested to provide a possible nucleation centre for plaque formation in AD via selective interaction with amyloid β/A4 protein (Aβ). We have shown previously that α-synuclein has experienced self-oligomerization when Aβ25-35 was present in an orientation-specific manner in the sequence. Here we examine this biochemically specific self-oligomerization with the use of various metals. Strikingly, copper(II) was the most effective metal ion affecting α-synuclein to form self-oligomers in the presence of coupling reagents such as dicyclohexylcarbodi-imide or N-(ethoxycarbonyl)-2-ethoxy-1,2-dihydroquinoline. The size distribution of the oligomers indicated that monomeric α-synuclein was oligomerized sequentially. The copper-induced oligomerization was shown to be suppressed as the acidic C-terminus of α-synuclein was truncated by treatment with endoproteinase Asp-N. In contrast, the Aβ25-35-induced oligomerizations of the intact and truncated forms of α-synuclein were not affected. This clearly indicated that the copper-induced oligomerization was dependent on the acidic C-terminal region and that its underlying biochemical mechanism was distinct from that of the Aβ25-35-induced oligomerization. Although the physiological or pathological relevance of the oligomerization remains currently elusive, the common outcome of α-synuclein on treatment with copper or Aβ25-35 might be useful in understanding neurodegenerative disorders in molecular terms. In addition, abnormal copper homoeostasis could be considered as one of the risk factors for the development of disorders such as AD or Parkinson's disease.