Calcium-Triggered Membrane Interaction of the α-Synuclein Acidic Tail

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Abstract
α-Synuclein (α-syn) is a 140-residue protein that aggregates in intraneuronal inclusions called Lewy bodies in Parkinson's disease (PD). It is composed of an N-terminal domain with a propensity to bind lipids and a C-terminal domain rich in acidic residues (the acidic tail). The objective of this study was to examine the effect of Ca2+ on the acidic tail conformation in lipid-bound α-syn. We exploit the extreme sensitivity of the band III fluorescence emission peak of the pyrene fluorophore to the polarity of its microenvironment to monitor subtle conformational response of the α-syn acidic tail to Ca2+. Using recombinant human α-syn bearing a pyrene to probe either the N-terminal domain or the acidic tail, we noted that lipid binding resulted in an increase in band III emission intensity in the pyrene probe tagging the N-terminal domain but not that in the acidic tail. This suggests that the protein is anchored to the lipid surface via the N-terminal domain. However, addition of Ca2+ caused an increase in band III emission intensity in the pyrene tagging the acidic tail, with a corresponding increased susceptibility to quenching by quenchers located in the lipid milieu, indicative of lipid interaction of this domain. Taken together with the increased β-sheet content of membrane-associated α-syn in the presence of Ca2+, we propose a model wherein initial lipid interaction occurs via the N-terminal domain, followed by a Ca2+-triggered membrane association of the acidic tail as a potential mechanism leading to α-syn aggregation. These observations have direct implications in the role of age-related oxidative stress and the attendant cellular Ca2+ dysregulation as critical factors in α-syn aggregation in PD.