Abstract
α-Synuclein (αS) is the primary protein associated with Parkinson's disease, and it undergoes aggregation from its intrinsically disordered monomeric form to a cross-β fibrillar form. The closely related homolog β-synuclein (βS) is essentially fibril-resistant under cytoplasmic physiological conditions. Toxic gain-of-function by βS has been linked to dysfunction, but the aggregation behavior of βS under altered pH is not well-understood. In this work, we compare fibril formation of αS and βS at pH 7.3 and mildly acidic pH 5.8, and we demonstrate that pH serves as an on/off switch for βS fibrillation. Using αS/βS domain-swapped chimera constructs and single residue substitutions in βS, we localized the switch to acidic residues in the N-terminal and non-amyloid component domains of βS. Computational models of βS fibril structures indicate that key glutamate residues (Glu-31 and Glu-61) in these domains may be sites of pH-sensitive interactions, and variants E31A and E61A show dramatically altered pH sensitivity for fibril formation supporting the importance of these charged side chains in fibril formation of βS. Our results demonstrate that relatively small changes in pH, which occur frequently in the cytoplasm and in secretory pathways, may induce the formation of βS fibrils and suggest a complex role for βS in synuclein cellular homeostasis and Parkinson's disease.
Highlights
␣-Synuclein (␣S) is the primary protein associated with Parkinson’s disease, and it undergoes aggregation from its intrinsically disordered monomeric form to a cross- fibrillar form
The key observation in our study is that mildly acidic pH 5.8 renders the otherwise non-fibrillogenic S fibrillogenic
At high pH the S protein is non-fibrillogenic as previously demonstrated in the literature [13, 62]
Summary
␣-Synuclein (␣S) is the primary protein associated with Parkinson’s disease, and it undergoes aggregation from its intrinsically disordered monomeric form to a cross- fibrillar form. Alternative topologies, derived from modeling based on experimentally determined structural restraints, have been proposed [45], and other experimental results suggest an inherent polymorphism and environmental sensitivity of the ␣S aggregate morphology (46 – 48) Environmental changes, such as those in pH, may exert their effects by stabilization of one or more of these fibrillar topologies similar to the effect of familial mutations switching the preference between two different fibril topologies, as recently proposed by Nussinov and co-workers [45]. Fibril models reveal two glutamate side chains that may underlie the observed pH-dependent fibrillation of S, and single substitutions to alanine at these positions lead to significantly altered aggregation behavior These findings provide insight into the complex interplay of charge and hydrophobicity for fibril formation in different cellular microenviron-. Ments, and they suggest that S may play a multifaceted role in disease and homeostasis in PD
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