Abstract

The in solution synchrotron small-angle X-ray scattering SAXS technique has been used to investigate an intrinsically disordered protein (IDP) related to Parkinson’s disease, the α-synuclein (α-syn), in prefibrillar diluted conditions. SAXS experiments have been performed as a function of temperature and concentration on the wild type (WT) and on the three pathogenic mutants G51D, E46K, and A53T. To identify the conformers that populate WT α-syn and the pathogenic mutants in prefibrillar conditions, scattering data have been analyzed by a new variational bayesian weighting method (VBWSAS) based on an ensemble of conformers, which includes unfolded monomers, trimers, and tetramers, both in helical-rich and strand-rich forms. The developed VBWSAS method uses a thermodynamic scheme to account for temperature and concentration effects and considers long-range protein–protein interactions in the framework of the random phase approximation. The global analysis of the whole set of data indicates that WT α-syn is mostly present as unfolded monomers and trimers (helical-rich trimers at low T and strand-rich trimers at high T), but not tetramers, as previously derived by several studies. On the contrary, different conformer combinations characterize mutants. In the α-syn G51D mutant, the most abundant aggregates at all the temperatures are strand-rich tetramers. Strand-rich tetramers are also the predominant forms in the A53T mutant, but their weight decreases with temperature. Only monomeric conformers, with a preference for the ones with the smallest sizes, are present in the E46K mutant. The derived conformational behavior then suggests a different availability of species prone to aggregate, depending on mutation, temperature, and concentration and accounting for the different neurotoxicity of α-syn variants. Indeed, this approach may be of pivotal importance to describe conformational and aggregational properties of other IDPs.

Highlights

  • Disordered proteins (IDPs) are a challenge for the biophysical community.[1]

  • Among the curves referring to the same α-syn mutant, the main differences at low q are due to the presence of a broad interference peak, which changes in position and height mainly, as expected, as a function of c

  • The possible presence of folded α-syn tetramers in prefibrillar conditions, together with unfolded monomers, is an issue widely discussed.[8,46−48] Some experiments have shown that the detection of such tetramers depends on the chemical−physical conditions and on the origin of α-syn, which can be produced using bacteria or isolated from mammalian cells as well as from red blood cells

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Summary

Introduction

Disordered proteins (IDPs) are a challenge for the biophysical community.[1]. Examples are Parkinson’s disease (PD), associated with the fibrillation of α-synuclein (αsyn),[14,15] Alzheimer’s disease, associated with the β-amyloid (Aβ) peptide, and Hungtington’s disease, in which huntingtin modifications are involved

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