Conformational equilibria in monomeric alpha-synuclein at the single molecule level

TL;DR

This study uses single-molecule force spectroscopy to characterize alpha-synuclein conformations, revealing beta-like structures linked to aggregation, which could inform early intervention strategies for Parkinson's disease.

Contribution

It provides the first direct observation and quantification of multiple conformational states of alpha-synuclein at the single-molecule level, linking specific structures to aggregation propensity.

Findings

Beta-like structures increase with aggregation-promoting conditions

Disordered and structured conformations coexist in native alpha-synuclein

Targeting the conformational equilibrium may prevent fibril formation

Abstract

Natively unstructured proteins defy the classical "one sequence-one structure" paradigm of protein science. Monomers of these proteins in pathological conditions can aggregate in the cell, a process that underlies socially relevant neurodegenerative diseases such as Alzheimer and Parkinson. A full comprehension of the formation and structure of the so-called misfolded intermediates from which the aggregated states ensue is still lacking. We characterized the folding and the conformational diversity of alpha-synuclein (aSyn), a natively unstructured protein involved in Parkinson disease, by mechanically stretching single molecules of this protein and recording their mechanical properties. These experiments permitted us to directly observe directly and quantify three main classes of conformations that, under in vitro physiological conditions, exist simultaneously in the aSyn sample, including disordered and "beta-like" structures. We found that this class of "beta-like" structures is directly related to aSyn aggregation. In fact, their relative abundance increases drastically in three different conditions known to promote the formation of aSyn fibrils: the presence of Cu2+, the occurrence of the pathogenic A30P mutation, and high ionic strength. We expect that a critical concentration of aSyn with a "beta-like" structure must be reached to trigger fibril formation. This critical concentration is therefore controlled by a chemical equilibrium. Novel pharmacological strategies can now be tailored to act upstream, before the aggregation process ensues, by targeting this equilibrium. To this end, Single Molecule Force Spectroscopy can be an effective tool to tailor and test new pharmacological agents.