  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.
