Taking Charge: Metal Ions Accelerate Amyloid Aggregation in Sequence Variants of ?-Synuclein

Alpha lpha-synuclein (alpha S) is an intrinsically disordered protein which exhibits a high degree of conformational heterogeneity. In vivo, alpha S experiences various environments which cause adaptation of its structural ensemble. Divalent metal ions are prominent in synaptic terminals where alpha S is located and are thought to bind to the alpha S C-terminal region. Herein, we used native nanoelectrospray ionization ion mobility-mass spectrometry to investigate changes in the charge state distribution and collision cross sections of wild-type N-terminally acetylated (NTA) alpha S, along with a deletion variant (Delta Delta NTA) which inhibits amyloid formation and a C-terminal truncated variant (119NTA) which increases the rate of amyloid formation. We also examine the effect of the addition of divalent metal ions, Ca2+, Mn2+, and Zn2+, and correlate the conformational properties of the alpha S monomer with the ability to aggregate into amyloid, measured using Thioflavin T fluorescence and negative stain transmission electron microscopy. We find a correlation between the population of species with a low collision cross section and accelerated amyloid assembly kinetics, with the presence of metal ions resulting in protein compaction and causing Delta Delta to regain its ability to form an amyloid. The results portray how the alpha S conformational ensemble is governed by specific intramolecular interactions that influence its amyloidogenic behavior.