Fibrillation of Alpha Synuclein: a view from the surface

RIAL HAWILA, MR; G.E. GÓMEZ; WIRTH MA; BINOLFI A; DELFINO J.M

Congreso; Biofísica en tiempos de COVID-19: Primeras Jornadas Virtuales SAB 2020; 2020

The nature and size of the accessible surface area (SASA) of the polypeptide chain plays a pivotal role in protein folding and complex formation. To investigate SASA, we employ diazirine (DZN), a minute precursor of the extremely reactive methylene carbene (:CH2). Methylation signatures left on the polypeptide provide telltale clues on conformation and interactions. The extent of methylation metric (EM) derived directly from mass spectra (ESI-MS) is able to discriminate between native and alternate states. The IDP human alpha synuclein (AS) aggregates into oligomers and amyloid fibrils, constituents of Lewy bodies, a cytosolic hallmark of Parkinson disease. DZN labeling proves particularly fit to analyzing the conformational plasticity inherent to this protein. Unlike well-structured proteins where the methylation signal differs strikingly between native and unfolded states, AS in buffer or equilibrated in 6 M GdmCl displays a similarly enhanced EM value, pointing to the high solvent exposure of AS under normal physiological conditions. Most remarkably, AS fibrils cause an enhanced EM value compare to the monomer, as a consequence of the occurrence of a hydrophobic interface in the fibrils. On the other hand, the feasibility to detect methylated products by multidimensional NMR is an approach that opens a potentially rich source of conformational information. The extent of reaction of (:CH2) at various sites across the surface of AS could be defined by 1H15NHSQC spectra. In AS fibrils, it is observed a decrease in the intensity profile -mostly in the N-terminus- due to the formation of a hydrophobic interface. These results are consistent with those obtained with ESI-MS. This information illuminates the role played by the constituent parts in the monomeric ensemble, in the less-known oligomers, as well as on those changes observed en route to the fibrillar aggregate.