Membrane-induced initial structure of α-synuclein control its amyloidogenesis on model membranes

Amyloid fibrillation causes serious neurodegenerative diseases and amyloidosis; however, the detailed mechanisms by which the structural states of precursor proteins in a lipid membrane-associated environment contribute to amyloidogenesis still remains to be elucidated. We examined the relationship between structural states of intrinsically-disordered wild-type and mutant alpha-synuclein (alpha SN) and amyloidogenesis on two-types of model membranes. Highly-unstructured wild-type aSN (alpha SNWT) and a C-terminally-truncated mutant lacking negative charges (alpha SN103) formed amyloid fibrils on both types of membranes, the model membrane mimicking presynaptic vesicles (Mimic membrane) and the model membrane of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC membrane). Unstructured alpha SNWT and alpha SN103 both bound to Mimic membranes in a helical conformation with similar binding affinity. Promotion and then inhibition of amyloidogenesis of alpha SNWT were observed as the concentration of Mimic lipids increased. We explain this by the two-state binding model: at lower lipid concentrations, binding of alpha SNWT to membranes enhances amyloidogenicity by increasing the local concentration of membrane-bound alpha SN and so promoting amyloid nucleation; at higher lipid concentrations, membrane-bound aSN is actually in a sense diluted by increasing the number of model membranes, which blocks amyloid fibrillation due to an insufficient bound population for productive nucleation. Meanwhile, alpha SN103 formed amyloid fibrils over the whole concentration of Mimic lipids used here without inhibition, revealing the importance of helical structures for binding affinity and negatively charged unstructured C-terminal region for modulating amyloidogenesis. We propose that membrane binding-induced initial conformations of alpha SN, its overall charge states, and the population of membrane-bound alpha SN are key determinants of amyloidogenesis on membranes.