We have carried out detailed investigations of membrane interactions and amyloid formation of alpha-syn that have provided residue-specific information and molecular insights into the mechanism of aggregation. Due to the complexity of the amyloid problem, the tools with which we attack have included molecular biology, steady-state and time-resolved fluorescence spectroscopy, nuclear magnetic resonance spectroscopy, electron microscopy, neutron reflectometry, and mass spectrometry. We are developing a chemical understanding in how specific phospholipids modulate protein structure, membrane binding, and aggregation propensity through different studies summarized below. In relating to the complex cellular lipid compositions, we are focusing towards understanding the effects of bilayer fluidity and phase state by changes in acyl chain length as well as chain saturation. Specifically, we have studied the effect of phosphatidylcholine (PC) membrane fluidity on the conformation and aggregation propensity of the physiologically relevant N-terminally acetylated (N-acetyl) alpha-syn. Using CD spectroscopy, we show that N-acetyl alpha-syn transitions from alpha-helical to disordered at the lipid melting temperature. We found that this fluidity sensing is a robust characteristic, unaffected by acyl chain length and preserved in its homologs beta- and gamma-syn. Interestingly, both N-acetyl alpha-syn membrane binding and amyloid formation trended with lipid order with gel-phase vesicles shortening aggregation kinetics and promoting fibril formation compared with fluid membranes. Furthermore, we found that acetylation enhances binding to PC micelles and small unilamellar vesicles with high curvature. Cholesterol concentration dependence results confirmed that the exposure of hydrocarbon chains (i.e. packing defects) is essential for binding to zwitterionic gel membranes. Collectively, our in vitro results suggest that N-acetyl alpha-syn localizes to highly curved, ordered membranes inside a cell. We propose that age-related changes in membrane fluidity can promote the formation of amyloid fibrils, insoluble materials associated with PD. To understand how membrane remodeling by alpha-syn affects amyloid formation, we have studied the alpha-syn aggregation process in the presence of phosphatidylglycerol (PG) micellar tubules, which were the first reported example of membrane tubulation by alpha-syn. Aggregation kinetics, beta-sheet content, and macroscopic protein-lipid structures were observed by Thioflavin T fluorescence, circular dichroism spectroscopy and transmission electron microscopy, respectively. Collectively, the presence of PG micellar tubules formed at a stochiometric ratio was found to stimulate alpha-syn fibril formation. Moreover, transmission electron microscopy and solid-state nuclear magnetic resonance spectroscopy revealed the co-assembly of PG and alpha-syn into fibril structures. However, isolated micellar tubules do not form fibrils by themselves, suggesting an important role of free alpha-syn monomers during amyloid formation. In contrast, fibrils did not form in the presence of excess PG lipids, where most of the alpha-syn molecules are in a membrane-bound alpha-helical form. Our results provide new mechanistic insights into how membrane tubules modulate alpha-syn amyloid formation and support a pivotal role of proteinlipid interaction in the dysfunction of alpha-syn.
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