Stimulation of α-synuclein amyloid formation by phosphatidylglycerol micellar tubules

alpha-Synuclein (alpha-Syn) is a presynaptic protein that is accumulated in its amyloid form in the brains of Parkinson's patients. Although its biological function remains unclear, alpha-syn has been suggested to bind to synaptic vesicles and facilitate neurotransmitter release. Recently, studies have found that alpha-syn induces membrane tubulation, highlighting a potential mechanism for alpha-syn to stabilize highly curved membrane structures which could have both functional and dysfunctional consequences. 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 (L/P = 1) 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 ( L/P = 50), where most of the a-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 protein lipid interaction in the dysfunction of alpha-syn.