Membrane association of α-synuclein (α-syn)， a neuronal protein associated with Parkinson's disease (PD)， is involved in α-syn function and pathology. Most previous studies on α-syn-membrane interactions have not used the physiologically relevant N-terminally acetylated (-acetyl) α-syn form nor the most naturally abundant cellular lipid， phosphatidylcholine (PC). Here， we report on how PC membrane fluidity affects the conformation and aggregation propensity of N-acetyl α-syn. It is well established that upon membrane binding， α-syn adopts an α-helical structure. Using CD spectroscopy， we show that N-acetyl α-syn transitions from α-helical to disordered at the lipid melting temperature ( ). We found that this fluidity sensing is a robust characteristic， unaffected by acyl chain length ( = 34-55 °C) and preserved in its homologs β- and γ-syn. Interestingly， both N-acetyl α-syn membrane binding and amyloid formation trended with lipid order (1，2-dipalmitoyl--glycero-3-phosphocholine (DPPC) > 1，2-dioleoyl--glycero-3-phosphocholine (DOPC)/sphingomyelin/cholesterol (2:2:1) ≥ DOPC)， with gel-phase vesicles shortening aggregation kinetics and promoting fibril formation compared to fluid membranes. Furthermore， we found that acetylation enhances binding to PC micelles and small unilamellar vesicles with high curvature ( ∼16-20 nm) and that DPPC binding is reduced in the presence of cholesterol. These results confirmed that the exposure of hydrocarbon chains ( packing defects) is essential for binding to zwitterionic gel membranes. Collectively， our results suggest that N-acetyl α-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.