Dynamic interactions between alpha-synuclein and curved membrane surfaces

Alpha-synuclein (α-syn) is an intrinsically disordered protein (IDP) that is abundant in neurons, where it is implicated in the pathogenesis of Parkinson's disease through cytosolic aggregation and formation of fibrils. In its healthy state, α-syn binds to synaptic lipid vesicles, transitioning to a membrane-inserting, α-helical structure in its N-terminal region while the C-terminal domain remains disordered in close proximity to the membrane. These types of disordered C-terminal domains have been shown to amplify the curvature sensitivity of several proteins through steric and/or electrostatic interactions with the membrane, which is essential for cellular function. However, little is known about the role of this disordered C-terminal domain in curvature sensing for α-syn. Additionally, little is known about the dynamic interactions of these types of disordered domains with membrane surfaces because traditional studies examine systems in thermodynamic equilibrium, which can take minutes to achieve. Conversely, many relevant cellular processes, such as cell signaling, occur on the order of seconds to milliseconds. Therefore, it is reasonable to infer that thermodynamic equilibrium does not fully encompass the complexities of α-syn's curvature sensing ability at physiologically relevant timescales. In this study, we utilize quantitative, in vitro microscopy and circular dichroism spectroscopy to examine both equilibrium and dynamic binding of α-syn to curved membranes. Our data suggests that the dissociation constant (Kd) and the association and dissociation rates (Kon and Koff) for α-syn depend on both membrane curvature and lipid composition. Both factors dictate steric and/or electrostatic interactions between C-terminal disordered domains and the membrane, as well as the density of lipid packing defects that are necessary for α-helical insertion. By employing novel strategies for investigating dynamic binding, this work unveils a previously unexplored and poorly understood category of biologically relevant IDP-membrane interactions.