Understanding the membrane biophysics of alpha-Synuclein and its role in membrane curvature induction and structural remodeling

Abstract

Alpha-Synuclein (aSyn) is a 140 amino acid, intrinsically disordered protein that adopts an extended amphipathic alpha-helical structure upon binding the membrane. aSyn is the major proteinaceous component of insoluble fibrillar Lewy bodies, a hallmark of Parkinson's disease (PD). The precise roles of both native and pathological forms of aSyn remain unclear. However, the interaction of aSyn with cellular membranes is now thought to be critical to its native function, and potentially to its role in PD. In vivo studies with overexpressed aSyn shows a stalling of vesicle fusion at the plasma membrane, whereas in vitro studies of small lipid vesicles and aSyn demonstrate an inhibition of vesicle fusion. In addition, numerous biophysical studies have identified potential curvature sensing and curvature inducing characteristics for aSyn, however the mechanism behind these processes is not well understood. The work in this thesis explores the membrane remodeling capacity of aSyn using a combination of computational (molecular dynamics simulation, MD) and experimental (x-ray scattering) methods to try to understand how aSyn interacts with lipid bilayers and potentially gain insight into the native function of the protein. Using a novel set of analysis algorithms we show that binding of aSyn to lipid bilayers thins the membrane and induces a stabilized intrinsic curvature field--whose magnitude matches the curvature of vesicles that aSyn has the highest binding affinity for. We also show that with equal surface density of protein, aSyn vesiculates giant unilamellar vesicles in a lipid-headgroup charge dependent manner. Using an extensive series of MD simulations we demonstrate that aSyn induced membrane remodeling is driven by the protein's binding affinity, partition depth, and induced inter-leaflet order asymmetry. In order to study the more physiologically relevant vesicle bound state, we have also simulated a series of lipid vesicles (with and without bound aSyn). Analysis of these systems required a new algorithm that employed spherical harmonics analysis to extract both structural and mechanical properties from the vesicles. We observe a reduction in bending rigidity and surface tension due to binding of aSyn. This result supports our hypothesis that aSyn stabilized highly curve vesicles--inhibiting vesicle fusion--through a relief of curvature stress (surface tension) inherent to the highly curved membrane.