α-Synuclein Induces Both Positive Mean Curvature and Negative Gaussian Curvature in Membranes

Abstract

Using a combination of X-ray scattering, fluorescence correlation spectroscopy, coarse-grained molecular dynamics (MD) simulations and potential of mean force calculations, we have explored the membrane remodeling effects of monomeric α-synuclein (αS). Our initial findings from multiple approaches are that αS (1) causes a significant thinning of the bilayer and (2) stabilizes positive mean curvature, such that the maximum principle curvature matches that of synaptic vesicles, αS-induced tubules, and the synthetic lipid vesicles to which the protein binds most tightly. This suggests that αS binding to synaptic vesicles likely stabilizes their intrinsic curvature. We then show that αS induces local negative Gaussian curvature, an effect that occurs in regions of αS shown previously via NMR and corroborated by MD simulation to have significant conformational flexibility. The induction of negative Gaussian curvature, which has implications for all curvature-sensing and curvature-generating amphipathic α-helices, supports a hypothesis that connects helix insertion to fusion and fission of vesicles, processes that have recently been linked to αS function. Then, in an effort to explain these biophysical properties of αS, we promote an intrinsic curvature-field model that recasts long-range protein–protein interactions in terms of the interactions between the local curvature fields generated by lipid–protein complexes.

α-Synuclein Reduces Tension and Increases Undulations in Simulations of Small Unilamellar Vesicles

“…As expected, the headgroup density is higher in the inner leaflet due to compaction in the convex vesicle. The protein has very little effect on bilayer structure, partitioning to a similar depth as previous studies (6,9,10,17), with no notable change in bilayer thickness (Fig. S1 in the Supporting Material).…”

“…The protein is simulated as a single, extended a-helix whose secondary structure is fixed. While conformational heterogeneity of the protein has recently been noted (15), we have shown that it has little effect on protein partition depth or membrane curvature (10). We have simulated DPPC vesicles in the fluid-phase, rather than the more complex quaternary mixture, for several reasons (discussed at length in the Supporting Material).…”

α-Synuclein Senses Lipid Packing Defects and Induces Lateral Expansion of Lipids Leading to Membrane Remodeling

“…This latter effect can also be related to the lateral expansion of the lipid molecules that induces membrane thinning and, with further insertion of protein into the membrane, causes lipid expansion out of the membrane plane of the bilayer or, as previously reported, formation of positive curvature to relieve excess surface pressure (56,57). A close relationship between amphipathic helix insertion and membrane deformation has been established (57), and ␣-Syn has been reported to induce positive curvature of the membrane and the formation of tubular structures (26,58,59). Recently, these remodeling processes caused by ␣-Syn binding have been proposed to induce the formation of lipoprotein nanoparticles (60).…”

N-terminal Acetylation Stabilizes N-terminal Helicity in Lipid- and Micelle-bound α-Synuclein and Increases Its Affinity for Physiological Membranes

“…Nevertheless, it has been shown that curvature selectivity by aSyn cannot be comprehensively characterized through measurements of affinity alone (54), and further studies will be required to evaluate more directly the curvature selectivity of Ac-aSyn. In addition, it is now generally accepted that aSyn can influence membrane properties, specifically curvature, in addition to sensing them (57,58,60,62,63,66,75,83), and the influence of N-terminal acetylation on this activity of the protein remains to be explored.…”