In dopaminergic neurons, a-synuclein (aS) partitions between a disordered cytosolic state and a lipid-bound state. Binding of aS to membrane phospholipids is implicated in its functional role of synaptic regulation, but also impacts fibril formation associated with Parkinson's disease. Previously we found by high-resolution NMR spectroscopy and circular dichroism (CD) measurements, that the natural post-translational mammalial modification where the N-terminus of the protein is acetylated, impacts the protein's structure and dynamics in free solution and also affects the protein's membrane binding properties. While no tetrameric form of acetylated aS could be isolated, N-terminal acetylation resulted in chemical shift perturbations of the first 12 residues of the protein which progressively decreased with distance from the N-terminus. The interaction between aS and lipid membranes is key to its role in synaptic vesicle homeostasis, and plays a role in initiating fibril formation which is implicated in Parkinson's disease. The natural state of aS inside the cell is generally believed to be intrinsically disordered, but chemical cross-linking experiments provided evidence for a tetrameric arrangement, which was reported to be rich in alpha-helical secondary structure based on circular dichroism (CD). Cross-linking relies on chemical modification of the proteins Lys Ce amino groups, commonly by glutaraldehyde, or by disuccinimidyl glutarate (DSG), with the latter agent preferred for cellular assays. We used ultra-high resolution homonuclear decoupled NMR experiments to probe the reactivity of the 15 aS Lys residues towards N-succinimidyl-acetate, effectively half the DSG cross linker, which results in acetylation of Lys. The intensities of both sidechain and backbone amide signals of acetylated Lys residues provide direct information on the reactivity, showing a difference by a factor 2.5 between the most reactive (K6) and the least reactive residue (K102). The presence of phospholipid vesicles decreases reactivity of most Lys residues by up to an order of magnitude at high lipid:protein stoichiometries (500:1), but only weakly at low ratios. The decrease of Lys reactivity is found to be impacted by lipid composition, even for vesicles that yield similar aS CD signatures. Our data provide new insight on the aS-bilayer interaction, including the pivotal state where the available lipid surface is limited. Protection of Lys Ce amino groups by aS-bilayer interaction will strongly impact quantitative interpretation of DSG cross-linking experiments.
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