N-Terminal Amine Chemistry Influences α-Synuclein Interactions with Lipid Bilayers.

α-Synuclein (αSyn) is an intrinsically disordered protein whose reversible membrane binding via amphipathic α-helix formation is central to its function and may also contribute to the pathology of Parkinson's disease. In mammals, αSyn is constitutively N-terminally acetylated, a modification that alters its structural propensity and interactions with lipid bilayers. Here, we decouple the structural and chemical contributions of N-terminal acetylation by engineering several αSyn variants that systematically vary N-terminal helix propensity and physicochemical properties: unmodified αSyn (αSynUnmod), αSyn with an N-terminal Asn (αSynAsn), N-terminally acetylated αSyn (αSynAcetyl), and N-terminally succinylated αSyn (αSynSuccinyl). Circular dichroism and NMR measurements show that all three modifications increase α-helical sampling within the first ∼10-12 residues, establishing a hierarchy of N-terminal helicity (αSynSuccinyl > αSynAcetyl > αSynAsn > αSynUnmod). However, fluorescence correlation spectroscopy reveals that enhanced helicity in solution does not strictly correlate with lipid bilayer affinity across several different lipid compositions. Instead, membrane binding is more strongly influenced by N-terminal chemistry, lipid composition, and bilayer curvature. Collectively, these results underscore how modifications tune αSyn's interactions with binding partners, providing mechanistic insight into how this modification modulates αSyn function. For lipid bilayers, N-terminal acetylation tunes both the structural and physicochemical properties of αSyn to enhance binding.