Residue-Specific Modulation of Aggregation-Associated Interactions by Spermine in Tau, α‑Synuclein, and Aβ40.

Preventing neurodegenerative diseases associated with intrinsically disordered proteins (IDPs) remains a major challenge due to the lack of a detailed, sequence-level picture of disease-relevant structure formation and the influence of cellular factors that modulate these transitions. Here, we probe spermine (Spm), a +4 charged polyamine abundant in cells, to determine how it reshapes the conformational ensembles and fibril-associated contact propensities of three disease-linked IDPs: the K18 domain of Tau, α-synuclein (αS), and amyloid-β40 (Aβ40). Using long all-atom molecular dynamics simulations across a range of Spm concentrations, we quantify residue-level changes in intrachain contacts relative to native contacts observed in fibrils and corroborate computational predictions with ThT fluorescence assays for Tau constructs. Spm acts in a sequence- and region-specific manner, not simply through the overall net charge. In K18, Spm binds near the fourth microtubule-binding repeat, disrupting intrachain contacts associated with Alzheimer's fibril structures and thereby inhibiting aggregation. In αS, Spm binds mainly to acidic residues in the C-terminal half of the sequence and redistributes intramolecular contacts to enhance aggregation-prone interactions in the central region, providing a residue-level mechanistic basis for previously reported Spm-enhanced αS aggregation. For Aβ40, Spm neutralizes acidic residues near positions 22-24 and shifts intrachain interactions toward its aggregation-prone core, resulting in a net promotion of fibril-like conformations. These divergent effects show that net charge alone cannot predict the polyamine influence on IDPs. Instead, residue-specific binding hotspots and local reweighting of aggregation-linked contacts determine whether Spm promotes or suppresses fibril-like conformations. This combined simulation-experimental framework provides a mechanistic basis for how small molecules reprogram IDP conformational ensembles and suggests principles for designing ligands that exploit similar residue-level modulation.