RNA interactions drive structural and functional diversification of alpha-synuclein fibrils

Nucleic acids, particularly RNA, exert multiple roles on protein aggregation, yet their mechanistic involvement remains poorly understood. Building on prior work demonstrating RNA-driven acceleration of protein aggregation, we now demonstrate that RNA binding markedly reshapes the structural and functional properties of alpha-synuclein (aS) fibrils, a protein implicated in Parkinsons disease (PD). Although monomeric aS exhibits negligible RNA-binding properties, aggregation markedly increases its RNA-binding capacity. Our kinetic analyses reveal that RNA incorporation during initial stages of aggregation alters fibril elongation dynamics and modulates surface-mediated secondary nucleation in a seed concentration-dependent manner. Spectroscopic and microscopic characterization (FTIR, micro-FTIR) demonstrate that RNA promotes the formation of structurally distinct, more compact aS fibrils with heterogeneous morphology. Exploiting the recently discovered catalytic hydrolase activity of aS fibrils, we show that direct RNA binding inhibits enzymatic function, consistent with RNA-induced conformational remodeling that limits the exposure of critical residues. Additionally, RNA binding to fibril surfaces impairs proteolytic degradation by proteinase K, suggesting RNA stabilizes fibrils against clearance mechanisms. Moreover, RNA sequencing of fibril-bound nucleic acids uncovers a preference towards uridine-rich motifs that computational docking locates within hydrophilic cavities of disease-associated fibril structures. Collectively, we identify RNA as an active co-factor that remodels structure, propagation kinetics, stability, and function of aS fibrils. We offer a mechanistic link between RNA interaction and amyloid polymorphism relevant to synucleinopathies and, in broader contexts, to amyloid-associated diseases. Significance statementOur work represents the first systematic evidence that RNA directly interacts with protein fibrils and modulates key functional properties such as aggregation kinetics, proteolytic resistance and catalytic activity. We demonstrate that RNA co-aggregates with alpha-synuclein inducing structural conformations that depend on RNA for efficient assembly. Given the resemblance of these RNA-associated fibrils to those found in multiple system atrophy, our findings suggest that the final fibril structures in the patient brain are influenced by multiple cofactors and may differ substantially from recombinant fibrils formed in vitro, even when seeded with patient-derived material.