Tyr39 Phosphorylation of α-Synuclein Accelerates Heterotypic Aggregation and Drives Toxic Amplification

c-Abl-mediated Tyr39 phosphorylation of α-synuclein correlates strongly with Parkinson's disease (PD) progression, yet the mechanistic basis of Tyr39-phosphorylated α-synuclein (pY39-α-syn) in synucleinopathies remains elusive. Here, we show that a minor fraction of pY39-α-syn markedly accelerates the aggregation of wild-type α-synuclein (WT-α-syn), overriding the inhibitory effect of Hsc70. Kinetics, continuous-wave electron paramagnetic resonance (EPR), cryo-transmission electron microscopy (cryo-TEM), and [¹⁵N,¹H]-NMR spectral analyses reveal that pY39-α-syn participates in primary nucleation, generating heterotypic nuclei that promote secondary nucleation predominantly through a fragmentation-based pathway. The resulting heterotypic aggregates are structurally unstable, forming short, preformed fibrillar-like assemblies that facilitate toxic amplification of α-synuclein species. Moreover, more toxic type-B oligomers arise from intermediate aggregates derived from these heterotypic assemblies. Notably, we further show that the heterotypic aggregates possess Fenton-like catalytic activity by binding and stabilizing Fe²⁺, providing new mechanistic insights into Fe²⁺-dependent oxidative toxicity during PD progression. Collectively, this study provides a systematic elucidation of how Tyr39 phosphorylation reprograms α-synuclein aggregation toward toxic amplification, offering new insight into PD-related α-synucleinopathies.