A Site-Specific Organometallic Approach for Installing Tyrosine Phosphorylation Mimics to Decipher the Role of Phosphorylation in Alpha-Synuclein (aSyn) Aggregation and Seeding

Several studies have identified phosphorylation of alpha-synuclein (aSyn) at multiple tyrosine residues (Y39, Y125, Y133, and Y136) within Lewy bodies, which are the pathological hallmarks of Parkinsons disease (PD). However, understanding the specific role of phosphorylation at each site, or how multiple phosphorylation sites interact, has been challenging. Herein, we present an efficient method that leverages an organometallic Pd-complex to site-specifically attach phosphomimetic groups via Cys-arylation, closely mimicking natural phosphorylation. Using this approach, we successfully incorporated native-like tyrosine phosphorylation mimics into cysteine-containing proteins like synthetic transcription factor Max and recombinant aSyn, in good yields. To demonstrate the methods versatility, we created a focused library of mono-, di-, and tri-phosphorylated aSyn analogues. This development enabled, for the first time, the investigation of the effect of site-specific phosphorylation at multiple C-terminal tyrosine residues on aSyn fibrillization and aggregation. Our results show that mono-phosphorylation at any of the C-terminal sites has little effect on aggregation, while di- and tri-phosphorylation slows down the process, extending the lag phase compared to wild-type aSyn. Additionally, phosphorylation at Y39 reduces the seeding activity of aSyn fibrils by almost threefold in both mammalian and neuronal models of synuclein pathology. Overall, our strategy provides a rapid, selective, and scalable way to introduce aromatic PTMs, producing homogeneous protein libraries essential for mechanistic studies, biomarker development, and exploring the therapeutic potential of targeting aSyn-phosphorylation for PD and related disorders.