Authentic Phosphorylation of α-Synuclein at Ser129 Reveals Functional Differences Not Captured by the S129D Phosphomimetic

Phosphorylation at serine 129 (pS129) is a dominant post-translational modification of α-synuclein (αSyn) and a widely used pathological marker in Parkinson's disease, yet its mechanistic consequences remain debated across physiological and pathological contexts. Most studies rely on phosphomimetic substitutions such as S129D, which approximate net charge but do not reproduce the steric, geometric, or hydrogen-bonding properties of authentic phosphorylation. Here, we establish a robust bacterial coexpression platform that generates homogeneous, site-specifically phosphorylated αSyn using its native kinase, Polo-like kinase 2. Using this system, we show that authentic pS129 differs fundamentally from S129D: it induces local, NMR-detectable perturbations within the C-terminal conformational ensemble, exhibits distinct and context-dependent aggregation behavior, and elicits neuronal responses and modest, reproducible toxicity not reproduced by phosphomimetics. These data resolve inconsistencies in the literature and highlight the importance of chemically authentic post-translational modification. More broadly, this platform provides a generalizable and scalable route to chemically faithful phosphorylated proteins, enabling more accurate interrogation of post-translationally regulated protein function.