Serine-129 phosphorylation of α-synuclein is a trigger for physiologic protein-protein interactions and synaptic function

Phosphorylation of α-synuclein at the Serine-129 site (α-syn Ser129P) is an established pathologic hallmark of synucleinopathies, and also a therapeutic target. In physiologic states, only a small fraction of total α-syn is phosphorylated at this site, and consequently, almost all studies to date have focused on putative pathologic roles of this post-translational modification. We noticed that unlike native (total) α-syn that is widely expressed throughout the brain, the overall pattern of α-syn Ser129P is restricted, suggesting intrinsic regulation and putative physiologic roles. Surprisingly, preventing phosphorylation at the Ser-129 site blocked the ability of α-syn to attenuate activity-dependent synaptic vesicle (SV) recycling – widely thought to reflect its normal function. Exploring mechanisms, we found that neuronal activity augments α-syn Ser-129P, and this phosphorylation is required for α-syn binding to VAMP2 and synapsin – two functional binding-partners that are necessary for α-syn function. AlphaFold2-driven modeling suggests a scenario where Ser129P induces conformational changes in the C-terminus that stabilizes this region and facilitates protein-protein interactions. Our experiments indicate that the pathology-associated Ser129P is an unexpected physiologic trigger of α-syn function, which has broad implications for pathophysiology and drug-development.