Phosphorylation of α-synuclein at the serine-129 site (α-syn Ser129P) is an established pathologic hallmark of synucleinopathies and a therapeutic target. In physiologic states, only a fraction of α-syn is phosphorylated at this site, and most studies have focused on the pathologic roles of this post-translational modification. We found that unlike wild-type (WT) α-syn, which is widely expressed throughout the brain, the overall pattern of α-syn Ser129P is restricted, suggesting intrinsic regulation. Surprisingly, preventing Ser129P blocked activity-dependent synaptic attenuation by α-syn—thought to reflect its normal function. Exploring mechanisms, we found that neuronal activity augments Ser129P, which is a trigger for protein-protein interactions that are necessary for mediating α-syn function at the synapse. AlphaFold2-driven modeling and membrane-binding simulations suggest a scenario where Ser129P induces conformational changes that facilitate interactions with binding partners. Our experiments offer a new conceptual platform for investigating the role of Ser129 in synucleinopathies, with implications for drug development.

α-突触核蛋白丝氨酸 129 位点 (α-syn Ser129P) 的磷酸化是突触核蛋白病的既定病理标志和治疗靶点。在生理状态下，只有一小部分 α-syn 在该位点被磷酸化，大多数研究都集中在这种翻译后修饰的病理作用上。我们发现，与在整个大脑中广泛表达的野生型 (WT) α-syn 不同，α-syn Ser129P 的整体模式受到限制，表明存在内在调节。令人惊讶的是，阻止 Ser129P 可以阻断 α-syn 的活动依赖性突触衰减，这被认为反映了其正常功能。通过探索机制，我们发现神经元活动增强了 Ser129P，这是蛋白质-蛋白质相互作用的触发因素，这对于介导突触处的 α-syn 功能是必需的。AlphaFold2 驱动的建模和膜结合模拟表明 Ser129P 诱导构象变化，从而促进与结合伙伴的相互作用。我们的实验为研究 Ser129 在突触核蛋白病中的作用提供了一个新的概念平台，并对药物开发具有影响。