A distinct PP2A subunit regulates local protein phosphorylation at the axon initial segment

Protein phosphorylation plays a crucial role in regulating the cytoskeletal and membrane proteins at the axon initial segment (AIS). However, our knowledge of AIS-specific kinases and phosphatases is very limited. Here, we report the identification of a protein phosphatase 2A (PP2A) B55 regulatory subunit enriched at the AIS in mice: Ppp2r2c. Our results demonstrate that PP2A-B55 subunits exhibit substantial heterogeneity in their subcellular localization and function. Notably, the Ppp2r2c subunit is selectively concentrated at the AIS, and this enrichment is driven by its unique structure. Utilizing a microelectrode array system (MEA), we show that Ppp2r2c modulates neuronal activity during in vitro development. With phosphoproteomics, we further reveal that the potassium channel Kv1.2 is one of the downstream targets that link Ppp2r2c activity to neuronal excitability. Together, these data provide a critical entry point for understanding the mechanisms of PP2A-mediated local phospho-regulation at the AIS.