Immunoproximity biotinylation reveals the axon initial segment proteome

Abstract

Axon initial segments (AISs) are specialized neuronal compartments crucial for action potential generation and neuronal polarity. However, a detailed understanding of the mechanisms regulating AIS structure and function has been hindered by an incomplete knowledge of its molecular composition. Here, we profile the AIS proteome and its dynamic changes in-depth during neuronal maturation. Our method capitalizes on antibody targeted proximity labeling with subcellular spatial specificity in fixed primary neurons. Among the AIS proteins labeled, we identified novel AIS components, including PHGDH and SCRIB. PHGDH and SCRIB are highly enriched in the AIS both in vitro and in vivo, and exhibit a periodic architecture like the axonal spectrin-based cytoskeleton. Furthermore, we show that neuronal PHGDH activity is necessary for AIS integrity and action potential initiation. This powerful and flexible approach defines the AIS proteome and provides a rich resource to elucidate the mechanisms regulating AIS structure and function.