Sodium channel β1 and β3 subunits associate with neurofascin through their extracellular immunoglobulin-like domain

Sequence homology predicts that the extracellular domain of the sodium channel β1 subunit forms an immunoglobulin (Ig) fold and functions as a cell adhesion molecule. We show here that β1 subunits associate with neurofascin, a neuronal cell adhesion molecule that plays a key role in the assembly of nodes of Ranvier. The first Ig-like domain and second fibronectin type III–like domain of neurofascin mediate the interaction with the extracellular Ig-like domain of β1, confirming the proposed function of this domain as a cell adhesion molecule. β1 subunits localize to nodes of Ranvier with neurofascin in sciatic nerve axons, and β1 and neurofascin are associated as early as postnatal day 5, during the period that nodes of Ranvier are forming. This association of β1 subunit extracellular domains with neurofascin in developing axons may facilitate recruitment and concentration of sodium channel complexes at nodes of Ranvier.