Voltage-gated sodium channels (Nas) are tightly regulated by multiple conserved auxiliary proteins, including the four fibroblast growth factor homologous factors (FGFs), which bind the Na EF-hand like domain (EFL), and calmodulin (CaM), a multifunctional messenger protein that binds the Na IQ motif. The EFL domain and IQ motif are contiguous regions of Na cytosolic C-terminal domains (CTD), placing CaM and FGF in close proximity. However, whether the FGFs and CaM act independently, directly associate, or operate through allosteric interactions to regulate channel function is unknown.