Sodium Channel β4, a New Disulfide-Linked Auxiliary Subunit with Similarity to β2

Abstract

The principal α subunit of voltage-gated sodium channels is associated with auxiliary β subunits that modify channel function and mediate protein-protein interactions. We have identified a new β subunit termed β4. Like the β1-β3 subunits, β4 contains a cleaved signal sequence, an extracellular Ig-like fold, a transmembrane segment, and a short intracellular C-terminal tail. Using TaqMan reverse transcription-PCR analysis,in situ hybridization, and immunocytochemistry, we show that β4 is widely distributed in neurons in the brain, spinal cord, and some sensory neurons.β4 is most similar to theβ2 subunit (35% identity), and, like theβ2 subunit, the Ig-like fold of β4 contains an unpaired cysteine that may interact with the α subunit. Under nonreducing conditions, β4 has a molecular mass exceeding 250 kDa because of its covalent linkage to Na v 1.2a, whereas on reduction, it migrates with a molecular mass of 38 kDa, similar to the mature glycosylated forms of the otherβ subunits. Coexpression ofβ4 with brain Na v 1.2a and skeletal muscle Na v 1.4 α subunits in tsA-201 cells resulted in a negative shift in the voltage dependence of channel activation, which overrode the opposite effects ofβ1 andβ3 subunits when they were present. This novel, disulfide-linked β subunit is likely to affect both protein-protein interactions and physiological function of multiple sodium channel α subunits.