Sodium Channel beta 4, a New Disulfide-Linked Auxiliary Subunit with Similarity to beta 2

The principal alpha subunit of voltage-gated sodium channels is associated with auxiliary beta subunits that modify channel function and mediate protein-protein interactions. We have identified a new beta subunit termed beta 4. Like the beta 1- beta 3 subunits, beta 4 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 beta 4 is widely distributed in neurons in the brain, spinal cord, and some sensory neurons. beta 4 is most similar to the beta 2 subunit (35% identity), and, like the beta 2 subunit, the Ig-like fold of beta 4 contains an unpaired cysteine that may interact with the alpha subunit. Under nonreducing conditions, beta 4 has a molecular mass exceeding 250 kDa because of its covalent linkage to Na sub(v)1.2a, whereas on reduction, it migrates with a molecular mass of 38 kDa, similar to the mature glycosylated forms of the other beta subunits. Coexpression of beta 4 with brain Na sub(v)1.2a and skeletal muscle Na sub(v)1.4 alpha subunits in tsA-201 cells resulted in a negative shift in the voltage dependence of channel activation, which overrode the opposite effects of beta 1 and beta 3 subunits when they were present. This novel, disulfide-linked beta subunit is likely to affect both protein-protein interactions and physiological function of multiple sodium channel alpha subunits.