Sodium channel beta4, a new disulfide-linked auxiliary subunit with similarity to beta2

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 beta4. Like the beta1-beta3 subunits, beta4 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 beta4 is widely distributed in neurons in the brain, spinal cord, and some sensory neurons.beta4 is most similar to the beta2 subunit (35% identity), and, like the beta2 subunit, the Ig-like fold of beta4 contains an unpaired cysteine that may interact with the alpha subunit. Under nonreducing conditions, beta4 has a molecular mass exceeding 250 kDa because of its covalent linkage to Nav1.2a, 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 beta4 with brain Nav1.2a and skeletal muscle Nav1.4 alpha subunits in tsA-201 cells resulted in a negative shift in the voltage dependence of channel activation, which overrode the opposite effects of beta1 and beta3 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.

Formation of oligomers containing the beta3 and beta4 subunits of the rat nicotinic receptor

The role of the beta3 and beta4 subunits of the nicotinic acetylcholine receptor in brain is still unclear. We investigated nicotinic receptor structure with antibodies directed against unique regions of the beta3 and beta4 subunits of the rat nicotinic acetylcholine receptor. Anti-beta4 detected a single band of 66 kDa in most regions of the brain that was strongest in striatum and cerebellum. The 60 kDa beta3 subunit was detected primarily in striatum and cerebellum, and faintly in hippocampus. Immunoprecipitation experiments established that the two subunits were coassembled in the cerebellum along with the beta2 subunit. Antibodies against the alpha4, beta2, beta3, and beta4 subunits immunoprecipitated approximately 75% of the bungarotoxin-insensitive nicotinic receptor from cerebellar extracts as determined by nicotine-dependent acetylcholine binding. Transfection of COS cells with cDNAs for these four subunits induced expression of a high affinity nicotinic receptor. Omission of only a single subunit from the transfection affected either the Bmax or the apparent KD of the receptor. Our data suggest that the beta3 subunit functions as a structural entity that links a relatively unstable alpha4beta2 heterodimer to a more stable alpha4beta4 heterodimer. The agonist-binding site formed by alpha4beta2 has a much greater affinity than does that formed by alpha4beta4. In this respect, nicotinic receptors that contain the beta3 subunit are structurally homologous to the muscle nicotinic receptor.