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

Abstract

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

Modulatory Proteins Can Rescue a Trafficking Defective Epileptogenic Nav1.1 Na+Channel Mutant

“…␤ subunits can increase Na ϩ current density and membrane insertion of wild-type ␣ subunits in some expression systems (Isom et al, 1992(Isom et al, , 1995 but not in others (Morgan et al, 2000;Qu et al, 2001;Yu et al, 2003). Thus, in some conditions, ␤ subunits can act as chaperones also for wild-type ␣ subunits, but in our experimental conditions, they did not have any significant effects.…”

“…In addition to highlighting the importance of mutations and polymorphisms in their promoters, this possibility opens a complex scenario, because expression patterns in CNS neurons are highly variable. For instance, ␤ subunits show neuron-specific expression in many brain areas (Morgan et al, 2000;Yu et al, 2003). Little is known about interneuron-specific expression levels of Na ϩ channel interacting proteins, and this is particularly important because Na v 1.1 loss-of-function mutations have a prominent effect on the excitable properties of GABAergic neurons (Yu et al, 2006).…”

Ion channel regulation by β-secretase BACE1 – enzymatic and non-enzymatic effects beyond Alzheimer's disease

“…Accessory subunits (b1-4, SCN1B-SCN4B) can associate with the a-subunit and influence channel expression, trafficking, and gating. [48][49][50] All b-subunits and Na v 1.1, Na v 1.2, Na v 1.3, and Na v 1.6 are expressed in the brain. 51 Mutations in both a-or b-subunits have been associated, among other disorders, with epilepsy.…”

Sodium channel Scn1b null mice exhibit prolonged QT and RR intervals

“…Mutations in SCN5A encoding Na v 1.5, the major cardiac channel, result in Long QT and Brugada syndromes [1]. Sodium channels are heterotrimers, composed of a single, pore-forming α subunit and two β subunits [3]: a noncovalently linked subunit (β1 or β3) [4,5] and a disulfide-linked subunit (β2 or β4) [6,7]. β subunits are multi-functional molecules that regulate channel cell-surface expression levels and modulate channel function, affecting channel kinetics and voltage-dependence in vitro [8].…”