Characterization of specific allosteric effects of the Na+ channel β1 subunit on the Nav1.4 isoform

The mechanism of inactivation of mammalian voltage-gated Na+ channels involves transient interactions between intracellular domains resulting in direct pore occlusion by the IFM motif and concomitant extracellular interactions with the beta 1 subunit. Na-v beta 1 subunits constitute single-pass transmembrane proteins that form protein-protein associations with pore-forming alpha subunits to allosterically modulate the Na+ influx into the cell during the action potential of every excitable cell in vertebrates. Here, we explored the role of the intracellular IFM motif of rNa(v)1.4 (skeletal muscle isoform of the rat Na+ channel) on the alpha-beta 1 functional interaction and showed for the first time that the modulation of beta 1 is independent of the IFM motif. We found that: (1) Na(v)1.4 channels that lack the IFM inactivation particle can undergo a C-type-like inactivation albeit in an ultraslow gating mode; (2) beta 1 can significantly accelerate the inactivation of Na(v)1.4 channels in the absence of the IFM motif. Previously, we identified two residues (T109 and N110) on the beta 1 subunit that disrupt the alpha-beta 1 allosteric modulation. We further characterized the electrophysiological effects of the double alanine substitution of these residues demonstrating that it decelerates inactivation and recovery from inactivation, abolishes the modulation of steady-state inactivation and induces a current rundown upon repetitive stimulation, thus causing a general loss of function. Our results contribute to delineating the process of the mammalian Na+ channel inactivation. These findings may be relevant to the design of pharmacological strategies, targeting beta subunits to treat pathologies associated to Na+ current dysfunction.