Eukaryotic voltage-gated sodium (Nav) channels contribute to the rising phase of action potentials and served as an early muse for biophysicists laying the foundation for our current understanding of electrical signaling. Given their central role in electrical excitability, it is not surprising that (a) inherited mutations in genes encoding for Nav channels and their accessory subunits have been linked to excitability disorders in brain, muscle, and heart; and (b) Nav channels are targeted by various drugs and naturally occurring toxins. Although the overall architecture and behavior of these channels are likely to be similar to the more well-studied voltage-gated potassium channels, eukaryotic Nav channels lack structural and functional symmetry, a notable difference that has implications for gating and selectivity. Activation of voltage-sensing modules of the first three domains in Nav channels is sufficient to open the channel pore, whereas movement of the domain IV voltage sensor is correlated with inactivation. Also, structure–function studies of eukaryotic Nav channels show that a set of amino acids in the selectivity filter, referred to as DEKA locus, is essential for Na+ selectivity. Structures of prokaryotic Nav channels have also shed new light on mechanisms of drug block. These structures exhibit lateral fenestrations that are large enough to allow drugs or lipophilic molecules to gain access into the inner vestibule, suggesting that this might be the passage for drug entry into a closed channel. In this Review, we will synthesize our current understanding of Nav channel gating mechanisms, ion selectivity and permeation, and modulation by therapeutics and toxins in light of the new structures of the prokaryotic Nav channels that, for the time being, serve as structural models of their eukaryotic counterparts.

Voltage-gated sodium (Na v ) channels are embedded in a multicomponent membrane signaling complex that plays a crucial role in cellular excitability. Although the mechanism remains unclear, β-subunits modify Na v channel function and cause debilitating disorders when mutated. While investigating whether β-subunits also influence ligand interactions, we found that β4 dramatically alters toxin binding to Na v 1.2. To explore these observations further, we solved the crystal structure of the extracellular β4 domain and identified 58 Cys as an exposed residue that, when mutated, eliminates the influence of β4 on toxin pharmacology. Moreover, our results suggest the presence of a docking site that is maintained by a cysteine bridge buried within the hydrophobic core of β4. Disrupting this bridge by introducing a β1 mutation implicated in epilepsy repositions the 58 Cys-containing loop and disrupts β4 modulation of Na v 1.2. Overall, the principles emerging from this work (i) help explain tissuedependent variations in Na v channel pharmacology; (ii) enable the mechanistic interpretation of β-subunit-related disorders; and (iii) provide insights in designing molecules capable of correcting aberrant β-subunit behavior.voltage-gated sodium channel | beta4 subunit | ProTx-II | X-ray structure | disease mutations

Hainantoxin-I is a novel peptide toxin, puri¢ed from the venom of the Chinese bird spider Selenocosmia hainana ( = Ornithoctonus hainana). It includes 33 amino acid residues with a disul¢de linkage of I^IV, II^V and III^VI, assigned by partial reduction and sequence analysis. Under two-electrode voltage-clamp conditions, hainantoxin-I can block rNa v 1.2/L L 1 and the insect sodium channel para/tipE expressed in Xenopus laevis oocytes with IC 50 values of 68 þ 6 W WM and 4.3 þ 0.3 W WM respectively. The three-dimensional solution structure of hainantoxin-I belongs to the inhibitor cystine knot structural family determined by two-dimensional 1 H nuclear magnetic resonance techniques. Structural comparison of hainantoxin-I with those of other toxins suggests that the combination of the charged residues and a vicinal hydrophobic patch should be responsible for ligand binding. This is the ¢rst report of an insect sodium channel blocker from spider venom and it provides useful information for the structure^function relationship studies of insect sodium channels. ß