Fast inactivation of voltage-gated sodium (Nav) channels is essential for electrical signaling but its mechanism remains poorly understood. Here, we determined the structures of a eukaryotic Nav channel alone and in complex with a lethal α-scorpion toxin, AaH2, by electron microscopy, both at 3.5-A resolution. AaH2 wedges into voltage-sensor domain IV (VSD4) to impede fast activation by trapping a deactivated state in which gating charge interactions bridge to the acidic intracellular C-terminal domain. In the absence of AaH2, the S4 helix of VSD4 undergoes a ~13-Å translation to unlatch the intracellular fast inactivation gating machinery. Highlighting the polypharmacology of α-scorpion toxins, AaH2 also targets an unanticipated receptor site on VSD1 and a pore-glycan adjacent to VSD4. Overall, this work provides key insights into fast inactivation, electromechanical coupling, and pathogenic mutations in Nav channels.

Voltage-gated Na⁺ channels are integral membrane proteins that function as a gateway for a selective permeation of sodium ions across biological membranes. In this way, they are crucial players for the generation of action potentials in excitable cells. Voltage-gated Na⁺ channels are encoded by at least nine genes in mammals. The different isoforms have remarkably similar functional properties, but small changes in function and pharmacology are biologically well-defined, as underscored by mutations that cause several diseases and by modulation of a myriad of compounds, respectively. This review will stress on the modulation of voltage-gated Na⁺ channels by scorpion α-toxins. Nature has designed these two classes of molecules as if they were predestined to each other: an inevitable ‘encounter’ between a voltage-gated Na⁺ channel isoform and an α-toxin from scorpion venom indeed results in a dramatically changed Na⁺ current phenotype with clear-cut consequences on electrical excitability and sometimes life or death. This fascinating aspect justifies an overview on scorpion venoms, their alpha-toxins and the Na⁺ channel targets they are built for, as well as on the molecular determinants that govern the selectivity and affinity of this ‘inseparable duo’.

The venom of the scorpion Androctonus mauretanicus mauretanicus was screened by use of a specific serum directed against AaH II, the scorpion alpha-toxin of reference, with the aim of identifying new analogues. This led to the isolation of Amm VIII (7382.57 Da), which gave a highly positive response in ELISA, but was totally devoid of toxicity when injected subcutaneously into mice. In voltage-clamp experiments with rat brain type II Na+ channel rNa(v)1.2 or rat skeletal muscle Na+ channel rNa(v)1.4, expressed in Xenopus oocytes, the EC50 values of the toxin-induced slowing of inactivation were: 29+/-5 and 416+/-14 nM respectively for AmmVIII and 2.6+/-0.3 nM and 2.2+/-0.2 nM, respectively, for AaH II interactions. Accordingly, Amm VIII clearly discriminates neuronal versus muscular Na+ channel. The Amm VIII cDNA was amplified from a venom gland cDNA library and its oligonucleotide sequence determined. It shows 87% sequence homology with AaH II, but carries an unusual extension at its C-terminal end, consisting of an additional Asp due to a point mutation in the cDNA penultimate codon. We hypothesized that this extra amino acid residue could induce steric hindrance and dramatically reduce recognition of the target by Amm VIII. We constructed a model of Amm VIII based on the X-ray structure of AaH II to clarify this point. Molecular modelling showed that this C-terminal extension does not lead to an overall conformational change in Amm VIII, but drastically modifies the charge repartition and, consequently, the electrostatic dipole moment of the molecule. At last, liquid-phase radioimmunassays with poly- and monoclonal anti-(AaH II) antibodies showed the loss of conformational epitopes between AaH II and Amm VIII.

1. The effects of toxin II from scorpion Androctonus australis Hector (AaH II) on the Na current of frog myelinated nerve fibres were analysed under voltage-clamp conditions. 2. Like other alpha-scorpion toxins and Anemonia toxin II, AaH II both increased the inactivation time constants of peak Na current and induced a non-inactivatable Na current (maintained current). 3. In the presence of AaH II, the slope of the maintained conductance-voltage curve was less steep than that corresponding to the peak conductance and the maintained current reversed at a voltage about 20 mV more negative than the peak current. 4. When the peak current was inactivated by pre-depolarizations, ‘on’ and ‘off’ relaxation kinetics of the maintained current were an exponential function whose time constant changed with voltage in a bell-shaped manner. At 0 mV, the time constant was about 10 ms. 5. The effects of AaH II could be decomposed into fast effects (increase in inactivation time constants of the peak current) which developed within about 5 s and slow effects (increase in maintained current and changes in initial amplitudes of fast and slow phases of peak current inactivation) which developed within about 30 s. 6. These two types of AaH II effects could be completely removed by conditioning depolarizations giving rise to outward currents. 7. A model is proposed in which the binding of the toxin with its receptor is modulated by membrane potential and internal cations, the appearance of the maintained current is modulated by the environment of channels and the change in inactivation time constants is modulated by membrane potential. The maintained current would correspond to the transformation of a fraction of channels into a non-inactivable (late) form.

The binding of a 3H-labeled ethylenediamine derivative of tetrodotoxin ([3H]EN-TTX) and 125I-labeled polypeptide neurotoxins, purified from the sea anemone Anenomia sulcata (ATXII) and the scorpion Androctonus australis Hector (AaHII), was studied using axolemma-enriched membrane fractions. The membrane fractions were derived from a purified preparation of myelinated axons fractionated via a linear sucrose gradient in a zonal rotor. The specific activity of Na+K+ ATPase in the axolemma-enriched preparation, found in the 28-32% sucrose region of the density gradient, was 59.1 mumol of ATP hydrolyzed/mg of protein/h. As estimated by 3H-specific ouabain binding, this fraction contained 183.6 pmol of Na+K+ ATPase/mg of protein. The 28-32% region of the density gradient was most enriched in the binding capacity for all neurotoxins, while the stoichiometry of the binding activities varied throughout the density gradient. The maximal binding (Bmax) of [3H]EN-TTX was 1 pmol/mg; the dissociation constant (KD) of the neurotoxin for its receptor was 2 X 10(-10) M. The comparable values for ATXII were 3.2 pmol/mg and 1.5 X 10(-7) M, respectively, while AaHII had a Bmax of 0.08 pmol/mg and a KD of 3.3 X 10(-9) M. The relationship of the binding of these neurotoxins to that observed in other axonal plasma membrane preparations is discussed.