Mapping the molecular structure of the voltage-dependent sodium channel. Distances between the tetrodotoxin and Leiurus quinquestriatus quinquestriatus scorpion toxin receptors

Three polymorphic genes encoding a depressant toxin from the Egyptian scorpion Leiurus quinquestriatus quinquestriatus

Four clones encoding the insect depressant toxin LqqIT2 have been isolated from the Egyptian scorpion Leiurus quinquestriatus quinquestriatus using RT-PCR. The four clones have been sequenced and their deduced amino acid sequences have been compared with the original amino acid sequence determined from the purified LqqIT2 protein and polymorphisms have been shown. This study succeeded in isolating more than one copy of the LqqIT2 gene, although only one amino acid sequence has been identified from the purified LqqIT2 toxin.

Purification of protein toxins from Leiurus quinquestriatus hebraeus that modify Na channels

Two protein toxins (Lqh1 and Lqh2) were purified from crude venom obtained from Middle Eastern scorpions, Leiurus quinquestriatus hebraeus, by using cationic exchange chromatography. Lqh1 and Lqh2 were compared to toxin V (Lqq5) obtained from the venom of the North African scorpion Leiurus quinquestriatus quinquestriatus. Lqh1 and Lqh2 were purified to homogeneity; they had mol. wts of 6390 and 5870, respectively; thus both toxins differ in size from Lqq5 (7462). Electrophysiological experiments also suggested that all three toxins are different. In a dose-dependent manner, Lqh1, Lqh2 and Lqq5 lengthened and attenuated propagated compound action potentials (AP) recorded from frog sciatic nerves using the single sucrose-gap technique. Toxins Lqh1 and Lqh2 were found to be more effective than Lqq5 in both lengthening and blocking APs. Voltage-clamp experiments using the vaseline-gap technique on frog skeletal muscle fibres showed that Lqh1 and Lqh2 attenuated the Na current amplitude and slowed inactivation, while Lqq5 primarily lengthened the Na current duration. Increases in the holding potential increase the current attenuation caused by all three toxins. Evidence from sucrose-gap and voltage-clamp experiments suggests that all three toxins bind to Na channels and block them, besides their well-known ability to slow inactivation kinetics. The increased effectiveness of Lqh1 appears to be produced by a slowed rate of exit of the toxin from its binding site.

Proton nuclear magnetic resonance studies on the variant-3 neurotoxin from Centruroides sculpturatus Ewing: sequential assignment of resonances

We report the sequential assignment of resonances to specific residues in the proton nuclear magnetic resonance spectrum of the variant-3 neurotoxin from the scorpion Centruroides sculpturatus Ewing (range southwestern U.S.A.). A combination of two-dimensional NMR experiments such as 2D-COSY, 2D-NOESY, and single- and double-RELAY coherence transfer spectroscopy has been employed on samples of the protein dissolved in D2O and in H2O for assignment purposes. These studies provide a basis for the determination of the solution-phase conformation of this protein and for undertaking detailed structure-function studies of these neurotoxins that modulate the flow of sodium current by binding to the sodium channels of excitable membranes.

Photochemically induced dynamic nuclear polarisation NMR study of the aromatic residues of sea-anemone polypeptide cardiac stimulants

One-dimensional and two-dimensional photochemically induced dynamic nuclear polarisation (photo-CIDNP) nuclear magnetic resonance spectra have been recorded for the sea-anemone polypeptide cardiac stimulants anthopleurin-A and Anemonia sulcata toxins I and II. In anthopleurin-A and toxin II, all three Trp residues are accessible to the flavin dye, although Trp-23 in anthopleurin-A shows a weaker photo-CIDNP response than Trp-33 and Trp-45. Tyr-25 in anthopleurin-A also shows a strong response. In toxin I, Trp-23, Trp-33 and Tyr-45 (which replaces Trp in this molecule) are accessible to the dye. The pH dependences of the photo-CIDNP spectra of all three polypeptides have been examined. The response of Trp-33 increases significantly with pH. The two His residues of anthopleurin-A and toxin II display a response in their imidazole forms, but not their imidazolium forms. The surface accessibilities of Trp-23 and Trp-33 are discussed in relation to the interaction of these polypeptides with the Na+ channel.

Activation of the voltage-sensitive sodium channel by a beta-scorpion toxin in rat brain nerve-ending particles

Neurotoxins purified from scorpion venoms previously had been divided into two classes according to their binding properties in rat brain synaptosomes. However, the pharmacological action of beta-scorpion toxin (beta-ScTx) on this preparation has not yet been described. In this report we show that a beta-ScTx induced an increase in 22Na+ uptake through synaptosomal voltage-sensitive sodium channels since this stimulation was abolished by tetrodotoxin (TTX). The increase was smaller than with veratridine and no synergy was observed between beta-ScTx and veratridine, as is the case for alpha-scorpion toxin (alpha-ScTx) and veratridine. The effects of alpha- and beta-ScTx were additive and the concentration-effect curves for each type of toxin were not modified by the other, suggesting that these two types of toxins act through distinct and noninteracting receptor sites. This was confirmed by the absence of mutual modification of the equilibrium and kinetic binding properties. beta-ScTx was shown to inhibit the uptake and to stimulate the release of [3H]gamma-aminobutyric acid. These effects were blocked by TTX, and no synergy was observed with veratridine. It was concluded that all these effects are mediated by the activation of voltage-sensitive sodium channels induced by the binding of beta-ScTx to a receptor site (site 4) distinct from those for other neurotoxins acting on sodium channels.

Changes in the ratio of two separate toxin binding sites on the sodium channel protein during rat brain development

The development of two separate toxin binding sites from sodium channels of rat brain was studied. Whereas [3H]saxitoxin binding developed gradually, reaching adult levels by 4.5 weeks after birth, the binding of [125I]scorpion toxin increased rapidly for the first 10 days after birth, then declined. The stoichiometric ratio of [3H]saxitoxin binding site to [125I]scorpion toxin binding sites changed during development from 0.5 in 8 day old rats to 1.9 in adult rats, implying separate biological regulation for each of these sites.