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

The Leiurus quinquestriatus quinquestriatus receptor site of the voltage-dependent sodium channel has been characterized using several fluorescent scorpion toxins. The derivatives show fluorescence enhancements upon binding to the receptor site on the channel together with blue shifts. The fluorescence properties of the bound probes indicate a conformationally flexible, hydrophobic site. Binding of tetrodotoxin has no effect on the fluorescence spectra of the bound derivatives, whereas binding of the allosteric activator batrachotoxin enhances the fluorescence about 2-fold and causes a red shift in the emission spectra, suggesting a batrachotoxin-induced conformational change in the scorpion toxin receptor. The distance between the tetrodotoxin receptor and the Leiurus scorpion toxin receptor on the channel was measured by fluorescence resonance energy transfer. Five different chromophoric scorpion toxin derivatives were used as energy transfer acceptors or donors with anthraniloyltetrodotoxin or N-methylanthraniloylglycine-tetrodotoxin as the energy donor or acceptor. Because of the presence of three tetrodotoxin receptors for each Leiurus receptor, the positions of the donors and acceptors were exchanged. Efficiencies of transfer were measured by both donor quenching and sensitized emission. The average distance of separation between these sites is 35 A. Upon batrachotoxin addition, this distance changes to 42 A indicating a conformational change in one subunit of the channel or a change in the interaction between two subunits coupled to the batrachotoxin-binding site. On the basis of these studies, we present a model suggesting that tetrodotoxin binds to a subunit/site which is extracellularly placed and is 35 A from the Leiurus subunit/site which is located in a protein cleft of the channel which extends partly into the membrane, and undergoes a neurotoxin and voltage-dependent conformational change.