Subtype Specificity of Scorpion β-Toxin Tz1 Interaction with Voltage-Gated Sodium Channels Is Determined by the Pore Loop of Domain 3

Abstract

Voltage-gated sodium (Na v ) channels are modulated by a variety of specific neurotoxins. Scorpion ␤-toxins affect the voltage-dependence of channel gating: In their presence, Na v channels activate at subthreshold membrane voltages. Previous mutagenesis studies have revealed that the ␤-toxin Css4

Mapping the Interaction Site for a β-Scorpion Toxin in the Pore Module of Domain III of Voltage-gated Na+ Channels

“…This conclusion is in agreement with our previous finding that transferring the entire IIISS2-S6 region of a ␤-scorpion toxin-insensitive Na v channel (Na v 1.5) to a ␤-scorpion toxin-sensitive Na v channel (Na v 1.2) markedly decreased the binding affinity of ␤-scorpion toxins in Na v channels (8). Similarly, our results agree with previous evidence that this segment of Na V channels determines the specificity for ␤-scorpion toxin interaction with different channel subtypes (12,23).…”

“…Therefore, the conformational change of the voltage-sensing module of one subunit in response to depolarization can be transmitted to the pore module of the neighboring subunit in clockwise direction to open the central ion conduction pore. Analysis of channel chimeras showed that the IIISS2-S6 region is important in binding of ␤-scorpion toxins (8) and in determining the specificity for binding of a ␤-scorpion toxin to individual Na v channel subtypes (12). These results led us to hypothesize that the single receptor site for the ␤-scorpion toxin CssIV in Na v channels (8) is formed by IIS1-S2, IIS3-S4, and IIISS2-S6 extracellular loops.…”

Mapping the receptor site for α-scorpion toxins on a Na + channel voltage sensor

“…This organization would place the IS5-S6 segment adjacent to the IVS1-S2 and IVS3-S4 segments in Na V channels. β-Scorpion toxins, which are similar to α-scorpion toxins in structure but enhance activation by binding to the S3-S4 loop on domain II, have been found to interact with amino acid residues in the SS2-S6 loop in the adjacent pore-forming module in domain III, as well as with amino acid residues in IIS3-S4 (34). Consistent with these previous studies, we found one amino acid residue (Thr393) in the ISS2-S6 loop that contributes significantly to α-scorpion toxin binding, although its contribution was substantially less than IVS1-S2 and IVS3-S4.…”

Substitutions in the Domain III Voltage-sensing Module Enhance the Sensitivity of an Insect Sodium Channel to a Scorpion β-Toxin

“…revealed that IS5-SS1, IIS1-S2, IIS3-S4, and IIISS2-S6, particularly a G845N substitution in IIS3-S4, play a role in determining toxin preference, suggesting that scorpion ␤-toxins bind to the S3-S4 loop (16). In a similar fashion, differences in potency of the toxin Tz1 on rat muscle, cardiac, and neuronal Na v channels motivated Heinemann and co-workers (18) to swap rNa v 1.2a sequences in the background of rNa v 1.4. Their analysis revealed that three amino acid residues in the C-terminal pore loop (SS2-S6) of domain III determine the Tz1 preference for the skeletal muscle Na v channel (18).…”

“…Rat brain (rNa v 1.2) 2 and skeletal muscle (rNa v 1.4) channels are often more sensitive than the cardiac channel (rNa v 1.5) to ␤-toxins of South American scorpions such as Css4 from Centruroides suffusus suffusus, Ts1 from Tityus serrulatus, and TdVIII from Tityus discrepans (16 -20). Moreover, Tz1 from Tityus zulianus is especially active at rNa v 1.4 compared with rNa v 1.2 and human Na v 1.5 (18). revealed that IS5-SS1, IIS1-S2, IIS3-S4, and IIISS2-S6, particularly a G845N substitution in IIS3-S4, play a role in determining toxin preference, suggesting that scorpion ␤-toxins bind to the S3-S4 loop (16).…”