Rosetta Structural Modeling of Tarantula Toxin Binding to Voltage Sensors

Abstract

Arachnids produce inhibitory cystine knot (ICK) peptide toxins that are potent allosteric modulators of ion channel voltage sensors. This study investigates the structural basis of voltage sensor toxin interaction. Guangxitoxin-1E (GxTX) and Scodra griseipes toxin (SgTX) inhibit Kv2.1 channels by binding to the third segment of the voltage sensor (S3b), but their sequences share only the cystine residues required for their folding motif. Using Rosetta structural modeling we constructed theoretical complexes of Kv2.1 with GxTX and SgTX. These toxins were docked to the α-helical tarantula toxin receptor site on the S3b region of the activated Kv2.1 voltage sensor paddle. These amphiphilic toxins partition into lipid bilayers and modulate channels by interacting with extracellular solution and membrane lipids as well as their receptor site. To better understand the importance of solution and lipid contacts, docking simulations were preformed in both aqueous and membrane-like environments. In aqueous environments, the complexes identified energetically favorable interfaces with the receptor site. Toxin docking with an implicit membrane yielded convergent structures with SgTX, but similar complexes with GxTX did not find energetic minima. The interaction surfaces in these membrane embedded models of SgTX compare favorably with key residues identified by experimental alanine scans. Voltage clamp recordings and fluorescent measurements of toxin binding to Kv2.1 reveal that GxTX has a greatly weakened affinity for activated voltage sensors. Our results are consistent with GxTX having a lower affinity for activated voltage sensors than SgTX. We propose the binding site for GxTX, but not SgTX, is occluded when voltage sensors are activated.