Binding of κ-Conotoxin-PVIIA to Open and Closed Shaker K-Channels Are Differentially Affected by the Ionic Strength

kappa-Conotoxin-PVIIA (kappa-PVIIA) is a potassium-channel blocking peptide from the venom of the fish-hunting snail, Conus purpurascens, which is essential for quick prey's excitotoxic immobilization. Binding of one kappa-PVIIA to Shaker K-channels occludes the K+-conduction pore without additional conformational effects. Because this 27-residue toxin is +4-charged at neutral pH, we asked if electrostatic interactions play a role in binding. With Voltage-Clamp electrophysiology, we tested how ionic strength (IS) affects kappa-PVIIA blockade to Shaker. When IS varied from similar to 0.06 to similar to 0.16 M, the dissociation constant for open and closed channels increased by similar to 5- and similar to 16-fold, respectively. While the association rates decreased equally, by similar to 4-fold, in open and closed channels, the dissociation rates increased 4-5-fold in closed channels but was IS-insensitive in open channels. To explain this differential IS-dependency, we propose that the bound kappa-PVIIA wobbles, so that in open channels the intracellular environment, via ion-conduction pore, buffers the imposed IS-changes in the toxin-channel interface. A Bronsted-Bjerrum analysis on the rates predicts that if, instead of fish, the snail preyed on organisms with seawater-like lymph ionic composition, a severely harmless toxin, with >100-fold diminished affinity, would result. Thus, considerations of the native ionic environment are essential for conotoxins evaluation as pharmacological leads.