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

• Main Authors: David Naranjo, Ignacio Díaz-Franulic
• Format: Article
• Language: English
• Published: MDPI AG 2020-10-01
• Series: Marine Drugs
• Subjects: Kv-channel; peptide toxin; conotoxin; association rate; dissociation rate; Brønsted-Bjerrum equation
• Online Access: https://www.mdpi.com/1660-3397/18/11/533

κ-Conotoxin-PVIIA (κ-PVIIA) is a potassium-channel blocking peptide from the venom of the fish-hunting snail, <i>Conus purpurascens</i>, which is essential for quick prey’s excitotoxic immobilization. Binding of one κ-PVIIA to <i>Shaker</i> 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 κ-PVIIA blockade to <i>Shaker</i>. When IS varied from ~0.06 to ~0.16 M, the dissociation constant for open and closed channels increased by ~5- and ~16-fold, respectively. While the association rates decreased equally, by ~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 κ-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 Brønsted-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.