Drug Interaction at the Lipid Bilayer-Potassium Channel Interface

The naturally occurring resin acid dehydroabietic acid (DHAA) opens a voltage-gated K channel by shifting the voltage dependence of the channel activation to more negative voltages. We synthesized 140 DHAA-derivatives and found that subtle alterations in the chemical structure of DHAA can have a large effect on the channel opening. Several of the channel-opening DHAA derivatives reduced excitability in dorsal root ganglion neurons, suggesting a possibility to act against diseases with increased cellular excitability. Here we searched for the mechanism and site of action for the DHAA derivatives. We expressed the non-inactivating Shaker K channel in Xenopus oocytes and studied the ion currents with a two-electrode voltage clamp technique. By utilizing the ILT-mutant, which separates the final channel-opening transition from the earlier transitions, we found that the DHAA derivatives had an almost isolated effect on the channel-opening transition. 100 μM of one DHAA derivative affected this last transition by −65 mV. We also found that the charge profile of the voltage sensor S4 was critical for the effect. Addition of positively charged arginines in specific positions increased the channel opening effects of the negatively charged DHAA derivatives, while addition of arginines on the opposite side of the S4 helix had opposing effects, suggesting that the DHAA derivatives support or prevent rotation of S4 to support or prevent channel opening. In general, the DHAA derivatives affected channel closing kinetic but not channel opening kinetic suggesting a rather specific effect and site of action. In-silico docking, molecular dynamics, and mutational analyses suggested a binding site in the cleft between the transmembrane segments S3 and S4. We conclude that certain resin-acid derivatives bind at the interface between the lipid bilayer and S3 and S4 of the channel's voltage-sensor domain, to electrostatically facilitate K channel opening.