Binding of κ-Conotoxin PVIIA to Shaker K⁺ Channels Reveals Different K⁺ and Rb⁺ Occupancies within the Ion Channel Pore

The x-ray structure of the KcsA channel at different [K⁺] and [Rb⁺] provided insight into how K⁺ channels might achieve high selectivity and high K⁺ transit rates and showed marked differences between the occupancies of the two ions within the ion channel pore. In this study, the binding of κ-conotoxin PVIIA (κ-PVIIA) to Shaker K⁺ channel in the presence of K⁺ and Rb⁺ was investigated. It is demonstrated that the complex results obtained were largely rationalized by differences in selectivity filter occupancy of this 6TM channels as predicted from the structural work on KcsA. κ-PVIIA inhibition of the Shaker K⁺ channel differs in the closed and open state. When K⁺ is the only permeant ion, increasing extracellular [K⁺] decreases κ-PVIIA affinity for closed channels by decreasing the “on” binding rate, but has no effect on the block of open channels, which is influenced only by the intracellular [K⁺]. In contrast, extracellular [Rb⁺] affects both closed- and open-channel binding. As extracellular [Rb⁺] increases, (a) binding to the closed channel is slightly destabilized and acquires faster kinetics, and (b) open channel block is also destabilized and the lowest block seems to occur when the pore is likely filled only by Rb⁺. These results suggest that the nature of the permeant ions determines both the occupancy and the location of the pore site from which they interact with κ-PVIIA binding. Thus, our results suggest that the permeant ion(s) within a channel pore can determine its functional and pharmacological properties.